TLR2 deletion promotes arthritis through reduction of IL-10

TLR2 signaling modulates K/BxN serum transfer arthritis by enhancing the expression of immune complex-induced IL-10. RA is a chronic inflammatory disease characterized by the persistent expression of inflammatory cytokines from macrophages, which may be mediated, in part, through TLR2 signaling. Earlier studies demonstrate a role for TLR2 signaling in dampening the arthritis in IL-1Ra−/− mice, which was mediated through T cells. This study was performed to determine whether TLR2 signaling plays a role in the pathogenesis of T cell-independent arthritis triggered by transferring serum from K/BxN mice. We documented more severe arthritis in Tlr2−/− mice compared with WT controls. The Tlr2−/− mice also demonstrated increased inflammation, erosion, pannus formation, and osteoclastogenesis, as well as increased IL-1β and decreased IL-10 within the joints. In vitro bone marrow-differentiated macrophages expressed comparable levels of activating and inhibitory FcγRs, however when stimulated with immune complexes, the Tlr2−/− macrophages expressed decreased IL-10 and reduced activation of Akt and ERK. Our findings indicate that Tlr2−/− promotes the effector phase of arthritis through decreased IL-10 by macrophages, which is important, not only as an anti-inflammatory cytokine but also in restraining the differentiation and activation of osteoclasts.