TH17 cells converted into exTH17 cells sustain rheumatoid-like IL-17–independent inflammatory arthritis

T helper 17 (TH17) cells are found in the periphery and synovium of patients with rheumatoid arthritis (RA); however, IL-17–targeted interventions have limited efficacy in established RA. Inflammation can induce TH17 cell transdifferentiation into IL-17–negative exTH17 cells, but the role of exTH17 cells in arthritis is unknown. We performed TH17 cell lineage tracing in the SKG mouse model of RA. In arthritic mice, synovial TH17 cells transdifferentiate into CD44+ exTH17 cells, which are more arthritogenic and sustain inflammation that is IL-17 independent. The exTH17 cell gene signature includes up-regulation of CD44 and sphingosine-1-phosphate receptor 4 (S1PR4) and correlates with the profile of human RA synovial CD4+ T cells. We demonstrate that cross-talk between TH17 cells and fibroblast-like synoviocytes (FLSs) via S1P promotes TH17-exTH17 cell conversion. CD44 is necessary for exTH17 cell–mediated arthritis. Our study suggests that FLS expansion during RA progression promotes TH17-exTH17 cell conversion. These results could potentially enable RA precision therapy.