Von Willebrand factor (VWF) and factor VIII (FVIII) circulate in a tight noncovalent complex. At present, the cells that contribute to the removal of FVIII and VWF are of unknown identity. Here, we analyzed spleen and liver tissue sections of VWFdeficient mice infused with recombinant VWF or recombinant FVIII. This analysis revealed that both proteins were targeted to cells of macrophage origin. When applied as a complex, both proteins were codirected to the same macrophages.Chemical inactivation of macrophages using gadolinium chloride resulted in doubling of endogenous FVIII levels in VWFnull mice, and of VWF levels in wild-type mice. Moreover, the survival of infused VWF was prolonged almost 2-fold in VWFdeficient mice after gadolinium chloride treatment. VWF and FVIII also bound to primary human macrophages in in vitro tests. In addition, radiolabeled VWF bound to human THP1 macrophages in a dosedependent, specific, and saturable manner (half-maximal binding at 0.014 mg/mL). Binding to macrophages was followed by a rapid uptake and subsequent degradation of the internalized protein. This process was also visualized using a VWFgreen fluorescent protein fusion protein.In conclusion, our data strongly indicate that macrophages play a prominent role in the clearance of the VWF/FVIII complex. (Blood. 2008;112:1704-1712)

P. falciparum induces systemic acute endothelial cell activation and release of activated vWF immediately after the onset of blood-stage infection. The resulting platelet agglutination may result in early thrombocytopenia and may play a role in the pathogenesis of malaria.

We demonstrate that C-terminal VWF fragments, as well as an antibody specifically directed toward the VWF D4 domain, inhibit VWF proteolysis by ADAMTS13 under shear conditions. We propose that this novel VWF C-terminal binding site may participate as the initial step of a multistep interaction ultimately leading to proteolysis of VWF by ADAMTS13. (Blood.

A deficiency in ADAMTS13 (a von Willebrand factor [VWF] cleaving protease) is associated with accumulation of prothrombogenic unusually large VWF multimers (UL-VWF) in plasma. We studied VWF release and proteolysis in patients with symptomatic Plasmodium falciparum or P. vivax malaria on the Indonesian island Sumba. Malaria patients had significantly lower platelet counts and higher VWF concentrations and VWF activation factors than healthy hospital staff controls. The latter indicates that a higher amount of circulating VWF was in a conformation enabling spontaneous platelet binding. In addition, ADAMTS13 activity and antigen levels were reduced in both malaria groups, and this was associated with the appearance of UL-VWF. The mechanism behind this reduction and the role in malaria pathogenesis needs to be further elucidated. In malaria, endothelial cell activation with increased circulating amounts of active and ultra-large VWF, together with reduced VWF inactivation by ADAMTS13, may result in intravascular platelet aggregation, thrombocytopenia, and microvascular disease.

Freshly secreted VWF consists of ultra-large multimers that interact spontaneously with platelets at the endothelial cell surface. Proteolysis of ultra-large VWF by a member of the disintegrin and metalloprotease with thrombospondin motif family (ADAMTS13) reduces both multimeric size and accessibility of platelet-adhesion sites. The resulting VWF molecules circulate as inactive multimers, which regain their platelet-adhesion capacity upon binding to the subendothelial matrix, in particular under conditions of high shear. Unfortunately, mechanisms responsible for suppression of circulating plasma levels of active VWF are hampered in a number of pathological conditions, leading to VWF-platelet aggregates associated with thrombotic complications or thrombocytopenia. A recently developed assay allowed us to monitor the presence of circulating active VWF and we found that several diseases are characterized by increased levels. Further analysis provided insight into mechanisms contributing to the presence of active VWF, which revealed that beta2-glycoprotein I may act as a natural regulator of VWF-platelet interactions.

Once thawed, fresh-frozen plasma (FFP) should be used, according to guidelines, within 24 h. In hospital practice, this may be associated with wastage. This study has been performed to investigate the coagulation levels of thawed quarantine FFP as used in the Netherlands. Five units of quarantine FFP, obtained by plasmapheresis, were thawed and by sterile docking divided into satellite bags (SB). SB 2-4 were stored at room temperature (RT) for, respectively, 1, 3 and 6 h and SB 5-9 at 4 degrees C for 6, 12 and 24 h and 1 and 2 weeks. At each time point, activated partial thromboplastin time (APTT), prothrombin time (PT), fibrinogen, factor V (FV), factor VIII (FVIII) and ADAMTS13 activity were measured. During storage at RT for up to 6 h, no major differences were found in the levels of FV, PT, fibrinogen and ADAMTS13 activity. FVIII activity showed a decrease of 16% and the APTT was prolonged by 6%. During storage at 4 degrees C for 2 weeks, FV and FVIII were reduced by 35 and 45%, respectively. The APTT and PT were prolonged by 17 and 15%, respectively. Fibrinogen was decreased by 8%. No change in ADAMTS13 activity was found. FFP stored at RT for 6 h or at 4 degrees C for 2 weeks can provide sufficient support for adequate haemostasis except for patients with a known deficiency for FVIII and can be used for plasmapheresis in patients with thrombotic thrombocytopenic purpura (TTP).

Abstract-The thrombotic microangiopathy observed in malignant hypertension is similar to that of thrombotic thrombocytopenic purpura, which is associated with a deficiency of ADAMTS13, a von Willebrand factor (VWF)-cleaving protease that cleaves large prothrombogenic multimers. We hypothesized that ADAMTS13 is deficient in malignant hypertension and that the severity of thrombotic microangiopathy is associated with decreased ADAMTS13 activity. We included 20 patients with malignant and 20 patients with severe hypertension, and 20 matched normotensive individuals served as control subjects. VWF, active VWF, and free hemoglobin were assessed to explore predictors of ADAMTS13 activity. Patients with malignant hypertension had lower ADAMTS13 activity (80%; interquartile range: 53% to 130%) compared with control subjects (99% interquartile range: 82% to 129%; PϽ0.01) but not compared with patients with severe hypertension (Pϭ0.14). ADAMTS13 activity negatively correlated with lactic dehydrogenase levels after logarithmic transformation (rϭϪ0.65; PϽ0.001) and was associated with platelet count (rϭ0.34; Pϭ0.04) and the presence of schistocytes (rϭϪ0.37; Pϭ0.02). Apart from the association with thrombotic microangiopathy, ADAMTS13 was inversely associated with creatinine (rϭϪ0.42; Pϭ0.008). Increasing levels of VWF were associated with a decrease in ADAMTS13 activity (rϭϪ0.34; Pϭ0.03). There was no significant association between ADAMTS13 activity and other parameters, including blood pressure. In conclusion, ADAMTS13 is decreased in malignant hypertension and associated with the severity of thrombotic microangiopathy, likely because of the release of VWF after endothelium stimulation. Key Words: hypertensive crisis Ⅲ thrombotic microangiopathy Ⅲ coagulation Ⅲ kidney failure Ⅲ endothelium M alignant hypertension is a condition characterized by severe hypertension and acute ischemic complications and frequently complicated by a thrombotic microangiopathy (TMA). TMA is characterized by thrombosis of small vessels, intravascular hemolysis with fragmentation of red blood cells (schistocytes), elevated lactic dehydrogenase (LDH) levels, and consumption of platelets. TMA was observed in 27% of patients with malignant hypertension presenting at the emergency department of our hospital and was associated with renal impairment at presentation but an increased recovery of renal function during follow-up. 1 The pathogenesis of TMA in malignant hypertension is incompletely understood. Several mechanisms may lead to the TMA associated with malignant hypertension.Malignant hypertension bears resemblance to thrombotic thrombocytopenic purpura (TTP). TTP is associated with a congenital or acquired deficiency of ADAMTS13, a zinccontaining metalloprotease that cleaves large von Willebrand factor (VWF) multimers, thereby decreasing their prothrombogenic properties. 2,3 Deficiency of ADAMTS13 (activity) leads to the appearance of unusually large prothrombogenic multimers (UL-VWF) in the circulation resulting in thrombocytopenia, intravascu...

Summary. Background: The effect of exercise on von Willebrand factor (VWF) and ADAMTS-13 levels in individuals with von Willebrand disease (VWD) has never been reported. Objectives: The aim was to quantify the effect of a standardized exercise protocol on individuals with type 1 and type 2B VWD. Patients/methods: Thirty individuals from three groups (10 controls, 11 with type 1 VWD and 9 with type 2B VWD) completed the Standard Bruce Protocol Treadmill Test. A bleeding questionnaire was administered and blood tests were performed pre-and immediately postexercise. The groups were well matched for age, gender and body mass index (BMI). Results: There was a correlation in all groups between the metabolic equivalents (METS) achieved and the degree of change of VWF and FVIII:C levels (P < 0.002, PearsonÕs correlation). There was a significant postexercise increase in VWF:Ag, VWF:RCo, FVIII:C and activated VWF levels in both the control group and in the type 2B VWD group, but not in the type 1 VWD group. Specific to the type 2B VWD group was an increase in the percentage of high molecular weight multimers (P = 0.022), a decrease in the mean platelet count compared with the other groups (P < 0.001) and an increase in the ADAMTS-13 level (P = 0.001). Conclusions: There are significant differences in the effects of exercise on individuals with type 1 and type 2B VWD compared with controls. Further clinical studies are necessary to evaluate exercise as a therapeutic option in VWD.