Quantitative and Qualitative Studies of Antibody to Streptococcal M-Protein

TLDR: Significant binding of 131 I-M12 was measurable with all type 12 sera examined and was present in early sera which lacked detectable precipitating antibody and also in sera with insignificant or equivocal bactericidal activity.

Abstract: Rabbit antibody to group A streptococcal M-protein was measured by incubating dilutions of unabsorbed serum with partially purified and 131 I-labeled M-protein from a type 12 strain ( 131 I-M12) and coprecipitating the resulting complexes with anti-rabbit γ globulin. The method was found to be sensitive, quantitative and reproducible, but no antigen preparations, all of which were purified from acid extracts of cell walls, were found to be free of significant reaction with heterologous, unabsorbed antisera. This limitation was overcome by preincubating serum with an excess of unlabeled type 6 antigen; heterologous sera so treated subsequently reacted with 131 I-M12 in a manner indistinguishable from normal rabbit serum (NRS). The specificity of the homologous reaction was indicated by its complete inhibition when serum was preincubated with unlabeled type 12 antigen and lack of inhibition with type 6 antigen. Significant binding of 131 I-M12 was measurable with all type 12 sera examined and was present in early sera (20 days) which lacked detectable precipitating antibody and also in sera with insignificant or equivocal bactericidal activity. There was generally a linear relationship between the antigen-binding and precipitating capacity of different sera but a definite correlation between antigen-binding capacity and bactericidal activity of individual sera was not apparent. On indirect radio-immunoelectrophoresis, binding of 131 I-M12 was observed only with IgG.