Deficient activity of von Willebrand factor-cleaving protease in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a dramatic intravascular platelet-clumping disorder characterized by microangiopathic hemolytic anemia, thrombocytopenia, neurologic abnormalities, renal insufficiency and fever. TTP is a rare disease but is almost always fatal if untreated. More than 80% of patients survive with plasma therapy. In healthy individuals, the proteolytic cleavage of ultralarge von Willebrand factor (vWF) multimers prevents spontaneous clumping of platelets in the microcirculation. Patients with TIP have either severe congenital deficiency of von Willebrand factor-cleaving protease (vWF-cp), or have autoantibodies that inhibit the protease. Determination of vWF-cp levels in patient plasma helps to distinguish between TTP and other thrombotic microangiopathies with similar clinical signs and symptoms. vWF-cp is a member of the ADAMTS family of metalloproteases and has been designated ADAMTS13.     

von Willebrand因子裂解酶活性水平的检测及其临床应用

血管性血友病因子裂解酶 (vWF cp)是新近发现的一个金属蛋白酶 ,其活性在多种生理病理状态下发生改变。为了研究vonWillebrand因子裂解酶活性检测及其临床应用 ,用ELISA法检测了vWF cp酶解前及裂解后血清 (浆 )中血管性血友病因子 (vWF)的胶原结合能力 ,两者之比作为该待测血清 (浆 )的相对vWF cp活性水平。同时观察血栓性血小板减少性紫癜 (TTP)患者与肿瘤患者体内vWF cp活性的改变。结果表明 ,该方法准确测出了87名健康成人和 79例患者血样的残余胶原结合力 (R CBA) ,批内变异和批间变异分别为 3.6 0 % (n =9)和 8.35 %(n =5 )。 5 3名健康成人血清vWF cp活性水平为 (79.4 7± 10 .78) % ,30名健康成人血浆vWF -cp活性水平为(78.79± 9.17) %。 6例TTP患者中 5例血浆vWF cp活性水平显著降低 ,2 5例良性肿瘤患者血清vWF cp水平轻度降低 ,4 9例恶性肿瘤患者则明显下降 (P值分别小于 0 .0 0 1,0 .0 3和 0 .0 0 1)。结论 :以R CBA检测vWF cp活性水平简单易行 ,可应用于临床检验。恶性肿瘤患者 ,尤其是TTP患者血浆 (清 )vWF cp活性水平显著降低。     

Cloning, expression and functional characterization of the full-length murine ADAMTS13.

Functional deficiency or absence of the human von Willebrand factor (VWF)-cleaving protease (VWF-cp), recently termed ADAMTS13, has been shown to cause acquired and congenital thrombotic thrombocytopenic purpura (TTP), respectively. As a first step towards developing a small animal model of TTP, we have cloned the complete (non-truncated) murine Adamts13 gene from BALB/c mice liver poly A+ mRNA. Murine ADAMTS13 is a 1426-amino-acid protein with a high homology and similar structural organization to the human ortholog. Transient expression of the murine Adamts13 cDNA in HEK 293 cells yielded a protein with a molecular weight of approximately 180 kDa which degraded recombinant murine VWF (rVWF) in a dose-dependent manner. The cleavage products of murine rVWF had the expected size of 140 and 170 kDa. Murine ADAMTS13 was inhibited by EDTA and the plasma from a TTP patient.     

血小板内血管性血友病因子裂解酶表达的鉴定

目的　血管性血友病因子裂解酶(vWF cp)是一种新发现的血浆金属蛋白酶,他在人外周血细胞中的 表达尚待确定。本研究确定了该酶在血小板中的表达。方法　应用逆转录 聚合酶链反应(RT PCR)、流式细胞 术(FCM)和免疫共沉淀鉴定了血小板内vWF cpmRNA和蛋白的表达情况。同时应用FCM检测了白细胞、红细 胞内vWF cp蛋白的表达。结果　RT PCR显示,血小板中存在该蛋白酶mRNA的表达。同时,免疫沉淀及蛋白 印迹结果表明,血小板内存在相对分子质量约200000的vWF cp蛋白。FCM结果显示,经穿膜处理的血小板可 检测到vWF cp蛋白的明显表达,而不经穿膜处理以及经过穿膜处理的白细胞和红细胞中不存在该酶的表达。 结论　本研究不仅证实了外周血血小板中存在vWF cp,由于血小板亦为血管性血友病因子的一个主要合成/贮 存场所,这将有助于深入阐明血小板在血栓形成与止血中的作用。     

血管性血友病因子裂解酶功能检测在血栓性血小板减少性紫癜诊断中的意义

目的 建立血栓性血小板减少性紫癜（TTP）的实验诊断方法,并用于TTP患者的诊断.方法 将微管透析前、后的正常人混合血浆、TTP患者血浆进行琼脂糖凝胶水平电泳与WesternBlot,分析TTP患者血浆中vWF多聚体的组成变化;以残余胶原结合力检测血管性血友病因子裂解酶（vWF-cp）的活性水平.结果 正常人混合血浆的vWF-cp活性为88.79%,4例TTP患者vWF-cp活性分别为8.90%,4.72%,28.73%,10.35%.正常混合血浆透析后只存在小分子量vWF;TTP患者血浆透析前、后的多聚体分布无变化或变化不明显.结论 残余胶原结合力测定结合裂解后的vWF多聚体电泳与Western Blot技术可准确分析vWF-cp活性与vWF多聚体结构组成的变化,为TTP诊断提供新的有用的实验诊断方法.     

A novel type 2A (Group II) von Willebrand disease mutation (L1503Q) associated with loss of the highest molecular weight von Willebrand factor multimers.

Type 2A von Willebrand disease (VWD) is characterized by decreased platelet-dependent function of von Willebrand factor (VWF) associated with an absence of high-molecular-weight multimers. In this study, sequence analysis of the VWF gene from a Type 2A VWD patient showed a novel, heterozygous T-->A transversion at nucleotide 4510, resulting in the non-conservative substitution of L1503Q in the mature VWF subunit. This substitution, which was not found in 55 unrelated normal individuals, was reproduced by in vitro site directed mutagenesis of a full-length VWF cDNA and was subsequently expressed in COS-7 cells. The corresponding recombinant mutant VWF protein was partially retained in COS-7 cells yet the full spectrum of multimers was observed, suggesting that the absence of the highest molecular weight multimers results from increased proteolysis. The recombinant mutant VWF protein was digested with the ADAMTS13 protease from VWF-depleted plasma and the aberrant VWF multimer pattern was observed. These results suggest that the L1503Q substitution induces a conformational change in the VWF protein, which increases the protein's susceptibility to proteolysis. A three-dimensional model of the A2 domain demonstrates that the L1503Q mutation and the physiological proteolytic cleavage site for ADAMTS13 (Y(1605)-M(1606)) are localized close together in two adjacent parallel beta-sheets. The mutation L1503Q does not significantly disrupt the conformation of the protein; thus the subtle loss of multimers in this patient may be due to altered interactions with the ADAMTS13 protease.     

血管性血友病因子裂解酶活性域的表达及其单克隆抗体的制备

血管性血友病因子裂解酶(vWF-cp)是一种新发现的金属蛋白酶,与血栓性微血管病的发生密切相关。本研究应用IPTG从pET28a( )-vWF-cp/MD质粒中诱导表达了vWF-cp的金属蛋白酶域,重组蛋白经Ni-NTA柱纯化,复性后免疫BALB/c小鼠,利用杂交瘤技术获得了稳定分泌抗vWF-cp金属蛋白酶域抗体的细胞株,进一步对其单克隆后,收集细胞接种于小鼠腹腔产生腹水,鉴定所获得的单克隆抗体,同时,利用单克隆抗体确定vWF-cp在组织细胞中的位置及其在各种正常组织中的表达情况。结果表明,重组蛋白主要以包涵体形式存在,占菌体总蛋白的28%。利用杂交瘤技术制备出3株抗vWF-cp单克隆抗体,对其中2株单克隆抗体作了进一步的鉴定。免疫双扩散和竞争ELISA结果显示,它们均属IgG1亚类,腹水中的含量约为4mg/ml,效价达1×10-5,而且识别vWF-cp金属蛋白酶域不同的表位。Westernblot和免疫沉淀结果表明,2株单克隆抗体能够识别重组蛋白和正常血小板中200kD的蛋白。在组织细胞中,2株单克隆抗体识别的蛋白位于细胞胞质内,而且在多种正常组织如肝、前列腺和卵巢等组织中表达,但脑组织中无该蛋白的表达。结论:本研究为了解vWF-cp在组织中的分布以及进一步研究该蛋白酶的结构和功能奠定了基础,并且可用所制备的单克隆抗体建立vWF-cp抗原的检测方法。     

ADAMTS13的结构、功能及其与TTP诊疗的关系

血管性血友病因子裂解蛋白酶（ADAMTS 13）是一种能够特异性裂解血管性血友病因子（vWF）的血浆金属蛋白酶,严重的ADAMTS13活性缺失可导致特发性血栓性血小板减少性紫癜（TTP）的发生.本文简要综述了ADAMTS13的结构和功能及其与TTP的发病机制、诊断和治疗之间的关系.     

Humoral immune response to ADAMTS13 in acquired thrombotic thrombocytopenic purpura

Summary. The apparently spontaneous development of autoantibodies to ADAMTS13 in previously healthy individuals is a major cause of thrombotic thrombocytopenic purpura (TTP). Epitope mapping studies have shown that in most patients antibodies directed towards the spacer domain of ADAMTS13 are present. A single antigenic surface comprising Arg⁶⁶⁰, Tyr⁶⁶¹ and Tyr⁶⁶⁵ that contributes to the productive binding of ADAMTS13 to unfolded von Willebrand factor is targeted by anti‐spacer domain antibodies. Antibodies directed to the carboxyl‐terminal CUB1–2 and TSP2–8 domains have also been observed in the plasma of patients with acquired TTP. As yet it has not been established whether this class of antibodies modulates ADAMTS13 activity. Inspection of the primary sequence of human monoclonal anti‐ADAMTS13 antibodies suggests that the variable heavy chain germline gene segment VH1–69 is frequently incorporated. We suggest a model in which ‘shape complementarity’ between the spacer domain and residues encoded by the VH1–69 gene segment explain the preferential use of this variable heavy chain gene segment. Finally, a model is presented for the development of anti‐ADAMTS13 antibodies in previously healthy individuals that incorporates the recent identification of HLA DRB1*11 as a risk factor for acquired TTP.     

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.     

血管性血友病因子vWF73和vWF114片段的表达及其在ADAMTS13活性测定中的应用

目的 构建血管性血友病因子(vWF)A2区片段vWF73和vWF114的表达质粒,在大肠杆菌中表达两种谷胱甘肽S转移酶(GST)融合蛋白,并探讨两种蛋白作为底物在测定ADAMTS13活性中的应用价值.方法 应用PCR的方法扩增vWF A2区内vWF73和vWF114的相应DNA片段,分别克隆至GST融合表达载体pGEX-6P-1进行诱导表达,Ni-NTA琼脂糖柱纯化可溶性蛋白部分.以Western blot法榆测正常人血浆、血栓性血小板减少性紫癜(TTP)患者血浆和重组ADAMTS13(rADAMTS13)水解两种重组vWF A2区片段的情况,并以此为底物用抗GST和抗His两种抗体建立酶联免疫吸附试验(ELISA)法测定血浆ADAMqS13活性的新方法.结果 成功表达和纯化了ADAMTS13的两种可溶性小分子底物GST-vWF73-H和GST-vWF114-H,均能被rADAMTS13或正常人血浆有效地水解,而遗传性和特发性TTP患者血浆则无此作用.同时,以此为底物建立了测定ADAMTS13活性的ELISA方法.结论 采用原核表达系统成功得到了vWF A2区片段vWF73和vWF114两种GST融合蛋白,可作为底物用于ADAMTS13活性的测定.

Abstract：

Objective To construct the expression vectors of vWF73 and vWF114 fragments of von Willebrand factor (vWF) A2 domain, and to express glutathione S-transferase (GST) fusion proteins in E.coli, and to explore their values in measuring ADAMTS13 activity as substrates. Methods The DNA fragments encoding vWF73 and vWF114 were generated using PCR and separately cloned into pGEX-6P-1 , a Schistosoma japonicum GST fusion expression vector. The expression of GST-vWF73-H and GST-vWF114-H was induced in liquid culture, followed by purification with Ni-NTA agarose column. The cleavage of two GST fusion proteins by recombinant ADAMTS13 ( rADAMTS13) or plasma from normal individuals and thrombotic thrombocytopenic purpura (TTP) patients were identified by Western blot. Based on an enzyme-linked immunosorbent assay ( ELISA) with anti-GST and anti-His monoclonal antibodies, GST-vWF73-H and GSTvWF114-H were used to measure plasma ADAMTS13 activity as substrates. Results Two small molecular substrates of ADAMTS13, GST-vWF73-H and GST-vWF114-H, are expressed and purified,which could be specifically cleaved by rADAMTS13 or plasma from healthy individuals, but not by plasma from congenital or idiopathic TTP patients. An ELISA assay was established to detect plasma ADAMTS13 activity using GSTvWF73-H and GST-vWF114-H as substrates. Conclusions Two GST fusion proteins in vWF A2 domain,vWF73 and vWF114, were expressed effectively using a prokaryotic expression system and could be used to detect ADAMTS13 activity as substrates.     

Ten candidate ADAMTS13 mutations in six French families with congenital thrombotic thrombocytopenic purpura (Upshaw–Schulman syndrome)

Summary.  ADAMTS13, the specific von Willebrand factor (VWF)-cleaving metalloprotease, prevents the spontaneous formation of platelet thrombi in the microcirculation by degrading the highly adhesive ultralarge VWF multimers into smaller forms. ADAMTS13 severe enzymatic deficiency and mutations have been described in the congenital thrombotic thrombocytopenic purpura (TTP or Upshaw–Schulman syndrome), a rare and severe disease related to multivisceral microvascular thrombosis. We investigated six French families with congenital TTP for ADAMTS13 enzymatic activity and gene mutations. Six probands with congenital TTP and their family were tested for ADAMTS13 activity in plasma using a two-site immunoradiometric assay and for ADAMTS13 gene mutations using polymerase chain reaction and sequencing. ADAMTS13 activity was severely deficient (< 5%) in the six probands and one mildly symptomatic sibling but normal (> 50%) in all the parents and the asymptomatic siblings. Ten novel candidate ADAMTS13 mutations were identified in all families, showing either a compound heterozygous or a homozygous status in all probands plus the previous sibling and a heterozygous status in the parents. The mutations were spread all over the gene, involving the metalloprotease domain (I79M, S203P, R268P), the disintegrin domain (29 bp deletion in intron/exon 8), the cystein-rich domain (acceptor splice exon 12, R507Q), the spacer domain (A596V), the 3rd TSP1 repeat (C758R), the 5th TSP1 repeat (C908S) and the 8th TSP1 repeat (R1096stop). This study emphasizes the role of ADAMTS13 mutations in the pathogenesis of congenital TTP and suggests that several structural domains of this metalloprotease are involved in both its biogenesis and its substrate recognition process.     

Therapeutic plasma exchange for thrombotic thrombocytopenic purpura with refractory thrombocytopenia

Thrombotic thrombocytopenic purpura (TTP) is an acute, life‐threatening illness with disseminated platelet‐rich thromboses of small vessels that variably presents with the classic clinical “pentad” of microangiopathic hemolytic anemia, thrombocytopenia, fever, altered mental status, and acute kidney injury. Most cases are caused by an acquired autoantibody to ADAMTS13, a metalloproteinase that cleaves large von Willebrand Factor (vWF) multimers. The mainstay of treatment is daily therapeutic plasma exchange (TPE), sometimes with adjunctive pharmacologic immunosuppression. TPE is generally continued until the platelet count is greater than 150 × 10³/µL and the lactate dehydrogenase is near normal for 2‐3 consecutive days. Unfortunately, there is no clear guidance for when thrombocytopenia is refractory for a prolonged period of time. The following case describes such a scenario in which consecutive ADAMTS13 activity and inhibitor levels were used to guide the decision to stop treatment with TPE in a patient who failed to recover their platelet count.     

Polyphosphate binds to human von Willebrand factor in vivo and modulates its interaction with glycoprotein Ib

Summary. Background: Polyphosphate, a phosphate polymer released by activated platelets, has recently been described as a potent modulator of blood coagulation and fibrinolysis. In blood plasma, polyphosphate binds to and alters the biological functions of factor XII, fibrin(ogen), thrombin and factor VII activating protease. Objectives: The aim of the present study is to investigate whether polyphosphate also binds to von Willebrand factor (VWF) and alters some of its activities. Methods/Results: When studying patients with type 1 von Willebrand disease (VWD) and their healthy relatives, we discovered a significant correlation between von Willebrand factor (VWF) and platelet polyphosphate levels. We have also found polyphosphate in preparations of VWF isolated from normal platelets and plasma. Surface plasmon resonance and electrophoretic mobility assays indicated that polyphosphate interacts with VWF in a dose‐ and time‐dependent manner. Treatment of normal plasma with active exopolyphosphatase decreased the VWF ristocetin cofactor (VWF:RCo) activity, a functional measure of VWF binding to platelet glycoprotein receptor Ib. VWF collagen binding and multimerization were unaltered after polyphosphate depletion. Moreover, addition of polyphosphate increased the deficient VWF:RCo activity presented by plasma from patients with type 1 VWD. Conclusions: Our results reveal that a new role is played by polyphosphate in hemostasis by its interaction with VWF, and suggest that this polymer may be effective in the treatment of some types of VWD.     

人vW因子A2区基因表达及其生物学活性的研究

血管性血友病因子（vWF）在血管性血友病（vWD）以及血栓性血小板减少性紫癜（TTP）的发病中起着重要的作用,并受血浆vWF裂解蛋白酶（ADAMTS-13）的调节.本研究表达vWF中的A2区蛋白,并研究ADAMTS-13对其裂解活性.首先,应用基因重组技术在大肠杆菌中表达人vWF-A2区蛋白,经纯化、复性获得重组蛋白（rvWF-A2）,并与正常人以及TYP患者血浆进行反应,分析rvWF-A2蛋白的生物学活性.结果表明：重组表达载体pQE-30-vWFA2在大肠杆菌M15中得到高效表达,目的蛋白表达量占菌体总蛋白的42%,Ni-NTA agrose柱层析纯化后,其纯度为98%,经复性的rvWF-A2可作为ADAMTS-13良好的酶解底物,可被枸橼酸钠抗凝的正常人血浆切割,其切割程度随时间的延长而增加,而EDTA抗凝的正常人血浆以及TTP患者血浆无法切割此蛋白.结论：在大肠杆菌中高效表达的rvWF-A2具有良好的生物学活性,为建立定量测定ADAMTS-13活性的方法奠定了坚实的基础.     

Two generations with familial thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a rare multi-system disease characterised by the pentad of microangiopathic haemolytic anaemia, thrombocytopenia, renal dysfunction, fever and neurologic changes. A hereditary form of recurrent familial TTP has been described, which usually presents in adolescence or early adulthood and can lead to recurrent or chronic relapsing TTP. Genetic analyses of patients with familial TTP have linked the disease to chromosome 9q34, and an increased incidence is seen in people with HLA-B40 group antigens. We describe here an 11-year-old Egyptian girl with no significant past medical history who presented with new onset of bruising, petechial rash, fatigue and fevers and was diagnosed with familial TTP. Further testing revealed that both the patient and her father had the HLA-B40 group antigen and also had ADAMTS-13 von Willebrand factor-cleaving protease deficiency as well as factor-H deficiency.     

ADAMTS13 and microvascular thrombosis

Interaction between platelet and von Willebrand factor, a circulating adhesive glycoprotein, is essential for hemostasis under the high shear environments of arterioles and capillaries. If unregulated, this interaction may lead to unwarranted platelet thrombosis. ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, number 13), a plasma zinc metalloprotease synthesized primarily in the stellate cells of the liver, cleaves shear stress-activated von Willebrand factor, thereby preventing the occurrence of von Willebrand factor–platelet interaction in the circulation. A profound deficiency of ADAMTS13, due to genetic mutations or autoimmune inhibition, results in intravascular von Willebrand factor platelet aggregation and widespread microvascular thrombosis characteristic of thrombotic thrombocytopenic purpura. Cloning of ADAMTS13 and structure–function analyses of the enzyme are leading to exciting advances in the diagnosis and therapy of this hitherto mysterious disease.     

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.     

Rituximab and intermediate‐purity plasma‐derived factor VIII concentrate (Koate®) as adjuncts to therapeutic plasma exchange for thrombotic thrombocytopenic purpura in patients with an ADAMTS13 inhibitor

Thrombotic thrombocytopenic purpura (TTP) results from a congenital or acquired deficiency of the von Willebrand factor (vWF)‐cleaving protease ADAMTS13. The disease can be fatal and hence treatment should be initiated promptly. Therapeutic plasma exchange (TPE) remains the standard treatment along with adjunct therapies including steroids and immunosuppressive drugs. Addition of rituximab to TPE has been shown to be beneficial in refractory/relapsing TTP; however, TPE results in removal of rituximab from the circulation requiring more frequent dosing of rituximab to achieve a favorable outcome. The intermediate‐purity plasma‐derived Factor VIII concentrate (FVIII) Koate® contains the highest amount of ADAMTS13 activity yet reported and has been used successfully in treating congenital TTP. Here we report our experience with addition of this FVIII concentrate to rituximab, corticosteroids and TPE in three TTP patients with an ADAMTS13 inhibitor to permit withholding TPE for 48 h after rituximab infusion. J. Clin. Apheresis 30:50–54, 2015. © 2014 Wiley Periodicals, Inc.     

Relapsed thrombotic thrombocytopenic purpura presenting as an acute cerebrovascular accident

Thrombotic thrombocytopenic purpura (TTP) is an uncommon but severe disorder that classically presents with microangiopathic hemolytic anemia (MAHA), thrombocytopenia, and fluctuating neurological changes. Previously, it was impossible to make a diagnosis of TTP in the absence of thrombocytopenia or microangiopathic hemolysis (MAHA). We describe two cases of relapsing TTP that presented with acute cerebrovascular accident (CVA) without concurrent thrombocytopenia or MAHA after initial classical presentation of TTP. In both cases, the diagnosis of TTP as the cause of the CVA was attributed to severe deficiency of the von Willebrand factor cleaving protease, ADAMTS13 in plasma (11 and 12%, normal 79-127%). Each patient had a dramatic clinical improvement in response to therapeutic plasma exchange. The experience in these two cases suggests that TTP should be considered as a potential cause among patients presenting with a CVA, particularly if the patients have a history of TTP.