Adhesive properties of plasma-circulating and platelet-derived microvesicles from healthy individuals

Background: Microvesicles (MVs) produced by platelets upon activation possess high procoagulant activity and represent a possible thrombotic risk marker. However, direct experimental evaluation of the adhesive properties of MVs and their potential role in thrombus growth is lacking. Objectives: We investigated integrin alpha IIb beta 3 status and adhesive properties of plasma-circulating and plateletderived MVs from healthy individuals. Methods: MVs were isolated from whole blood or produced from activated platelets. Flow cytometry was used for quantification of fluorescently labeled PAC-1 and fibrinogen binding to MVs. Confocal microscopy was used for evaluation of MVs adhesion to fibrinogen and for estimation of their involvement in whole blood thrombus formation in a parallel-plate flow chambers under arterial shear conditions. Results and conclusions: Neither circulating plasma MVs, nor platelet-activation-produced MVs bound PAC-1. However, both types of MVs specifically and weakly bound fibrinogen (about 400 molecules of bound fibrinogen per MV versus >100,000 per non-procoagulant activated platelet). Still, the MVs did not adhere stably to the immobilized fibrinogen. Both types of MVs were weakly incorporated into a thrombus and did not affect thrombus formation: average thrombus height in the recalcified whole blood in the presence of plateletactivation-produced MVs was 4.19 +/- 1.38 mu m versus 4.87 +/- 1.72 mu m (n = 6, p > 0.05) in the control experiments. This suggests that MVs present in plasma of healthy individuals are not likely to be directly involved in thrombus formation under arterial flow conditions.

Automated summary

In this study, we investigated the adhesive properties of circulating plasma-derived and platelet-derived microvesicles (MVs) and their potential role in thrombus growth. The results showed that both types of MVs weakly bound fibrinogen but did not stably adhere to immobilized fibrinogen. Moreover, the presence of these MVs did not significantly affect thrombus formation. Therefore, MVs present in the plasma of healthy individuals are unlikely to directly contribute to thrombus formation under arterial flow conditions.