Inflammation reprograms fibro-adipogenic progenitors to sustain immunopathogenic niches in myositis.

Idiopathic inflammatory myopathies (IIMs) are autoimmune disorders defined by persistent muscle inflammation, fibrosis, and frequent resistance to current therapies. However, the mechanisms perpetuating disease activity despite immunosuppressive treatment remain elusive. Here, we describe a novel role for tissue-resident stromal cells, specifically fibro-adipogenic progenitors (FAPs), in sustaining skeletal muscle inflammation. Utilizing single-nucleus and spatial transcriptomics in 24 IIM patients and six non-diseased controls, we describe how FAPs adapt to their tissue context, favoring T-cell-centric programs in T-cell environments and myeloid programs in macrophage environments. At the spatial level, FAPs form inflammatory niches by co-localizing with muscle stem cells and activated macrophages, positioning them to participate in cell-to-cell communication with both immune and muscle cells. Trajectory and ligand-receptor analyses suggest a dual-input mechanism whereby infiltrating immune cells (via TGF-β) and myofibers (via epidermal growth factor (EGF)) converge on the AP-1 transcription factor to drive FAP differentiation toward a pro-inflammatory and pro-fibrotic phenotype. Mechanistically, exposure of primary human FAPs to TGF-β and EGF induces a primed state by altering the accessibility to AP-1 regulatory elements. Together, our findings reveal a previously unrecognized role of tissue-resident stromal cells in IIM, highlighting microenvironmental cross-talk centered on FAPs as a promising and actionable therapeutic target.