Activation of human synovial mast cells from rheumatoid arthritis or osteoarthritis patients in response to aggregated IgG through Fc? receptor I and Fc? receptor II

Objective Substantial evidence suggests that human synovial mast cells (MCs) are involved in the pathogenesis of rheumatoid arthritis (RA). A plausible pathway for the activation of synovial MCs is through IgG receptors, given the prevalence of circulating IgG isotype autoantibodies and synovial immune complexes in patients with RA. However, IgG receptor expression on human synovial MCs remains uncharacterized. The aim of this study was to identify which IgG receptor(s) on synovial MCs are responsible for MC activation in immune complexes. Methods Synovial tissue specimens were obtained from patients with RA or patients with osteoarthritis (OA) who were undergoing joint replacement surgery, and synovial MCs were enzymatically dispersed. Cultured synovium-derived MCs were generated by culturing synovial cells with stem cell factor, and receptor expression was analyzed using fluorescence-activated cell sorting. Mediators released from MCs were measured using enzyme immunoassays or enzyme-linked immunosorbent assays. Results Primary synovial MCs and cultured synovium-derived MCs obtained from both patients with RA and patients with OA expressed Fce receptor I (FceRI), Fc?RI, and Fc?RII but not Fc?RIII. Cultured synovium-derived MCs induced degranulation and the production of prostaglandin D2 and tumor necrosis factor a (TNFa) through Fc?RI. The aggregation of Fc?RII caused histamine release from cultured MCs but not from primary MCs. Histamine release induced by aggregated IgG was significantly inhibited by neutralizing anti-Fc?RI monoclonal antibody and anti-Fc?RII monoclonal antibody. Conclusion With regard to the FcR expression profile, synovial MCs from patients with RA and patients with OA were similar. Fc?RI was responsible for producing abundant TNFa from synovial MCs in response to aggregated IgG. Immune complexes may activate synovial MCs through Fc?RI and Fc?RII.