Distinct platelet interactions with soluble and immobilized von Willebrand factor modulate platelet adhesion and aggregation with differential impact on hemostasis and thrombosis

Arterial thrombosis is a prevailing and lethal pathological condition that remains difficult to treat or prevent without potentially serious side effects, mostly hemorrhagic in nature. Platelets and von Willebrand factor (VWF) have a recognized major role in the pathogenesis of arterial thrombosis. Platelets bind to surface immobilized VWF for initial adhesion to injured vascular sites, but also interact with soluble VWF to aggregate into thrombi, particularly under flow conditions creating elevated shear stress. Whether the binding of immobilized and soluble VWF to platelets is regulated by separate mechanisms and how they respectively regulate hemostasis and thrombosis remains unclear. Using targeted mutagenesis we engineered VWF to achieve modified binding kinetics with the platelet receptor glycoprotein (GP) Ib and discovered that the interactions of immobilized and soluble VWF with platelets can be differentially regulated with distinct consequences on platelet adhesion and aggregation. Based on these results, we studied a monoclonal antibody, NMC4, known to bind to an epitope in the VWFA1 domain and to inhibit preferentially platelet aggregation under elevated shear stress conditions. We found that NMC4 was less efficient in reducing platelet adhesion to immobilized VWF than platelet aggregation mediated by soluble VWF and, surprisingly, also inhibited arterial thrombosis in a mouse model of ferric chloride-induced carotid artery occlusion at a dose that failed to prolong post-injury bleeding. Thus, our current findings help delineate interrelated biochemical and biophysical mechanisms underlying VWF function in vascular health; and suggest selective inhibition of VWF-mediated platelet aggregation as opposed to adhesion as a strategy to prevent arterial thrombosis while minimizing bleeding complications.