Protein A effect on alternative pathway complement activation and opsonization of Staphylococcus aureus.

Twelve Staphylococcus aureus strains with known amounts of protein A were compared with regard to alternative pathway complement activation and opsonization in human serum. "Protein A-poor" strains (less than or equal to 0.16 ng/10(6) bacteria) were, on the average, 3. 4-fold more efficient in alternative pathway complement activation than "protein A-rich" strains (greater than or equal to 0.625 ng/10(6) bacteria) (P less than 0.001). Protein A-poor strains were significantly better phagocytized by human polymorphonuclear leukocytes after opsonization in magnesium-ethylene glycol-bis (beta-amino-ethyl ether)-N, N-tetraacetic acid-chelated serum than were the protein A-rich strains (P less than 0.001). No significant differences between protein A-poor and -rich strains were found in complement activation and opsonization in normal serum. Cell wall-bound protein A appeared to hinder alternative pathway complement activation by S. aureus, which resulted in decreased opsonization of these bacteria in the absence of an intact classical pathway. These studies suggest that protein A may cover alternative pathway complement-activating sites within the peptidoglycan matrix of the staphylococcal cell wall.