Requirement of local synovial IL-17 receptor signaling in the progression of chronic synovitis and bone erosion in arthritis

IL-17 is a proinflammatory cytokine that is suspected to be involved in development of rheumatoid arthritis (RA). However, the role of IL-17–IL-17 receptor signaling in the effector phase of arthritis, including requirement of this signaling pathway in synoviocytes, is still not fully elucidated. Here we demonstrate, using IL-17 receptor deficient (IL-17R^-/-) mice, a requirement for IL-17R signaling in immune reactivity and progression of synovitis and bone erosion in a T cell mediated mBSA arthritis model and chronic streptococcal cell wall arthritis. Of great interest, chimeric mice of host wild-type (wt) and donor IL-17R^-/- bone marrow (BM) cells developed chronic synovitis similar to that in wt/wt chimeras. In contrast, chimeric mice of host IL-17R^-/- and donor wt BM cells were protected from progressive synovitis similar to IL-17R^-/-/ IL-17R^-/- chimeras, suggesting a crucial role of resident synovial cells. Weakened mobilization of neutrophils into the joint and downregulation of synovial mRNA expression for leukocyte specific chemokines and selectins, and MMP-1 was found in IL-17R^-/- mice. These data suggest that, in addition to the role played by IL-17–IL-17R signaling in development of immunity, synovial IL-17–IL-17R signaling plays a critical role in the effector phase of arthritis. Activation of IL-17R on fibroblast-like synoviocytes appears pivotal.