CD4+ tissue resident memory Th17 cells drive IL-17A-mediated joint pathology in Spondyloarthritis

Interleukin (IL)-17A plays a central role in driving joint pathology in Spondyloarthritis (SpA). In this study, synovial tissues from patients with Axial SpA and Psoriatic arthritis were analyzed using single-cell RNA sequencing and spatial RNA profiling to pinpoint the cellular source of IL-17A. The results revealed that IL-17A expression is exclusively localized to CD4⁺CXCR6⁺ tissue resident memory Th17 (TRM17) cells, which interact with activated CLEC10A⁺ dendritic cells within the joint. These interactions coincide with an enhanced IL-17A response signature in both sublining and lining fibroblasts. Furthermore, in vitro-generated TRM17-like cells from blood memory CD4⁺ T cells recapitulated the in situ characteristics by producing IL-17A in response to T cell receptor stimulation, while IL-23 selectively amplified TCR-mediated IL-17F and IFN-γ production. Importantly, perturbation of the epigenetic regulator BRD1 impaired the generation of TRM17-like cells. These findings underscore the predominant role of TRM17 cells as the source of IL-17A in SpA and suggest that targeting BRD1 or depleting TRM17 cells may offer promising therapeutic strategies for achieving long-term disease remission.