A fascinating new study reframes idiopathic inflammatory myopathies (IIMs): the key drivers of chronic muscle inflammation may not be immune cells alone, but tissue-resident fibro-adipogenic progenitors (FAPs).
Using single-nucleus RNA-seq, spatial transcriptomics, ATAC-seq, and primary human FAP cultures, researchers analyzed muscle biopsies from anti-synthetase syndrome (ASYS), inclusion body myositis (IBM), and immune-mediated necrotizing myopathy (IMNM). They found that FAPs adopt disease-specific inflammatory phenotypes that mirror the dominant immune environment.
Key findings:
🔹 IBM FAPs acquired T-cell–oriented programs, expressing mediators such as IL7 and CCL13.
🔹 IMNM FAPs preferentially engaged macrophage-associated inflammatory pathways.
🔹 ASYS FAPs displayed humoral immunity signatures and elevated IL6 expression.
🔹 Across all IIM subtypes, FAPs lost homeostatic markers (e.g., COL15A1) and shifted toward pro-inflammatory and pro-fibrotic states.
Trajectory analysis revealed two major FAP fates:
Homeostatic FAPs supporting muscle structure and regeneration.
Pro-inflammatory FAPs characterized by CXCL1, CCL2, IL18, LIF, COL1A1, COL1A2, LOX, and extracellular matrix remodeling programs.
Importantly, the proportion of pro-inflammatory FAPs increased with disease duration, suggesting progressive stromal reprogramming during chronic inflammation.
Mechanistically, the study identifies a dual-input signaling axis:
• Immune cells provide TGF-β signals.
• Damaged myofibers provide EGF signals.
Both converge on AP-1 transcription factor activity (JUN/FOS family), driving chromatin remodeling and establishment of a pathogenic FAP state. ATAC-seq demonstrated increased AP-1 accessibility after TGF-β EGF stimulation, while AP-1 inhibition reduced inflammatory cytokine induction.
Spatial transcriptomics further showed that FAPs form inflammatory niches by co-localizing with macrophages, muscle stem cells, and—in IBM—T cells, positioning them as organizers of local immune microenvironments rather than passive bystanders.
The broader implication is significant: chronic autoimmune muscle disease may involve a form of "stromal memory" or tissue priming, analogous to pathogenic fibroblasts in rheumatoid arthritis. If so, targeting FAP reprogramming, AP-1 signaling, BET proteins, or TGF-β pathways could complement conventional immunosuppression and potentially address treatment-refractory disease.
Reference
Nelke C et al. Inflammation reprograms fibro-adipogenic progenitors to sustain immunopathogenic niches in myositis. Cell Death & Disease (2026). DOI: 10.1038/s41419-026-08966-w.