M+ group A streptococci are phagocytized and killed in whole blood by C5a-activated polymorphonuclear leukocytes

Historically, resistance to phagocytosis has been determined by incubating group A streptococci in human blood and comparing the numbers of CFU before and after incubation. Utilizing a flow cytometry-based technique, we have investigated the phagocytosis of M+ group A streptococci by polymorphonuclear leukocytes (PMNs) in heparinized human peripheral whole blood. Intracellular labeling of streptococci with a nontoxic fluorescent dye allowed us to quantify the association and phagocytosis of M+ streptococci by PMNs in whole blood in the presence or absence of C5a, a physiologically important chemotactic activator of PMNs. We found that wild-type strains of group A streptococci that are resistant to phagocytosis (determined by the classical Lancefield method) readily associate with C5a-activated whole-blood PMNs. In the absence of opsonizing M-type-specific antibodies, the M+ streptococci associated with PMNs are phagocytized and killed. In addition, blockade of the beta (2) integrin, CD11b/CD18, with anti-human CD11b monoclonal antibody inhibited association between M+ streptococci and C5a-activated PMNs. These findings establish a new relationship between M+ streptococci and PMNs, in which C5a-activated PMNs have the capacity to kill M+ streptococci in whole blood through a receptor-mediated phagocytic mechanism.