Arthritis: where are the T cells?

T-helper (Th) lymphocytes contribute to arthritis pathogenesis by helping B cells to produce antibodies, by producing cytokines that activate effector cells involved in the destruction of cartilage and bone, and by contributing to osteoclast differentiation. There are murine models of arthritis, most notably collagen- and proteoglycan-induced arthritis, in which arthritis depends on T-cell recognition of antigens that are expressed in the joints. In spite of this, we still do not know the antigens recognised by arthritogenic Th cells in humans. Moreover, current evidence for Th cells exerting arthritogenic effector functions within the joints is only indirect.