Role of immunoglobulin G in platelet aggregation by viridans group streptococci

The aggregation of human platelets by the viridans group streptococci requires both direct platelet-bacterium binding and plasma components. Some of these extracellular constituents (e.g., fibrinogen) are cofactors for ADP, which mediates the terminal events in platelet activation by these organisms. In addition, other plasma components which are specific for viridans group streptococci are necessary. To better define these latter cofactors, we examined the role of immunoglobulin G (IgG) in platelet aggregation by two strains of viridans group streptococci. The addition of either strain to washed human platelets suspended in normal plasma resulted in a 5- to 12-min lag phase, followed by brisk and irreversible platelet aggregation. In contrast, neither strain aggregated platelets suspended in IgG-depleted plasma (IgG concentration, less than or equal to 6.7 micrograms/ml). The addition of IgG (1.0 mg/ml) to the platelet suspension restored normal aggregation. Absorption of the IgG with intact bacteria abolished its ability to support aggregation. Preincubation of washed platelets with a murine monoclonal antibody to the 40,000-Mr platelet Fc receptor blocked aggregation by both strains, but had no effect on aggregation by ADP (5 microM) or collagen (200 micrograms/ml). Neither strain aggregated gel-filtered platelets supplemented with fibrinogen (100 micrograms/ml), whereas ADP induced a maximal platelet response. When IgG (1.0 mg/ml) was added to the suspension of gel-filtered platelets, both strains produced normal aggregation. These results indicate that specific IgG is required for platelet aggregation by viridans group streptococci and that platelet activation is mediated through the 40,000-Mr Fc receptor on the platelet surface.