The biological activity of human monoclonal IgG anti-D is reduced by beta-galactosidase treatment.

The contribution to IgG effector function of exposed galactose residues on the oligosaccharide chains in IgG has been tested experimentally. We studied a human monoclonal antibody (BRAD-5) to the Rh D blood group antigen. The preparation used contained a very low percentage of agalactosyl IgG (3.6%). After digestion with beta-galactosidase there was an increase in terminal GlcNAc indicating an increase in % agalactosyl IgG to approximately 30%. Comparison was made of the Fc receptor-(Fc gamma R)-mediated functional interactions of the two glycoforms of BRAD-5 in assays which measured the recognition and destruction of sensitised erythrocytes by various effector cells. After beta-galactosidase treatment, there was a slight reduction in Fc gamma RI-mediated adherence of erythrocytes to U937 cells and phagocytosis of erythrocytes by monocytes. Fc gamma RII-mediated binding of erythrocytes to K562 cells was also reduced. However there was little difference in adherence of erythrocytes to either Daudi cells (via Fc gamma RII) or NK cells (via Fc gamma RIII). There was a consistent reduction in lysis of erythrocytes mediated through Fc gamma RIII on K cells with the beta-galactosidase treated anti-D. Overall the results showed that reduced levels of galactose on IgG anti-D were associated with reduced biological activity in these assays, but the experiments failed to cast light on the physiological role of the agalactosyl glycoform of IgG.