Role of heavy chain constant domains in antibody-antigen interaction. Apparent specificity differences among streptococcal IgG antibodies expressing identical variable domains.

In this report, we examine the influence of CH domains on antibody specificity, in the context of variable epitope density on bacteria and synthetic glycoconjugates. Hybridomas secreting IgG1 and IgG2b mAb, specific for the N-acetyl-glucosamine (GlcNAc) residues of streptococcal group A carbohydrate, were previously generated from a hybridoma secreting a mouse IgG3 mAb. We show that these three mAb have identical H and L chain V domains, as determined by 1) cDNA sequencing, 2) binding to soluble Ag, and 3) binding to nine monoclonal anti-idiotopes. Nevertheless, the IgG3 mAb binds more effectively than the V region-identical IgG1 or IgG2b mAb to each of three strains of group A streptococci that display different amounts of terminal GlcNAc residues on their cell walls. The magnitude of the subclass-associated differential in binding varies with the target strain, and, whereas the IgG3 mAb binds best to the strain expressing an intermediate amount of GlcNAc, the IgG1 and IgG2b mAb and IgG3-derived F(ab')2 fragments bind best to the strain expressing the highest amount of GlcNAc. The IgG3 mAb also binds better than the IgG1 and IgG2b mAb to solid-phase GlcNAc50-BSA, but the IgG2b mAb binds best to otherwise identical conjugates with lower ratios of GlcNAc to BSA (20:1, 10:1, 5:1, and 1:1). These results suggest that epitope density can significantly influence the magnitude of IgG subclass-associated binding differences, and that structural differences in the CH regions, particularly the CH2 and CH3 domains, can influence the apparent specificities of IgG molecules for multivalent Ag.