Restricted deposition of C3 on M⁺ group A streptococci: Correlation with resistance to phagocytosis

In this report, the mechanism of resistance to phagocytosis by group A streptococci and the participation of M protein in this process were studied. A quantitative fluorometric immunoassay was used to determine the amount of the opsonic third component of complement, C3, fixed by group A streptococci in the presence and absence of M protein. In the absence of M protein, phagocytosis was found to be dependent on the amount of C3 fixed. However, when M protein was present, the streptococci still bound sufficient C3 to have promoted phagocytosis, yet none was observed. Further investigation using immunofluorescent microscopy demonstrated that the C3 fixed to M⁺ chains was confined to dense patches, interspaced by areas devoid of detectable C3. This uneven distribution of C3 was dependent on the presence of a trypsin-sensitive structure; most likely the M protein. The significance of this restricted C3 deposition with respect to resistance to phagocytosis by group A streptococci is discussed.