Independent binding of native and aggregated IgG in group A streptococci.

Irrespective of IgG Fc-receptor activity, earlier characterized, many group A streptococci were recently found to bind aggregated IgG Fab and/or light chains. In the present study, binding of glutaraldehyde-aggregated, radiolabelled, intact human IgG (aIgG) to group A streptococci was tested, and strains representing several M-types were found reactive. In particular, high binding was observed among type M12 strains, earlier found devoid of Fc-receptors for monomeric IgG; accordingly, unlabelled, native IgG had little influence on the binding. The sites binding aIgG were highly sensitive to trypsin and relatively resistant to heat treatment. The binding to M12 was inhibited by human fibrinogen and, to a lesser extent, by heat-aggregated serum albumin. Rabbit antiserum to M12 was more inhibitory than antiserum to a heterologous type of group A streptococci or normal rabbit serum. Our results indicate that streptococcal M-protein binds aIgG by a multipoint requiring interaction of low specificity and that previously described Fc-receptors binding native IgG are not involved. For comparison, in Cowan I staphylococci and one strain of group G streptococci tested, high binding of aIgG was also observed; however, this binding was inhibited by native IgG, indicating that protein A and group G streptococcal Fc-receptor, earlier known to bind untreated IgG, also bound a*IgG.