Diagnosis of thrombotic thrombocytopenic purpura: easy-to-use fiber optic surface plasmon resonance immunoassays for automated ADAMTS-13 antigen and conformation evaluation.

BACKGROUND: Laboratory diagnosis of immune-mediated thrombotic thrombocytopenic purpura (iTTP) remains challenging when ADAMTS-13 activity ranges between 10% and 20%. To prevent misdiagnosis, open ADAMTS-13 conformation gained clinical attention as a novel biomarker, especially to diagnose acute iTTP in patients with diagnostic undecisive ADAMTS-13 activity. Plasma ADAMTS-13 conformation analysis corrects for ADAMTS-13 antigen, with both parameters being characterized in enzyme-linked immunosorbent assay (ELISA)-based reference assays requiring expert technicians. OBJECTIVES: To design ADAMTS-13 antigen and conformation assays on automated, easy-to-use fiber optic surface plasmon resonance (FO-SPR) technology to promote assay accessibility and diagnose challenging iTTP patients. METHODS: ADAMTS-13 antigen and conformation assays were designed on FO-SPR technology. Plasma of 20 healthy donors and 20 acute iTTP patients were quantified, and data from FO-SPR and ELISA reference assays were compared. RESULTS: Following assay design, both antigen and conformation FO-SPR assays were optimized and characterized, presenting strong analytical sensitivity (detection limit of 0.001 µg/mL) and repeatability (interassay variation of 14.4%). Comparative analysis suggested positive correlation (Spearman r of 0.92) and good agreement between FO-SPR and ELISA assays. As expected, FO-SPR assays showed a closed or open ADAMTS-13 conformation in healthy donors and acute iTTP patients, respectively. CONCLUSION: Both ADAMTS-13 antigen and conformation assays were transferred onto automated, easy-to-use FO-SPR technology, displaying potent analytical sensitivity and reproducibility. ADAMTS-13 antigen and conformation were determined for healthy donors and acute iTTP patients showing strong correlation with ELISA reference. Introducing FO-SPR technology in clinical context could support routine diagnosis of acute iTTP patients, notably when ADAMTS-13 activity fluctuates between 10% and 20%.

Measuring ADAMTS-13 activity to diagnose thrombotic thrombocytopenic purpura: a novel, fast fiber-optic surface plasmon resonance immunoassay.

Background: Thrombotic thrombocytopenic purpura (TTP) is characterized by severe ADAMTS-13 activity deficiency (<10%). Diagnostic testing is challenging because of unavailability, high cost, and expert technician requirement of ADAMTS-13 enzyme assays. Cost-effective, automated fiber-optic surface plasmon resonance (FO-SPR) platforms show potential for developing diagnostic tests. Yet, FO-SPR has never been explored to measure enzymatic activities. Objectives: To develop an easy-to-use ADAMTS-13 activity assay utilizing optical fibers to rapidly diagnose TTP. Methods: The ADAMTS-13 activity assay was designed and optimized using FO-SPR technology based on a previously described enzyme-linked immunosorbent assay setup. A calibration curve was generated to quantify ADAMTS-13 activity in plasma of healthy donors and patients with acute immune-mediated TTP (iTTP), hemolytic uremic syndrome, or sepsis. ADAMTS-13 activity data from FO-SPR and fluorescence resonance energy transfer-based strategies (FRETS)-VWF73 reference assays were compared. Results: After initial assay development, optimization improved read-out magnitude and signal-to-noise ratio and reduced variation. Further characterization demonstrated a detection limit (6.8%) and inter-assay variation (Coefficient of variation, 7.2%) that showed good analytical sensitivity and repeatability. From diverse plasma samples, only plasma from patients with acute iTTP showed ADAMTS-13 activities below 10%. Strong Pearson correlation (r = 0.854) between FO-SPR and reference FRETS-VWF73 assays were observed for all measured samples. Conclusions: A fast ADAMTS-13 activity assay was designed onto automated FO-SPR technology. Optimization resulted in sensitive ADAMTS-13 activity measurements with a detection limit enabling clinical diagnosis of TTP within 3 hours. The FO-SPR assay proved strong correlation with the reference FRETS-VWF73 assay. For the first time, this assay demonstrated the capacity of FO-SPR technology to measure enzymatic activity in pre-clinical context.

A novel von Willebrand factor multimer ratio as marker of disease activity in thrombotic thrombocytopenic purpura.

Immune-mediated thrombotic thrombocytopenic purpura (iTTP), an autoantibody-mediated severe ADAMTS13 deficiency, is caused by insufficient proteolytic processing of von Willebrand factor (VWF) multimers (MMs) and microvascular thrombi. Recurrence of acute iTTP is associated with persistence or reappearance of ADAMTS13 deficiency. Some patients remain in remission despite recurring or persisting severe ADAMTS13 deficiency. In a prospective 2-year observational study, we investigated VWF MM patterns and ADAMTS13 in patients with iTTP in remission and at acute episodes. Of the 83 patients with iTTP, 16 suffered 22 acute episodes whereas 67 remained in clinical remission during follow-up, including 13 with ADAMTS13 <10% and 54 with ADAMTS13 ≥10%. High -molecular weight to low-molecular weight VWF MM ratio based on sodium dodecyl sulfate-agarose gel electrophoresis was compared with ADAMTS13 activity. VWF MM ratio was significantly higher in patients in remission with <10% compared with ≥10% ADAMTS13 activity. Fourteen samples obtained from 13 to 50 days (interquartile range; median, 39) before acute iTTP onset (ADAMTS13 <10% in 9 patients and 10%-26% in 5) showed VWF MM ratios significantly higher than those from 13 patients remaining in remission with ADAMTS13 <10%. At acute iTTP onset, VWF MM ratio decreased significantly and was low in all patients despite <10% ADAMTS13. The VWF MM ratio does not depend exclusively on ADAMTS13 activity. The disappearance of high molecular weight VWF MMs resulting in low VWF MM ratio at iTTP onset may be explained by consumption of larger VWF MMs in the microcirculation. The very high VWF MM ratio preceding acute iTTP recurrence suggests that VWF processing is hampered more than in patients remaining in remission.

Toward gene therapy for congenital thrombotic thrombocytopenic purpura.

Congenital thrombotic thrombocytopenic purpura (cTTP) is caused by a severe deficiency in the plasma metalloprotease ADAMTS-13. The current management of cTTP is dependent on the prophylactic administration of ADAMTS-13 via plasma infusion. This is a demanding therapy for patients because transfusions are lifelong and time-consuming and allergic reactions frequently occur. Although current management of cTTP controls acute episodes, it does not provide a long-lasting cure for this disease. The endogenous expression of ADAMTS-13 after gene transfer would provide a curative therapy and ongoing research explores various preclinical gene therapeutic approaches for cTTP. This review focuses on the current state of the literature regarding preclinical gene therapy studies for cTTP and on the challenges of developing a gene therapy medicinal product for cTTP.

ADAMTS13 conformation and immunoprofiles in Japanese patients with immune-mediated thrombotic thrombocytopenic purpura.

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is an ultrarare thrombotic disease caused by autoantibody-induced ADAMTS13 deficiency. Open ADAMST13 conformation, induced by autoantibodies, was identified as a novel biomarker for iTTP. Determining immunoprofiles in patients with iTTP has been shown to guide the development of novel targeted therapies. However, these studies were done in mainly Caucasian iTTP cohorts. To validate those findings across other ethnic cohorts, we investigated 195 acute TTP plasma samples from the Japanese iTTP registry. Seventy-six of the 195 samples had detectable ADAMTS13 antigen levels, of which 94.7% were shown to have an open ADAMTS13 conformation. A positive correlation was observed between ADAMTS13 inhibitor titers (a diagnostic parameter in Japan) and anti-ADAMTS13 immunoglobulin G autoantibody titers. Studying anti-M, anti-DT, anti-CS, anti-T2-T5, anti-T6-T8, anti-CUB1-2 autoantibodies and the corresponding immunoprofile showed that 73% of the patients had anti-CS autoantibodies and 25.8% had anti-M autoantibodies, with the latter being higher than in Caucasians. Stratifying patients according to their immunoprofiles revealed that the profile with only anti-CS autoantibodies was the most common immunoprofile similar to that in Caucasians (28.9%). Although this profile did not affect the 1-year TTP-related mortality rate, patients with autoantibodies against all 6 ADAMTS13 fragments had a higher risk for TTP-related death than other patients (P = .02). We here validated open ADAMTS13 as a novel biomarker for acute iTTP and determined the dominant immunoprofiling in the Japanese cohort, contributing to setting up the diagnosis and managing guidelines across different ethnic cohorts and developing ADAMTS13 variants that do not bind to the anti-CS autoantibodies.

Improvement of recombinant ADAMTS13 production through a more optimal signal peptide or an N-terminal fusion protein.

BACKGROUND: Recombinant human ADAMTS13 (rADAMTS13) is a key protein in fundamental research for investigating its mode of action and the pathophysiology of thrombotic thrombocytopenic purpura (TTP). However, the expression of rADAMTS13 is quite low in mammalian cells, which makes the production of the protein time-consuming and labor-intensive. OBJECTIVES: We aimed at increasing the yield of rADAMTS13 by (1) using a more optimal signal peptide (SP) and (2) constructing an N-terminal fusion protein of ADAMTS13 with human serum albumin domain 1 (AD1-ADAMTS13). METHODS: Six SPs were investigated to select the most optimal SP. Expression plasmids containing the most optimal SP and ADAMTS13 cDNA or the fusion construct AD1-ADAMTS13 were generated and transiently transfected into CHOEBNALT85 cell-line. Expression levels of rADAMTS13 in expression medium were analyzed and compared with the expression level of rADAMTS13 with native SP (nat-SP). RESULTS: Expression of rADAMTS13 with coagulation factor VII (FVII) SP was 3-fold higher (16.00 µg/ml) compared with the expression with nat-SP (5.03 µg/ml). The highest yields were obtained with AD1-ADAMTS13 protein with a 15-fold higher concentration (78.22 µg/ml) compared with the expression with nat-SP. The rADAMTS13 expressed with FVII-SP retained its activity (104.0%) to cleave von Willebrand factor, whereas AD1-ADAMTS13 demonstrated even higher activity (144.3%). CONCLUSION: We succeeded in generating expression vectors that yield (1) rADAMTS13 at higher levels because of more optimal FVII-SP and (2) high levels of AD1-ADAMTS13 N-terminal fusion protein. The highest expression levels were obtained with AD1-ADAMTS13 N-terminal fusion protein, which is paving the way for highly efficient protein production.

Effect of red blood cell transfusion on inflammation, endothelial cell activation and coagulation in the critically ill.

BACKGROUND AND OBJECTIVES: Red blood cell (RBC) transfusion is a frequently applied intervention in an intensive care unit. However, transfusion is associated with adverse outcomes including organ failure and thrombo-embolic events. Mechanisms of these effects are not known but may be related to activation of the endothelium or of the coagulation or inflammatory system. We hypothesized that a RBC transfusion in the critically ill would result in further activation of these systems. MATERIALS AND METHODS: In 74 non-bleeding critically ill patients receiving one RBC unit, markers of inflammation, endothelial cell activation and coagulation were measured before transfusion, at 1 h after transfusion and 24 h after transfusion. The impact of disease severity of the recipient on these changes was assessed by comparing septic and non-septic patients (according to sepsis-3 definition) and by correlation of biomarkers with the sequential organ failure assessment (SOFA) score. RESULTS: Levels of von Willebrand Factor (vWF), soluble ICAM-1, soluble thrombomodulin, fibrinogen and d-dimer were already high at baseline, whereas ADAMTS13 levels were low. VWF levels increased significantly 24 h after RBC transfusion (median 478% (338-597) vs. 526% (395-623), p = 0.009). The other biomarkers did not change significantly. Post transfusion change was not dependent on the presence of sepsis and was not correlated with SOFA score. CONCLUSION: RBC transfusion in critically ill patients was associated with an increase in circulating vWF levels, suggesting a further increase in activation of the endothelium, a finding that was independent of the presence of sepsis or organ injury level.

Plasma and rhADAMTS13 reduce trauma-induced organ failure by restoring the ADAMTS13-VWF axis.

Trauma-induced organ failure is characterized by endothelial dysfunction. The aim of this study was to investigate the role of von Willebrand factor (VWF) and its cleaving enzyme, ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs, member 13) in the occurrence of endothelial permeability and organ failure in trauma. In an observational study in a level-1 trauma center, 169 adult trauma patients with clinical signs of shock and/or severe injuries were included. Trauma was associated with low ADAMTS13 and high VWF antigen levels, thus generating an imbalance of ADAMTS13 to VWF. Patients who developed organ failure (23%) had greater ADAMTS13-to-VWF imbalances, persistently lower platelet counts, and elevated levels of high-molecular-weight VWF multimers compared with those without organ failure, suggesting microthrombi formation. To investigate the effect of replenishing low ADAMTS13 levels on endothelial permeability and organ failure using either recombinant human ADAMTS13 (rhADAMTS13) or plasma transfusion, a rat model of trauma-induced shock and transfusion was used. Rats in traumatic hemorrhagic shock were randomized to receive crystalloids, crystalloids supplemented with rhADAMTS13, or plasma transfusion. A 70-kDa fluorescein isothiocyanate-labeled dextran was injected to determine endothelial leakage. Additionally, organs were histologically assessed. Both plasma transfusion and rhADAMTS13 were associated with a reduction in pulmonary endothelial permeability and organ injury when compared with resuscitation with crystalloids, but only rhADAMTS13 resulted in significant improvement of a trauma-induced decline in ADAMTS13 levels. We conclude that rhADAMTS13 and plasma transfusion can reduce organ failure following trauma. These findings implicate the ADAMTS13-VWF axis in the pathogenesis of organ failure.

Imbalance of von Willebrand factor and ADAMTS13 axis is rather a biomarker of strong inflammation and endothelial damage than a cause of thrombotic process in critically ill COVID-19 patients.

BACKGROUND: Critically ill patients with coronavirus disease 2019 (COVID-19) are prone to developing macrothrombosis and microthrombosis. COVID-19 has been reported to be rarely associated with thrombotic microangiopathies. A disintegrin and metalloprotease with thrombospondin type I repeats, member 13 (ADAMTS13) severe deficiency, the hallmark of thrombotic thrombocytopenic purpura (TTP), induces the formation of platelet, unusually large von Willebrand factor (VWF) multimer microthrombi. In immune-mediated TTP, ADAMTS13 adopts specifically an open conformation. The VWF/ADAMTS13 couple may contribute to the microthrombi formation in pulmonary alveolar capillaries in COVID-19. OBJECTIVE: To investigate clinical features, hemostatic laboratory parameters, VWF/ADAMTS13 axis, and ADAMTS13 conformation in critically ill COVID-19 patients at admission. METHODS: Fifty three critically ill COVID-19 patients were enrolled between March 18 and May 9 2020 in a monocentric hospital. RESULTS: The median age was 59 years and the male-to-female ratio was 2.8/1. We reported seven pulmonary embolisms and 15 deaths. Biological investigations showed increased fibrinogen and factor V levels, and strongly increased D-dimers correlated with mortality. No patient presented severe thrombocytopenia nor microangiopathic hemolytic anemia. An imbalance between high VWF antigen levels and normal or slightly decreased ADAMTS13 activity levels (strongly elevated VWF/ADAMTS13 ratio) was correlated with mortality. Three patients had a partial quantitative deficiency in ADAMTS13. We also reported a closed conformation of ADAMTS13 in all patients, reinforcing the specificity of an open conformation of ADAMTS13 as a hallmark of TTP. CONCLUSION: We suggest that slightly decreased or normal ADAMTS13 activity and highly elevated VWF are rather biomarkers reflecting both the strong inflammation and the endothelial damage rather than drivers of the thrombotic process of COVID-19.

Determination of anti-ADAMTS-13 autoantibody titers in ELISA: Influence of ADAMTS-13 presentation and autoantibody detection.

BACKGROUND: Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is caused by inhibitory and/or clearing anti-ADAMTS-13 (A Disintegrin and Metalloprotease with ThromboSpondin type 1 repeats, member 13) autoantibodies. To determine the presence and total level of anti-ADAMTS-13 autoantibodies, commercial and in-house developed ELISAs are performed. However, different ELISA methods vary in relation to the presentation of recombinant (r)ADAMTS-13 and the detection method of the anti-ADAMTS-13 autoantibodies. Currently, the influence of those different approaches on anti-ADAMTS-13 autoantibody titers is not known. OBJECTIVES: To assess the influence of different ADAMTS-13 presentation- and autoantibody detection methods on anti-ADAMTS-13 autoantibody titers in ELISA. MATERIALS/METHODS: Anti-ADAMTS-13 autoantibody titers from 18 iTTP patients were determined using four different set-ups of anti-ADAMTS-13 autoantibody ELISAs. The ELISAs varied in the used presentation of rADAMTS-13 (directly coated full-length rADAMTS-13, directly coated rMDTCS and rT2C2, or antibody-captured full-length rADAMTS-13) and the detection antibodies (polyclonal anti-human IgG or monoclonal anti-human IgG1-4 antibodies). RESULTS: Strong correlations between the different anti-ADAMTS-13 autoantibody ELISA approaches were observed, when using polyclonal anti-human IgG detection antibodies recognizing all IgG subclasses similarly, independent of the method of rADAMTS-13 presentation. Anti-ADAMTS-13 autoantibody titers correlated less when using a mixture of monoclonal anti-human IgG1-4 , because not all IgG subclasses were recognized with similar affinities. CONCLUSION: Anti-ADAMTS-13 autoantibody levels using different methods of rADAMTS-13 presentation strongly correlate. However, the levels of anti-ADAMTS-13 autoantibodies are highly dependent on the detection antibody used, which should detect all IgG subclasses (IgG1-4 ) equally well.

Co(III)-NTA Mediated Antigen Immobilization on a Fiber Optic-SPR Biosensor for Detection of Autoantibodies in Autoimmune Diseases: Application in Immune-Mediated Thrombotic Thrombocytopenic Purpura.

Autoantibodies are key biomarkers in clinical diagnosis of autoimmune diseases routinely detected by enzyme-linked immunosorbent assays (ELISAs). However, the complexity of these assays is limiting their use in routine diagnostics. Fiber optic-surface plasmon resonance (FO-SPR) can overcome these limitations, but improved surface chemistries are still needed to guarantee detection of autoantibodies in complex matrices. In this paper, we describe the development of an FO-SPR immunoassay for the detection of autoantibodies in plasma samples from immune-mediated thrombotic thrombocytopenic purpura (iTTP) patients. Hereto, hexahistidine-tagged recombinant ADAMTS13 (rADAMTS13-His6) was immobilized on nitrilotriacetic acid (NTA)-coated FO probes chelated by cobalt (Co(III)) and exposed to anti-ADAMTS13 autoantibodies. Initial studies were performed to optimize rADAMTS13-His6 immobilization and to confirm the specificity of the immunoassay for detection of anti-ADAMTS13 autoantibodies with FO-SPR. The performance of the immunoassay was then evaluated by comparing Co(III)- and nickel (Ni(II))-NTA stabilized surfaces, confirming the stable immobilization of the antigen in Co(III)-NTA-functionalized FO probes. A calibration curve was prepared with a dilution series of a cloned human anti-ADAMTS13 autoantibody in ADAMTS13-depleted plasma resulting in an average interassay coefficient of variation of 7.1% and a limit of detection of 0.24 ng/mL. Finally, the FO-SPR immunoassay was validated using seven iTTP patient plasma samples, resulting in an excellent correlation with an in-house-developed ELISA (r = 0.973). In summary, the specificity and high sensitivity in combination with a short time-to-result (2.5 h compared to 4-5 h for a regular ELISA) make the FO-SPR immunoassay a powerful assay for routine diagnosis of iTTP and with extension for any other autoimmune disease.

Open ADAMTS13, induced by antibodies, is a biomarker for subclinical immune-mediated thrombotic thrombocytopenic purpura.

Recently, we showed that ADAMTS13 circulates in an open conformation during the acute phase of immune-mediated thrombotic thrombocytopenic purpura (iTTP). Although the cause of this conformational change remains elusive, ADAMTS13 is primarily closed in iTTP patients in remission with ADAMTS13 activity >50% and undetectable anti-ADAMTS13 autoantibodies, as well as after rituximab treatment, suggesting a role for anti-ADAMTS13 autoantibodies. Therefore, immunoglobulin G from 18 acute iTTP patients was purified and added to closed ADAMTS13 in healthy donor plasma. This resulted in open ADAMTS13 in 14 of 18 (78%) samples, proving that anti-ADAMTS13 autoantibodies can induce an open ADAMTS13 conformation. To further elucidate the conformation of ADAMTS13 in iTTP patients, we studied a novel iTTP patient cohort (n = 197) that also included plasma samples from iTTP patients in remission in whom ADAMTS13 activity was <50%. The open ADAMTS13 conformation was found during acute iTTP, as well as in patients in remission with ADAMTS13 activity <50% and in half of the patients with ADAMTS13 activity >50%, although free anti-ADAMTS13 autoantibodies were not always detected. Thus, open ADAMTS13 is a hallmark of acute iTTP, as well as a novel biomarker that can be used to detect subclinical iTTP in patients in remission. Finally, a long-term follow-up study in 1 iTTP patient showed that the open conformation precedes a substantial drop in ADAMTS13 activity. In conclusion, we have shown that anti-ADAMTS13 autoantibodies from iTTP patients induce an open ADAMTS13 conformation. Most importantly, an open ADAMTS13 conformation is a biomarker for subclinical iTTP and could become an important tool in TTP management.

Anti-ADAMTS13 autoantibodies in immune-mediated thrombotic thrombocytopenic purpura do not hamper ELISA-based quantification of ADAMTS13 antigen.

BACKGROUND: The biological diagnosis of immune-mediated thrombotic thrombocytopenic purpura (iTTP) is based on determination of ADAMTS13 activity (<10%) and anti-ADAMTS13 autoantibodies. ADAMTS13 antigen levels are not routinely measured in iTTP patients, but studies have shown that antigen levels are a valuable prognostic factor. OBJECTIVES: To (a) report the validation of our in-house developed ADAMTS13 antigen enzyme-linked immunosorbent assay (ELISA) and determine ADAMTS13 antigen in a large cohort of healthy donor and iTTP patient plasma samples; and (b) to investigate whether ADAMTS13 antigen determination is not disturbed by the presence of anti-ADAMTS13 autoantibodies. METHODS: Our in-house ADAMTS13 antigen ELISA was validated in terms of sensitivity, repeatability, and reproducibility. ADAMTS13 antigen levels were determined in plasma samples from 423 healthy donors and 112 acute iTTP patients. Purified IgGs from iTTP patients were added to normal human plasma to determine whether anti-ADAMTS13 autoantibodies hampered ADAMTS13 antigen determination. RESULTS: Our in-house ADAMTS13 antigen ELISA has a detection limit of 3% and low intra-assay (coefficient of variation, %CV < 10%) and inter-assay (%CV < 18%) variability. ADAMTS13 antigen levels were significantly reduced (P < .0001) in acute iTTP patients (15 ± 18%) compared to healthy donors (101 ± 18%). The anti-ADAMTS13 autoantibodies in plasma of iTTP patients did not impede ADAMTS13 antigen determinations using our in-house ELISA. CONCLUSIONS: Our in-house ADAMT13 antigen ELISA is a powerful tool to correctly determine ADAMTS13 antigen levels in iTTP patients, which supports routine ADAMTS13 antigen measurements in these patients to have better insight into disease prognosis.

N-acetylcysteine in preclinical mouse and baboon models of thrombotic thrombocytopenic purpura.

Thrombotic thrombocytopenic purpura (TTP) is a microangiopathic disorder diagnosed by thrombocytopenia and hemolytic anemia, associated with a deficiency in von Willebrand factor (VWF)-cleaving protease ADAMTS13. Current treatment is based on plasma infusion for congenital TTP, or plasma exchange, often in combination with immunosuppressive agents, for acquired TTP. These treatment methods are not always effective; therefore, new treatment methods are highly necessary. N-acetylcysteine (NAC), an FDA-approved anti-mucolytic agent, is a possible new treatment strategy for TTP, as it was demonstrated to reduce disulfide bonds in VWF, thereby decreasing VWF multimers size and hence their prothrombotic potential. We investigated whether NAC, without concurrent plasma exchange and immunosuppressive therapy, is effective in preventing and resolving TTP signs, using well-established murine and baboon models for TTP. In mice, prophylactic administration of NAC was effective in preventing severe TTP signs. This in vivo finding was supported by in vitro data demonstrating the VWF multimer-reducing properties of NAC in solution. Nonetheless, in both mice and baboons, administration of NAC was not effective in resolving preexisting TTP signs; thrombocytopenia, hemolytic anemia, and organ damage could not be reversed, as thrombus resolution was not achieved. Failure to improve clinical outcome occurred even though a reduction in VWF multimers was observed, demonstrating that NAC was efficient in reducing disulfide bonds in circulating VWF multimers. In conclusion, prophylactic administration of NAC, without concurrent plasma exchange, was effective in preventing severe TTP signs in mice, but NAC was not effective in resolving preexistent acute TTP signs in mice and baboons.