Acute Activation of Endothelium Induces Circulation of Active Von Willebrand Factor in HELLP Sydrome.

Von Willebrand Factor (VWF) is unable to interact spontaneously with platelets under physiological conditions. To induce this interaction, a conversion of the VWF A1-domain into a Glycoprotein Ibα (GpIbα)-binding conformation is required. We recently described a nanobody, designated AU/VWFa-11 that preferentially recognizes the GpIbα-binding conformation (Hulstein et al. 2005 Blood, in press). Using an AU/VWFa-11 based immunosorbant assay, we found that the active form of VWF circulates in VWD type 2B and Thrombotic Thrombocytopenic Purpura. Although these diseases have different phenotypical appearances, both are characterized by thrombocytopenia. Another disease associated with a low platelet count is HELLP (hemolysis, elevated liver enzymes and low platelets) syndrome, a severe form of preeclampsia that compromises pregnancy. In this study, we have compared the levels of activated VWF in the plasma of healthy pregnant women (n=9), patients suffering from preeclampsia (n=6) and patients with HELLP syndrome (n=14) at similar gestational age. In HELLP syndrome, the levels of activated VWF were increased 1.8-fold compared to healthy volunteers (p=0.0001). Moreover, these levels were also increased 1.5-fold compared to the patients suffering from preeclampsia. In order to get insight into the origin of the increased activated VWF levels, a number of other parameters were determined. VWF antigen levels were found to be increased in normal pregnancy (165% ± 32.5), as expected. In preeclampsia and HELLP syndrome these levels were even further increased (178% ± 89.8 and 338% ± 89.3, respectively). In healthy pregnant women and in preeclampsia, VWF propeptide levels were concomitantly increased. In contrast, in HELLP syndrome propeptide levels were increased up to 3-fold compared to propeptide levels in healthy pregnant women (p<0.0001) and 1.8-fold compared to patients with preeclampsia (p<0.04). When we calculated the propeptide /VWF antigen ratio, this ratio was found to be within the normal range in healthy pregnant women (0.11 ± 0.04) and preeclampsia (0.12 ± 0.03). However, this ratio was significantly increased in HELLP syndrome (0.17 ± 0.05, p<0.05). Because these increased ratios may represent secretion of VWF from the Weibel Palade bodies, these data suggests that acute activation of the endothelium occurs in HELLP syndrome and is the source of circulating active VWF. Therefore, we determined the amounts of active VWF secreted by human umbilical vein endothelial cells (HUVEC) in vitro. Medium containing constitutionally secreted VWF and medium from PMA stimulated endothelial cells was collected and the amount of active VWF was measured in the AU/VWFa-11-based immunosorbant assay. A 1.6-fold increase in activated VWF levels was found in medium containing constitutionally secreted VWF compared to normal pool plasma (p<0.05). The amount of activated VWF was increased even up to 2.2-fold in medium of the stimulated endothelial cells (p<0.001) indicating that activation of endothelium in HELLP could indeed result in increased levels of activated VWF. In conclusion, acute activation of the endothelium in HELLP syndrome results in increased amounts of activated VWF. This might well explain the thrombocytopenia associated with HELLP syndrome.