Factor Va, bound to microparticles released during platelet storage, is resistant to inactivation by activated protein C.

Background: Microparticles (MPs) support coagulation and can be helpful in restoring the hemostatic system in thrombocytopenic patients. The anticoagulant properties of MPs shed during storage of platelets (PLTs) have not been studied yet.

Study Design And Methods: Storage-induced MPs were harvested from outdated PLT concentrates. Whether factor (F)Va was present on the surface of these MPs was investigated. The activated protein C (APC)-catalyzed inactivation of MP-bound FVa was further determined. Also, inactivation of FVa at the surface of thrombin-activated PLTs and synthetic vesicles was determined.

Results: MPs in stored PLT products carry FVa at their surface. APC-catalyzed inactivation of MP-bound FVa resulted in 42 +/- 2 percent residual FVa activity after 20 minutes. The residual activity of FVa on thrombin-activated PLTs was 25 +/- 3 percent. Plasma-derived FVa was rapidly inactivated in the presence of synthetic vesicles, with 5 +/- 4 percent residual FVa activity. When synthetic vesicles were added to the inactivation mixture of MP- or thrombin-activated PLTs, a residual activity of 5 to 10 percent was found. Furthermore, addition of excess plasma-FVa to storage-induced MPs resulted in a residual activity of 26 +/- 2 percent. Moreover, the APC-resistant phenotype of MPs was confirmed in plasma in which thrombin generation was measured in the absence and presence of APC. Residual FVa activity in the presence of MPs, PLTs, or synthetic vesicles was 87 +/- 6, 65 +/- 3, and 8 +/- 19 percent, respectively.

Conclusion: Together, these results suggest that the MP surface environment renders FVa resistant to APC. It is further concluded that the APC resistance of FVa at the surface of storage-induced MPs enhances their procoagulant nature.