Opsonization of encapsulated Staphylococcus aureus: The role of specific antibody and complement

Previous studies of encapsulated Staphylococcus aureus have shown that the opsonins of normal, nonimmune human serum (complement factor C3 and IgG) bind beneath the capsule, i.e., on the cell wall, and when bound at this site these opsonins are not effective in promoting phagocytosis of the bacteria by polymorphonuclear leukocytes (PMN). In this investigation immune antibody was added to human serum to effect opsonization of encapsulated S. aureus. Opsonization was assessed by quantitating the uptake of ³H-labeled staphylococci by human PMN, and the amount of C3 fixation to bacteria was measured in a quantitative fluorescent immunoassay. Low levels of immune antibody (IgG) effectively opsonized encapsulated S. aureus when added to fresh but not to heated serum; phagocytosis of the staphylococci was mediated via pronase-sensitive membrane receptors (presumably C3b receptors) of PMN. Experiments with C2-, C3-, or C5-deficient human sera revealed that C3 was required for opsonization and that activation of C3 was mediated via the alternative complement pathway. Encapsulated S. aureus bound significantly less C3 than unencapsulated strains in diluted normal serum; addition of immune antibody, however, increased C3 fixation 4.7-fold (p < 0.005). Immunoelectron microscopy localized C3 throughout the capsule as well as on the staphylococcus cell wall when bacteria had been opsonized in human serum with immune antibody. Without immune antibody, C3 binding was restricted to the cell wall. At approximately 10-fold higher levels of immune antibody, opsonization and phagocytosis of encapsulated S. aureus was independent of complement and pronase-sensitive receptors on PMN. These studies show that, in addition to immune antibody, the alternative pathway of complement plays an important role in the opsonization of encapsulated S. aureus and suggest that complement may be crucial to the in vivo clearance of these organisms.