CD32 Antibodies Protect FCGR2A Mice from IGG-mediated Hypersensitivity Reactions and Thrombosis

Abstract

Background: CD32a activation by IgG immune complexes (IC) mediates thrombosis and shock in CD32A transgenic (Tg) mice. FCGR2A mice carry the CD32A transgene and are immune-competent, whereas endogenous IgG receptor (FcgR) knockout mice crossed with CD32A Tg mice are selectively immunodeficient. Hypersensitivity Reactions (HR) to IgG are mediated by FcgRs such as CD32a and are Type II (HR-T2; cell-bound IgG) or Type III (HR-T3; IgG IC). Certain IgGs induce in CD32A mice either HR-T2 reactions and thrombocytopenia without thrombosis, or HR-T3 reactions with concomitant thrombosis. CD32a monoclonal antibody (mAb) blockade, such as by IV.3, could prevent such reactions, but induces anaphylaxis in immunodeficient CD32A Tg mice, suggesting mAb CD32a targeting is not feasible. Aims: (1) to determine whether immune-competent CD32A Tg mice react to CD32a mAbs as do immunodeficient CD32A Tg mice and whether such reactions mechanistically are HR-T2 or T3; (2) to test the effect of CD32a blockade by mAbs on IgG IC-induced thrombotic shock. Methods: Various CD32a mAbs were injected into FCGR2A mice. Mice were challenged with IgGs that induce thrombocytopenia with or without thrombotic shock. Platelets were counted before and after CD32a mAb injection or exposure to active IgGs. Animals were observed for hypothermia and shock symptoms. Lung thrombosis was assessed histologically. Results: CD32a mAbs induced thrombocytopenia independently of and concomitantly with HR-T2 (but not T3) reactions without thrombosis in FCGR2A mice. Such reactions were not observed with corresponding aglycosyl CD32a mAbs, which also completely prevented IgG IC-induced thrombosis. Conclusion: IgG-induced thrombocytopenic, thrombotic, and hypersensitivity reactions can be differentiated in FCGR2A mice. CD32 mAbs cause HR-T2 without thrombosis which is avoided by deglycosylation. Whole aglycosyl CD32a mAbs can thus be safely infused into CD32A Tg mice and completely prevent HR-T2 and T3, and thrombosis.