Strain-specific restriction of the antiphagocytic property of group A streptococcal M proteins

Group A streptococcal M proteins are type-specific virulence factors that i nhibit phagocytosis. We used two M proteins, M5 and Emm22, to analyze the i nfluence of genetic background on the properties of M proteins. Mutant stra ins, engineered to lack these M proteins, were complemented with genes enco ding the homologous or heterologous M protein, and the complemented strains were analyzed for phagocytosis resistance. Neither the M5 nor the Emm22 pr otein conferred phagocytosis resistance in the heterologous background, but they did do so in the homologous background. This was not due to lack of s urface expression in the heterologous background. Moreover, the M5 and Emm2 2 proteins expressed in heterologous background appeared to have normal str ucture, since they were not affected in their ability to bind different hum an plasma proteins. In particular, M5 or Emm22 had normal ability to bind h uman complement inhibitors, a property that has been implicated in phagocyt osis resistance. Results similar to those obtained with M5 and Emm22 were o btained in experiments with the M6 and Emm4 proteins. Together, these data suggest that the surface expression of M protein alone may not be sufficien t to confer phagocytosis resistance and consequently that strain-specific f actors other than M and Emm proteins may contribute to the ability of group A streptococci to resist phagocytosis.