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Keywords:

  • blood coagulation;
  • platelets;
  • microparticles;
  • factor XI;
  • propagation of thrombin formation

Summary

Platelets shed microparticles, which support haemostasis via adherence to the damaged vasculature and by promoting blood coagulation. We investigated mechanisms through which storage-induced microparticles might support blood coagulation. Flow cytometry was used to determine microparticle number, cellular origin and surface expression of tissue factor (TF), procoagulant phosphatidylserine (PtdSer) and glycoprotein (GP) Ib-α. The influence of microparticles on initiation and propagation of coagulation were examined in activated factor X (factor Xa; FXa) and thrombin generation assays and compared with that of synthetic phospholipids. About 75% of microparticles were platelet derived and their number significantly increased during storage of platelet concentrates. About 10% of the microparticles expressed functionally active TF, as measured in a FXa generation assay. However, TF-driven thrombin generation was only found in plasma in which tissue factor pathway inhibitor (TFPI) was neutralised, suggesting that microparticle-associated TF in platelet concentrates is of minor importance. Furthermore, 60% of all microparticles expressed PtdSer. In comparison with synthetic procoagulant phospholipids, the maximal rate of thrombin formation in TF-activated plasma was 15-fold higher when platelet-free plasma was titrated with microparticles. This difference could be attributed to the ability of microparticles to propagate thrombin generation by thrombin-activated FXI. Collectively, our findings indicate a role of microparticles in supporting haemostasis by enhancement of the propagation phase of blood coagulation.