Antigenicity of the M proteins of group A hemolytic streptococci: further evidence for shared determinants among serotypes.

Shared antigenic determinants between M proteins of group A streptococci appear to be widespread among serotypes. This is demonstrated by the ability of purified M proteins to absorb opsonic antibody from a variety of heterologous antisera prepared against whole cells or purified M proteins. This absorption procedure has the capacity to separate passive mouse protecting and passive hemagglutinating antibodies from opsonic antibodies measured in vitro. When whole cells or M proteins are used as heterologous absorbents, immunoglobulin may be recovered from the cells or precipitates. The recovered antibody has most of the opsonic and some of the precipitating qualities of the original unabsorbed serum, but hemagglutinating titers are significantly lower. These data provide additional evidence that shared antigenicity among M proteins can be the result of common antigenic determinants. Arguments are presented that these cross-reactions are not the result of a nonspecific protein fraction associated with purified M proteins.