Idiotypic specificity of rabbit antibodies to streptococcal group polysaccharides.

Anti-idiotypic antisera against six restricted rabbit streptococcal group specific antibodies have been raised in rabbits matched for allotypes. All these antisera reacted specifically with their homologous idiotypes on double-diffusion tests in agarose gel. In addition, they showed a high incidence of cross-specificities with group-specific hyperimmune sera induced in both closely related and unrelated individuals. These precipitating cross-specificities could be explained for two systems by the interference of rheumatoid factor. Two idiotypic antibody systems have been analyzed in detail; these were restricted antibodies produced in a father and in one of his offspring. The methods employed included binding inhibition of radio-labeled homologous Fab fragments and hemagglutination inhibition with homologous idiotypic coat. The data demonstrated that only related rabbits produced, besides non-cross-reacting antibodies, idiotypically similar antibodies raised to the same antigen. About one-third of the cross-reactive idiotypes showed binding inhibition between 31 and 92%. Inhibition of binding above 50% in the paternal idiotypic system was only achieved by one offspring antibody whereas the F(1) progeny idiotypic system was inhibited to this extent by seven antibodies of related rabbits. In contrast, 87.5% and 91.7% of antibodies of unrelated rabbits were less than 20% inhibitory. Within this study two idiotypically identical antibodies have not been found. This implies that A-variant-specific antibodies of related rabbits which produced antipolysaccharide antibodies were structurally different. Cross-reaction, even if greater than 90% by binding inhibition, appears to involve only part and not all of the variable regions.