Shear Induced Reversible Conglomerates of Von Willebrand Factor Networks with Aggregates of Inactivated Platelets - a Fast Switchable Plugging Mechanism

Abstract 2104

Thrombus growth rate has been reported to be a linear function of very high shear rates beyond 40,000 s^-1 (Ku DN, Bioreology. 2007;44(4):273-84), which are thought to occur in severe arterial stenosis. We therefore applied shear rates continuously increasing from 2,000 s^-1 to 40,000 s^-1 and examined platelet and von Willebrand factor (VWF) interaction in a parallel plate flow chamber under direct visualization. Above a critical threshold of 10,000 to 15,000 s^-1 aggregates of inactivated platelets formed when VWF was present in solution and immobilized on the perfused surface (Ruggeri, … Reininger; Blood, 108: 1903–1910, 2006). A new finding was the shear dependent simultaneous formation of VWF strands in and around those platelet aggregates, growing to strands up to several microns thick and up to several tens of microns long, thus forming large VWF networks. Platelets appeared to be enmeshed in the VWF networks but were nevertheless a prerequsite for their formation, mutually fostering each other. Platelet-VWF conglomerates formed at the perfused VWF surface, were constantly rolling in the flow direction, grew in size with increasing shear rate, and were completely reversible below the critical shear rate threshold of 10,000 s^-1. VWF networks assembled in whole blood as well as in washed blood cell suspensions reconstituted with VWF. Recombinant full length VWF generated larger VWF networks than a commercial VWF preparation from pooled human plasma. Perfusion of collagen coated surfaces caused assembly of VWF networks anchored to the collagen fibrils and capture of activation-blocked platelets within them at shear rates of 2,000 s^-1, leading to residence times of platelet-VWF conglomerates of more than a minute. Newly formed platelet conglomerates could be visualized rolling on the surfaces of already attached conglomerates. When we used a gain of function mutation of VWF, rolling platelet aggregates could be detected at a shear rate as low as 1,500 s-1. We conclude that shear rate activation of VWF leads to reversible platelet-VWF network generation, which may be a crucial mechanism of fast platelet accrual and prolonged arrest times at sites of thrombosis as well as physiological hemostasis.