SummaryAll virulent group A streptococcal isolates bind fibrinogen, a property that is closely linked to expression of type-specific antiphagocytic surface molecules designated M proteins. Here we show that although the M proteins from two different strains, M1 and M5, both bind fibrinogen with high affinity, they interact with different regions in the ligand. Moreover, mapping experiments demonstrated that the fibrinogen-binding regions in the M1 and M5 proteins are quite dissimilar at the amino acid sequence level and that they bind to different regions in the plasma protein. In spite of these differences, the fibrinogenbinding regions of M1 and M5 could both be shown to contribute to streptococcal survival in human blood, providing evidence for the distinct function of a plasma protein interaction in bacterial pathogenesis.

During phagocytosis, surface receptors on neutrophils interact with pathogens opsonized with complement factor C3b/iC3b and in some cases with antibodies. In human immune sera antibodies directed against surface‐bound M proteins mediated killing of Streptococcus pyogenes by neutrophils. Surprisingly, blocking of the Fc receptors had little effect on the killing. In contrast, inhibition of C3b/iC3b generation, or blocking of the major neutrophil iC3b receptor CD11b/CD18, enabled S. pyogenes to grow efficiently in immune sera. Inhibition of CD11b/CD18, but not of CD32, the major neutrophil signaling Fc receptor, prevented Streptococcus‐induced NADPH oxidase‐dependent respiratory burst, and blocking of C3b/iC3b formation inhibited Streptococcus‐induced activation of Cdc42, a small GTPase critically involved in transmitting pro‐inflammatory signals to the cytoskeleton. Consequently, ligation of CD11b/CD18 by bacteria‐bound iC3b is necessary for inducing a neutrophil response leading to elimination of S. pyogenes in immune human serum.

Streptococcus pyogenes is an important Gram-positive pathogen that is strictly limited to infections in humans. Here we report that streptolysin O (SLO), a cytolytic exotoxin secreted by S. pyogenes, activates human polymorphonuclear neutrophils (PMNs) by perforating these cells. This appears to be followed by an influx of Ca(2+) and p38 MAPK activation. As a consequence, PMNs secrete heparin-binding protein, a potent inducer of vascular leakage, and neutrophil-borne proteins, including LL-37, alpha-defensins, and elastase. The results of the present work therefore suggest that the interaction between SLO and PMNs evokes an exaggerated host response which may contribute to the pathogenesis of local and generalized S. pyogenes infections.

Group A streptococci (GAS) are common human pathogens that express major surface-associated virulence factors designated M proteins. In this study, we explored directly the cellular mechanisms behind their supposed ability to prevent phagocytosis. Isolated human neutrophils killed an M-negative GAS mutant (ΔM5), but not the wild-type parent strain (M5). After 3 h, 3–4 times as many ΔM5 as M5 bacteria were associated with the neutrophils, and more ΔM5 than M5 bacteria were ingested. However, there was no statistically significant difference between ΔM5 and M5 bacteria in regard to the percentage of the neutrophil-associated bacteria that were ingested, indicating that M5 protein prevents an adhesion receptor-dependent association with neutrophils and not the phagocytic machinery per se. Different Abs against CD11b/CD18 (CR3) blocked adhesion and killing of ΔM5 bacteria, whereas the blocking of two other complement receptors, CD11c/CD18 (CR4) and CD35 (CR1), did not. The CD11b/CD18-mediated killing of ΔM5 bacteria resulted in protein tyrosine phosphorylations and Cdc42 activation. Furthermore, inhibition of CD11b/CD18 receptor engagement or tyrosine kinase activity blocked the ΔM5-induced activation of Cdc42 as well as the killing of these bacteria. We conclude that M5 protein interferes with the CD11b/CD18-dependent association between GAS and neutrophils, and thereby blocks subsequent ingestion of the bacteria.