Mechanisms of microvascular thrombosis in thrombotic thrombocytopenic purpura

VWF/ADAMTS13 Ratio as a Potential Predictive Biomarker for Acute Kidney Injury Onset in Cirrhosis

AIM

We investigated the von Willebrand factor to ADAMTS13 ratio (von Willebrand factor [VWF]:Ag/ADAMTS13:AC) as a potential biomarker for the outcomes of acute kidney injury (AKI) in liver cirrhosis (LC).

METHODS

This retrospective cross-sectional study included patients with LC who developed AKI (AKI group: n = 91) and patients with LC who did not develop AKI [non-AKI (NAKI) group, n = 91] as a control group. Plasma levels of the von Willebrand factor antigen (Ag) and ADAMTS13 activity (AC) were measured in patients with AKI or NAKI. Moreover, risk factors for onset of AKI, AKI-associated 90-day mortality, and poor AKI treatment response were identified.

RESULTS

The AKI group had a significantly higher VWF:Ag/ADAMTS13:AC than the NAKI group. Values of VWF:Ag/ADAMTS13:AC ≥ 5.7 were identified as risk factors for AKI onset in patients with LC (odds ratio [OR] 2.56; 95% CI 1.26-4.99; p < 0.001). Among patients with AKI, values of VWF:Ag/ADAMTS13:AC ≥ 9.0 were identified as risk factors for 90-day mortality (OR 6.83; 95% CI 2.32-20.10; p < 0.001). Cumulative survival was significantly lower in those with high (≥ 9.0) than in those with low (< 9.0) VWF:Ag/ADAMTS13:AC. Furthermore, values of VWF:Ag/ADAMTS13:AC ≥ 7.4 were identified as risk factors for poor treatment response (OR 4.2; 95% CI 1.39-12.70; p < 0.001). The treatment response rates were significantly higher in those with low (< 7.4) VWF:Ag/ADAMTS13:AC than in those with high (≥ 7.4) VWF:Ag/ADAMTS13:AC.

CONCLUSION

VWF:Ag/ADAMTS13:AC potentially predicts the onset, prognosis, and treatment response of AKI in patients with LC.

Evaluation and Management of Thrombotic Thrombocytopenic Purpura in the Emergency Department

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is a dangerous condition that can be misdiagnosed in the emergency department.

OBJECTIVE

The purpose of this narrative review article is to provide a summary of the background, pathophysiology, diagnosis, and management of TTP, with a focus on emergency clinicians.

DISCUSSION

TTP is a disorder with microangiopathic hemolytic anemia, severe thrombocytopenia, and multiorgan ischemic injury. It may be acquired or hereditary, and is caused by a reduced amount or function of a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13), which is an enzyme involved in cleaving von Willebrand factor. The classic presentation of TTP includes fever, neurologic abnormalities, thrombocytopenia with purpura, microangiopathic hemolytic anemia, and acute renal injury. However, < 7% of cases have all of these findings present. Testing should include a complete blood count, complete metabolic panel, blood smear, coagulation panel, fibrinogen, D-dimer, lactate dehydrogenase, ADAMTS13 level, troponin, human immunodeficiency virus assessment, urinalysis, pregnancy test as appropriate, and electrocardiogram. Management includes hematology consultation if available, plasma exchange and corticosteroids, and treatment of end-organ complications. All patients require admission for treatment and close monitoring.

CONCLUSION

TTP is a potentially dangerous medical condition requiring rapid diagnosis and management. It is essential for emergency clinicians to know how to diagnose and treat this disorder.

Formation and Resolution of Pial Microvascular Thrombosis in a Mouse Model of Thrombotic Thrombocytopenic Purpura

OBJECTIVE

Microvascular thrombosis is the hallmark pathology of thrombotic thrombocytopenic purpura (TTP), a rare life-threatening disease. Neurological dysfunction is present in over 90% of patients with TTP, and TTP can cause long-lasting neurological damage or death. However, the pathophysiology of microvascular thrombosis in the brain is not well studied to date. Here, we investigate the formation and resolution of thrombosis in pial microvessels. Approach and Results: Using a cranial intravital microscopy in well-established mouse models of congenital TTP induced by infusion of recombinant VWF (von Willebrand factor), we found that soluble VWF, at high concentration, adheres to the endothelium of the vessel wall, self-associates, and initiates platelet adhesion resulting in the formation of pial microvascular thrombosis in ADAMTS13^-/- (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) mice. Importantly, VWF-mediated pial microvascular thrombosis occurred without vascular injury to the brain, and thrombi consisted of resting platelets adhered onto ultra-large VWF without fibrin in the brain in rVWF (recombinant VWF) challenged ADAMTS13^-/- mice. Prophylactic treatment with recombinant ADAMTS13 (BAX930) effectively prevented the onset of the VWF-mediated microvascular thrombosis and therapeutic treatment with BAX930 acutely resolved the preexisting or growing thrombi in the brain of ADAMTS13^-/- mice after rVWF challenge. The absence of platelet activation and fibrin formation within VWF-mediated thrombi and efficacy of BAX930 was confirmed with an endothelial-driven VWF-mediated microvascular thrombosis model in mice.

CONCLUSIONS

Our results provide important insight into the initiation and development of microvascular thrombi in mouse models that mimics TTP and indicate that rADAMTS13 could be an effective interventional therapy for microvascular thrombosis, the hallmark pathology in TTP.

Potential influences of complement factor H in autoimmune inflammatory and thrombotic disorders

Complement system homeostasis is important for host self-protection and anti-microbial immune surveillance, and recent research indicates roles in tissue development and remodelling. Complement also appears to have several points of interaction with the blood coagulation system. Deficiency and altered function due to gene mutations and polymorphisms in complement effectors and regulators, including Factor H, have been associated with familial and sporadic autoimmune inflammatory - thrombotic disorders, in which autoantibodies play a part. These include systemic lupus erythematosus, rheumatoid arthritis, atypical haemolytic uremic syndrome, anti-phospholipid syndrome and age-related macular degeneration. Such diseases are generally complex - multigenic and heterogeneous in their symptoms and predisposition/susceptibility. They usually need to be triggered by vascular trauma, drugs or infection and non-complement genetic factors also play a part. Underlying events seem to include decline in peripheral regulatory T cells, dendritic cell, and B cell tolerance, associated with alterations in lymphoid organ microenvironment. Factor H is an abundant protein, synthesised in many cell types, and its reported binding to many different ligands, even if not of high affinity, may influence a large number of molecular interactions, together with the accepted role of Factor H within the complement system. Factor H is involved in mesenchymal stem cell mediated tolerance and also contributes to self-tolerance by augmenting iC3b production and opsonisation of apoptotic cells for their silent dendritic cell engulfment via complement receptor CR3, which mediates anti-inflammatory-tolerogenic effects in the apoptotic cell context. There may be co-operation with other phagocytic receptors, such as complement C1q receptors, and the Tim glycoprotein family, which specifically bind phosphatidylserine expressed on the apoptotic cell surface. Factor H is able to discriminate between self and nonself surfaces for self-protection and anti-microbe defence. Factor H, particularly as an abundant platelet protein, may also modulate blood coagulation, having an anti-thrombotic role. Here, we review a number of interaction pathways in coagulation and in immunity, together with associated diseases, and indicate where Factor H may be expected to exert an influence, based on reports of the diversity of ligands for Factor H.

Idiopathic thrombotic thrombocytopenic purpura: strongest risk factor for relapse from remission is having had a relapse

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is a rare, episodic clinical syndrome involving the production of thrombi in the microvasculature accompanied by thrombocytopenia and symptoms of organ ischemia. Idiopathic TTP develops when a patient produces antibodies that react with the protease ADAMTS13. The course after an episode is unpredictable; patients may relapse frequently or never. There is no laboratory value that can reliably predict potential relapse.

STUDY DESIGN AND METHODS

To assess diagnostic and predictive values for risk of relapse, plasma samples from 27 patients with idiopathic TTP in remission were analyzed for anti-ADAMTS13 immunoglobulin (Ig)G, ADAMTS13 activity, and ADAMTS13 inhibitor titer. Patients were recruited at the Department of Hematology at the University Medical Center, Johannes Gutenberg University Mainz, Mainz, Germany.

RESULTS

Anti-ADAMTS13 IgG was detected in 12 patients (44%); their median level of ADAMTS13 activity was nondetectable. Patients with anti-ADAMTS13 IgG had a median number of three previous relapses, whereas the 15 patients without presence of IgG (56%) had a median number of one previous relapse (p < 0.001; Mann-Whitney U test). The concentration of free anti-ADAMTS13 IgG and the levels measuring inhibitory activity (Bethesda unit) were positively correlated.

CONCLUSION

A subgroup of TTP patients in remission with anti-ADAMTS13 IgG and nondetectable ADAMTS13 activity showed an increased risk for relapsing disease as demonstrated by their number of past relapses. The positive correlation we observed between anti-ADAMTS13 IgG and inhibitor levels supports the theory of ADAMTS13 inhibition as the crucial mechanism causing severe deficiency in ADAMTS13 activity in TTP.

Thrombotic thrombocytopenic purpura and the atypical hemolytic uremic syndrome: an update

Since the last review in 2007 of thrombotic thrombocytopenic purpura (TTP) and microangiopathic hemolytic anemia in the Clinics, further understanding of the nature of TTP and atypical hemolytic uremic syndrome (aHUS) has led to increasing use of rituximab in the treatment of TTP and the approval in 2011 of eculizumab for the treatment of aHUS. With this new armamentarium, distinction of aHUS from TTP has become more critical than ever. This article updates the new knowledge, highlights the difference between aHUS and TTP, and presents a scheme for their diagnosis and management.

On the versatility of von Willebrand factor

Von Willebrand factor (VWF) is a large multimeric protein, the function of which has been demonstrated to be pivotal to the haemostatic system. Indeed, quantitative and/or qualitative abnormalities of VWF are associated with the bleeding disorder Von Willebrand disease (VWD). Moreover, increased plasma concentrations of VWF have been linked to an increased risk for thrombotic complications. In the previous decades, many studies have contributed to our understanding of how VWF is connected to the haemostatic system, particularly with regard to structure-function relationships. Interactive sites for important ligands of VWF (such as factor VIII, collagen, glycoprotein Ibα, integrin αIIbβ3 and protease ADAMTS13) have been identified, and mutagenesis studies have confirmed the physiological relevance of the interactions between VWF and these ligands. However, we have also become aware that VWF has a more versatile character than previously thought, given its potential role in various non-hemostatic processes, like intimal thickening, tumor cell apoptosis and inflammatory processes. In the presence review, a summary of our knowledge on VWF structure-function relationships is provided in the context of the “classical” haemostatic task of VWF and in perspective of pathological processes beyond haemostasis.

Ultralarge von Willebrand factor fibers mediate luminal Staphylococcus aureus adhesion to an intact endothelial cell layer under shear stress

BACKGROUND

During pathogenesis of infective endocarditis, Staphylococcus aureus adherence often occurs without identifiable preexisting heart disease. However, molecular mechanisms mediating initial bacterial adhesion to morphologically intact endocardium are largely unknown.

METHODS AND RESULTS

Perfusion of activated human endothelial cells with fluorescent bacteria under high-shear-rate conditions revealed 95% attachment of the S aureus by ultralarge von Willebrand factor (ULVWF). Flow experiments with VWF deletion mutants and heparin indicate a contribution of the A-type domains of VWF to bacterial binding. In this context, analyses of different bacterial deletion mutants suggest the involvement of wall teichoic acid but not of staphylococcal protein A. The presence of inactivated platelets and serum increased significantly ULVWF-mediated bacterial adherence. ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin motifs 13) caused a dose-dependent reduction of bacterial binding and a reduced length of ULVWF, but single cocci were still tethered by ULVWF at physiological levels of ADAMTS13. To further prove the role of VWF in vivo, we compared wild-type mice with VWF knockout mice. Binding of fluorescent bacteria was followed in tumor necrosis factor-α-stimulated tissue by intravital microscopy applying the dorsal skinfold chamber model. Compared with wild-type mice (n=6), we found less bacteria in postcapillary (60±6 versus 32±5 bacteria) and collecting venules (48±5 versus 18±4 bacteria; P<0.05) of VWF knockout mice (n=5).

CONCLUSIONS

Our data provide the first evidence that ULVWF contributes to the initial pathogenic step of S aureus-induced endocarditis in patients with an apparently intact endothelium. An intervention reducing the ULVWF formation with heparin or ADAMTS13 suggests novel therapeutic options to prevent infective endocarditis.

Double filtration plasmapheresis in different diseases in Thailand

Double filtration plasmapheresis (DFPP) was applied to the treatment of two different categories from 100 cases that had been collected over a 5 year period (2007-2011). These categories were allocated into groups by size of toxic substances, which were classified as two different kinds of diseases. Group I comprised diseases that were caused by alloimmunity in transplantation, autoimmune diseases, complicated nephrotic syndrome, pure red cell aplasia, and toxemia of pregnancy. This group was treated with a plasma separator (plasmaflow-05, Asahi Kasei) and plasma fractionators, EC-20W. The second group, which included hyperviscosity syndrome, was treated by the same plasma separator, but with different plasma fractionators using EC-40W. This group included diabetes nephropathy, hyperlipidemia, peripheral arterial diseases, and neurosensory hearing loss. Both groups used 1.5 plasma volumes in each treatment for three sessions in two consecutive weeks. The result of treatment in group I showed that plasma immunoglobulin G (IgG) was decreased substantially by 66% in either transplant or lupus nephritis patients after the third session. In the second group, IgM, fibrinogen, and lipid markedly responded to the treatment. Two diabetes nephropathy patients showed stable renal function for more than 12 months. Peripheral arterial disease was shown to benefit from significantly decreasing fibrinogen and IgM, which resulted in clinical tissue oxygenation. Neither bleeding diathesis nor membrane anaphylaxis were reported from the treatment. In summary, apheresis patients were shown to benefit in hypersensitized and hyperviscosity syndrome.

Thrombotic microangiopathy in two tiger snake envenomations

Thrombotic microangiopathies are a rare group of disorders with features such as microangiopathic haemolytic anaemia, thrombocytopenia and renal failure. Thrombotic microangiopathy has been previously reported in association with envenomation from a number of snake species. We present the first two reported cases of thrombotic microangiopathy caused by envenomation from the common tiger snake (Notechis scutatus). Both patients had classical features of thrombotic microangiopathy with microangiopathic haemolytic anaemia, thrombocytopenia and renal failure commencing in the first 48 hours after envenomation. The presentations and recovery were similar to case presentations of other snakebite envenomation associated thrombotic microangiopathies. Normal ADAMTS13 activity suggests that plasmapharesis may not be beneficial, although this needs further investigation.

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.

Excessive activation of the complement system in atypical hemolytic uremic syndrome: is it ready for prime time?

Complement factor I (CFI) mutations are implicated in the pathogenesis of atypical hemolytic uremic syndrome (aHUS). Nevertheless, there is evidence that CFI deficiency is a weak effector of aHUS. Bienaime et al. report that homozygous deletion of CFHR-1 in the RCA gene cluster of chromosome 1q is a major risk factor for poor outcome for patients with CFI mutations. The basic and clinical implications of the findings are further elaborated here.

Production, crystallization and preliminary crystallographic analysis of an exosite-containing fragment of human von Willebrand factor-cleaving proteinase ADAMTS13

ADAMTS13 is a reprolysin-type metalloproteinase belonging to the ADAMTS (a disintegrin and metalloproteinase with thrombospondin type 1 motif) family. It specifically cleaves plasma von Willebrand factor (VWF) and regulates platelet adhesion and aggregation. ADAMTS13 is a multi-domain enzyme. In addition to the N-terminal metalloproteinase domain, the ancillary domains, including a disintegrin-like domain, a thrombospondin-1 type 1 repeat, a Cys-rich domain and a spacer domain, are required for VWF recognition and cleavage. In the present study, a fragment of the ADAMTS13 ancillary domains (ADAMTS13-DTCS; residues 287-685) was expressed using CHO Lec cells, purified and crystallized. Diffraction data sets were collected using the SPring-8 beamline. Two ADAMTS13-DTCS crystals with distinct unit-cell parameters generated data sets to 2.6 and 2.8 A resolution, respectively.