Beta2-Glycoprotein I Interferes with von Willebrand Factor-Dependent Platelet Adhesion.

Patients with the antiphospholipid syndrome are characterized by the association of thrombosis or pregnancy morbidity with the presence of antibodies against phospholipid-binding proteins, such as beta2-Glycoprotein (beta2-GPI) or prothrombin. In particular, antibodies against beta2-GPI strongly correlate with thrombotic complications. One model that explains this correlation involves an antibody-induced gain-of-function of beta2-GPI, which results in prothrombotic properties of this plasma protein. In an alternative model, beta2-GPI may display anti-thrombotic properties that disappear in an antibody-dependent manner. Of course, both possibilities may occur simultaneously. It should be noted, however, that little is known about the physiological function of beta2-GPI. Several in vitro-based studies have pointed to beta2-GPI as a potential inhibitor of fibrinolysis and/or the intrinsic pathway of coagulation. In the present study, we investigated the hypothesis that beta2-GPI affects platelet function. Addition or immune-depletion of beta2-GPI from platelet-rich plasma had little effect on ADP- or collagen-induced platelet aggregation, whereas it did modulate ristocetin-induced platelet aggregation. In a system employing washed platelets, beta2-GPI completely inhibited platelet aggregation induced by VWF/ristocetin or by a recombinant VWF type 2B mutant (VWF/R1306Q). This suggests that beta2-GPI is directed to the von Willebrand (VWF)-glycoprotein Ib axis. Indeed, beta2-GPI interfered with platelet adhesion to VWF-coated coverslips under conditions of flow as well, resulting in a 2-fold reduction of platelet coverage. In direct binding studies, wt-VWF displayed weak binding to immobilized beta2-GPI. However, conversion of latent VWF into a platelet-binding conformation (either via ristocetin or via a type 2B mutation) induced efficient, dose-dependent and saturable binding of VWF to beta2-GPI. By using a number of recombinant deletion-mutants and individual domains, we found that binding was mediated by the VWF A1 domain. Interestingly, anti-beta2-GPI antibodies isolated from patients with the antiphospholipid syndrome not only interfered with the interaction between beta2-GPI and VWF, but also neutralized the inhibitory effect of beta2-GPI towards VWF-dependent platelet aggregation. Finally, we analysed plasma of antiphospholid-syndrome patients for the presence of VWF that is in its platelet-binding conformation by using a specific nanobody (