Active Von Willebrand Factor in Plasma of Sickle Cell Patients Is An Indicator for Severity of Sickle Cell Anemia

Vaso-occlusion, hemolysis and oxidative stress are hallmarks of sickle cell anemia. We and others have shown that oxidants generated in sickle cell anemia can stimulate endothelial cells to release ultra-large von Willebrand factor (ULVWF), which is hyperactive compared to the normal plasma VWF in binding platelets and red blood cells, and is capable of binding sickle erythrocytes especially well. For this reason, we surveyed several parameters of VWF quantity and function in plasma from patients with sickle cell anemia (9 individuals, some with multiple determinations). These parameters included VWF antigen levels, VWF multimer composition, ADAMTS13 (the VWF-cleaving metalloprotease) activity, VWF activation factor, and total active VWF. VWF activation factor was measured using a llama antibody (nanobody) that detects an active conformation in the VWF A1 domain that correlates with an enhanced ability to bind platelets. The activation factor is defined as the ratio of nanobody binding of patient plasma to that of pooled normal plasma at a given VWF antigen level. Thus, the activation factor of pooled normal plasma is 1. Elevated VWF activation factor (≥ 1.5, Curr. Opin. Hematol. 2007 14:284–289) has been demonstrated in the plasma of patients with type 2B von Willebrand disease, thrombotic thrombocytopenic purpura, HELLP syndrome, and other prothrombotic disorders. In this study, plasma was collected from patients with sickle cell anemia at baseline during routine outpatient visits. In all of the patients, the VWF antigen level was higher than in the pooled normal plasma (1.5–3.6 fold) or an ethnically matched control. Activation factors were in the range of 0.2 to 6.8. The total quantity of active VWF in plasma was then determined by multiplying VWF antigen level and activation factor. The total active VWF in plasma correlated with the severity of the patient’s disease, as determined by the number of hospital admissions over the previous year and elevated plasma lactate dehydrogenase (LDH). Elevated LDH is a risk factor for hemolysis associated pulmonary hypertension, leg ulcers, priapism and death in sickle cell anemia. In addition, we found that a sub-set of patents had reduced ADAMTS13 activity, which was correlated with higher VWF multimer composition on agarose gels. This indicates that accumulation of active VWF in patients with sickle cell anemia arises not only from increased endothelial secretion but also from decreased ADAMTS13 activity. These findings suggest that high levels of active VWF predispose patients with sickle cell anemia to vasoocclusive complications and correlate with increased hemolysis.