Direct demonstration of binding of aggregated mouse IgG1 to the 40-kDa Fc receptor for IgG (FcγRII) in both high and low responders

Abstract Several directly fluorochrome-conjugated murine monoclonal antibodies (mAb) of the IgG1 subclass and directed against B or T cell antigens were found to bind to monocytes via the 40-kDa Fc receptor for IgG (FcγRII). As expected from the established polymorphism of FcγRII, strong staining was observed in about 75% of individuals. In the remaining 25% staining was clearly weaker, but could be definitely demonstrated with a mAb against the B cell-specific CD19 differentiation antigen. Specificity of binding to FcγRII was confirmed by the ability to block the binding of the CD19 mAb by pre-incubation with aggregated IgG1 or with mAb against FcγRII. The extent of T cell proliferation induced with a CD3 mAb of the IgG1 isotype (a-Leu 4), which is dependent on the interaction of monocyte FcγRII with the Fc portion of the CD3 mAb, exactly correlated with the amount of binding to FcγRII in all individuals. Proliferation was dose-dependent for both high and low responders; cells of low responders did not proliferate at concentrations below 16 ng/ml of mAb, whereas there was a small but unequivocal proliferation at higher concentrations. These results confirm that monocytes from previously characterized “non-responders” are able to bind aggregated murine IgG1 via FcγRII. They also demonstrate that directly labeled mAb can cause extensive nonspecific staining which may not be excluded by the use of control antibodies of the same isotype.