Murine myeloma proteins of the four IgG subclasses and of the IgA and IgM classes have been examined for their ability to inhibit the binding of affinity cross-linked trimeric rabbit IgG to various types of murine cells. In addition, proteins of the IgG1, IgG2a, and IgG2b subclasses have been cross-linked with dimethyl suberimidate and the purified oligomers have been tested for direct binding. With a macrophage line (P388D1), a lymphocyte line (AKTB-1), and normal spleen cells, proteins of the IgG1, IgG2a, and IgG2b subclasses all compete for binding at the same site. Differences in binding affinities between proteins of the IgG1, IgG2a, and IgG2b subclasses are small and are not greater than differences in affinities when proteins within a subclass are compared. Proteins of the IgG3 subclass and the IgM and IgA classes do not significantly inhibit IgG binding at the highest concentrations tested.
P388D1 cells are also able to bind iodinated monomeric IgG2a proteins with higher affinity than expected from inhibition studies. This binding is inhibitable by proteins of the IgG1, 2a, and 2b subclasses, and is trypsin sensitive. These results could be explained by the existence of a second type of receptor, or by assuming that the iodination procedure increases the affinity of IgG2a proteins for P388D1 cells.