Alternate complement pathway activation by group A streptococci: role of M-protein

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Abstract
Avirulent strains of group A streptococci readily activate the complement [C] system in normal human serum via the alternate C pathway (ACP). Virulent M -positive group A streptococci are much less potent as activators of the ACP. Ability of M-positive streptococci to activate the ACP is enhanced by trypsinization or mild peptic digestion. The latter treatment removes the serologically active and anti-phagocytic type-specific moieties of M protein but retains the surface fuzzy layer. Phagocytosis of avirulent streptocci is markedly enhanced by pre-opsonization in serum chelated with Mg-ethylene glycol tetraacetic acid (classic C pathway blocked) but not in serum devoid of heat-labile factors. The function of M protein as a virulence factor may be at least partly mediated by its ability to retard interaction of ACP components with structures present on the streptococcal cell surface.