Macrophages contribute to joint inflammation in RA by a number of functions. The typeI IgG receptor (FcγRI) has been shown to be upregulated on inflammatory macrophages and can very efficiently endocytose FcγRI-targeted antigens. Based on its unique function and distribution it was tried to selectively eliminate inflammatory macrophages by a FcγRI-directed humanized antibody (H22) conjugated to the toxin Ricin A (anti-FcγRI-RiA).
FcγRI expression (CD64) was determined on RA monocytes/ macrophages (mo/mac's) and granulocytes from peripheral blood (PB) and synovial fluid (SF). Anti-FcγRI-RiA was tested on mo/mac's from PB and SF of 12 RA patients. Cell death of mo/mac's (vs. lymphocytes) was assessed by morphological changes, changes in CD14 expression and nuclear DNA fragmentation (measuring apoptosis) after 24 hours. The anti-inflammatory effect in vitro was tested by the effect of anti-FcγRI-RiA on TNFα production, antigen-induced Th1-mediated proliferation and T-cell survival in the context of SFMC. In addition the effect on cytokine production by synovial tissue explants was studied (n = 6).
FcγRI was exclusively expressed on mo/mac's and was higher in SF than PB (MFI 232 vs. 59, P < 0.01). Treatment with anti-FcγRI-RiA reduced high FcγRI-expressing mo/mac's and was associated and was associated by selective cell death of mo/mac's (death of lymphocytes was not observed after 24 hours). Immunotoxin-induced cell death was higher in SF than in PB (15% in PB vs. 61% in SF, P < 0.01). High expression of FcγRI correlated with increased capacity of anti-FcγRI to induce mo/mac cell death. Induction of macrophage cell death was mediated via apoptosis as measured by nuclear DNA fragmentation. After prolonged culture (3–5 days) macrophage death was associated with significant reductions in TNFα production, Th1-induced lymphocyte proliferation and T-cell survival in the context of SFMC (42%, 63%, 72%, respectively). Furthermore, IL-1 production by RA synovial tissue explants was inhibited (63%, P < 0.01). Macrophages from RA patients can be selectively eliminated by FcγRI-directed immunotoxins. Considering the important role of these cells in RA, this type of elimination may be a novel concept in the treatment of RA.