Selective elimination of synovial inflammatory macrophages in rheumatoid arthritis by an Fcγ receptor I-directed immunotoxin

Objective. To determine whether monocyte/macrophages from rheumatoid arthritis (RA) patients can be selectively eliminated by a toxin-conjugated antibody CD64-ricin A (CD64-RiA) directed toward the highaffinity receptor for IgG (FcγRI), exploiting the capacity of FcγRI to efficiently endocytose antibody which it has bound. Methods. Mononuclear cells from peripheral blood (PB) and synovial fluid (SF) obtained from RA patients were cultured in the presence of CD64-RiA. Cell death of monocyte/macrophages was measured by phenotypic changes (light-scatter patterns and CD14 and FcγRI expression) and apoptosis (nuclear DNA fragmentation). We then tested whether CD64-RiAinduced cell death of macrophages affected their capacity to stimulate antigen-induced lymphocyte proliferation and to secrete cytokines. Additionally, the capacity of CD64-RiA to inhibit proinflammatory activity and cartilage degradation by RA synovial tissue explants was evaluated. Results. Inflammatory macrophages from RA SF expressed elevated levels of FcγRI and were selectively eliminated by CD64-RiA via apoptotic cell death. Monocyte/macrophages from RA PB, which had lower levels of FcγRI expression, were much less affected. Induction of SF macrophage apoptosis was associated with efficient inhibition of antigen-induced lymphocyte proliferation and a reduction in tumor necrosis factor α (TNFα) release. Consistent with these effects on SF macrophages, CD64-RiA also inhibited TNFα production, interleukin-1β production, and cartilagedegrading activity of RA synovial tissue explants. Conclusion. Together, these data underscore the crucial role of synovial macrophages in RA joint inflammation and indicate that selective elimination of these cells through FcγRI-directed immunotoxins could be a novel approach to the treatment of RA.