Predominance of immunoglobulin G sub-class 3 among the complement-fixing antibodies to streptococcal M-associated protein.

During investigation of the absorption of group-A streptococcal antibodies from human sera by a protein A-positive Staphylococcus aureus strain, we found that the complement-fixing antibodies to M-associated protein (MAP) were only partially absorbed from the majority of sera tested, although they were shown to belong to the immunoglobulin G (IgG) class by density gradient centrifugation. In contrast, other streptococcal antibodies: anti-streptolysin O (ASO), anti-deoxyribonuclease B (anti-DNAase B), 'bactericidal' M antibody and anti-opacity factor (anti-OF), were completely absorbed from all but a minority of sera. We suggest that the complement-fixing antibodies to MAP may be of restricted heterogeneity and have an abnormal IgG sub-class distribution, with a marked predominance of IgG3 (the only sub-class that does not interact with protein A) over the IgG1 and IgG2 sub-class; IgG4 does not participate in complement fixation. The concentration and relative porportions of IgG sub-classes are believed to be genetically influenced, so our findings may have some bearing on the immune responsiveness of different individuals to streptococcal infection, and possibly have important implications in the development of the secondary sequelae.