Inhibitory effects of staphylococcal enterotoxin type B on human platelet adhesion in vitro

Abstract

Septic shock was formerly recognized as a consequence of Gram-negative bacteraemia, but at present the incidence of Gram-positive sepsis seems to be more relevant, contributing for more than 50% of cases. Staphylococcal aureus can induce toxic shock in humans through the production of potent toxins termed Staphylococcal enterotoxins, from which Staphylococcal enterotoxin type B (SEB) is one of most studied. Platelets are reported to participate in pathogenesis of severe sepsis, but the exact role of platelets in this event is poorly investigated, particularly that caused by Gram-positive bacteria. Therefore, we have used the model of platelet adhesion to fibrinogen-coated plates to investigate the actions of SEB on human platelets. Ninety-six-well microtiter plates were coated with human fibrinogen (50 µg/mL), and human washed platelet suspension (6 × 106 platelets) was added to each well. Adherent platelets were quantified through measurement of acid phosphatase activity. Staphylococcal enterotoxin B (0.0001–30 µg/mL, incubated for 5 to 60 min) time- and dose-dependently inhibited platelet adhesion. This response was modified neither by the protein synthesis inhibitor puromycin (0.01 and 0.1 mM) nor by the superoxide scavengers superoxide dismutase (SOD, 100 units/mL) and polyethylene glycol-SOD (30 U/mL). The peroxide hydrogen (H2O2) scavenger catalase polyethylene glycol (1000 U/mL) significantly attenuated the platelet adhesion inhibition by SEB. The cAMP and cGMP levels were not changed by SEB (0.0001–30 µg/mL, 60 min). Our findings suggest that H2O2 at least partly contributes to the inhibitory responses of human platelet adhesion by SEB.