Type-specific inhibition of preopsonization versus immunoprecipitation by Streptococcal M proteins.

Purified M protein of group A streptococci inhibits in vitro phagocytosis of bacteria nonspecifically when it is mixed directly with the test blood. We devised a method of preopsonization inhibition that avoids direct contact between the test blood and the opsonic inhibitory agent. Each of the M proteins tested by this method inhibited opsonization and phagocytosis of only homologous-type streptococci. Titration of the M antigen in various purified preparations demonstrated a clear dissociation between titers of type-specific precipitating M antigen and type-specific opsonic inhibitory M antigen. Since opsonization reflects protective antibody, assays of M protein based on reaction with opsonic antibody should reflect more accurately the presence of the type-specific M determinant involved in protective immunity against streptococcal infections.