PURPOSE: Staphylococcus aureus induces a proinflammatory and prothrombotic response causing micro- and macrovascular thrombosis. Understanding the key effector cells and cellular mechanisms in these responses are critical to developing effective treatments.
METHODS: We drew whole blood from healthy subjects through standard venipuncture. Platelets were isolated from whole blood using a negative bead isolation protocol and were free of contaminating leukocytes. Whole blood or freshly-isolated platelets were incubated with methicillin resistant S. aureus (MRSA, 9x106 cfu/mL) for up to 8 hours. Thrombin generation (TG) was measured using a fluorogenic substrate in platelet-poor plasma isolated from whole blood. Platelet activation was assessed using flow cytometry for CD62p (platelet surface p-selectin expression, P-SEL), and sandwich ELISA for platelet-factor 4 release (PF4).
RESULTS: When MRSA was incubated in whole blood for 4 hours, mean (±SEM) P-SEL expression increased (2.2 ± 0.5% vs. 8.1 ± 1.7%, p<0.05) and PF4 release was greater (367 ± 83 ng/mL vs. 3587 ± 884 ng/mL, p <0.05) compared to untreated samples, demonstrating the ability of MRSA to induce platelet activation. In addition, rates of thrombin generation were significantly accelerated (8.9 ± 1.9 nM/min vs. 35.1 ± 10.6 nM/min, p<0.05). The addition of abciximab (136 nM), a glycoprotein IIb/IIIa receptor antagonist, to MRSA-stimulated whole blood inhibited the expression of P-SEL and the release of PF4, suggesting that the cellular responses to MRSA in whole blood are mediated, in part, through GP IIb/IIIa. Freshly-isolated platelets incubated with MRSA also demonstrated evidence of activation with increased P-SEL expression and PF4 release, but longer time courses (≥8 hours) were required to detect platelet activation. As we confirmed by western blotting that our MRSA strain produced alpha-toxin within 4 hours, these findings may reflect platelet-mediated bacterial killing, as has been previously described.
CONCLUSIONS: These data support the ability of MRSA to activate platelets and induce a hypercoagulable state, in part through the GP IIb/IIIa receptor.
CLINICAL IMPLICATIONS: A better understanding of the cellular responses in MRSA sepsis will contribute to novel therapeutic strategies.
DISCLOSURE: The following authors have nothing to disclose: Zechariah Franks, Robert Campbell, Andy Weyrich, Guy Zimmerman, Matthew Rondina
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