R1205H (Vicenza) causes conformational changes in the von Willebrand factor D'D3 domains and enhances von Willebrand factor binding to clearance receptors LRP1 and SR-AI.

BACKGROUND: von Willebrand factor (VWF)-R1205H variant (Vicenza) results in markedly enhanced VWF clearance in humans that has been shown to be largely macrophage-mediated. However, the biological mechanisms underlying this enhanced clearance remain poorly understood. OBJECTIVES: This study aimed to investigate the roles of (i) specific VWF domains and (ii) different macrophage receptors in regulating enhanced VWF-R1205H clearance. METHODS: In vivo clearance of full-length and truncated wild-type (WT)-VWF and VWF with R1205 substitutions was investigated in VWF-/- mice. Plate-binding assays were employed to characterize VWF binding to purified scavenger receptor class A member 1 (SR-AI), low-density lipoprotein receptor-related protein-1 (LRP1) cluster II or cluster IV receptors, and macrophage galactose-type lectin. RESULTS: In full-length VWF missing the A1 domain, introduction of R1205H led to significantly enhanced clearance in VWF-/- mice compared with WT-VWF missing the A1 domain. Importantly, R1205H in a truncated VWF-D'D3 fragment also triggered increased clearance compared with WT-VWF-D'D3. Additional in vivo studies demonstrated that VWF-R1205K (which preserves the positive charge at 1205) exhibited normal clearance, whereas VWF-R1205E (which results in loss of the positive charge) caused significantly enhanced clearance, pinpointing the importance of the positive charge at VWF-R1205. In vitro plate-binding studies confirmed increased VWF-R1205H interaction with SR-AI compared with WT-VWF. Furthermore, significantly enhanced VWF-R1205H binding to LRP1 cluster IV (P < .001) and less marked enhanced binding to LRP1 cluster II (P = .034) was observed. In contrast, VWF-R1205H and WT-VWF demonstrated no difference in binding affinity to macrophage galactose-type lectin. CONCLUSION: Disruption of the positive charge at amino acid R1205 causes conformational changes in the VWF-D'D3 domains and triggers enhanced LRP1-mediated and SR-AI-mediated clearance.

Macrophage Galactose Lectin Contributes to the Regulation of FVIII (Factor VIII) Clearance in Mice-Brief Report.

BACKGROUND: Although most plasma FVIII (Factor VIII) circulates in complex with VWF (von Willebrand factor), a minority (3%-5%) circulates as free-FVIII, which is rapidly cleared. Consequently, 20% of total FVIII may be cleared as free-FVIII. Critically, the mechanisms of free-FVIII clearance remain poorly understood. However, recent studies have implicated the MGL (macrophage galactose lectin) in modulating VWF clearance. METHODS: Since VWF and FVIII share similar glycosylation, we investigated the role of MGL in FVIII clearance. FVIII binding to MGL was assessed in immunosorbent and cell-based assays. In vivo, FVIII clearance was assessed in MGL1-/- and VWF-/-/FVIII-/- mice. RESULTS: In vitro-binding studies identified MGL as a novel macrophage receptor that binds free-FVIII in a glycan-dependent manner. MGL1-/- and MGL1-/- mice who received an anti-MGL1/2 blocking antibody both showed significantly increased endogenous FVIII activity compared with wild-type mice (P=0.036 and P<0.0001, respectively). MGL inhibition also prolonged the half-life of infused FVIII in FVIII-/- mice. To assess whether MGL plays a role in the clearance of free FVIII in a VWF-independent manner, in vivo clearance experiments were repeated in dual VWF-/-/FVIII-/- mice. Importantly, the rapid clearance of free FVIII in VWF-/-/FVIII-/- mice was significantly (P=0.012) prolonged in the presence of anti-MGL1/2 antibodies. Finally, endogenous plasma FVIII levels in VWF-/- mice were significantly increased following MGL inhibition (P=0.016). CONCLUSIONS: Cumulatively, these findings demonstrate that MGL plays an important role in regulating macrophage-mediated clearance of both VWF-bound FVIII and free-FVIII in vivo. We propose that this novel FVIII clearance pathway may be of particular clinical importance in patients with type 2N or type 3 Von Willebrand disease.

von Willebrand factor links primary hemostasis to innate immunity.

The plasma multimeric glycoprotein von Willebrand factor (VWF) plays a critical role in primary hemostasis by tethering platelets to exposed collagen at sites of vascular injury. Recent studies have identified additional biological roles for VWF, and in particular suggest that VWF may play an important role in regulating inflammatory responses. However, the molecular mechanisms through which VWF exerts its immuno-modulatory effects remain poorly understood. In this study, we report that VWF binding to macrophages triggers downstream MAP kinase signaling, NF-κB activation and production of pro-inflammatory cytokines and chemokines. In addition, VWF binding also drives macrophage M1 polarization and shifts macrophage metabolism towards glycolysis in a p38-dependent manner. Cumulatively, our findings define an important biological role for VWF in modulating macrophage function, and thereby establish a novel link between primary hemostasis and innate immunity.

Sustained VWF-ADAMTS-13 axis imbalance and endotheliopathy in long COVID syndrome is related to immune dysfunction.

BACKGROUND: Prolonged recovery is common after acute SARS-CoV-2 infection; however, the pathophysiological mechanisms underpinning Long COVID syndrome remain unknown. VWF/ADAMTS-13 imbalance, dysregulated angiogenesis, and immunothrombosis are hallmarks of acute COVID-19. We hypothesized that VWF/ADAMTS-13 imbalance persists in convalescence together with endothelial cell (EC) activation and angiogenic disturbance. Additionally, we postulate that ongoing immune cell dysfunction may be linked to sustained EC and coagulation activation. PATIENTS AND METHODS: Fifty patients were reviewed at a minimum of 6 weeks following acute COVID-19. ADAMTS-13, Weibel Palade Body (WPB) proteins, and angiogenesis-related proteins were assessed and clinical evaluation and immunophenotyping performed. Comparisons were made with healthy controls (n = 20) and acute COVID-19 patients (n = 36). RESULTS: ADAMTS-13 levels were reduced (p = 0.009) and the VWF-ADAMTS-13 ratio was increased in convalescence (p = 0.0004). Levels of platelet factor 4 (PF4), a putative protector of VWF, were also elevated (p = 0.0001). A non-significant increase in WPB proteins Angiopoietin-2 (Ang-2) and Osteoprotegerin (OPG) was observed in convalescent patients and WPB markers correlated with EC parameters. Enhanced expression of 21 angiogenesis-related proteins was observed in convalescent COVID-19. Finally, immunophenotyping revealed significantly elevated intermediate monocytes and activated CD4+ and CD8+ T cells in convalescence, which correlated with thrombin generation and endotheliopathy markers, respectively. CONCLUSION: Our data provide insights into sustained EC activation, dysregulated angiogenesis, and VWF/ADAMTS-13 axis imbalance in convalescent COVID-19. In keeping with the pivotal role of immunothrombosis in acute COVID-19, our findings support the hypothesis that abnormal T cell and monocyte populations may be important in the context of persistent EC activation and hemostatic dysfunction during convalescence.

The role of VWF/FVIII in thrombosis and cancer progression in multiple myeloma and other hematological malignancies.

Cancer associated thrombosis (CAT) is associated with significant morbidity and mortality, highlighting an unmet clinical need to improve understanding of the pathophysiology of CAT. Multiple myeloma (MM) is associated with one of the highest rates of thrombosis despite widespread use of thromboprophylactic agents. The pathophysiology of thrombosis in MM is multifactorial and patients with MM appear to display a hypercoagulable phenotype with potential contributory factors including raised von Willebrand factor (VWF) levels, activated protein C resistance, impaired fibrinolysis, and abnormal thrombin generation. In addition, the toxic effect of anti-myeloma therapies on the endothelium and contribution to thrombosis has been widely described. Elevated VWF/factor VIII (FVIII) plasma levels have been reported in heterogeneous cohorts of patients with MM and other hematological malignancies. In specific studies, high plasma VWF levels have been shown to associate with VTE risk and reduced overall survival. While the mechanisms underpinning this remain unclear, dysregulation of the VWF and A Disintegrin And Metalloprotease Thrombospondin type 1, motif 13 (ADAMTS-13) axis is evident in certain solid organ malignancies and correlates with advanced disease and thrombosis. Furthermore, thrombotic microangiopathic conditions arising from deficiencies in ADAMTS-13 and thus an accumulation of prothrombotic VWF multimers have been reported in patients with MM, particularly in association with specific myeloma therapies. This review will discuss current evidence on the pathophysiological mechanisms underpinning thrombosis in MM and in particular summarize the role of VWF/FVIII in hematological malignancies with a focus on thrombotic risk and emerging evidence for contribution to disease progression.

Breast cancer cells mediate endothelial cell activation, promoting von Willebrand factor release, tumor adhesion, and transendothelial migration.

BACKGROUND: Breast cancer results in a three- to four-fold increased risk of venous thromboembolism (VTE), which is associated with reduced patient survival. Despite this, the mechanisms underpinning breast cancer-associated thrombosis remain poorly defined. Tumor cells can trigger endothelial cell (EC) activation resulting in increased von Willebrand factor (VWF) secretion. Importantly, elevated plasma VWF levels constitute an independent biomarker for VTE risk. Moreover, in a model of melanoma, treatment with low molecular weight heparin (LMWH) negatively regulated VWF secretion and attenuated tumor metastasis. OBJECTIVE: To investigate the role of VWF in breast cancer metastasis and examine the effect of LMWH in modulating EC activation and breast tumor transmigration. METHODS: von Willebrand factor levels were measured by ELISA. Primary ECs were used to assess tumor-induced activation, angiogenesis, tumor adhesion, and transendothelial migration. RESULTS AND CONCLUSION: Patients with metastatic breast cancer have markedly elevated plasma VWF:Ag levels that also correlate with poorer survival. MDA-MB-231 and MCF-7 breast cancer cells induce secretion of VWF, angiopoietin-2, and osteoprotegerin from ECs, which is further enhanced by the presence of platelets. Vascular endothelial growth factor-A (VEGF-A) plays an important role in modulating breast cancer-induced VWF release. Moreover, VEGF-A from breast tumor cells also contributes to a pro-angiogenic effect on ECs. VWF multimers secreted from ECs, in response to tumor-VEGF-A, mediate adhesion of breast tumor cells along the endothelium. LMWH inhibits VWF-breast tumor adhesion and transendothelial migration. Our findings highlight the significant crosstalk between tumor cells and the endothelium including increased VWF secretion which may contribute to tumor metastasis.

Potential mechanisms of resistance to current anti-thrombotic strategies in Multiple Myeloma.

Multiple Myeloma (MM) is a common haematological malignancy that is associated with a high rate of venous thromboembolism (VTE) with almost 10% of patients suffering thrombosis during their disease course. Recent studies have shown that, despite current thromboprophylaxis strategies, VTE rates in MM remain disappointingly high. The pathophysiology behind this consistently high rate of VTE is likely multifactorial. A number of factors such as anti-thrombin deficiency or raised coagulation Factor VIII levels may confer resistance to heparin in these patients, however, the optimal method of clinically evaluating this is unclear at present, though some groups have attempted its characterisation with thrombin generation testing (TGT). In addition to testing for heparin resistance, TGT in patients with MM has shown markedly varied abnormalities in both endogenous thrombin potential and serum thrombomodulin levels. Apart from these thrombin-mediated processes, other mechanisms potentially contributing to thromboprophylaxis failure include activated protein C resistance, endothelial toxicity secondary to chemotherapy agents, tissue factor abnormalities and the effect of immunoglobulins/"M-proteins" on both the endothelium and on fibrin fibre polymerisation. It thus appears clear that there are a multitude of factors contributing to the prothrombotic milieu seen in MM and further work is necessitated to elucidate which factors may directly affect and inhibit response to anticoagulation and which factors are contributing in a broader fashion to the hypercoagulability phenotype observed in these patients so that effective thromboprophylaxis strategies can be employed.

Sialylation on O-linked glycans protects von Willebrand factor from macrophage galactose lectin-mediated clearance.

Terminal sialylation determines the plasma half-life of von Willebrand factor (VWF). A role for macrophage galactose lectin (MGL) in regulating hyposialylated VWF clearance has recently been proposed. In this study, we showed that MGL influences physiological plasma VWF clearance. MGL inhibition was associated with a significantly extended mean residence time and 3-fold increase in endogenous plasma VWF antigen levels (P<0.05). Using a series of VWF truncations, we further demonstrated that the A1 domain of VWF is predominantly responsible for enabling the MGL interaction. Binding of both full-length and VWF-A1-A2-A3 to MGL was significantly enhanced in the presence of ristocetin (P<0.05), suggesting that the MGL-binding site in A1 is not fully accessible in globular VWF. Additional studies using different VWF glycoforms demonstrated that VWF O-linked glycans, clustered at either end of the A1 domain, play a key role in protecting VWF against MGLmediated clearance. Reduced sialylation has been associated with pathological, increased clearance of VWF in patients with von Willebrand disease. Herein, we demonstrate that specific loss of α2-3 linked sialylation from O-glycans results in markedly increased MGL-binding in vitro, and markedly enhanced MGL-mediated clearance of VWF in vivo. Our data further show that the asialoglycoprotein receptor (ASGPR) does not have a significant role in mediating the increased clearance of VWF following loss of O-sialylation. Conversely however, we observed that loss of N-linked sialylation from VWF drives enhanced circulatory clearance predominantly via the ASGPR. Collectively, our data support the hypothesis that in addition to regulating physiological VWF clearance, the MGL receptor works in tandem with ASGPR to modulate enhanced clearance of aberrantly sialylated VWF in the pathogenesis of von Willebrand disease.

Illustrated State-of-the-Art Capsules of the ISTH 2020 Congress.

This year's Congress of the International Society of Thrombosis and Haemostasis (ISTH) was hosted virtually from Philadelphia July 17-21, 2021. The conference, now held annually, highlighted cutting-edge advances in basic, population and clinical sciences of relevance to the Society. Despite being held virtually, the 2021 congress was of the same scope and quality as an annual meeting held in person. An added feature of the program is that talks streamed at the designated times will then be available on-line for asynchronous viewing. The program included 77 State of the Art (SOA) talks, thematically grouped in 28 sessions, given by internationally recognized leaders in the field. The SOA speakers were invited to prepare brief illustrated reviews of their talks that were peer reviewed and are included in this article. The topics, across the main scientific themes of the congress, include Arterial Thromboembolism, Coagulation and Natural Anticoagulants, COVID-19 and Coagulation, Diagnostics and Omics, Fibrinogen, Fibrinolysis and Proteolysis, Hemophilia and Rare Bleeding Disorders, Hemostasis in Cancer, Inflammation and Immunity, Pediatrics, Platelet Disorders, von Willebrand Disease and Thrombotic Angiopathies, Platelets and Megakaryocytes, Vascular Biology, Venous Thromboembolism and Women's Health. These illustrated capsules highlight the major scientific advances with potential to impact clinical practice. Readers are invited to take advantage of the excellent educational resource provided by these illustrated capsules. They are also encouraged to use the image in social media to draw attention to the high quality and impact of the science presented at the congress.

The role of von Willebrand factor in breast cancer metastasis.

Breast cancer is the most common female cancer globally, with approximately 12% of patients eventually developing metastatic disease. Critically, limited effective treatment options exist for metastatic breast cancer. Recently, von Willebrand factor (VWF), a haemostatic plasma glycoprotein, has been shown to play an important role in tumour progression and metastasis. In breast cancer, a significant rise in the plasma levels of VWF has been reported in patients with malignant disease compared to benign conditions and healthy controls, with an even greater increase seen in patients with disseminated disease. Direct interactions between VWF, tumour cells, platelets and endothelial cells may promote haematogenous dissemination and thus the formation of metastatic foci. Intriguingly, patients with metastatic disease have unusually large VWF multimers. This observation has been proposed to be a result of a dysfunctional or deficiency of VWF-cleaving protease activity, ADAMTS-13 activity, which may then regulate the platelet-tumour adhesive interactions in the metastatic process. In this review, we provide an overview of VWF in orchestrating the pathological process of breast cancer dissemination, and provide supporting evidence of the role of VWF in mediating metastatic breast cancer.

Advances in the Management of Cancer-Associated Thrombosis.

The association between cancer and venous thromboembolism (VTE) has been established for more than 150 years. Nevertheless, cancer-associated thrombosis still remains a major clinical challenge and is associated with significant morbidity and mortality for patients with cancer. The clinical presentation of cancer-associated thrombosis can be distinct from that of a patient without an underlying malignancy. Moreover, specific cancer types, including pancreatic cancer and hematological malignancies, as well as advanced stage disease can confer a significant thrombotic risk. This risk is further augmented by specific anticancer treatment modalities. The pathophysiology of cancer-associated thrombosis is complex and multifactorial. However, understanding the biological mechanisms underpinning VTE risk may provide insight into novel targeted prophylaxis in cancer patients. Over the last decade, low-molecular-weight heparin has been the preferred anticoagulant agent for patients with cancer-associated thrombosis due to improved efficacy compared with vitamin K antagonists. However, the advent of direct oral anticoagulants (DOACs) has added to the repertoire of ammunition now at the disposal of clinicians to aid in the management of cancer-associated thrombosis. Several randomized controlled trials have now been published, demonstrating DOAC as a noninferior alternative for both the treatment and prevention of cancer-associated thrombosis. Notwithstanding this, limitations for their widespread use remain, with the potential for increased bleeding risk, drug interactions, and poor DOAC metabolism. This review discusses the evidence base for the incidence and risk factors associated with VTE in cancer, development, and refinement of risk prediction models and novel advances in the therapeutic management of cancer-associated thrombosis.

Management of elective procedures in low von Willebrand factor patients in the LoVIC study.

BACKGROUND: Most individuals with mild to moderate reductions in plasma von Willebrand factor (VWF) levels do not demonstrate increased bleeding. However, some patients with plasma VWF levels of 30-50 IU/dl do have a significant bleeding phenotype. Management of these "low VWF" patients, who may have significant bleeding scores >10, around times of elective procedures continues to pose a common clinical challenge because of a lack of evidence. OBJECTIVE: To investigate the safety and efficacy of different periprocedural management options for adult patients with low VWF. METHODS: Treatment and outcomes were retrospectively reviewed for 160 invasive procedures performed in 60 patients with well characterized low VWF enrolled in the previously described Low Von Willebrand factor Ireland Cohort study. RESULTS: We demonstrate that 1-desamino-8-D-arginine vasopressin is efficacious in preventing bleeding for both minor or major elective procedures in adult low VWF patients, even in those with significant bleeding histories. In addition, tranexamic acid alone is effective for low VWF patients undergoing nondental minor procedures. Importantly, age-related increases in plasma VWF:antigen levels above 50 IU/dl were not necessarily associated with complete correction of bleeding phenotype. Procedure-related bleeding complications were increased in low VWF patients who did not receive any hemostatic cover before their procedure. CONCLUSION: Elective procedures in adult patients with low VWF should be managed in liaison with a comprehensive care tertiary referral center so that personalized treatment plans may be implemented before all minor or major elective procedures.

Von Willebrand factor and cancer; metastasis and coagulopathies.

Von Willebrand factor (VWF) is a multimeric procoagulant plasma glycoprotein that mediates platelet adhesion along the endothelium. In addition to its role maintaining normal hemostasis, more recently novel biological functions for VWF have been described, including inflammation, angiogenesis, and metastasis. Significantly increased plasma VWF levels have been reported across a variety of cancer patient cohorts. Given that VWF is established as a risk factor for venous thrombosis, this is of direct clinical importance. Moreover, elevated VWF has also been observed localized within the tumor microenvironment, correlating with advanced disease stage and poorer clinical outcome. Critically, evidence suggests that elevated VWF levels in cancer patients may not only contribute to cancer associated coagulopathies but may also mediate cancer progression and metastasis. Studies have shown that VWF can promote pro-inflammatory signaling, regulate angiogenesis and vascular permeability, which may facilitate tumor cell growth and extravasation across the vessel wall. Endothelial secreted VWF multimers contribute to the adhesion and transendothelial migration of tumor cells key for tumor dissemination. In support of this, VWF inhibition attenuated metastasis in vivo. Perhaps most intriguingly, specific tumor cells have been reported to acquire de novo VWF expression which increases tumor-platelet heteroaggregates and confers enhanced metastatic activity. Current knowledge on the roles of VWF in cancer and in particular its contribution to metastasis and cancer associated coagulopathies is summarized in this review.

A novel role for the macrophage galactose-type lectin receptor in mediating von Willebrand factor clearance.

Previous studies have shown that loss of terminal sialic acid causes enhanced von Willebrand factor (VWF) clearance through the Ashwell-Morrell receptor (AMR). In this study, we investigated (1) the specific importance of N- vs O-linked sialic acid in protecting against VWF clearance and (2) whether additional receptors contribute to the reduced half-life of hyposialylated VWF. α2-3-linked sialic acid accounts for <20% of total sialic acid and is predominantly expressed on VWF O-glycans. Nevertheless, specific digestion with α2-3 neuraminidase (α2-3Neu-VWF) was sufficient to cause markedly enhanced VWF clearance. Interestingly, in vivo clearance experiments in dual VWF-/-/Asgr1-/- mice demonstrated enhanced clearance of α2-3Neu-VWF even in the absence of the AMR. The macrophage galactose-type lectin (MGL) is a C-type lectin that binds to glycoproteins expressing terminal N-acetylgalactosamine or galactose residues. Importantly, the markedly enhanced clearance of hyposialylated VWF in VWF-/-/Asgr1-/- mice was significantly attenuated in the presence of an anti-MGL inhibitory antibody. Furthermore, dose-dependent binding of human VWF to purified recombinant human MGL was confirmed using surface plasmon resonance. Additionally, plasma VWF:Ag levels were significantly elevated in MGL1-/- mice compared with controls. Collectively, these findings identify MGL as a novel macrophage receptor for VWF that significantly contributes to the clearance of both wild-type and hyposialylated VWF.

Emerging Roles for von Willebrand Factor in Cancer Cell Biology.

von Willebrand factor (VWF) is a complex multimeric plasma glycoprotein that plays critical roles in normal hemostasis. However, additional novel roles for VWF in modulating cancer cell biology, and in particular tumor metastasis, have recently been reported. Markedly elevated plasma VWF levels were associated with advanced tumor stage and metastatic disease. These observations have raised the question of whether VWF may be involved in regulating tumor progression. Interestingly, novel findings indicate that VWF is expressed by a variety of tumor cells of nonendothelial origin. Critically, tumor cells that exhibit de novo acquired VWF expression demonstrate enhanced binding to endothelial cells (EC) and platelets, and increased extravasation through EC barriers. Furthermore, in vitro studies have shown that VWF can bind a variety of different tumor cells mediated by specific receptors expressed on the tumor cell surface. The concept that VWF is important in modulating tumor metastasis is further supported by in vivo experiments demonstrating that antibody-mediated VWF inhibition significantly attenuated murine metastasis. Intriguingly, however, VWF binding to specific human tumor cell lines results in apoptosis. In this study, the authors provide an overview of recent advances supporting a role for VWF in regulating multiple aspects of cancer cell biology.

N-linked glycans within the A2 domain of von Willebrand factor modulate macrophage-mediated clearance.

Enhanced von Willebrand factor (VWF) clearance is important in the etiology of von Willebrand disease. However, the molecular mechanisms underlying VWF clearance remain poorly understood. In this study, we investigated the role of VWF domains and specific glycan moieties in regulating in vivo clearance. Our findings demonstrate that the A1 domain of VWF contains a receptor-recognition site that plays a key role in regulating the interaction of VWF with macrophages. In A1-A2-A3 and full-length VWF, this macrophage-binding site is cryptic but becomes exposed following exposure to shear or ristocetin. Previous studies have demonstrated that the N-linked glycans within the A2 domain play an important role in modulating susceptibility to ADAMTS13 proteolysis. We further demonstrate that these glycans presented at N1515 and N1574 also play a critical role in protecting VWF against macrophage binding and clearance. Indeed, loss of the N-glycan at N1515 resulted in markedly enhanced VWF clearance that was significantly faster than that observed with any previously described VWF mutations. In addition, A1-A2-A3 fragments containing the N1515Q or N1574Q substitutions also demonstrated significantly enhanced clearance. Importantly, clodronate-induced macrophage depletion significantly attenuated the increased clearance observed with N1515Q and N1574Q in both full-length VWF and A1-A2-A3. Finally, we further demonstrate that loss of these N-linked glycans does not enhance clearance in VWF in the presence of a structurally constrained A2 domain. Collectively, these novel findings support the hypothesis that conformation of the VWF A domains plays a critical role in modulating macrophage-mediated clearance of VWF in vivo.

A novel role for von Willebrand factor in the pathogenesis of experimental cerebral malaria.

Plasmodium falciparum malaria infection is associated with an early marked increase in plasma von Willebrand factor (VWF) levels, together with a pathological accumulation of hyperreactive ultra-large VWF (UL-VWF) multimers. Given the established critical role of platelets in malaria pathogenesis, these increases in plasma VWF raise the intriguing possibility that VWF may play a direct role in modulating malaria pathogenesis. To address this hypothesis, we used an established murine model of experimental cerebral malaria (ECM), in which wild-type (WT) C57BL/6J mice were infected with Plasmodium berghei ANKA. In keeping with findings in children with P falciparum malaria, acute endothelial cell activation was an early and consistent feature in the murine model of cerebral malaria (CM), resulting in significantly increased plasma VWF levels. Despite the fact that murine plasma ADAMTS13 levels were not significantly reduced, pathological UL-VWF multimers were also observed in murine plasma following P berghei infection. To determine whether VWF plays a role in modulating the pathogenesis of CM in vivo, we further investigated P berghei infection in VWF(-/-) C57BL/6J mice. Clinical ECM progression was delayed, and overall survival was significantly prolonged in VWF(-/-) mice compared with WT controls. Despite this protection against ECM, no significant differences in platelet counts or blood parasitemia levels were observed between VWF(-/-) and WT mice. Interestingly, however, the degree of ECM-associated enhanced blood-brain barrier permeability was significantly attenuated in VWF(-/-) mice compared with WT controls. Given the significant morbidity and mortality associated with CM, these novel data may have direct translational significance.