Linker regions and flexibility around the metalloprotease domain account for conformational activation of ADAMTS-13

Comparison of the Novel Thrombolytic Constitutively Active ADAMTS13 With Clinical Thrombolytics in a Murine Stroke Model

BACKGROUND

r-tPA (recombinant tissue-type plasminogen activator) and its variant, TNK (tenecteplase), are the currently approved thrombolytic drugs for the treatment of acute ischemic stroke, but they are ineffective in a proportion of patients due to r-tPA resistance of platelet-rich thrombi. A novel thrombolytic, caADAMTS13 (constitutively active a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) has been shown to improve experimental stroke outcomes where platelet-rich thrombi are present but have not been directly compared with r-tPA or TNK.

METHODS

We conducted a direct comparison of caADAMTS13 versus r-tPA versus TNK versus vehicle control in the ferric chloride-mediated distal middle cerebral artery occlusion model in mice, which features platelet and VWF (von Willebrand Factor)-rich thrombi that reproduce r-tPA-resistant occlusion. Treatments were administered intravenously 1 hour after ferric chloride application by bolus injection or bolus followed by infusion, as translationally applicable. Laser speckle contrast imaging measured early reperfusion over the hour following treatment, and magnetic resonance imaging measured cerebral blood flow and lesion volume at 24 hours.

RESULTS

Reperfusion 1 hour after treatment was greatest in caADAMTS13-treated animals. Later cerebral blood flow, 24 hours post-treatment, within the stroke-affected hypoperfused area was higher in caADAMTS13 and r-tPA but not TNK-treated mice. Functionally, this led to the absence of an initial behavioral deficit in caADAMTS13-treated mice, alongside a smaller lesion volume at 24 hours and reduced extent of bleeding.

CONCLUSIONS

These findings demonstrate an overall suggestion that caADAMTS13 has improved thrombolytic efficacy, compared with current stroke treatments, against platelet-rich thrombi, for which there is currently an unmet clinical need.

A Disintegrin and Metalloprotease with Thrombospondin Motif, Member 13, and Von Willebrand Factor in Relation to the Duality of Preeclampsia and HIV Infection

Normal pregnancy is associated with multiple changes in the coagulation and the fibrinolytic system. In contrast to a non-pregnant state, pregnancy is a hypercoagulable state where the level of VWF increases by 200-375%, affecting coagulation activity. Moreover, in this hypercoagulable state of pregnancy, preeclampsia is exacerbated. ADAMTS13 cleaves the bond between Tyr1605 and Met1606 in the A2 domain of VWF, thereby reducing its molecular weight. A deficiency of ADAMTS13 originates from mutations in gene or autoantibodies formed against the protease, leading to defective enzyme production. Von Willebrand protein is critical for hemostasis and thrombosis, promoting thrombus formation by mediating the adhesion of platelets and aggregation at high shear stress conditions within the vessel wall. Mutations in VWF disrupts multimer assembly, secretion and/or catabolism, thereby influencing bleeding. VWF is the primary regulator of plasma ADAMTS13 levels since even minute amounts of active ADAMTS13 protease have a significant inhibitory effect on inflammation and thrombosis. VWF is released as a result of endothelial activation brought on by HIV infection. The SARS-CoV-2 infection promotes circulating proinflammatory cytokines, increasing endothelial secretion of ultra large VWF that causes an imbalance in VWF/ADAMTS13. Raised VWF levels corresponds with greater platelet adhesiveness, promoting a thrombotic tendency in stenotic vessels, leading to increased shear stress conditions.

Unraveling antibody-induced structural dynamics in the ADAMTS13 CUB1-2 domains via HDX-MS

ABSTRACT

Allosteric regulation of ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type-1 motif, member 13) activity involves an interaction between its spacer (S) and 2 complement C1r/C1s, Uegf and BMP1 (CUB; CUB1-2) domains to keep the enzyme in a closed, latent conformation. Monoclonal antibodies (mAbs) uncouple the S-CUB interaction to open the ADAMTS13 conformation and thereby disrupt the global enzyme latency. The molecular mechanism behind this mAb-induced allostery remains poorly understood. To gain insights in the mAb-induced S-CUB uncoupling and global latency disruption, we combined hydrogen/deuterium exchange mass spectrometry (HDX-MS) with structural analysis of ADAMTS13 CUB1-2 mutants. Thereby, the CUB1 L3 and L9 loops were fine-mapped as the 17G2 mAb binding epitope. Indirect S-CUB uncoupling was observed as mAb binding-induced extensive structural dynamics within both CUB1-2 domains without directly targeting the contiguous CUB1 surface that engages with the ADAMTS13 S domain. HDX-MS analysis revealed the short interdomain linker to structurally cover the central CUB1-2 domain interface, which also showed some protein regions that became more exposed upon mAb binding. Therefore, repositioning of the central CUB1-2 interface appears crucial to transfer structural dynamics between both domains. Nevertheless, mutagenesis of the short linker did not disrupt the ADAMTS13 global latency because its closed conformation was preserved. Presumably, allosteric disruption of the global latency requires a structural impact extending beyond the central interface repositioning. Because anti-ADAMTS13 autoantibodies from patients with immune-mediated thrombotic thrombocytopenic purpura (iTTP) also induce an open ADAMTS13 conformation, our novel insights in the antibody-mediated global latency disruption boost our understanding of the iTTP disease pathology.

N-glycan shielded CUB domains of ADAMTS13 prevent binding of C-terminal antibodies in patients with immune-mediated TTP

ABSTRACT

In immune-mediated thrombotic thrombocytopenic purpura (iTTP), patients develop antibodies against ADAMTS13. Most patients exhibit inhibitory antispacer antibodies. Noninhibitory antibodies binding to the carboxy-terminal CUB domains have been suggested to enhance the clearance of ADAMTS13 in iTTP. Furthermore, anti-CUB antibodies induce an open conformation, which has been shown to be an important biomarker for disease severity and relapse risk. We explored whether the introduction of N-glycans in the CUB domains of ADAMTS13 can reduce the binding of pathogenic anti-CUB autoantibodies. The binding of a panel of anti-CUB monoclonal antibodies derived from patients with iTTP, to newly designed N-glycan modified ADAMTS13 CUB domain variants, was assessed by enzyme-linked immunosorbent assay. In addition, a subset of these variants was screened against plasma samples from patients with iTTP, which primarily contain antibodies directed toward the carboxy-terminal domains of ADAMTS13. Introduction of N-glycans at amino acid positions of 1251, 1255, and 1368 in the CUB1/2 domains of ADAMTS13 can effectively reduce the binding of 6 out of 7 anti-CUB antibodies derived from patients with iTTP. Reduced binding to CUB N-glycan variants was observed in 8 out of 9 patient samples. Binding was decreased from 81% to 47% for NGLY3+CUB-NGLY and 60% to 28% for 5ALA+CUB-NGLY variants. Collectively our findings show that the introduction of N-glycans within the CUB domain of ADAMTS13 can prevent the binding of anti-CUB antibodies in patients with iTTP. Based on these findings, we propose that CUB-NGLY modified ADAMTS13 variants can be used for improved treatment of patients with iTTP.

ADAMTS13 Improves Endothelial Function and Reduces Inflammation in Diabetic Retinopathy

The protease, a disintegrin and metalloproteinase with thrombospondin type 1 motif member 13 (ADAMTS13), known to cleave only the von Willebrand factor (VWF), has powerful regulatory effects on microvascular platelet adhesion, thrombosis, inflammation, and endothelial dysfunction. We study the protection against diabetes-induced retinal injury in experimental rats by supplementation with recombinant ADAMTS13. We compare human epiretinal membranes and vitreous samples from nondiabetic subjects and patients with proliferative diabetic retinopathy (PDR) and extend in vitro analyses with the use of various immunodetection and spectrofluorimetric methods on rat retina and human retinal glial and endothelial cell cultures. Functional studies include the assessment of the blood-retinal barrier (BRB), cell adhesion, and in vitro angiogenesis. In epiretinal membranes, endothelial cells and monocytes/macrophages express ADAMTS13. The levels of VWF, the platelet marker CD41, ADAMTS13, and the biomarkers of endothelial cell injury soluble VE-cadherin and soluble syndecan-1 are increased in PDR vitreous. ADAMTS13 is downregulated in diabetic rat retinas. The intravitreal administration of ADAMTS13 attenuates diabetes-induced BRB breakdown, the downregulation of VE-cadherin and β-catenin, and the upregulation of VWF, CD41, phospho-ERK1/2, HMGB1, VCAM-1, and ICAM-1. In Müller cells, ADAMTS13 attenuates MCP-1, MMP-9, and ROS upregulation induced by diabetic mimetic conditions. In HRMECs, ADAMTS13 attenuates the shedding of the soluble VE-cadherin and soluble syndecan-1 and the levels of phospho-ERK1/2, MCP-1, fractalkine, and ROS induced by diabetic mimetic conditions, the upregulation of ICAM-1 and VCAM-1 elicited by TNF-α, the adherence of monocytes induced by TNF-α, and VEGF-induced migration of human retinal microvascular endothelial cells. Our findings suggest that enhancing ADAMTS13 levels in situ ameliorates diabetes-induced retinal inflammation and vascular dysfunction.

Immune Thrombotic Thrombocytopenic Purpura: Pathophysiology, Diagnosis, Therapy and Open Issues

Immune thrombotic thrombocytopenic purpura (iTTP) is a life-threatening thrombotic microangiopathy characterized by microangiopathic hemolytic anemia, thrombocytopenia, and ischemic end-organ injury due to microvascular platelet-rich thrombi. iTTP pathophysiology is based on a severe ADAMTS13 deficiency, the specific von Willebrand factor (vWF)-cleaving protease, due to anti-ADAMTS13 autoantibodies. Early diagnosis and treatment reduce the mortality. Frontline therapy includes daily plasma exchange (PEX) with fresh frozen plasma replacement and immunosuppression with corticosteroids. Caplacizumab has recently been added to frontline therapy. Caplacizumab is a nanobody that binds to the A1 domain of vWF, blocking the interaction of ultra-large vWF multimers with the platelet and thereby preventing the formation of platelet-rich thrombi. Caplacizumab reduces mortality due to ischemic events, refractoriness, and exacerbations after PEX discontinuation. Until now, the criteria for response to treatment mainly took into account the normalization of platelet count and discontinuation of PEX; with the use of caplacizumab leading to rapid normalization of platelet count, it has been necessary to redefine the response criteria, taking into account also the underlying autoimmune disease. Monitoring of ADAMTS13 activity is important to identify cases with a low value of activity (<10IU/L), requiring the optimization of immunosuppressive therapy with the addition of Rituximab. Rituximab is effective in patients with refractory disease or relapsing disease. Currently, the use of Rituximab has expanded, both in frontline treatment and during follow-up, as a pre-emptive approach. Some patients do not achieve ADAMTS13 remission following the acute phase despite steroids and rituximab treatment, requiring an individualized immunosuppressive approach to prevent clinical relapse. In iTTP, there is an increased risk of venous thrombotic events (VTEs) as well as arterial thrombotic events, and most occur after platelet normalization. Until now, there has been no consensus on the use of pharmacological thromboprophylaxis in patients on caplacizumab because the drug is known to increase bleeding risk.

Recombinant ADAMTS13 for Immune Thrombotic Thrombocytopenic Purpura

In patients with immune thrombotic thrombocytopenic purpura (iTTP), autoantibodies against the metalloprotease ADAMTS13 lead to catastrophic microvascular thrombosis. However, the potential benefits of recombinant human ADAMTS13 (rADAMTS13) in patients with iTTP remain unknown. Here, we report the clinical use of rADAMTS13, which resulted in the rapid suppression of disease activity and complete recovery in a critically ill patient whose condition had proved to be refractory to all available treatments. We also show that rADAMTS13 causes immune complex formation, which saturates the autoantibody and may promote its clearance. Our data support the role of rADAMTS13 as a novel adjunctive therapy in patients with iTTP.

ADAMTS-13 conformation influences autoimmune recognition in immune thrombotic thrombocytopenic purpura

BACKGROUND

Patients with immune-mediated thrombotic thrombocytopenic purpura (iTTP) have anti-ADAMTS-13 immunoglobulin G (IgG) autoantibodies that enhance ADAMTS-13 clearance and/or inhibit its function. ADAMTS-13 normally circulates in a closed conformation, which is manifested by the interaction of the CUB domains with the central spacer domain. Disruption of the spacer-CUB interaction opens ADAMTS-13, which augments its proteolytic function but may also expose cryptic autoimmune epitopes that promote further autoantibody recognition.

OBJECTIVES

To explore differences in autoantibody binding to ADAMTS-13 in its closed or open conformations in patients with iTTP and to correlate these differences with disease-related parameters.

METHODS

We developed a novel assay to measure autoantibodies binding to closed and open ADAMTS-13. Autoantibody titer and IgG subclass binding to open or closed ADAMTS-13 were measured in 70 iTTP first presentation samples and correlated with clinical data, remission, and relapse.

RESULTS

In 70 patients with iTTP, the mean autoantibody titer against open ADAMTS-13 was, on average, approximately 2-fold greater than that against closed ADAMTS-13, suggesting that ADAMTS-13 opening increases epitope exposure and immune complex formation. Autoantibody titer against closed/open ADAMTS-13 and IgG subclass did not correlate with ADAMTS-13 antigen at presentation. Two patients with iTTP and persistent autoantibodies lost specificity for closed ADAMTS-13 in remission. Recognition of closed/open ADAMTS-13 and autoantibody IgG subclass between the first and second iTTP episodes were very similar.

CONCLUSION

ADAMTS-13 autoantibody binding is highly influenced by ADAMTS-13 conformation. Although this does not appear to modify the pathogenicity of autoantibodies, the autoantibody signature at relapse suggests that relapse represents re-emergence of the original autoimmune response rather than de novo presentation.

Open ADAMTS-13 conformation index predicts earlier relapse in immune-mediated thrombotic thrombocytopenic purpura

BACKGROUND

ADAMTS-13 adopts an open conformation in patients with immune-mediated thrombotic thrombocytopenic purpura (iTTP) in acute phase while being closed in healthy donors. We reported that a substantial number of patients with iTTP in remission with restored ADAMTS-13 activity (>50%) still had an open ADAMTS-13 conformation, although a closed conformation is expected given the extent of remission.

OBJECTIVES

To investigate whether open ADAMTS-13, represented by a conformation index >0.5, is associated with a risk of earlier ADAMTS-13 and/or clinical relapse.

METHODS

We collected follow-up data (ADAMTS-13 parameters, ADAMTS-13 and clinical relapse, and treatment) from 81 patients with iTTP in remission with ADAMTS-13 activity >50%.

RESULTS

During follow-up, 19 ADAMTS-13 and 10 clinical relapses were reported (median follow-up period, 20 months). First, open or closed ADAMTS-13 conformation was dichotomized based on the 0.5 conformation index cutoff. Open ADAMTS-13 (conformation index, >0.5) was not identified as a risk factor for ADAMTS-13 and clinical relapse (log-rank test and Cox regression model). In contrast, by identifying the optimal conformation index cutoff for relapse prediction, using classification and regression tree analysis, a conformation index >0.645 and >0.835 was shown to be a risk factor for ADAMTS-13 relapse (hazard ratio, 3.3; 95% CI, 1.3-8.3; P = .01) and clinical relapse (hazard ratio, 4.4; 95% CI, 1.3-15.3; P = .02), respectively.

CONCLUSION

Patients with open ADAMTS-13 with a conformation index >0.645 and >0.835 have a >3- and >4-fold higher risk of earlier ADAMTS-13 and clinical relapse, respectively. Hence, ADAMTS-13 conformation index could be used to complement ADAMTS-13 activity monitoring to timely notice ADAMTS-13 relapse and prevent clinical relapse.

Novel ADAMTS13 mutations in a patient with congenital thrombotic thrombocytopenic purpura

Congenital thrombotic thrombocytopenic purpura (TTP) is a rare autosomal recessive genetic disorder caused by mutations in the ADAMTS13 gene. Approximately 200 mutations of the ADAMTS-13 gene have been identified, although only a few have been characterized through in vitro expression studies. We conducted an investigation on a male congenital TTP patient with reduced plasma levels of ADAMTS13 activity. DNA sequence analysis revealed two mutations on chromosome 9 (1.9q34.2) in the patient's ADAMTS13 gene. One mutation was a non-synonymous mutation (exon 5: c.A530G: p.Y177C), while the other was a nonsense mutation (exon 21: c.G2651A: p.W884X). Both mutations were found to be heterozygous. The patient's parents had no history of thrombocytopenia or neurological symptoms. DNA sequence analysis showed the patient's father was a heterozygote for the nonsense mutation of the ADAMTS13 gene (exon 21: c.G2651A: p.W884X), while the mother was a heterozygote for the non-synonymous mutation of the ADAMTS13 gene (exon 5: c.A530G: p.Y177C). To investigate the mechanism behind ADAMTS13 deficiency in this patient, wild type (WT), ADAMTS13 p.Y177C, and ADAMTS13 p.W884X were transiently expressed in 293-6E cells. Expression studies revealed a significant reduction in enzyme activity and secretion, although the protease was detected within the cells. The 3D structures of the natural and mutated ADAMTS-13 proteins were partially reconstructed using the Phyre2 web server. The mutation that replaces the tyrosine residue at amino acid position 177 with cysteine may result in decreased steric hindrance and a looser structure. This mutation affects the binding of calcium ions and the secretion of the enzyme from intracellular to extracellular compartments.

Immune thrombotic thrombocytopenic purpura: pathogenesis and novel therapies: a narrative review

Background and Objectives

Immune thrombotic thrombocytopenic purpura (iTTP) is a rare, but potentially fatal blood disease, resulting from autoantibodies against A Disintegrin and Metalloprotease with ThromboSpondin Type 1 Repeats, 13 (ADAMTS13). While major progress has been made in past decades concerning early diagnosis and management of iTTP, the mechanisms underlying the formation and the mechanism of action of these autoantibodies against ADMATS13 are still unknown. This review will provide a narrative review of pathogenesis and novel therapeutics of iTTP.

Methods

We did PubMed literature search using a combination of thrombotic thrombocytopenic purpura and treatment or pathogenesis from 1955 to November 2022. A total of 4,767 articles with full text were found and only relevant articles in English were further reviewed and summarized.

Key Content and Findings

We found that the primary mechanism underlying severe ADAMTS13 deficiency in patients with iTTP is autoantibody-mediated inhibition and/or accelerated clearance of ADAMTS13 metalloprotease. Other factors including allosteric regulation and post-translational modifications (i.e., glycosylation and citrullination, and arginine methylation, etc.) may affect ADAMTS13 secretion and function and also contribute to the pathogenesis of iTTP. The standard of care for iTTP today consists of therapeutic plasma exchange, anti-von Willebrand factor (vWF) caplacizumab, and immunosuppressives (e.g., corticosteroids and rituximab), known as the triple therapy, which has significantly reduced exacerbation and mortality rates.

Conclusions

We hope that the information provided in the review article helps better understand the pathogenesis of iTTP, which may guide design novel and more effective therapeutics for this potentially fatal disorder.

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.

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.

Metalloprotease domain latency protects ADAMTS13 against broad-spectrum inhibitors of metalloproteases while maintaining activity toward VWF

BACKGROUND

ADAMTS13 is a circulating metalloprotease that cleaves von Willebrand factor (VWF) in a shear-dependent manner. ADAMTS13 is secreted as an active protease but has a long half-life, suggesting that it is resistant to circulating protease inhibitors. These zymogen-like properties indicate that ADAMTS13 exists as a latent protease that is activated by its substrate.

OBJECTIVES

To investigate the mechanism of ADAMTS13 latency and resistance to metalloprotease inhibitors.

METHODS

Probe the active site of ADAMTS13 and variants using alpha-2 macroglobulin (A2M), tissue inhibitors of metalloproteases (TIMPs), and Marimastat.

RESULTS

ADAMTS13 and C-terminal deletion mutants are not inhibited by A2M, TIMPs, or Marimastat, but cleave FRETS-VWF73, suggesting that the metalloprotease domain is latent in the absence of substrate. Within the metalloprotease domain, mutating the gatekeeper triad (R193, D217, D252) or substituting the calcium-binding (R180-R193) or the variable (G236-S263) loops with corresponding features from ADAMTS5 did not sensitize MDTCS to inhibition. However, substituting the calcium-binding loop and an extended variable loop (G236-S263) corresponding to the S1-S1' pockets with those from ADAMTS5, resulted in MDTCS-GVC5 inhibition by Marimastat, but not by A2M or TIMP3. Substituting the MD domains of ADAMTS5 into full-length ADAMTS13 resulted in a 50-fold reduction in activity compared with the substitution into MDTCS. However, both chimeras were susceptible to inhibition, suggesting that the closed conformation does not contribute to the latency of the metalloprotease domain.

CONCLUSION

The metalloprotease domain protects ADAMTS13 from inhibitors and exists in a latent state that is partially maintained by loops flanking the S1 and S1' specificity pockets.

In vitro characterization of a novel Arg102 mutation in the ADAMTS13 metalloprotease domain

BACKGROUND

Congenital thrombotic thrombocytopenic purpura is caused by defects in the ADAMTS13 gene. ADAMTS13 is normally preactivated by conformational changes of the Metalloprotease (M) domain. Studying a novel congenital thrombotic thrombocytopenic purpura p.R102S mutation in the M domain, which results in undetectable ADAMTS13 activity in the patient, could help to explain the patients' phenotype and to elucidate the currently unclear mechanism of allosteric preactivation.

OBJECTIVES

To investigate the in vitro effect of p.R102S mutation on ADAMTS13 secretion, activity, and allosteric preactivation.

METHODS

Molecular modeling was used to study the effect of the mutation on the stability of ADAMTS13. Recombinant mutant ADAMTS13 was generated by transient and stable transfection of, respectively, CHO K1 and HEK293-T cells. ADAMTS13 antigen was measured in enzyme-linked immunosorbent assay. ADAMTS13 activity was measured in a FRETS-VWF73 assay. Allosteric preactivation was assessed in FRETS-VWF73 assay, using monoclonal antibody (mAb) 17G2 that normally induces a ∼2-fold increase in activity, and in enzyme-linked immunosorbent assay using mAb 6A6 recognizing a cryptic epitope in the M domain that becomes exposed after binding of 17G2.

RESULTS

p.R102S mutation destabilizes the interactions between the M and Disintegrin-like (D) domain. p.R102S mutant secretion was impaired (35% of wild type) and activity was severely reduced (12% of wild type). p.R102S mutant could still be activated and the cryptic epitope of 6A6 was still fully exposed by 17G2 addition.

CONCLUSION

p.R102S mutation destabilizes the M-D domain interactions, causing impaired ADAMTS13 secretion and activity, which explains the patients' phenotype. Allosteric preactivation of ADAMTS13 remains conserved in the presence of the p.R102S mutation.

Frontiers in pathophysiology and management of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a fatal disease in which platelet-rich microthrombi cause end-organ ischemia and damage. TTP is caused by markedly reduced ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) activity. Hereditary or congenital TTP (cTTP) is caused by ADAMTS13 gene mutations. In acquired or immune TTP (iTTP), ADAMTS13 activity is reduced by anti-ADAMTS13 autoantibodies. TTP is characterized by thrombocytopenia, hemolytic anemia, fever, renal dysfunction, and neuropsychiatric symptoms. Therapeutic plasma exchange (TPE) and immunosuppressive therapy are the mainstays of treatment. As untreated TTP has a high mortality rate, immediate initiation of TPE is recommended when TTP is suspected. Conventionally, corticosteroids have been used for immunosuppressive therapy. Current drug therapies include rituximab, an anti-CD20 antibody that is effective in newly diagnosed cases and refractory cases, as well as for relapse prevention, and caplacizumab, an anti- von Willebrand factor (VWF) nanobody that inhibits the binding of platelets to VWF and prevents microthrombi formation. Recombinant human ADAMTS13 is a promising treatment for cTTP. Although these therapeutic advances have improved the outcomes of TTP, early diagnosis and prompt initiation of appropriate therapy are necessary to achieve these outcomes.

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.

ADAMTS13 conformations and mechanism of inhibition in immune thrombotic thrombocytopenic purpura

ADAMTS13, a plasma metalloprotease that cleaves von Willebrand factor, is crucial for normal hemostasis. Acquired autoantibody-mediated deficiency of plasma ADAMTS13 results in a potentially fatal blood disorder, immune thrombotic thrombocytopenic purpura (iTTP). Plasma ADAMTS13 protease appears to exist in multiple conformations. Under physiological conditions, plasma ADAMTS13 exists predominantly in its "closed" conformation (or latent form), which may be activated by lowering pH, ligand binding, and binding of an antibody against the distal domains of ADAMTS13. In patients with iTTP, polyclonal antibodies target at various domains of ADAMTS13. However, nearly all inhibitory antibodies bind the spacer domain, whereas antibodies that bind the distal C-terminal domains may activate ADAMTS13 through removing its allosteric inhibition. Additionally, the anti-C-terminal antibodies may alter the potency of inhibitory antibodies towards ADAMTS13 activity. This review summarizes some of the most recent knowledge about the ADAMTS13 conformation and its mechanism of inhibition by its autoantibodies.

Differential Expression and Localization of ADAMTS Proteinases in Proliferative Diabetic Retinopathy

We analyzed the expression of ADAMTS proteinases ADAMTS-1, -2, -4, -5 and -13; their activating enzyme MMP-15; and the degradation products of proteoglycan substrates versican and biglycan in an ocular microenvironment of proliferative diabetic retinopathy (PDR) patients. Vitreous samples from PDR and nondiabetic patients, epiretinal fibrovascular membranes from PDR patients, rat retinas, retinal Müller glial cells and human retinal microvascular endothelial cells (HRMECs) were studied. The levels of ADAMTS proteinases and MMP-15 were increased in the vitreous from PDR patients. Both full-length and cleaved activation/degradation fragments of ADAMTS proteinases were identified. The amounts of versican and biglycan cleavage products were increased in vitreous from PDR patients. ADAMTS proteinases and MMP-15 were localized in endothelial cells, monocytes/macrophages and myofibroblasts in PDR membranes, and ADAMTS-4 was expressed in the highest number of stromal cells. The angiogenic activity of PDR membranes correlated significantly with levels of ADAMTS-1 and -4 cellular expression. ADAMTS proteinases and MMP-15 were expressed in rat retinas. ADAMTS-1 and -5 and MMP-15 levels were increased in diabetic rat retinas. HRMECs and Müller cells constitutively expressed ADAMTS proteinases but not MMP-15. The inhibition of NF-κB significantly attenuated the TNF-α-and-VEGF-induced upregulation of ADAMTS-1 and -4 in a culture medium of HRMECs and Müller cells. In conclusion, ADAMTS proteinases, MMP-15 and versican and biglycan cleavage products were increased in the ocular microenvironment of patients with PDR.

Robust thrombolytic and anti-inflammatory action of a constitutively active ADAMTS13 variant in murine stroke models

Advances in our understanding of ADAMTS13 structure, and the conformation changes required for full activity, have rejuvenated the possibility of its use as a thrombolytic therapy. We have tested a novel Ala1144Val ADAMTS13 variant (constitutively active [ca] ADAMTS13) that exhibits constitutive activity, characterized using in vitro assays of ADAMTS13 activity, and greatly enhanced thrombolytic activity in 2 murine models of ischemic stroke, the distal FeCl3 middle cerebral artery occlusion (MCAo) model and transient middle cerebral artery occlusion (tMCAO) with systemic inflammation and ischemia/reperfusion injury. The primary measure of efficacy in both models was restoration of regional cerebral blood flow (rCBF) to the MCA territory, which was determined using laser speckle contrast imaging. The caADAMTS13 variant exhibited a constitutively active conformation and a fivefold enhanced activity against fluorescence resonance energy transfer substrate von Willebrand factor 73 (FRETS-VWF73) compared with wild-type (wt) ADAMTS13. Moreover, caADAMTS13 inhibited VWF-mediated platelet capture at subphysiological concentrations and enhanced t-PA/plasmin lysis of fibrin(ogen), neither of which were observed with wtADAMTS13. Significant restoration of rCBF and reduced lesion volume was observed in animals treated with caADAMTS13. When administered 1 hour after FeCl3 MCAo, the caADAMTS13 variant significantly reduced residual VWF and fibrin deposits in the MCA, platelet aggregate formation, and neutrophil recruitment. When administered 4 hours after reperfusion in the tMCAo model, the caADAMTS13 variant induced a significant dissolution of platelet aggregates and a reduction in the resulting tissue hypoperfusion. The caADAMTS13 variant represents a potentially viable therapeutic option for the treatment of acute ischemic stroke, among other thrombotic indications, due to its enhanced in vitro and in vivo activities that result from its constitutively active conformation.

Anti-cysteine/spacer antibodies that open ADAMTS13 are a common feature in iTTP

An open ADAMTS13 conformation is a novel biomarker for iTTP and is induced by anti-ADAMTS13 autoantibodies.

The autoantibodies against the CS region play an important role in the appearance of an open ADAMTS13 conformation.

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is caused by an autoantibody-mediated deficiency in ADAMTS13. In healthy individuals, ADAMTS13 has a folded conformation in which the central spacer (S) domain interacts with the C-terminal CUB domains. We recently showed that ADAMTS13 adopts an open conformation in iTTP and that patient immunoglobulin G antibodies (IgGs) can open ADAMTS13. Anti-ADAMTS13 autoantibodies in patients with iTTP are directed against the different ADAMTS13 domains, but almost all patients have autoantibodies binding to the cysteine/spacer (CS) domains. In this study, we investigated whether the autoantibodies against the CS and CUB domains can disrupt the S-CUB interaction of folded ADAMTS13, thereby opening ADAMTS13. To this end, we purified anti-CS and anti-CUB autoantibodies from 13 patients with acute iTTP by affinity chromatography. The successfully purified anti-CS (10/13 patients) and anti-CUB (4/13 patients) autoantibody fractions were tested further in our ADAMTS13 conformation enzyme-linked immunosorbent assay to study whether they could open ADAMTS13. Interestingly, all purified anti-CS fractions (10/10 patients) were able to open ADAMTS13. On the other hand, only half of the purified anti-CUB fractions (2/4 patients) opened ADAMTS13. Our finding highlights that anti-CS autoantibodies that open ADAMTS13 are a common feature of the autoimmune response in iTTP.

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.

Conformational plasticity of ADAMTS13 in hemostasis and autoimmunity

A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) is a multidomain metalloprotease for which until now only a single substrate has been identified. ADAMTS13 cleaves the polymeric force-sensor von Willebrand factor (VWF) that unfolds under shear stress and recruits platelets to sites of vascular injury. Shear force–dependent cleavage at a single Tyr–Met peptide bond in the unfolded VWF A2 domain serves to reduce the size of VWF polymers in circulation. In patients with immune-mediated thrombotic thrombocytopenic purpura (iTTP), a rare life-threatening disease, ADAMTS13 is targeted by autoantibodies that inhibit its activity or promote its clearance. In the absence of ADAMTS13, VWF polymers are not adequately processed, resulting in spontaneous adhesion of blood platelets, which presents as severe, life-threatening microvascular thrombosis. In healthy individuals, ADAMTS13–VWF interactions are guided by controlled conversion of ADAMTS13 from a closed, inactive to an open, active conformation through a series of interdomain contacts that are now beginning to be defined. Recently, it has been shown that ADAMTS13 adopts an open conformation in the acute phase and during subclinical disease in iTTP patients, making open ADAMTS13 a novel biomarker for iTTP. In this review, we summarize our current knowledge on ADAMTS13 conformation and speculate on potential triggers inducing conformational changes of ADAMTS13 and how these relate to the pathogenesis of iTTP.

Gold nanoparticle enhanced multiplexed biosensing on a fiber optic surface plasmon resonance probe

We present an innovative multiplexing concept on a fiber optic surface plasmon resonance (FO-SPR) platform and demonstrate for the first time the simultaneous detection of two targets using the same FO sensor probe. Co(III)-NTA chemistry was used for oriented and stable co-immobilization of two different His₆-tagged bioreceptors. T2C2 and MDTCS (i.e. fragments of the ADAMTS13 metalloprotease linked to the thrombotic thrombocytopenic purpura disorder) served as model system bioreceptors together with their respective targets (4B9 and II-1 antibodies). Gold nanoparticles were used here in an original way for discriminating the two targets in the same sample, in addition to their traditional signal amplification-role. After verifying the specificity of the selected model system, we studied the bioreceptor surface density and immobilization order. Innovative approach to lower the bioreceptor concentration below surface saturation resulted in an optimal detection of both targets, whereas the order of immobilization of the two bioreceptors did not give any significant difference. By sequentially immobilizing the T2C2 and MDTC bioreceptors, we established calibration curves in buffer and 100-fold diluted human blood plasma. This resulted in calculated limits of detection of 3.38 and 2.31 ng/mL in diluted plasma for 4B9 and II-1, respectively, indicating almost the same sensitivity as in buffer. Importantly, we also proved the applicability of the established calibration curves for quantifying the targets at random and more realistic ratios, directed by the design of experiments. This multiplexing study further expands the repertoire of applications on the FO-SPR biosensing platform, which together with its intrinsic features opens up great opportunities for diagnostics and life sciences.

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.

Antibodies that conformationally activate ADAMTS13 allosterically enhance metalloprotease domain function

Plasma ADAMTS13 circulates in a folded conformation that is stabilized by an interaction between the central Spacer domain and the C-terminal CUB (complement components C1r and C1s, sea urchin protein Uegf, and bone morphogenetic protein-1) domains. Binding of ADAMTS13 to the VWF D4(-CK) domains or to certain activating murine monoclonal antibodies (mAbs) induces a structural change that extends ADAMTS13 into an open conformation that enhances its function. The objective was to characterize the mechanism by which conformational activation enhances ADAMTS13-mediated proteolysis of VWF. The activating effects of a novel anti-Spacer (3E4) and the anti-CUB1 (17G2) mAbs on the kinetics of proteolysis of VWF A2 domain fragments by ADAMTS13 were analyzed. mAb-induced conformational changes in ADAMTS13 were investigated by enzyme-linked immunosorbent assay. Both mAbs enhanced ADAMTS13 catalytic efficiency (kcat/Km) by ∼twofold (3E4: 2.0-fold; 17G2: 1.8-fold). Contrary to previous hypotheses, ADAMTS13 activation was not mediated through exposure of the Spacer or cysteine-rich domain exosites. Kinetic analyses revealed that mAb-induced conformational extension of ADAMTS13 enhances the proteolytic function of the metalloprotease domain (kcat), rather than augmenting substrate binding (Km). A conformational effect on the metalloprotease domain was further corroborated by the finding that incubation of ADAMTS13 with either mAb exposed a cryptic epitope in the metalloprotease domain that is normally concealed when ADAMTS13 is in a closed conformation. We show for the first time that the primary mechanism of mAb-induced conformational activation of ADAMTS13 is not a consequence of functional exosite exposure. Rather, our data are consistent with an allosteric activation mechanism on the metalloprotease domain that augments active site function.

Crystal structure of ADAMTS13 CUB domains reveals their role in global latency

ADAMTS13 is a plasma metalloprotease that is essential for the regulation of von Willebrand factor (VWF) function, mediator of platelet recruitment to sites of blood vessel damage. ADAMTS13 function is dynamically regulated by structural changes induced by VWF binding that convert it from a latent to active conformation. ADAMTS13 global latency is manifest by the interaction of its C-terminal CUB1-2 domains with its central Spacer domain. We resolved the crystal structure of the ADAMTS13 CUB1-2 domains revealing a previously unreported configuration for the tandem CUB domains. Docking simulations between the CUB1-2 domains with the Spacer domain in combination with enzyme kinetic functional characterization of ADAMTS13 CUB domain mutants enabled the mapping of the CUB1-2 domain site that binds the Spacer domain. Together, these data reveal the molecular basis of the ADAMTS13 Spacer-CUB interaction and the control of ADAMTS13 global latency.

Thrombotic Thrombocytopenic Purpura: Pathophysiology, Diagnosis, and Management

Thrombotic thrombocytopenic purpura (TTP) is a rare thrombotic microangiopathy characterized by microangiopathic hemolytic anemia, severe thrombocytopenia, and ischemic end organ injury due to microvascular platelet-rich thrombi. TTP results from a severe deficiency of the specific von Willebrand factor (VWF)-cleaving protease, ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13). ADAMTS13 deficiency is most commonly acquired due to anti-ADAMTS13 autoantibodies. It can also be inherited in the congenital form as a result of biallelic mutations in the ADAMTS13 gene. In adults, the condition is most often immune-mediated (iTTP) whereas congenital TTP (cTTP) is often detected in childhood or during pregnancy. iTTP occurs more often in women and is potentially lethal without prompt recognition and treatment. Front-line therapy includes daily plasma exchange with fresh frozen plasma replacement and immunosuppression with corticosteroids. Immunosuppression targeting ADAMTS13 autoantibodies with the humanized anti-CD20 monoclonal antibody rituximab is frequently added to the initial therapy. If available, anti-VWF therapy with caplacizumab is also added to the front-line setting. While it is hypothesized that refractory TTP will be less common in the era of caplacizumab, in relapsed or refractory cases cyclosporine A, N-acetylcysteine, bortezomib, cyclophosphamide, vincristine, or splenectomy can be considered. Novel agents, such as recombinant ADAMTS13, are also currently under investigation and show promise for the treatment of TTP. Long-term follow-up after the acute episode is critical to monitor for relapse and to diagnose and manage chronic sequelae of this disease.

Immunogenic hotspots in the spacer domain of ADAMTS13 in immune-mediated thrombotic thrombocytopenic purpura

BACKGROUND

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is caused by anti-ADAMTS13 autoantibodies inducing a severe deficiency of ADAMTS13. Epitope mapping studies on samples obtained during acute iTTP episodes have shown that the iTTP immune response is polyclonal, with almost all patients having autoantibodies targeting the spacer domain of ADAMTS13.

OBJECTIVES

To identify the immunogenic hotspots in the spacer domain of ADAMTS13.

PATIENTS/METHODS

A library of 11 full-length ADAMTS13 spacer hybrids was created in which amino acid regions of the spacer domain of ADAMTS13 were exchanged by the corresponding region of the spacer domain of ADAMTS1. Next, the full-length ADAMTS13 spacer hybrids were used in enzyme-linked immunosorbent assay to epitope map anti-spacer autoantibodies in 138 samples from acute and remission iTTP patients.

RESULTS

Sixteen different anti-spacer autoantibody profiles were identified with a similar distribution in acute and remission patients. There was no association between the anti-spacer autoantibody profiles and disease severity. Almost all iTTP samples contained anti-spacer autoantibodies against the following three regions: amino acid residues 588-592, 602-610, and 657-666 (hybrids E, G, and M). Between 31% and 57% of the samples had anti-spacer autoantibodies against amino acid regions 572-579, 629-638, 667-676 (hybrids C, J, and N). In contrast, none of the samples had anti-spacer autoantibodies against amino acid regions 556-563, 564-571, 649-656, and 677-685 (hybrids A, B, L, and O).

CONCLUSION

We identified three hotspot regions (amino acid regions 588-592, 602-610, and 657-666) in the spacer domain of ADAMTS13 that are targeted by anti-spacer autoantibodies found in a large cohort of iTTP patients.

[Diagnosis of thrombotic thrombocytopenic purpura]

Thrombotic thrombocytopenic purpura (TTP) is a rare, life-threatening disease, disease, characterised by microangiopathic hemolytic anaemia, consumption thrombocytopenia, and organ dysfunction. The pathogenesis of TTP is attributed to the deficiency in the activity of the metalloproteinase ADAMTS13, specific von Willebrand factor cleaving protease. TTP is suspected when detecting microangiopathic hemolytic anemia, thrombocytopenia, damage to various organs. Diagnosis of TTP is confirmed by the detection of ADAMTS13 activity in plasma less than 10%. Plasma samples for the study of ADAMTS13 activity should be taken before the start of plasma transfusions or plasma exchange. In patients with severe ADAMTS-13 deficiency autoantibodies anti-ADAMTS13 and inhibitor ADAMTS13 should be investigated. Anti-ADAMTS13 antibodies belonging to IgG not always have inhibitory effects. The inhibitory effect of anti-ADAMTS13 antibodies is confirmed by mixing test. All patients with the first established diagnosis of TTP should be examined for mutations of the ADAMTS13 gene.

Insights Into Immunothrombosis: The Interplay Among Neutrophil Extracellular Trap, von Willebrand Factor, and ADAMTS13

Both neutrophil extracellular traps (NETs) and von Willebrand factor (VWF) are essential for thrombosis and inflammation. During these processes, a complex series of events, including endothelial activation, NET formation, VWF secretion, and blood cell adhesion, aggregation and activation, occurs in an ordered manner in the vasculature. The adhesive activity of VWF multimers is regulated by a specific metalloprotease ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs, member 13). Increasing evidence indicates that the interaction between NETs and VWF contributes to arterial and venous thrombosis as well as inflammation. Furthermore, contents released from activated neutrophils or NETs induce the reduction of ADAMTS13 activity, which may occur in both thrombotic microangiopathies (TMAs) and acute ischemic stroke (AIS). Recently, NET is considered as a driver of endothelial damage and immunothrombosis in COVID-19. In addition, the levels of VWF and ADAMTS13 can predict the mortality of COVID-19. In this review, we summarize the biological characteristics and interactions of NETs, VWF, and ADAMTS13, and discuss their roles in TMAs, AIS, and COVID-19. Targeting the NET-VWF axis may be a novel therapeutic strategy for inflammation-associated TMAs, AIS, and COVID-19.

Generation and validation of small ADAMTS13 fragments for epitope mapping of anti-ADAMTS13 autoantibodies in immune-mediated thrombotic thrombocytopenic purpura

BACKGROUND

In immune-mediated thrombotic thrombocytopenic purpura (iTTP), patients develop an immune response against the multidomain enzyme ADAMTS13. ADAMTS13 consists of a metalloprotease (M) and disintegrin-like (D) domain, 8 thrombospondin type 1 repeats (T1-T8), a cysteine-rich (C), a spacer (S), and 2 CUB domains (CUB1-2). Previous epitope mapping studies have used relatively large overlapping ADAMTS13 fragments.

OBJECTIVES

We aimed at developing small nonoverlapping ADAMTS13 fragments to fine map anti-ADAMTS13 autoantibodies in iTTP patients.

METHODS

A library of 16 ADAMTS13 fragments, comprising several small (M, DT, C, S, T2-T5, T6-T8, CUB1, CUB2), and some larger fragments with overlapping domains (MDT, MDTC, DTC, CS, T2-T8, CUB1-2, MDTCS, T2-C2), were generated. All fragments, and ADAMTS13, were expressed as a fusion protein with albumin domain 1, and purified. The folding of the fragments was tested using 17 anti-ADAMTS13 monoclonal antibodies with known epitopes. An epitope mapping assay using small ADAMTS13 fragments was set up, and validated by analyzing 18 iTTP patient samples.

RESULTS

Validation with the monoclonal antibodies demonstrated that single S and CUB1 were not correctly folded, and therefore CS and CUB1-2 fragments were selected instead of single C, S, CUB1, and CUB2 fragments. Epitope mapping of antibodies of patients with iTTP confirmed that 6 nonoverlapping ADAMTS13 fragments M, DT, CS, T2-T5, T6-T8, and CUB1-2 were sufficient to accurately determine the antibody-binding sites.

CONCLUSION

We have developed a tool to profile patients with iTTP according to their anti-ADAMTS13 antibodies for a better insight in their immune response.

Expanding a Portfolio of (FO-) SPR Surface Chemistries with the Co(III)-NTA Oriented Immobilization of His6-Tagged Bioreceptors for Applications in Complex Matrices

Cobalt-nitrilotriacetic acid (Co(III)-NTA) chemistry is a recognized approach for oriented patterning of His₆-tagged bioreceptors. We have applied the matching strategy for the first time on a surface plasmon resonance (SPR) platform, namely, the commercialized fiber optic (FO)-SPR. To accomplish this, His₆-tagged bioreceptor (scFv-33H1F7) and its target PAI-1 were used as a model system, after scrutinizing the specificity of their interaction. When benchmarked to traditional carboxyl-based self-assembled monolayers (SAM), NTA allowed (1) more efficient FO-SPR surface coverage with bioreceptors compared with the former and (2) realization of thus far difficult-to-attain label-free bioassays on the FO-SPR platform in both buffer and 20-fold diluted human plasma. Moreover, Co(III)-NTA surface proved to be compatible with traditional gold nanoparticle-mediated signal amplification in the buffer as well as in 10-fold diluted human plasma, thus expanding the dynamic detection range to low ng/mL. Both types of bioassays revealed that scFv-33H1F7 immobilized on the FO-SPR surface using different concentrations (20, 10, or 5 μg/mL) had no impact on the bioassay sensitivity, accuracy, or reproducibility despite the lowest concentration effectively resulting in close to 20% fewer bioreceptors. Collectively, these results highlight the importance of Co(III)-NTA promoting the oriented patterning of bioreceptors on the FO-SPR sensor surface for securing robust and sensitive bioassays in complex matrices, both in label-free and labeled formats.

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.

Updated overview on von Willebrand disease: focus on the interest of genotyping

Introduction: Von Willebrand disease (VWD) is the most common inherited bleeding disorder, characterized by a quantitative or qualitative defect of von Willebrand factor (VWF), a multimeric glycoprotein crucial for primary hemostasis and coagulation. VWD pathophysiology is heterogeneous as it includes several types and subtypes which therapeutic management is different. The mainstays of VWD treatment are desmopressin and replacement therapy based on both plasma-derived concentrates and a recently developed recombinant VWF. VWD definitive diagnosis is achieved by a battery of phenotypic biologic assays and genotyping is currently performed mostly for research.Areas covered: This narrative review will firstly present a general overview on VWD epidemiology, pathophysiology, classification, clinics, phenotypic biologic diagnosis, and treatment. Secondly, a focus on VWD genotyping will be presented with specific emphasis on the evolution of its technical aspects, its applications for research dedicated to a better understanding of VWD pathophysiology and epidemiology and its interest in both a faster diagnosis and an optimal treatment of VWD.Expert opinion: Based on analysis of the literature, it can be concluded that the fast evolution of genetic techniques together with the development of innovating treatments may significantly change diagnostic flow charts for VWD and their use for specific and personalized treatment.

Crystal structure and substrate-induced activation of ADAMTS13

Platelet recruitment to sites of blood vessel damage is highly dependent upon von Willebrand factor (VWF). VWF platelet-tethering function is proteolytically regulated by the metalloprotease ADAMTS13. Proteolysis depends upon shear-induced conformational changes in VWF that reveal the A2 domain cleavage site. Multiple ADAMTS13 exosite interactions are involved in recognition of the unfolded A2 domain. Here we report through kinetic analyses that, in binding VWF, the ADAMTS13 cysteine-rich and spacer domain exosites bring enzyme and substrate into proximity. Thereafter, binding of the ADAMTS13 disintegrin-like domain exosite to VWF allosterically activates the adjacent metalloprotease domain to facilitate proteolysis. The crystal structure of the ADAMTS13 metalloprotease to spacer domains reveals that the metalloprotease domain exhibits a latent conformation in which the active-site cleft is occluded supporting the requirement for an allosteric change to enable accommodation of the substrate. Our data demonstrate that VWF functions as both the activating cofactor and substrate for ADAMTS13.

Exploring the "minimal" structure of a functional ADAMTS13 by mutagenesis and small-angle X-ray scattering

Human ADAMTS13 is a multidomain protein with metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains, followed by 7 additional T domains and 2 CUB (complement components C1r and C1s, sea urchin protein Uegf, and bone morphogenetic protein-1) domains. ADAMTS13 inhibits the growth of von Willebrand factor (VWF)-platelet aggregates by cleaving the cryptic Tyr1605-Met1606 bond in the VWF A2 domain. ADAMTS13 is regulated by substrate-induced allosteric activation; without shear stress, the distal T8-CUB domains markedly inhibit VWF cleavage, and binding of VWF domain D4 or selected monoclonal antibodies (MAbs) to distal ADAMTS13 domains relieves this autoinhibition. By small angle X-ray scattering (SAXS), ADAMTS13 adopts a hairpin-like conformation with distal T7-CUB domains close to the proximal MDTCS domains and a hinge point between T4 and T5. The hairpin projects like a handle away from the core MDTCS and T7-CUB complex and contains distal T domains that are dispensable for allosteric regulation. Truncated constructs that lack the T8-CUB domains are not autoinhibited and cannot be activated by VWF D4 but retain the hairpin fold. Allosteric activation by VWF D4 requires T7, T8, and the 58-amino acid residue linker between T8 and CUB1. Deletion of T3 to T6 produced the smallest construct (delT3-6) examined that could be activated by MAbs and VWF D4. Columba livia (pigeon) ADAMTS13 (pADAMTS13) resembles human delT3-6, retains normal activation by VWF D4, and has a SAXS envelope consistent with amputation of the hairpin containing the dispensable T domains of human ADAMTS13. Our findings suggest that human delT3-6 and pADAMTS13 approach a "minimal" structure for allosterically regulated ADAMTS13.

Phylogenetic and functional analysis of ADAMTS13 identifies highly conserved domains essential for allosteric regulation

The metalloprotease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats member 13) prevents microvascular thrombosis by cleaving von Willebrand factor (VWF) within platelet-rich thrombi, and cleavage depends on allosteric activation of ADAMTS13 by the substrate VWF. Human ADAMTS13 has a short propeptide, metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains (proximal domains), followed by 7 T and 2 CUB (complement components C1r and C1s, sea urchin protein Uegf, and bone morphogenetic protein-1) domains (distal domains). Distal domains inhibit the catalytic proximal domains; binding of distal T8-CUB domains to the VWF D4 domain relieves autoinhibition and promotes cleavage of the nearby VWF A2 domain. However, the role of specific ADAMTS13 distal domains in this allosteric mechanism is not established. Assays of plasma ADAMTS13 from 20 placental mammals, birds, and amphibians show that allosteric regulation is broadly conserved, and phylogenetic analysis of 264 vertebrates shows the long propeptide, T3, T4, T6, and T6a domains have been deleted several times in placental mammals, birds, and fish. Notably, pigeon ADAMTS13 has only 3 distal T domains but was activated normally by human VWF D4 and cleaved VWF multimers, preferentially under fluid shear stress. Human ADAMTS13 constructed to resemble pigeon ADAMTS13 retained normal allosteric regulation and shear-dependent cleavage of VWF. Thus, the T3-T6 domains of human ADAMTS13 are dispensable. Conversely, deletion of T7 or T8 abolished allosteric activation. For most species, some sequence changes in the VWF substrate can markedly increase the rate of cleavage, suggesting that ADAMTS13 and VWF have not evolved to be optimal enzyme-substrate pairs. These properties may reflect evolutionary pressure to balance the risk for VWF-dependent bleeding and thrombosis.

AFM Imaging Reveals Multiple Conformational States of ADAMTS13

Background

ADAMTS13 (A disintegrin and metalloprotease with a thrombospondin type 1 motif 13) cleaves Von Willebrand factor (VWF) to regulate its size, thereby preventing aberrant platelet aggregation and thrombus. Deficiency of ADAMTS13 caused by either genetic mutations or by inhibitory autoantibodies against ADAMTS13 leads to thrombotic thrombocytopenic purpura (TTP). Recently, ADAMTS13 was reported to adopt a “closed” conformation with lower activity and an “open” one resulting from the engagements of VWF D4-CK domains or antibodies to the distal domains of ADAMTS13, or mutations in its spacer domain. These engagements or mutations increase ADAMTS13 activity by ~ 2.5-fold. However, it is less known whether the conformation of ADAMTS13 is dynamic or stable.

Results

Wild type ADAMTS13 (WT-ADAMTS13) and the gain-of-function variant (GOF-ADAMTS13) with five mutations (R568K / F592Y / R660K / Y661F / Y665F) in spacer domain were imaged by atomic force microscopy (AFM) at pH 6 and pH 7.5. The data revealed that at both pH 6 and pH 7.5, WT-ADAMTS13 adopted two distinct conformational states (state I and state II), while an additional state (state III) was observed in GOF-ADAMTS13. In the present study, we propose that state I is the “closed” conformation, state III is the “open” one, and state II is an intermediate one. Comparing to pH 7.5, the percentages of state II of WT-ADAMTS13 and state III of GOF-ADAMTS13 increased at pH 6, with the decrease in the state I for WT-ADAMTS13 and state I and state II for GOF-ADAMTS13, suggesting lower pH extended the conformation of ADAMTS13.

Conclusion

Both WT- and GOF-ADAMTS13 exist multiple conformational states and lower pH might alter the tertiary structure and/or disrupt the intra-domain interactions, increasing the flexibility of ADAMTS13 molecules.

Electronic supplementary material

The online version of this article (10.1186/s13036-018-0102-y) contains supplementary material, which is available to authorized users.

Enhanced activity of an ADAMTS-13 variant (R568K/F592Y/R660K/Y661F/Y665F) against platelet agglutination in vitro and in a murine model of acute ischemic stroke

Essentials ADAMTS13 requires a substrate-induced conformational change to attain full activity in vitro. The efficacy of wild type ADAMTS13 in models of thrombosis/stroke may be enhanced by pre-activation. A pre-activated ADAMTS13 variant exhibits enhanced proteolysis of platelet agglutinates. This ADAMTS13 variant is protective in a murine model of stroke at a lower dose than WT ADAMTS13. SUMMARY: Background ADAMTS-13 circulates in a closed conformation, only achieving full proteolytic activity against von Willebrand factor (VWF) following a substrate-induced conformational change. A gain-of-function (GoF) ADAMTS-13 variant (R568K/F592Y/R660K/Y661F/Y665F) is conformationally preactivated. Objectives To establish how the hyperactivity of GoF ADAMTS-13 is manifested in experimental models mimicking the occlusive arterial thrombi present in acute ischemic stroke. Methods The ability of GoF ADAMTS-13 to dissolve VWF-platelet agglutinates was examined with an assay of ristocetin-induced platelet agglutination and in parallel-flow models of arterial thrombosis. A murine model of focal ischemia was used to assess the thrombolytic potential of GoF ADAMTS-13. Results Wild-type (WT) ADAMTS-13 required conformational activation to attain full activity against VWF-mediated platelet capture under flow. In this assay, GoF ADAMTS-13 had an EC50 value more than five-fold lower than that of WT ADAMTS-13 (0.73 ± 0.21 nm and 3.81 ± 0.97 nm, respectively). The proteolytic activity of GoF ADAMTS-13 against preformed platelet agglutinates under flow was enhanced more than four-fold as compared with WT ADAMTS-13 (EC50 values of 2.5 ± 1.1 nm and 10.2 ± 5.6 nm, respectively). In a murine stroke model, GoF ADAMTS-13 restored cerebral blood flow at a lower dose than WT ADAMTS-13, and partially retained the ability to recanalize vessels when administration was delayed by 1 h. Conclusions The limited proteolytic activity of WT ADAMTS-13 in in vitro models of arterial thrombosis suggests an in vivo requirement for conformational activation. The enhanced activity of the GoF ADAMTS-13 variant translates to a more pronounced protective effect in experimental stroke.

Dissecting the pathophysiology of immune thrombotic thrombocytopenic purpura: interplay between genes and environmental triggers

Although outstanding progress has been made in understanding the pathophysiology of thrombotic thrombocytopenic purpura (TTP), knowledge of the immunopathogenesis of the disease is only at an early stage. Anti-ADAMTS13 auto-antibodies were shown to block proteolysis of von Willebrand factor and/or induce ADAMTS13 clearance from the circulation. However, it still remains to identify which immune cells are involved in the production of anti-ADAMTS13 autoantibodies, and therefore account for the remarkable efficacy of the B-cell depleting agents in this disease. The mechanisms leading to the loss of tolerance of the immune system towards ADAMTS13 involve the predisposing genetic factors of the human leukocyte antigen class II locus DRB1*11 and DQB1*03 alleles as well as the protective allele DRB1*04, and modifying factors such as ethnicity, sex and obesity. Future studies have to identify why these identified genetic risk factors are also frequently to be found in the healthy population although the incidence of immune-mediated thrombotic thrombocytopenic purpura (iTTP) is extremely low. Moreover, the development of recombinant ADAMTS13 opens a new therapeutic era in the field. Interactions of recombinant ADAMTS13 with the immune system of iTTP patients will require intensive investigation, especially for its potential immunogenicity. Better understanding of iTTP immunopathogenesis should, therefore, provide a basis for the development of novel therapeutic approaches to restore immune tolerance towards ADAMTS13 and thereby better prevent refractoriness and relapses in patients with iTTP. In this review, we address these issues and the related challenges in this field.

Monoclonal antibodies against metzincin targets

The metzincin clan of metalloproteinases includes the MMP, disintegrin and metalloproteinase (ADAM) and ADAM with thrombospondin motifs families, which cleave extracellular targets in a wide range of (patho)physiological processes. Antibodies constitute a powerful tool to modulate the activity of these enzymes for both therapeutic and research purposes. In this review, we give an overview of monoclonal antibodies (mAbs) that have been tested in preclinical disease models, human trials and important studies of metzincin structure and function. Initial attempts to develop therapeutic small molecule inhibitors against MMPs were hampered by structural similarities between metzincin active sites and, consequently, off-target effects. Therefore, more recently, mAbs have been developed that do not bind to the active site but bind to surface-exposed loops that are poorly conserved in closely related family members. Inhibition of protease activity by these mAbs occurs through a variety of mechanisms, including (i) barring access to the active site, (ii) disruption of exosite binding, and (iii) prevention of protease activation. These different modes of inhibition are discussed in the context of the antibodies' potency, selectivity and, importantly, the effects in models of disease and clinical trials. In addition, various innovative strategies that were used to generate anti-metzincin mAbs are discussed. LINKED ARTICLES: This article is part of a themed section on Translating the Matrix. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.1/issuetoc.

Pathophysiology of thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

Thrombotic microangiopathies are rare disorders characterized by the concomitant occurrence of severe thrombocytopenia, microangiopathic hemolytic anemia, and a variable degree of ischemic end-organ damage. The latter particularly affects the brain, the heart, and the kidneys. The primary forms, thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS), although their clinical presentations often overlap, have distinctive pathophysiologies. TTP is the consequence of a severe ADAMTS-13 deficiency, either immune-mediated as a result of circulating autoantibodies, or caused by mutations in ADAMTS-13. HUS develops following an infection with Shiga-toxin producing bacteria, or as the result of excessive activation of the alternative pathway of the complement system because of mutations in genes encoding complement system proteins.

An open conformation of ADAMTS-13 is a hallmark of acute acquired thrombotic thrombocytopenic purpura

Essentials Conformational changes in ADAMTS-13 are part of its mode-of-action. The murine anti-ADAMTS-13 antibody 1C4 discriminates between folded and open ADAMTS-13. ADAMTS-13 conformation is open in acute acquired thrombotic thrombocytopenic purpura (TTP). Our study forms an important basis to fully elucidate the pathophysiology of TTP.

SUMMARY

Background Acquired thrombotic thrombocytopenic purpura (aTTP) is an autoimmune disorder characterized by absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies. Recently, it was shown that ADAMTS-13 adopts a folded or an open conformation. Objectives As conformational changes in self-antigens play a role in the pathophysiology of different autoimmune diseases, we hypothesized that the conformation of ADAMTS-13 changes during acute aTTP. Methods Antibodies recognizing cryptic epitopes in the spacer domain were generated. Next, the conformation of ADAMTS-13 in 40 healthy donors (HDs), 99 aTTP patients (63 in the acute phase versus 36 in remission), 12 hemolytic-uremic syndrome (HUS) patients and 63 sepsis patients was determined with ELISA. Results The antibody 1C4 recognizes a cryptic epitope in ADAMTS-13. Therefore, we were able to discriminate between a folded and an open ADAMTS-13 conformation. We showed that ADAMTS-13 in HDs does not bind to 1C4, indicating that ADAMTS-13 circulates in a folded conformation. Similar results were obtained for HUS and sepsis patients. In contrast, ADAMTS-13 of acute aTTP patients bound to 1C4 in 92% of the cases, whereas, in most cases, this binding was abolished during remission, showing that the conformation of ADAMTS-13 is open during an acute aTTP episode. Conclusions Our study shows that, besides absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies, an open ADAMTS-13 conformation is also a hallmark of acute aTTP. Demonstrating this altered ADAMTS-13 conformation in acute aTTP will help to further unravel the pathophysiology of aTTP and lead to improved therapy and diagnosis.

Anti-ADAMTS13 Antibodies and a Novel Heterozygous p.R1177Q Mutation in a Case of Pregnancy-Onset Immune-Mediated Thrombotic Thrombocytopenic Purpura

In this study, we investigated a case of pregnancy-onset thrombotic thrombocytopenic purpura (TTP). The patient had severely decreased ADAMTS13 ( ad isintegrin a nd m etalloprotease with t hrombo s pondin type 1 motif, member 13) activity levels during acute phase and the presence of inhibitory anti-ADAMTS13 autoantibodies was demonstrated, which led to the diagnosis of immune-mediated TTP. However, ADAMTS13 activity was only mildly restored during remission, although inhibitory anti-ADAMTS13 antibodies were no longer detected. We hypothesized that genetic abnormalities could account for this discrepancy between ADAMTS13 activity and antigen. Genetic analysis revealed the presence of two heterozygous substitutions on the same allele: a single nucleotide polymorphism (SNP) c.2699C > T (p.A900V), located in the beginning of the T5 domain, and a mutation c.3530G > A (p.R1177Q) located in the third linker region of ADAMTS13. In vitro testing of those substitutions by expression of recombinant proteins revealed a normal secretion but a reduced ADAMTS13 activity by the novel p.R1177Q mutation, which could partially explain the subnormal activity levels found during remission. Although changes in the linker region might induce conformational changes in ADAMTS13, the p.R1177Q mutation in the third linker region of ADAMTS13 did not expose a cryptic epitope in the metalloprotease domain. In conclusion, we report on an immune-mediated pregnancy-onset TTP patient who had inhibitory anti-ADAMTS13 autoantibodies during acute phase, but not during remission. Genetic analysis confirmed the diagnosis of immune-mediated TTP and revealed the novel p.R1177Q mutation which mildly impaired ADAMTS13 activity.