Background: Microvesicles produced by platelets upon activation or circulating in plasma are known to have high procoagulant activity, but their adhesive properties remain poorly understood. High concentrations of circulating microvesicles are reported to be a thrombosis marker in patients with cardiovascular diseases; still, direct experimental evaluation of the contribution of microvesicles to thrombus growth is lacking.

Aims: Study of the adhesive properties of circulating and platelet-derived microvesicles and their contribution to thrombus growth.

Methods: Flow cytometry was used for detection of PAC-1 and fibrinogen binding with microvesicles. Confocal microscopy was used for evaluation of microvesical adhesion to fibrinogen and visualization of thrombus formation in parallel-plate flow chambers. The experiments with flow chambers were produced on heparinized blood at shear rate 1000 s^-1, while immobilized type I collagen was used as an activator. The isolated microvesicles were stained with annexin V-Alexa Fluor 647, added to blood and the blood was perfused through flow chamber.

Results: No specific binding of PAC-1 antibody to the active form of integrin α_IIbβ₃ to the circulating and platelet-derived microvesicles was detected, and binding of fibrinogen was very low. The ability of microvesicles to adhere to immobilized fibrinogen was comparable with the non-specific ability of microvesicles to adhere to BSA and was lower than their specific ability to adhere to Monafram (F(ab)2 fragments of anti-integrin α_IIbβ₃ monoclonal antibody CRC64). We did not observe the signficant difference in the thrombus growth between two studied groups of thrombi as with or without circulating or platelet-derived microvesicles. The possibility of binding of microvesicles to a growing thrombus under flow conditions was studied, and we did not observe the circulating and platelet-derived microvesicles in the growing thrombi.

Conclusions: Our results suggest that the circulating and platelet-derived microvesicles are not involved in adhesion to a growing thrombus and not affect the thrombus growth.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/adhesive-properties-of-plasma-circulating-and-platelet-derived-microvesicles/