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Interleukin‐17: the missing link between T‐cell accumulation and effector cell actions in rheumatoid arthritis?

The prominence of T cells and monocyte/macrophages in rheumatoid synovium suggests T cells may localize and amplify the effector functions of monocyte/macrophages in rheumatoid disease. However, while T cells are abundant in rheumatoid joints, classic T‐cell derived cytokines are scarce, especially when compared to the levels of monokines IL‐1β and TNF‐α. For this reason, it has been speculated that monocyte/macrophages may act independently of T cells in rheumatoid disease and that the role of T cells may be more or less irrelevant to core disease mechanisms. The question of T‐cell influence requires re‐evaluation in light of the characterization of IL‐17, a T‐cell derived cytokine that is abundant in rheumatoid synovium and synovial fluid. IL‐17 has a number of pro‐inflammatory effects, both directly and through amplification of the effects of IL‐1β and TNF‐α. IL‐17 is able to induce expression of pro‐inflammatory cytokines and stimulate release of eicosanoids by monocytes and synoviocytes. Furthermore, IL‐17 has been implicated in the pathogenesis of inflammatory bone and joint damage through induction of matrix metalloproteinases and osteoclasts, as well as inhibition of proteoglycan synthesis. In animal models of arthritis, intra‐articular injection of IL‐17 results in joint inflammation and damage. The recognition of IL‐17 as a pro‐inflammatory T cell derived cytokine, and its abundance within rheumatoid joints, provides the strongest candidate mechanism to date through which T cells can capture and localize macrophage effector functions in rheumatoid arthritis. As such, IL‐17 warrants consideration for its potential as a therapeutic target in rheumatoid arthritis.