Bovine IgG1, but not IgG2, binds to human B cells and inhibits antibody secretion

We previously observed that milk-derived bovine IgG, but not serum-derived bovine IgG, strongly inhibits antibody secretion by pokeweed mitogen (PWM)-stimulated human peripheral blood mononuclear cells (PBMC). Bovine milk contains a greater percentage of IgG1 (90%) than does bovine serum (53%). To determine whether bovine IgG subclasses have different functional capabilities, we have examined the effects of bovine IgG1 and IgG2 subclasses upon not only antibody secretion but also mitogenesis by human PBMC. Both bovine IgG subclasses markedly inhibited PWM-stimulated mitogenesis. However, only bovine IgG1, and not IgG2, inhibited antibody secretion during a 14-day in vitro culture period. Also, antibody secretion was inhibited following a 24-hr preincubation of human PBMC with bovine IgG1, but not with IgG2. To determine whether these differences corresponded to specificities of human Fc gamma receptors on subsets of mononuclear cells, fluorescence-activated cell sorter (FACS) analyses were performed. Both bovine IgG subclasses bound to human monocytes. However, only bovine IgG1 bound to human B cells, and bovine IgG1 bound more avidly to human B cells than did human IgG. One model to explain these findings is that inhibition of mitogenesis may be due to the binding of both bovine IgG1 and IgG2 subclasses to monocytes; whereas subclass-specific inhibition of antibody secretion may result from the selective binding of bovine IgG1, but not bovine IgG2, to B cells. The observation that bovine IgG1 has a greater avidity for human B lymphocyte Fc receptors than human IgG may have important implications for future studies of Fc gamma receptors on human leucocytes.