Although interleukin‐17 (IL‐17)–producing γ/δ T cells were reported to play pathogenic roles in collagen‐induced arthritis (CIA), their characteristics remain unknown. The aim of this study was to clarify whether γ/δ T cells or CD4+ T cells are the predominant IL‐17–producing cells, and to determine what stimulates γ/δ T cells to secret IL‐17 in mice with CIA. The involvement of IL‐17–producing γ/δ T cells in SKG mice with autoimmune arthritis and patients with rheumatoid arthritis (RA) was also investigated.

IL‐17–producing cells in the affected joints of mice with CIA were counted by intracellular cytokine staining during 6 distinct disease phases, and these cells were stimulated with various combinations of cytokines or specific antigens to determine the signaling requirements. Similar studies were performed using SKG mice with arthritis and patients with RA.

Gamma/delta T cells were the predominant population in IL‐17–producing cells in the swollen joints of mice with CIA, and the absolute numbers of these cells increased in parallel with disease activity. IL‐17–producing γ/δ T cells expressed CC chemokine receptor 6, were maintained by IL‐23 but not by type II collagen in vitro, and were induced antigen independently in vivo. Furthermore, IL‐17 production by γ/δ T cells was induced by IL‐1β plus IL‐23 independently of T cell receptor. In contrast to what was observed in mice with CIA, IL‐17–producing γ/δ T cells were nearly absent in the affected joints of SKG mice and patients with RA, and Th1 cells were predominant in the joints of patients with RA.

Gamma/delta T cells were antigen independently stimulated by inflammation at affected joints and produced enhanced amounts of IL‐17 to exacerbate arthritis in mice with CIA but not in SKG mice with arthritis or patients with RA.