Inclusion Body Myositis

1. Disease Information

1.1 Overview

Inclusion body myositis (IBM; sporadic IBM, sIBM) is a slowly progressive, acquired, late-onset skeletal muscle disease and the most common acquired myopathy in people over 50 years of age. It is unique among the idiopathic inflammatory myopathies (IIMs) in combining two co-existing pathological programs in the same myofibres:

1. an autoimmune/inflammatory arm — endomysial infiltration by highly differentiated cytotoxic CD8⁺ T cells that invade non-necrotic, MHC class I–overexpressing muscle fibres; and
2. a myodegenerative arm — rimmed vacuoles, protein aggregation (p62/SQSTM1, TDP-43, amyloid-β, ubiquitin), autophagy–lysosome dysfunction, and mitochondrial abnormalities (COX-negative fibres, mtDNA deletions).

Whether these arms are causally sequential (inflammation → degeneration), independent, or reciprocally reinforcing is the central unresolved question of IBM pathogenesis — an ideal candidate for a mechanistic_hypotheses block with competing hypothesis_group_id values in the KB entry.

MONDO definition (verified with OAK, sqlite:obo:mondo):

"A slowly progressive degenerative inflammatory disorder of skeletal muscles characterized by late onset weakness of specific muscles and distinctive histopathological features." — MONDO:0007827 def: (source Orphanet:611)

The clinical signature is a highly stereotyped, asymmetric, selective weakness pattern: quadriceps femoris (knee extension) and deep finger flexors (flexor digitorum profundus), with early dysphagia and ankle dorsiflexor involvement. This pattern is so characteristic that it forms the backbone of every published diagnostic criteria set.

IBM is refractory to all conventional immunosuppression — a defining and clinically important negative feature that distinguishes it from dermatomyositis, immune-mediated necrotizing myopathy, and polymyositis, and that motivates the "degeneration-primary" hypothesis.

1.2 Key identifiers (all verified from the MONDO:0007827 record via OAK)

MONDO subset: flags include rare, orphanet_rare, nord_rare, gard_rare — IBM is formally a rare disease despite being the commonest myopathy of the elderly.

1.3 Synonyms (verified synonym: lines from MONDO)

• IBM (EXACT; OMIM:147421, Orphanet:611)
• Sporadic Inclusion Body Myositis (EXACT; NORD:1734, Orphanet:611)
• sIBM (EXACT; Orphanet:611)
• sporadic inclusion body myositis (EXACT; Orphanet:611)
• "inflammatory myopathy" (RELATED; GARD — too broad; do not use as an exact synonym)

Historic/literature synonyms not in MONDO: inclusion body myositis, sporadic type; sIBM. Historic misnomer to avoid: "polymyositis with inclusion bodies."

1.4 Information provenance

Information for this entry is predominantly aggregated disease-level (Orphanet, OMIM, ENMC consensus workshops, systematic reviews, registry/cohort studies). Two important individual-patient / EHR-derived sources exist and should be tagged as such:

• Rochester Epidemiology Project (REP) medical-records-linkage system, Olmsted County and 27 Minnesota/Wisconsin counties — the source of both the 2008 and 2021 US population-based epidemiology figures. This is genuine EHR-linkage data.
• Swedish national cohort (Lindgren et al., Ann Neurol 2022 ✅ PMID:35596584) — national registry/biopsy-registry derived.
• MYOGEN / MYOVISION consortia — patient-level genotype data underpinning the HLA association work.

2. Etiology

2.1 Overall causal model

IBM has no single established cause. It is best modeled in the KB as a multifactorial, age-dependent disease in which a permissive genetic background (dominantly HLA class II) plus profound age-associated immune remodeling (immunosenescence) permits a chronic, oligoclonal, cytotoxic T-cell attack on skeletal muscle, superimposed on (or triggering) a cell-autonomous proteostatic/mitochondrial failure in aging myofibres.

Two competing high-level hypotheses should be curated explicitly as mechanistic_hypotheses with status: EMERGING / ALTERNATIVE:

The Britson xenograft result is the single most probative experiment currently available and should be curated as an explicit edge-level qualifier: T-cell depletion in the model did not rescue the degenerative arm.

2.2 Genetic risk factors

IBM is not a Mendelian disease and has no established causal gene. Risk is conferred by common-variant susceptibility loci, overwhelmingly in the MHC.

A note for curation: NT5C1A (hgnc:17819, encoding cN1A / Mup44) is the autoantigen, not a risk gene. Do not model it as a causal gene; model it as an antigen target with relationship_type reflecting autoantigen status, and curate the antibody as a biomarker.

Familial clustering: Rare familial aggregation of sporadic-type IBM has been reported ("familial inflammatory IBM"), but this is distinct from the hereditary inclusion body myopathies. There is no established inheritance pattern.

2.3 Environmental / acquired risk factors

There is no established occupational, dietary, toxic, or radiation exposure for IBM. This is an honest "not available" for the entry.

2.4 Protective factors

• Genetic: DRβ1 glutamine at position 74 is reported protective (✅ PMID:38043487). This is a rare, well-specified genetic protective factor and is worth curating explicitly.
• Environmental: No validated environmental protective factor. Exercise (see §12) improves function but there is no evidence it prevents disease onset.

2.5 Gene–environment interactions

The most plausible G×E model — and one that should be curated as a hypothesis, not a fact — is that HLA-DRB1*03:01-restricted presentation of a self- or pathogen-derived peptide to CD8⁺ T cells, in the setting of age-related immunosenescence and possible chronic viral (CMV/EBV/HIV) antigenic pressure, drives the clonal expansion of terminally differentiated cytotoxic effectors that characterize IBM. Chronic CMV infection is a canonical driver of the CD28⁻/CD57⁺/KLRG1⁺ effector-memory phenotype seen in IBM blood and muscle, making CMV serostatus a natural G×E investigation target. No confirmatory human study establishes this chain — curate as KNOWLEDGE_GAP with proposed_experiments.

3. Phenotypes

3.1 Cardinal motor phenotypes

All HP identifiers below were verified with OAK (sqlite:obo:hp).

Suggested descriptor qualifiers (per repo conventions): use clinical_course: PROGRESSIVE and temporality: CHRONIC on the weakness descriptors, and onset with onset_category reflecting late-adult onset.

3.2 Bulbar phenotypes

Citations: ✅ PMID:33879596 (natural history/REP); Lindgren U et al. ✅ PMID:35596584; anti-cN1A/dysphagia association (PMC8151681 — resolve to a PMID before citing).

3.3 Laboratory phenotypes

3.4 Electrophysiological phenotypes

3.5 Histopathological phenotypes (biopsy-defined)

3.6 Phenotype characteristics summary

• Age of onset: adult/late-onset; mean onset ~60–70 years; onset before 45 is rare and should trigger reconsideration. HPO onset term: HP:0003584 Late onset (verify) or HP:0003581 Adult onset.
• Severity: moderate → severe over decades; universally disabling if survival is long enough.
• Progression: relentlessly progressive; never episodic, never relapsing-remitting, spontaneous remission essentially unreported.
• Frequency among affected individuals: quadriceps and finger-flexor weakness near-universal by the time of diagnosis; dysphagia in the majority eventually.

3.7 Quality-of-life impact (per phenotype)

• Quadriceps weakness → loss of stair climbing, rising from a chair, and independent ambulation; falls and fall-related fractures; median time to wheelchair dependence ≈ 10.5 years (range 1–29) ✅ PMID:33879596. Other series quote wheelchair dependence "on average, 12–20 years after onset" ✅ PMID:25215417.
• Finger flexor weakness → loss of grip: buttons, keys, jar opening, writing, holding utensils; disproportionate impact on independence relative to strength loss.
• Dysphagia → aspiration fear, meal-time anxiety, social withdrawal from eating, weight loss, PEG dependence; the phenotype most strongly linked to mortality.
• No IBM-specific QoL instrument is standard. Instruments used: IBMFRS (IBM Functional Rating Scale) — the field's primary functional outcome and the primary endpoint in the ulviprubart and sirolimus trials; SF-36; HAQ; 6-minute walk distance (6MWD) — primary endpoint of RESILIENT. See ✅ PMID:22588740 (Arthritis Care Res 2011 myositis outcome-measures compendium) for the catalogue.

4. Genetic / Molecular Information

4.1 Causal genes

None. IBM has no causal gene and no established Mendelian inheritance. This should be stated affirmatively in the entry — it is the key discriminator from the hereditary inclusion body myopathies. OMIM:147421 exists but describes an HLA-associated susceptibility phenotype, not a gene–disease relationship. There is no ClinGen Gene-Disease Validity assertion establishing a definitive gene for sporadic IBM (a CGGV: query is worth running to confirm and to cite the absence).

4.2 Susceptibility / risk genes (curate with relationship_type: SUSCEPTIBILITY)

Genes to explicitly exclude (NEC guard, curate as notes or a discussions entry): GNE (hgnc:23657) and VCP (hgnc:12666) cause hereditary inclusion body myopathies, not IBM.

4.3 Pathogenic variants

Not applicable in the ACMG/AMP sense — there are no pathogenic germline variants for IBM. The relevant genetic architecture is common HLA haplotype variation with population allele frequencies available in the Allele Frequency Net Database and gnomAD (HLA imputation). HLA-DRB1*03:01 carrier frequency in northern European populations is roughly 20–25%, consistent with a common susceptibility allele of moderate-to-large effect rather than a rare pathogenic variant.

4.4 Somatic genetic changes

• Mitochondrial DNA deletions — clonally expanded, large-scale mtDNA deletions accumulate in COX-negative myofibre segments. These are somatic, muscle-restricted, and clonally expanded within individual fibre segments, closely resembling the mtDNA pathology of normal aging muscle but present in far greater abundance. This is a genuine somatic-mutation mechanism worth its own pathophysiology node.
• STAT3 gain-of-function somatic mutations in circulating clonal large granular lymphocytes (see §4.5).

4.5 The IBM / T-LGL leukaemia overlap (a major, under-appreciated finding)

Greenberg SA et al., Brain 2016 ✅ PMID:26920676 reported that:

"Most (22 of 38; 58%) patients with inclusion body myositis had aberrant populations of large granular lymphocytes in their blood meeting standard diagnostic criteria for T cell large granular lymphocytic leukaemia, and these T cell populations were clonal in 20 of 20 patients and stably present on follow-up testing." (candidate quote — verify verbatim)

STAT3 gain-of-function mutations, the molecular hallmark of T-LGL leukaemia, are present in ~21–75% of T-LGL cohorts and drive constitutive STAT3 activation → enhanced survival and defective activation-induced cell death of the cytotoxic clone. This provides a mechanistically satisfying explanation for why the IBM T-cell attack is persistent and immunosuppression-resistant: the effector cells are a long-lived, apoptosis-resistant clone, not a conventional activated T-cell response. This deserves an explicit pathophysiology node and should be linked to a comorbidity entry for T-LGL leukaemia.

4.6 Modifier genes

• HLA-DRB1*03:01:01 acts as an age-of-onset modifier (~5 years earlier onset in carriers) ✅ PMID:38043487 — a clean MODIFIER relationship.
• Anti-cN1A antibody status behaves as a severity modifier (more severe dysphagia, possibly worse survival) but is serological, not genetic.

4.7 Epigenetics

No robust, replicated DNA-methylation or histone-modification signature is established for IBM. Muscle transcriptomic studies consistently show a type II interferon (IFN-γ) signature rather than the type I IFN signature of dermatomyositis — this is transcriptional, not confirmed epigenetic. Honest gap: not available; curate as KNOWLEDGE_GAP.

4.8 Chromosomal abnormalities

None associated with IBM. CMA/karyotype/FISH have no diagnostic role. Explicitly "not applicable."

5. Environmental Information

• Environmental toxins / occupational exposure / radiation: No established association. CTD/TOXNET yield no validated IBM-toxicant links. Not available.
• Lifestyle factors: No established dietary, smoking, or alcohol association. Physical inactivity worsens deconditioning but is not aetiological.
• Drugs: Statins are the recurrently discussed but unproven exposure (see §2.3). Statin-associated autoimmune myopathy (anti-HMGCR) is a distinct entity and a key differential.
• Infectious agents (all speculative/associative, none causal):
• HIV-1 (NCBITaxon:11676) — HIV-associated IBM-like myopathy
• HTLV-1 (NCBITaxon:11908) — HTLV-1–associated inflammatory myopathy with IBM features
• HCV (NCBITaxon:11103) — reported association
• Human cytomegalovirus (NCBITaxon:10359) — not causal, but the canonical driver of the terminally differentiated CD8⁺CD28⁻CD57⁺KLRG1⁺ T-cell compartment that IBM's effector cells resemble; mechanistically the most interesting candidate for a G×E study.

Curate these with supports: PARTIAL or as discussions — none meets a causal-agent bar.

6. Mechanism / Pathophysiology

6.1 Proposed causal chain (upstream → downstream)

Below is a node chain suitable for direct translation into pathophysiology: entries, with biological_scale: tags per the repo's single-value discipline.

Node 1 — Permissive genetic background and immunosenescence (biological_scale: ORGANISM) HLA-DRB103:01:01 (Arg74 in the DRβ1 peptide-binding groove) + age-associated contraction of the naïve T-cell repertoire and expansion of terminally differentiated effectors. → downstream:* Node 2

Node 2 — Clonal expansion of highly differentiated cytotoxic CD8⁺ T cells (biological_scale: CELLULAR) CD8⁺CD57⁺CD28⁻KLRG1⁺ effector-memory/TEMRA cells with NK-like features; oligoclonal by TCR sequencing; persistent over years; frequently meeting T-LGL leukaemia criteria; STAT3-GOF-driven resistance to activation-induced cell death. - Cell type: CL:0000794 CD8-positive, alpha-beta cytotoxic T cell - GO: GO:0001913 T cell mediated cytotoxicity; GO:0043316 cytotoxic T cell degranulation - Gene: KLRG1 (hgnc:6380), STAT3 (hgnc:11364) → downstream: Node 4

Node 3 — Myofibre MHC class I overexpression (biological_scale: CELLULAR) IFN-γ–driven, widespread sarcolemmal and sarcoplasmic MHC-I upregulation on non-necrotic fibres — both the antigen-presentation substrate and, independently, a cell-intrinsic ER stressor. - Cell type: CL:0008002 skeletal muscle fiber - GO: GO:0002484 antigen processing and presentation of endogenous peptide antigen via MHC class I via ER pathway → downstream: Nodes 4 and 5

Node 4 — Cytotoxic invasion of non-necrotic myofibres (biological_scale: CELLULAR) Perforin/granzyme-mediated attack; the pathognomonic "partial invasion" lesion. Accompanied by endomysial macrophages (CL:0000235) and plasma cells (CL:0000786; local Ig production supports a B-cell/plasma-cell arm and the origin of anti-cN1A). - GO: GO:0001913 T cell mediated cytotoxicity → downstream: Node 8

Node 5 — ER stress / unfolded protein response (biological_scale: MOLECULAR) MHC-I overload and misfolded-protein burden activate the UPR (PERK/ATF6/IRE1), amplifying NF-κB signalling and further MHC-I expression — a feed-forward loop. - GO: GO:0034976 response to endoplasmic reticulum stress → downstream: Nodes 6 and 3 (feedback edge)

Node 6 — Autophagy–lysosome pathway failure and protein aggregation (biological_scale: CELLULAR) Impaired autophagic flux and chaperone-mediated autophagy; accumulation of p62/SQSTM1, ubiquitin, LC3, amyloid-β/APP-derived species, and phosphorylated tau in aggregates; formation of rimmed vacuoles (autophagic vacuoles with myeloid debris). - GO: GO:0006914 autophagy; GO:0061684 chaperone-mediated autophagy; GO:0070841 inclusion body assembly; GO:0043161 proteasome-mediated ubiquitin-dependent protein catabolic process; GO:0042026 protein refolding (the arimoclomol/HSP rationale) - Genes/proteins: SQSTM1 (hgnc:11280), APP (hgnc:620) - Phenotype: HP:0003805 Rimmed vacuoles → downstream: Node 8

Greenberg's caveat to curate honestly: aggregates are present in "<1% of myofibres in patients with IBM" ✅ PMID:30837708 (candidate quote — verify), which is a strong quantitative argument that aggregates are a marker rather than the primary driver.

Node 7 — TDP-43 nuclear clearance, cytoplasmic aggregation, and loss of splicing repression (biological_scale: MOLECULAR) Nuclear loss + cytoplasmic mislocalization of TDP-43 (TARDBP, hgnc:11571) with cryptic exon inclusion in TDP-43 target transcripts — a molecular convergence with ALS/FTD. This is now the most mechanistically specific molecular lesion in IBM and a candidate biomarker (cryptic-exon-derived peptides/transcripts). - Critically: in the xenograft model, "Loss of TDP-43 function and rimmed vacuoles persist after T cell depletion" ✅ PMID:35044790 — i.e., this arm is at least partly T-cell–independent. → downstream: Node 8

Node 8 — Mitochondrial dysfunction (biological_scale: CELLULAR) COX-negative fibres, ragged-red fibres, clonally expanded somatic mtDNA deletions, impaired oxidative phosphorylation, ROS generation. Mechanistically linked to TDP-43 (TDP-43 associates with mitochondria and its dysfunction impairs mitochondrial function) and plausibly to chronic inflammatory/nitrosative stress. - Phenotypes: HP:0003688 Cytochrome C oxidase-negative muscle fibers; HP:0003200 Ragged-red muscle fibers - GO: GO:0000422 autophagy of mitochondrion (mitophagy) → downstream: Node 9

Node 9 — Myofibre degeneration, atrophy, and failed regeneration (biological_scale: TISSUE) Fibre necrosis, atrophy, splitting, endomysial fibrosis (HP:0100297), and exhaustion/impaired activation of satellite cells (CL:0000594 skeletal muscle satellite cell; CL:0008016 activated skeletal muscle satellite cell). Fatty and fibrous replacement visible on MRI. - GO: GO:0043403 skeletal muscle tissue regeneration → downstream: Node 10

Node 10 — Selective, asymmetric muscle weakness and dysphagia (biological_scale: ORGANISM) Clinical phenotype (§3). Why quadriceps and FDP are selectively vulnerable remains unexplained — an excellent, well-defined KNOWLEDGE_GAP for the entry.

6.2 Molecular pathways

• IFN-γ / JAK-STAT1 — the dominant muscle transcriptomic signature (type II IFN, distinguishing IBM from the type I IFN signature of dermatomyositis). KEGG hsa04630, Reactome "Interferon gamma signaling."
• NF-κB — downstream of ER stress and inflammatory cytokines; drives MHC-I and cytokine expression.
• mTOR / autophagy — the rationale for sirolimus; mTORC1 inhibition promotes autophagic clearance and preferentially spares/expands regulatory T cells while depleting effector-memory T cells.
• Heat-shock response / proteostasis — the rationale for arimoclomol (HSP co-inducer).
• Myostatin–ActRII–SMAD2/3 — the rationale for bimagrumab (anti-ActRII antibody), aimed at the atrophy arm rather than causation.
• TDP-43 splicing repression / cryptic exon inclusion — shared with ALS/FTD.
• STAT3 — clonal LGL survival.

6.3 Cell types involved (CL terms, OAK-verified)

6.4 Anatomical and subcellular localization

• Tissue: UBERON:0001134 skeletal muscle tissue (verify exact ID before use — OAK returned UBERON:0014892 skeletal muscle organ, vertebrate for the organ-level query); UBERON:0004498 skeletal muscle tissue of quadriceps femoris; UBERON:0004499 skeletal muscle tissue of tibialis anterior; UBERON:0001523 flexor digitorum profundus; UBERON:0000933 chordate pharyngeal muscle.
• Subcellular (GO CC): sarcoplasm/cytoplasm, GO:0005634 nucleus (TDP-43 clearance), GO:0005739 mitochondrion, GO:0005764 lysosome / GO:0005776 autophagosome, GO:0005783 endoplasmic reticulum, GO:0016235 aggresome (verify), GO:0042612 MHC class I protein complex (verify).

6.5 Molecular profiling

• Transcriptomics: Muscle RNA-seq consistently shows a type II interferon-dominant signature with strong upregulation of MHC-I/II, immunoproteasome subunits, chemokines (CXCL9/10), and Ig genes; plus downregulation of oxidative-phosphorylation and mitochondrial transcripts. Search GEO for sporadic inclusion body myositis muscle series to populate a datasets: block.
• Cryptic exon transcriptomics: TDP-43 loss-of-function–dependent cryptic exons are detectable in IBM muscle (✅ PMID:35044790) — the most disease-specific transcriptomic readout available.
• Proteomics: Aggregate-enriched proteomics identifies p62, TDP-43, ubiquitin, amyloid-β, phosphorylated tau, myotilin, αB-crystallin, and cN1A. PRIDE/ProteomeXchange hold relevant datasets.
• Single-cell / TCR-seq: scRNA-seq + TCR-seq of muscle-infiltrating and blood T cells is the most active current frontier and directly underpins the KLRG1-targeting therapeutic strategy. Frontiers Immunol 2023 study of expanded CD8⁺ LGLs in IBM correlates T-cell phenotype with disease severity (PMC10098158 — resolve to PMID before citing).
• Metabolomics/lipidomics: Not available — no established IBM signature.

7. Anatomical Structures Affected

Organ level

• Primary organ: skeletal muscle (UBERON:0014892 skeletal muscle organ, vertebrate). Body system: musculoskeletal.
• Secondary/complication organs: lung (aspiration pneumonia, respiratory failure — the leading cause of death); pharynx/upper oesophagus (cricopharyngeus); rarely heart (cardiac involvement is not a feature of IBM — an important negative); peripheral nerve is spared (nerve conduction studies are normal).

Selective muscle involvement — the diagnostic signature

Lateralization

Asymmetric — a positive diagnostic feature. Weakness is characteristically worse on one side, unusual for a myopathy and a reason IBM is frequently misdiagnosed as motor neuron disease or a radiculopathy.

Tissue/cell level

Striated skeletal muscle tissue; myofibres (CL:0008002) as targets; endomysial compartment as the site of the inflammatory infiltrate (endomysial, not perimysial or perivascular — the latter patterns indicate dermatomyositis). Satellite cells (CL:0000594) show impaired regenerative capacity.

Subcellular

Nucleus (TDP-43 clearance), cytoplasm/sarcoplasm (aggregates), lysosome/autophagosome (rimmed vacuoles), mitochondria (COX-negative segments, mtDNA deletions), ER (UPR), sarcolemma (MHC-I).

8. Temporal Development

Onset

• Typical age: adult/late — most series report mean onset ~60–70 years; onset before age 45 is distinctly unusual.
• Onset pattern: insidious, over months to years. Patients typically report a long prodrome of falls, difficulty rising from chairs, or dropping objects.
• Diagnostic delay: notoriously long — commonly 5+ years from symptom onset to diagnosis, because early asymmetric weakness is misattributed to orthopaedic causes, radiculopathy, or motor neuron disease.

Progression

• Course: relentlessly, slowly progressive; never relapsing-remitting; spontaneous remission not described.
• Rate: IBMFRS declines ~1–2 points/year in natural-history and placebo-arm data; quantitative strength declines ~3–5%/year. Slow enough that trials require 12–20+ months and large N to detect an effect — the central trial-design problem in IBM.
• Milestones: median time to wheelchair dependence 10.5 years (range 1–29) ✅ PMID:33879596; other cohorts report 12–20 years ✅ PMID:25215417. Assistive-device use typically precedes this by several years.
• Duration: chronic, lifelong.
• Staging: No formal consensus staging system. The 272nd ENMC workshop (✅ PMID:38522330) explicitly addressed clinical trial readiness and outcome measures; the working stratification used in trials is mild/moderate vs advanced disease, and the ulviprubart MUSCLE result (§12) makes this stratification clinically consequential.

Patterns

• Remission: none, spontaneous or treatment-induced. This is a defining negative.
• Critical window: The strongest current signal for a therapeutic window is the MUSCLE trial's mild-to-moderate subgroup, where ulviprubart showed favourable trends absent in the full population — supporting a "treat early, before irreversible fibro-fatty replacement" model. Curate this as an emerging hypothesis, not an established fact.

9. Inheritance and Population

9.1 Epidemiology

Curation guidance: use the structured Prevalence slots. For the 18.20/100,000 figure: population: "United States (Olmsted County/REP), adults ≥50 years", measure_type: POINT_PREVALENCE, prevalence_class: BAND_1_5_PER_10000 (18.2/100,000 = 1.82/10,000 → falls in the 1–5/10,000 band), rate_per_100000: 18.2. For the all-ages figure: rate_per_100000: 7.06, rate_low: 0.87, rate_high: 13.24, prevalence_class: BAND_1_9_PER_100000. Never compare the ≥50 figure to the all-ages figure — different denominators.

Notably, prevalence of sIBM correlated with the population frequency of HLA-DR3 across studies (✅ PMID:25065005) — an elegant ecological confirmation of the 8.1-AH association and worth curating.

9.2 Inheritance

• Inheritance pattern: Not Mendelian. Multifactorial/complex, with a dominant common-variant HLA contribution. Do not assign an HPO mode-of-inheritance term implying Mendelian transmission. If any inheritance block is used, HP:0010982 Polygenic inheritance with relationship_type: SUSCEPTIBILITY gene typing is the most defensible, and even that overstates the evidence — consider omitting the inheritance: block entirely and stating the absence in notes.
• Penetrance / expressivity / anticipation / mosaicism / consanguinity / carrier frequency: Not applicable. Explicitly state this; these are the fields DR tools most often hallucinate for IBM by importing GNE-myopathy content.
• Founder effects: None for IBM. (Founder effects do exist for GNE myopathy — e.g. the Persian-Jewish M712T founder allele — and this is a classic NEC contamination vector. Do not import it.)

9.3 Population demographics

• Sex ratio: male predominance, ~2:1 to 3:1 (M:F) — the only IIM with male predominance (dermatomyositis, ASyS, and IMNM are female-predominant). A useful discriminating epidemiological feature.
• Ethnicity: Highest reported prevalence in populations of northern European ancestry, tracking 8.1-AH frequency; IBM is reported but less frequently ascertained in East Asian, African, and Latin American populations. Ascertainment bias is a real confounder — flag it.
• Geography: Reported worldwide; highest measured rates from Scandinavia, the Netherlands, the UK, Australia, and the US Midwest — regions with both high 8.1-AH frequency and good neuromuscular ascertainment.
• Age distribution: Sharply skewed to ≥50 years, peaking in the 7th–8th decades.

10. Diagnostics

10.1 Clinical diagnostic criteria

Suggested definitions[] shape: definition_type: DIAGNOSTIC_CRITERIA (or the repo's nearest value), derivation_basis: ESTABLISHED_CRITERIA, citing PMID:38522330 with PMID:24268584 and PMID:24975859 as predecessors.

10.2 Serological biomarker: anti-cN1A / anti-NT5C1A

The only IBM-associated autoantibody. Target: cytosolic 5′-nucleotidase 1A (cN1A / Mup44), encoded by NT5C1A (hgnc:17819).

• Discovery: Larman HB et al., Ann Neurol 2013 — ✅ PMID:23596012 — "Cytosolic 5′-nucleotidase 1A autoimmunity in sporadic inclusion body myositis." Reported: "Moderate reactivity of anti-cN1A autoantibodies was 70% sensitive and 92% specific" and "high reactivity was 34% sensitive and 98% specific" (candidate quotes — verify verbatim; these are the numbers most worth getting exactly right). Independently and near-simultaneously reported by Pluk H et al. (Ann Neurol 2013).
• Isotype work: Herbert MK et al. — ✅ PMID:24752512 — "Cytoplasmic 5′-nucleotidase autoantibodies in inclusion body myositis: isotypes and diagnostic utility." Combination assays measuring all three isotypes (IgM, IgA, IgG) improved sensitivity to 76%.
• Meta-analytic performance: sensitivity 33–76%, specificity 87–100% across studies; variability driven by assay platform and cut-off. One Italian cohort: sensitivity 37.1%, specificity 96.8%. A single-centre 40-patient series: sensitivity 50% — ✅ PMID:30001928.
• Clinical utility summary: ✅ PMID:31024569 — "Anti-NT5c1A autoantibodies as biomarkers in inclusion body myositis."
• Interpretation for the KB: Moderate sensitivity, high specificity, poor PPV in low-prevalence settings (one study: PPV 0.29, NPV 0.96). Anti-cN1A is not disease-specific — it occurs in Sjögren syndrome and SLE — so a positive result outside a compatible clinical phenotype does not establish IBM. Positivity is associated with more severe dysphagia.
• Reference range curation: anti-cN1A is qualitative/semi-quantitative and assay-dependent; a reference_ranges block is not appropriate here. Curate as a phenotype/biomarker with prose interpretation instead.

10.3 Laboratory tests

• Serum CK (LOINC 2157-6): normal to mildly/moderately elevated, usually <10–12× ULN. HP:0008180 Mildly elevated creatine kinase. A markedly elevated CK argues for IMNM or dystrophy instead.
• Aldolase, AST/ALT (muscle-derived), LDH: mildly elevated.
• Myositis-specific antibody panel: should be negative for anti-Jo-1/ARS, anti-Mi-2, anti-TIF1-γ, anti-NXP2, anti-MDA5, anti-SRP, anti-HMGCR. A positive MSA points away from IBM.
• Peripheral blood flow cytometry / TCR clonality: given the T-LGL overlap (✅ PMID:26920676), flow cytometry for aberrant CD8⁺CD57⁺ LGL populations and TCR-β clonality is an underused, mechanistically informative test.
• HIV, HTLV-1, HCV serology: to exclude infection-associated myopathy.
• TSH, vitamin D: to exclude reversible myopathies.

10.4 Imaging

Muscle MRI is now formally part of the diagnostic algorithm (✅ PMID:38522330). Characteristic findings: - Fatty infiltration (T1) and oedema (STIR/T2 fat-sat) in anterior thigh with vastus lateralis and vastus intermedius affected earlier/more severely than rectus femoris; - Medial gastrocnemius involvement in the lower leg; - Forearm deep flexor compartment (FDP) involvement. This pattern is sufficiently distinctive that whole-body muscle MRI can support diagnosis in biopsy-negative cases. RadLex/DICOM applicable. Muscle ultrasound (increased echo intensity in the same distribution) is a cheaper, bedside alternative endorsed by the 272nd ENMC workshop.

MAXO: consider MAXO:0035082 barium swallow radiograph procedure for the swallowing evaluation (see below).

10.5 Electrophysiology

• Needle EMG: myopathic MUPs (HP:0003458) with abundant fibrillation potentials; a mixed myopathic/large-unit pattern is characteristic and a classic source of misdiagnosis as ALS. Short MUP duration correlated with all clinical measures in a 50-patient series ✅ PMID:34617994.
• Nerve conduction studies: normal or mild age-related changes — used to exclude neuropathy/motor neuron disease.

10.6 Muscle biopsy (the historical gold standard)

Site selection matters: biopsy an affected but not end-stage muscle (commonly vastus lateralis or biceps; avoid severely atrophic muscle, which yields only fibro-fatty tissue).

Canonical findings: 1. Endomysial inflammatory infiltrate with CD8⁺ T-cell invasion of non-necrotic fibres (the immunological hallmark); 2. Sarcolemmal/sarcoplasmic MHC class I overexpression (immunohistochemistry) — highly sensitive, present even when infiltrate is sparse; 3. Rimmed vacuoles (HP:0003805) on modified Gomori trichrome — specific, insensitive; 4. Mitochondrial pathology: COX-negative fibres (HP:0003688), ragged-red fibres (HP:0003200), SDH-positive/COX-negative fibres on dual staining; 5. Protein aggregates: p62/SQSTM1 (the most practical and sensitive aggregate stain), TDP-43 (cytoplasmic, with nuclear clearance), ubiquitin, amyloid-β (Congo red/crystal violet — technically demanding, poor reproducibility); 6. Increased endomysial connective tissue (HP:0100297).

Important: absence of rimmed vacuoles does not exclude IBM. COX-deficient fibres and p62/TDP-43 immunostaining rescue many vacuole-negative biopsies. In inflammatory myopathy without rimmed vacuoles, COX-deficient fibres were reported 100% sensitive and 73% specific for IBM (paraphrase — locate and verify the primary source).

10.7 Swallow assessment

Videofluoroscopic swallow study / modified barium swallow (MAXO:0035082 barium swallow radiograph procedure) and fibreoptic endoscopic evaluation of swallowing (FEES); manometry to document cricopharyngeal non-relaxation. Speech-language pathologist evaluation: MAXO:0000733.

10.8 Genetic testing

Genetic testing has no role in diagnosing sporadic IBM, and this negative should be stated explicitly. Its role is exclusionary, to rule out mimics with rimmed vacuoles or late-onset selective weakness: - GNE sequencing (GNE myopathy — spares quadriceps, a key clinical discriminator); - VCP (multisystem proteinopathy/IBMPFD — look for Paget disease, FTD, family history); - MYH2, DES, MATR3, SQSTM1, HNRNPA1/A2B1, TIA1 (rimmed-vacuolar myopathies); - DMPK CTG repeat (myotonic dystrophy type 1 — distal weakness, but with myotonia and multisystem features); - GAA (late-onset Pompe disease — a treatable mimic; dried blood spot enzyme assay is the first-line test and should be done in essentially every case); - FKRP, ANO5, CAPN3, DYSF (LGMDs).

Approach: targeted gene panel (limb-girdle/distal/rimmed-vacuolar myopathy panel), escalating to WES/WGS only in atypical or familial cases. CMA, karyotype, FISH, mtDNA testing, and repeat-expansion testing have no routine diagnostic role (with the DMPK exception). GTR/GeneReviews are the relevant resources.

10.9 Omics-based diagnostics

• RNA-seq for TDP-43–dependent cryptic exons is the most promising emerging molecular diagnostic, arising directly from ✅ PMID:35044790. Not yet clinically deployed.
• Proteomics, metabolomics, epigenomics, liquid biopsy: research-only; no validated clinical assay.

10.10 Differential diagnosis

10.11 Screening

No population screening exists or is justified for IBM. No newborn screening, no carrier screening, no cascade screening — there is no Mendelian gene to screen. Explicitly "not applicable."

11. Outcome / Prognosis

11.1 Survival and mortality

Evidence from the Mayo/REP cohort (Rheumatology (Oxford) 2022;61(5):2016, "Survival and associated comorbidities in inclusion body myositis"; 50 IBM patients, 65 IIM controls, 294 population controls):

(Note the 10-year IIM > controls inversion — verify these figures against the primary abstract before curating; the pattern is unusual and may reflect a summarization artefact.)

• Leading cause of death: respiratory failure or pneumonia (44%) — i.e., aspiration secondary to dysphagia is the dominant mortality mechanism. This is the single most actionable prognostic fact in IBM and should anchor the prognosis section.
• The 40-year population-based study concluded that "Patients with sIBM have similar risk of cancer, but slightly shorter life expectancy compared to matched patients without sIBM" ✅ PMID:33879596 (candidate quote — verify).
• Older literature asserting that IBM "does not reduce life expectancy" (e.g. ✅ PMID:25215417) is now superseded by population-based data. Curate the older claim, if at all, with supports: REFUTE or PARTIAL and an explanation — this is exactly the kind of superseded claim the KB should represent explicitly rather than silently drop.

11.2 Morbidity and function

• Median time to wheelchair dependence 10.5 years (range 1–29) ✅ PMID:33879596.
• Progressive loss of ambulation, grip function, and independent feeding.
• Fall-related fractures and head injury.
• Aspiration pneumonia (recurrent), malnutrition, weight loss.
• Respiratory muscle weakness in advanced disease (less prominent than in other myopathies, but present).
• Functional instruments: IBMFRS (primary), MMT-8, 6MWD, HAQ, SF-36, quantitative dynamometry. See ✅ PMID:22588740 for the outcome-measure compendium; ✅ PMID:38522330 for the current trial-readiness consensus.

11.3 Complications

Aspiration pneumonia; respiratory failure; falls and fractures; deep vein thrombosis from immobility; pressure injury; malnutrition; depression and social isolation. Not complications of IBM: cardiomyopathy, interstitial lung disease, malignancy (cancer incidence not increased ✅ PMID:33879596) — important negatives that distinguish IBM from dermatomyositis and the anti-synthetase syndrome.

11.4 Recovery potential

None. No treatment has been shown to halt or reverse progression. Recovery of lost strength does not occur. This should be stated plainly.

11.5 Prognostic factors

Prognostic biomarkers: No validated molecular prognostic biomarker exists. MRI fat fraction and IBMFRS slope are the best current predictors. Honest gap.

12. Treatment

The central fact of IBM therapeutics: there is no disease-modifying therapy and no approved drug. Every immunosuppressive and immunomodulatory agent trialled has failed. Management is supportive and rehabilitative. This is not a curation gap — it is the state of the field, and the KB entry should say so directly.

12.1 Failed / not recommended pharmacotherapy

12.2 Completed and ongoing clinical trials

Curation note on ulviprubart: this is the most mechanistically important trial in IBM history — a direct test of the autoimmune-primary hypothesis with a precision T-cell-depleting agent. Its primary-endpoint failure in the overall population is meaningful negative evidence for the autoimmune-primary model and should be curated as such (supports: PARTIAL or REFUTE against the autoimmune_primary hypothesis group), while the mild-to-moderate subgroup signal is curated as EMERGING with a clear "subgroup analysis, not confirmatory" explanation. Do not present the subgroup finding as efficacy.

Other agents in earlier-phase development or of historical interest: follistatin gene therapy (AAV1-FS344, phase I/II), rapamycin analogues, ABC008 follow-ons, and anti-CD8/anti-senescent-T-cell approaches. clinicaltrials.gov should be queried for the current pipeline and cached via just fetch-reference NCT<...>.

12.3 Supportive and rehabilitative management (the actual standard of care)

12.4 Interventional / surgical management of dysphagia

Reserved for cricopharyngeal dysfunction refractory to conservative measures: - Cricopharyngeal myotomy (no specific MAXO term found via OAK — use MAXO:0000004 surgical procedure or NCIT:C15329 Surgical Procedure with therapeutic_modality: SURGERY); - Endoscopic/balloon dilation of the upper oesophageal sphincter; - Botulinum toxin injection into the cricopharyngeus — therapeutic_modality: SMALL_MOLECULE/protein; therapeutic_agent bindable to a CHEBI/NCIT botulinum toxin term (verify with OAK).

Evidence for all three is uncontrolled case series with variable and often transient benefit; curate with supports: PARTIAL.

12.5 Pharmacogenomics

No IBM-specific pharmacogenomic guidance exists (no CPIC guideline, no FDA PGx biomarker for any IBM-relevant agent). If sirolimus enters practice, CYP3A4/CYP3A5 metabolism and therapeutic drug monitoring become relevant, but this is general sirolimus pharmacology, not IBM-specific. Not available.

12.6 Treatment strategy / algorithm

1. Establish the diagnosis (272nd ENMC criteria; exclude Pompe and other treatable mimics).
2. Do not initiate chronic immunosuppression — it is ineffective and adds steroid myopathy, infection, and osteoporosis risk. Deprescribe if already started.
3. Refer immediately to PT/OT and start a supervised aerobic + resistance program.
4. Screen for dysphagia at every visit and refer to SLP at first symptom; escalate to VFSS → dietary modification → myotomy/dilation/botulinum → PEG.
5. Falls prevention, orthoses, assistive devices proactively.
6. Refer to a clinical trial — this is an explicit standard-of-care recommendation in IBM given the absence of approved therapy.
7. Advance-care planning around respiratory and feeding decisions.
8. Consider peripheral blood flow cytometry for T-LGL given the ~58% overlap ✅ PMID:26920676.

No personalized/genotype-guided treatment approach exists.

13. Prevention

• Primary prevention: None available. Aetiology is unknown; the dominant risk factor (age) and the dominant genetic factor (HLA haplotype) are unmodifiable. No vaccine, no risk-factor modification, no chemoprophylaxis. State this explicitly.
• Secondary prevention (early detection): No population screening. The realistic secondary-prevention target is reducing diagnostic delay — increasing clinician recognition of the quadriceps + finger-flexor + asymmetry pattern so patients are diagnosed years earlier. Given the MUSCLE-trial mild-to-moderate subgroup signal, earlier diagnosis may become therapeutically consequential.
• Tertiary prevention (the substantive, evidence-supported arm):
• Dysphagia surveillance and management to prevent aspiration pneumonia — the highest-value preventive intervention in IBM, given that respiratory failure/pneumonia causes ~44% of deaths.
• Falls-prevention programs, home safety assessment, orthoses, gait aids.
• Maintenance exercise to prevent superimposed disuse atrophy and cardiovascular deconditioning.
• Vaccination against influenza, pneumococcus, COVID-19, and RSV (MAXO:0001017 vaccination) — indicated to reduce respiratory-infection mortality in a population whose leading cause of death is pneumonia. Note this is generic preventive care applied to a high-risk group, not IBM-specific evidence.
• Osteoporosis and fracture prevention in patients with reduced mobility.
• Genetic screening / counselling: Not indicated. IBM is not Mendelian; there is no carrier state, no prenatal testing, no PGD, and no cascade screening. MAXO:0000079 genetic counseling applies only when a hereditary inclusion body myopathy is in the differential — and that is a different disease.
• Public health / environmental interventions: Not applicable.

14. Other Species / Natural Disease

• Taxonomy: Human — Homo sapiens, NCBITaxon:9606.
• Naturally occurring IBM in other species: No established naturally occurring animal homologue of sporadic IBM has been described. An OMIA (Online Mendelian Inheritance in Animals) query should be run to confirm and to cite the absence explicitly.
• Do not conflate IBM with the immune-mediated myopathies that do occur naturally in animals — notably canine masticatory muscle myositis (autoantibodies to type 2M myofibres) and equine immune-mediated myositis — which share the "immune attack on muscle" concept but have no rimmed-vacuole/TDP-43/aggregate pathology and are mechanistically distinct. VBO breed identifiers would apply to those, not to IBM.
• Orthologous genes (NCBI Gene / Alliance of Genome Resources): mouse Nt5c1a, Tardbp, Sqstm1, App, Klrg1, Mstn all have clear orthologues. Mouse lacks a direct HLA-DRB1 orthologue (H2 class II is the functional analogue), which is a fundamental limitation for modelling IBM's dominant genetic risk factor.
• Comparative biology: The degenerative arm has strong evolutionary conservation — TDP-43 proteinopathy, autophagy failure, and mtDNA-deletion accumulation are conserved from invertebrates to humans and are extensively modelled in Drosophila and C. elegans. The inflammatory arm — human-specific HLA restriction, human immunosenescence, and the CMV-driven terminally differentiated T-cell compartment — is poorly conserved, which is precisely why the human-muscle xenograft model was needed.
• Zoonotic potential / cross-species transmission: Not applicable — IBM is not transmissible.

15. Model Organisms

IBM modelling is a genuine HUMAN_MODEL_MISMATCH case rather than a plain knowledge gap: models exist, but no non-xenograft model reproduces both arms of the disease, and the field's most important therapeutic inferences depend on which arm a given model captures. Curate a discussions entry with kind: HUMAN_MODEL_MISMATCH.

15.1 The xenograft model (the current best model)

Britson KA et al., Science Translational Medicine 2022 — ✅ PMID:35044790 — "Loss of TDP-43 function and rimmed vacuoles persist after T cell depletion in a xenograft model of sporadic inclusion body myositis."

• Design: human IBM muscle transplanted into the hindlimb of immunodeficient mice; human myofibres regenerate in situ.
• Recapitulation: "Xenografts from subjects with IBM displayed robust regeneration of human myofibers and recapitulated both inflammatory and degenerative features of the disease" (candidate quote — verify). Specifically: invasion by human oligoclonal CD8⁺ T cells, MHC-I upregulation, rimmed vacuoles, mitochondrial pathology, p62⁺ inclusions, and nuclear clearance with cytoplasmic aggregation of TDP-43 associated with cryptic exon inclusion.
• Significance: the first animal model to recapitulate both the degenerative and inflammatory hallmarks of IBM.
• Key experimental result: after T-cell depletion, TDP-43 loss of function and rimmed vacuoles persisted — the strongest available evidence that the degenerative arm is not merely downstream of the T-cell attack.
• Limitations: requires fresh human IBM muscle (scarce); immunodeficient host lacks a complete immune system; not a genetic model; low throughput; does not model disease initiation.

15.2 Transgenic mouse models

15.3 Non-mammalian and in vitro models

• Drosophila melanogaster and C. elegans TDP-43 and autophagy models — used for the degenerative arm and for genetic modifier screens; no immune arm.
• Human myoblast / myotube cultures, including patient-derived iPSC-derived myotubes — used to study ER stress, MHC-I induction (IFN-γ stimulation), autophagic flux, and aggregate formation. These are the right substrate for CRISPR/RNAi functional-genomics screens (DepMap/GenomeRNAi have no IBM-specific screens).
• Co-culture systems of patient CD8⁺ T cells with autologous myotubes — a promising route to model the cytotoxic synapse in vitro.

15.4 Databases and resources

MGI (mouse), IMSR/JAX (strain availability), Alliance of Genome Resources (orthology), Cellosaurus/ATCC (cell lines), DepMap and GenomeRNAi (screens), and The Myositis Association (TMA) and Cure IBM for funded-project registries and patient-facing trial listings.

15.5 Research applications

The xenograft model is the only system currently suitable for preclinical testing of agents targeting both arms; MHC-I transgenics remain useful for the inflammation-to-ER-stress axis; TDP-43 models for splicing/cryptic-exon biology; iPSC myotubes for high-throughput proteostasis screening.

Appendix A — Verified identifier quick-reference for KB curation

Disease: MONDO:0007827

Genes (HGNC, lowercase prefix per repo convention, all OAK-verified): hgnc:4948 HLA-DRB1 · hgnc:17819 NT5C1A · hgnc:11571 TARDBP · hgnc:11280 SQSTM1 · hgnc:620 APP · hgnc:6380 KLRG1 · hgnc:4223 MSTN · hgnc:11364 STAT3 · hgnc:1323 C4A · hgnc:14673 FYCO1 · (exclusion guards: hgnc:23657 GNE, hgnc:12666 VCP)

Phenotypes (HP, OAK-verified): HP:0003731 Quadriceps muscle weakness · HP:0031177 Finger flexor weakness · HP:0002460 Distal muscle weakness · HP:0009063 Progressive distal muscle weakness · HP:0003376 Steppage gait · HP:0002359 Frequent falls · HP:0009050 Quadriceps muscle atrophy · HP:0002015 Dysphagia · HP:0200136 Oral-pharyngeal dysphagia · HP:0002068 Neuromuscular dysphagia · HP:0003236 Elevated circulating creatine kinase concentration · HP:0008180 Mildly elevated creatine kinase · HP:0003458 EMG: myopathic abnormalities · HP:0003805 Rimmed vacuoles · HP:0003688 Cytochrome C oxidase-negative muscle fibers · HP:0003200 Ragged-red muscle fibers · HP:0100297 Increased endomysial connective tissue · HP:0030057 Autoimmune antibody positivity · HP:0002960 Autoimmunity

Cell types (CL, OAK-verified): CL:0000794 CD8-positive, alpha-beta cytotoxic T cell · CL:0008002 skeletal muscle fiber · CL:0000235 macrophage · CL:0000786 plasma cell · CL:0000594 skeletal muscle satellite cell · CL:0008016 activated skeletal muscle satellite cell

Biological processes (GO, OAK-verified): GO:0001913 T cell mediated cytotoxicity · GO:0043316 cytotoxic T cell degranulation · GO:0002484 antigen processing and presentation of endogenous peptide antigen via MHC class I via ER pathway · GO:0034976 response to endoplasmic reticulum stress · GO:0006914 autophagy · GO:0061684 chaperone-mediated autophagy · GO:0000422 autophagy of mitochondrion · GO:0070841 inclusion body assembly · GO:0043161 proteasome-mediated ubiquitin-dependent protein catabolic process · GO:0042026 protein refolding · GO:0043403 skeletal muscle tissue regeneration

Anatomy (UBERON, OAK-verified): UBERON:0014892 skeletal muscle organ, vertebrate · UBERON:0001377 quadriceps femoris · UBERON:0004498 skeletal muscle tissue of quadriceps femoris · UBERON:0001523 flexor digitorum profundus · UBERON:0001385 tibialis anterior · UBERON:0004499 skeletal muscle tissue of tibialis anterior · UBERON:0000933 chordate pharyngeal muscle

Treatments (MAXO, OAK-verified): MAXO:0000011 physical therapy · MAXO:0000065 aerobic exercise therapy · MAXO:0000465 aquatic exercise therapy · MAXO:0001351 occupational therapy · MAXO:0000733 speech-language pathologist evaluation · MAXO:0000930 speech therapy · MAXO:0001346 gastrostomy · MAXO:0035082 barium swallow radiograph procedure · MAXO:0001480 immunoglobulin infusion therapy · MAXO:0000950 supportive care · MAXO:0000088 dietary intervention · MAXO:0001017 vaccination

Clinical trial: clinicaltrials:NCT05721573 (MUSCLE / ulviprubart)

Appendix B — PMID verification status

✅ Confirmed by direct NCBI E-utilities lookup (title/journal/year checked during this research):

⚠️ Cited but NOT verified by direct lookup — resolve and confirm before use: 30001928 (cN1A sensitivity, 40-patient single-centre series) · 31024569 (Anti-NT5c1A autoantibodies as biomarkers in IBM) · 30136253 (IBM: Update on Pathogenesis and Treatment) · 36237625 (IBM: update on diagnostic and therapeutic landscape) · Pluk H et al. 2013 (independent cN1A discovery, Ann Neurol) · Griggs RC et al. 1995 (original criteria) · Mayo "Survival and associated comorbidities in IBM," Rheumatology (Oxford) 2022;61(5):2016 · PMC8151681 (anti-cN1A and dysphagia severity) · PMC10098158 (expanded CD8⁺ LGLs in IBM, Front Immunol 2023) · Nagaraju et al. MHC-I transgenic mouse

Non-peer-reviewed sources (curate as evidence_source: OTHER with explicit caveats): Abcuro corporate press releases on the MUSCLE trial (24 Feb 2026 topline; 26 Mar 2026 GCOM presentation).

Appendix C — Recommended discussions entries (knowledge gaps and model mismatches)

1. KNOWLEDGE_GAP — Why quadriceps and flexor digitorum profundus? No mechanism explains IBM's stereotyped, near-pathognomonic muscle selectivity. Proposed experiments: comparative single-nucleus transcriptomics/proteomics of affected vs spared muscles from the same patient; fibre-type composition and mitochondrial-load analysis.
2. KNOWLEDGE_GAP — Inflammation-first vs degeneration-first. Attach to the causal edges between the T-cell-cytotoxicity node and the proteostasis/TDP-43 nodes; link to the two mechanistic_hypotheses groups.
3. HUMAN_MODEL_MISMATCH — No genetic animal model reproduces both arms. MHC-I transgenics give degeneration without vacuoles/TDP-43; TDP-43 and APP models give degeneration without inflammation; mouse lacks an HLA-DRB1 orthologue, so the dominant human genetic risk factor cannot be modelled. The xenograft model is the only dual-arm system and depends on scarce fresh human tissue.
4. KNOWLEDGE_GAP — Is anti-cN1A pathogenic or an epiphenomenon? No passive-transfer or in vivo pathogenicity evidence exists; cN1A is intracellular, and the antibody is not disease-specific.
5. KNOWLEDGE_GAP — Does the T-LGL clone cause IBM, or does IBM drive clonal expansion? The 58% overlap with clonal LGL populations and STAT3 GOF mutations is unexplained directionally.
6. KNOWLEDGE_GAP — Why does IBM not respond to any immunosuppression, if it is autoimmune? The MUSCLE trial's primary-endpoint failure sharpens rather than resolves this.

Sources: - Inclusion body myositis: clinical features and pathogenesis — Nature Reviews Rheumatology - Safety and efficacy of arimoclomol for inclusion body myositis — Lancet Neurology - Safety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT) — PubMed - Epidemiology and Natural History of Inclusion Body Myositis: A 40-Year Population-Based Study — PubMed - Epidemiology, Survival, and Clinical Characteristics of Inclusion Body Myositis — Annals of Neurology - Incidence and prevalence of inflammatory myopathies: a systematic review — Rheumatology - Cytosolic 5'-nucleotidase 1A autoimmunity in sporadic inclusion body myositis — PubMed - Anti-NT5c1A Autoantibodies as Biomarkers in Inclusion Body Myositis — PubMed - High-resolution HLA genotyping in inclusion body myositis refines 8.1 ancestral haplotype association to DRB1*03:01:01 — PubMed - Immune-Array Analysis in Sporadic Inclusion Body Myositis Reveals HLA-DRB1 Amino Acid Heterogeneity — Arthritis & Rheumatology - Association of inclusion body myositis with T cell large granular lymphocytic leukaemia — Brain - Loss of TDP-43 function and rimmed vacuoles persist after T cell depletion in a xenograft model of sporadic inclusion body myositis — Science Translational Medicine - 272nd ENMC international workshop: revision of the ENMC 2013 diagnostic criteria for inclusion body myositis — Neuromuscular Disorders - 188th ENMC International Workshop: Inclusion Body Myositis — Neuromuscular Disorders - Abcuro Announces Topline Results from the MUSCLE Study of Ulviprubart in Patients with Inclusion Body Myositis - Abcuro Presents Results from Phase 2/3 MUSCLE Study of Ulviprubart at GCOM 2026 — BioSpace - Mitochondrial defects in sporadic inclusion body myositis—causes and consequences — Frontiers in Cell and Developmental Biology - Uncovering the significance of expanded CD8+ large granular lymphocytes in inclusion body myositis — Frontiers in Immunology - Inclusion body myositis and immunosenescence: current evidence and future perspectives — PMC - Sensitivity and clinical utility of the anti-cN1A antibody test in sporadic inclusion body myositis — PubMed