From psoriatic plaque to synovium: decoding the skin-joint axis in psoriatic arthritis.

Psoriatic arthritis (PsA) develops in up to 30% of individuals with psoriasis, but the mechanisms that drive progression from skin-limited disease to musculoskeletal disease remain incompletely understood. Emerging evidence supports a functional skin-joint axis in which psoriatic plaques function not only as sites of local inflammation but also as sources of immune cells capable of shaping musculoskeletal pathology. Myeloid progenitors that reside in the inflamed skin can migrate to synovial compartments; however, cell trafficking alone is insufficient to induce arthritis, as the fate of these cells is dictated by the stromal microenvironment of the musculoskeletal niche. Data from single-cell RNA sequencing, imaging mass cytometry and mitochondrial DNA lineage tracing now provide direct evidence that skin-derived myeloid precursors populate synovial tissue in people with early PsA. In parallel, T cell receptor analyses indicate that clonally related T cells are shared between psoriatic skin and inflamed joints, indicating that skin-derived T cells migrate between tissues. Together, these findings fuel a model in which PsA emerges through the convergence of high-risk skin lesions, a systemic milieu permissive to immune cell trafficking and a receptive joint stromal niche, with implications for biomarker discovery, risk stratification and disease interception.