Strain-Specific Restriction of the Antiphagocytic Property of Group A Streptococcal M Proteins

Abstract

Group A streptococcal M proteins are type-specific virulence factors that inhibit phagocytosis. We used two M proteins, M5 and Emm22, to analyze the influence of genetic background on the properties of M proteins. Mutant strains, engineered to lack these M proteins, were complemented with genes encoding the homologous or heterologous M protein, and the complemented strains were analyzed for phagocytosis resistance. Neither the M5 nor the Emm22 protein conferred phagocytosis resistance in the heterologous background, but they did do so in the homologous background. This was not due to lack of surface expression in the heterologous background. Moreover, the M5 and Emm22 proteins expressed in heterologous background appeared to have normal structure, since they were not affected in their ability to bind different human plasma proteins. In particular, M5 or Emm22 had normal ability to bind human complement inhibitors, a property that has been implicated in phagocytosis resistance. Results similar to those obtained with M5 and Emm22 were obtained in experiments with the M6 and Emm4 proteins. Together, these data suggest that the surface expression of M protein alone may not be sufficient to confer phagocytosis resistance and consequently that strain-specific factors other than M and Emm proteins may contribute to the ability of group A streptococci to resist phagocytosis.Group A streptococcus (Streptococcus pyogenes) is the cause of tonsillitis and impetigo and also causes severe diseases, including necrotizing fasciitis and the streptococcal toxic shock syndrome (2). In addition, group A streptococcal infections are sometimes complicated by one of the postinfectious sequelae, rheumatic fever and glomerulonephritis.The M proteins of group A streptococci are major virulence factors that confer resistance to phagocytosis (14). The Nterminal sequences of the M proteins are highly variable, giving rise to the existing ϳ100 serotypes. Protection against phagocytosis is usually serotype specific. It should be noted that a large fraction of all group A streptococcal strains encode three proteins with structural features typical for M proteins (12, 18). These proteins, designated Mrp, Emm, and Enn, are encoded by adjacent genes on the chromosome and are regulated by a common positive regulatory gene element now designated mga (7,20). At least two of the gene products (Mrp and Emm) from a single strain can contribute to resistance against phagocytosis, although Emm appears to be more important (22, 32). Strains with three genes encoding M-like proteins differ from those with a single gene in that they usually express opacity factor (OF), a lipoproteinase that is both surface bound and secreted (25,26). Recent evidence indicates that OF contributes to group A streptococcal virulence (4).Streptococcal mutants lacking M protein(s) are readily phagocytosed. However, the antiphagocytic property can be restored by complementation with the homologous M protein (19). Moreover, there is evidence that introduction of DNA encoding heter...

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