Polymyositis

Autoimmune MONDO:0019127 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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Inheritance

Histopathology

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Subtypes

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References

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🏷

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Harrison's Chapter

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Pathophysiology

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 link Skeletal Muscle Cell link

T Cell Cytotoxicity link

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 link

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 link

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 link

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 link

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 link

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 link

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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Phenotypes

Digestive 1

Dysphagia FREQUENT Dysphagia (HP:0002015)

Pharyngeal muscle involvement

Limbs 1

Proximal Muscle Weakness VERY_FREQUENT Limb muscle weakness (HP:0003690)

Symmetric proximal weakness, difficulty rising from chairs

Show evidence (1 reference)

PMID:31257434 SUPPORT

"The level of serum creatinine kinase was higher in patients with CD8+ T cells in muscle cells than those without these cells."

Links the presence of CD8+ T cell invasion to elevated muscle enzymes and clinical muscle damage manifestations.

Musculoskeletal 1

Arthritis FREQUENT Arthritis (HP:0001369)

Part of antisynthetase syndrome

Respiratory 1

Pulmonary Fibrosis FREQUENT Pulmonary fibrosis (HP:0002206)

Associated with antisynthetase syndrome

🧬

Genetic Associations

HLA-DRB1*0301 (Risk Factor)

💊

Treatments

Corticosteroids

First-line therapy; high-dose initially with gradual taper.

Methotrexate

Most commonly used steroid-sparing agent.

Azathioprine

Alternative immunosuppressant for maintenance.

IVIG

For refractory cases or when immunosuppression contraindicated.

🔬

Biochemical Markers

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

{ }

Source YAML

click to show

name: Polymyositis
creation_date: '2025-12-19T01:12:52Z'
updated_date: '2026-02-17T21:53:14Z'
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.
- 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.
- 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
- 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
- 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.
- 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.
- 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: Proximal Muscle Weakness
  category: Musculoskeletal
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Limb Muscle Weakness
    term:
      id: HP:0003690
      label: Limb muscle weakness
  notes: Symmetric proximal weakness, difficulty rising from chairs
  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 level of serum creatinine kinase was higher in patients with CD8+
      T cells in muscle cells than those without these cells."
    explanation: Links the presence of CD8+ T cell invasion to elevated muscle
      enzymes and clinical muscle damage manifestations.
- 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
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.
- name: Methotrexate
  description: Most commonly used steroid-sparing agent.
- name: Azathioprine
  description: Alternative immunosuppressant for maintenance.
- name: IVIG
  description: For refractory cases or when immunosuppression contraindicated.
classifications:
  harrisons_chapter:
  - classification_value: musculoskeletal system disorder
  - classification_value: connective tissue disease
  - classification_value: autoimmune disease
references:
- reference: DOI:10.1002/art.21103
  title: 'Activation of the endoplasmic reticulum stress response in autoimmune myositis:
    Potential role in muscle fiber damage and dysfunction'
  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: []

📚

References & Deep Research

References

DOI:10.1002/art.21103

Activation of the endoplasmic reticulum stress response in autoimmune myositis: Potential role in muscle fiber damage and dysfunction