Depletion of synovial macrophages in rheumatoid arthritis by an anti-FcγRI-calicheamicin immunoconjugate

Background: Monocytes/macrophages have an important and versatile role in joint inflammation and destruction in rheumatoid arthritis (RA). Objective: To determine the efficiency of monocyte/macrophage elimination by a new drug conjugated antibody (CD64-calicheamicin (CD64-CaMi)) directed to the high affinity receptor for IgG (FcγRI). Methods: Mononuclear cells from peripheral blood and synovial fluid of patients with RA were cultured in the presence of CD64-CaMi. Cell death of monocytes/macrophages was measured by analysis of phenotypic changes (light scatter patterns, CD14 expression, and FcγRI expression) and nuclear DNA fragmentation. The selectivity of CD64-CaMi was checked by using FcγRI deficient and FcγRI transfected cell lines. In addition, the indirect effect of CD64-CaMi-induced macrophage cell death on arthritogenic T(h1) cell activity was determined. Results: Inflammatory macrophages from RA synovial fluid, expressing increased FcγRI levels, were efficiently killed by CD64-CaMi through induction of DNA fragmentation. CD64-CaMi-induced cell death of monocytes/macrophages from peripheral blood of patients with RA proved less efficient. Induction of synovial macrophage death by CD64-CaMi was accompanied by efficient inhibition of proinflammatory T(h1) cytokine production. Conclusion: Together, the presented data suggest that elimination of macrophages through a new FcγRI directed CD64-CaMi is feasible. Because monocytes from peripheral blood are also eliminated by this immunoconjugate, additional experimental studies should validate its potential for local (intra-articular) application in the treatment of RA.