C D69 is induced after activation of leukocytes at inflammatory sites, but its physiological role during inflammation remains unknown. We explored the role of CD69 in autoimmune reactivity by analyzing a model of collagen-induced arthritis (CIA) in WT and CD69-deficient mice. CD69^–/– mice showed higher incidence and severity of CIA, with exacerbated T and B cell immune responses to type II collagen. Levels of TGF-β1 and TGF-β2, which act as protective agents in CIA, were reduced in CD69^–/– mice inflammatory foci, correlating with the increase in the proinflammatory cytokines IL-1β and RANTES. Local injection of blocking anti–TGF-β antibodies increased CIA severity and proinflammatory cytokine mRNA levels in CD69^+/+ but not in CD69^–/– mice. Moreover, in vitro engagement of CD69 induced total and active TGF-β1 production in Concanavalin A–activated splenocyte subsets, mouse and human synovial leukocytes, and Jurkat stable transfectants of human CD69 but not in the parental CD69 negative cell line. Our results show that CD69 is a negative modulator of autoimmune reactivity and inflammation through the synthesis of TGF-β, a cytokine that in turn downregulates the production of various proinflammatory mediators.