Active Von Willebrand Factor, thrombocytopenia and thrombosis

Abstract

Platelets and von Willebrand factor (VWF) are unable to interact in circulation. To induce an interaction, a conversion of VWF to a platelet-binding conformation is required. At higher shear stresses, the first step in thrombus formation is binding of VWF to the subendothelium. This results in exposure of the binding site for platelet-receptor glycoprotein (GP)-Iba, inducing an interaction between platelets and VWF. In some pathological conditions VWF circulates in an “active”, GpIba-binding conformation, which binds spontaneously to platelets. We developed a novel llama antibody-fragment that specifically recognizes the GpIba-binding conformation of VWF. The unique properties of this antibody-fragment allowed us to study the conformation of VWF in various diseases associated with a consumptive thrombocytopenia: von Willebrand disease (VWD)-type 2B, thrombotic thrombocytopenic purpura (TTP), the syndrome of hemolysis, elevated liver enzymes and low platelets (HELLP), and the antiphospholipid syndrome. VWF was found to circulate in an active conformation in these four diseases, which could explain the presence of a consumptive thrombocytopenia. Moreover, it may also account the presence of a thrombotic microangiopathy in TTP, HELLP syndrome and the antiphospholipid syndrome. We further investigated the source of active VWF in the different diseases. The active conformation of VWF in VWD type 2B is induced by gain-of-function mutations in the A1-domain. We found a negative correlation between the amount of active VWF and the platelet count in patients with VWD type 2B, suggesting a direct role for active VWF in the onset of thrombocytopenia. In HELLP syndrome, acutely activated endothelium is the source of active VWF. VWF stored in the Weibel Palade bodies of endothelial cells is enriched in ultralarge (UL)-VWF multimers, which have a higher haemostatic potential. These UL-VWF multimers are proteolysed into smaller, less reactive multimers by ADAMTS13 (A Disintegrin And Metalloprotease with ThromboSpondin motif). However, in HELLP syndrome, where there is a massive acute endothelial activation and a reduced ADAMTS13 activity, the circulating active VWF causes a thrombotic microangiopathy. Due to a complete absence of ADAMTS13 activity regulation of the activity of UL-VWF multimers is abolished in TTP, resulting in the formation of VWF-rich thrombi in the microvasculature of many organs, consumptive thrombocytopenia and microangiopathic hemolytic anemia. In search for the cause of the increased amounts of active VWF in the antiphospholipid syndrome, we found a new regulator of VWF activity, b2-glycoprotein (GP)-I. Patients with the antiphospholipid syndrome are characterized by the presence of thrombosis or pregnancy morbidity and anti-phospholipid antibodies. Particularly, antibodies against b2-GPI correlate with thrombosis. We found that b2-GPI is a biological relevant inhibitor of VWF function and as such inhibits platelet adhesion. Anti-b2-GPI antibodies neutralize this inhibitory function of b2-GPI, resulting in increased levels of active VWF. This might explain thrombosis and consumptive thrombocytopenia observed in patients with anti-b2-GPI antibodies. Deficiencies of ADAMTS13 or b2-GPI alone do not directly induce TTP or antiphospholipid syndrome. Apparently, a second hit is necessary to induce the syndromes. The balance between b2-GPI and active VWF might be very important in protection from diseases associated with thrombocytopenia. Moreover, this balance could also play an important role in the onset of cardiovascular disease.