Polymyositis

Autoimmune MONDO:0019127 Pathograph 9 Show in embeddings browser Autoimmune Disease Inflammatory Myopathy

An idiopathic inflammatory myopathy characterized by symmetric proximal muscle weakness without the skin manifestations of dermatomyositis. CD8+ T cell-mediated attack on muscle fibers is the primary pathogenic mechanism.

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7
Pathophys.
19
Phenotypes
3
Gaps
9
Pathograph
1
Genes
4
Medical Actions
4
Differentials
3
Datasets
15
References
2
Deep Research
🏷

Classifications

Harrison's Part
IMMUNE RHEUMATOLOGIC
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Discussions and Knowledge Gaps

3
Is polymyositis a distinct disease entity, or a residual label for idiopathic inflammatory myopathy that has not yet been resolved into antisynthetase syndrome, immune-mediated necrotizing myopathy, or inclusion body myositis?
CONTROVERSY OPEN pm_nosological_status
Attached to
pathophysiology#CD8+ T Cell-Mediated Cytotoxicity pathophysiology#Myositis-Specific Autoantibodies
This entry models polymyositis as a standalone entity with a CD8+/MHC-I mechanism. That mechanism is real and is what defines PM histologically, but the *scope* of the label is contested: a follow-up study of 165 myositis patients could reclassify most PM diagnoses, and contemporary classifications carve antisynthetase syndrome, immune-mediated necrotizing myopathy and overlap myositis out of the same population. The unresolved question determines what belongs in this entry — in particular whether the antisynthetase content (pulmonary fibrosis, arthritis, anti-Jo-1, and the `Myositis-Specific Autoantibodies` node) should move to Antisynthetase_Syndrome, and whether an Immune-Mediated Necrotizing Myopathy entry is needed to receive the anti-SRP content.
Raised as item 3 of dismech#6968, which asks explicitly whether this entry should stay as-is with a scoping note, gain a discussion, or be repositioned relative to its sibling IIM entries. This discussion records the uncertainty; the repositioning decision is left to a maintainer. Related to dismech#306 (lumping/splitting).
Show evidence (3 references)
PMID:12913190 SUPPORT Human Clinical
"Polymyositis is an overdiagnosed entity."
The primary source for the claim that the label is applied far more often than it applies.
PMID:41068877 SUPPORT Other
"recent classifications based on autoantibodies and gene expression have proposed new categories, such as antisynthetase syndrome, clinical amyopathic dermatomyositis, immune-mediated necrotizing myopathy, and inclusion body myositis, resulting in the concept of IIM as a spectrum of diseases..."
Documents the reclassification of the IIM space into which polymyositis is being subdivided.
PMID:39132766 SUPPORT Other
"there is a need to revise the criteria with regard to the addition of new myositis-specific autoantibodies, newly defined subgroups (especially antisynthetase syndrome, immune medicated necrotizing myopathy and overlap myositis)"
Shows the question is live rather than settled — the 2017 EULAR/ACR criteria are themselves under revision on exactly this point.
Does any immunosuppressive or immunomodulatory therapy have reliable randomized evidence of benefit in polymyositis specifically, as opposed to in dermatomyositis or in idiopathic inflammatory myopathy pooled?
KNOWLEDGE GAP OPEN pm_treatment_evidence_base
Attached to
treatments#Corticosteroids treatments#Methotrexate treatments#Azathioprine treatments#IVIG
All four treatments curated here are standard of care, and none rests on polymyositis-specific randomized evidence. Glucocorticoids are used empirically; the Cochrane review finds the azathioprine and methotrexate trials very-low-certainty; and while IVIg has moderate-certainty benefit in refractory IIM pooled and in refractory dermatomyositis, the review states that the polymyositis data are not reliable. This is not merely a citation gap — it is plausibly downstream of `pm_nosological_status`, since trials enrolling "PM" have been enrolling a mixed population.
Show evidence (2 references)
PMID:40787733 SUPPORT Human Clinical
"For the other agents, the small number of trials of immunosuppressive and immunomodulatory therapies is inadequate to decide whether these agents are beneficial in IIM (excluding IBM)."
The review's own summary of the size of this gap.
PMID:40787733 SUPPORT Human Clinical
"The risk of bias in all but one study was high or unclear."
Characterizes the quality of the 16 trials that do exist.
Are myositis-specific autoantibodies pathogenic drivers of muscle injury, or diagnostic markers of an immune response to regenerating muscle?
KNOWLEDGE GAP OPEN pm_msa_pathogenic_role
Attached to
pathophysiology#Myositis-Specific Autoantibodies
This is why the `Myositis-Specific Autoantibodies` node carries no `downstream` edges. The antibodies are well established as classifiers, but no route from antibody to fiber injury has been demonstrated, and the leading alternative — that the autoantigens are simply overexpressed in regenerating fibers — would make the antibodies a consequence of muscle damage rather than a cause. Drawing a causal edge either way would assert more than the literature supports.
Show evidence (1 reference)
PMID:23568993 SUPPORT Other
"Although the pathogenic role of MSAs remains unknown, recent studies have shown that myositis autoantigens are expressed at high levels in regenerating muscle fibers, which may initiate or amplify autoimmune responses in idiopathic inflammatory myopathies."
States both the gap and the leading alternative interpretation in one sentence.
⚙

Pathophysiology

7
CD8+ T Cell-Mediated Cytotoxicity
Clonally expanded CD8+ cytotoxic T cells infiltrate muscle fibers and directly attack myocytes expressing MHC class I. Perforin and granzyme release leads to muscle fiber necrosis.
CD8+ T Cell CL:0000625 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves CD8+ T Cell, annotated with CD8-positive, alpha-beta T cell (CL:0000625). CL:0000625 is a cell type from the Cell Ontology. Skeletal Muscle Cell CL:0000188 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Skeletal Muscle Cell, annotated with cell of skeletal muscle (CL:0000188). CL:0000188 is a cell type from the Cell Ontology.
T Cell Cytotoxicity GO:0001913 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves T Cell Cytotoxicity, annotated with T cell mediated cytotoxicity (GO:0001913). GO:0001913 is a biological process from the Gene Ontology.
Downstream
Apoptosis of Muscle Fibers Cytotoxic T cells kill neighbouring fibers by an apoptotic route running in parallel with the partial-invasion route.
Necroptosis and DAMP Release Fiber death in myositis includes a necroptotic component, but what couples the cytotoxic infiltrate to necroptosis induction has not been identified.
Show evidence (2 references)
PMID:31257434 SUPPORT
"The hallmark histopathology of PM is the presence of CD8+ T cells in the non-necrotic muscle cells."
Demonstrates that CD8+ T cell invasion into muscle fibers is the defining pathological feature of polymyositis.
PMID:31257434 SUPPORT
"CD8+ T cells invade into muscle cells and contribute to muscle injury in PM."
Confirms the direct cytotoxic role of CD8+ T cells in causing muscle fiber damage through invasion and attack.
Aberrant MHC Class I Expression
Muscle fibers, which normally do not express MHC class I, show aberrant upregulation making them targets for CD8+ T cell recognition and attack.
MHC Class I Antigen Presentation GO:0002474 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves MHC Class I Antigen Presentation, annotated with antigen processing and presentation of peptide antigen via MHC class I (GO:0002474). GO:0002474 is a biological process from the Gene Ontology.
Downstream
CD8+ T Cell-Mediated Cytotoxicity Sarcolemmal class I MHC is the ligand that turns an otherwise immunologically inconspicuous fiber into a target for antigen-specific CD8+ T cells.
ER Stress and Unfolded Protein Response Class I MHC overexpression is itself sufficient to trigger the UPR in muscle, giving the entry its non-immune arm of fiber damage.
Show evidence (2 references)
PMID:15934115 SUPPORT
"Up-regulation of class I MHC in skeletal muscle fibers was an early and consistent feature of human inflammatory myopathies."
Demonstrates that MHC class I upregulation is a hallmark feature of polymyositis pathogenesis.
PMID:15934115 SUPPORT
"Class I MHC staining in muscle fibers of myositis patients showed both cell surface and a reticular pattern of internal reactivity."
Shows that aberrant MHC I expression occurs both on the sarcolemma and within the endoplasmic reticulum of muscle fibers.
Myositis-Specific Autoantibodies
Anti-synthetase antibodies (anti-Jo-1, anti-PL-7, anti-PL-12) define antisynthetase syndrome with myositis, ILD, and arthritis. Anti-SRP antibodies associate with necrotizing myopathy.
Immunoglobulin Production GO:0002377 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves Immunoglobulin Production (GO:0002377). GO:0002377 is a biological process from the Gene Ontology.
Show evidence (2 references)
PMID:23568993 SUPPORT Other
"Anti-Jo-1, anti-PL-7, anti-PL-12, anti-EJ, anti-KS, anti-OJ, anti-Ha, and anti-Zo antibodies target aminoacyl tRNA synthetases, and represent anti-synthetase syndrome."
Sources the antisynthetase-antibody group named in this node.
PMID:23568993 SUPPORT Other
"anti-SRP and anti-3-hydroxy-3-methylglutaryl-coenzyme A (anti-HMG-CoA) antibodies have been found in patients with necrotizing myopathy"
Sources the anti-SRP / necrotizing-myopathy association named in this node.
Pro-inflammatory Cytokine Production
IL-1, TNF-alpha, and IFN-gamma produced by infiltrating immune cells perpetuate muscle inflammation and contribute to muscle weakness independent of direct fiber destruction.
Inflammatory Response GO:0006954 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves Inflammatory Response (GO:0006954). GO:0006954 is a biological process from the Gene Ontology.
Downstream
Aberrant MHC Class I Expression Feed-forward amplification: cytokines released by the infiltrate drive the class I MHC upregulation that recruits further cytotoxic attack, closing a loop with the CD8+ arm.
Show evidence (1 reference)
PMID:9655472 SUPPORT In Vitro
"Treatment of myoblasts with TNFalpha or IFNgamma led to the expression of a range of immunostimulatory molecules including MHC class I and II, and CD95 (Fas), but not B7 family molecules."
Human muscle cells respond to TNF-alpha and IFN-gamma by becoming immunostimulatory, which is the mechanism this node asserts.
ER Stress and Unfolded Protein Response
Overexpression of MHC class I in muscle fibers triggers endoplasmic reticulum stress and activation of the unfolded protein response (UPR). This includes upregulation of GRP78/BiP and NF-kappaB pathway activation, contributing to muscle fiber dysfunction independent of immune-mediated damage.
Endoplasmic Reticulum Stress Response GO:0034976 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves Endoplasmic Reticulum Stress Response, annotated with response to endoplasmic reticulum stress (GO:0034976). GO:0034976 is a biological process from the Gene Ontology.
Downstream
Proximal muscle weakness The non-immune route from class I MHC overexpression to weakness, invoked to explain why the degree of inflammation does not track disease severity.
Show evidence (2 references)
PMID:15934115 SUPPORT
"The pathways of endoplasmic reticulum (ER) stress response, the unfolded protein response (glucose-regulated protein 78 pathway), and the ER overload response (NF-kappaB pathway) were significantly activated in muscle tissue of human myositis patients and in the mouse model."
Demonstrates that ER stress pathways are activated in polymyositis muscle tissue as a consequence of MHC I overexpression.
PMID:15934115 SUPPORT
"These results indicate that the ER stress response may be a major nonimmune mechanism responsible for skeletal muscle damage and dysfunction in autoimmune myositis."
Identifies ER stress as a non-immune mechanism contributing to muscle pathology, explaining why inflammation doesn't always correlate with disease severity.
Necroptosis and DAMP Release
Muscle fibers undergo necroptosis, a form of regulated cell death, releasing damage-associated molecular patterns (DAMPs) including HMGB1. This amplifies inflammation and contributes to persistent muscle weakness even after initial immune attack.
Necroptotic Process GO:0070266 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves Necroptotic Process (GO:0070266). GO:0070266 is a biological process from the Gene Ontology.
Downstream
Pro-inflammatory Cytokine Production DAMPs released from necroptotic fibers — HMGB1 chief among them — drive the inflammatory mediator output that sustains the lesion.
Proximal muscle weakness The arm that accounts for weakness persisting after inflammation has been suppressed; blocking necroptosis or HMGB1 restores strength in the murine polymyositis model.
Show evidence (2 references)
PMID:37483632 SUPPORT
"Recent studies have revealed that muscle fibers of IIMs undergo necroptosis, a newly recognized form of regulated cell death, and promote muscle inflammation and dysfunction through releasing inflammatory mediators such as damage-associated molecular patterns (DAMPs)."
Establishes necroptosis as a key mechanism of muscle fiber death in polymyositis that perpetuates inflammation.
PMID:37483632 SUPPORT
"The research on murine model of polymyositis, a subtype of IIM, revealed that the inhibition of necroptosis or HMGB1, one of major DAMPs released from muscle fibers undergoing necroptosis, ameliorated muscle inflammation and recovered muscle weakness."
Demonstrates that blocking necroptosis or DAMP signaling improves both inflammation and muscle function, highlighting therapeutic potential.
Apoptosis of Muscle Fibers
Apoptotic cell death occurs in muscle fibers surrounding those undergoing partial invasion by CD8+ T cells. Fibers with apoptotic nuclei are surrounded by CD8+ T cells, granzyme B-positive cells, and macrophages, suggesting cytotoxic T cell-mediated apoptosis occurs in parallel to direct invasion.
Apoptotic Process GO:0006915 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves Apoptotic Process (GO:0006915). GO:0006915 is a biological process from the Gene Ontology.
Show evidence (2 references)
PMID:32936839 SUPPORT
"Apoptotic myonuclei were found in muscle with partial invasion, but not in the invaded fibres. Fibres with TUNEL positive nuclei were surrounded by CD8+ T-cells, granzyme B+ cells and macrophages, but lacked FAS receptor expression."
Shows that apoptosis occurs in polymyositis muscle fibers adjacent to those with CD8+ T cell invasion, mediated by cytotoxic mechanisms.
PMID:32936839 SUPPORT
"The findings confirm that apoptosis occurs in idiopathic inflammatory myopathies and support that it is mediated by CD8+ cytotoxic T- cells, acting in parallel to the process of partial invasion."
Establishes that CD8+ T cell-mediated apoptosis is a distinct pathway of muscle damage occurring alongside direct fiber invasion.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Polymyositis Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

19
Digestive 3
Dysphagia FREQUENT HP:0002015 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Dysphagia (HP:0002015). HP:0002015 is a phenotype from the Human Phenotype Ontology.
Pharyngeal muscle involvement
Constipation FREQUENT HP:0002019 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Constipation (HP:0002019). HP:0002019 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0002019 | Constipation | Frequent (79-30%)"
Orphanet records constipation as a frequent (30-79%) phenotype of polymyositis.
Anorexia FREQUENT HP:0002039 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Anorexia (HP:0002039). HP:0002039 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0002039 | Anorexia | Frequent (79-30%)"
Orphanet records anorexia as a frequent (30-79%) phenotype of polymyositis.
Immune 1
Autoimmunity VERY_FREQUENT HP:0002960 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Autoimmunity (HP:0002960). HP:0002960 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0002960 | Autoimmunity | Very frequent (99-80%)"
Orphanet records autoimmunity as a very frequent (80-99%) phenotype of polymyositis.
Metabolism 2
Elevated circulating creatine kinase concentration VERY_FREQUENT HP:0003236 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Elevated circulating creatine kinase concentration (HP:0003236). HP:0003236 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
ORPHA:732 SUPPORT Other
"HP:0003236 | Elevated circulating creatine kinase concentration | Very frequent (99-80%)"
Orphanet records elevated circulating creatine kinase concentration as a very frequent (80-99%) phenotype of polymyositis.
PMID:31257434 SUPPORT Human Clinical
"The level of serum creatinine kinase was higher in patients with CD8+ T cells in muscle cells than those without these cells."
Links serum creatine kinase, the readout curated here, to the degree of CD8+ T cell invasion of muscle fibers.
Fever FREQUENT HP:0001945 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Fever (HP:0001945). HP:0001945 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0001945 | Fever | Frequent (79-30%)"
Orphanet records fever as a frequent (30-79%) phenotype of polymyositis.
Musculoskeletal 3
Arthritis FREQUENT HP:0001369 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Arthritis (HP:0001369). HP:0001369 is a phenotype from the Human Phenotype Ontology.
Part of antisynthetase syndrome
Hypotonia VERY_FREQUENT HP:0001252 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypotonia (HP:0001252). HP:0001252 is a phenotype from the Human Phenotype Ontology.
Frequency band inherited from the ORPHA:732 bulk phenotype import and not corroborated by any clinical source. Hypotonia at VERY_FREQUENT is clinically surprising for an adult-onset inflammatory myopathy, where the cardinal finding is proximal weakness rather than reduced muscle tone; read this as Orphanet's coding choice rather than as an independently evidenced band, and re-check it against a clinical cohort before relying on it (dismech#6968, item 4).
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0001252 | Hypotonia | Very frequent (99-80%)"
Orphanet records hypotonia as a very frequent (80-99%) phenotype of polymyositis.
Proximal muscle weakness VERY_FREQUENT HP:0003701 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Proximal muscle weakness (HP:0003701). HP:0003701 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0003701 | Proximal muscle weakness | Very frequent (99-80%)"
Orphanet records proximal muscle weakness as a very frequent (80-99%) phenotype of polymyositis.
Respiratory 3
Pulmonary Fibrosis FREQUENT HP:0002206 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Pulmonary Fibrosis (HP:0002206). HP:0002206 is a phenotype from the Human Phenotype Ontology.
Associated with antisynthetase syndrome
Cough VERY_FREQUENT HP:0012735 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cough (HP:0012735). HP:0012735 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0012735 | Cough | Very frequent (99-80%)"
Orphanet records cough as a very frequent (80-99%) phenotype of polymyositis.
Respiratory insufficiency FREQUENT HP:0002093 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Respiratory insufficiency (HP:0002093). HP:0002093 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0002093 | Respiratory insufficiency | Frequent (79-30%)"
Orphanet records respiratory insufficiency as a frequent (30-79%) phenotype of polymyositis.
Constitutional 2
Arthralgia VERY_FREQUENT HP:0002829 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Arthralgia (HP:0002829). HP:0002829 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0002829 | Arthralgia | Very frequent (99-80%)"
Orphanet records arthralgia as a very frequent (80-99%) phenotype of polymyositis.
Myalgia FREQUENT HP:0003326 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Myalgia (HP:0003326). HP:0003326 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0003326 | Myalgia | Frequent (79-30%)"
Orphanet records myalgia as a frequent (30-79%) phenotype of polymyositis.
Growth 1
Weight loss FREQUENT HP:0001824 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Weight loss (HP:0001824). HP:0001824 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0001824 | Weight loss | Frequent (79-30%)"
Orphanet records weight loss as a frequent (30-79%) phenotype of polymyositis.
Other 4
EMG abnormality VERY_FREQUENT HP:0003457 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is EMG abnormality (HP:0003457). HP:0003457 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0003457 | EMG abnormality | Very frequent (99-80%)"
Orphanet records eMG abnormality as a very frequent (80-99%) phenotype of polymyositis.
Elevated circulating aldolase concentration VERY_FREQUENT HP:0012544 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Elevated circulating aldolase concentration (HP:0012544). HP:0012544 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0012544 | Elevated circulating aldolase concentration | Very frequent (99-80%)"
Orphanet records elevated circulating aldolase concentration as a very frequent (80-99%) phenotype of polymyositis.
Exertional dyspnea FREQUENT HP:0002875 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Exertional dyspnea (HP:0002875). HP:0002875 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0002875 | Exertional dyspnea | Frequent (79-30%)"
Orphanet records exertional dyspnea as a frequent (30-79%) phenotype of polymyositis.
Antinuclear antibody positivity FREQUENT HP:0003493 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Antinuclear antibody positivity (HP:0003493). HP:0003493 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:732 SUPPORT Other
"HP:0003493 | Antinuclear antibody positivity | Frequent (79-30%)"
Orphanet records antinuclear antibody positivity as a frequent (30-79%) phenotype of polymyositis.
🧬

Genetic Associations

1
HLA-DRB1*0301 (Risk Factor)
💊

Medical Actions

4
Corticosteroids
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: prednisone CHEBI:8382 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses prednisone (CHEBI:8382). CHEBI:8382 is a therapeutic agent from Chemical Entities of Biological Interest. prednisolone CHEBI:8378 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses prednisolone (CHEBI:8378). CHEBI:8378 is a therapeutic agent from Chemical Entities of Biological Interest.
First-line therapy; high-dose initially with gradual taper. Used empirically rather than on the strength of randomized evidence, and the adverse-effect burden is itself the reason a steroid-sparing agent is usually added early.
Show evidence (1 reference)
PMID:29669460 SUPPORT Other
"glucocorticoids are used empirically as the first-line treatment despite their various adverse effects"
Supports first-line status while being explicit that the basis is empirical practice rather than trial evidence.
Methotrexate
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: methotrexate CHEBI:44185 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses methotrexate (CHEBI:44185). CHEBI:44185 is a therapeutic agent from Chemical Entities of Biological Interest.
Most commonly used steroid-sparing agent. Added to reduce cumulative glucocorticoid exposure rather than on demonstrated independent efficacy in adult polymyositis.
Show evidence (2 references)
PMID:29669460 SUPPORT Other
"Concomitant treatment with steroid-sparing immunosuppressive agents, including methotrexate, azathioprine, calcineurin inhibitors, mycophenolate mofetil, and cyclophosphamide, reduces successfully initial glucocorticoid doses for the remission induction, the relapse risk during glucocorticoid..."
Supports the steroid-sparing rationale for methotrexate in PM/DM.
PMID:40787733 SUPPORT Human Clinical
"For methotrexate, there may be little or no improvement in adults with DM or PM in function"
Cochrane's very-low-certainty finding qualifies the steroid-sparing rationale: the single 27-participant adult RCT showed no functional benefit.
Azathioprine
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: azathioprine CHEBI:2948 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses azathioprine (CHEBI:2948). CHEBI:2948 is a therapeutic agent from Chemical Entities of Biological Interest.
Alternative steroid-sparing immunosuppressant for maintenance. The only randomized placebo comparison is a 16-participant study from which no reliable conclusion can be drawn.
Show evidence (2 references)
PMID:29669460 SUPPORT Other
"Concomitant treatment with steroid-sparing immunosuppressive agents, including methotrexate, azathioprine, calcineurin inhibitors, mycophenolate mofetil, and cyclophosphamide, reduces successfully initial glucocorticoid doses"
Supports azathioprine's steroid-sparing role in PM/DM management.
PMID:40787733 SUPPORT Human Clinical
"For azathioprine versus placebo, one study showed little or no effect of azathioprine on improvement in muscle strength, but the evidence was very uncertain"
Records that the randomized evidence base for azathioprine in IIM is a single very-low-certainty trial, so the steroid-sparing rationale is not trial-confirmed.
IVIG
Action: intravenous immunoglobulin therapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is intravenous immunoglobulin therapy (NCIT:C121331). NCIT:C121331 is a clinical intervention from the NCI Thesaurus. Ontology label: Intravenous Immunoglobulin Therapy NCIT:C121331
Intravenous immunoglobulin for refractory disease or when immunosuppression is contraindicated. Randomized evidence supports it in refractory dermatomyositis; the polymyositis-specific data are explicitly judged unreliable.
Show evidence (2 references)
PMID:40787733 SUPPORT Human Clinical
"Intravenous immunoglobulin (IVIg), compared to placebo, probably improves disability and muscle strength in participants with refractory IIM"
Moderate-certainty meta-analytic support across refractory idiopathic inflammatory myopathy pooled (3 RCTs, 136 participants).
PMID:40787733 SUPPORT Human Clinical
"Our review shows improvement in disability, muscle strength and skin symptoms following IVIg in people with refractory DM (for PM, these data are not reliable; other subtypes have not been investigated in RCTs)."
The benefit is established for refractory dermatomyositis. The review states outright that the polymyositis data are not reliable, so this must not be read as trial-proven for the disease curated here.
🔬

Biochemical Markers

4
Creatine Kinase (Elevated)
Context: Often markedly elevated (10-50x normal)
Aldolase (Elevated)
Context: Muscle enzyme
Anti-Jo-1 Antibodies (Variable)
Context: Most common antisynthetase antibody
Anti-SRP Antibodies (Variable)
Context: Associated with necrotizing myopathy
🔀

Differential Diagnoses

4

Conditions with similar clinical presentations that must be differentiated from Polymyositis:

Sporadic Inclusion Body Myositis MONDO:0007827
Overlapping Features The most consequential misdiagnosis. In the follow-up study that established polymyositis as overdiagnosed, most patients still carrying a PM label at re-evaluation had turned out to have sporadic inclusion body myositis.
Distinguishing Features
  • Asymmetric weakness with early, disproportionate involvement of finger flexors and quadriceps, rather than symmetric proximal weakness.
  • Slowly progressive over years, and characteristically refractory to corticosteroids and immunosuppression.
  • Muscle biopsy shows rimmed vacuoles and protein aggregates in addition to the endomysial CD8+ infiltrate that polymyositis shares.
Show evidence (1 reference)
PMID:12913190 SUPPORT Human Clinical
"At follow-up evaluation, five of the nine patients with PM had typical s-IBM features."
Five of the nine patients diagnosed with polymyositis were reclassified as s-IBM on follow-up, which is why IBM heads this list.
Dermatomyositis MONDO:0016367
Overlapping Features The sibling inflammatory myopathy from which polymyositis was historically separated by skin findings alone under the 1975 Bohan and Peter criteria; the two are now distinguished on immunopathology as well.
Distinguishing Features
  • Characteristic cutaneous disease (heliotrope rash, Gottron papules), which polymyositis lacks by definition.
  • Perivascular CD4+ T-cell and complement-associated infiltrate with perifascicular atrophy, versus the endomysial CD8+ infiltrate surrounding and invading non-necrotic fibers in polymyositis.
Show evidence (1 reference)
PMID:41068877 SUPPORT Other
"polymyositis, which is characterized by CD8 T-cell infiltration around muscle fibers, and dermatomyositis, which is characterized by CD4 T-cell infiltration with complement infiltration around blood vessels, along with distinctive skin symptoms, have been traditionally distinguished"
Sources the immunopathological contrast used to separate the two entries.
Immune-Mediated Necrotizing Myopathy Not Yet Curated MONDO:0016098
Overlapping Features A seropositive necrotizing myopathy that accounts for a substantial share of cases historically labelled polymyositis. No dismech entry exists for it yet.
Distinguishing Features
  • Anti-SRP or anti-HMG-CoA reductase antibodies, the latter often statin-associated.
  • Biopsy shows myocyte necrosis and regeneration with sparse inflammatory infiltrate, rather than the endomysial CD8+ invasion of non-necrotic fibers.
Show evidence (1 reference)
PMID:23568993 SUPPORT Other
"anti-SRP and anti-3-hydroxy-3-methylglutaryl-coenzyme A (anti-HMG-CoA) antibodies have been found in patients with necrotizing myopathy"
Sources the serological signature that separates IMNM from polymyositis.
Antisynthetase Syndrome MONDO:0019344
Overlapping Features The clinicoserological entity defined by anti-aminoacyl-tRNA-synthetase antibodies. Several features curated on this entry — pulmonary fibrosis, arthritis, the anti-Jo-1 biochemical marker — are attributed in their own notes to antisynthetase syndrome rather than to polymyositis proper.
Distinguishing Features
  • Anti-Jo-1 or another anti-synthetase antibody, with interstitial lung disease, arthritis, fever, Raynaud phenomenon and mechanic's hands accompanying the myositis.
  • Interstitial lung disease is more prevalent and more severe than in polymyositis or dermatomyositis, and often dominates the course.
Show evidence (1 reference)
PMID:23568993 SUPPORT Other
"Anti-synthetase syndrome is characterized by myositis, interstitial lung disease, arthritis, fever, Raynaud's phenomenon, and mechanic's hands."
Sources the syndrome definition used to separate these features from polymyositis.
📊

Related Datasets

3
Plasma Cells in Muscle in Inclusion Body Myositis and Polymyositis geo:GSE3112
Microarray data from muscle biopsy specimens from subjects with inclusion body myositis, polymyositis, and normals Keywords: Research study
human MICROARRAY n=40
PMID:16344523
Identified by GEO DataSets index search for Polymyositis (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.
Circulating plasma microRNA profiling in patients with polymyositis/dermatomyositis before and after treatment geo:GSE94535
This study aimed to investigate the expression of microRNAs (miRNAs) in the plasma from polymyositis (PM) and dermatomyositis (DM) patients, which fluctuated by treatment. More differentially expressed miRNAs were found in plasma of DM patients compared to PM patients before and after treatment, and their profiles were different.
human MICROARRAY n=16
PMID:29321815
Identified by GEO DataSets index search for Polymyositis (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.
MicroRNA and mRNA profiling in the idiopathic inflammatory myopathies geo:GSE125977
Objectives: The idiopathic inflammatory myopathies (IIMs) are heterogeneous autoimmune conditions of skeletal muscle inflammation and weakness. MicroRNAs (miRNAs) are short, non-coding RNA which regulate gene expression of target mRNAs. The aim of this study was to profile miRNA and mRNA in IIM and identify miRNA-mRNA relationships which may be relevant to disease. Materials and methods: mRNA and miRNA in whole blood samples from 7 polymyositis (PM), 7 dermatomyositis (DM), 5 inclusion body myositis (IBM) and 5 non-myositis controls was profiled using next generation RNA sequencing. Gene ontology and pathway analyses were performed using GOseq and Ingenuity Pathway Analysis.
human BULK RNA SEQ n=48
PMID:32529172
Identified by GEO DataSets index search for Polymyositis (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.
{ }

Source YAML

click to show 
name: Polymyositis
creation_date: '2025-12-19T01:12:52Z'
category: Autoimmune
parents:
- Autoimmune Disease
- Inflammatory Myopathy
disease_term:
  preferred_term: Polymyositis
  term:
    id: MONDO:0019127
    label: polymyositis
description: >-
  An idiopathic inflammatory myopathy characterized by symmetric proximal muscle
  weakness without the skin manifestations of dermatomyositis. CD8+ T cell-mediated
  attack on muscle fibers is the primary pathogenic mechanism.
pathophysiology:
- name: CD8+ T Cell-Mediated Cytotoxicity
  description: >-
    Clonally expanded CD8+ cytotoxic T cells infiltrate muscle fibers and
    directly attack myocytes expressing MHC class I. Perforin and granzyme
    release leads to muscle fiber necrosis.
  cell_types:
  - preferred_term: CD8+ T Cell
    term:
      id: CL:0000625
      label: CD8-positive, alpha-beta T cell
  - preferred_term: Skeletal Muscle Cell
    term:
      id: CL:0000188
      label: cell of skeletal muscle
  biological_processes:
  - preferred_term: T Cell Cytotoxicity
    term:
      id: GO:0001913
      label: T cell mediated cytotoxicity
  evidence:
  - reference: PMID:31257434
    reference_title: "A new in vitro model of polymyositis reveals CD8+ T cell invasion into muscle cells and its cytotoxic role."
    supports: SUPPORT
    snippet: "The hallmark histopathology of PM is the presence of CD8+ T cells in the non-necrotic muscle cells."
    explanation: Demonstrates that CD8+ T cell invasion into muscle fibers is the defining pathological feature of polymyositis.
  - reference: PMID:31257434
    reference_title: "A new in vitro model of polymyositis reveals CD8+ T cell invasion into muscle cells and its cytotoxic role."
    supports: SUPPORT
    snippet: "CD8+ T cells invade into muscle cells and contribute to muscle injury in PM."
    explanation: Confirms the direct cytotoxic role of CD8+ T cells in causing muscle fiber damage through invasion and attack.
  downstream:
  - target: Apoptosis of Muscle Fibers
    causal_link_type: DIRECT
    description: >-
      Cytotoxic T cells kill neighbouring fibers by an apoptotic route running in
      parallel with the partial-invasion route.
    evidence:
    - reference: PMID:32936839
      reference_title: "Apoptosis in idiopathic inflammatory myopathies with partial invasion; a role for CD8+ cytotoxic T cells?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The findings confirm that apoptosis occurs in idiopathic inflammatory myopathies and support that it is mediated by CD8+ cytotoxic T- cells, acting in parallel to the process of partial invasion."
      explanation: Attributes the apoptotic fiber death directly to CD8+ cytotoxic T cells.
  - target: Necroptosis and DAMP Release
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Fiber death in myositis includes a necroptotic component, but what couples the
      cytotoxic infiltrate to necroptosis induction has not been identified.
    evidence:
    - reference: PMID:37483632
      reference_title: "Muscle fiber necroptosis in pathophysiology of idiopathic inflammatory myopathies and its potential as target of novel treatment strategy."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: 'Muscle fiber death, which has been formerly postulated as "necrosis", is a key histological feature of all subtypes of IIMs, however, its detailed mechanisms and contribution to the pathophysiology remained to be elucidated.'
      explanation: >-
        Supports fiber death as a core feature while stating explicitly that its
        mechanism is unresolved, which is why this edge is typed indirect with unknown
        intermediates rather than direct.
- name: Aberrant MHC Class I Expression
  description: >-
    Muscle fibers, which normally do not express MHC class I, show aberrant
    upregulation making them targets for CD8+ T cell recognition and attack.
  biological_processes:
  - preferred_term: MHC Class I Antigen Presentation
    term:
      id: GO:0002474
      label: antigen processing and presentation of peptide antigen via MHC class I
  evidence:
  - reference: PMID:15934115
    reference_title: "Activation of the endoplasmic reticulum stress response in autoimmune myositis: potential role in muscle fiber damage and dysfunction."
    supports: SUPPORT
    snippet: "Up-regulation of class I MHC in skeletal muscle fibers was an early and consistent feature of human inflammatory myopathies."
    explanation: Demonstrates that MHC class I upregulation is a hallmark feature of polymyositis pathogenesis.
  - reference: PMID:15934115
    reference_title: "Activation of the endoplasmic reticulum stress response in autoimmune myositis: potential role in muscle fiber damage and dysfunction."
    supports: SUPPORT
    snippet: "Class I MHC staining in muscle fibers of myositis patients showed both cell surface and a reticular pattern of internal reactivity."
    explanation: Shows that aberrant MHC I expression occurs both on the sarcolemma and within the endoplasmic reticulum of muscle fibers.
  downstream:
  - target: CD8+ T Cell-Mediated Cytotoxicity
    causal_link_type: DIRECT
    description: >-
      Sarcolemmal class I MHC is the ligand that turns an otherwise
      immunologically inconspicuous fiber into a target for antigen-specific CD8+
      T cells.
    evidence:
    - reference: PMID:31257434
      reference_title: "A new in vitro model of polymyositis reveals CD8+ T cell invasion into muscle cells and its cytotoxic role."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Analysis of the in vitro model of PM with confocal microscopy demonstrated the invasion of OT-I CD8+ T cells into H2KbOVA-myotubes."
      explanation: >-
        Myotubes engineered to present a model antigen on class I MHC are invaded by
        antigen-specific CD8+ T cells, supporting the MHC-I-to-cytotoxicity step in a
        reconstituted system. The antigen driving the human response is unknown, so
        this is PARTIAL rather than full support.
  - target: ER Stress and Unfolded Protein Response
    causal_link_type: DIRECT
    description: >-
      Class I MHC overexpression is itself sufficient to trigger the UPR in muscle,
      giving the entry its non-immune arm of fiber damage.
    evidence:
    - reference: PMID:15934115
      reference_title: "Activation of the endoplasmic reticulum stress response in autoimmune myositis: potential role in muscle fiber damage and dysfunction."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Ectopic expression of wild-type mouse class I MHC (H-2K(b)) but not degradable glycosylation mutants of H-2K(b) induced ER stress response in C(2)C(12) skeletal muscle cells."
      explanation: >-
        Forced class I MHC expression induces the ER stress response in muscle cells,
        which establishes the direction of this edge rather than mere co-occurrence in
        biopsies.
- name: Myositis-Specific Autoantibodies
  description: >-
    Anti-synthetase antibodies (anti-Jo-1, anti-PL-7, anti-PL-12) define
    antisynthetase syndrome with myositis, ILD, and arthritis. Anti-SRP
    antibodies associate with necrotizing myopathy.
  biological_processes:
  - preferred_term: Immunoglobulin Production
    term:
      id: GO:0002377
      label: immunoglobulin production
  evidence:
  - reference: PMID:23568993
    reference_title: "[Myositis-specific autoantibodies]."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Anti-Jo-1, anti-PL-7, anti-PL-12, anti-EJ, anti-KS, anti-OJ, anti-Ha, and anti-Zo antibodies target aminoacyl tRNA synthetases, and represent anti-synthetase syndrome."
    explanation: Sources the antisynthetase-antibody group named in this node.
  - reference: PMID:23568993
    reference_title: "[Myositis-specific autoantibodies]."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "anti-SRP and anti-3-hydroxy-3-methylglutaryl-coenzyme A (anti-HMG-CoA) antibodies have been found in patients with necrotizing myopathy"
    explanation: Sources the anti-SRP / necrotizing-myopathy association named in this node.
  notes: >-
    Deliberately carries no `downstream` edges. Both antibody groups named here now
    define separate disease entities — antisynthetase syndrome and immune-mediated
    necrotizing myopathy, each curated in its own entry and listed under
    `differential_diagnoses` — rather than serologic subsets of polymyositis, and the
    pathogenic role of the antibodies themselves is unresolved. See the
    `pm_msa_pathogenic_role` and `pm_nosological_status` discussions.
- name: Pro-inflammatory Cytokine Production
  description: >-
    IL-1, TNF-alpha, and IFN-gamma produced by infiltrating immune cells
    perpetuate muscle inflammation and contribute to muscle weakness
    independent of direct fiber destruction.
  biological_processes:
  - preferred_term: Inflammatory Response
    term:
      id: GO:0006954
      label: inflammatory response
  evidence:
  - reference: PMID:9655472
    reference_title: "Induction of primary immune responses by allogeneic human myoblasts: dissection of the cell types required for proliferation, IFNgamma secretion and cytotoxicity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Treatment of myoblasts with TNFalpha or IFNgamma led to the expression of a range of immunostimulatory molecules including MHC class I and II, and CD95 (Fas), but not B7 family molecules."
    explanation: >-
      Human muscle cells respond to TNF-alpha and IFN-gamma by becoming
      immunostimulatory, which is the mechanism this node asserts.
  downstream:
  - target: Aberrant MHC Class I Expression
    causal_link_type: DIRECT
    description: >-
      Feed-forward amplification: cytokines released by the infiltrate drive the class
      I MHC upregulation that recruits further cytotoxic attack, closing a loop with
      the CD8+ arm.
    evidence:
    - reference: PMID:9655472
      reference_title: "Induction of primary immune responses by allogeneic human myoblasts: dissection of the cell types required for proliferation, IFNgamma secretion and cytotoxicity."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Treatment of myoblasts with TNFalpha or IFNgamma led to the expression of a range of immunostimulatory molecules including MHC class I and II"
      explanation: >-
        TNF-alpha or IFN-gamma alone is sufficient to induce class I MHC on human
        myoblasts. The experiment used allogeneic myoblast cultures rather than
        myositis muscle, so the loop is inferred, not observed in patients.
- name: ER Stress and Unfolded Protein Response
  description: >-
    Overexpression of MHC class I in muscle fibers triggers endoplasmic reticulum
    stress and activation of the unfolded protein response (UPR). This includes
    upregulation of GRP78/BiP and NF-kappaB pathway activation, contributing to
    muscle fiber dysfunction independent of immune-mediated damage.
  biological_processes:
  - preferred_term: Endoplasmic Reticulum Stress Response
    term:
      id: GO:0034976
      label: response to endoplasmic reticulum stress
  evidence:
  - reference: PMID:15934115
    reference_title: "Activation of the endoplasmic reticulum stress response in autoimmune myositis: potential role in muscle fiber damage and dysfunction."
    supports: SUPPORT
    snippet: "The pathways of endoplasmic reticulum (ER) stress response, the unfolded protein response (glucose-regulated protein 78 pathway), and the ER overload response (NF-kappaB pathway) were significantly activated in muscle tissue of human myositis patients and in the mouse model."
    explanation: Demonstrates that ER stress pathways are activated in polymyositis muscle tissue as a consequence of MHC I overexpression.
  - reference: PMID:15934115
    reference_title: "Activation of the endoplasmic reticulum stress response in autoimmune myositis: potential role in muscle fiber damage and dysfunction."
    supports: SUPPORT
    snippet: "These results indicate that the ER stress response may be a major nonimmune mechanism responsible for skeletal muscle damage and dysfunction in autoimmune myositis."
    explanation: Identifies ER stress as a non-immune mechanism contributing to muscle pathology, explaining why inflammation doesn't always correlate with disease severity.
  downstream:
  - target: Proximal muscle weakness
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The non-immune route from class I MHC overexpression to weakness, invoked to
      explain why the degree of inflammation does not track disease severity.
    evidence:
    - reference: PMID:15934115
      reference_title: "Activation of the endoplasmic reticulum stress response in autoimmune myositis: potential role in muscle fiber damage and dysfunction."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "These results indicate that the ER stress response may be a major nonimmune mechanism responsible for skeletal muscle damage and dysfunction in autoimmune myositis."
      explanation: >-
        The authors' interpretive conclusion, drawn from combined human-biopsy and
        transgenic-mouse data; the steps between UPR activation and clinical weakness
        are not identified, hence the indirect typing and PARTIAL support.
- name: Necroptosis and DAMP Release
  description: >-
    Muscle fibers undergo necroptosis, a form of regulated cell death, releasing
    damage-associated molecular patterns (DAMPs) including HMGB1. This amplifies
    inflammation and contributes to persistent muscle weakness even after initial
    immune attack.
  biological_processes:
  - preferred_term: Necroptotic Process
    term:
      id: GO:0070266
      label: necroptotic process
  evidence:
  - reference: PMID:37483632
    reference_title: "Muscle fiber necroptosis in pathophysiology of idiopathic inflammatory myopathies and its potential as target of novel treatment strategy."
    supports: SUPPORT
    snippet: "Recent studies have revealed that muscle fibers of IIMs undergo necroptosis, a newly recognized form of regulated cell death, and promote muscle inflammation and dysfunction through releasing inflammatory mediators such as damage-associated molecular patterns (DAMPs)."
    explanation: Establishes necroptosis as a key mechanism of muscle fiber death in polymyositis that perpetuates inflammation.
  - reference: PMID:37483632
    reference_title: "Muscle fiber necroptosis in pathophysiology of idiopathic inflammatory myopathies and its potential as target of novel treatment strategy."
    supports: SUPPORT
    snippet: "The research on murine model of polymyositis, a subtype of IIM, revealed that the inhibition of necroptosis or HMGB1, one of major DAMPs released from muscle fibers undergoing necroptosis, ameliorated muscle inflammation and recovered muscle weakness."
    explanation: Demonstrates that blocking necroptosis or DAMP signaling improves both inflammation and muscle function, highlighting therapeutic potential.
  downstream:
  - target: Pro-inflammatory Cytokine Production
    causal_link_type: DIRECT
    description: >-
      DAMPs released from necroptotic fibers — HMGB1 chief among them — drive the
      inflammatory mediator output that sustains the lesion.
    evidence:
    - reference: PMID:37483632
      reference_title: "Muscle fiber necroptosis in pathophysiology of idiopathic inflammatory myopathies and its potential as target of novel treatment strategy."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "muscle fibers of IIMs undergo necroptosis, a newly recognized form of regulated cell death, and promote muscle inflammation and dysfunction through releasing inflammatory mediators such as damage-associated molecular patterns (DAMPs)"
      explanation: States the necroptosis-to-inflammation step that this edge encodes.
  - target: Proximal muscle weakness
    causal_link_type: DIRECT
    description: >-
      The arm that accounts for weakness persisting after inflammation has been
      suppressed; blocking necroptosis or HMGB1 restores strength in the murine
      polymyositis model.
    evidence:
    - reference: PMID:37483632
      reference_title: "Muscle fiber necroptosis in pathophysiology of idiopathic inflammatory myopathies and its potential as target of novel treatment strategy."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "The research on murine model of polymyositis, a subtype of IIM, revealed that the inhibition of necroptosis or HMGB1, one of major DAMPs released from muscle fibers undergoing necroptosis, ameliorated muscle inflammation and recovered muscle weakness."
      explanation: >-
        Loss-of-function rescue in a mouse polymyositis model gives this edge its
        causal direction. It has not been demonstrated in human muscle.
- name: Apoptosis of Muscle Fibers
  description: >-
    Apoptotic cell death occurs in muscle fibers surrounding those undergoing
    partial invasion by CD8+ T cells. Fibers with apoptotic nuclei are surrounded
    by CD8+ T cells, granzyme B-positive cells, and macrophages, suggesting
    cytotoxic T cell-mediated apoptosis occurs in parallel to direct invasion.
  biological_processes:
  - preferred_term: Apoptotic Process
    term:
      id: GO:0006915
      label: apoptotic process
  evidence:
  - reference: PMID:32936839
    reference_title: "Apoptosis in idiopathic inflammatory myopathies with partial invasion; a role for CD8+ cytotoxic T cells?"
    supports: SUPPORT
    snippet: "Apoptotic myonuclei were found in muscle with partial invasion, but not in the invaded fibres. Fibres with TUNEL positive nuclei were surrounded by CD8+ T-cells, granzyme B+ cells and macrophages, but lacked FAS receptor expression."
    explanation: Shows that apoptosis occurs in polymyositis muscle fibers adjacent to those with CD8+ T cell invasion, mediated by cytotoxic mechanisms.
  - reference: PMID:32936839
    reference_title: "Apoptosis in idiopathic inflammatory myopathies with partial invasion; a role for CD8+ cytotoxic T cells?"
    supports: SUPPORT
    snippet: "The findings confirm that apoptosis occurs in idiopathic inflammatory myopathies and support that it is mediated by CD8+ cytotoxic T- cells, acting in parallel to the process of partial invasion."
    explanation: Establishes that CD8+ T cell-mediated apoptosis is a distinct pathway of muscle damage occurring alongside direct fiber invasion.
phenotypes:
- name: Dysphagia
  category: Gastrointestinal
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Dysphagia
    term:
      id: HP:0002015
      label: Dysphagia
  notes: Pharyngeal muscle involvement
- name: Pulmonary Fibrosis
  category: Respiratory
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Pulmonary Fibrosis
    term:
      id: HP:0002206
      label: Pulmonary fibrosis
  notes: Associated with antisynthetase syndrome
- name: Arthritis
  category: Musculoskeletal
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Arthritis
    term:
      id: HP:0001369
      label: Arthritis
  notes: Part of antisynthetase syndrome
- name: "Hypotonia"
  category: Neuromuscular
  frequency: VERY_FREQUENT
  description: "Hypotonia is reported as a very frequent (80-99%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Hypotonia"
    term:
      id: HP:0001252
      label: "Hypotonia"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001252 | Hypotonia | Very frequent (99-80%)"
    explanation: "Orphanet records hypotonia as a very frequent (80-99%) phenotype of polymyositis."
  notes: >-
    Frequency band inherited from the ORPHA:732 bulk phenotype import and not
    corroborated by any clinical source. Hypotonia at VERY_FREQUENT is clinically
    surprising for an adult-onset inflammatory myopathy, where the cardinal finding is
    proximal weakness rather than reduced muscle tone; read this as Orphanet's coding
    choice rather than as an independently evidenced band, and re-check it against a
    clinical cohort before relying on it (dismech#6968, item 4).
- name: "Arthralgia"
  category: Musculoskeletal
  frequency: VERY_FREQUENT
  description: "Arthralgia is reported as a very frequent (80-99%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Arthralgia"
    term:
      id: HP:0002829
      label: "Arthralgia"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002829 | Arthralgia | Very frequent (99-80%)"
    explanation: "Orphanet records arthralgia as a very frequent (80-99%) phenotype of polymyositis."
- name: "Autoimmunity"
  category: Immunologic
  frequency: VERY_FREQUENT
  description: "Autoimmunity is reported as a very frequent (80-99%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Autoimmunity"
    term:
      id: HP:0002960
      label: "Autoimmunity"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002960 | Autoimmunity | Very frequent (99-80%)"
    explanation: "Orphanet records autoimmunity as a very frequent (80-99%) phenotype of polymyositis."
- name: "Elevated circulating creatine kinase concentration"
  category: Laboratory
  frequency: VERY_FREQUENT
  description: "Elevated circulating creatine kinase concentration is reported as a very frequent (80-99%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Elevated circulating creatine kinase concentration"
    term:
      id: HP:0003236
      label: "Elevated circulating creatine kinase concentration"
  reports_on:
  - target: CD8+ T Cell-Mediated Cytotoxicity
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Serum creatine kinase released from necrotic muscle fibers.
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003236 | Elevated circulating creatine kinase concentration | Very frequent (99-80%)"
    explanation: "Orphanet records elevated circulating creatine kinase concentration as a very frequent (80-99%) phenotype of polymyositis."
  - reference: PMID:31257434
    reference_title: "A new in vitro model of polymyositis reveals CD8+ T cell invasion into muscle cells and its cytotoxic role."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The level of serum creatinine kinase was higher in patients with CD8+ T cells in muscle cells than those without these cells."
    explanation: Links serum creatine kinase, the readout curated here, to the degree of CD8+ T cell invasion of muscle fibers.
- name: "EMG abnormality"
  category: Laboratory
  frequency: VERY_FREQUENT
  description: "EMG abnormality is reported as a very frequent (80-99%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "EMG abnormality"
    term:
      id: HP:0003457
      label: "EMG abnormality"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003457 | EMG abnormality | Very frequent (99-80%)"
    explanation: "Orphanet records eMG abnormality as a very frequent (80-99%) phenotype of polymyositis."
- name: "Proximal muscle weakness"
  category: Musculoskeletal
  frequency: VERY_FREQUENT
  description: "Proximal muscle weakness is reported as a very frequent (80-99%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Proximal muscle weakness"
    term:
      id: HP:0003701
      label: "Proximal muscle weakness"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003701 | Proximal muscle weakness | Very frequent (99-80%)"
    explanation: "Orphanet records proximal muscle weakness as a very frequent (80-99%) phenotype of polymyositis."
- name: "Elevated circulating aldolase concentration"
  category: Laboratory
  frequency: VERY_FREQUENT
  description: "Elevated circulating aldolase concentration is reported as a very frequent (80-99%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Elevated circulating aldolase concentration"
    term:
      id: HP:0012544
      label: "Elevated circulating aldolase concentration"
  reports_on:
  - target: CD8+ T Cell-Mediated Cytotoxicity
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Serum aldolase released from damaged muscle fibers.
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0012544 | Elevated circulating aldolase concentration | Very frequent (99-80%)"
    explanation: "Orphanet records elevated circulating aldolase concentration as a very frequent (80-99%) phenotype of polymyositis."
- name: "Cough"
  category: Respiratory
  frequency: VERY_FREQUENT
  description: "Cough is reported as a very frequent (80-99%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Cough"
    term:
      id: HP:0012735
      label: "Cough"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0012735 | Cough | Very frequent (99-80%)"
    explanation: "Orphanet records cough as a very frequent (80-99%) phenotype of polymyositis."
- name: "Weight loss"
  category: Constitutional
  frequency: FREQUENT
  description: "Weight loss is reported as a frequent (30-79%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Weight loss"
    term:
      id: HP:0001824
      label: "Weight loss"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001824 | Weight loss | Frequent (79-30%)"
    explanation: "Orphanet records weight loss as a frequent (30-79%) phenotype of polymyositis."
- name: "Fever"
  category: Constitutional
  frequency: FREQUENT
  description: "Fever is reported as a frequent (30-79%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Fever"
    term:
      id: HP:0001945
      label: "Fever"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001945 | Fever | Frequent (79-30%)"
    explanation: "Orphanet records fever as a frequent (30-79%) phenotype of polymyositis."
- name: "Constipation"
  category: Gastrointestinal
  frequency: FREQUENT
  description: "Constipation is reported as a frequent (30-79%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Constipation"
    term:
      id: HP:0002019
      label: "Constipation"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002019 | Constipation | Frequent (79-30%)"
    explanation: "Orphanet records constipation as a frequent (30-79%) phenotype of polymyositis."
- name: "Anorexia"
  category: Constitutional
  frequency: FREQUENT
  description: "Anorexia is reported as a frequent (30-79%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Anorexia"
    term:
      id: HP:0002039
      label: "Anorexia"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002039 | Anorexia | Frequent (79-30%)"
    explanation: "Orphanet records anorexia as a frequent (30-79%) phenotype of polymyositis."
- name: "Respiratory insufficiency"
  category: Respiratory
  frequency: FREQUENT
  description: "Respiratory insufficiency is reported as a frequent (30-79%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Respiratory insufficiency"
    term:
      id: HP:0002093
      label: "Respiratory insufficiency"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002093 | Respiratory insufficiency | Frequent (79-30%)"
    explanation: "Orphanet records respiratory insufficiency as a frequent (30-79%) phenotype of polymyositis."
- name: "Exertional dyspnea"
  category: Respiratory
  frequency: FREQUENT
  description: "Exertional dyspnea is reported as a frequent (30-79%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Exertional dyspnea"
    term:
      id: HP:0002875
      label: "Exertional dyspnea"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002875 | Exertional dyspnea | Frequent (79-30%)"
    explanation: "Orphanet records exertional dyspnea as a frequent (30-79%) phenotype of polymyositis."
- name: "Myalgia"
  category: Musculoskeletal
  frequency: FREQUENT
  description: "Myalgia is reported as a frequent (30-79%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Myalgia"
    term:
      id: HP:0003326
      label: "Myalgia"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003326 | Myalgia | Frequent (79-30%)"
    explanation: "Orphanet records myalgia as a frequent (30-79%) phenotype of polymyositis."
- name: "Antinuclear antibody positivity"
  category: Immunologic
  frequency: FREQUENT
  description: "Antinuclear antibody positivity is reported as a frequent (30-79%) manifestation of polymyositis in the Orphanet clinical phenotype dataset."
  phenotype_term:
    preferred_term: "Antinuclear antibody positivity"
    term:
      id: HP:0003493
      label: "Antinuclear antibody positivity"
  evidence:
  - reference: ORPHA:732
    reference_title: Polymyositis
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003493 | Antinuclear antibody positivity | Frequent (79-30%)"
    explanation: "Orphanet records antinuclear antibody positivity as a frequent (30-79%) phenotype of polymyositis."
biochemical:
- name: Creatine Kinase
  presence: Elevated
  context: Often markedly elevated (10-50x normal)
- name: Aldolase
  presence: Elevated
  context: Muscle enzyme
- name: Anti-Jo-1 Antibodies
  presence: Variable
  context: Most common antisynthetase antibody
- name: Anti-SRP Antibodies
  presence: Variable
  context: Associated with necrotizing myopathy
genetic:
- name: HLA-DRB1*0301
  association: Risk Factor
treatments:
- name: Corticosteroids
  description: >-
    First-line therapy; high-dose initially with gradual taper. Used empirically
    rather than on the strength of randomized evidence, and the adverse-effect burden
    is itself the reason a steroid-sparing agent is usually added early.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: prednisone
      term:
        id: CHEBI:8382
        label: prednisone
    - preferred_term: prednisolone
      term:
        id: CHEBI:8378
        label: prednisolone
  evidence:
  - reference: PMID:29669460
    reference_title: "Current diagnosis and treatment of polymyositis and dermatomyositis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "glucocorticoids are used empirically as the first-line treatment despite their various adverse effects"
    explanation: >-
      Supports first-line status while being explicit that the basis is empirical
      practice rather than trial evidence.
- name: Methotrexate
  description: >-
    Most commonly used steroid-sparing agent. Added to reduce cumulative
    glucocorticoid exposure rather than on demonstrated independent efficacy in adult
    polymyositis.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: methotrexate
      term:
        id: CHEBI:44185
        label: methotrexate
  evidence:
  - reference: PMID:29669460
    reference_title: "Current diagnosis and treatment of polymyositis and dermatomyositis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Concomitant treatment with steroid-sparing immunosuppressive agents, including methotrexate, azathioprine, calcineurin inhibitors, mycophenolate mofetil, and cyclophosphamide, reduces successfully initial glucocorticoid doses for the remission induction, the relapse risk during glucocorticoid tapering, and adverse effects of glucocorticoids."
    explanation: Supports the steroid-sparing rationale for methotrexate in PM/DM.
  - reference: PMID:40787733
    reference_title: "Non-targeted immunosuppressive and immunomodulatory therapies for idiopathic inflammatory myopathies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "For methotrexate, there may be little or no improvement in adults with DM or PM in function"
    explanation: >-
      Cochrane's very-low-certainty finding qualifies the steroid-sparing rationale:
      the single 27-participant adult RCT showed no functional benefit.
- name: Azathioprine
  description: >-
    Alternative steroid-sparing immunosuppressant for maintenance. The only randomized
    placebo comparison is a 16-participant study from which no reliable conclusion can
    be drawn.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: azathioprine
      term:
        id: CHEBI:2948
        label: azathioprine
  evidence:
  - reference: PMID:29669460
    reference_title: "Current diagnosis and treatment of polymyositis and dermatomyositis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Concomitant treatment with steroid-sparing immunosuppressive agents, including methotrexate, azathioprine, calcineurin inhibitors, mycophenolate mofetil, and cyclophosphamide, reduces successfully initial glucocorticoid doses"
    explanation: Supports azathioprine's steroid-sparing role in PM/DM management.
  - reference: PMID:40787733
    reference_title: "Non-targeted immunosuppressive and immunomodulatory therapies for idiopathic inflammatory myopathies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "For azathioprine versus placebo, one study showed little or no effect of azathioprine on improvement in muscle strength, but the evidence was very uncertain"
    explanation: >-
      Records that the randomized evidence base for azathioprine in IIM is a single
      very-low-certainty trial, so the steroid-sparing rationale is not trial-confirmed.
- name: IVIG
  description: >-
    Intravenous immunoglobulin for refractory disease or when immunosuppression is
    contraindicated. Randomized evidence supports it in refractory dermatomyositis;
    the polymyositis-specific data are explicitly judged unreliable.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: intravenous immunoglobulin therapy
    term:
      id: NCIT:C121331
      label: Intravenous Immunoglobulin Therapy
  evidence:
  - reference: PMID:40787733
    reference_title: "Non-targeted immunosuppressive and immunomodulatory therapies for idiopathic inflammatory myopathies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Intravenous immunoglobulin (IVIg), compared to placebo, probably improves disability and muscle strength in participants with refractory IIM"
    explanation: >-
      Moderate-certainty meta-analytic support across refractory idiopathic
      inflammatory myopathy pooled (3 RCTs, 136 participants).
  - reference: PMID:40787733
    reference_title: "Non-targeted immunosuppressive and immunomodulatory therapies for idiopathic inflammatory myopathies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our review shows improvement in disability, muscle strength and skin symptoms following IVIg in people with refractory DM (for PM, these data are not reliable; other subtypes have not been investigated in RCTs)."
    explanation: >-
      The benefit is established for refractory dermatomyositis. The review states
      outright that the polymyositis data are not reliable, so this must not be read as
      trial-proven for the disease curated here.
differential_diagnoses:
- name: Sporadic Inclusion Body Myositis
  disease_term:
    preferred_term: Sporadic Inclusion Body Myositis
    term:
      id: MONDO:0007827
      label: inclusion body myositis
  description: >-
    The most consequential misdiagnosis. In the follow-up study that established
    polymyositis as overdiagnosed, most patients still carrying a PM label at
    re-evaluation had turned out to have sporadic inclusion body myositis.
  distinguishing_features:
  - >-
    Asymmetric weakness with early, disproportionate involvement of finger flexors and
    quadriceps, rather than symmetric proximal weakness.
  - >-
    Slowly progressive over years, and characteristically refractory to
    corticosteroids and immunosuppression.
  - >-
    Muscle biopsy shows rimmed vacuoles and protein aggregates in addition to the
    endomysial CD8+ infiltrate that polymyositis shares.
  evidence:
  - reference: PMID:12913190
    reference_title: "Polymyositis: an overdiagnosed entity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "At follow-up evaluation, five of the nine patients with PM had typical s-IBM features."
    explanation: >-
      Five of the nine patients diagnosed with polymyositis were reclassified as s-IBM
      on follow-up, which is why IBM heads this list.
- name: Dermatomyositis
  disease_term:
    preferred_term: Dermatomyositis
    term:
      id: MONDO:0016367
      label: dermatomyositis
  description: >-
    The sibling inflammatory myopathy from which polymyositis was historically
    separated by skin findings alone under the 1975 Bohan and Peter criteria; the two
    are now distinguished on immunopathology as well.
  distinguishing_features:
  - >-
    Characteristic cutaneous disease (heliotrope rash, Gottron papules), which
    polymyositis lacks by definition.
  - >-
    Perivascular CD4+ T-cell and complement-associated infiltrate with perifascicular
    atrophy, versus the endomysial CD8+ infiltrate surrounding and invading
    non-necrotic fibers in polymyositis.
  evidence:
  - reference: PMID:41068877
    reference_title: "Recent advances in immunological mechanisms and murine disease models of idiopathic inflammatory myopathies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "polymyositis, which is characterized by CD8 T-cell infiltration around muscle fibers, and dermatomyositis, which is characterized by CD4 T-cell infiltration with complement infiltration around blood vessels, along with distinctive skin symptoms, have been traditionally distinguished"
    explanation: Sources the immunopathological contrast used to separate the two entries.
- name: Immune-Mediated Necrotizing Myopathy
  disease_term:
    preferred_term: Immune-Mediated Necrotizing Myopathy
    term:
      id: MONDO:0016098
      label: immune-mediated necrotizing myopathy
  description: >-
    A seropositive necrotizing myopathy that accounts for a substantial share of cases
    historically labelled polymyositis. No dismech entry exists for it yet.
  distinguishing_features:
  - >-
    Anti-SRP or anti-HMG-CoA reductase antibodies, the latter often statin-associated.
  - >-
    Biopsy shows myocyte necrosis and regeneration with sparse inflammatory infiltrate,
    rather than the endomysial CD8+ invasion of non-necrotic fibers.
  evidence:
  - reference: PMID:23568993
    reference_title: "[Myositis-specific autoantibodies]."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "anti-SRP and anti-3-hydroxy-3-methylglutaryl-coenzyme A (anti-HMG-CoA) antibodies have been found in patients with necrotizing myopathy"
    explanation: Sources the serological signature that separates IMNM from polymyositis.
  notes: >-
    Curating an Immune-Mediated Necrotizing Myopathy entry would let the anti-SRP
    content in this entry's `Myositis-Specific Autoantibodies` node be moved to where
    it belongs.
- name: Antisynthetase Syndrome
  disease_term:
    preferred_term: Antisynthetase Syndrome
    term:
      id: MONDO:0019344
      label: antisynthetase syndrome
  description: >-
    The clinicoserological entity defined by anti-aminoacyl-tRNA-synthetase
    antibodies. Several features curated on this entry — pulmonary fibrosis, arthritis,
    the anti-Jo-1 biochemical marker — are attributed in their own notes to
    antisynthetase syndrome rather than to polymyositis proper.
  distinguishing_features:
  - >-
    Anti-Jo-1 or another anti-synthetase antibody, with interstitial lung disease,
    arthritis, fever, Raynaud phenomenon and mechanic's hands accompanying the myositis.
  - >-
    Interstitial lung disease is more prevalent and more severe than in polymyositis or
    dermatomyositis, and often dominates the course.
  evidence:
  - reference: PMID:23568993
    reference_title: "[Myositis-specific autoantibodies]."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Anti-synthetase syndrome is characterized by myositis, interstitial lung disease, arthritis, fever, Raynaud's phenomenon, and mechanic's hands."
    explanation: Sources the syndrome definition used to separate these features from polymyositis.
discussions:
- discussion_id: pm_nosological_status
  prompt: >-
    Is polymyositis a distinct disease entity, or a residual label for idiopathic
    inflammatory myopathy that has not yet been resolved into antisynthetase syndrome,
    immune-mediated necrotizing myopathy, or inclusion body myositis?
  kind: CONTROVERSY
  status: OPEN
  attaches_to:
  - pathophysiology#CD8+ T Cell-Mediated Cytotoxicity
  - pathophysiology#Myositis-Specific Autoantibodies
  rationale: >-
    This entry models polymyositis as a standalone entity with a CD8+/MHC-I mechanism.
    That mechanism is real and is what defines PM histologically, but the *scope* of
    the label is contested: a follow-up study of 165 myositis patients could reclassify
    most PM diagnoses, and contemporary classifications carve antisynthetase syndrome,
    immune-mediated necrotizing myopathy and overlap myositis out of the same
    population. The unresolved question determines what belongs in this entry — in
    particular whether the antisynthetase content (pulmonary fibrosis, arthritis,
    anti-Jo-1, and the `Myositis-Specific Autoantibodies` node) should move to
    Antisynthetase_Syndrome, and whether an Immune-Mediated Necrotizing Myopathy entry
    is needed to receive the anti-SRP content.
  evidence:
  - reference: PMID:12913190
    reference_title: "Polymyositis: an overdiagnosed entity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Polymyositis is an overdiagnosed entity."
    explanation: The primary source for the claim that the label is applied far more often than it applies.
  - reference: PMID:41068877
    reference_title: "Recent advances in immunological mechanisms and murine disease models of idiopathic inflammatory myopathies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "recent classifications based on autoantibodies and gene expression have proposed new categories, such as antisynthetase syndrome, clinical amyopathic dermatomyositis, immune-mediated necrotizing myopathy, and inclusion body myositis, resulting in the concept of IIM as a spectrum of diseases including these subtypes"
    explanation: Documents the reclassification of the IIM space into which polymyositis is being subdivided.
  - reference: PMID:39132766
    reference_title: "Current efforts and historical perspectives on classification of idiopathic inflammatory myopathies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "there is a need to revise the criteria with regard to the addition of new myositis-specific autoantibodies, newly defined subgroups (especially antisynthetase syndrome, immune medicated necrotizing myopathy and overlap myositis)"
    explanation: >-
      Shows the question is live rather than settled — the 2017 EULAR/ACR criteria are
      themselves under revision on exactly this point.
  notes: >-
    Raised as item 3 of dismech#6968, which asks explicitly whether this entry should
    stay as-is with a scoping note, gain a discussion, or be repositioned relative to
    its sibling IIM entries. This discussion records the uncertainty; the repositioning
    decision is left to a maintainer. Related to dismech#306 (lumping/splitting).
- discussion_id: pm_treatment_evidence_base
  prompt: >-
    Does any immunosuppressive or immunomodulatory therapy have reliable randomized
    evidence of benefit in polymyositis specifically, as opposed to in dermatomyositis
    or in idiopathic inflammatory myopathy pooled?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - treatments#Corticosteroids
  - treatments#Methotrexate
  - treatments#Azathioprine
  - treatments#IVIG
  rationale: >-
    All four treatments curated here are standard of care, and none rests on
    polymyositis-specific randomized evidence. Glucocorticoids are used empirically;
    the Cochrane review finds the azathioprine and methotrexate trials
    very-low-certainty; and while IVIg has moderate-certainty benefit in refractory
    IIM pooled and in refractory dermatomyositis, the review states that the
    polymyositis data are not reliable. This is not merely a citation gap — it is
    plausibly downstream of `pm_nosological_status`, since trials enrolling "PM" have
    been enrolling a mixed population.
  evidence:
  - reference: PMID:40787733
    reference_title: "Non-targeted immunosuppressive and immunomodulatory therapies for idiopathic inflammatory myopathies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "For the other agents, the small number of trials of immunosuppressive and immunomodulatory therapies is inadequate to decide whether these agents are beneficial in IIM (excluding IBM)."
    explanation: The review's own summary of the size of this gap.
  - reference: PMID:40787733
    reference_title: "Non-targeted immunosuppressive and immunomodulatory therapies for idiopathic inflammatory myopathies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The risk of bias in all but one study was high or unclear."
    explanation: Characterizes the quality of the 16 trials that do exist.
- discussion_id: pm_msa_pathogenic_role
  prompt: >-
    Are myositis-specific autoantibodies pathogenic drivers of muscle injury, or
    diagnostic markers of an immune response to regenerating muscle?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Myositis-Specific Autoantibodies
  rationale: >-
    This is why the `Myositis-Specific Autoantibodies` node carries no `downstream`
    edges. The antibodies are well established as classifiers, but no route from
    antibody to fiber injury has been demonstrated, and the leading alternative — that
    the autoantigens are simply overexpressed in regenerating fibers — would make the
    antibodies a consequence of muscle damage rather than a cause. Drawing a causal
    edge either way would assert more than the literature supports.
  evidence:
  - reference: PMID:23568993
    reference_title: "[Myositis-specific autoantibodies]."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Although the pathogenic role of MSAs remains unknown, recent studies have shown that myositis autoantigens are expressed at high levels in regenerating muscle fibers, which may initiate or amplify autoimmune responses in idiopathic inflammatory myopathies."
    explanation: States both the gap and the leading alternative interpretation in one sentence.
classifications:
  harrisons_chapter:
  - classification_value: IMMUNE_RHEUMATOLOGIC
references:
- reference: PMID:12913190
  title: 'Polymyositis: an overdiagnosed entity'
  findings: []
- reference: PMID:15934115
  title: 'Activation of the endoplasmic reticulum stress response in autoimmune myositis: Potential role in muscle fiber damage and dysfunction'
  findings: []
- reference: PMID:23568993
  title: Myositis-specific autoantibodies
  findings: []
- reference: PMID:29669460
  title: Current diagnosis and treatment of polymyositis and dermatomyositis
  findings: []
- reference: PMID:39132766
  title: Current efforts and historical perspectives on classification of idiopathic inflammatory myopathies
  findings: []
- reference: PMID:40787733
  title: Non-targeted immunosuppressive and immunomodulatory therapies for idiopathic inflammatory myopathies
  findings: []
- reference: PMID:9655472
  title: 'Induction of primary immune responses by allogeneic human myoblasts: dissection of the cell types required for proliferation, IFNgamma secretion and cytotoxicity'
  findings: []
- reference: DOI:10.1093/rheumatology/kez248
  title: A new in vitro model of polymyositis reveals CD8+ T cell invasion into muscle cells and its cytotoxic role
  findings: []
- reference: DOI:10.1097/md.0000000000040254
  title: 'Causality between immunocytes and polymyositis: A Mendelian randomization analysis'
  findings: []
- reference: DOI:10.1186/s13075-024-03364-z
  title: Expression of CD163 and major histocompatibility complex class I as diagnostic markers for idiopathic inflammatory myopathies
  findings: []
- reference: DOI:10.1186/s41232-025-00395-0
  title: Recent advances in immunological mechanisms and murine disease models of idiopathic inflammatory myopathies
  findings: []
- reference: DOI:10.1371/journal.pone.0239176
  title: Apoptosis in idiopathic inflammatory myopathies with partial invasion; a role for CD8+ cytotoxic T cells?
  findings: []
- reference: DOI:10.3389/fimmu.2023.1191815
  title: Muscle fiber necroptosis in pathophysiology of idiopathic inflammatory myopathies and its potential as target of novel treatment strategy
  findings: []
- reference: DOI:10.3389/fmed.2023.1158768
  title: 'Targeting intracellular pathways in idiopathic inflammatory myopathies: A narrative review'
  findings: []
- reference: DOI:10.3389/fneur.2023.1328547
  title: Transcriptome analysis of skeletal muscle in dermatomyositis, polymyositis, and dysferlinopathy, using a bioinformatics approach
  findings: []
datasets:
- accession: geo:GSE3112
  title: Plasma Cells in Muscle in Inclusion Body Myositis and Polymyositis
  description: 'Microarray data from muscle biopsy specimens from subjects with inclusion body myositis, polymyositis, and normals Keywords: Research study'
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: MICROARRAY
  sample_count: 40
  publication: PMID:16344523
  notes: Identified by GEO DataSets index search for Polymyositis (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.
- accession: geo:GSE94535
  title: Circulating plasma microRNA profiling in patients with polymyositis/dermatomyositis before and after treatment
  description: This study aimed to investigate the expression of microRNAs (miRNAs) in the plasma from polymyositis (PM) and dermatomyositis (DM) patients, which fluctuated by treatment. More differentially expressed miRNAs were found in plasma of DM patients compared to PM patients before and after treatment, and their profiles were different.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: MICROARRAY
  sample_count: 16
  publication: PMID:29321815
  notes: Identified by GEO DataSets index search for Polymyositis (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.
- accession: geo:GSE125977
  title: MicroRNA and mRNA profiling in the idiopathic inflammatory myopathies
  description: 'Objectives: The idiopathic inflammatory myopathies (IIMs) are heterogeneous autoimmune conditions of skeletal muscle inflammation and weakness. MicroRNAs (miRNAs) are short, non-coding RNA which regulate gene expression of target mRNAs. The aim of this study was to profile miRNA and mRNA in IIM and identify miRNA-mRNA relationships which may be relevant to disease. Materials and methods: mRNA and miRNA in whole blood samples from 7 polymyositis (PM), 7 dermatomyositis (DM), 5 inclusion body myositis (IBM) and 5 non-myositis controls was profiled using next generation RNA sequencing. Gene ontology and pathway analyses were performed using GOseq and Ingenuity Pathway Analysis.'
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: BULK_RNA_SEQ
  sample_count: 48
  publication: PMID:32529172
  notes: Identified by GEO DataSets index search for Polymyositis (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.
📚

References & Deep Research

References

15
PMID:12913190
Polymyositis: an overdiagnosed entity
No top-level findings curated for this source.
PMID:15934115
Activation of the endoplasmic reticulum stress response in autoimmune myositis: Potential role in muscle fiber damage and dysfunction
No top-level findings curated for this source.
PMID:23568993
Myositis-specific autoantibodies
No top-level findings curated for this source.
PMID:29669460
Current diagnosis and treatment of polymyositis and dermatomyositis
No top-level findings curated for this source.
PMID:39132766
Current efforts and historical perspectives on classification of idiopathic inflammatory myopathies
No top-level findings curated for this source.
PMID:40787733
Non-targeted immunosuppressive and immunomodulatory therapies for idiopathic inflammatory myopathies
No top-level findings curated for this source.
PMID:9655472
Induction of primary immune responses by allogeneic human myoblasts: dissection of the cell types required for proliferation, IFNgamma secretion and cytotoxicity
No top-level findings curated for this source.
DOI:10.1093/rheumatology/kez248
A new in vitro model of polymyositis reveals CD8+ T cell invasion into muscle cells and its cytotoxic role
No top-level findings curated for this source.
DOI:10.1097/md.0000000000040254
Causality between immunocytes and polymyositis: A Mendelian randomization analysis
No top-level findings curated for this source.
DOI:10.1186/s13075-024-03364-z
Expression of CD163 and major histocompatibility complex class I as diagnostic markers for idiopathic inflammatory myopathies
No top-level findings curated for this source.
DOI:10.1186/s41232-025-00395-0
Recent advances in immunological mechanisms and murine disease models of idiopathic inflammatory myopathies
No top-level findings curated for this source.
DOI:10.1371/journal.pone.0239176
Apoptosis in idiopathic inflammatory myopathies with partial invasion; a role for CD8+ cytotoxic T cells?
No top-level findings curated for this source.
DOI:10.3389/fimmu.2023.1191815
Muscle fiber necroptosis in pathophysiology of idiopathic inflammatory myopathies and its potential as target of novel treatment strategy
No top-level findings curated for this source.
DOI:10.3389/fmed.2023.1158768
Targeting intracellular pathways in idiopathic inflammatory myopathies: A narrative review
No top-level findings curated for this source.
DOI:10.3389/fneur.2023.1328547
Transcriptome analysis of skeletal muscle in dermatomyositis, polymyositis, and dysferlinopathy, using a bioinformatics approach
No top-level findings curated for this source.

Deep Research

2
Disorder ▸

Disorder

  • Name: Polymyositis
  • Category: Autoimmune
  • Existing deep-research providers: falcon
  • Existing evidence reference count in YAML: 20

Key Pathophysiology Nodes

  • CD8+ T Cell-Mediated Cytotoxicity
  • Aberrant MHC Class I Expression
  • Myositis-Specific Autoantibodies
  • Pro-inflammatory Cytokine Production
  • ER Stress and Unfolded Protein Response
  • Necroptosis and DAMP Release
  • Apoptosis of Muscle Fibers
  • Deep research literature mapping

Citation Inventory (for evidence mapping)

  • DOI:10.1002/art.21103
  • DOI:10.1093/rheumatology/kez248
  • DOI:10.1097/md.0000000000040254
  • DOI:10.1186/s13075-024-03364-z
  • DOI:10.1186/s41232-025-00395-0
  • DOI:10.1371/journal.pone.0239176
  • DOI:10.3389/fimmu.2023.1191815
  • DOI:10.3389/fmed.2023.1158768
  • DOI:10.3389/fneur.2023.1328547
Falcon ▸
Disease Pathophysiology Research Report
Edison Scientific Literature 32 citations 2025-12-18T09:55:06.559540

Disease Pathophysiology Research Report

Target Disease - Disease Name: Polymyositis (PM) - MONDO ID: MONDO:0005294 - Category: Autoimmune

Overview and current understanding Polymyositis is an idiopathic inflammatory myopathy characterized pathologically by endomysial inflammation in skeletal muscle with CD8+ cytotoxic T cells invading non-necrotic myofibers that aberrantly express MHC class I, consistent with an antigen-specific, cell-mediated autoimmune process (kamiya2020anewin pages 1-1, pandya2014tcellsubsets pages 20-22, nishidate2025recentadvancesin pages 1-2). Recent transcriptomic analyses and immunohistologic studies reinforce dysregulation of antigen processing/presentation pathways, interferon-related signaling, and stress responses within myofibers, and support diagnostic utility for immunostains (e.g., MHC class I, CD163) in clinical practice (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9, ghang2024expressionofcd163 pages 1-2, nagaraju2005activationofthe pages 5-8).

Core pathophysiology 1) Primary mechanisms - CD8+ T cell cytotoxicity against non-necrotic fibers: Muscle biopsies in PM show endomysial CD8+ T cells that “invade non-necrotic muscle fibers,” expressing cytotoxic molecules (perforin/granzyme). An antigen-specific in vitro model demonstrated CD8+ T cell invasion into myotubes via MHC class I, with earlier death of invaded myotubes, supporting direct cytotoxicity (Rheumatology 2020; https://doi.org/10.1093/rheumatology/kez248) (kamiya2020anewin pages 1-1, pandya2014tcellsubsets pages 20-22). “CD8 T-cell infiltration around muscle fibers” with direct damage via myocyte MHC class I is a defining feature (Inflammation & Regeneration 2025; https://doi.org/10.1186/s41232-025-00395-0) (nishidate2025recentadvancesin pages 1-2). - MHC class I upregulation on myofibers and antigen presentation: PM muscles overexpress MHC class I; omics identify antigen-presentation hub genes (HLA-A/B/C, B2M, TAP1). RNA-seq in PM found upregulation of B2M, TAP1, HLA-C, and TAPBPL (Front Neurol 2023; https://doi.org/10.3389/fneur.2023.1328547) (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9, nishidate2025recentadvancesin pages 1-2). - Interferon signatures and JAK/STAT signaling: Interferon pathways drive MHC I and immune activation in IIM; JAK/STAT is central to IFN signaling, with clinical targeting under evaluation (Front Med 2023; https://doi.org/10.3389/fmed.2023.1158768) (rocca2023targetingintracellularpathways pages 3-4). Although type I IFN signatures are more prominent in dermatomyositis, IFN-γ/STAT1 signaling is implicated in IIM muscle immune activation and MHC I induction (rocca2023targetingintracellularpathways pages 3-4).

2) Key molecular pathways and stress responses - ER stress/Unfolded Protein Response (UPR): Overexpressed MHC class I localizes to ER (HLA-ABC colocalized with calnexin) and activates robust UPR in myositis muscle (elevated Grp78/BiP; upregulation of PERK/ATF genes; NF-κB activation), linking antigen presentation overload to myofiber stress and dysfunction (Arthritis Rheum 2005; https://doi.org/10.1002/art.21103) (nagaraju2005activationofthe pages 5-8). These mechanisms mechanistically connect MHC I upregulation to ER stress and inflammatory amplification (nagaraju2005activationofthe pages 5-8). - Regulated cell death and DAMPs: Apoptotic myonuclei occur in IIM with partial invasion; fibres surrounded by CD8+ and granzyme B+ cells and macrophages were enriched for apoptosis, supporting CD8+ T cell–mediated death pathways parallel to partial invasion (PLoS One 2020; https://doi.org/10.1371/journal.pone.0239176) (danielsson2020apoptosisinidiopathic pages 2-3). Necroptosis of myofibers contributes to inflammation and weakness in IIM models of polymyositis; inhibition of necroptosis or the DAMP HMGB1 ameliorated inflammation and weakness, implicating DAMP-driven vicious cycles (Front Immunol 2023; https://doi.org/10.3389/fimmu.2023.1191815) (kamiya2023musclefibernecroptosis pages 11-11, kamiya2023musclefibernecroptosis pages 2-3).

Key molecular players and entities - Genes/Proteins (HGNC where applicable): • HLA-A, HLA-B, HLA-C; B2M (HGNC:914); TAP1; PDIA3; immunoproteasome PSMB8 (HGNC:9536), PSMB9 (HGNC:9538): implicated in PM antigen processing/presentation; hub genes identified in PM (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9, nishidate2025recentadvancesin pages 1-2). • Cytotoxicity molecules: perforin/granzyme in CD8+ T cells invading fibers (pandya2014tcellsubsets pages 20-22, kamiya2020anewin pages 1-1). • Interferon pathway/JAK-STAT: IFN-γ/STAT1 and JAK/STAT signaling implicated in muscle immune activation (rocca2023targetingintracellularpathways pages 3-4). • ER stress/UPR: GRP78/BiP, PERK/ATF genes; NF-κB activation downstream of ER overload (nagaraju2005activationofthe pages 5-8). • DAMPs: HMGB1 implicated in necroptosis-driven inflammation and muscle dysfunction (kamiya2023musclefibernecroptosis pages 11-11). • Autoantigens/autoantibodies relevant to PM subsets: FHL1 (~20% across IIM; enriched in PM/IBM) and TRIM72 (>10% in PM/DM) (nishidate2025recentadvancesin pages 1-2). - Chemical entities (CHEBI): Not consistently implicated across PM-specific evidence; DAMPs (e.g., HMGB1) function as endogenous inflammatory mediators (kamiya2023musclefibernecroptosis pages 11-11). - Cell types (CL): CD8+ T cells (principal effectors); macrophages; dendritic cells; B cells/plasma cells may be present in muscle/blood; regulatory T cell alterations suggested by Mendelian randomization (kamiya2020anewin pages 1-1, pandya2014tcellsubsets pages 20-22, nishidate2025recentadvancesin pages 1-2, yang2024causalitybetweenimmunocytes pages 3-5). - Anatomical locations (UBERON): Endomysium within skeletal muscle is the site of CD8+ T cell infiltration and myofiber invasion (kamiya2020anewin pages 1-1, nishidate2025recentadvancesin pages 1-2).

Biological processes (GO) disrupted - Antigen processing and presentation of peptide antigen via MHC class I [GO:0002474] is upregulated in PM muscle (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9). - T cell–mediated cytotoxicity [GO:0001913] is central to myofiber injury in PM (kamiya2020anewin pages 1-1, pandya2014tcellsubsets pages 20-22). - ER stress and UPR pathways drive inflammatory signaling (NF-κB) and contribute to muscle dysfunction (nagaraju2005activationofthe pages 5-8). - Interferon signaling and JAK/STAT activation in myofibers and the muscle microenvironment (rocca2023targetingintracellularpathways pages 3-4). - Regulated cell death (apoptosis, necroptosis) and DAMP release perpetuate inflammation (danielsson2020apoptosisinidiopathic pages 2-3, kamiya2023musclefibernecroptosis pages 11-11).

Cellular components (GO) - MHC class I protein complex [GO:0042612] upregulated at the sarcolemma/ER of myofibers; colocalization with ER markers observed (nagaraju2005activationofthe pages 5-8, jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9).

Disease progression sequence - Proposed mechanistic cascade: Environmental/immune triggers induce MHC class I upregulation on myofibers → ER stress/UPR and NF-κB activation within muscle → enhanced antigen processing/presentation (HLA-A/B/C, B2M, TAP1) → recruitment and activation of antigen-specific CD8+ T cells (endomysial infiltration) → partial invasion of non-necrotic myofibers and cytotoxic injury (perforin/granzyme) → regulated myofiber death (apoptosis and necroptosis) and DAMP release (e.g., HMGB1), amplifying local inflammation and sustaining weakness (nagaraju2005activationofthe pages 5-8, jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9, kamiya2020anewin pages 1-1, danielsson2020apoptosisinidiopathic pages 2-3, kamiya2023musclefibernecroptosis pages 11-11).

Phenotypic manifestations and correlations - Clinical: Symmetric proximal muscle weakness; elevated CK is typical. In a PM biopsy series, patients with intrafiber CD8+ T cells showed higher CK, linking cytotoxic invasion to biochemical injury (Rheumatology 2020; https://doi.org/10.1093/rheumatology/kez248) (kamiya2020anewin pages 1-1). - Histopathology: Endomysial CD8+ T cell infiltrates; invasion of non-necrotic myofibers; diffuse myofiber MHC class I upregulation; presence of macrophages; variable B cell/plasma cells (kamiya2020anewin pages 1-1, pandya2014tcellsubsets pages 20-22, nagaraju2005activationofthe pages 5-8, nishidate2025recentadvancesin pages 1-2). - Molecular signatures: Upregulated antigen presentation genes (B2M, HLA-C, TAP1, TAPBPL) in PM; IFN-responsive transcripts variably increased; ER stress markers elevated with MHC I overload (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9, nagaraju2005activationofthe pages 5-8).

Recent developments (2023–2024) and expert analysis - PM muscle transcriptomics (2023): PM exhibits enrichment of MHC I antigen processing/presentation with hub genes PDIA3, HLA-C, B2M, and TAP1; provides molecular specificity distinguishing PM from DM and dysferlinopathy (Front Neurol 2023; 12/2023; https://doi.org/10.3389/fneur.2023.1328547) (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9). - Necroptosis as a therapeutic target (2023): Polymyositis models implicate myofiber necroptosis and DAMPs (HMGB1) in muscle inflammation/weakness; inhibition ameliorates disease, suggesting new treatment avenues beyond immunosuppression (Front Immunol 2023; 07/2023; https://doi.org/10.3389/fimmu.2023.1191815) (kamiya2023musclefibernecroptosis pages 11-11). - Immunostaining-based diagnostics (2024): An algorithm combining muscular CD163 and MHC class I immunostaining achieved 95.5% diagnostic accuracy (sensitivity 96.1%, specificity 94.5%) in IIM versus controls and correctly classified 94.1% of expert-diagnosed IIM cases not meeting ACR/EULAR criteria (Arthritis Res Ther 2024; 07/2024; https://doi.org/10.1186/s13075-024-03364-z) (ghang2024expressionofcd163 pages 1-2). While this analysis was across IIM, it supports real-world implementation of objective pathology markers that capture the immunopathology relevant to PM. - Immune-cell causal signals (2024): Mendelian randomization prioritizes CD8+ T cell phenotypes, myeloid markers (CD11b, CD33dim, HLA-DR), and B-cell activation (CD20+, IgD−CD38+) in PM susceptibility/activity, highlighting the combined roles of cytotoxic, myeloid, and B-cell compartments (Medicine 2024; 10/2024; https://doi.org/10.1097/md.0000000000040254). Authors note results did not survive stringent multiple testing but provide directional hypotheses for biomarkers and pathogenesis (yang2024causalitybetweenimmunocytes pages 3-5). - Interferon/JAK–STAT axis (2023): Review synthesis underscores JAK/STAT coupling to IFN signaling in IIM pathogenesis and therapeutic developments targeting this pathway (Front Med 2023; 03/2023; https://doi.org/10.3389/fmed.2023.1158768) (rocca2023targetingintracellularpathways pages 3-4).

Current applications and implementations - Diagnostic pathology: Routine assessment of myofiber MHC class I and CD163+ macrophage staining improves diagnostic performance for IIM in clinical practice; algorithmic use can assist pathologists in borderline cases (ghang2024expressionofcd163 pages 1-2). - Mechanism-informed targets: Evidence supports exploring inhibitors of necroptosis or DAMP pathways (e.g., HMGB1) to complement T cell–directed therapies, particularly where inflammation persists or weakness remains despite immunosuppression (kamiya2023musclefibernecroptosis pages 11-11). - Pathway-directed therapies: Given IFN/JAK–STAT involvement, judicious consideration of JAK inhibition and interferon-pathway–targeted agents is mechanistically supported in IIM, with strongest evidence in DM but biologically plausible in PM subsets with IFN/STAT signatures (rocca2023targetingintracellularpathways pages 3-4).

Relevant statistics and data - Immunostaining algorithm (IIM cohort): CD163 positivity 99.2% vs 20.8% in controls; MHC class I 87.6% vs 23.1% (p<0.001); combined CD163+MHC I algorithm accuracy 95.5% (sensitivity 96.1%, specificity 94.5%) (ghang2024expressionofcd163 pages 1-2). - Autoantibody prevalence (IIM/PM): Anti-FHL1 detected in approximately 20% of IIM, enriched in PM/IBM; anti-TRIM72 in >10% of PM and DM (Inflammation & Regeneration 2025; 10/2025; https://doi.org/10.1186/s41232-025-00395-0) (nishidate2025recentadvancesin pages 1-2). - Transcriptomic hub genes in PM: Upregulated antigen-presentation genes include B2M, HLA-C, TAP1, TAPBPL; pathway enrichment for antigen processing/presentation (Front Neurol 2023; 12/2023; https://doi.org/10.3389/fneur.2023.1328547) (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9). - Apoptosis in partial invasion IIM: TUNEL+ myonuclei present in biopsies with partial invasion; fibres with TUNEL+ nuclei were surrounded by CD8+ and granzyme B+ cells and macrophages (PLoS One 2020; 09/2020; https://doi.org/10.1371/journal.pone.0239176) (danielsson2020apoptosisinidiopathic pages 2-3).

Selected evidence quotations - “The hallmark histopathology of PM is the presence of CD8+ T cells in the non‑necrotic muscle cells.” (Rheumatology 2020; https://doi.org/10.1093/rheumatology/kez248) (kamiya2020anewin pages 1-1). - “PM is primarily characterized by endomysial CD8+ T-cell infiltration… causing direct cellular damage through interaction with muscle cell MHC class I.” (Inflammation & Regeneration 2025; https://doi.org/10.1186/s41232-025-00395-0) (nishidate2025recentadvancesin pages 1-2). - “Overexpression of genes related to… MHC class I formation was identified in PM” with hub genes PDIA3, HLA‑C, B2M, TAP1 (Front Neurol 2023; https://doi.org/10.3389/fneur.2023.1328547) (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9). - “HLA‑ABC colocalized with the ER marker calnexin… Grp78 levels were increased… demonstrating activation of the ER stress response” (Arthritis Rheum 2005; https://doi.org/10.1002/art.21103) (nagaraju2005activationofthe pages 5-8). - “Apoptotic myonuclei were found… fibres with TUNEL positive nuclei were surrounded by CD8+ T-cells, granzyme B+ cells and macrophages” (PLoS One 2020; https://doi.org/10.1371/journal.pone.0239176) (danielsson2020apoptosisinidiopathic pages 2-3).

Ontology-linked annotations | Category | Entity (Name) | Ontology ID | Role/Association in PM | Evidence (citation id) | |---|---|---:|---|---| | Gene / Protein | HLA-A | | Upregulated/implicated in MHC class I antigen presentation in PM muscle (supports CD8+ T cell recognition) | (nishidate2025recentadvancesin pages 1-2) | | Gene / Protein | HLA-B | | Upregulated/implicated in MHC class I antigen presentation in PM muscle (supports CD8+ T cell recognition) | (nishidate2025recentadvancesin pages 1-2) | | Gene / Protein | HLA-C | | Upregulated/implicated in MHC class I antigen presentation in PM muscle (supports CD8+ T cell recognition) | (nishidate2025recentadvancesin pages 1-2) | | Gene / Protein | B2M | HGNC:914 | Component of MHC class I complex; implicated in antigen presentation in PM muscle | (nishidate2025recentadvancesin pages 1-2) | | Gene / Protein | PSMB8 | HGNC:9536 | Proteasome subunit (immunoproteasome) — hub gene linked to antigen processing for MHC I in PM | (nishidate2025recentadvancesin pages 1-2) | | Gene / Protein | PSMB9 | HGNC:9538 | Proteasome subunit (immunoproteasome) — hub gene linked to antigen processing for MHC I in PM | (nishidate2025recentadvancesin pages 1-2) | | Gene / Protein | FHL1 | HGNC:3700 | Muscle antigen reported as autoantibody target (associated with PM/IBM subsets) | (nishidate2025recentadvancesin pages 1-2) | | Gene / Protein | TRIM72 / MG53 | HGNC:16265 | Muscle antigen reported as autoantibody target in a subset of PM/DM patients | (nishidate2025recentadvancesin pages 1-2) | | Process (GO) | Antigen processing and presentation of peptide antigen via MHC class I | GO:0002474 | Central disrupted process in PM: myofiber MHC I upregulation enables CD8+ T cell recognition | (nishidate2025recentadvancesin pages 1-2) | | Process (GO) | T cell mediated cytotoxicity | GO:0001913 | Effector mechanism: CD8+ cytotoxic T cells invade non-necrotic fibers and mediate cytotoxicity (perforin/granzyme implied) | (yang2024causalitybetweenimmunocytes pages 3-5, nishidate2025recentadvancesin pages 1-2) | | Cellular Component (GO) | MHC class I protein complex | GO:0042612 | Localized to sarcolemma/myofiber surface when upregulated; enables antigen display to CD8+ T cells | (nishidate2025recentadvancesin pages 1-2) | | Cell Type (CL) | CD8+ T cell | CL:0000625 | Primary effector cell in PM endomysial infiltrates; mediates invasion/partial invasion of non-necrotic fibers | (yang2024causalitybetweenimmunocytes pages 3-5, nishidate2025recentadvancesin pages 1-2) | | Cell Type (CL) | Macrophage | CL:0000235 | Present in infiltrates; contribute to inflammation, phagocytosis and cytokine milieu in muscle | (nishidate2025recentadvancesin pages 1-2, yang2024causalitybetweenimmunocytes pages 3-5) | | Cell Type (CL) | B cell | CL:0000236 | B-cell activation / plasma cells reported in muscle and blood; B-cell phenotypes increased in MR analysis (CD20+) | (yang2024causalitybetweenimmunocytes pages 3-5) | | Cell Type (CL) | Dendritic cell | CL:0000451 | Antigen-presenting cells that may prime/autonomously present antigen to T cells in muscle microenvironment | (nishidate2025recentadvancesin pages 1-2) | | Anatomy (UBERON) | Skeletal muscle tissue | UBERON:0001134 | Primary anatomical site of inflammation and antigen presentation in PM | (nishidate2025recentadvancesin pages 1-2) | | Anatomy (UBERON) | Endomysium | UBERON:0002396 | Microanatomical compartment with endomysial CD8+ T cell infiltrates and fiber invasion in PM | (nishidate2025recentadvancesin pages 1-2, yang2024causalitybetweenimmunocytes pages 3-5) |

Table: Compact, evidence-linked ontology table mapping genes/proteins, processes, cellular components, cell types and anatomical sites implicated in polymyositis, with supporting context citations to the gathered evidence (nishidate2025recentadvancesin pages 1-2, yang2024causalitybetweenimmunocytes pages 3-5). This table is useful for populating structured disease-knowledge entries.

Gene/protein annotations (HGNC) and pathways - HLA-A/B/C (MHC I alpha chains), B2M (HGNC:914), TAP1; PDIA3; immunoproteasome PSMB8 (HGNC:9536), PSMB9 (HGNC:9538): involved in GO:0002474 antigen processing/presentation via MHC I; localized to GO:0042612 MHC I complex (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9, nishidate2025recentadvancesin pages 1-2). - Cytotoxic effectors: perforin/granzyme expressed by CD8+ T cells invading myofibers; process GO:0001913 T cell–mediated cytotoxicity (pandya2014tcellsubsets pages 20-22, kamiya2020anewin pages 1-1). - ER stress/UPR: GRP78/BiP upregulation; PERK/ATF responses; NF-κB activation; links to MHC I overexpression (nagaraju2005activationofthe pages 5-8). - Interferon/JAK–STAT: pathway activation contributes to MHC I induction and inflammatory milieu (rocca2023targetingintracellularpathways pages 3-4).

Phenotype associations (HP terms; representative) - HP:0003323 Proximal muscle weakness of the limbs; HP:0008990 Elevated serum creatine kinase; HP:0003391 Myopathy with endomysial inflammation; Histologic partial invasion of non-necrotic fibers (supported mechanistically by CD8+ cytotoxicity) (kamiya2020anewin pages 1-1, pandya2014tcellsubsets pages 20-22, nishidate2025recentadvancesin pages 1-2).

Cell types (CL terms) - CL:0000625 CD8+ T cell (principal effector); CL:0000235 macrophage; CL:0000451 dendritic cell; CL:0000236 B cell (kamiya2020anewin pages 1-1, pandya2014tcellsubsets pages 20-22, nishidate2025recentadvancesin pages 1-2, yang2024causalitybetweenimmunocytes pages 3-5).

Anatomical locations (UBERON terms) - UBERON:0001134 skeletal muscle tissue; UBERON:0002396 endomysium (kamiya2020anewin pages 1-1, nishidate2025recentadvancesin pages 1-2).

Chemical entities (CHEBI) - DAMP mediator HMGB1 (endogenous alarmin) implicated in necroptosis-driven inflammation; no PM-specific exogenous chemical drivers consistently implicated in the gathered evidence (kamiya2023musclefibernecroptosis pages 11-11).

Evidence items with PMIDs/DOIs/URLs - Kamiya M et al. A new in vitro model of polymyositis… Rheumatology (Oxford). 2020-06; doi:10.1093/rheumatology/kez248; URL: https://doi.org/10.1093/rheumatology/kez248 (kamiya2020anewin pages 1-1). - Jeong H-N et al. Transcriptome analysis… Front Neurol. 2023-12; doi:10.3389/fneur.2023.1328547; URL: https://doi.org/10.3389/fneur.2023.1328547 (jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9). - Ghang B et al. Expression of CD163 and MHC class I… Arthritis Res Ther. 2024-07; doi:10.1186/s13075-024-03364-z; URL: https://doi.org/10.1186/s13075-024-03364-z (ghang2024expressionofcd163 pages 1-2). - Danielsson O et al. Apoptosis in IIM with partial invasion… PLoS One. 2020-09; doi:10.1371/journal.pone.0239176; URL: https://doi.org/10.1371/journal.pone.0239176 (danielsson2020apoptosisinidiopathic pages 2-3). - Nagaraju K et al. Activation of the ER stress response… Arthritis Rheum. 2005-06; doi:10.1002/art.21103; URL: https://doi.org/10.1002/art.21103 (nagaraju2005activationofthe pages 5-8). - La Rocca G et al. Targeting intracellular pathways… Front Med. 2023-03; doi:10.3389/fmed.2023.1158768; URL: https://doi.org/10.3389/fmed.2023.1158768 (rocca2023targetingintracellularpathways pages 3-4). - Nishidate A et al. Recent advances… Inflammation & Regeneration. 2025-10; doi:10.1186/s41232-025-00395-0; URL: https://doi.org/10.1186/s41232-025-00395-0 (nishidate2025recentadvancesin pages 1-2). - Yang N et al. Causality between immunocytes and polymyositis… Medicine (Baltimore). 2024-10; doi:10.1097/MD.0000000000040254; URL: https://doi.org/10.1097/md.0000000000040254 (yang2024causalitybetweenimmunocytes pages 3-5).

Conclusions Polymyositis pathogenesis is centered on MHC class I–restricted CD8+ T cell–mediated cytotoxicity against non-necrotic myofibers, supported by molecular upregulation of antigen processing/presentation machinery and by ER stress responses within muscle. Interferon/JAK–STAT signaling and regulated cell-death pathways (apoptosis and necroptosis with DAMP release) amplify and sustain tissue injury. Diagnostic immunostaining for MHC class I and CD163 has strong real-world utility to classify IIM—including PM—in pathology workflows. These convergent lines of evidence provide mechanistic anchors for developing targeted therapies (e.g., JAK/STAT modulation, necroptosis/DAMP pathway inhibition) and for biomarker-driven diagnostics in PM (kamiya2020anewin pages 1-1, jeong2023transcriptomeanalysisof pages 1-2, jeong2023transcriptomeanalysisof pages 7-9, nagaraju2005activationofthe pages 5-8, rocca2023targetingintracellularpathways pages 3-4, danielsson2020apoptosisinidiopathic pages 2-3, kamiya2023musclefibernecroptosis pages 11-11, ghang2024expressionofcd163 pages 1-2, nishidate2025recentadvancesin pages 1-2, yang2024causalitybetweenimmunocytes pages 3-5).

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