Demonstration of the existence of two distinct Fc receptors for IgG isotypes on guinea-pig macrophages by the use of monoclonal antibodies.

As reported in a previous paper by the authors (J. Biochem. 99, 227-235, 1986), the Fab' of a monoclonal antibody, VIA2 IgG1, prepared by fusion of splenic cells of a mouse immunized with guinea-pig peritoneal macrophages with a myeloma cells line, completely inhibits the binding of ovalbumin (OA)-complexed IgG1 antibody to macrophages, but only partially the binding of OA-complexed IgG2 antibody. Based on these results, it was proposed that the cells have at least two types of Fc receptor (FcR) for homologous IgG isotypes: FcR2 for IgG2 and FcR1.2 for both IgG2 and IgG1, and also that VIA2 IgG1 is anti-FcR1.2 antibody. Thereafter, complete inhibition of the binding of OA-complexed IgG2 antibody to macrophages occurred when the Fab' of another monoclonal antibody, VIIA1 IgG1 was added to the Fab' of VIA2 IgG1, whereas the former did not affect the binding of OA-complexed IgG1 antibody. This effect of the Fab' of VIIA1 IgG1 indicates that VIIA1 IgG1 is a monoclonal antibody capable of selectively blocking the binding of OA-complexed IgG2 antibody to FcR2. When the antigen of VIIA1 IgG1 was isolated by affinity chromatography on the F(ab')2 of the antibody coupled to Sepharose, it gave a single band with a mol. wt of 52,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It moved slightly faster than the FcR1.2 with a mol. wt of 55,000, which was isolated by the use of VIA2 IgG1, and corresponded to the fast moving portion of the broad band of FcRs isolated with OA-complexed IgG2 antibody. These results strongly suggest that VIIA1 IgG1 is a monoclonal antibody to FcR2.