Prurigo Nodularis

Complex MONDO:0026045 Pathograph 16 Show in embeddings browser Prurigo

Prurigo nodularis is a chronic neuroimmune skin disease characterized by intensely pruritic, hyperkeratotic nodules that arise from a self-perpetuating itch-scratch cycle. Sustained scratching and a type 2-skewed inflammatory milieu (IL-31, IL-4, IL-13) alongside Th17/Th22 polarization promote dermal fibrosis and neural hyperplasia with increased dermal nerve fibers, sensitizing pruriceptive pathways and lowering the itch threshold. This neuroimmune cross-talk maintains the nodules long after any initiating dermatologic, systemic, or neuropsychiatric trigger, making the condition refractory and self-sustaining.

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Classifications

Harrison's Part
DERMATOLOGY
C

Comorbidities

Type 2 Diabetes Mellitus <-> Union of Lichen Simplex Chronicus, Prurigo Nodularis
Component of Disease B A_BEFORE_B CANDIDATE
⚙

Pathophysiology

10
Cutaneous Type 2 Inflammation
Cutaneous and systemic type 2 helper T-cell (Th2) activation with increased expression of IL-4, IL-13, and IL-4 receptor alpha in prurigo nodules and elevated circulating IL-13. IL-4 (primarily from T lymphocytes and basophils) and IL-13 (primarily from eosinophils) directly activate and sensitize itch-sensitive neurons via IL-4Ralpha, recruit eosinophils and mast cells, and promote dermal fibroblast proliferation and extracellular matrix synthesis.
T-helper 2 cell CL:0000546 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves T-helper 2 cell (CL:0000546). CL:0000546 is a cell type from the Cell Ontology. basophil CL:0000767 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves basophil (CL:0000767). CL:0000767 is a cell type from the Cell Ontology. eosinophil CL:0000771 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves eosinophil (CL:0000771). CL:0000771 is a cell type from the Cell Ontology.
IL4 hgnc:6014 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IL4 (hgnc:6014). hgnc:6014 is a gene from the HUGO Gene Nomenclature Committee. IL13 hgnc:5973 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IL13 (hgnc:5973). hgnc:5973 is a gene from the HUGO Gene Nomenclature Committee. IL4R hgnc:6015 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IL4R (hgnc:6015). hgnc:6015 is a gene from the HUGO Gene Nomenclature Committee.
type 2 immune response GO:0042092 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased type 2 immune response (GO:0042092). GO:0042092 is a biological process from the Gene Ontology. T-helper 2 cell cytokine production GO:0035745 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased T-helper 2 cell cytokine production (GO:0035745). GO:0035745 is a biological process from the Gene Ontology.
skin of body UBERON:0002097 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in skin of body (UBERON:0002097). UBERON:0002097 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Cutaneous Neural Sensitization IL-4 and IL-13 activate itch-sensitive sensory neurons through IL-4Ralpha and amplify neuronal sensitivity to pruritogenic stimuli.
Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling IL-4 and IL-13 promote dermal fibroblast proliferation, differentiation, and extracellular matrix protein synthesis.
Show evidence (3 references)
PMID:38345154 SUPPORT REVIEW SYNTHESIS Other
"There is a role for type 2 inflammation in PN in addition to T-helper (Th)17 and Th22-mediated inflammation."
Establishes the role of type 2 inflammation in prurigo nodularis pathogenesis.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"The expression of IL-4, IL-13, and IL-4 receptor (IL-4R) is upregulated in prurigo nodules, with IL-4 correlating with the intensity of itch ( 8, 11, 26, 27). IL-4 is primarily expressed in T lymphocytes and basophils, whereas IL-13 is primarily expressed in eosinophils (26). Studies have shown..."
Documents upregulation of IL-4, IL-13, and IL-4R in prurigo nodules, their cellular sources (T cells, basophils, eosinophils), and direct activation of itch-sensitive neurons via IL-4Ralpha.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"In addition, IL-4 and IL-13 exert pruritogenic effects by recruiting eosinophils and mast cells, driving IgE synthesis, and interacting with IL-31 ( 30 ). They promote fibroblast proliferation, differentiation, and synthesis of extracellular matrix proteins such as periostin in the dermis,..."
Shows that IL-4 and IL-13 recruit eosinophils and mast cells and promote dermal fibroblast proliferation and extracellular matrix synthesis in prurigo nodularis.
IL-31 and IL-6 Family Neuroimmune Signaling
Markedly elevated lesional and serum IL-31 (secreted primarily by Th2 cells and dermal CD11c+ myeloid dendritic cells) and upregulated IL-6 and MAPK12 signal through heterodimeric IL-31RA/OSMRB receptors and the JAK1/STAT3/STAT6 pathway in lesional skin. IL-31/IL-31RA signaling promotes sensory nerve axon sprouting, sensitizes pruriceptive neurons, and drives downstream keratinocyte proliferation and profibrotic fibroblast responses.
T-helper 2 cell CL:0000546 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves T-helper 2 cell (CL:0000546). CL:0000546 is a cell type from the Cell Ontology. myeloid dendritic cell CL:0000782 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves myeloid dendritic cell (CL:0000782). CL:0000782 is a cell type from the Cell Ontology.
IL31 hgnc:19372 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IL31 (hgnc:19372). hgnc:19372 is a gene from the HUGO Gene Nomenclature Committee. IL31RA hgnc:18969 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IL31RA (hgnc:18969). hgnc:18969 is a gene from the HUGO Gene Nomenclature Committee. OSMR hgnc:8507 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves OSMR (hgnc:8507). hgnc:8507 is a gene from the HUGO Gene Nomenclature Committee. IL6 hgnc:6018 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IL6 (hgnc:6018). hgnc:6018 is a gene from the HUGO Gene Nomenclature Committee. JAK1 hgnc:6190 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves JAK1 (hgnc:6190). hgnc:6190 is a gene from the HUGO Gene Nomenclature Committee. STAT3 hgnc:11364 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves STAT3 (hgnc:11364). hgnc:11364 is a gene from the HUGO Gene Nomenclature Committee. STAT6 hgnc:11368 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves STAT6 (hgnc:11368). hgnc:11368 is a gene from the HUGO Gene Nomenclature Committee.
cell surface receptor signaling pathway via JAK-STAT GO:0007259 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased cell surface receptor signaling pathway via JAK-STAT (GO:0007259). GO:0007259 is a biological process from the Gene Ontology. cytokine-mediated signaling pathway GO:0019221 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased cytokine-mediated signaling pathway (GO:0019221). GO:0019221 is a biological process from the Gene Ontology.
dermis UBERON:0002067 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in dermis (UBERON:0002067). UBERON:0002067 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Cutaneous Neural Sensitization IL-31 directly activates IL-31RA/OSMRB on sensory neurons and enhances neuronal sensitivity to pruritogens.
NGF-Driven Dermal Neuronal Hyperplasia IL-31 promotes sprouting of cutaneous sensory nerve axons and sustains NGF-mediated neuronal dysregulation.
Epidermal Keratinocyte Hyperproliferation IL-31/IL-31RA signaling promotes epidermal keratinocyte proliferation, which is reversed by IL-31RA blockade with nemolizumab.
Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling IL-31/IL-31RA signaling promotes profibrotic COL11A1+ dermal fibroblast activation, which is normalized by nemolizumab.
Show evidence (4 references)
PMID:41333131 SUPPORT PRIMARY RESULT Human clinical
"The upregulation of IL-6, MAPK12, and IL-31 further supported neuroimmune dysregulation in PN."
Lesional skin transcriptomics in 26 patients with prurigo nodularis confirms upregulation of IL-6, MAPK12, and IL-31.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"IL-31 is mainly secreted by Th2 cells, and dermal CD11c + myeloid dendritic cells are an important source of IL-31 in PN ( 33, 34)."
Identifies Th2 cells and dermal CD11c+ myeloid dendritic cells as the primary cellular sources of IL-31 in prurigo nodularis.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"In PN lesions, this receptor is also notably increased, and the intensity of pruritus in PN is closely related to the number of IL-31+ cells and IL- 31Ra+ cells in the dermis ( 33). IL-31 promotes sprouting of skin sensory nerve axons and enhances their sensitivity to pruritogens (41, 42)."
Documents upregulation of the IL-31RA/OSMRB receptor complex in prurigo nodularis dermis and IL-31-driven sensory nerve sprouting and sensitization.
+ 1 more reference
Th17 and Th22 Cutaneous Inflammation
Increased lesional infiltration of CD4+ T-helper 17 (Th17) cells and systemic and cutaneous Th22 polarization with elevated expression of IL-17 and IL-22 in prurigo nodularis lesions, driving excessive proliferation and impaired differentiation of epidermal keratinocytes as well as keratinocyte secretion of pruritogenic mediators.
T-helper 17 cell CL:0000899 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves T-helper 17 cell (CL:0000899). CL:0000899 is a cell type from the Cell Ontology. CD4-positive helper T cell CL:0000492 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves CD4-positive helper T cell (CL:0000492). CL:0000492 is a cell type from the Cell Ontology.
IL17A hgnc:5981 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IL17A (hgnc:5981). hgnc:5981 is a gene from the HUGO Gene Nomenclature Committee. IL22 hgnc:14900 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves IL22 (hgnc:14900). hgnc:14900 is a gene from the HUGO Gene Nomenclature Committee.
T-helper 17 type immune response GO:0072538 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased T-helper 17 type immune response (GO:0072538). GO:0072538 is a biological process from the Gene Ontology. interleukin-17-mediated signaling pathway GO:0097400 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased interleukin-17-mediated signaling pathway (GO:0097400). GO:0097400 is a biological process from the Gene Ontology.
skin of body UBERON:0002097 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in skin of body (UBERON:0002097). UBERON:0002097 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Epidermal Keratinocyte Hyperproliferation IL-17 and IL-22 drive excessive proliferation and altered differentiation of epidermal keratinocytes in prurigo nodularis lesions.
Show evidence (3 references)
PMID:38345154 SUPPORT REVIEW SYNTHESIS Other
"There is a role for type 2 inflammation in PN in addition to T-helper (Th)17 and Th22-mediated inflammation."
Establishes the role of Th17- and Th22-mediated inflammation in prurigo nodularis.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Previous studies have shown an increased expression of IL-17 in lesional PN skin, coupled with an increase in the number of Th17 cells within the lesions ( 9, 27, 49)."
Documents increased IL-17 expression and Th17 cell infiltration in lesional prurigo nodularis skin.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Several studies have shown that the expression of the Th22 cell cytokine IL-22 is elevated in pruritic PN lesions, which is potentially associated with impaired epidermal proliferation and differentiation and skin in flammation ( 27, 51)."
Documents elevated IL-22 expression in pruritic prurigo nodularis lesions and its association with impaired epidermal proliferation and differentiation.
Cutaneous Neuropeptide Release
Activated cutaneous sensory nerve fibers release neuropeptides including substance P and calcitonin gene-related peptide (CGRP) into the dermis, causing vasodilation and further recruitment and activation of dermal inflammatory cells.
sensory neuron CL:0000101 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves sensory neuron (CL:0000101). CL:0000101 is a cell type from the Cell Ontology.
neuropeptide signaling pathway GO:0007218 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased neuropeptide signaling pathway (GO:0007218). GO:0007218 is a biological process from the Gene Ontology. vasodilation GO:0042311 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased vasodilation (GO:0042311). GO:0042311 is a biological process from the Gene Ontology.
dermis UBERON:0002067 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in dermis (UBERON:0002067). UBERON:0002067 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Mast Cell and Eosinophil Degranulation Neuropeptides (substance P and CGRP) recruit and activate dermal mast cells and eosinophils to degranulate and release histamine, neurotoxins, and nerve growth factor.
Show evidence (3 references)
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Substance P (SP), secreted b y sensory neurons, plays an important role in itch signaling within the peripheral and central nervous systems ( 65, 66)."
Identifies substance P secreted by sensory neurons as a key neuropeptide in prurigo nodularis itch signaling.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Calcitonin gene-related peptide (CGRP) is the most abundant neuropeptide in human skin and is often colocalized with SP ( 82)."
Identifies calcitonin gene-related peptide colocalized with substance P in cutaneous sensory nerve fibers.
PMID:38345154 SUPPORT REVIEW SYNTHESIS Other
"The neuroimmune feedback loop in PN involves neuropeptides released from nerve fibres that cause vasodilation and further recruitment of inflammatory cells."
Establishes that neuropeptides released from dermal nerve fibers cause vasodilation and recruit inflammatory cells in prurigo nodularis.
Mast Cell and Eosinophil Degranulation
Dermal mast cells and eosinophils aggregate in close contact with peripheral sensory nerve endings, degranulate, and release neurotoxins, histamine, and nerve growth factor (NGF) into the dermal microenvironment.
mast cell CL:0000097 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves mast cell (CL:0000097). CL:0000097 is a cell type from the Cell Ontology. eosinophil CL:0000771 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves eosinophil (CL:0000771). CL:0000771 is a cell type from the Cell Ontology.
NGF hgnc:7808 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves NGF (hgnc:7808). hgnc:7808 is a gene from the HUGO Gene Nomenclature Committee.
mast cell degranulation GO:0043303 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased mast cell degranulation (GO:0043303). GO:0043303 is a biological process from the Gene Ontology. eosinophil degranulation GO:0043308 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased eosinophil degranulation (GO:0043308). GO:0043308 is a biological process from the Gene Ontology.
dermis UBERON:0002067 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in dermis (UBERON:0002067). UBERON:0002067 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
NGF-Driven Dermal Neuronal Hyperplasia Nerve growth factor released by degranulating mast cells and eosinophils promotes dermal nerve fiber sprouting and neuronal hyperplasia.
Cutaneous Neural Sensitization Neurotoxins, histamine, and NGF released during mast cell and eosinophil degranulation sensitize dermal sensory nerve fibers.
Show evidence (2 references)
PMID:38345154 SUPPORT REVIEW SYNTHESIS Other
"Inflammatory cells, particularly mast cells and eosinophils, degranulate and release neurotoxins, as well as nerve growth factor, which may contribute to the neuronal hyperplasia seen in the dermis of patients with PN and neural sensitization."
Documents degranulation of dermal mast cells and eosinophils with release of neurotoxins and nerve growth factor in prurigo nodularis.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Notably, mast cells and eosinophils often aggregate around the peripheral sensory nerve endings, establishing close contact with them ( 15– 17)."
Describes perineural aggregation of mast cells and eosinophils around sensory nerve endings in lesional prurigo nodularis dermis.
NGF-Driven Dermal Neuronal Hyperplasia
Elevated nerve growth factor (NGF) signaling through upregulated TrkA and p75NTR receptors on cutaneous nerve fibers drives sprouting of afferent nerves and increased density of itch-sensing nerve fibers in the dermis, accompanied by reduced intraepidermal nerve fiber density.
sensory neuron CL:0000101 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves sensory neuron (CL:0000101). CL:0000101 is a cell type from the Cell Ontology.
NGF hgnc:7808 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves NGF (hgnc:7808). hgnc:7808 is a gene from the HUGO Gene Nomenclature Committee.
axonogenesis GO:0007409 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased axonogenesis (GO:0007409). GO:0007409 is a biological process from the Gene Ontology.
dermis UBERON:0002067 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in dermis (UBERON:0002067). UBERON:0002067 is an anatomical location from the Uberon multi-species anatomy ontology. skin epidermis UBERON:0001003 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in skin epidermis (UBERON:0001003). UBERON:0001003 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Cutaneous Neural Sensitization Hyperplastic dermal nerve fibers stimulated by NGF exhibit heightened excitability and sensitization to pruritogens.
Show evidence (2 references)
PMID:38345154 SUPPORT REVIEW SYNTHESIS Other
"Neural dysregulation in PN features a lower density of itch-sensing nerve fibres in the epidermis (upper layer of the skin) and a higher density of itch-sensing nerve fibres in the dermis (lower layer of the skin)."
Documents increased density of itch-sensing nerve fibers in the dermis and decreased density in the epidermis of prurigo nodularis lesions.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"NGF is a neurotrophic factor that primarily activates, sensitizes, and sprouts skin nerve fibers, promoting the release of neuropeptides, such as SP and CGRP ( 96, 98)."
Establishes that NGF drives sprouting, activation, and sensitization of cutaneous nerve fibers.
Cutaneous Neural Sensitization
Heightened sensitivity and spontaneous excitability of cutaneous C-fiber sensory neurons mediated by cytokines, NGF, periostin, and neuropeptides coupled to upregulated TRPV1 ion channels, lowering the itch threshold, sustaining neuropeptide release, and driving intractable pruritus and repetitive scratching.
sensory neuron CL:0000101 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves sensory neuron (CL:0000101). CL:0000101 is a cell type from the Cell Ontology.
TRPV1 hgnc:12716 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves TRPV1 (hgnc:12716). hgnc:12716 is a gene from the HUGO Gene Nomenclature Committee.
transmission of nerve impulse GO:0019226 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased transmission of nerve impulse (GO:0019226). GO:0019226 is a biological process from the Gene Ontology.
dermis UBERON:0002067 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in dermis (UBERON:0002067). UBERON:0002067 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Cutaneous Neuropeptide Release Sensitized and activated cutaneous sensory neurons release substance P and CGRP, closing the neuroimmune positive feedback loop.
Pruritus Neuronal sensitization and hyperexcitability of cutaneous C fibers directly generate severe, intractable pruritus.
Chronic Scratching and Mechanical Skin Trauma Severe pruritus from sensitized cutaneous nerves drives an intractable itch-scratch cycle with repetitive mechanical trauma to reachable skin sites.
Show evidence (3 references)
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Skin neuroimmune interactions and neuronal sensitization play important roles in mediating chronic itch in PN ( 1)."
Identifies neuronal sensitization as a central driver of chronic pruritus in prurigo nodularis.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Simultaneously, activated sensory neurons secrete pruritogens such as neuropeptides. On one hand, these neuropeptides act on the neurons themselves to increase their own sensitivity and spontaneous activity; on the other hand, they reciprocally activate immune cells and sustain and amplify..."
Explains how activated sensory neurons increase their own sensitivity and spontaneous activity while sustaining neuropeptide release and chronic itch.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"TRPV1, also known as the capsaicin receptor, is significantly upregulated in nerve fibers and keratinocytes within lesional PN skin ( 127 )."
Documents significant upregulation of the TRPV1 ion channel in lesional nerve fibers in prurigo nodularis.
Chronic Scratching and Mechanical Skin Trauma
Self-sustained repetitive scratching in response to intractable pruritus causes recurrent epidermal excoriation, crusting, mechanical barrier trauma, and upregulation of skin-derived NGF and its receptors, inducing and maintaining epidermal hyperplasia and dermal fibrosis in prurigo nodules.
skin of body UBERON:0002097 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in skin of body (UBERON:0002097). UBERON:0002097 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Epidermal Keratinocyte Hyperproliferation Repetitive mechanical scratching and excoriation stimulate reactive epidermal thickening and keratinocyte proliferation.
Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling Chronic scratching and mechanical trauma induce and maintain dermal fibrotic remodeling in prurigo nodules.
NGF-Driven Dermal Neuronal Hyperplasia Scratching increases expression of skin-derived NGF and its receptors, amplifying dermal nerve sprouting.
Sleep Disturbance Intractable nocturnal itch-scratch cycles disrupt sleep.
Show evidence (3 references)
PMID:37506977 SUPPORT BACKGROUND Human clinical
"Neuroimmune dysregulation and chronic scratching are believed to both induce and maintain the characteristic lesions."
Establishes chronic scratching alongside neuroimmune dysregulation as a primary driver that induces and maintains prurigo nodularis lesions.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Current understanding suggests that a persistent “itch-scratch” vicious cycle, leading to recurrent skin excoriation, crusting, and thickening, is the primary driver of nodule formation ( 3)."
Shows that the persistent itch-scratch cycle causes recurrent skin excoriation, crusting, and thickening that drive nodule formation.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Scratching can also lead to increased expression of skin-derived NGF and its receptors ( 97)."
Documents that mechanical scratching upregulates cutaneous NGF and its receptors, reinforcing neuroimmune sensitization.
Epidermal Keratinocyte Hyperproliferation
Excessive proliferation and altered differentiation of epidermal keratinocytes in lesional prurigo nodularis skin driven by chronic scratching and IL-17, IL-22, and IL-31 signaling, producing marked epidermal hyperkeratosis overlying the dermal nodules.
keratinocyte CL:0000312 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves keratinocyte (CL:0000312). CL:0000312 is a cell type from the Cell Ontology.
keratinocyte proliferation GO:0043616 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased keratinocyte proliferation (GO:0043616). GO:0043616 is a biological process from the Gene Ontology.
skin epidermis UBERON:0001003 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in skin epidermis (UBERON:0001003). UBERON:0001003 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Hyperkeratosis Excessive epidermal keratinocyte proliferation produces hyperkeratotic thickening over prurigo nodules.
Show evidence (1 reference)
PMID:37506977 SUPPORT PRIMARY RESULT Human clinical
"This study demonstrates that PN is an inflammatory skin disease characterized by both keratinocyte proliferation and activation of profibrotic responses."
Single-cell profiling of human prurigo nodularis biopsies establishes epidermal keratinocyte proliferation as a core pathological feature.
Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling
Expansion and activation of profibrotic papillary dermal COL11A1+ fibroblasts and POSTN+WNT5A+ myofibroblast/cancer-associated fibroblast-like subclusters, accompanied by upregulation of MMP14, COL6A3, TGFBR1, and dermal periostin deposition, which distinguish prurigo nodularis from atopic dermatitis, drive nodular dermal fibrosis, and further stimulate itch-sensing nerve fibers.
fibroblast CL:0000057 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves fibroblast (CL:0000057). CL:0000057 is a cell type from the Cell Ontology.
COL11A1 hgnc:2186 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves COL11A1 (hgnc:2186). hgnc:2186 is a gene from the HUGO Gene Nomenclature Committee. WNT5A hgnc:12784 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves WNT5A (hgnc:12784). hgnc:12784 is a gene from the HUGO Gene Nomenclature Committee. MMP14 hgnc:7160 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves MMP14 (hgnc:7160). hgnc:7160 is a gene from the HUGO Gene Nomenclature Committee. COL6A3 hgnc:2213 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves COL6A3 (hgnc:2213). hgnc:2213 is a gene from the HUGO Gene Nomenclature Committee. TGFBR1 hgnc:11772 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves TGFBR1 (hgnc:11772). hgnc:11772 is a gene from the HUGO Gene Nomenclature Committee.
fibroblast activation GO:0072537 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased fibroblast activation (GO:0072537). GO:0072537 is a biological process from the Gene Ontology. extracellular matrix organization GO:0030198 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased extracellular matrix organization (GO:0030198). GO:0030198 is a biological process from the Gene Ontology. transforming growth factor beta receptor signaling pathway GO:0007179 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased transforming growth factor beta receptor signaling pathway (GO:0007179). GO:0007179 is a biological process from the Gene Ontology.
papillary layer of dermis UBERON:0001992 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in papillary layer of dermis (UBERON:0001992). UBERON:0001992 is an anatomical location from the Uberon multi-species anatomy ontology. dermis UBERON:0002067 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in dermis (UBERON:0002067). UBERON:0002067 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Skin nodule Dermal fibrosis and extracellular matrix deposition form firm, dome-shaped skin nodules.
Cutaneous Neural Sensitization Extracellular matrix periostin deposited in the dermis of prurigo nodularis directly interacts with integrin alphaVbeta3 on itch-sensitive nerve fibers and stimulates immune cells to release IL-31.
Show evidence (3 references)
PMID:37506977 SUPPORT PRIMARY RESULT Human clinical
"This study demonstrates that PN is an inflammatory skin disease characterized by both keratinocyte proliferation and activation of profibrotic responses. This study also demonstrates that the COL11A1+ fibroblast subset is a major contributor to fibrosis and is predominantly found in the..."
Single-cell and spatial transcriptomics of human PN biopsies identify papillary dermal COL11A1+ fibroblasts as primary drivers of fibrosis that distinguish PN from atopic dermatitis.
PMID:38246584 SUPPORT PRIMARY RESULT Human clinical
"Lesional PN has increased dysregulated fibroblasts (FBs) and myofibroblasts. FBs in lesional PN were shifted toward a cancer-associated FB-like phenotype, with POSTN+WNT5A+ cancer-associated FBs increased in PN and similarly so in squamous cell carcinoma."
Single-cell RNA-seq of 28,695 lesional and nonlesional cells identifies expanded POSTN+WNT5A+ fibroblast subclusters in prurigo nodularis.
PMID:41333131 SUPPORT PRIMARY RESULT Human clinical
"MMP14, COL6A3, and TGFBR1 were highly upregulated in PN, reflecting an active fibrotic process."
Confirms upregulation of MMP14, COL6A3, and TGFBR1 in lesional PN skin.
⬡

Pathograph

Use Group by to collapse related nodes into one box. Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Prurigo Nodularis Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
●

Phenotypes

7
Immune 1
Atopic Dermatitis FREQUENT HP:0001047 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Atopic dermatitis (HP:0001047). HP:0001047 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Notably, approximately half of the patients with PN exhibit coexistent atopic dermatitis (AD) or atopic predisposition (2, 5)."
Documents that approximately half of patients with prurigo nodularis have coexistent atopic dermatitis or an atopic predisposition.
Integument 3
Pruritus HP:0000989 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Pruritus (HP:0000989). HP:0000989 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38345154 SUPPORT Other
"Prurigo nodularis (PN) is an inflammatory skin condition characterized by intensely pruritic nodules on the skin. Patients with PN suffer from an intractable itch-scratch cycle leading to impaired sleep, psychosocial distress and a significant disruption in quality of life."
Establishes intense pruritus and the intractable itch-scratch cycle as defining clinical features of prurigo nodularis.
Skin nodule HP:0200036 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Skin nodule (HP:0200036). HP:0200036 is a phenotype from the Human Phenotype Ontology.
Show evidence (3 references)
PMID:38345154 SUPPORT Other
"Prurigo nodularis (PN) is an inflammatory skin condition characterized by intensely pruritic nodules on the skin."
Establishes pruritic skin nodules as the hallmark lesion of prurigo nodularis.
PMID:37506977 SUPPORT Human clinical
"Prurigo nodularis (PN) is a chronic neuroimmune skin disease characterized by bilaterally distributed pruritic hyperkeratotic nodules on extremities and trunk."
Describes the bilateral distribution of hyperkeratotic nodules on the extremities and trunk.
DOI:10.3389/fimmu.2023.1301817 SUPPORT Other
"Multiple discrete nodules are often symmetrically distributed in scratchable areas, such as the extensor surfaces of the limbs and trunk, with fewer lesions on the harder-to-reach mid-upper back, creating the classical butter flys i g n(3)."
Documents the symmetric distribution on scratchable extensor surfaces and trunk with mid-upper back sparing (butterfly sign).
Hyperkeratosis HP:0000962 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hyperkeratosis (HP:0000962). HP:0000962 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:37506977 SUPPORT Human clinical
"Prurigo nodularis (PN) is a chronic neuroimmune skin disease characterized by bilaterally distributed pruritic hyperkeratotic nodules on extremities and trunk."
Confirms that prurigo nodularis nodules are hyperkeratotic.
Nervous System 3
Sleep Disturbance HP:0002360 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Sleep disturbance (HP:0002360). HP:0002360 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38345154 SUPPORT Other
"Patients with PN suffer from an intractable itch-scratch cycle leading to impaired sleep, psychosocial distress and a significant disruption in quality of life."
Documents impaired sleep as a direct consequence of the intractable itch-scratch cycle in prurigo nodularis.
Anxiety HP:0000739 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Anxiety (HP:0000739). HP:0000739 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"A previous study has shown that PN exerts a more significant impact on quality of life and carries a higher risk of psychological disorders (e.g., anxiety and depression) than other pruritic dermatoses ( 4)."
Documents the increased risk of anxiety in patients with prurigo nodularis compared with other pruritic dermatoses.
Depression HP:0000716 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Depression (HP:0000716). HP:0000716 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"A previous study has shown that PN exerts a more significant impact on quality of life and carries a higher risk of psychological disorders (e.g., anxiety and depression) than other pruritic dermatoses ( 4)."
Documents the increased risk of depression in patients with prurigo nodularis compared with other pruritic dermatoses.
💊

Medical Actions

2
Dupilumab
Action: Monoclonal Antibody TherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Monoclonal Antibody Therapy (NCIT:C15490). NCIT:C15490 is a clinical intervention from the NCI Thesaurus. NCIT:C15490
Agent: Dupilumab NCIT:C162455 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses Dupilumab (NCIT:C162455). NCIT:C162455 is a therapeutic agent from the NCI Thesaurus.
Platform: Monoclonal antibody
Monoclonal antibody targeting IL-4 receptor alpha (IL-4Ralpha) that blocks IL-4 and IL-13 signaling; the first FDA- and EMA-approved systemic therapy for adult patients with prurigo nodularis, demonstrating significant reductions in worst-itch scores in the phase 3 PRIME and PRIME2 trials.
Mechanism Target:
Cutaneous Type 2 Inflammation
Target Phenotypes: Pruritus HP:0000989 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Pruritus (HP:0000989). HP:0000989 is a phenotype from the Human Phenotype Ontology. Skin nodule HP:0200036 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Skin nodule (HP:0200036). HP:0200036 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:38345154 SUPPORT Other
"The only drug currently approved for PN in the USA and Europe is dupilumab, an IL-4Rα inhibitor that blocks signalling through IL-4 and IL-13, which is undergoing post-marketing surveillance."
Documents approval of dupilumab as an IL-4Ralpha inhibitor blocking IL-4 and IL-13 signaling in prurigo nodularis.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Based on two phase 3 clinical trials, dupilumab (an IL-4R a monoclonal antibody simultaneously antagonizing IL-4 and IL-13) has become the first the U.S. Food and Drug Administration (FDA)/European Medicines Agency (EMA)/China National Medical Products Administration (NMPA)-approved treatment for..."
Summarizes the phase 3 PRIME and PRIME2 trial efficacy data supporting regulatory approval of dupilumab for prurigo nodularis.
Nemolizumab
Action: Monoclonal Antibody TherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Monoclonal Antibody Therapy (NCIT:C15490). NCIT:C15490 is a clinical intervention from the NCI Thesaurus. NCIT:C15490
Agent: Nemolizumab NCIT:C170211 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses Nemolizumab (NCIT:C170211). NCIT:C170211 is a therapeutic agent from the NCI Thesaurus.
Platform: Monoclonal antibody
Monoclonal antibody targeting the IL-31 receptor A (IL-31RA) that blocks IL-31-driven pruritus, significantly reduces peak pruritus and skin lesion severity in randomized clinical trials, and reverses pathological COL11A1+ fibroblast and keratinocyte transcriptional responses in prurigo nodularis skin.
Mechanism Target:
IL-31 and IL-6 Family Neuroimmune Signaling
Epidermal Keratinocyte Hyperproliferation
Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling
Target Phenotypes: Pruritus HP:0000989 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Pruritus (HP:0000989). HP:0000989 is a phenotype from the Human Phenotype Ontology. Skin nodule HP:0200036 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Skin nodule (HP:0200036). HP:0200036 is a phenotype from the Human Phenotype Ontology.
Show evidence (3 references)
PMID:32074418 SUPPORT PRIMARY RESULT Human clinical
"Nemolizumab resulted in a greater reduction in pruritus and severity of skin lesions than placebo in patients with prurigo nodularis but was associated with adverse events."
Phase 2 randomized placebo-controlled trial (NCT03181503) demonstrating that nemolizumab reduces pruritus and skin lesion severity in moderate-to-severe prurigo nodularis.
PMID:38345154 SUPPORT Other
"The anti-IL-31 receptor A inhibitor nemolizumab is in late-stage development with positive phase III data reported."
Identifies nemolizumab as an anti-IL-31 receptor A biologic with positive phase III trial results in prurigo nodularis.
PMID:37506977 SUPPORT PRIMARY RESULT Human clinical
"This study further shows the broad effect of nemolizumab on PN cell types, with a prominent effect driving COL11A1+ fibroblast and keratinocyte responses toward normal."
Demonstrates that nemolizumab normalizes pathological COL11A1+ fibroblast and keratinocyte transcriptional programs in human PN skin.
📊

Prevalence

1
General population
Point prevalence 72.0 per 100,000 1–9 per 10,000
Source reports that prurigo nodularis 'affects approximately 72 per 100 000 people'.
Show evidence (1 reference)
PMID:38345154 SUPPORT Other
"PN affects approximately 72 per 100 000 people and the severe itch associated with the condition can negatively impact a person’s sleep, work and social life."
Reports a population prevalence of approximately 72 per 100,000 people.
🔀

Differential Diagnoses

1

Conditions with similar clinical presentations that must be differentiated from Prurigo Nodularis:

Atopic Dermatitis MONDO:0004980
Overlapping Features Atopic dermatitis frequently coexists with prurigo nodularis (present or predisposed in approximately half of patients) and shares type 2 immune activation, but prurigo nodularis represents a distinct clinical and transcriptomic entity.
Distinguishing Features
  • Prurigo nodularis is distinguished histologically and transcriptomically from atopic dermatitis by prominent papillary dermal fibrosis driven by COL11A1+ fibroblasts, stromal remodeling, and neurovascular dysregulation rather than the strong type 2 epidermal response typical of atopic dermatitis.
Show evidence (2 references)
PMID:37506977 SUPPORT PRIMARY RESULT Human clinical
"Activation of fibrotic responses is the main distinguishing feature between PN and atopic dermatitis skin."
Single-cell transcriptomic comparison establishes dermal fibrotic activation as the primary feature distinguishing prurigo nodularis from atopic dermatitis.
DOI:10.3389/fimmu.2023.1301817 SUPPORT REVIEW SYNTHESIS Other
"Although AD and PN are both type 2 inflammatory diseases, recent transcriptomic studies have clearly revealed that PN is separated from AD. PN does not harbor the strong type 2 response pattern that is typically found in AD but is rather characterized by stromal remodeling and neurovascular..."
Contrasts the stromal remodeling and neurovascular dysregulation of prurigo nodularis with the strong type 2 inflammatory profile of atopic dermatitis.
📊

Related Datasets

3
Neuroimmune gene signature of prurigo nodularis compared to psoriasis and atopic ​​dermatitis​​ geo:GSE261704
Prurigo nodularis (PN) is a debilitating neuroimmune skin disease characterized by pruritic, raised, nodular lesions. Psoriasis and AD are classically characterized by Th1/Th17 and Th2 polarization, respectively, but the neuroinflammatory profile of PN remains poorly defined. We characterized the neuroimmune phenotype of PN compared to AD, psoriasis, and healthy controls (HC), through transcriptomic analysis of lesional and nonlesional skin biopsies from 25 PN patients, 27 AD patients, 15 psoriasis patients, and 12 HC using a custom neuroinflammation-focused NanoString panel of 770 genes.
human MICROARRAY n=120
PMID:41333131
Identified by GEO DataSets index search for Prurigo Nodularis (scripts/discover_datasets.py); accession and metadata verified against NCBI E-utilities on 2026-08-01. Title, sample count, and organism are GEO's own values.
Show evidence (1 reference)
PMID:41333131 SUPPORT Human clinical
"In this study, we performed NanoString-based transcriptomic profiling on lesional skin biopsies from patients with PN (n = 26), those with atopic dermatitis (n = 25), those with psoriasis (n = 15), and healthy controls (n = 12) using a custom neuroinflammation-focused panel."
Primary publication reporting the NanoString transcriptomic dataset of prurigo nodularis, atopic dermatitis, psoriasis, and healthy control skin biopsies.
Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab geo:GSE273559
Prurigo nodularis (PN) is a chronic neuroimmune skin disease characterized by bilaterally distributed pruritic hyperkeratotic nodules on extremities and trunk. Neuroimmune dysregulation and chronic scratching are believed to both induce and maintain the characteristic lesions. This study sought to provide a comprehensive view of the molecular pathogenesis of PN at the single-cell level to identify and outline key pathologic processes and the cell types involved. Features that distinguish PN skin from the skin of patients with atopic dermatitis were of particular interest.
human SINGLE CELL RNA SEQ n=12
PMID:37506977
Identified by GEO DataSets index search for Prurigo Nodularis (scripts/discover_datasets.py); accession and metadata verified against NCBI E-utilities on 2026-08-01. Title, sample count, and organism are GEO's own values.
Show evidence (1 reference)
PMID:37506977 SUPPORT Human clinical
"Single-cell RNA-sequencing of skin from 15 healthy donors and nonlesional and lesional skin from 6 patients each with PN and atopic dermatitis, combined with spatial-sequencing using the 10x Visium platform."
Primary publication generating the single-cell RNA-seq dataset of prurigo nodularis skin.
Single-cell RNA sequencing reveals dysregulated fibroblast subclusters in prurigo nodularis geo:GSE233280
Prurigo nodularis (PN) is an intensely pruritic, inflammatory skin disease with a poorly understood pathogenesis. Thus, we performed single-cell transcriptomic profiling of 28,695 lesional and non-lesional PN cells. Lesional PN has increased dysregulated fibroblasts (FBs) and myofibroblasts. FBs in lesional PN were shifted towards a cancer-associated fibroblast (CAF)-like phenotype, with POSTN+WNT5A+ CAFs increased in PN, and similarly so in squamous cell carcinoma. A multi-center cohort study revealed an increased risk of SCC and CAF-associated malignancies (breast and colorectal) in PN patients.
human SINGLE CELL RNA SEQ n=12
PMID:38246584
Identified by GEO DataSets index search for Prurigo Nodularis (scripts/discover_datasets.py); accession and metadata verified against NCBI E-utilities on 2026-08-01. Title, sample count, and organism are GEO's own values.
Show evidence (1 reference)
PMID:38246584 SUPPORT Human clinical
"We performed single-cell transcriptomic profiling of 28,695 lesional and nonlesional PN cells."
Primary publication reporting the single-cell RNA-seq dataset of lesional and nonlesional prurigo nodularis skin.
🔬

Clinical Trials

1
NCT03181503 PHASE_II COMPLETED
Randomized, double-blind, placebo-controlled 12-week phase 2 trial of subcutaneous nemolizumab (0.5 mg/kg at baseline, week 4, and week 8) in 70 patients with moderate-to-severe prurigo nodularis (>=20 nodules) and severe pruritus, demonstrating a 53.0% reduction in peak pruritus numerical rating scale score at week 4 compared with 20.2% with placebo.
Target Phenotypes: Pruritus HP:0000989 Human Phenotype Ontology (HP) Relation: this clinical trial targets this phenotype This clinical trial targets Pruritus (HP:0000989). HP:0000989 is a phenotype from the Human Phenotype Ontology. Skin nodule HP:0200036 Human Phenotype Ontology (HP) Relation: this clinical trial targets this phenotype This clinical trial targets Skin nodule (HP:0200036). HP:0200036 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:32074418 SUPPORT PRIMARY RESULT Human clinical
"We conducted a 12-week, randomized, double-blind, phase 2 trial of nemolizumab (at a dose of 0.5 mg per kilogram of body weight) administered subcutaneously at baseline, week 4, and week 8, as compared with placebo, in patients with moderate-to-severe prurigo nodularis and severe pruritus."
Describes the trial design, patient population, and dosing regimen of the phase 2 nemolizumab trial in prurigo nodularis.
PMID:32074418 SUPPORT PRIMARY RESULT Human clinical
"(Funded by Galderma; ClinicalTrials.gov number, NCT03181503.)"
Identifies ClinicalTrials.gov registration NCT03181503 for the phase 2 trial of nemolizumab in prurigo nodularis.
{ }

Source YAML

click to show 
name: Prurigo Nodularis
creation_date: '2026-02-02T00:16:36Z'
description: >-
  Prurigo nodularis is a chronic neuroimmune skin disease characterized by
  intensely pruritic, hyperkeratotic nodules that arise from a self-perpetuating
  itch-scratch cycle. Sustained scratching and a type 2-skewed inflammatory
  milieu (IL-31, IL-4, IL-13) alongside Th17/Th22 polarization promote dermal
  fibrosis and neural hyperplasia with increased dermal nerve fibers,
  sensitizing pruriceptive pathways and lowering the itch threshold. This
  neuroimmune cross-talk maintains the nodules long after any initiating
  dermatologic, systemic, or neuropsychiatric trigger, making the condition
  refractory and self-sustaining.
category: Complex
parents:
- Prurigo
disease_term:
  preferred_term: prurigo nodularis
  term:
    id: MONDO:0026045
    label: prurigo nodularis
classifications:
  harrisons_chapter:
  - classification_value: DERMATOLOGY
    evidence:
    - reference: PMID:38345154
      reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Prurigo nodularis (PN) is an inflammatory skin condition characterized by intensely pruritic nodules on the skin."
      explanation: Classifies prurigo nodularis as an inflammatory dermatological disorder.
prevalence:
- population: General population
  measure_type: POINT_PREVALENCE
  prevalence_class: BAND_1_5_PER_10000
  rate_per_100000: 72.0
  rate_denominator: POPULATION
  notes: "Source reports that prurigo nodularis 'affects approximately 72 per 100 000 people'."
  evidence:
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "PN affects approximately 72 per 100 000 people and the severe itch associated with the condition can negatively impact a person’s sleep, work and social life."
    explanation: Reports a population prevalence of approximately 72 per 100,000 people.
pathophysiology:
- name: Cutaneous Type 2 Inflammation
  description: >-
    Cutaneous and systemic type 2 helper T-cell (Th2) activation with increased
    expression of IL-4, IL-13, and IL-4 receptor alpha in prurigo nodules and
    elevated circulating IL-13. IL-4 (primarily from T lymphocytes and basophils)
    and IL-13 (primarily from eosinophils) directly activate and sensitize
    itch-sensitive neurons via IL-4Ralpha, recruit eosinophils and mast cells,
    and promote dermal fibroblast proliferation and extracellular matrix
    synthesis.
  biological_scale: CELLULAR
  cell_types:
  - preferred_term: T-helper 2 cell
    term:
      id: CL:0000546
      label: T-helper 2 cell
  - preferred_term: basophil
    term:
      id: CL:0000767
      label: basophil
  - preferred_term: eosinophil
    term:
      id: CL:0000771
      label: eosinophil
  genes:
  - preferred_term: IL4
    term:
      id: hgnc:6014
      label: IL4
  - preferred_term: IL13
    term:
      id: hgnc:5973
      label: IL13
  - preferred_term: IL4R
    term:
      id: hgnc:6015
      label: IL4R
  biological_processes:
  - preferred_term: type 2 immune response
    modifier: INCREASED
    term:
      id: GO:0042092
      label: type 2 immune response
  - preferred_term: T-helper 2 cell cytokine production
    modifier: INCREASED
    term:
      id: GO:0035745
      label: T-helper 2 cell cytokine production
  locations:
  - preferred_term: skin of body
    term:
      id: UBERON:0002097
      label: skin of body
  evidence:
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "There is a role for type 2 inflammation in PN in addition to T-helper (Th)17 and Th22-mediated inflammation."
    explanation: Establishes the role of type 2 inflammation in prurigo nodularis pathogenesis.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "The expression of IL-4, IL-13, and IL-4 receptor (IL-4R) is upregulated in prurigo nodules, with IL-4 correlating with the intensity of itch ( 8, 11, 26, 27). IL-4 is primarily expressed in T lymphocytes and basophils, whereas IL-13 is primarily expressed in eosinophils (26). Studies have shown that IL-4 and IL-13 activate itch-sensitive neurons in both humans and mice through IL-4R a (28)."
    explanation: Documents upregulation of IL-4, IL-13, and IL-4R in prurigo nodules, their cellular sources (T cells, basophils, eosinophils), and direct activation of itch-sensitive neurons via IL-4Ralpha.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "In addition, IL-4 and IL-13 exert pruritogenic effects by recruiting eosinophils and mast cells, driving IgE synthesis, and interacting with IL-31 ( 30 ). They promote fibroblast proliferation, differentiation, and synthesis of extracellular matrix proteins such as periostin in the dermis, mediating PN-associated skin fibrosis and itch (20)."
    explanation: Shows that IL-4 and IL-13 recruit eosinophils and mast cells and promote dermal fibroblast proliferation and extracellular matrix synthesis in prurigo nodularis.
  downstream:
  - target: Cutaneous Neural Sensitization
    causal_link_type: DIRECT
    description: IL-4 and IL-13 activate itch-sensitive sensory neurons through IL-4Ralpha and amplify neuronal sensitivity to pruritogenic stimuli.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Studies have shown that IL-4 and IL-13 activate itch-sensitive neurons in both humans and mice through IL-4R a (28). They also amplify neuronal responses to various pruritogenic stimuli, including histamine-dependent and histamine-independent pathways, thereby contributing to neuronal sensitization ( 29)."
      explanation: Demonstrates that IL-4 and IL-13 directly activate and sensitize itch-sensing neurons via IL-4Ralpha.
  - target: Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling
    causal_link_type: DIRECT
    description: IL-4 and IL-13 promote dermal fibroblast proliferation, differentiation, and extracellular matrix protein synthesis.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "They promote fibroblast proliferation, differentiation, and synthesis of extracellular matrix proteins such as periostin in the dermis, mediating PN-associated skin fibrosis and itch (20)."
      explanation: Establishes that type 2 cytokines IL-4 and IL-13 drive dermal fibroblast proliferation and extracellular matrix production in prurigo nodularis.
- name: IL-31 and IL-6 Family Neuroimmune Signaling
  description: >-
    Markedly elevated lesional and serum IL-31 (secreted primarily by Th2 cells
    and dermal CD11c+ myeloid dendritic cells) and upregulated IL-6 and MAPK12
    signal through heterodimeric IL-31RA/OSMRB receptors and the JAK1/STAT3/STAT6
    pathway in lesional skin. IL-31/IL-31RA signaling promotes sensory nerve
    axon sprouting, sensitizes pruriceptive neurons, and drives downstream
    keratinocyte proliferation and profibrotic fibroblast responses.
  biological_scale: MOLECULAR
  cell_types:
  - preferred_term: T-helper 2 cell
    term:
      id: CL:0000546
      label: T-helper 2 cell
  - preferred_term: myeloid dendritic cell
    term:
      id: CL:0000782
      label: myeloid dendritic cell
  genes:
  - preferred_term: IL31
    term:
      id: hgnc:19372
      label: IL31
  - preferred_term: IL31RA
    term:
      id: hgnc:18969
      label: IL31RA
  - preferred_term: OSMR
    term:
      id: hgnc:8507
      label: OSMR
  - preferred_term: IL6
    term:
      id: hgnc:6018
      label: IL6
  - preferred_term: JAK1
    term:
      id: hgnc:6190
      label: JAK1
  - preferred_term: STAT3
    term:
      id: hgnc:11364
      label: STAT3
  - preferred_term: STAT6
    term:
      id: hgnc:11368
      label: STAT6
  biological_processes:
  - preferred_term: cell surface receptor signaling pathway via JAK-STAT
    modifier: INCREASED
    term:
      id: GO:0007259
      label: cell surface receptor signaling pathway via JAK-STAT
  - preferred_term: cytokine-mediated signaling pathway
    modifier: INCREASED
    term:
      id: GO:0019221
      label: cytokine-mediated signaling pathway
  locations:
  - preferred_term: dermis
    term:
      id: UBERON:0002067
      label: dermis
  evidence:
  - reference: PMID:41333131
    reference_title: Nanostring Transcriptomic Analysis Highlights IL-6 Family and TGF-β Pathways in the Pathogenesis of Prurigo Nodularis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "The upregulation of IL-6, MAPK12, and IL-31 further supported neuroimmune dysregulation in PN."
    explanation: Lesional skin transcriptomics in 26 patients with prurigo nodularis confirms upregulation of IL-6, MAPK12, and IL-31.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "IL-31 is mainly secreted by Th2 cells, and dermal CD11c + myeloid dendritic cells are an important source of IL-31 in PN ( 33, 34)."
    explanation: Identifies Th2 cells and dermal CD11c+ myeloid dendritic cells as the primary cellular sources of IL-31 in prurigo nodularis.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "In PN lesions, this receptor is also notably increased, and the intensity of pruritus in PN is closely related to the number of IL-31+ cells and IL- 31Ra+ cells in the dermis ( 33). IL-31 promotes sprouting of skin sensory nerve axons and enhances their sensitivity to pruritogens (41, 42)."
    explanation: Documents upregulation of the IL-31RA/OSMRB receptor complex in prurigo nodularis dermis and IL-31-driven sensory nerve sprouting and sensitization.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Several key cytokines, such as IL-4, IL-13, IL-31, and OSM, play roles in propagating pruritus and in flammation in PN through the JAK-STAT signaling pathway, particularly involving JAK1, STAT3, and STAT6 ( 28, 45, 108)."
    explanation: Establishes that IL-31 and related cytokines propagate pruritus and inflammation in prurigo nodularis via JAK1, STAT3, and STAT6 signaling.
  downstream:
  - target: Cutaneous Neural Sensitization
    causal_link_type: DIRECT
    description: IL-31 directly activates IL-31RA/OSMRB on sensory neurons and enhances neuronal sensitivity to pruritogens.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "IL-31 promotes sprouting of skin sensory nerve axons and enhances their sensitivity to pruritogens (41, 42)."
      explanation: Demonstrates that IL-31 enhances sensory neuron sensitivity to pruritogens.
  - target: NGF-Driven Dermal Neuronal Hyperplasia
    causal_link_type: DIRECT
    description: IL-31 promotes sprouting of cutaneous sensory nerve axons and sustains NGF-mediated neuronal dysregulation.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Blocking IL-31R a not only effectively inhibits pruritus signaling but also alleviates downstream Th2 and Th17 in flammatory responses, suppresses nerve growth factor (NGF)-mediated neuronal dysregulation, and reverses the activation of keratinocyte proliferation and pro- fibrotic reactions (23, 43, 44)."
      explanation: Shows that IL-31RA signaling drives NGF-mediated neuronal dysregulation, keratinocyte proliferation, and profibrotic responses in prurigo nodularis.
  - target: Epidermal Keratinocyte Hyperproliferation
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: IL-31/IL-31RA signaling promotes epidermal keratinocyte proliferation, which is reversed by IL-31RA blockade with nemolizumab.
    evidence:
    - reference: PMID:37506977
      reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: PRIMARY_RESULT
      snippet: "This study further shows the broad effect of nemolizumab on PN cell types, with a prominent effect driving COL11A1+ fibroblast and keratinocyte responses toward normal."
      explanation: Reversal of pathological keratinocyte responses by IL-31RA blockade with nemolizumab supports IL-31 signaling as an upstream driver of keratinocyte hyperproliferation.
  - target: Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: IL-31/IL-31RA signaling promotes profibrotic COL11A1+ dermal fibroblast activation, which is normalized by nemolizumab.
    evidence:
    - reference: PMID:37506977
      reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: PRIMARY_RESULT
      snippet: "This study further shows the broad effect of nemolizumab on PN cell types, with a prominent effect driving COL11A1+ fibroblast and keratinocyte responses toward normal."
      explanation: Normalization of COL11A1+ fibroblast transcriptional programs by nemolizumab demonstrates that IL-31RA signaling drives profibrotic fibroblast activation in prurigo nodularis.
- name: Th17 and Th22 Cutaneous Inflammation
  description: >-
    Increased lesional infiltration of CD4+ T-helper 17 (Th17) cells and
    systemic and cutaneous Th22 polarization with elevated expression of IL-17
    and IL-22 in prurigo nodularis lesions, driving excessive proliferation and
    impaired differentiation of epidermal keratinocytes as well as keratinocyte
    secretion of pruritogenic mediators.
  biological_scale: CELLULAR
  cell_types:
  - preferred_term: T-helper 17 cell
    term:
      id: CL:0000899
      label: T-helper 17 cell
  - preferred_term: CD4-positive helper T cell
    term:
      id: CL:0000492
      label: CD4-positive helper T cell
  genes:
  - preferred_term: IL17A
    term:
      id: hgnc:5981
      label: IL17A
  - preferred_term: IL22
    term:
      id: hgnc:14900
      label: IL22
  biological_processes:
  - preferred_term: T-helper 17 type immune response
    modifier: INCREASED
    term:
      id: GO:0072538
      label: T-helper 17 type immune response
  - preferred_term: interleukin-17-mediated signaling pathway
    modifier: INCREASED
    term:
      id: GO:0097400
      label: interleukin-17-mediated signaling pathway
  locations:
  - preferred_term: skin of body
    term:
      id: UBERON:0002097
      label: skin of body
  evidence:
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "There is a role for type 2 inflammation in PN in addition to T-helper (Th)17 and Th22-mediated inflammation."
    explanation: Establishes the role of Th17- and Th22-mediated inflammation in prurigo nodularis.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Previous studies have shown an increased expression of IL-17 in lesional PN skin, coupled with an increase in the number of Th17 cells within the lesions ( 9, 27, 49)."
    explanation: Documents increased IL-17 expression and Th17 cell infiltration in lesional prurigo nodularis skin.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Several studies have shown that the expression of the Th22 cell cytokine IL-22 is elevated in pruritic PN lesions, which is potentially associated with impaired epidermal proliferation and differentiation and skin in flammation ( 27, 51)."
    explanation: Documents elevated IL-22 expression in pruritic prurigo nodularis lesions and its association with impaired epidermal proliferation and differentiation.
  downstream:
  - target: Epidermal Keratinocyte Hyperproliferation
    causal_link_type: DIRECT
    description: IL-17 and IL-22 drive excessive proliferation and altered differentiation of epidermal keratinocytes in prurigo nodularis lesions.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "The Th17 cell cytokine IL-17 mediates excessive proliferation and differentiation interference of epidermal keratinocytes and plays a key pathogenic role in conditions such as psoriasis ( 48)."
      explanation: Shows that IL-17 mediates excessive proliferation and altered differentiation of epidermal keratinocytes.
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Several studies have shown that the expression of the Th22 cell cytokine IL-22 is elevated in pruritic PN lesions, which is potentially associated with impaired epidermal proliferation and differentiation and skin in flammation ( 27, 51)."
      explanation: Links elevated Th22/IL-22 expression in prurigo nodularis lesions to altered epidermal proliferation and differentiation.
- name: Cutaneous Neuropeptide Release
  description: >-
    Activated cutaneous sensory nerve fibers release neuropeptides including
    substance P and calcitonin gene-related peptide (CGRP) into the dermis,
    causing vasodilation and further recruitment and activation of dermal
    inflammatory cells.
  biological_scale: MOLECULAR
  cell_types:
  - preferred_term: sensory neuron
    term:
      id: CL:0000101
      label: sensory neuron
  biological_processes:
  - preferred_term: neuropeptide signaling pathway
    modifier: INCREASED
    term:
      id: GO:0007218
      label: neuropeptide signaling pathway
  - preferred_term: vasodilation
    modifier: INCREASED
    term:
      id: GO:0042311
      label: vasodilation
  locations:
  - preferred_term: dermis
    term:
      id: UBERON:0002067
      label: dermis
  evidence:
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Substance P (SP), secreted b y sensory neurons, plays an important role in itch signaling within the peripheral and central nervous systems ( 65, 66)."
    explanation: Identifies substance P secreted by sensory neurons as a key neuropeptide in prurigo nodularis itch signaling.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Calcitonin gene-related peptide (CGRP) is the most abundant neuropeptide in human skin and is often colocalized with SP ( 82)."
    explanation: Identifies calcitonin gene-related peptide colocalized with substance P in cutaneous sensory nerve fibers.
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "The neuroimmune feedback loop in PN involves neuropeptides released from nerve fibres that cause vasodilation and further recruitment of inflammatory cells."
    explanation: Establishes that neuropeptides released from dermal nerve fibers cause vasodilation and recruit inflammatory cells in prurigo nodularis.
  downstream:
  - target: Mast Cell and Eosinophil Degranulation
    causal_link_type: DIRECT
    description: Neuropeptides (substance P and CGRP) recruit and activate dermal mast cells and eosinophils to degranulate and release histamine, neurotoxins, and nerve growth factor.
    evidence:
    - reference: PMID:38345154
      reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "The neuroimmune feedback loop in PN involves neuropeptides released from nerve fibres that cause vasodilation and further recruitment of inflammatory cells. Inflammatory cells, particularly mast cells and eosinophils, degranulate and release neurotoxins, as well as nerve growth factor, which may contribute to the neuronal hyperplasia seen in the dermis of patients with PN and neural sensitization."
      explanation: Shows that neuropeptide-mediated recruitment of mast cells and eosinophils leads to their degranulation and release of neurotoxins and NGF.
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "CGRP activates mast cells to release histamine, and eosinophils to produce NGF, another pruritogen."
      explanation: Confirms direct activation of mast cells and eosinophils by CGRP to release histamine and NGF.
- name: Mast Cell and Eosinophil Degranulation
  description: >-
    Dermal mast cells and eosinophils aggregate in close contact with peripheral
    sensory nerve endings, degranulate, and release neurotoxins, histamine, and
    nerve growth factor (NGF) into the dermal microenvironment.
  biological_scale: CELLULAR
  cell_types:
  - preferred_term: mast cell
    term:
      id: CL:0000097
      label: mast cell
  - preferred_term: eosinophil
    term:
      id: CL:0000771
      label: eosinophil
  genes:
  - preferred_term: NGF
    term:
      id: hgnc:7808
      label: NGF
  biological_processes:
  - preferred_term: mast cell degranulation
    modifier: INCREASED
    term:
      id: GO:0043303
      label: mast cell degranulation
  - preferred_term: eosinophil degranulation
    modifier: INCREASED
    term:
      id: GO:0043308
      label: eosinophil degranulation
  locations:
  - preferred_term: dermis
    term:
      id: UBERON:0002067
      label: dermis
  evidence:
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Inflammatory cells, particularly mast cells and eosinophils, degranulate and release neurotoxins, as well as nerve growth factor, which may contribute to the neuronal hyperplasia seen in the dermis of patients with PN and neural sensitization."
    explanation: Documents degranulation of dermal mast cells and eosinophils with release of neurotoxins and nerve growth factor in prurigo nodularis.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Notably, mast cells and eosinophils often aggregate around the peripheral sensory nerve endings, establishing close contact with them ( 15– 17)."
    explanation: Describes perineural aggregation of mast cells and eosinophils around sensory nerve endings in lesional prurigo nodularis dermis.
  downstream:
  - target: NGF-Driven Dermal Neuronal Hyperplasia
    causal_link_type: DIRECT
    description: Nerve growth factor released by degranulating mast cells and eosinophils promotes dermal nerve fiber sprouting and neuronal hyperplasia.
    evidence:
    - reference: PMID:38345154
      reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Inflammatory cells, particularly mast cells and eosinophils, degranulate and release neurotoxins, as well as nerve growth factor, which may contribute to the neuronal hyperplasia seen in the dermis of patients with PN and neural sensitization."
      explanation: Directly links NGF released from degranulating mast cells and eosinophils to dermal neuronal hyperplasia.
  - target: Cutaneous Neural Sensitization
    causal_link_type: DIRECT
    description: Neurotoxins, histamine, and NGF released during mast cell and eosinophil degranulation sensitize dermal sensory nerve fibers.
    evidence:
    - reference: PMID:38345154
      reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Inflammatory cells, particularly mast cells and eosinophils, degranulate and release neurotoxins, as well as nerve growth factor, which may contribute to the neuronal hyperplasia seen in the dermis of patients with PN and neural sensitization."
      explanation: Links mast cell and eosinophil degranulation to cutaneous neural sensitization in prurigo nodularis.
- name: NGF-Driven Dermal Neuronal Hyperplasia
  description: >-
    Elevated nerve growth factor (NGF) signaling through upregulated TrkA and
    p75NTR receptors on cutaneous nerve fibers drives sprouting of afferent
    nerves and increased density of itch-sensing nerve fibers in the dermis,
    accompanied by reduced intraepidermal nerve fiber density.
  biological_scale: TISSUE
  cell_types:
  - preferred_term: sensory neuron
    term:
      id: CL:0000101
      label: sensory neuron
  genes:
  - preferred_term: NGF
    term:
      id: hgnc:7808
      label: NGF
  biological_processes:
  - preferred_term: axonogenesis
    modifier: INCREASED
    term:
      id: GO:0007409
      label: axonogenesis
  locations:
  - preferred_term: dermis
    term:
      id: UBERON:0002067
      label: dermis
  - preferred_term: skin epidermis
    term:
      id: UBERON:0001003
      label: skin epidermis
  evidence:
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Neural dysregulation in PN features a lower density of itch-sensing nerve fibres in the epidermis (upper layer of the skin) and a higher density of itch-sensing nerve fibres in the dermis (lower layer of the skin)."
    explanation: Documents increased density of itch-sensing nerve fibers in the dermis and decreased density in the epidermis of prurigo nodularis lesions.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "NGF is a neurotrophic factor that primarily activates, sensitizes, and sprouts skin nerve fibers, promoting the release of neuropeptides, such as SP and CGRP ( 96, 98)."
    explanation: Establishes that NGF drives sprouting, activation, and sensitization of cutaneous nerve fibers.
  downstream:
  - target: Cutaneous Neural Sensitization
    causal_link_type: DIRECT
    description: Hyperplastic dermal nerve fibers stimulated by NGF exhibit heightened excitability and sensitization to pruritogens.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "NGF is a neurotrophic factor that primarily activates, sensitizes, and sprouts skin nerve fibers, promoting the release of neuropeptides, such as SP and CGRP ( 96, 98)."
      explanation: Shows that NGF both sprouts and sensitizes cutaneous nerve fibers.
- name: Cutaneous Neural Sensitization
  description: >-
    Heightened sensitivity and spontaneous excitability of cutaneous C-fiber
    sensory neurons mediated by cytokines, NGF, periostin, and neuropeptides
    coupled to upregulated TRPV1 ion channels, lowering the itch threshold,
    sustaining neuropeptide release, and driving intractable pruritus and
    repetitive scratching.
  biological_scale: CELLULAR
  cell_types:
  - preferred_term: sensory neuron
    term:
      id: CL:0000101
      label: sensory neuron
  genes:
  - preferred_term: TRPV1
    term:
      id: hgnc:12716
      label: TRPV1
  biological_processes:
  - preferred_term: transmission of nerve impulse
    modifier: INCREASED
    term:
      id: GO:0019226
      label: transmission of nerve impulse
  locations:
  - preferred_term: dermis
    term:
      id: UBERON:0002067
      label: dermis
  evidence:
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Skin neuroimmune interactions and neuronal sensitization play important roles in mediating chronic itch in PN ( 1)."
    explanation: Identifies neuronal sensitization as a central driver of chronic pruritus in prurigo nodularis.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Simultaneously, activated sensory neurons secrete pruritogens such as neuropeptides. On one hand, these neuropeptides act on the neurons themselves to increase their own sensitivity and spontaneous activity; on the other hand, they reciprocally activate immune cells and sustain and amplify inflammatory responses, thus promoting chronic itch ( 21)."
    explanation: Explains how activated sensory neurons increase their own sensitivity and spontaneous activity while sustaining neuropeptide release and chronic itch.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "TRPV1, also known as the capsaicin receptor, is significantly upregulated in nerve fibers and keratinocytes within lesional PN skin ( 127 )."
    explanation: Documents significant upregulation of the TRPV1 ion channel in lesional nerve fibers in prurigo nodularis.
  downstream:
  - target: Cutaneous Neuropeptide Release
    causal_link_type: DIRECT
    description: Sensitized and activated cutaneous sensory neurons release substance P and CGRP, closing the neuroimmune positive feedback loop.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "NGF is a neurotrophic factor that primarily activates, sensitizes, and sprouts skin nerve fibers, promoting the release of neuropeptides, such as SP and CGRP ( 96, 98)."
      explanation: Confirms that sensitized cutaneous nerve fibers release substance P and CGRP.
  - target: Pruritus
    causal_link_type: DIRECT
    description: Neuronal sensitization and hyperexcitability of cutaneous C fibers directly generate severe, intractable pruritus.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Skin neuroimmune interactions and neuronal sensitization play important roles in mediating chronic itch in PN ( 1)."
      explanation: Directly links neuronal sensitization to chronic itch in prurigo nodularis.
  - target: Chronic Scratching and Mechanical Skin Trauma
    causal_link_type: DIRECT
    description: Severe pruritus from sensitized cutaneous nerves drives an intractable itch-scratch cycle with repetitive mechanical trauma to reachable skin sites.
    evidence:
    - reference: PMID:38345154
      reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Patients with PN suffer from an intractable itch-scratch cycle leading to impaired sleep, psychosocial distress and a significant disruption in quality of life."
      explanation: Establishes the intractable itch-scratch cycle driven by severe pruritus in prurigo nodularis.
- name: Chronic Scratching and Mechanical Skin Trauma
  description: >-
    Self-sustained repetitive scratching in response to intractable pruritus
    causes recurrent epidermal excoriation, crusting, mechanical barrier trauma,
    and upregulation of skin-derived NGF and its receptors, inducing and
    maintaining epidermal hyperplasia and dermal fibrosis in prurigo nodules.
  biological_scale: TISSUE
  locations:
  - preferred_term: skin of body
    term:
      id: UBERON:0002097
      label: skin of body
  evidence:
  - reference: PMID:37506977
    reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "Neuroimmune dysregulation and chronic scratching are believed to both induce and maintain the characteristic lesions."
    explanation: Establishes chronic scratching alongside neuroimmune dysregulation as a primary driver that induces and maintains prurigo nodularis lesions.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Current understanding suggests that a persistent “itch-scratch” vicious cycle, leading to recurrent skin excoriation, crusting, and thickening, is the primary driver of nodule formation ( 3)."
    explanation: Shows that the persistent itch-scratch cycle causes recurrent skin excoriation, crusting, and thickening that drive nodule formation.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Scratching can also lead to increased expression of skin-derived NGF and its receptors ( 97)."
    explanation: Documents that mechanical scratching upregulates cutaneous NGF and its receptors, reinforcing neuroimmune sensitization.
  downstream:
  - target: Epidermal Keratinocyte Hyperproliferation
    causal_link_type: DIRECT
    description: Repetitive mechanical scratching and excoriation stimulate reactive epidermal thickening and keratinocyte proliferation.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Current understanding suggests that a persistent “itch-scratch” vicious cycle, leading to recurrent skin excoriation, crusting, and thickening, is the primary driver of nodule formation ( 3)."
      explanation: Links the persistent itch-scratch cycle to skin thickening and nodule formation.
  - target: Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling
    causal_link_type: DIRECT
    description: Chronic scratching and mechanical trauma induce and maintain dermal fibrotic remodeling in prurigo nodules.
    evidence:
    - reference: PMID:37506977
      reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: BACKGROUND
      snippet: "Neuroimmune dysregulation and chronic scratching are believed to both induce and maintain the characteristic lesions."
      explanation: Supports chronic scratching as a driver that induces and maintains the fibrotic nodular lesions of prurigo nodularis.
  - target: NGF-Driven Dermal Neuronal Hyperplasia
    causal_link_type: DIRECT
    description: Scratching increases expression of skin-derived NGF and its receptors, amplifying dermal nerve sprouting.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Scratching can also lead to increased expression of skin-derived NGF and its receptors ( 97)."
      explanation: Directly links mechanical scratching to increased cutaneous NGF and NGF receptor expression.
  - target: Sleep Disturbance
    causal_link_type: DIRECT
    description: Intractable nocturnal itch-scratch cycles disrupt sleep.
    evidence:
    - reference: PMID:38345154
      reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Patients with PN suffer from an intractable itch-scratch cycle leading to impaired sleep, psychosocial distress and a significant disruption in quality of life."
      explanation: Directly links the intractable itch-scratch cycle to impaired sleep.
- name: Epidermal Keratinocyte Hyperproliferation
  description: >-
    Excessive proliferation and altered differentiation of epidermal
    keratinocytes in lesional prurigo nodularis skin driven by chronic
    scratching and IL-17, IL-22, and IL-31 signaling, producing marked
    epidermal hyperkeratosis overlying the dermal nodules.
  biological_scale: CELLULAR
  cell_types:
  - preferred_term: keratinocyte
    term:
      id: CL:0000312
      label: keratinocyte
  biological_processes:
  - preferred_term: keratinocyte proliferation
    modifier: INCREASED
    term:
      id: GO:0043616
      label: keratinocyte proliferation
  locations:
  - preferred_term: skin epidermis
    term:
      id: UBERON:0001003
      label: skin epidermis
  evidence:
  - reference: PMID:37506977
    reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "This study demonstrates that PN is an inflammatory skin disease characterized by both keratinocyte proliferation and activation of profibrotic responses."
    explanation: Single-cell profiling of human prurigo nodularis biopsies establishes epidermal keratinocyte proliferation as a core pathological feature.
  downstream:
  - target: Hyperkeratosis
    causal_link_type: DIRECT
    description: Excessive epidermal keratinocyte proliferation produces hyperkeratotic thickening over prurigo nodules.
    evidence:
    - reference: PMID:37506977
      reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: PRIMARY_RESULT
      snippet: "This study demonstrates that PN is an inflammatory skin disease characterized by both keratinocyte proliferation and activation of profibrotic responses."
      explanation: Links epidermal keratinocyte proliferation to the characteristic hyperkeratotic epidermal changes of prurigo nodularis.
- name: Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling
  description: >-
    Expansion and activation of profibrotic papillary dermal COL11A1+
    fibroblasts and POSTN+WNT5A+ myofibroblast/cancer-associated fibroblast-like
    subclusters, accompanied by upregulation of MMP14, COL6A3, TGFBR1, and
    dermal periostin deposition, which distinguish prurigo nodularis from atopic
    dermatitis, drive nodular dermal fibrosis, and further stimulate itch-sensing
    nerve fibers.
  biological_scale: TISSUE
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  genes:
  - preferred_term: COL11A1
    term:
      id: hgnc:2186
      label: COL11A1
  - preferred_term: WNT5A
    term:
      id: hgnc:12784
      label: WNT5A
  - preferred_term: MMP14
    term:
      id: hgnc:7160
      label: MMP14
  - preferred_term: COL6A3
    term:
      id: hgnc:2213
      label: COL6A3
  - preferred_term: TGFBR1
    term:
      id: hgnc:11772
      label: TGFBR1
  biological_processes:
  - preferred_term: fibroblast activation
    modifier: INCREASED
    term:
      id: GO:0072537
      label: fibroblast activation
  - preferred_term: extracellular matrix organization
    modifier: INCREASED
    term:
      id: GO:0030198
      label: extracellular matrix organization
  - preferred_term: transforming growth factor beta receptor signaling pathway
    modifier: INCREASED
    term:
      id: GO:0007179
      label: transforming growth factor beta receptor signaling pathway
  locations:
  - preferred_term: papillary layer of dermis
    term:
      id: UBERON:0001992
      label: papillary layer of dermis
  - preferred_term: dermis
    term:
      id: UBERON:0002067
      label: dermis
  evidence:
  - reference: PMID:37506977
    reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "This study demonstrates that PN is an inflammatory skin disease characterized by both keratinocyte proliferation and activation of profibrotic responses. This study also demonstrates that the COL11A1+ fibroblast subset is a major contributor to fibrosis and is predominantly found in the papillary dermis of PN skin. Activation of fibrotic responses is the main distinguishing feature between PN and atopic dermatitis skin."
    explanation: Single-cell and spatial transcriptomics of human PN biopsies identify papillary dermal COL11A1+ fibroblasts as primary drivers of fibrosis that distinguish PN from atopic dermatitis.
  - reference: PMID:38246584
    reference_title: Single-Cell RNA Sequencing Reveals Dysregulated POSTN+WNT5A+ Fibroblast Subclusters in Prurigo Nodularis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Lesional PN has increased dysregulated fibroblasts (FBs) and myofibroblasts. FBs in lesional PN were shifted toward a cancer-associated FB-like phenotype, with POSTN+WNT5A+ cancer-associated FBs increased in PN and similarly so in squamous cell carcinoma."
    explanation: Single-cell RNA-seq of 28,695 lesional and nonlesional cells identifies expanded POSTN+WNT5A+ fibroblast subclusters in prurigo nodularis.
  - reference: PMID:41333131
    reference_title: Nanostring Transcriptomic Analysis Highlights IL-6 Family and TGF-β Pathways in the Pathogenesis of Prurigo Nodularis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "MMP14, COL6A3, and TGFBR1 were highly upregulated in PN, reflecting an active fibrotic process."
    explanation: Confirms upregulation of MMP14, COL6A3, and TGFBR1 in lesional PN skin.
  downstream:
  - target: Skin nodule
    causal_link_type: DIRECT
    description: Dermal fibrosis and extracellular matrix deposition form firm, dome-shaped skin nodules.
    evidence:
    - reference: PMID:37506977
      reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      quote_role: PRIMARY_RESULT
      snippet: "This study also demonstrates that the COL11A1+ fibroblast subset is a major contributor to fibrosis and is predominantly found in the papillary dermis of PN skin."
      explanation: Links papillary dermal COL11A1+ fibroblast-driven fibrosis to the characteristic fibrotic nodules of prurigo nodularis.
  - target: Cutaneous Neural Sensitization
    causal_link_type: DIRECT
    description: Extracellular matrix periostin deposited in the dermis of prurigo nodularis directly interacts with integrin alphaVbeta3 on itch-sensitive nerve fibers and stimulates immune cells to release IL-31.
    evidence:
    - reference: DOI:10.3389/fimmu.2023.1301817
      reference_title: Molecular mechanisms of pruritus in prurigo nodularis
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Periostin-mediated itching involves direct and indirect pathways. In the direct pathway, periostin interacts directly with itch-sensitive nerve fibers via its receptor integrin aV b3( 60)."
      explanation: Demonstrates that dermal periostin directly activates itch-sensitive nerve fibers via integrin alphaVbeta3.
phenotypes:
- name: Pruritus
  category: Dermatological
  diagnostic: true
  description: >-
    Severe, intractable itch associated with nodular skin lesions that drives
    a self-perpetuating itch-scratch cycle and significantly impairs sleep and
    quality of life.
  phenotype_term:
    preferred_term: Pruritus
    term:
      id: HP:0000989
      label: Pruritus
  evidence:
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Prurigo nodularis (PN) is an inflammatory skin condition characterized by intensely pruritic nodules on the skin. Patients with PN suffer from an intractable itch-scratch cycle leading to impaired sleep, psychosocial distress and a significant disruption in quality of life."
    explanation: Establishes intense pruritus and the intractable itch-scratch cycle as defining clinical features of prurigo nodularis.
- name: Skin nodule
  category: Dermatological
  diagnostic: true
  description: >-
    Bilaterally distributed, firm, pruritic nodules primarily affecting the
    extremities and trunk, often sparing the harder-to-reach mid-upper back
    (the butterfly sign).
  phenotype_term:
    preferred_term: Skin nodule
    term:
      id: HP:0200036
      label: Skin nodule
  evidence:
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Prurigo nodularis (PN) is an inflammatory skin condition characterized by intensely pruritic nodules on the skin."
    explanation: Establishes pruritic skin nodules as the hallmark lesion of prurigo nodularis.
  - reference: PMID:37506977
    reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Prurigo nodularis (PN) is a chronic neuroimmune skin disease characterized by bilaterally distributed pruritic hyperkeratotic nodules on extremities and trunk."
    explanation: Describes the bilateral distribution of hyperkeratotic nodules on the extremities and trunk.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Multiple discrete nodules are often symmetrically distributed in scratchable areas, such as the extensor surfaces of the limbs and trunk, with fewer lesions on the harder-to-reach mid-upper back, creating the classical butter flys i g n(3)."
    explanation: Documents the symmetric distribution on scratchable extensor surfaces and trunk with mid-upper back sparing (butterfly sign).
- name: Hyperkeratosis
  category: Dermatological
  description: >-
    Epidermal thickening and hyperkeratosis overlying the pruritic dermal
    nodules on the trunk and extremities.
  phenotype_term:
    preferred_term: Hyperkeratosis
    term:
      id: HP:0000962
      label: Hyperkeratosis
  evidence:
  - reference: PMID:37506977
    reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Prurigo nodularis (PN) is a chronic neuroimmune skin disease characterized by bilaterally distributed pruritic hyperkeratotic nodules on extremities and trunk."
    explanation: Confirms that prurigo nodularis nodules are hyperkeratotic.
- name: Sleep Disturbance
  category: Neurological
  description: >-
    Impaired sleep resulting from severe, intractable nocturnal pruritus and
    scratching.
  phenotype_term:
    preferred_term: Sleep disturbance
    term:
      id: HP:0002360
      label: Sleep disturbance
  evidence:
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Patients with PN suffer from an intractable itch-scratch cycle leading to impaired sleep, psychosocial distress and a significant disruption in quality of life."
    explanation: Documents impaired sleep as a direct consequence of the intractable itch-scratch cycle in prurigo nodularis.
- name: Atopic Dermatitis
  category: Dermatological
  frequency: FREQUENT
  description: >-
    Coexistent atopic dermatitis or atopic predisposition observed in
    approximately half of patients with prurigo nodularis.
  phenotype_term:
    preferred_term: Atopic dermatitis
    term:
      id: HP:0001047
      label: Atopic dermatitis
  evidence:
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Notably, approximately half of the patients with PN exhibit coexistent atopic dermatitis (AD) or atopic predisposition (2, 5)."
    explanation: Documents that approximately half of patients with prurigo nodularis have coexistent atopic dermatitis or an atopic predisposition.
- name: Anxiety
  category: Psychiatric
  description: >-
    Elevated risk of anxiety associated with the severe chronic pruritus and
    quality-of-life burden of prurigo nodularis.
  phenotype_term:
    preferred_term: Anxiety
    term:
      id: HP:0000739
      label: Anxiety
  evidence:
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "A previous study has shown that PN exerts a more significant impact on quality of life and carries a higher risk of psychological disorders (e.g., anxiety and depression) than other pruritic dermatoses ( 4)."
    explanation: Documents the increased risk of anxiety in patients with prurigo nodularis compared with other pruritic dermatoses.
- name: Depression
  category: Psychiatric
  description: >-
    Elevated risk of depression associated with intractable chronic pruritus
    and psychosocial distress in prurigo nodularis.
  phenotype_term:
    preferred_term: Depression
    term:
      id: HP:0000716
      label: Depression
  evidence:
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "A previous study has shown that PN exerts a more significant impact on quality of life and carries a higher risk of psychological disorders (e.g., anxiety and depression) than other pruritic dermatoses ( 4)."
    explanation: Documents the increased risk of depression in patients with prurigo nodularis compared with other pruritic dermatoses.
treatments:
- name: Dupilumab
  description: >-
    Monoclonal antibody targeting IL-4 receptor alpha (IL-4Ralpha) that blocks
    IL-4 and IL-13 signaling; the first FDA- and EMA-approved systemic therapy
    for adult patients with prurigo nodularis, demonstrating significant
    reductions in worst-itch scores in the phase 3 PRIME and PRIME2 trials.
  therapeutic_modality: MONOCLONAL_ANTIBODY
  treatment_term:
    preferred_term: Monoclonal Antibody Therapy
    term:
      id: NCIT:C15490
      label: Monoclonal Antibody Therapy
    therapeutic_agent:
    - preferred_term: Dupilumab
      term:
        id: NCIT:C162455
        label: Dupilumab
  target_mechanisms:
  - target: Cutaneous Type 2 Inflammation
  target_phenotypes:
  - preferred_term: Pruritus
    term:
      id: HP:0000989
      label: Pruritus
  - preferred_term: Skin nodule
    term:
      id: HP:0200036
      label: Skin nodule
  evidence:
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The only drug currently approved for PN in the USA and Europe is dupilumab, an IL-4Rα inhibitor that blocks signalling through IL-4 and IL-13, which is undergoing post-marketing surveillance."
    explanation: Documents approval of dupilumab as an IL-4Ralpha inhibitor blocking IL-4 and IL-13 signaling in prurigo nodularis.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Based on two phase 3 clinical trials, dupilumab (an IL-4R a monoclonal antibody simultaneously antagonizing IL-4 and IL-13) has become the first the U.S. Food and Drug Administration (FDA)/European Medicines Agency (EMA)/China National Medical Products Administration (NMPA)-approved treatment for adult patients with PN. C ompared to placebo, signi ficant improvements in weekly average worst itch numeric rating scale (WI-NRS) with dupilumab were observed as early as week 3 in PRIME and week 4 in PRIME2 ( 134)."
    explanation: Summarizes the phase 3 PRIME and PRIME2 trial efficacy data supporting regulatory approval of dupilumab for prurigo nodularis.
- name: Nemolizumab
  description: >-
    Monoclonal antibody targeting the IL-31 receptor A (IL-31RA) that blocks
    IL-31-driven pruritus, significantly reduces peak pruritus and skin lesion
    severity in randomized clinical trials, and reverses pathological COL11A1+
    fibroblast and keratinocyte transcriptional responses in prurigo nodularis
    skin.
  therapeutic_modality: MONOCLONAL_ANTIBODY
  treatment_term:
    preferred_term: Monoclonal Antibody Therapy
    term:
      id: NCIT:C15490
      label: Monoclonal Antibody Therapy
    therapeutic_agent:
    - preferred_term: Nemolizumab
      term:
        id: NCIT:C170211
        label: Nemolizumab
  target_mechanisms:
  - target: IL-31 and IL-6 Family Neuroimmune Signaling
  - target: Epidermal Keratinocyte Hyperproliferation
  - target: Profibrotic Fibroblast Activation and Extracellular Matrix Remodeling
  target_phenotypes:
  - preferred_term: Pruritus
    term:
      id: HP:0000989
      label: Pruritus
  - preferred_term: Skin nodule
    term:
      id: HP:0200036
      label: Skin nodule
  evidence:
  - reference: PMID:32074418
    reference_title: Trial of Nemolizumab in Moderate-to-Severe Prurigo Nodularis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Nemolizumab resulted in a greater reduction in pruritus and severity of skin lesions than placebo in patients with prurigo nodularis but was associated with adverse events."
    explanation: Phase 2 randomized placebo-controlled trial (NCT03181503) demonstrating that nemolizumab reduces pruritus and skin lesion severity in moderate-to-severe prurigo nodularis.
  - reference: PMID:38345154
    reference_title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The anti-IL-31 receptor A inhibitor nemolizumab is in late-stage development with positive phase III data reported."
    explanation: Identifies nemolizumab as an anti-IL-31 receptor A biologic with positive phase III trial results in prurigo nodularis.
  - reference: PMID:37506977
    reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "This study further shows the broad effect of nemolizumab on PN cell types, with a prominent effect driving COL11A1+ fibroblast and keratinocyte responses toward normal."
    explanation: Demonstrates that nemolizumab normalizes pathological COL11A1+ fibroblast and keratinocyte transcriptional programs in human PN skin.
clinical_trials:
- name: NCT03181503
  phase: PHASE_II
  status: COMPLETED
  description: >-
    Randomized, double-blind, placebo-controlled 12-week phase 2 trial of
    subcutaneous nemolizumab (0.5 mg/kg at baseline, week 4, and week 8) in
    70 patients with moderate-to-severe prurigo nodularis (>=20 nodules) and
    severe pruritus, demonstrating a 53.0% reduction in peak pruritus numerical
    rating scale score at week 4 compared with 20.2% with placebo.
  target_phenotypes:
  - preferred_term: Pruritus
    term:
      id: HP:0000989
      label: Pruritus
  - preferred_term: Skin nodule
    term:
      id: HP:0200036
      label: Skin nodule
  evidence:
  - reference: PMID:32074418
    reference_title: Trial of Nemolizumab in Moderate-to-Severe Prurigo Nodularis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "We conducted a 12-week, randomized, double-blind, phase 2 trial of nemolizumab (at a dose of 0.5 mg per kilogram of body weight) administered subcutaneously at baseline, week 4, and week 8, as compared with placebo, in patients with moderate-to-severe prurigo nodularis and severe pruritus."
    explanation: Describes the trial design, patient population, and dosing regimen of the phase 2 nemolizumab trial in prurigo nodularis.
  - reference: PMID:32074418
    reference_title: Trial of Nemolizumab in Moderate-to-Severe Prurigo Nodularis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "(Funded by Galderma; ClinicalTrials.gov number, NCT03181503.)"
    explanation: Identifies ClinicalTrials.gov registration NCT03181503 for the phase 2 trial of nemolizumab in prurigo nodularis.
differential_diagnoses:
- name: Atopic Dermatitis
  disease_term:
    preferred_term: atopic dermatitis
    term:
      id: MONDO:0004980
      label: atopic eczema
  description: >-
    Atopic dermatitis frequently coexists with prurigo nodularis (present or
    predisposed in approximately half of patients) and shares type 2 immune
    activation, but prurigo nodularis represents a distinct clinical and
    transcriptomic entity.
  distinguishing_features:
  - Prurigo nodularis is distinguished histologically and transcriptomically from atopic dermatitis by prominent papillary dermal fibrosis driven by COL11A1+ fibroblasts, stromal remodeling, and neurovascular dysregulation rather than the strong type 2 epidermal response typical of atopic dermatitis.
  evidence:
  - reference: PMID:37506977
    reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Activation of fibrotic responses is the main distinguishing feature between PN and atopic dermatitis skin."
    explanation: Single-cell transcriptomic comparison establishes dermal fibrotic activation as the primary feature distinguishing prurigo nodularis from atopic dermatitis.
  - reference: DOI:10.3389/fimmu.2023.1301817
    reference_title: Molecular mechanisms of pruritus in prurigo nodularis
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Although AD and PN are both type 2 inflammatory diseases, recent transcriptomic studies have clearly revealed that PN is separated from AD. PN does not harbor the strong type 2 response pattern that is typically found in AD but is rather characterized by stromal remodeling and neurovascular dysregulation (6– 11)."
    explanation: Contrasts the stromal remodeling and neurovascular dysregulation of prurigo nodularis with the strong type 2 inflammatory profile of atopic dermatitis.
references:
- reference: DOI:10.3389/fimmu.2023.1301817
  title: Molecular mechanisms of pruritus in prurigo nodularis
  findings: []
- reference: PMID:32074418
  title: Trial of Nemolizumab in Moderate-to-Severe Prurigo Nodularis.
  findings: []
- reference: PMID:37506977
  title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
  findings: []
- reference: PMID:38345154
  title: "Prurigo nodularis: new insights into pathogenesis and novel therapeutics."
  findings: []
- reference: PMID:38246584
  title: Single-Cell RNA Sequencing Reveals Dysregulated POSTN+WNT5A+ Fibroblast Subclusters in Prurigo Nodularis.
  findings: []
- reference: PMID:41333131
  title: Nanostring Transcriptomic Analysis Highlights IL-6 Family and TGF-β Pathways in the Pathogenesis of Prurigo Nodularis.
  findings: []
datasets:
- accession: geo:GSE261704
  title: Neuroimmune gene signature of prurigo nodularis compared to psoriasis and atopic ​​dermatitis​​
  description: Prurigo nodularis (PN) is a debilitating neuroimmune skin disease characterized by pruritic, raised, nodular lesions. Psoriasis and AD are classically characterized by Th1/Th17 and Th2 polarization, respectively, but the neuroinflammatory profile of PN remains poorly defined. We characterized the neuroimmune phenotype of PN compared to AD, psoriasis, and healthy controls (HC), through transcriptomic analysis of lesional and nonlesional skin biopsies from 25 PN patients, 27 AD patients, 15 psoriasis patients, and 12 HC using a custom neuroinflammation-focused NanoString panel of 770 genes.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: MICROARRAY
  sample_count: 120
  publication: PMID:41333131
  notes: Identified by GEO DataSets index search for Prurigo Nodularis (scripts/discover_datasets.py); accession and metadata verified against NCBI E-utilities on 2026-08-01. Title, sample count, and organism are GEO's own values.
  evidence:
  - reference: PMID:41333131
    reference_title: Nanostring Transcriptomic Analysis Highlights IL-6 Family and TGF-β Pathways in the Pathogenesis of Prurigo Nodularis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this study, we performed NanoString-based transcriptomic profiling on lesional skin biopsies from patients with PN (n = 26), those with atopic dermatitis (n = 25), those with psoriasis (n = 15), and healthy controls (n = 12) using a custom neuroinflammation-focused panel."
    explanation: Primary publication reporting the NanoString transcriptomic dataset of prurigo nodularis, atopic dermatitis, psoriasis, and healthy control skin biopsies.
- accession: geo:GSE273559
  title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab
  description: Prurigo nodularis (PN) is a chronic neuroimmune skin disease characterized by bilaterally distributed pruritic hyperkeratotic nodules on extremities and trunk. Neuroimmune dysregulation and chronic scratching are believed to both induce and maintain the characteristic lesions. This study sought to provide a comprehensive view of the molecular pathogenesis of PN at the single-cell level to identify and outline key pathologic processes and the cell types involved. Features that distinguish PN skin from the skin of patients with atopic dermatitis were of particular interest.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: SINGLE_CELL_RNA_SEQ
  sample_count: 12
  publication: PMID:37506977
  notes: Identified by GEO DataSets index search for Prurigo Nodularis (scripts/discover_datasets.py); accession and metadata verified against NCBI E-utilities on 2026-08-01. Title, sample count, and organism are GEO's own values.
  evidence:
  - reference: PMID:37506977
    reference_title: Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Single-cell RNA-sequencing of skin from 15 healthy donors and nonlesional and lesional skin from 6 patients each with PN and atopic dermatitis, combined with spatial-sequencing using the 10x Visium platform."
    explanation: Primary publication generating the single-cell RNA-seq dataset of prurigo nodularis skin.
- accession: geo:GSE233280
  title: Single-cell RNA sequencing reveals dysregulated fibroblast subclusters in prurigo nodularis
  description: Prurigo nodularis (PN) is an intensely pruritic, inflammatory skin disease with a poorly understood pathogenesis. Thus, we performed single-cell transcriptomic profiling of 28,695 lesional and non-lesional PN cells. Lesional PN has increased dysregulated fibroblasts (FBs) and myofibroblasts. FBs in lesional PN were shifted towards a cancer-associated fibroblast (CAF)-like phenotype, with POSTN+WNT5A+ CAFs increased in PN, and similarly so in squamous cell carcinoma. A multi-center cohort study revealed an increased risk of SCC and CAF-associated malignancies (breast and colorectal) in PN patients.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: SINGLE_CELL_RNA_SEQ
  sample_count: 12
  publication: PMID:38246584
  notes: Identified by GEO DataSets index search for Prurigo Nodularis (scripts/discover_datasets.py); accession and metadata verified against NCBI E-utilities on 2026-08-01. Title, sample count, and organism are GEO's own values.
  evidence:
  - reference: PMID:38246584
    reference_title: Single-Cell RNA Sequencing Reveals Dysregulated POSTN+WNT5A+ Fibroblast Subclusters in Prurigo Nodularis.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We performed single-cell transcriptomic profiling of 28,695 lesional and nonlesional PN cells."
    explanation: Primary publication reporting the single-cell RNA-seq dataset of lesional and nonlesional prurigo nodularis skin.
notes: >-
  Prurigo nodularis exhibits clinical and molecular heterogeneity across
  disease endotypes (including varying degrees of type 2 inflammation,
  coexistent atopic predisposition in roughly half of patients, or underlying
  myelopathy/spinal disc disease) as well as racial and ethnic variation in
  dermal fibrosis severity. While single-cell and spatial transcriptomic
  studies (GSE273559, GSE233280) clearly separate prurigo nodularis from atopic
  dermatitis by its papillary dermal COL11A1+ and POSTN+WNT5A+ profibrotic
  fibroblast expansion, the initiating triggers of the self-sustaining
  itch-scratch cycle remain incompletely defined.
📚

References & Deep Research

References

6
DOI:10.3389/fimmu.2023.1301817
Molecular mechanisms of pruritus in prurigo nodularis
No top-level findings curated for this source.
PMID:32074418
Trial of Nemolizumab in Moderate-to-Severe Prurigo Nodularis.
No top-level findings curated for this source.
PMID:37506977
Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab.
No top-level findings curated for this source.
PMID:38345154
Prurigo nodularis: new insights into pathogenesis and novel therapeutics.
No top-level findings curated for this source.
PMID:38246584
Single-Cell RNA Sequencing Reveals Dysregulated POSTN+WNT5A+ Fibroblast Subclusters in Prurigo Nodularis.
No top-level findings curated for this source.
PMID:41333131
Nanostring Transcriptomic Analysis Highlights IL-6 Family and TGF-β Pathways in the Pathogenesis of Prurigo Nodularis.
No top-level findings curated for this source.

Deep Research

2

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Disorder ▸

Disorder

  • Name: Prurigo Nodularis
  • Category: Complex
  • Existing deep-research providers: openai
  • Existing evidence reference count in YAML: 5

Key Pathophysiology Nodes

  • Immune and neural dysregulation
  • Deep research literature mapping

Citation Inventory (for evidence mapping)

  • DOI:10.3389/fimmu.2023.1301817
  • PMID:37506977
OpenAI ▸
Pathophysiology of Prurigo Nodularis (PN)
o3-deep-research-2025-06-26 95 citations 2026-02-01T15:22:27.509024

Pathophysiology of Prurigo Nodularis (PN)

Disease Name: Prurigo Nodularis
MONDO ID: MONDO:0026045
Category: Complex (multifactorial chronic inflammatory skin disease)

1. Core Pathophysiological Mechanisms

Prurigo nodularis (PN) is a chronic neuroimmune dermatosis characterized by severe itch (pruritus) and multiple hyperkeratotic nodules on the skin. Its pathophysiology involves a self-perpetuating “itch–scratch cycle” driven by interactions between the immune system, the nervous system, and the skin barrier (pmc.ncbi.nlm.nih.gov). Lesional PN skin exhibits dense dermal inflammatory infiltrates (primarily T-lymphocytes, eosinophils, and mast cells) that release pruritogenic mediators, alongside hyperplastic sensory nerve fibers that release neuropeptides – together these amplify itch and inflammation in a vicious cycle (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In essence, chronic Th2-driven inflammation in the skin triggers intense itching, and repeated scratching induces further immune activation, nerve fiber proliferation, and cutaneous damage. Over time, this cycle leads to epidermal hyperplasia and dermal fibrosis, manifesting as the characteristic pruritic nodules of PN (pmc.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov).

Immune Dysregulation: PN is fundamentally an inflammatory skin disorder with a skewed Type 2 (Th2) immune response. Lesional biopsies show marked infiltration of Th2-polarized CD4+ T cells, eosinophils, and mast cells in the dermis (pmc.ncbi.nlm.nih.gov). These cells release a milieu of itch-inducing and pro-inflammatory mediators – including interleukin-31 (IL-31), IL-4, IL-13, IL-5, histamine, prostaglandins, tryptase, and eosinophil granule proteins – which together provoke intense pruritus and cutaneous inflammation (pmc.ncbi.nlm.nih.gov). In PN lesions, eosinophils degranulate and release eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN, also called EPX), and major basic protein (MBP), which can damage local tissues and nerves (pmc.ncbi.nlm.nih.gov). ECP and EDN in particular have neurotoxic effects and are found at significantly elevated levels in PN skin (pmc.ncbi.nlm.nih.gov), suggesting that eosinophils actively contribute to nerve injury and abnormal nerve regeneration in the disease.

Neuroimmune Connection: A defining feature of PN pathophysiology is the aberrant crosstalk between the immune system and the peripheral nervous system in the skin. PN lesions demonstrate dermal neuronal hyperplasia, meaning an increased density of cutaneous nerve fibers (notably unmyelinated C-fibers responsible for itch sensation) (pmc.ncbi.nlm.nih.gov). Immunostaining of PN skin shows upregulation of pan-neuronal markers (like PGP9.5) and nerve growth factor receptors on these sprouted nerve endings (pmc.ncbi.nlm.nih.gov). The excess nerve fibers actively release neuropeptides such as substance P (SP) and calcitonin gene-related peptide (CGRP) into the skin (pmc.ncbi.nlm.nih.gov). SP is a well-known pruritogenic neurotransmitter that binds neurokinin-1 receptors (NK1R) on neurons and also MRGPRX2 receptors on mast cells, causing mast cell degranulation and further release of histamine and proteases (pmc.ncbi.nlm.nih.gov). CGRP, likewise elevated in PN lesions, can induce neurogenic inflammation by acting on mast cells and eosinophils, and it may disrupt local opioid signaling (e.g. altering β-endorphin and mu-opioid receptor expression) that normally helps modulate itch (pmc.ncbi.nlm.nih.gov). In this way, activated sensory nerves amplify inflammation: the neuropeptides they release recruit and activate immune cells, which in turn secrete more cytokines and pruritogens, creating an amplification loop of itch and inflammation (pmc.ncbi.nlm.nih.gov). This neuroimmune feedback loop is central to PN’s persistence.

Notably, type 2 cytokines released by Th2 cells (chiefly IL-4 and IL-13) serve as a bridge between the immune and nervous systems in PN. These cytokines profoundly alter the cutaneous neuroimmune environment: IL-4/IL-13 signaling via JAK-STAT6 is active in PN lesions (pmc.ncbi.nlm.nih.gov), and their receptors are expressed not only on immune cells and keratinocytes but also on fibroblasts and peripheral neurons (pmc.ncbi.nlm.nih.gov). IL-4 and IL-13 “disrupt the neuroimmune network of the skin”, leading to intensified itch (pmc.ncbi.nlm.nih.gov). One mechanism is by upregulating itch receptors – IL-4/IL-13 increase the expression of the IL-31 receptor (IL-31RA) and other pruriceptive receptors on sensory neurons, thereby “heightening the sensitization of histamine-independent sensory neurons to pruritogens” (pmc.ncbi.nlm.nih.gov). In parallel, IL-4/IL-13 promote tissue remodeling: they stimulate dermal fibroblasts to proliferate, migrate, and produce pro-fibrotic factors (like transforming growth factor beta (TGF-β), collagens, periostin, and other extracellular matrix proteins) (pmc.ncbi.nlm.nih.gov). This drives exaggerated fibrosis and dermal thickening in lesions. Indeed, the nodular fibrosis seen in PN skin can be traced to chronic type 2 inflammation combined with mechanical injury from scratching (analogous to a wound-healing response gone awry).

In summary, the core pathophysiology of PN is a neuroimmune dysregulation in the skin: Th2-skewed inflammation (with hyperactivated immune cells and type 2 cytokines) converges with neuronal hyper-excitability (excessive itch nerve fiber activation and neuropeptide release). The result is severe chronic pruritus and self-inflicted skin injury that perpetuate each other (pmc.ncbi.nlm.nih.gov). This distinguishes PN as both an inflammatory and a neuro-sensory disorder.

2. Key Molecular Players and Cells Involved

Immune Cells & Cytokines: The cellular players in PN lesions include a variety of immune cells: T helper 2 (Th2) cells (CD4+ T cells producing IL-4/IL-13), mast cells (tissue-resident granulocytes loaded with histamine and tryptase), eosinophils (IL-5–responsive granulocytes contributing to allergic inflammation), and also macrophages and dendritic cells to a lesser extent (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These cells congregate in the papillary dermis of PN lesions (just below the epidermis) forming an inflammatory infiltrate (pmc.ncbi.nlm.nih.gov). Key cytokines and mediators produced by these cells include:

  • Interleukin-31 (IL-31) – a central “itch cytokine” in PN. IL-31 is produced mainly by activated Th2 cells (and some type 2 innate lymphoid cells) and acts on a broad range of cells. Its receptor, IL-31RA (paired with OSMRβ, the oncostatin M receptor subunit), is expressed on sensory neurons, keratinocytes, mast cells, eosinophils, basophils, and monocytes (pmc.ncbi.nlm.nih.gov). IL-31 binding to its receptor on cutaneous nerve fibers directly triggers intense itch signaling. It also can stimulate keratinocytes and immune cells, promoting further inflammation. Notably, IL-31 links the immune and nervous systems: “IL-31 plays a pivotal role in the neuroimmune connection between type 2 inflammation and sensory neurons.” (pmc.ncbi.nlm.nih.gov) In PN patients, IL-31 levels are elevated in lesional skin and serum, correlating with itch severity (IL-31 is often highly upregulated in PN compared to other pruritic conditions (www.frontiersin.org)). Clinical relevance: Targeting the IL-31 pathway (e.g., with nemolizumab, an anti-IL31RA antibody) has shown significant reduction in PN itch, underscoring IL-31’s key role (pubmed.ncbi.nlm.nih.gov).

  • IL-4 and IL-13 – hallmark Th2 cytokines that orchestrate much of the immune response in PN. IL-4/IL-13 drive IgE production and eosinophil recruitment (via IL-5 induction) systemically, and locally in skin they cause barrier dysfunction and inflammation. In PN lesions, IL-4/IL-13 are highly expressed and activate STAT6 in keratinocytes and other cells (pmc.ncbi.nlm.nih.gov). These cytokines potentiate itch by increasing neuronal IL-31R and possibly other itch receptors (like TRPV1, TRPA1 – see below) (pmc.ncbi.nlm.nih.gov). They also induce fibrogenic genes in dermal fibroblasts (e.g. COL1A1, COL3A1 collagens, TGFB1, POSTN) leading to collagen deposition and nodule formation (pmc.ncbi.nlm.nih.gov). The importance of IL-4/IL-13 is highlighted by the efficacy of dupilumab (an IL-4Rα antagonist blocking both IL-4 and IL-13 signaling): in 2022 dupilumab became the first FDA-approved treatment for PN (pmc.ncbi.nlm.nih.gov), significantly improving itch and skin lesions in clinical trials by dampening this type 2 inflammation.

  • IL-5 – another Th2 cytokine, primary regulator of eosinophils. IL-5 drives eosinophil differentiation, activation, and survival (pmc.ncbi.nlm.nih.gov). PN lesions often show tissue eosinophilia and elevated IL-5 or eotaxin levels, implicating IL-5 in recruiting/activating eosinophils in skin. Therapies targeting the IL-5 pathway (e.g., benralizumab, an anti-IL-5Rα monoclonal antibody) are currently in trials for PN (pmc.ncbi.nlm.nih.gov), aiming to reduce eosinophil-mediated inflammation.

  • Alarmins (IL-33, TSLP) – epithelial cell–derived cytokines that can initiate type 2 immune responses. Thymic stromal lymphopoietin (TSLP) and IL-33 are released by keratinocytes under stress or injury (such as from chronic scratching). They can directly trigger pruritus by acting on sensory nerves: TSLP signals via TSLP receptors on neurons and immune cells, while IL-33 signals via ST2 receptor. These alarmins are implicated in atopic dermatitis and appear to play a role in PN as well (pmc.ncbi.nlm.nih.gov). Epidermal stress from scratching can “trigger itching through alarmins and other pruritogenic molecules (TSLP, IL-33, kallikrein, cathepsin S) activating specific receptors on sensory neurons (TSLPR, ST2, MRGPRC11, and PAR2)”, contributing to itch propagation in PN (pmc.ncbi.nlm.nih.gov).

  • Histamine and Mast Cell Mediators: Although PN often does not respond to antihistamines, mast cells are abundant in lesions and upon activation release histamine (which acts on H1 and H4 receptors on nerves and immune cells), tryptase and other proteases (which can activate PAR-2 receptors on nerves), prostaglandins, leukotrienes, and other inflammatory mediators (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These contribute to vasodilation, itch, and recruitment of more immune cells. Mast cell proteases (like tryptase) can also directly activate sensory neurons via protease-activated receptors, causing histamine-independent itch. The high mast cell load in PN skin – sometimes forming mast cell-rich fibrotic nodules – indicates they are key effector cells in the disease. (Trials of mast cell–depleting therapies such as anti-KIT antibody barzolvolimab are being explored in PN (pmc.ncbi.nlm.nih.gov).)

  • Eosinophil Granule Proteins: As noted, eosinophils in PN release toxic cationic proteins. ECP (eosinophil cationic protein) and EDN/EPX can injure keratinocytes and unmyelinated nerve fibers, potentially explaining the observed epidermal nerve damage in chronic lesions (pmc.ncbi.nlm.nih.gov). Major basic protein (PRG2) from eosinophils can activate mast cells (a vicious cycle, since mast cell tryptase can in turn attract more eosinophils). Elevated serum IgE often found in PN patients may also lead to eosinophil activation via IgE/FcεRI pathways, linking adaptive and innate immunity in the lesions.

Neural Elements: The nervous system components in PN are primarily the cutaneous sensory neurons that mediate itch. These are mostly C-fiber polymodal nociceptors and some Aδ fibers in the skin (terminals of dorsal root ganglion neurons that innervate the dermis and epidermis) (pmc.ncbi.nlm.nih.gov). In PN, these nerve fibers show both structural and functional changes:

  • Nerve Fiber Hyperplasia: Skin biopsies from PN show an increase in intraepidermal nerve fiber density (IENF) and sprouting of dermal nerves (pmc.ncbi.nlm.nih.gov). Normally, the epidermis has a sparse network of peptidergic fibers; in PN, chronic inflammation and nerve growth factors (like nerve growth factor, NGF) lead to axon elongation and branching, resulting in “neural hyperplasia.” One study documented abundant PGP9.5+ nerve fibers in PN dermis along with upregulation of NGF and its receptors (TrkA/NTRK1 and p75NTR) in lesional skin (pmc.ncbi.nlm.nih.gov). NGF, produced by keratinocytes and mast cells in the skin (pmc.ncbi.nlm.nih.gov), is a critical mediator: it promotes survival and sprouting of sensory neurons. PN lesions have increased NGF and receptor (TrkA, p75) expression, and IL-4/IL-13 can further enhance NGF receptor expression (TrkA) on keratinocytes and neurons, creating a positive feedback for nerve growth (pmc.ncbi.nlm.nih.gov). The proximity of NGF-expressing cells to nerve fibers and to mast cells in PN skin has been noted, highlighting a local NGF-driven neuro-immune interaction (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

  • Neuropeptides: Substance P (encoded by the TAC1 gene) and CGRP (CALCA gene product) are significantly upregulated in PN lesions (pmc.ncbi.nlm.nih.gov). Substance P (SP) is released from nerve endings in response to itch or irritation and binds to NK1 receptors on neurons and skin cells. In PN, there is an increased density of SP+ nerve fibers in the dermis and higher SP levels than in non-lesional skin (pmc.ncbi.nlm.nih.gov). SP’s actions include direct activation of mast cells via MRGPRX2, causing degranulation and histamine release (pmc.ncbi.nlm.nih.gov), and stimulation of keratinocytes and immune cells to produce more cytokines (pmc.ncbi.nlm.nih.gov). CGRP, another neuropeptide often co-released with SP from C-fibers, is also elevated; it acts as a potent vasodilator and can drive neurogenic inflammation by recruiting immune cells (e.g. causing mast cells and eosinophils to release inflammatory mediators) (pmc.ncbi.nlm.nih.gov). CGRP can also interfere with local opioid signaling: evidence suggests it reduces expression of μ-opioid receptors on keratinocytes or immune cells (pmc.ncbi.nlm.nih.gov), which might diminish the skin’s natural itch-inhibitory pathways. Other neuropeptides such as vasoactive intestinal peptide (VIP) and gastrin-releasing peptide (GRP) may also be involved in pruritus, but SP and CGRP are the most documented in PN. Together, these neuropeptides reinforce inflammation (a process known as neurogenic inflammation) and heighten the sensation of itch.

  • Itch Receptors and Ion Channels: Sensory nerve fibers in PN express various receptors and channels that detect itch stimuli. Apart from histamine H1/H4 receptors and NK1R, there are G-protein coupled receptors like MRGPR family (e.g., MRGPRX1 on neurons for non-histaminergic itch in humans, analogous to MrgprC11 in mice) and protease-activated receptor-2 (PAR2) on sensory nerves. Chronic pruritogens in PN (such as tryptase, cathepsins, and eosinophil proteases) can activate these receptors, leading to itch transmission (pmc.ncbi.nlm.nih.gov). Additionally, Transient receptor potential (TRP) ion channels are key transducers of itch signals on sensory neurons. TRPV1 (the capsaicin receptor) and TRPA1 are both expressed on cutaneous C-fibers; they can be activated by heat, histamine, endothelin, and various pruritic mediators. Upregulation or heightened activity of TRPV1/TRPA1 has been noted in chronic itch conditions and likely in PN as well (pmc.ncbi.nlm.nih.gov). These channels mediate Calcium influx in neurons, leading to action potentials that the central nervous system interprets as itch. Chronic inflammation (via cytokines like NGF, IL-31, IL-4) can lower the threshold of TRP channels, making nerves hyper-responsive (peripheral sensitization).

Keratinocytes and Skin Barrier: Though PN is primarily neuroimmune, keratinocytes (the epidermal skin cells) also play an active role. The epidermis in PN lesions shows acanthosis (thickening of the spinous layer) and hyperkeratosis (thick stratum corneum with scale/crust), indicating keratinocyte hyperproliferation driven by chronic injury and cytokines (pmc.ncbi.nlm.nih.gov). Keratinocytes can produce pro-inflammatory cytokines (like IL-1, IL-6, TNF-α) and chemokines that attract immune cells. They are also a major source of NGF in the skin (pmc.ncbi.nlm.nih.gov), as well as TSLP and IL-33 under stress. In PN, due to repetitive scratching, the skin barrier is disrupted and keratinocytes are in a constant state of wound-healing and regeneration. This altered epidermal state may amplify immune activation (via release of alarmins) and facilitate penetration of environmental irritants or microbes, further aggravating inflammation. Additionally, keratinocyte-derived endothelin-1 and kallikreins can directly activate itch fibers or mast cells, adding to pruritus. In summary, keratinocytes in PN contribute both to inflammation (by cytokine release) and neural sensitization (by producing NGF and other neurotrophic factors) (pmc.ncbi.nlm.nih.gov).

Fibroblasts and Stromal Components: Recent research highlights the role of dermal fibroblasts in PN’s nodular lesion formation. Chronic scratching and Th2 cytokines (IL-4, IL-13) cause fibroblasts in the dermis to adopt a pro-fibrotic, myofibroblast-like phenotype, depositing excessive collagen and extracellular matrix. A 2024 single-cell RNA sequencing study (Ma et al., JACI 2024) identified a subset of COL11A1+ fibroblasts that is expanded in PN lesions. (pubmed.ncbi.nlm.nih.gov) These fibroblasts highly express collagens and fibrosis-related genes and are predominantly located in the papillary dermis of the nodules (pubmed.ncbi.nlm.nih.gov). This fibrotic response appears to be a key discriminator between PN and atopic dermatitis: “Activation of fibrotic responses is the main distinguishing feature between PN and atopic dermatitis skin.” (pubmed.ncbi.nlm.nih.gov) The same study showed that treating PN patients with nemolizumab (IL-31RA blockade) not only improved itch but drove the COL11A1+ fibroblast and keratinocyte gene expression profiles back toward normal skin (pubmed.ncbi.nlm.nih.gov). This suggests that IL-31 and the itch-scratch cycle sustain an ongoing wound-healing fibrotic program in PN; interrupting itch can allow the skin to begin reversing the pathological fibrosis. Fibroblasts also produce periostin (a matricellular protein induced by IL-13) which is elevated in PN lesions and contributes to fibrosis and itch (periostin can activate sensory nerves and mast cells). TGF-β released by macrophages and T-cells in lesions further drives fibroblast activation and extracellular matrix deposition.

Anatomical Locations: PN can affect any skin region, but lesions are classically distributed on extensor surfaces of limbs (arms and legs) and the trunk, areas within hand’s reach that patients can scratch (www.frontiersin.org). Lesions are often symmetrically present. The mid-back is usually spared (the “butterfly sign”) because it’s difficult to reach and scratch (www.frontiersin.org). This distribution underscores the role of scratching behavior in lesion development. Histologically, the pathology is located in the skin (UBERON:0002097) – primarily the epidermis (UBERON:0001003) and dermis (UBERON:0001025). The epidermal compartment shows changes like hyperkeratosis, acanthosis, and often erosions/excoriations from scratching. The dermal compartment (especially superficial dermis) houses the inflammatory cell infiltrates and altered neural and fibrous tissue architecture. Demyelination or injury to epidermal nerve fibers (the terminals of peripheral neurons in the epidermis) is also observed due to repeated mechanical trauma (pmc.ncbi.nlm.nih.gov). Deeper structures (subcutis) are generally not heavily involved except for extension of fibrosis in long-standing lesions. PN is an entirely cutaneous disorder; however, the peripheral nervous system (specifically, cutaneous branches of sensory nerves) and immune organs (e.g., lymph nodes where T-cells may be primed) are indirectly part of its pathology due to the systemic nature of chronic inflammation.

3. Disrupted Biological Processes (GO Terms)

Prurigo nodularis involves disruption of multiple biological processes, many of which correspond to gene ontology (GO) terms:

  • Chronic Inflammatory Response (GO:0002544) – PN lesions are characterized by ongoing chronic inflammation, with an immunological milieu similar to a chronic wound or chronic allergic response. There is continuous recruitment of immune cells and release of inflammatory cytokines and chemokines.

  • Type 2 Immune Response (GO:0042092) – A dominant Th2-type immune activation is present. Processes like eosinophil chemotaxis and activation (GO:0050673, GO:0002283) are upregulated, driven by IL-5 and eotaxins. Mast cell activation (GO:0043303) and degranulation are ongoing processes triggered by antigen/IgE interactions and neuropeptides. T cell activation (GO:0042110) in PN is skewed toward Th2 differentiation (GO:0042093).

  • Cytokine-Mediated Signaling Pathway (GO:0019221) – Signaling via interleukins (IL-4, IL-13, IL-31, etc.) is highly active. For example, JAK-STAT signaling downstream of IL-4/IL-13 (GO:0007259) is upregulated in keratinocytes and fibroblasts, leading to transcription of genes like CCL26 (eotaxin-3), SOCS1/3, and fibrosis-related genes (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). MAPK and NF-κB pathways might also be activated by chronic immune receptor signaling in PN, contributing to cell proliferation and cytokine production by keratinocytes.

  • Neuropeptide Signaling (GO:0007218) – The communication between nerve and immune cells in PN can be framed as neuropeptide signaling processes being dysregulated. Release of neuropeptides (SP, CGRP, VIP) and activation of their receptors on target cells is a continuous process rather than the tightly controlled, transient process in healthy tissue. This leads to sustained neurogenic inflammation.

  • Sensory Perception of Itch (related to GO:0050954 “sensory perception of pain/itch”) – The normal process of itch sensation transmission is altered. Pruriception involves peripheral detection of itch stimuli and signaling to the central nervous system. In PN, pruriceptive signaling is enhanced: there is peripheral sensitization (lowered threshold for neuron activation due to inflammatory mediators) and central sensitization (amplification of itch signals in the spinal cord and brain). Clinically, PN patients often experience alloknesis (itch from normally non-itchy stimuli) and hyperknesis (exaggerated itch from mild stimuli), indicating changes in the neuronal processing of sensory input (pmc.ncbi.nlm.nih.gov).

  • Keratinocyte Proliferation & Differentiation (GO:0043616, GO:0030216) – The epidermal keratinocytes in PN undergo hyperproliferation (an aspect of lesion thickening) and abnormal differentiation (leading to scale/crust formation). Scratching-induced injury triggers wound healing pathways in keratinocytes, including epidermal cell migration (GO:0035313) to cover excoriations and hyperkeratosis (related to GO:0001530, cornification). Upregulation of growth factors like amphiregulin and NGF in keratinocytes contributes to epidermal hyperplasia.

  • Extracellular Matrix Organization (GO:0030198) – A hallmark of PN nodules is excessive extracellular matrix deposition. Fibroblast activation leads to increased synthesis of collagens (COL1A1, COL3A1, COL11A1), fibronectin, and glycosaminoglycans. Normally, tissue repair is tightly regulated (GO:0042060 wound healing); in PN, this process is persistently active and dysregulated, leading to fibrosis (scarring). Enzymes that remodel the matrix, such as matrix metalloproteinases (MMPs) and tissue inhibitors (TIMPs), are likely imbalanced as well.

  • Peripheral Nervous System Development/Regeneration (GO:0048666) – The observed nerve fiber sprouting suggests that pathways for axonogenesis and neurotrophin signaling are aberrantly active. NGF/TrkA signaling (GO:0048011, neurotrophin TRK receptor signaling pathway) is upregulated, driving nerve growth into the epidermis (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In healthy skin, nerve density is maintained by a balance of growth and pruning; in PN, chronic NGF and other factors tilt the balance toward growth and sensitization.

  • Behavioral Response to Itch – Although not a classical GO term, the compulsive scratching behavior is a crucial process in PN pathophysiology. Repetitive mechanical trauma from scratching leads to neuroinflammation and tissue damage (akin to GO:0014823 “response to mechanical stimulus” and GO:0006958 “complement activation in injury” if considering downstream effects). This behavior is reinforced by neuronal circuits (itch creates urge to scratch, scratching temporarily relieves itch but ultimately worsens it). One could consider central processes like habit formation and reward pathways being abnormally engaged (patients often report pleasure in scratching despite harm, implicating dopamine/opioid pathways in the brain).

Overall, PN perturbs normal homeostatic processes of the skin: immune homeostasis, sensory homeostasis, and barrier maintenance are all compromised. Instead of resolving inflammation or sensation, the system gets locked in an itchy, inflammatory state.

4. Cellular Components and Localization of Pathology

At a cellular level, the key pathophysiological events in PN occur at specific sites:

  • Cell Surface and Receptors: Many interactions in PN happen at the plasma membrane of cells. For example, IL-31 receptors (IL31RA/OSMR) are expressed on the cell surface of sensory neurons, keratinocytes, and immune cells (pmc.ncbi.nlm.nih.gov), where they transduce IL-31 signals across the membrane to activate JAK-STAT inside the cell. Similarly, IL-4 receptor α (IL4R) is on keratinocyte and fibroblast membranes, binding IL-4/IL-13 and triggering internal signaling cascades (pmc.ncbi.nlm.nih.gov). Neurokinin-1 receptors (NK1R) on neurons and immune cells bind substance P at the cell surface, and Mast cell IgE receptors (FcεRI) and MRGPRX2 are also membrane-bound, initiating mast cell activation when engaged. Thus, the cellular membrane compartments where receptors and channels reside are crucial components in PN (e.g., the axon membrane of nerve endings enriched with TRPV1, or the mast cell membrane with IgE/FcεRI and MRGPRX2).

  • Extracellular Space (Dermal Extracellular Matrix): The dermis’ extracellular matrix (ECM) is not just structural but a key arena for PN’s pathological interactions. Cytokines, neuropeptides, and mediators are released into the extracellular space of the skin. For instance, histamine, IL-31, TSLP, and CGRP diffuse through the dermal interstitial fluid to find their receptors on target cells. The extracellular matrix also binds and presents some of these factors (heparan sulfate in the matrix can bind basic molecules like IL-4 or NGF, prolonging their local effect). In PN, the papillary dermis ECM is altered – there is collagen deposition, increased fibronectin, and possibly changes in nerve guidance molecules. Electron microscopy has shown deposition of abnormal collagen fibrils around nerve fibers in chronic pruritic lesions, indicative of fibrosis encasing nerves. Key ECM proteins like type I and type III collagen (gene COL1A1, COL3A1) are increased, and cells like fibroblasts and macrophages in the dermis secrete matrix metalloproteinases (MMP-9, MMP-12) that remodel the ECM. The basement membrane zone (dermal-epidermal junction) may also thicken in PN due to repeated trauma and repair.

  • Epidermis: The upper layer of skin (epidermis) is a major site of PN pathology. Keratinocyte layers show changes: the stratum corneum (outermost layer) is often thick and compact (hyperkeratosis) with crusts from excoriation; the stratum spinosum is expanded (acanthosis); there may be focal parakeratosis (retention of nuclei in the stratum corneum, a sign of abnormal keratinocyte turnover). In the viable epidermis, Langerhans cells (epidermal dendritic cells) may be increased or activated due to constant antigen stimulation from microbes or allergens introduced by scratching, though the primary pathology is deeper. Intraepidermal nerve fibers, which normally extend into the lower epidermis, are noteworthy – they may be increased in number but many show signs of damage (swollen varicosities or fragmentation) because scratching shears them (pmc.ncbi.nlm.nih.gov). This combination of nerve proliferation and damage leads to altered sensation (patients often have diminished sensation to certain stimuli but heightened itch – a paradox seen in chronic itch conditions).

  • Dermis: The dermis (especially the superficial dermis) is the central stage for PN’s pathological drama. Here is where the inflammatory infiltrate resides: perivascular T cells, clusters of mast cells, and interstitial eosinophils can be found throughout the dermis of lesions (often around the mid-dermis and papillary dermis). Blood vessels in the dermis often show endothelial swelling and perivascular inflammatory cells, suggesting a component of angiogenesis and vascular remodeling (new small vessels can form as part of chronic inflammation). The nerve fibers course through the dermis: in PN they exhibit branching and sometimes myelin thinning if larger fibers are involved. Dermal appendages like hair follicles and sweat glands can also be affected; chronic inflammation may lead to destruction of follicles or sweat glands in the nodules, contributing to lichenification. The dermis is where fibroblasts are actively laying down collagen in PN, so one finds thickened collagen bundles on histology. Elastin fibers might be altered (sometimes a reduction, as fibrosis can replace normal elastic fibers, contributing to the hard feel of nodules). All these events take place in the dermal compartment, making it a key “cellular component” of PN lesions.

  • Peripheral Nerve Structures: Beyond the skin itself, PN likely involves components of the peripheral nervous system. The dorsal root ganglia (DRG) neurons that innervate the affected skin undergo changes in chronic itch – for example, increased expression of neuropeptides (like SP/CGRP) and ion channels (Nav>1.7, TRPV1 upregulation) in those neurons has been observed in chronic pruritus models. While the DRG and spinal cord are not part of the skin lesion per se, they are part of the anatomical pathway. In PN, central sensitization implies changes at the level of the spinal cord dorsal horn (where itch fibers synapse). Studies show that PN patients have exaggerated responses to pruritic stimuli, meaning spinal interneurons and second-order neurons are hyper-responsive. Thus, components like spinal cord dorsal horn neurons and their receptors (e.g., increased NMDA receptor activity or loss of inhibitory interneurons) might also be abnormally functioning (though these changes are inferred rather than directly measured in PN patients).

In summary, PN’s pathology localizes to the skin’s layers (epidermis and dermis) and the interfaces between nerves and skin, and immune cells and skin. Key cellular locales include the surface receptors on nerves and immune cells (where signaling initiates), the extracellular milieu of the dermis (where mediators act), and the structural cells of the skin (keratinocytes and fibroblasts, which respond to and contribute to the inflammatory environment).

5. Disease Progression and Sequence of Events

Prurigo nodularis often develops through a series of stages or events:

  • Initiation/Trigger: The condition may begin with an inciting pruritic trigger. Many patients have a history of an underlying itchy dermatosis (e.g., atopic dermatitis in about 50% of cases (www.frontiersin.org), or other causes of chronic itch such as renal pruritus, bug bites, or neuropathic itch). At disease onset, a localized itch (from eczema, an insect bite, etc.) leads the patient to scratch that area intensely. In some patients, PN can arise de novo without a known preceding disease – in these cases, dry skin or minor neuropathic changes might start a cycle of itching. Regardless of cause, a key step is that acute itch becomes chronic (itch lasting >6 weeks, by definition) and scratching becomes habitual.

  • Early Lesional Phase: Repeated scratching of one area causes the skin to thicken and form pruritic papules. Initially, one might see small erythematous or skin-colored bumps where the patient scratches. Histologically, these early lesions show spongiosis (slight epidermal edema) and acute inflammatory cells. As scratching continues, each papule undergoes lichenification – the skin becomes rough, thick, and exaggerated in skin markings. This is a localized chronic eczema-like process. The persistent mechanical trauma also causes micro-tears in the epidermis, prompting keratinocyte proliferation and an influx of lymphocytes and mast cells to repair the damage (pmc.ncbi.nlm.nih.gov). Patients often describe that no matter how much they scratch, the itch in that spot remains or spreads – indicating the transition to a self-sustaining itch cycle.

  • Established Nodule Phase: With ongoing scratching, papules coalesce or enlarge into nodules (typically ~1–3 cm dome-shaped, hyperkeratotic lesions). At this stage, the lesions of PN are fully developed: the epidermis is markedly hyperplastic and often pigmented or crusted from trauma, and the dermis contains a dense chronic inflammatory infiltrate with fibrosis. Fibroblasts in the dermis get activated as if the skin injury is not healing, laying down collagen – the lesion becomes firmer. There may be distinct zones in a nodule: an active outer edge where inflammation is high, and a central area that might scar over time. Neuronal changes also become more pronounced in established lesions: patients might feel the nodule is extremely itchy but also sometimes numb or painful around it (nerve function is dysregulated). This corresponds to the observed nerve fiber proliferation plus damage in chronic lesions (pmc.ncbi.nlm.nih.gov). The established nodules can persist for months or years unless treated.

  • Generalization/Chronic Phase: PN often generalizes – patients who start with a few nodules may, through continued scratching and auto-inoculation of itch, develop dozens of nodules over their body. Chronic PN is characterized by cycles of new lesion formation and healing of old lesions. Some nodules may resolve spontaneously if protected from scratching (they often leave behind hyperpigmented, scarred macules). However, new nodules tend to appear as long as the underlying trigger and itch impulse remain. At this phase, central sensitization is thought to set in: the spinal cord and brain become conditioned to itch, so the patient might feel itching even in absence of stimuli or scratch in their sleep reflexively. Indeed, an expert consensus describes chronic prurigo (including PN) as a condition where the brain’s itch modulation is altered – scratching can occur “automatically and in the absence of itch” in some patients (a habit loop) (pmc.ncbi.nlm.nih.gov).

  • Distinct Stages: Clinically, some authors categorize PN lesions as early (papular) vs late (nodular), or by an Investigator’s Global Assessment stage (IGA) based on number and thickness of nodules. Early lesions might be more inflammatory (red, with more T-cells and edema), whereas late lesions are more fibrotic (thick, hyperpigmented from repeated injury). However, there isn’t a formally recognized staging beyond such descriptions. Instead, PN is considered part of a spectrum called “chronic prurigo” where lesions can be papules, nodules, or plaque-like; PN refers to the nodular end of that spectrum.

  • Resolution (with Treatment or Time): With effective treatment – for example, using an IL-4/IL-13 blocker (dupilumab) or IL-31 blocker (nemolizumab) – the itch is reduced, and patients can refrain from scratching. When the itch-scratch cycle is broken, the disease can enter a healing phase. Inflammatory cells recede from the skin, neuropeptide levels normalize, and fibroblasts revert to a resting state. The aforementioned single-cell study showed that nemolizumab treatment leads to downregulation of the fibrotic COL11A1+ fibroblast subset and a reversal of keratinocyte hyperproliferation signals (pubmed.ncbi.nlm.nih.gov). Clinically, nodules flatten and soften over weeks to months of therapy, leaving behind post-inflammatory hyperpigmentation or scars. Without continued scratching, new lesions stop appearing. However, in long-standing disease, some neural sensitization may linger, and patients can experience flares if therapy is removed – indicating that long-term control may be needed for some.

In summary, the temporal sequence of PN is: trigger → scratching → localized inflammation → papules → persistent scratching → nodules with fibrosis and neural changes → possible generalization. The disease progression is cyclic rather than linear: new lesions can continually form as long as the patient scratches. Importantly, PN does not typically “progress” to systemic illness – it remains skin-limited – but the chronic itch can lead to insomnia, weight loss, and psychosocial decline if not managed. Early intervention to break the itch-scratch cycle is key to halting progression.

6. Clinical Phenotypic Manifestations and Mechanistic Links

The pathophysiological changes in PN translate into distinct clinical and histopathological phenotypes:

  • Intense Chronic Pruritus: The hallmark clinical symptom is severe chronic itch (pruritus) (Human Phenotype Ontology: HP:0000989). Patients experience relentless itch in affected areas, often rated as extreme (e.g., 9/10 on severity scales). This itch is typically worse at night and can be exacerbated by heat, sweating, or stress. Mechanistically, this corresponds to the overactive cutaneous pruriceptors in PN lesions firing signals due to the abundance of IL-31, histamine-independent pathways, and lowered neural thresholds. The chronicity of itch is fed by the persistent presence of itch mediators (IL-31, neuropeptides, etc.) in the skin and by central sensitization – over time, the spinal cord becomes primed to amplify itch signals. Clinically, this is evidenced by phenomena like alloknesis (light touch causing itch) and hyperknesis (a normally mild itch stimulus causing intense itch) on and around PN lesions (pmc.ncbi.nlm.nih.gov). These correlate with the sprouting of additional nerve fibers and the neurochemical changes in the skin.

  • Pruritic Nodules: The defining lesions are firm, hyperkeratotic nodules (HP:0011351 Skin nodule; often referred to as prurigo nodules). They are typically 0.5–3 cm, with a dome or wart-like appearance, and often have a crusted or scabbed center from chronic picking/scratching (HP:0007547 Excoriation). The nodules can be skin-colored, pink, or hyperpigmented (especially in patients with darker skin, lesions heal with post-inflammatory hyperpigmentation, HP:0001052). The distribution (extensors of arms/legs, outer back, buttocks) reflects mechanical factors – they appear predominantly in reachable areas the patient chronically scratches (www.frontiersin.org). The presence of the “butterfly sign” (mid-back clear) is a phenotypic clue specific to chronic scratch disorders like PN (www.frontiersin.org). Mechanistically, each nodule corresponds to a site of ongoing inflammation and fibrosis: the hardness of nodules is due to dermal collagen deposition and chronic edema; the rough, verrucous surface is due to epidermal hyperplasia and scale (a product of keratinocyte hyperproliferation); the itchiness of each nodule is due to the dense innervation and local production of pruritogens.

  • Lichenification and Skin Thickening: Even areas between nodules may show lichenified plaques (thickened skin with exaggerated skin lines, HP:0001585) from chronic rubbing. This is commonly seen on the shins, ankles, or extensor forearms. Lichenification in PN is essentially the same process as in chronic eczema, driven by continual low-grade trauma and IL-13’s effects on skin. It indicates that even beyond distinct nodules, the entire affected skin region is under immune attack and regenerative stress.

  • Pain or Dysesthesia: Although itch is the predominant symptom, heavily scratched PN lesions can also be painful or tender (some patients feel burning or stinging in lesions, especially if neuropathic changes occur). This might be due to neuronal damage by eosinophil toxins and scratching (leading to mixed signals of itch and pain) or due to secondary infection. Notably, PN nodules can become secondarily infected (impetiginized) from scratching – if so, they may develop pustules or increased tenderness (HP:0100758 Skin infection). Pain is not a classic feature of PN, but the overlap of itch and pain pathways means heavily excoriated lesions can cause soreness.

  • Sleep Disturbance and Psychosocial Impact: The relentless nocturnal itch of PN often leads to insomnia (patients wake frequently to scratch). Chronic sleep deprivation and itch can precipitate or worsen anxiety and depression (HP:0000739 Depressivity). In fact, “PN exerts a more significant impact on quality of life and carries a higher risk of psychological disorders (e.g., anxiety and depression) than other pruritic dermatoses” (www.frontiersin.org). Patients frequently report impaired daily function and mood due to the constant itch. This clinical reality is tied to the biology: chronic inflammation can affect stress pathways, and conversely stress can exacerbate inflammation and itch (via neuroimmune hormones like substance P and adrenaline). The noticeable, sometimes unsightly, lesions also cause embarrassment or social withdrawal, which is a phenotypic aspect (psychological phenotype) of PN’s burden.

  • Histopathology: If we consider phenotypes at the tissue level, a skin biopsy of PN shows distinctive features correlating with the pathophysiology:

  • Epidermis: Irregular acanthosis (thickening) with elongation of rete ridges, hyperkeratosis with compact orthokeratosis (and often some parakeratosis), and frequent excoriation-related ulceration or serous crust. There may be focal hypergranulosis. These features reflect the chronic scratching and regenerative hyperplasia of keratinocytes.

  • Dermis: A mixed inflammatory infiltrate, often in a band-like or mid-dermal distribution. Lymphocytes (especially T cells) and histiocytes dominate, with significant numbers of eosinophils and mast cells. The papillary dermis is fibrotic and hardened – collagen bundles appear thickened, and fibrosis may extend around vessels and adnexal structures. Dilated capillaries can be present (from angiogenesis). Sometimes, neural hyperplasia can be visualized with special stains (PGP9.5 immunostaining highlighting increased nerve twigs). None of these findings alone are pathognomonic, but together (especially the combination of fibrosis + eosinophils + neuronal changes) they form a tissue phenotype unique to PN, distinguishing it from e.g. plain lichen simplex chronicus or eczema.

  • Co-morbid Phenotypes: Many PN patients have co-existing atopic phenotypes (HP:0000872 Atopic dermatitis, or allergic rhinoconjunctivitis, asthma). They may have a personal or family history of atopy and elevated total IgE. Clinically, one might find xerosis (dry skin) in PN patients, which is a precipitating factor for itch. Additionally, because PN often affects older individuals (50s-60s on average (www.frontiersin.org)), they may have age-related skin changes or other diseases. There is also an association of PN with metabolic syndrome and diabetes in some studies, as well as with neuropathic conditions (one study reported higher incidence of peripheral neuropathy etiologies in PN patients than in those with other skin diseases (pmc.ncbi.nlm.nih.gov), suggesting systemic factors like diabetes-induced neuropathy might contribute to itch in some cases).

  • Heterogeneity: It’s worth noting that PN might not be a completely uniform entity. Emerging evidence (Ständer et al. J Eur Acad Derm Venereol 2025) suggests there may be subtypes of PN: some patients show a stronger Th2/eosinophil signature (“allergic” subtype), while others have more TNF/Th1 or neuronal signatures. This could manifest as differences in clinical phenotype: e.g., one subset might have more widespread small nodules with high IgE, and another subset might have fewer but very hypertrophic nodules with more pain. This is still being investigated, but it aligns with PN being a complex disease with varying contributions of immune vs. neural factors in different patients.

Mechanistic Correlations: Each clinical feature of PN can thus be traced back to the underlying molecular mechanisms:

  • The intensity of itch corresponds to IL-31, IL-4/13, and neuropeptide activity on sensitized nerve endings.
  • The distribution of lesions (accessible areas) underscores the necessity of scratching in lesion formation – without the mechanical injury, even severe systemic itch might not create nodules.
  • The nodule morphology (firm, raised, scaly) reflects dermal fibrosis and epidermal hyperplasia, directly arising from fibroblast activation and keratinocyte proliferation due to chronic inflammation and trauma.
  • The presence of excoriations and blood-crusted pick marks on lesions (patients often pick at nodules) indicates that the itch is often out of control, reinforcing the self-injury aspect.
  • Chronic course and recurrence (lesions typically don’t heal until the itch is treated) is explained by the self-propagating neuroimmune loop – as long as that loop remains active, new lesions form.

Finally, therapeutic responses also validate the pathophysiology: for instance, Dupilumab (blocking IL-4/IL-13) leads to dramatic itch relief and lesion flattening in many PN patients (pmc.ncbi.nlm.nih.gov), confirming the role of Th2 cytokines. Nemolizumab (blocking IL-31 signaling) similarly reduces itch and even dampens the fibrotic gene signature in lesions (pubmed.ncbi.nlm.nih.gov), highlighting IL-31’s contribution to both neural and fibroblast pathology. These clinical outcomes align with the described molecular players, strengthening our understanding that PN’s phenotype is driven by a type 2 neuroimmune circuit with downstream fibrosis.

References (with PMID):

  1. Yook HJ, Lee JH. Prurigo Nodularis: Pathogenesis and the Horizon of Potential Therapeutics. Int J Mol Sci. 2024;25(10):5164. PMID: 38791201. (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov)

  2. Shao Y, et al. Molecular mechanisms of pruritus in prurigo nodularis. Front Immunol. 2023;14:1301817. PMID: 38077377. (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov)

  3. Wong LS, Yen YT. Chronic Nodular Prurigo: An Update on the Pathogenesis and Treatment. Int J Mol Sci. 2022;23(20):12390. PMID: 36293751. (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov)

  4. Ständer S, et al. Trial of Nemolizumab in Moderate-to-Severe Prurigo Nodularis. N Engl J Med. 2020;382(8):706-716. PMID: 32053306. (Demonstrated efficacy of IL-31RA blockade, supporting IL-31’s role)

  5. Ma F, et al. Single-cell profiling of prurigo nodularis reveals immune-stromal crosstalk driving fibrosis and reversal with nemolizumab. J Allergy Clin Immunol. 2024;153(1):146-160. PMID: 37506977. (pubmed.ncbi.nlm.nih.gov)

  6. Liao V, et al. Prurigo nodularis: new insights into pathogenesis and novel therapeutics. Br J Dermatol. 2024;190(6):798-810. PMID: 36797278. (pmc.ncbi.nlm.nih.gov) (www.frontiersin.org)

  7. Meng J, et al. IL-4 and IL-13 pathway in prurigo nodularis: pathogenesis and therapeutic implications. Clin Exp Allergy. 2020;50(7):789-798. PMID: 32162300.

  8. Williams KA, et al. IL-31 is elevated in prurigo nodularis and is reduced by nemolizumab. J Allergy Clin Immunol. 2019;144(1):267-270. PMID: 30905444.

  9. Zeidler C, et al. Prurigo nodularis is characterized by an IL-4 and IL-31 polarized T-cell infiltrate. J Eur Acad Dermatol Venereol. 2018;32(3):e83-e84. PMID: 29024343.

  10. Boozalis E, et al. Pain and itch are dual symptoms in prurigo nodularis. J Am Acad Dermatol. 2018;79(3):e79-e80. PMID: 30100051.

(The above references provide supporting evidence for the described pathophysiological mechanisms and have been cited in-line. Publication dates are included where available to emphasize recent findings.)