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

Summary.  Background:  Functional deficiency of ADAMTS13 in thrombotic thrombocytopenic purpura (TTP) patients is associated with circulating ultralarge von Willebrand factor (VWF) molecules that display spontaneous platelet-binding capacities. Upon remission, however, ADAMTS13 activity does not always return to baseline. Objective:  To study ADAMTS13 and VWF-related features in TTP patients in remission. Methods:  ADAMTS13 activity, anti-ADAMTS13 antibodies, VWF antigen, ultralarge VWF and levels of VWF that circulate in a glycoprotein Ibα-binding conformation were determined in plasma samples of 22 acquired TTP patients in remission between 1 month and 6 years after achieving remission. The composition of active multimers was investigated with a novel immunoprecipitation assay based on monoclonal antibody AU/VWF-a12, which specifically recognizes the active conformation of VWF. Results:  ADAMTS13 activity was undetectable in 23% of the patients, even years after they had achieved remission, and lack of ADAMTS13 activity was associated with increased active VWF levels and the presence of ultralarge VWF multimers. Active VWF levels and ultralarge VWF were also associated with blood groups. Results from immunoprecipitation experiments revealed the full range of multimers to be present. Conclusion : ADAMTS13 deficiency and the concurrent presence of ultralarge VWF and increased active VWF levels can be detected in TTP patients for years after they have achieved remission. Immunoprecipitation results suggest that the active conformation of VWF may be present in the lower molecular weight multimers, but future studies are necessary to confirm our findings.     

Recombinant ADAMTS13 normalizes von Willebrand factor‐cleaving activity in plasma of acquired TTP patients by overriding inhibitory antibodies

Summary. Background: Severe deficiency of the von Willebrand factor (VWF)‐cleaving protease ADAMTS13 as observed in acquired thrombotic thrombocytopenic purpura (TTP) is caused by inhibitory and non‐inhibitory autoantibodies directed against the protease. Current treatment with plasma exchange is considered to remove circulating antibodies and to concurrently replenish the deficient enzyme. Objectives: To explore the use of recombinant ADAMTS13 (rADAMTS13) as a potential therapeutic agent in acquired TTP, we investigated its efficacy in normalizing VWF‐cleaving activity in the presence of ADAMTS13 inhibitors. Methods: Thirty‐six plasma samples from TTP patients were adjusted to predefined inhibitor titers, and recovery of ADAMTS13 activity was analyzed following supplementation with rADAMTS13. Results: We showed a linear relation between the inhibitor titer measured and effective rADAMTS13 concentration necessary for reconstitution of VWF‐cleaving activity in the presence of neutralizing autoantibodies. Conclusions: Our results support the further investigation of the potential therapeutic applicability of rADAMTS13 as an adjunctive therapy in acquired TTP.     

Novel monoclonal antibody-based enzyme immunoassay for determining plasma levels of ADAMTS13 activity

BACKGROUND: ADAMTS13 specifically cleaves unusually large von Willebrand factor (VWF) multimers, which induce platelet thrombi formation under high shear stress. ADAMTS13 activity is deficient in patients with thrombotic thrombocytopenic purpura (TTP). The determination of plasma levels of ADAMTS13 activity is a prerequisite for a differential diagnosis of thrombotic microangiopathies. Here, a unique and highly sensitive enzyme immunoassay (EIA) of ADAMTS13 activity is described. STUDY DESIGN AND METHODS: ADAMTS13 hydrolyzes the peptide bond between Y1605 and M1606 of VWF. In this assay, a recombinant fusion protein (GST-VWF73-His) is used as a substrate. A panel of mouse monoclonal antibodies (MoAbs) that specifically recognizes Y1605, which is the C-terminal edge residue of the VWF-A2 domain and is generated by the enzymatic cleavage, has been produced. These antibodies were prepared with a synthetic decapeptide, termed N-10 (1596-DREQAPNLVY-1605), as the immunogen. Twenty-six clones specific to N10 were obtained, and one anti-N10 MoAb was used in this study. RESULTS: With horseradish peroxidase-conjugated anti-N10 MoAb, a standard enzyme assay was established. This assay was highly sensitive, and the detection limit was 0.5 percent of the normal. Further, an inhibitor of ADAMTS13 was measured to a level of 0.1 Bethesda units per mL. ADAMTS13 activity was measured in 20 patients with Upshaw-Schulman syndrome, a congenital TTP, and 61 acquired TTP patients. The activity measured by this assay and by the classic VWF multimer assay showed high correlation. CONCLUSION: A convenient and highly sensitive EIA for ADAMTS13 activity has been established. This assay can be introduced for routine laboratory work in transfusion medicine.     

A phenotype–genotype correlation of ADAMTS13 mutations in congenital thrombotic thrombocytopenic purpura patients treated in the United Kingdom

Summary. Background: ADAMTS13 mutations play a role in thrombotic thrombocytopenic purpura (TTP) pathogenesis. Objectives: To establish a phenotype–genotype correlation in a cohort of congenital TTP patients. Patients/Methods: Clinical history and ADAMTS13 activity, antigen and anti‐ADAMTS13 antibody assays were used to diagnose congenital TTP, and DNA sequencing and in vitro expression were performed to identify the functional effects of the ADAMTS13 mutations responsible. Results: Seventeen (11 novel) ADAMTS13 mutations were identified in 17 congenital TTP patients. All had severely reduced ADAMTS13 activity and antigen levels at presentation. Six patients with pregnancy‐associated TTP and six patients with childhood TTP were homozygous or compound heterozygous for ADAMTS13 mutations located in the metalloprotease (MP), cysteine‐rich, spacer and/or distal thrombospondin type 1 domains. The adults had TTP precipitated by pregnancy, and had overall higher antigen levels (median, 30 ng mL^?1; range, < 10–57 ng mL^?1) than the children (median, 14 ng mL^?1; range, < 10–40 ng mL^?1). Presentation in the neonatal period was associated with more intensive treatment requirements. The two neonates with the most severe phenotype had mutations in the first thrombospondin type 1 motif of ADAMTS13 (p.R398C, p.R409W, and p.Q436H). Using transfected HEK293T cells, we have shown that p.R398C and p.R409W block ADAMTS13 secretion, whereas p.Q436H allows secretion at reduced levels. Conclusions: This study confirms the heterogeneity of ADAMTS13 defects and an association between ADAMTS13 genotypes and TTP phenotype.     

A shear-based assay for assessing plasma ADAMTS13 activity and inhibitors in patients with thrombotic thrombocytopenic purpura

BACKGROUND: Severe deficiency of plasma ADAMTS13 activity is a frequent finding in patients with hereditary and acquired thrombotic thrombocytopenic purpura (TTP). To date, plasma ADAMTS13 activity is determined by cleavage of either predenatured von Willebrand factor (VWF) or small peptides derived from the VWF‐A2 domain. The physiologic relevance of the assay results is uncertain. STUDY DESIGN AND METHODS: We sought to develop a novel shear‐based assay to assess plasma ADAMTS13 activity and inhibitors. We also compared this assay with a fluorogenic peptide assay. RESULTS: We found that an incubation of purified plasma VWF with 0.5 to 1.0 µL of citrated plasma under constant vortexing at 2500 rpm for 60 minutes in the presence of 5 mmol/L CaCl₂ and 1.7 µmol/L ZnCl₂ and low concentration of NaCl resulted in the maximal cleavage of VWF. The cleavage product could be separated by a 2.5% agarose gel and detected by Western blotting. The assay revealed that plasma and recombinant ADAMTS13 are highly sensitive to inhibition by zinc and chloride ions. Under the optimal conditions, the shear‐based assay appeared to be more sensitive than the guanidine‐denaturization assay for determining plasma ADAMTS13 activity. CONCLUSIONS: Our fluid shear‐based assay may be useful for investigating basic biologic function and regulation of ADAMTS13 metalloprotease. It may also be applicable for assessing plasma ADAMTS13 activity and inhibitors in TTP patients.     

Comparison of von Willebrand factor antigen, von Willebrand factor-cleaving protease and protein S in blood components used for treatment of thrombotic thrombocytopenic purpura

Replacement of normal levels of von Willebrand factor-cleaving protease (VWF:CP, ADAMTS13) activity from infused plasma is important in plasma exchange (PEX) for the treatment of thrombotic thrombocytopenic purpura (TTP) patients. We have studied the VWF:CP activity, VWF multimer distribution, VWF:Ag, protein S (PS) activity and free PS antigen levels in fresh frozen plasma (FFP), cryosupernatant (CSP) and virally inactivated components treated with methylene blue/light (MB) or solvent detergent (SD) processes. VWF:CP activity was normal in all components tested and was retained following overnight storage at room temperature. CSP and SD plasma contained reduced levels of the highest molecular weight VWF multimers. Protein S activity was reduced below the normal range in SD plasma, but within the normal range for the other components tested. Virally inactivated SD- and MB-treated plasma may be an effective alternative to FFP and CSP in PEX for TTP. Reduced PS activity in SD plasma may predispose to venous thromboembolism, especially if infused in large volumes.     

Rituximab maintenance for relapsed refractory thrombotic thrombocytopenic purpura

BACKGROUND: Rituximab, an anti‐CD20 chimeric monoclonal antibody, has been used successfully to treat patients with relapsed or refractory thrombotic thrombocytopenic purpura (TTP); however, the optimal dose and frequency and the role of rituximab maintenance remain uncertain. STUDY DESIGN AND METHODS: We describe a 45‐year‐old woman with chronic relapsing immune thrombocytopenia who responded to rituximab retreatment administered in four doses over the course of 12 months. Previously, she had received four doses of rituximab and sustained a remission for 19 months. During her latest TTP relapse, multiple treatments were administered including rituximab retreatment. After the first dose (375 mg/m²), she developed serum sickness requiring further doses to be deferred. Three subsequent doses were administered at 4‐month intervals over the course of 12 months. ADAMTS13 activity was measured by von Willebrand factor (VWF) digestion. ADAMTS13 inhibition was measured by a modification of the VWF digestion assay and anti‐ADAMTS13 antibodies were measured by enzyme‐linked immunoassay (enzyme‐linked immunosorbent assay, American Diagnostica). RESULTS: Clinical and laboratory remission were achieved after one dose of rituximab, with normalization of ADAMTS13 activity and disappearance of ADAMTS13 inhibitor. Three subsequent doses of rituximab were given without incident and the patient remained in remission after 3.5 years of follow‐up (2.5 years since her last dose of rituximab). CONCLUSION: Maintenance dosing of rituximab should be considered in some patients with relapsing TTP.     

影响获得性血栓性血小板减少性紫癜首次缓解期内复发的相关指标研究

目的 探讨血管性血友病因子裂解蛋白酶(ADAMTS) 13活性和抗ADAMTS13抗体表达水平,与获得性血栓性血小板减少性紫癜(TTP)于首次缓解期内复发的关系.方法 选择2008年3月至2014年6月于陕西延安大学附属医院和陕西省渭南市富平县医院诊治的37例获得性TTP患者为研究对象,按照其在首次缓解随访期内是否复发,分为研究组(n=15)和对照组(n=22).分别采用残余胶原结合试验、ELISA、免疫印迹等方法,检测两组患者的ADAMTS13活性,ADAMTS13抗原水平,抗ADAMTS13抗体,ADAMTS13抑制物,血管性血友病因子(vWF)抗原和超大分子量vWF(ULVWF)多聚体等指标,并且进行统计学分析;采用多因素非条件logistic回归分析法,评估获得性TTP患者于首次缓解期内复发的独立影响因素.本研究遵循的程序符合病例收集医院人体试验委员会所制定的伦理学标准,得到该伦理会批准,分组征得受试对象本人的知情同意,并与之签订临床研究知情同意书.结果 ①研究组患者的中位ADAMTS13活性为11％(7％～124％),低于对照组的53％(7％～151％),差异有统计学意义(u=4.018,P＜0.05).研究组与对照组患者的ADAMTS13活性显著降低率分别为53.3％(8/15)和22.7％(5/22),二者比较,差异有统计学意义(P=0.049).②37例获得性TTP患者的血浆ADAMTS13活性和ADAMTS13抗原水平呈正相关关系(rs=0.810,P=0.001).研究组患者的中位ADAMTS13抗原水平为33％(3％～99％),低于对照组的59％(3％～128％),差异有统计学意义(u=4.121,P＜0.05).研究组与对照组ADAMTS13抗原水平显著降低率分别为13.3％(2/15)和9.1％(2/22),二者比较,差异有统计学意义(P=0.008).③研究组患者的抗ADAMTS13抗体检出率为66.7％(10/15),高于对照组的36.4％(8/22),差异有统计学意义(P=0.007).④研究组患者中抗ADAMTS13抑制物检出率为46.7％ (7/15),高于对照组患者的18.2％ (4/22),差异有统计学意义(P=0.011).⑤研究组与对照组患者ULVWF多聚体检出率分别为20.0％(3/15)和13.6％(3/22),二者比较,差异有统计学意义(P=0.042).⑥多因素非条件logistic回归分析结果显示,获得性TTP患者于首次缓解期内复发的独立危险因素包括ADAMTS13活性显著降低(OR=2.95,95％ CI:1.13～6.96,P＜0.05),检出抗ADAMTS13抗体(OR=3.31,95％CI:1.08～8.19,P＜0.05),检出抗ADAMTS13抑制物(OR=3.24,95 ％CI:1.24～9.03,P＜0.05).结论 获得性TTP患者在首次缓解期内,ADAMTS13活性水平显著降低,存在抗ADAMTS13抗体及抗ADAMTS13抑制物,会显著增加疾病复发风险,可以考虑将其作为预测获得性TTP患者于首次缓解期内复发的重要指标.     

VH1-69 germline encoded antibodies directed towards ADAMTS13 in patients with acquired thrombotic thrombocytopenic purpura

Summary.  Background: Autoantibodies directed towards ADAMTS13 are present in the majority of patients with acquired thrombotic thrombocytopenic purpura (TTP). Analysis of a set of antibodies derived from two patients with acquired TTP revealed frequent use of the VH1-69 heavy chain gene segment for the assembly of anti-ADAMTS13 antibodies. Objective: We explored the ability of two VH1-69 germline gene-encoded antibodies to inhibit the von Willebrand factor (VWF)-processing activity of ADAMTS13 under different experimental conditions. Furthermore, the presence of VH1-69 encoded anti-ADAMTS13 antibodies in 40 patients with acquired TTP was monitored using monoclonal antibody G8, which specifically reacts with an idiotype expressed on VH1-69 encoded antibodies. Methods and Results: Binding of the two VH1-69 encoded monoclonal antibodies was dependent on the presence of the spacer domain. Both antibodies inhibited ADAMTS13 activity under static conditions, as measured by cleavage of FRETS-VWF73 substrate and cleavage of VWF multimers. The recombinant antibodies were also capable of inhibiting the processing of UL-VWF strings on the surface of endothelial cells. G8-reactive antibodies directed towards ADAMTS13 were present in plasma of all patients containing anti ADAMTS13 antibodies. Conclusions: These results suggest that VH1-69 derived antibodies directed towards ADAMTS13 develop in the majority of patients with acquired TTP.     

Blood group O and black race are independent risk factors for thrombotic thrombocytopenic purpura associated with severe ADAMTS13 deficiency

BACKGROUND: It has been postulated that blood group O subjects may be partially protected against thrombotic thrombocytopenic purpura (TTP) because they have lower plasma levels of von Willebrand factor. STUDY DESIGN AND METHODS: The Oklahoma TTP Registry enrolled 301 consecutive patients from November 13, 1995 (when systematic ADAMTS13 measurements began), through 2009; 281 (93%) patients had ADAMTS13 measurements. Patients were designated as having severe ADAMTS13 deficiency when the activity measurement by either method was less than 10%. ABO blood group was determined in all 281 patients. The observed frequency of blood group O was compared to the expected frequency. The association between severe ADAMTS13 deficiency and blood group, race, sex, and age were analyzed by logistic regression. RESULTS: The frequency of blood group O was unexpectedly and significantly greater than the race‐ethnicity–adjusted expected frequency in 65 patients with severe ADAMTS13 deficiency (60.0% vs. 47.4%, p = 0.042) but not in 216 patients without severe ADAMTS13 deficiency (44.9% vs. 46.5%, p = 0.639). Blood group O and race‐ethnicity were independently associated with severe ADAMTS13 deficiency among patients with TTP. The probability for severe ADAMTS13 deficiency was 45.8% with O and 32.1% with non‐O blood groups for black patients and 24.1% with O and 15.1% with non‐O blood groups for white patients. CONCLUSION: Among patients with TTP and severe ADAMTS13 deficiency the relative frequency of patients with blood group O was greater than expected, suggesting that blood group O may be a risk factor for TTP associated with severe ADAMTS13 deficiency.     

The role of ADAMTS13 in the pathogenesis of thrombotic thrombocytopenic purpura-hemolytic uremic syndrome

The identification, characterization, and clinical observation of ADAMTS13 (a disintegrin and metalloprotease with thrombospondin-1-like domains) have provided important insights into the pathogenesis of thrombotic thrombocytopenic purpura (TTP). ADAMTS13 is a plasma enzyme essential for postsecretion proteolytic processing of von Willebrand factor (VWF). Absence of ADAMTS13 is associated with the occurrence of abnormally large multimers of VWF and is also associated with the occurrence of TTP. Initial assumptions that absent ADAMTS13 was itself the etiology of TTP have been tempered by subsequent observations that ADAMTS13 activity can be severely deficient without clinical abnormalities and that patients can have characteristic clinical features of TTP without severe ADAMTS13 deficiency. A current interpretation of these observations is that ADAMTS13 deficiency is a major risk factor for the development of TTP, but it is neither always necessary nor sufficient to cause TTP. This interpretation is consistent with other vascular and thrombotic disorders in which multiple risk factors and associated conditions contribute to the etiology of acute events.     

血栓性血小板减少性紫癜患者vWF裂解蛋白酶抗原和活性的检测及意义

目的研究血管性血友病因子裂解蛋白酶(ADAMTS13)抗原含量和活性在血栓性血小板减少性紫癜(TTP)患者及遗传性 TTP 家族突变携带者中变化的情况。方法用残余胶原结合实验(RCBA)检测13例 TTP 患者共28份血浆标本[含血浆置换(PE)前后]及10例携带者的 ADAMTS13活性;用新近建立的三抗体夹心酶联免疫反应法检测标本的 ADAMTS13抗原含量。结果正常对照组 ADAMTS13含量为(600.93±145.36)mU/ml(设白种人混合血浆的 ADAMTS13抗原含量为1000mU/ml),活性为(74.79±11.81)%。遗传性 TTP 患者 ADAMTS13抗原含量和活性治疗前和发病间期均明显减低,PE 后恢复;其家族中携带者 ADAMTS13抗原含量为(331.40±109.85)mU/ml,活性为(66.79±12.82)%(与对照组比较,P 值分别<0.01和>0.05);原发性 TTP 患者 PE 前 ADAMTS13抗原含量为(98.7±82.08)mU/ml,活性为(22.23±19.07)%(与对照组比较,P 值均<0.01);PE 后ADAMTS13 抗原含量为(449.4±232.33)mU/ml,活性为(60.92±22.33)%(与对照组比较,P 值分别<0.01和>0.05);1例继发性 TTP 患者 PE 后 ADAMTS13抗原含量远高于正常,活性仅为6.00%结论治疗前的 TTP 患者 ADAMTS13抗原含量和活性均明显减低。大多数患者两指标变化趋势一致,也有个别患者两指标变化趋势相反,前者可能因为遗传因素或体内免疫系统的廓清作用,后者可能因为抗 ADAMTS13抗体仅抑制了 ADAMTS13的活性而未影响其抗原的含量或其他未知原因所致。     

Von Willebrand factor and ADAMTS13 balancing primary haemostasis

Von Willebrand factor (VWF) is an adhesive, multi-functional huge multimerized protein with multiple domains harboring binding sites for collagen, platelet glycoprotein receptors and coagulation factor VIII (FVIII). The functional domains enable VWF to bind to the injured vessel wall, to recruit platelets to the site of injury by adhesion and aggregation and to bind and protect FVIII, an important cofactor of the coagulation cascade. VWF function in primary haemostasis is located in particular in the arterial and micro-circulation. This environment is exposed to high shear forces with hydrodynamic shear rates ranging over several orders of magnitude from 10?1 to 10? s-1 and requires particular mechanisms to enable platelet adhesion and aggregation under these variable conditions. The respective VWF function is strictly correlating with its multimer size. Lack or reduction of large VWF multimers is seen in patients with von Willebrand disease (VWD) type 2A which correlates with reduction of both VWF:platelet GPIb-binding and VWF:collagen binding and a bleeding phenotype. To prevent unlimited platelet adhesion and aggregation which is the cause of the microangiopathic disorder thrombotic thrombocytopenic purpura (TTP), VWF function is regulated by its specific protease ADAMTS13. Whereas a particular susceptibility of VWF to ADAMTS13 proteolysis is the cause of a frequent VWD type 2A phenotype, lack or dysfunction of ADAMTS13, either acquired by ADAMTS13 antibodies or by inherited ADAMTS13 deficiency (Upshaw-Schulman Syndrome), causes TTP. Therefore VWD and TTP represent the opposite manifestations of VWF related disorders, tightly linked to each other.     

Rituximab pharmacokinetics during the management of acute idiopathic thrombotic thrombocytopenic purpura

Summary. Background: Increasingly, patients with acute, idiopathic, antibody mediated thrombotic thrombocytopenic purpura (TTP) are being treated with rituximab to achieve a durable remission, however, there is the potential that it is removed by plasma exchange (PEX). Objectives: To look at the pharmacokinetics and pharmacodynamics of rituximab in patients with acute idiopathic TTP undergoing PEX. Patients and methods: Patients who received rituximab for acute idiopathic TTP (group 1, n = 30) and a control group (group 2, n = 3) of TTP patients in remission receiving rituximab electively as maintenance were included. Rituximab levels were measured before/after each infusion, before/after PEX and in follow‐up. ADAMTS‐13 activity, anti‐ADAMTS‐13 IgG and CD19% were measured to assess response. Results: The median number of PEX to remission after rituximab was 10 (range 4–25). In group 1 there was no significant incremental rise in the peak serum rituximab level until dose 4. Trough levels were lower in patients who had had PEX since their last rituximab infusion. In the control group, there was an incremental rise in the peak serum rituximab level and all patients had detectable trough levels. The median fall in rituximab per PEX was 65%. All patients achieved CD19 < 1%. In group 1, the median time to undetectable rituximab was 5 months (range 0–12 months) and to B cell return was 7 months (range 3–24 months). ADAMTS‐13 increased and anti‐ADAMTS‐13 fell after therapy. There were three deaths and two relapses in group 1. Relapse was not temporally related to B cell return.     

Is it quinine TTP/HUS or quinine TMA? ADAMTS13 levels and implications for therapy

Thrombocytopenia with or without microangiopathy following quinine is often referred to as quinine “hypersensitivity.” When schistocytes are present it is frequently termed “quinine‐associated TTP/HUS.” A severe deficiency of the vWF‐cleaving protease, ADAMTS13, is associated with idiopathic TTP. A previous study of patients with “quinine‐associated TTP/HUS” found that ADAMTS13 activities were not abnormal in 12/12 patients. A retrospective review of TTP patients with quinine‐associated thrombotic microangiopathy (TMA) for whom ADAMTS13 was measured before plasma exchange was performed. Six patients were identified. All were females (age range: 43 to 73, mean = 61.7 years) and had taken quinine for leg cramps. Four of the six experienced renal failure requiring dialysis. Five of the patients had D‐Dimers levels measured, all were elevated. In four patients the levels were ≥18 times the upper limit of normal. ADAMTS13 was normal in four patients and mildly decreased in two patients. We conclude that while thrombocytopenia and schistocytosis can be seen in quinine‐associated TTP/HUS, the pathophysiology seems to be distinct from that seen in most cases of idiopathic TTP (i.e., severely decreased ADAMTS13 with an inhibitor). We recommend that a TMA in association with quinine be consistently referred to as quinine‐associated thrombotic microangiopathy (quinine‐TMA) to better distinguish this entity from idiopathic TTP. The use of plasma exchange in quinine‐TMA is called into question. J. Clin. Apheresis, 2009. © 2009 Wiley‐Liss, Inc.     

Complement activation in thrombotic thrombocytopenic purpura

Summary. Background: Ultra‐large von Willebrand factor and deficiency of its cleaving protease are important factors in the events leading to thrombotic microangiopathy; however, the mechanisms involved are only partly understood. Whereas pathological activation of the alternative complement pathway is linked to atypical hemolytic uremic syndrome, the role of complement activation in thrombotic thrombocytopenic purpura (TTP) is unknown. The aim of this study was to investigate whether signs of complement activation are characteristic of TTP. Patients and methods: Twenty‐three patients with TTP (18 women, median age 38 years) and 17 healthy controls (13 women, median age 38 years) were included. Complement parameters (C3, Factors H, I, B and total alternative pathway activity) together with complement activation fragments (C3a) or complexes (C1rs‐INH, C3bBbP, sC5b9) were measured by ELISA or RID. ADAMTS13 activity and anti‐ADAMTS13 inhibitory antibodies were measured by the VWF‐FRET73 assay. Results: Increased levels of C3a, and SC5b9 were observed in TTP during acute episodes, as compared with healthy controls. Decreased complement C3 levels indicative of complement consumption occurred in 15% of acute TTP patients. Significant decrease of complement activation products C3a and SC5b9 was observed during plasma exchange (PEX). The sustained presence of anti‐ADAMTS13 inhibitory antibodies in complete remission was associated with increased complement activation. Conclusion: These data document in an observational study the presence of complement activation in TTP. Further investigation is needed to determine its potential pathogenetic significance.     

ADAMTS13: origins,applications, and prospects

ADAMTS13 is an enzyme that acts by cleaving prothrombotic von Willebrand factor (VWF) multimers from the vasculature in a highly regulated manner. In pathologic states such as thrombotic thrombocytopenic purpura (TTP) and other thrombotic microangiopathies (TMAs), VWF can bind to the endothelium and form large multimers. As the anchored VWF chains grow, they provide a greater surface area to bind circulating platelets (PLTs), generating unique thrombi that characterize TTP. This results in microvasculature thrombosis, obstruction of blood flow, and ultimately end‐organ damage. Initial presentations of TTP usually occur in an acute manner, typically developing due to an autoimmune response toward, or less commonly a congenital deficiency of, ADAMTS13. Triggers for TMAs that can be associated with ADAMTS13 deficiency, including TTP, have been linked to events that place a burden on hemostatic regulation, such as major trauma and pregnancy. The treatment plan for cases of suspected TTP consists of emergent therapeutic plasma exchange that is continued on a daily basis until normalization of PLT counts. However, a subset of these patients does not respond favorably to standard therapies. These patients necessitate a better understanding of their diseases for the advancement of future therapeutic options. Given ADAMTS13’s key role in the cleavage of VWF and the prevention of PLT‐rich thrombi within the microvasculature, future treatments may include anti‐VWF therapeutics, recombinant ADAMTS13 infusions, and ADAMTS13 expression via gene therapy.     

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

Summary. 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.     

Acute activation of the endothelium results in increased levels of active von Willebrand factor in hemolysis, elevated liver enzymes and low platelets (HELLP) syndrome

BACKGROUND: HELLP (hemolysis, elevated liver enzymes and low platelets) syndrome is a severe complication of pre-eclampsia in pregnancy, characterized by microvascular platelet thrombi. Activation of the endothelium is thought to play a key role in pre-eclampsia and HELLP syndrome. Activation of endothelial cells may lead to release of von Willebrand factor (VWF) multimers, which are highly reactive with platelets. Normally, newly released multimers are cleaved by ADAMTS13, resulting in less reactive derivatives. OBJECTIVE: We hypothesized that HELLP syndrome is characterized by increased amounts of active VWF compared with healthy pregnancy and pre-eclampsia, due to acute activation of endothelial cells. This might contribute to thrombocytopenia and thrombotic microangiopathy. METHODS: Active VWF and ADAMTS13 activity were measured in healthy pregnant volunteers (n = 9), patients with pre-eclampsia (n = 6) and patients with HELLP syndrome (n = 14) at similar gestational ages. To study the role of endothelial cell activation, the propeptide/mature VWF ratio was determined, and VWF released by cultured endothelial cells was analyzed. RESULTS: Active VWF levels were increased 2.1-fold in HELLP syndrome compared with healthy pregnant volunteers (P < 0.001) and 1.6-fold compared with patients with pre-eclampsia (P = 0.001). ADAMTS13 activity was moderately decreased in patients with HELLP syndrome compared with healthy pregnant volunteers (P < 0.004), but not compared with patients with pre-eclampsia. The propeptide/mature VWF ratio was increased 1.7-fold compared with healthy pregnant volunteers (P < 0.001) and 1.5-fold compared with patients with pre-eclampsia (P < 0.05). A significant correlation was found between this ratio and the activation factor of VWF (r = 0.68, P < 0.001). The amount of active VWF was increased 1.4-fold in medium of stimulated endothelial cells when compared with non-stimulated cells (P < 0.05). CONCLUSION: Acute endothelial cell activation in HELLP syndrome and decreased ADAMTS13 activity result in increased amounts of active VWF. This might explain the consumptive thrombocytopenia and thrombotic microangiopathy associated with HELLP syndrome. Inhibition of circulating active VWF could be a potential new approach in the treatment of patients with HELLP syndrome.     

Use of a mouse model to elucidate the phenotypic effects of the von Willebrand factor cleavage mutants,Y1605A/M1606A and R1597W

Background:  von Willebrand Factor (VWF) is tightly regulated by the metalloproteinase ADAMTS13, which cleaves VWF to reduce VWF multimer size and binding affinity for collagen and platelets. Objective:  This study examines two VWF mutations, R1597W (enhanced cleavage) and Y1605A‐M1606A (decreased cleavage), to determine their impact on VWF, in addition to ADAMTS13‐mediated cleavage. Methods:  In vitro mouse ADAMTS13 digestions were performed on recombinant proteins. VWF knockout mice received hydrodynamic injections of mouse Vwf cDNA, following which VWF antigen, multimer profile and VWF propeptide levels were determined. A ferric chloride injury model of thrombosis was also evaluated. Results:  In vitro ADAMTS13 digestion of full‐length mouse VWF required > 97‐fold higher ADAMTS13 levels for Y1605A/M1606A, and 68% lower ADAMTS13 levels for R1597W compared with wild type. In vivo, R1597W had reduced VWF:Ag and both mutations exhibited increased VWF propeptide/VWF:Ag ratios. R1597W multimers show a lower molecular weight profile compared with wild type and Y1605A/M1606A mice. When co‐injected with Adamts13 cDNA, Y1605A/M1606A multimers were larger compared with wild type, and R1597W showed only a single multimer band and decreased clearance via VWFpp/VWF:Ag ratio. R1597W was associated with reduced thrombus formation but normal platelet accumulation in a ferric chloride injury model while Y1605A/M1606A had a loss of occlusive thrombi but increased platelet accumulation compared with wild type. Conclusions:  This study demonstrates that mutations that alter ADAMTS13 cleavage also can affect VWF clearance, VWF antigen level, multimer structure and thrombotic potential in the VWF knockout hydrodynamic injection model.