The active conformation of von Willebrand factor in patients with thrombotic thrombocytopenic purpura in remission

Numerical study of ultra-large von Willebrand factor multimers in coagulopathy

An excessive von Willebrand factor (VWF) secretion, coupled with a moderate to severe deficiency of ADAMTS13 activity, serves as a linking mechanism between inflammation to thrombosis. The former facilitates platelet adhesion to the vessel wall and the latter is required to cleave VWF multimers. As a result, the ultra-large VWF (UL-VWF) multimers released by Weibel-Palade bodies remain uncleaved. In this study, using a computational model based on first principles, we quantitatively show how the uncleaved UL-VWF multimers interact with the blood cells to initiate microthrombosis. We observed that platelets first adhere to unfolded and stretched uncleaved UL-VWF multimers anchored to the microvessel wall. By the end of this initial adhesion phase, the UL-VWF multimers and platelets make a mesh-like trap in which the red blood cells increasingly accumulate to initiate a gradually growing microthrombosis. Although high-shear rate and blood flow velocity are required to activate platelets and unfold the UL-VWFs, during the initial adhesion phase, the blood velocity drastically drops after thrombosis, and as a result, the wall shear stress is elevated near UL-VWF roots, and the pressure drops up to 6 times of the healthy condition. As the time passes, these trends progressively continue until the microthrombosis fully develops and the effective size of the microthrombosis and these flow quantities remain almost constant. Our findings quantitatively demonstrate the potential role of UL-VWF in coagulopathy.

Hemostatic Profile and Serological Response of Patients with Immune Thrombotic Thrombocytopenic Purpura after Receiving BNT162b2 Vaccine: A Prospective Study

INTRODUCTION

 Coronavirus disease is a clinical challenge for patients with autoimmune conditions. Patients affected by immune thrombotic thrombocytopenic purpura (iTTP) are particularly vulnerable to SARS-CoV-2 infection. Protecting these patients with vaccination is therefore mandatory, although concerns may exist on a possible increased thrombotic risk or risk of disease relapse after vaccine exposure. So far, there is no information on serological response and hemostatic activation in iTTP patients after SARS-CoV-2 vaccination.

MATERIALS AND METHODS

 In this study, in April 2021, we enrolled iTTP patients in clinical remission and on regular outpatient follow-up to receive the first and second dose BNT162b2 vaccine as a part of a prospective trial aimed at monitoring for 6 months after vaccination the occurrence of subclinical laboratory signs of clotting activation, as well as overt thrombotic complications or disease relapse. The seroconversion response was monitored in parallel. The results were compared with those of control non-iTTP subjects.

RESULTS

 A moderate decrease of ADAMTS-13 activity was recorded at 3 and 6 months in five patients with normal values at baseline, while an ADAMTS-13 relapse occurred at 6 months in one patient. Abnormalities in the endothelium activation biomarkers postvaccination were observed in iTTP patients compared with controls. The immunological response to vaccine was overall positive. No clinical iTTP relapses or thrombotic events manifested in the 6 month-follow-up after vaccination.

CONCLUSION

 The results of this study are in favor of efficacy and safety of mRNA vaccines in patients with iTTP, and highlight the importance of long-term monitoring of iTTP patients.

Osteoprotegerin modulates platelet adhesion to von Willebrand factor during release from endothelial cells

BACKGROUND

Platelet-binding Von Willebrand Factor (VWF) strings assemble upon stimulated secretion from endothelial cells.

OBJECTIVES

To investigate the efficiency of platelet binding to multi-molecular VWF bundles secreted from endothelial cells and to investigate the role of osteoprotegerin, a protein located in Weibel-Palade bodies that interacts with the VWF platelet binding domain.

METHODS

The nanobody VWF/AU-a11 that specifically binds to VWF in its active platelet-binding conformation was used to investigate the conformation of VWF.

RESULTS

Upon stimulated secretion from endothelial cells, VWF strings were only partially covered with platelets, while a VWD-type 2B mutation or ristocetin enhanced platelet binding by 2-3-fold. Osteoprotegrin, reduces platelet adhesion to VWF by 40% ± 18% in perfusion assays. siRNA-mediated down-regulation of endothelial osteoprotegerin expression resulted in a 1.8-fold increase in platelet adhesion to VWF strings. Upon viral infection, there is a concordant rise in VWF and osteoprotegerin plasma levels. Unexpectedly, no such increase was observed in plasma of desmopressin-treated hemophilia A-patients. In a mouse model, osteoprotegerin expression was low in liver endothelial cells of vehicle-treated mice, and concanavalin A-treatment increased VWF and osteoprotegerin expression 4- and 40-fold, respectively. This increase was translated in a 30-fold increased osteoprotegerin/VWF ratio in plasma.

CONCLUSIONS

Release of VWF from endothelial cells opens the platelet-binding site, irrespective of the presence of flow. However, not all available platelet-binding sites are being occupied, suggesting some extent of regulation. Part of this regulation involves endothelial proteins that are co-secreted with VWF, like osteoprotegerin. This regulatory mechanism may be of more relevance under inflammatory conditions.

Targeting shear gradient activated von Willebrand factor by the novel single-chain antibody A1 reduces occlusive thrombus formation in vitro

Intraluminal thrombus formation precipitates conditions such as acute myocardial infarction and disturbs local blood flow resulting in areas of rapidly changing blood flow velocities and steep gradients of blood shear rate. Shear rate gradients are known to be pro-thrombotic with an important role for the shear-sensitive plasma protein von Willebrand factor (VWF). Here, we developed a single-chain antibody (scFv) that targets a shear gradient specific conformation of VWF to specifically inhibit platelet adhesion at sites of SRGs but not in areas of constant shear. Microfluidic flow channels with stenotic segments were used to create shear rate gradients during blood perfusion. VWF-GPIbα interactions were increased at sites of shear rate gradients compared to constant shear rate of matched magnitude. The scFv-A1 specifically reduced VWF-GPIbα binding and thrombus formation at sites of SRGs but did not block platelet deposition and aggregation under constant shear rate in upstream sections of the channels. Significantly, the scFv A1 attenuated platelet aggregation only in the later stages of thrombus formation. In the absence of shear, direct binding of scFv-A1 to VWF could not be detected and scFV-A1 did not inhibit ristocetin induced platelet agglutination. We have exploited the pro-aggregatory effects of SRGs on VWF dependent platelet aggregation and developed the shear-gradient sensitive scFv-A1 antibody that inhibits platelet aggregation exclusively at sites of shear rate gradients. The lack of VWF inhibition in non-stenosed vessel segments places scFV-A1 in an entirely new class of anti-platelet therapy for selective blockade of pathological thrombus formation while maintaining normal haemostasis.

In vivo modulation of a dominant-negative variant in mouse models of von Willebrand disease type 2A

Essentials Treatment options for von Willebrand disease (VWD) patients are limited. The p.P1127_C1948delinsR deletion/variant is a useful model to study VWD in vitro and in vivo. Counteracting dominant-negative effects restores von Willebrand factor multimerization in mice. This is the first siRNA-based treatment applied to a mouse model of VWD-type 2A. ABSTRACT: Background Treatment options for patients suffering from von Willebrand disease (VWD) are limited. Von Willebrand factor (VWF) is a polymeric protein that undergoes regulated dimerization and subsequent multimerization during its biosynthesis. Numerous heterozygous variants within the VWF gene display a dominant-negative effect and result in severe VWD. Previous studies have suggested that preventing the assembly of wild-type and mutant heteropolymers using siRNAs may have beneficial effects on VWF phenotypes in vitro. Objectives To study heterozygous dominant-negative variants in vivo, we developed a mouse model of VWD-type 2A and tested two independent strategies to modulate its detrimental effect. Methods The p.P1127_C1948delinsR deletion/variant, causing defective VWF multimerization, was expressed in mice as a model of VWD-type 2A variant. Two corrective strategies were applied. For the first time in a mouse model of VWD, we applied siRNAs selectively inhibiting translation of the mutant transcripts and we combined the VWD-type 2A deletion with the Cys to Arg substitution at position 2773, which is known to prevent dimerization. Results The RNA silencing approach induced a modest but consistent improvement of the VWF multimer profile. However, due to incomplete efficiency, the dominant-negative effect of the original variant could not be completely prevented. In contrast, the DNA approach resulted in increased antigen levels and restoration of a normal multimer profile. Conclusions Our data showed that preventing the detrimental impact of dominant-negative VWF variants by independent molecular mechanisms has beneficial consequences in vivo, in mouse models of dominant VWD.

A framework for the optimal design of a minimum set of clinical trials to characterize von Willebrand disease

BACKGROUND AND OBJECTIVE

Von Willebrand disease (VWD) is one of the most severe inherited bleeding disorder in humans, and it is associated with a qualitative and/or quantitative deficiency of von Willebrand factor, a multimeric glycoprotein fundamental in the coagulation process. At present, the diagnosis of VWD is extremely challenging and mostly based on clinical experience. Kinetic models have been recently proposed and applied to help in the diagnosis and characterization of VWD, but the complexity of these models is such that they requires long and stressful clinical tests, such as the desmopressin response test (DDAVP), to achieve a satisfactory estimation of the individual haemostatic parameters. The goal of this paper is to design a minimal set of clinical tests for the identification of akinetic model to decrease the required time and effort for the characterization and diagnosis of VWD.

METHODS

A model proposed in the literature is used as a building block to develop a new model, where response surface methodologies have been applied to determine a set of explicit correlations linkingkinetic model parameters to basal clinical trials data. Model-based design of experiments techniques are then used to devise optimally informative tests for model validation which are shorter and easier to implement.

RESULTS

Results show an excellent agreement between the original model for VWD and the new proposed model on representing healthy and VWD subjects. The application of experimental design techniques for model validation shows the possibility to drastically reduce the duration of DDAVP tests from 24 h-3 h by exploiting complementary information from basal clinical tests.

CONCLUSIONS

Basal clinical tests can be used alongside a time-reduced DDAVP test to validate pharmacokinetic models for a quantitative characterisation of subjects affected by VWD and for a quicker and easier diagnosis of the disease.

Sepsis-associated disseminated intravascular coagulation and its differential diagnoses

Disseminated intravascular coagulation (DIC) is a common complication in sepsis. Since DIC not only promotes organ dysfunction but also is a strong prognostic factor, its diagnosis at the earliest possible timing is important. Thrombocytopenia is often present in patients with DIC but can also occur in a number of other critical conditions. Of note, many of the rare thrombocytopenic diseases require prompt diagnoses and specific treatments. To differentiate these diseases correctly, the phenotypic expressions must be considered and the different disease pathophysiologies must be understood. There are three major players in the background characteristics of thrombocytopenia: platelets, the coagulation system, and vascular endothelial cells. For example, the activation of coagulation is at the core of the pathogenesis of sepsis-associated DIC, while platelet aggregation is the essential mechanism in thrombotic thrombocytopenic purpura and endothelial damage is the hallmark of hemolytic uremic syndrome. Though each of the three players is important in all thrombocytopenic diseases, one of the three dominant players typically establishes the individual features of each disease. In this review, we introduce the pathogeneses, symptoms, diagnostic measures, and recent therapeutic advances for the major diseases that should be immediately differentiated from DIC in sepsis.

Analytical characterization and reference interval of an enzyme-linked immunosorbent assay for active von Willebrand factor

BACKGROUND

Interaction of von Willebrand factor (VWF) with platelets requires a conformational change that exposes an epitope within the VWF A1 domain, enabling platelet glycoprotein Ibα binding. Quantification of this ''active" conformation of VWF has been shown to provide pathophysiological insight into conditions characterized by excessive VWF-platelet interaction.

METHODS

We developed an immunosorbent assay based on a variable heavy chain antibody fragment against the VWF A1 domain as a capture antibody. Assay performance in terms of specificity (binding to active R1306W- and sheared VWF), precision, accuracy, linearity, limits of detection and stability were determined. Active VWF, VWF antigen, VWF ristocetin cofactor activity, VWF:GP1bM and VWF propeptide were measured in citrated plasma and platelet-VWF binding in whole blood from 120 healthy individuals to establish a reference interval for active VWF and to assess associations with other VWF parameters.

RESULTS

Intra- and inter-assay CVs were between 2.4-7.2% and 4.1-9.4%, depending on the level. Mean recovery of spiked recombinant R1306W VWF was 103±3%. The assay was linear in the range of 90.1-424.5% and had a limit of quantification of 101%. The reference interval for active VWF was 91.6-154.8% of NPP. Significant, positive correlations between active VWF and all other VWF parameters were found, with the strongest correlation with VWF:GP1bM binding.

CONCLUSIONS

We developed and validated an immunosorbent assay for the accurate detection of active VWF levels in plasma. The assay fulfilled all analytical criteria in this study and a reference interval was established, allowing its use to quantify active VWF in pathological conditions for future research.

An open conformation of ADAMTS-13 is a hallmark of acute acquired thrombotic thrombocytopenic purpura

Essentials Conformational changes in ADAMTS-13 are part of its mode-of-action. The murine anti-ADAMTS-13 antibody 1C4 discriminates between folded and open ADAMTS-13. ADAMTS-13 conformation is open in acute acquired thrombotic thrombocytopenic purpura (TTP). Our study forms an important basis to fully elucidate the pathophysiology of TTP.

SUMMARY

Background Acquired thrombotic thrombocytopenic purpura (aTTP) is an autoimmune disorder characterized by absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies. Recently, it was shown that ADAMTS-13 adopts a folded or an open conformation. Objectives As conformational changes in self-antigens play a role in the pathophysiology of different autoimmune diseases, we hypothesized that the conformation of ADAMTS-13 changes during acute aTTP. Methods Antibodies recognizing cryptic epitopes in the spacer domain were generated. Next, the conformation of ADAMTS-13 in 40 healthy donors (HDs), 99 aTTP patients (63 in the acute phase versus 36 in remission), 12 hemolytic-uremic syndrome (HUS) patients and 63 sepsis patients was determined with ELISA. Results The antibody 1C4 recognizes a cryptic epitope in ADAMTS-13. Therefore, we were able to discriminate between a folded and an open ADAMTS-13 conformation. We showed that ADAMTS-13 in HDs does not bind to 1C4, indicating that ADAMTS-13 circulates in a folded conformation. Similar results were obtained for HUS and sepsis patients. In contrast, ADAMTS-13 of acute aTTP patients bound to 1C4 in 92% of the cases, whereas, in most cases, this binding was abolished during remission, showing that the conformation of ADAMTS-13 is open during an acute aTTP episode. Conclusions Our study shows that, besides absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies, an open ADAMTS-13 conformation is also a hallmark of acute aTTP. Demonstrating this altered ADAMTS-13 conformation in acute aTTP will help to further unravel the pathophysiology of aTTP and lead to improved therapy and diagnosis.

von Willebrand activation factor as a marker of mortality, cardiovascular events, and bleeding complications in patients treated with oral anticoagulants

BACKGROUND

Serious bleeding is a frequent and feared treatment complication in patients treated with oral anticoagulants (OACs). Levels of von Willebrand factor (VWF) antigen have been linked to the risk of bleeding complications, mortality, and cardiovascular events.

OBJECTIVES

In this longitudinal cohort study of evaluating patients treated with OACs, we aimed to evaluate the relationship between VWF displaying a glycoprotein Ib binding conformation (VWF activation factor) and the risk of cardiovascular events, bleeding complications, or all-cause mortality.

MATERIALS AND METHODS

Blood samples were collected at baseline in 356 patients on OACs. Patients were followed for an average of 48 months and bleeding complications leading to admission to hospital or death, cardiovascular events (myocardial infarction, ischemic stroke, and peripheral arterial emboli), and all-cause mortality were recorded and classified.

RESULTS

During the study period, 47 bleeding complications, 84 cardiovascular events, and 97 deaths occurred. In multivariate Cox regression analyses, VWF activation factor was significantly associated with all-cause mortality (HR 1.62; 95% CI: 1.25-2.08) and cardiovascular events (HR 1.28; 95% CI: 1.01-1.63). There was no association observed between VWF activation factor and bleeding complications.

CONCLUSIONS

Patients with high levels of VWF activation factor had an increased risk of cardiovascular events and all-cause mortality during OAC treatment. The selectivity for thrombotic complications adds to the potential value of VWF activation factor as a biomarker or pharmacological target.

Antibody-based prevention of von Willebrand factor degradation mediated by circulatory assist devices

Haemorrhagic episodes in patients carrying circulatory assist devices represent a severe life-threatening clinical complication. These bleeding episodes may originate from a reduced functionality of von Willebrand factor (VWF), a multimeric protein pertinent to the formation of a haemostatic plug. It has been reported that the reduced functionality is due to increased proteolytic degradation by the enzyme ADAMTS13, a phenomenon that is facilitated by device-induced increases in shear stress to which VWF is exposed. Here, we have tested a series of VWF-derived protein fragments and monoclonal murine anti-VWF antibodies for their capacity to reduce shear stress-dependent degradation of VWF. Via direct binding experiments, we identified an anti-VWF antibody that partially blocked VWF-ADAMTS13 interactions (46 ± 14%). Epitope mapping experiments revealed that the antibody, designated mAb508, is directed against the distal portion of the VWF D4-domain (residues 2134-2301) and recognises a synthetic peptide encompassing residues 2158-2169. Consistent with its partial inhibition of VWF-ADAMTS13 interactions in binding assays, mAb508 reduced ADAMTS13-mediated VWF degradation in a vortex-based degradation assay by 48 ± 10%. In a HeartMateII-based whole blood-perfusion system, mAb508 was able to reduce degradation of high-molecular-weight (HMW)-VWF-multimers dose-dependently, with a maximal inhibition (83 ± 8%) being reached at concentrations of 10 μg/ml or higher. In conclusion, we report that partial inhibition of VWF-ADAMTS13 interactions using an anti-VWF antibody can prevent excessive degradation of HMW-VWF multimers. This strategy may be used for the development of therapeutic options to treat bleeding episodes due to shear stress-dependent VWF degradation, for instance in patients carrying circulatory assist devices.

Plasmin cleavage of von Willebrand factor as an emergency bypass for ADAMTS13 deficiency in thrombotic microangiopathy

BACKGROUND

Von Willebrand factor (VWF) multimer size is controlled through continuous proteolysis by ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type I motif, member 13). This prevents spontaneous platelet agglutination and microvascular obstructions. ADAMTS13 deficiency is associated with thrombotic thrombocytopenic purpura, in which life-threatening episodes of microangiopathy damage kidneys, heart, and brain. Enigmatically, a complete ADAMTS13 deficiency does not lead to continuous microangiopathy. We hypothesized that plasmin, the key enzyme of the fibrinolytic system, serves as a physiological backup enzyme for ADAMTS13 in the degradation of pathological platelet-VWF complexes.

METHODS AND RESULTS

Using real-time microscopy, we determined that plasmin rapidly degrades platelet-VWF complexes on endothelial cells in absence of ADAMTS13, after activation by urokinase-type plasminogen activator or the thrombolytic agent streptokinase. Similarly, plasmin degrades platelet-VWF complexes in platelet agglutination studies. Plasminogen directly binds to VWF and its A1 domain in a lysine-dependent manner, as determined by enzyme-linked immunosorbent assay. Plasma levels of plasmin-α2-antiplasmin complexes increase with the extent of thrombocytopenia in patients with acute episodes of thrombotic thrombocytopenic purpura, independent of ADAMTS13 activity. This indicates that plasminogen activation takes place during microangiopathy. Finally, we show that the thrombolytic agent streptokinase has therapeutic value for Adamts13(-/-) mice in a model of thrombotic thrombocytopenic purpura.

CONCLUSIONS

We propose that plasminogen activation on endothelial cells acts as a natural backup for ADAMTS13 to degrade obstructive platelet-VWF complexes. Our findings indicate that thrombolytic agents may have therapeutic value in the treatment of microangiopathies and may be useful to bypass inhibitory antibodies against ADAMTS13 that cause thrombotic thrombocytopenic purpura.

The prognostic value of ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13) deficiency in septic shock patients involves interleukin-6 and is not dependent on disseminated intravascular coagulation

Introduction

ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13) deficiency has been reported in patients with sepsis but its clinical relevance and pathophysiology remain unclear. Our objectives were to assess the clinical significance, prognostic value and pathophysiology of ADAMTS13 deficiency in patients with septic shock with and without disseminated intravascular coagulation (DIC).

Methods

This was a prospective monocenter cohort study of patients with septic shock. Von Willebrand Factor, ADAMTS13-related parameters and plasma IL-6 concentration were measured at inclusion to the study. Patients were categorized into three groups according to the presence of ADAMT13 deficiency (<30%) or DIC.

Results

This study included 72 patients with a median age of 59 years (interquartile range (IQR) 50 to 71). Each of the included patients received vasopressors; 55 (76%) were under mechanical ventilation and 22 (33%) underwent renal replacement therapy. Overall, 19 patients (26%) had DIC, and 36 patients had ADMTS13 deficiency (50%). Patients with DIC, ADAMTS13 deficiency or both were more severe at ICU admission. Mortality was higher in septic shock patients from group one. By multivariate analysis, Simplified Acute Physiology Score 2 (SAPS2) score (odds ratio (OR) 1.11/point; 95% CI 1.01 to 1.24) and ADAMTS13 activity <30% (OR 11.86; 95% CI 1.36 to 103.52) were independently associated with hospital mortality. There was no correlation between ADAMTS13 activity and the International Society for Thrombosis and Haemostasis (ISTH) score (r_{s =} -0.97, P = 0.41) suggesting that ADAMTS13 functional deficiency and DIC were independent parameters. IL-6 level was higher in patients with ADAMTS13 activity <30% [895 (IQR 330 to 1843) pg/mL versus 83 (IQR 43 to 118), P = 0.0003).

Conclusions

Septic shock was associated with a functional deficiency of ADAMTS13, independently of DIC. ADAMTS13 functional deficiency is then a prognostic factor for mortality in septic shock patients, independently of DIC.

Macrophage LRP1 contributes to the clearance of von Willebrand factor

The relationship between low-density lipoprotein receptor-related protein-1 (LRP1) and von Willebrand factor (VWF) has remained elusive for years. Indeed, despite a reported absence of interaction between both proteins, liver-specific deletion of LRP1 results in increased VWF levels. To investigate this discrepancy, we used mice with a macrophage-specific deficiency of LRP1 (macLRP1(-)) because we previously found that macrophages dominate VWF clearance. Basal VWF levels were increased in macLRP1(-) mice compared with control mice (1.6 ± 0.4 vs 1.0 ± 0.4 U/mL). Clearance experiments revealed that half-life of human VWF was significantly increased in macLRP1(-) mice. Ubiquitous blocking of LRP1 or additional lipoprotein receptors by overexpressing receptor-associated protein in macLRP1(-) mice did not result in further rise of VWF levels (0.1 ± 0.2 U/mL), in contrast to macLRP1(+) mice (rise in VWF, 0.8 ± 0.4 U/mL). This points to macLRP1 being the only lipoprotein receptor regulating VWF levels. When testing the mechanism(s) involved, we observed that VWF-coated beads adhered efficiently to LRP1 but only when exposed to shear forces exceeding 2.5 dyne/cm(2), implying the existence of shear stress-dependent interactions. Furthermore, a mechanism involving β2-integrins that binds both VWF and LRP1 also is implicated because inhibition of β2-integrins led to increased VWF levels in control (rise, 0.19 ± 0.16 U/mL) but not in macLRP1(-) mice (0.08 ± 0.15 U/mL).

Platelet reactive conformation and multimeric pattern of von Willebrand factor in acquired thrombotic thrombocytopenic purpura during acute disease and remission

BACKGROUND

Binding of von Willebrand factor (VWF) multimers of ultra-large size to platelets is considered the triggering mechanism of microvascular thrombosis in thrombotic thrombocytopenic purpura (TTP).

OBJECTIVE

To assess the potential of VWF-related measurements as markers of disease activity and severity in TTP.

METHODS

VWF antigen (VWF:Ag), platelet glycoprotein-Ib-α binding-conformation (GPIb-α/BC) and multimeric pattern were investigated in 74 patients with acquired TTP during acute disease, remission or both and 73 healthy controls. In patients with both acute and remission samples available, VWF ristocetin co-factor activity (VWF:RCo) and collagen binding (VWF:CB) were also measured. The relationships of study measurements with the presence of acute disease and remission and with markers of disease severity were assessed.

RESULTS

VWF:Ag and VWF-GPIb-α/BC were higher in TTP patients than controls (P < 0.001 and 0.004). However, there was no statistically significant difference in VWF-GPIb-α/BC between samples obtained during acute TTP and remission. Larger VWF multimers were frequently lacking in acute TTP patients, who displayed ultra-large multimers at remission. The degree of loss of larger VWF multimers correlated with the degree of abnormality of hemoglobin, platelet counts and serum lactate dehydrogenase (LDH) and was associated with low levels of both VWF:RCo/Ag and VWF:CB/Ag ratios.

CONCLUSIONS

In TTP the platelet-binding conformation of VWF is not exclusively present in acute disease, nor is it associated with its clinical and laboratory severity. The loss of larger VWF multimers, accompanied by low VWF:RCo/Ag and VWF:CB/Ag ratio values, represents an index of disease activity and severity of acute TTP in patients with severe ADAMTS-13 deficiency.

Cleavage of von Willebrand factor by granzyme M destroys its factor VIII binding capacity

Von Willebrand factor (VWF) is a pro-hemostatic multimeric plasma protein that promotes platelet aggregation and stabilizes coagulation factor VIII (FVIII) in plasma. The metalloproteinase ADAMTS13 regulates the platelet aggregation function of VWF via proteolysis. Severe deficiency of ADAMTS13 is associated with thrombotic thrombocytopenic purpura, but does not always correlate with its clinical course. Therefore, other proteases could also be important in regulating VWF activity. In the present study, we demonstrate that VWF is cleaved by the cytotoxic lymphocyte granule component granzyme M (GrM). GrM cleaved both denaturated and soluble plasma-derived VWF after Leu at position 276 in the D3 domain. GrM is unique in that it did not affect the multimeric size and pro-hemostatic platelet aggregation ability of VWF, but instead destroyed the binding of VWF to FVIII in vitro. In meningococcal sepsis patients, we found increased plasma GrM levels that positively correlated with an increased plasma VWF/FVIII ratio in vivo. We conclude that, next to its intracellular role in triggering apoptosis, GrM also exists extracellularly in plasma where it could play a physiological role in controlling blood coagulation by determining plasma FVIII levels via proteolytic processing of its carrier VWF.

Decreased active von Willebrand factor level owing to shear stress in aortic stenosis patients

BACKGROUND

Aortic stenosis patients often show bleeding complications. Previously, a prolonged platelet function analyzer (PFA-100) closure time was observed with plasma of severe aortic stenosis patients. To elucidate a possible role of circulating preactivated von Willebrand factor (VWF), we determined the level of VWF in its active, platelet-binding conformation in plasma of patients with aortic stenosis.

PATIENTS/METHODS

Sixty-two aortic stenosis patients were included in this study. VWF and related parameters were measured, and the results were related to severity of aortic stenosis.

RESULTS

VWF activation factor, indicating the proportion of circulating VWF able to bind to platelets, correlated negatively with peak transvalvular gradient and PFA-100 closure time. No correlation was observed between ADAMTS13 activity and peak transvalvular gradient or PFA-100 closure time. Both VWF antigen and VWF propeptide levels were significantly higher in patients with mild and moderate aortic stenosis, but not in those with severe stenosis.

CONCLUSIONS

Our data demonstrate that the aortic pressure gradient is inversely associated with VWF activation factor, but not with VWF antigen or VWF multimerization in patients with aortic stenosis. These findings might have implications for the bleeding observed in patients with aortic stenosis.

The rate of hemolysis in sickle cell disease correlates with the quantity of active von Willebrand factor in the plasma

Vaso-occlusion, hemolysis, and oxidative stress are hallmarks of sickle cell disease (SCD). This pathology is accompanied by systemic endothelial activation, rendering the endothelium more adhesive for blood cells, including sickle erythrocytes. Activated endothelial cells display or secrete several adhesive molecules, including von Willebrand factor (VWF). We assessed several VWF parameters in SCD patients at baseline: multimer pattern, antigen concentration (VWF:Ag), activation factor (VWF:AF), and total active VWF (VWF:TA). VWF:AF was determined using a llama nanobody (AU/VWFa-11) that detects a platelet-binding conformation of the A1 domain; VWF:TA was calculated by multiplying VWF:Ag by VWF:AF. SCD plasma contained elevated VWF:Ag and ultralarge VWF multimers. VWF:TA, a measure of total VWF reactivity, correlated closely with hemolysis, as determined by serum lactate dehydrogenase. ADAMTS13 activity and antigen were normal in all patients. These findings suggest an important role for hyperreactive VWF in SCD pathology and connect SCD to other microangiopathies, particularly thrombotic thrombocytopenic purpura.

Differential modulation of adhesion molecule expression by hydroxycarbamide in human endothelial cells from the micro- and macrocirculation: potential implications in sickle cell disease vasoocclusive events

BACKGROUND

All the cellular partners of the vascular system and especially endothelial cells are involved in the pathophysiology of the vasoocclusive crises associated with sickle cell disease. In sickle cell disease, circulating cells adhere abnormally to endothelial cells in a chronic pro-inflammatory context. Hydroxycarbamide is the only drug with demonstrated efficacy to reduce the frequency of vasoocclusive crises. Here, we investigated the effects of hydroxycarbamide and/or cytokines on the expression of genes related to adhesion events in endothelial cells from three different vascular sites.

DESIGN AND METHODS

Endothelial cells representative of the macro- (HUVEC) or microcirculation (TrHBMEC and HPMEC) were grown in the presence or absence of hydroxycarbamide and/or cytokines (TNFα and IFNγ). Expression of genes encoding adhesion proteins was analyzed by RQ-PCR, ELISA, flow cytometry, in situ ELISA for extracellular matrix proteins, and Western blot.

RESULTS

In cells from the microcirculation, expression of TSP-1, vWF, and PECAM-1 genes was decreased by hydroxycarbamide and/or cytokine treatment at the mRNA level. In the macro-circulation their expression was unaffected or increased. Hydroxycarbamide significantly decreased vWF incorporated in the TrHBMEC extracellular matrix. CD36 mRNA was strongly down-regulated by cytokines in HPMEC, the only cell type in which it is expressed. Hydroxycarbamide decreased soluble PECAM-1 in HUVEC supernatants.

CONCLUSIONS

Our results highlight the heterogeneity of vascular endothelial cell responses to hydroxycarbamide and/or cytokines depending upon their origin. They also suggest that hydroxycarbamide has an anti-adhesogenic effect on endothelial cells, but by mechanisms which could vary according to their macro- or microcirculation and organ origin.

Mutation and ADAMTS13-dependent modulation of disease severity in a mouse model for von Willebrand disease type 2B

Von Willebrand disease (VWD)–type 2B originates from a gain-of-function mutation in von Willebrand factor (VWF), resulting in enhanced platelet binding. Clinical manifestations include increased bleeding tendency, loss of large multimers, thrombocytopenia, and circulating platelet aggregates. We developed a mouse model to study phenotypic consequences of VWD-type 2B mutations in murine VWF: mVWF/R1306Q and mVWF/V1316M. Both mutations allow normal multimerization but are associated with enhanced ristocetin-induced platelet aggregation, typical for VWD-type 2B. In vivo expression resulted in thrombocytopenia and circulating aggregates, both of which were more pronounced for mVWF/V1316M. Furthermore, both mutants did not support correction of bleeding time or arterial vessel occlusion in a thrombosis model. They further displayed a 2- to 3-fold reduced half-life and induced a 3- to 6-fold increase in number of giant platelets compared with wild-type VWF. Loss of large multimers was observed in 50% of the mice. The role of ADAMTS13 was investigated by expressing both mutants in VWF/ADAMTS13 double-deficient mice. ADAMTS13 deficiency resulted in more and larger circulating platelet aggregates for both mutants, whereas the full multimer range remained present in all mice. Thus, we established a mouse model for VWD-type 2B and found that phenotype depends on mutation and ADAMTS13.

An autoantibody epitope comprising residues R660, Y661, and Y665 in the ADAMTS13 spacer domain identifies a binding site for the A2 domain of VWF

In the majority of patients with acquired thrombotic thrombocytopenic purpura (TTP), antibodies are directed toward the spacer domain of ADAMTS13. We have previously shown that region Y658-Y665 is involved. We now show that replacement of R660, Y661, or Y665 with alanine in ADAMTS13 reduced/abolished the binding of 2 previously isolated human monoclonal antibodies and polyclonal antibodies derived from plasma of 6 patients with acquired TTP. We investigated whether these residues also influenced cleavage of short von Willebrand factor (VWF) fragment substrate VWF115. An ADAMTS13 variant (R660A/Y661A/Y665A, ADAMTS13-RYY) showed a 12-fold reduced catalytic efficiency (k(cat)/K(m)) arising from greatly reduced (> 25-fold) binding, demonstrated by surface plasmon resonance. The influence of these residue changes on full-length VWF was determined with denaturing and flow assays. ADAMTS13-RYY had reduced activity in both, with proteolysis of VWF unaffected by autoantibody. Binding of ADAMTS13-RYY mutant to VWF was, however, similar to normal. Our results demonstrate that residues within Y658-Y665 of the ADAMTS13 spacer domain that are targeted by autoantibodies in TTP directly interact with a complementary exosite (E1660-R1668) within the VWF A2 domain. Residues R660, Y661, and Y665 are critical for proteolysis of short VWF substrates, but wider domain interactions also make important contributions to cleavage of full-length VWF.