Low- affinity FcγR interactions can decide the fate of novel human IgG-sensitised red blood cells and platelets

G1nab is a mutant human IgG1 constant region with a lower ability to interact with FcR than the natural IgG constant regions. Radiolabelled RBCs and platelets sensitised with specific G1nab Abs were cleared more slowly from human circulation than IgG1-sensitised counterparts. However, non-destructive splenic retention of G1nab-coated RBCs required investigation and plasma radioactivities now suggest this also occurred for platelets sensitised with an IgG1/G1nab mixture. In vitro assays with human cells showed that G1nab-sensitised RBCs did not cause FcRI-mediated monocyte activation, FcRIIIa-mediated antibody-dependent cell-mediated cytotoxicity (ADCC) or macrophage phagocytosis although they did adhere to macrophages. Thus, FcRII was implicated in the adhesion despite the nab mutation reducing the already low-affinity binding to this receptor class. Additional contacts via P-selectin enhance the interaction of sensitised platelets with monocytes and this system provided evidence of FcRII-dependent activation by G1nab. These results emphasise the physiological relevance of low-affinity interactions: It appears that FcRII interactions of G1nab allowed splenic retention of G1nab-coated RBCs with inhibitory FcRIIb binding preventing RBC destruction and that FcRIIb engagement by G1nab on IgG1/G1nab-sensitised platelets overcame activation by IgG1. Considering therapeutic blocking Abs, G1nab offers lower FcR binding and a greater bias towards inhibition than IgG2 and IgG4 constant regions.