Interleukin 17-expressing Innate Synovial Cells Drive K/BxN Serum-induced Arthritis

K/BxN serum can induce arthritis in normal mice because of abundant autoantibodies that trigger an innate inflammatory response in joints. To determine whether IL-17 is involved in the pathogenesis of serum-induced arthritis, we injected wild-type and $IL-17^{-/-}$ mice with K/BxN serum and evaluated them for signs of arthritis. Unlike wild-type mice, $IL-17^{-/-}$ mice did not show any signs of arthritis. IL-17 was produced predominantly by $CD3^-CD4^-gdTCR^-NK1.1^-Sca1^{int}Thy1^{hi}$ cells residing in the inflamed synovial tissue. When synovial cells extracted from normal joints were stimulated with IL-23 or autoantibody-containing immune complexes, a substantial fraction of $Sca1^{int}Thy1^{hi}$ cells produced IL-17. Thus, we have identified a novel population of IL-17-producing innate synovial cells that play a crucial role in the development of K/BxN serum-induced arthritis.