Evidence for a role of anti-ADAMTS13 autoantibodies despite normal ADAMTS13 activity in recurrent thrombotic thrombocytopenic purpura

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.

Cattle-FRETS71, a novel fluorogenic substrate with broad applicability for characterizing ADAMTS13 properties and function

BACKGROUND

Current ADAMTS13 activity assays are important for diagnosing thrombotic thrombocytopenic purpura (TTP) but are unreliable to assay ADAMTS13 activity in animal models. The Cattle-FRETS71 assay is capable of detecting ADAMTS13 activity in plasma from multiple animal species, making it a potentially useful reagent at all stages of clinical research. The performance of Cattle-FRETS71 in TTP diagnosis is not yet known.

OBJECTIVES

We evaluated the performance of the Cattle-FRETS71 substrate against the human FRETS-rVWF71 and the FRETS-VWF73 commercial substrates in human plasma and serum samples to validate its utility in diagnosing TTP in patients.

METHODS

Internal validation was performed using heparinized plasma samples (n = 81). External validation was a blinded study using serum samples from the Oklahoma TTP Registry (n = 118, collected 2004-2014) that had been initially assayed by FRETS-VWF73 within 1 year of collection. Additional validation was performed with citrated plasma samples with variable ADAMTS13 activities (n = 32) that were analyzed by FRETS-VWF73.

RESULTS

There was an excellent correlation (r = 0.94) between Cattle-FRETS71 and FRETS-rVWF71 for assayed heparinized plasma samples (n = 81). Assay results between Cattle-FRETS71 and FRETS-VWF73 of Oklahoma TTP Registry serum samples (n = 118) and citrated plasma samples (n = 32) were comparably good (r = 0.81 and r = 0.85, respectively).

CONCLUSION

The Cattle-FRETS71 assay is comparable with other assays in quantifying ADAMTS13 activity in human plasma collected from patients with documented or suspected TTP. The versatility of Cattle-FRETS71, combined with its specificity and sensitivity, makes it a useful tool for the standardization of ADAMTS13 activity across basic and clinical research paradigms.

A validation and modification of PLASMIC score by adjusting the criteria of mean corpuscular volume and international normalized ratio

BACKGROUND

The PLASMIC score was developed for distinguishing thrombotic thrombocytopenic purpura (TTP) from other types of thrombotic microangiopathy. However, two components of the PLASMIC score, mean corpuscular volume (MCV) and international normalized ratio (INR), showed non-significant differences between TTP and non-TTP patients in previous validations. Here, we validate the PLASMIC score and aim to modify it by adjusting the criteria of MCV and INR.

MATERIALS AND METHODS

A retrospective validation of suspected TTP patients was performed by reviewing electronic medical records from two medical centers in Taiwan. The performance of different modified types of the PLASMIC score was carried out.

RESULTS

Among 50 patients included in the final analysis, 12 were diagnosed with TTP based on deficiency of ADAMTS13 activity and clinical judgement. When stratified by high (score ≥ 6) and low-intermediate risk (score < 6), the positive predictive value (PPV) of the PLASMIC score to predict TTP was 0.45 (95% confidence interval [CI]: 0.29-0.61). The area under curve (AUC) was 0.70 (95% CI: 0.56-0.82). When adjusting the criteria of the PLASMIC score from MCV < 90 fL to MCV ≥ 90 fL, the PPV increased to 0.57 (95% CI: 0.37-0.75). The AUC was 0.75 (95% CI: 0.61-0.87). When adjusting the INR from >1.5 to >1.1, the PPV increased to 0.56 (95% CI: 0.39-0.71). The AUC was 0.81 (95% CI: 0.68-0.90).

CONCLUSION

MCV ≥ 90 fL and/or INR > 1.1 might be suitable modifications for PLASMIC score but should be validated in a larger sample size.

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.

ADAMTS13 Antibody and Inhibitor Assays

A finding of an ADAMTS13 (a disintegrin-like and metalloprotease with thrombospondin type 1 motif, member 13) activity level of <10% of normal is usually sufficient to distinguish thrombotic thrombocytopenic purpura (TTP) from other thrombotic microangiopathies. TTP can be congenital or acquired, the most common form being acquired immune-mediated TTP caused by autoantibodies than inhibit ADAMTS13 function and/or increase its clearance. Basic 1 + 1 mixing tests can detect the presence of inhibitory antibodies, and quantification can be achieved with Bethesda-type assays that measure loss of function in a series of mixtures of test plasma and normal plasma. Not all patients present with inhibitory antibodies, and here the ADAMTS13 deficiency may be caused by clearing antibodies alone, which are not detectable in functional assays. ELISA assays are commonly used to detect clearing antibodies via capture with recombinant ADAMTS13. Since they also detect inhibitory antibodies, they are the preferred assay, although they cannot distinguish between inhibitory and clearing antibodies. The present chapter describes principles, performance, and practical aspects of a commercial ADAMTS13 antibody ELISA and a generic approach to Bethesda-type assays for detecting inhibitory ADAMTS13 antibodies.

Recombinant ADAMTS13 for Hereditary Thrombotic Thrombocytopenic Purpura

A 27-year-old patient with a history of severe obstetrical complications and arterial thrombosis received a diagnosis of hereditary thrombotic thrombocytopenic purpura (TTP) due to severe ADAMTS13 deficiency when she presented with an acute episode in the 30th week of her second pregnancy. When the acute episode of hereditary TTP became plasma-refractory and fetal death was imminent, weekly injections of recombinant ADAMTS13 at a dose of 40 U per kilogram of body weight were initiated. The patient's platelet count normalized, and the growth of the fetus stabilized. At 37 weeks 1 day of gestation, a small-for-gestational-age boy was delivered by cesarean section. At the time of this report, the patient and her son were well, and she continued to receive injections of recombinant ADAMTS13 every 2 weeks. (Funded by the Swiss National Science Foundation.).

Thrombotic Thrombocytopenic Purpura: From 1972 to 2022 and Beyond

This review tells the story of my personal experience with thrombotic thrombocytopenic purpura (TTP). It begins with my first encounter with TTP 50 years ago when 2 sisters presented 2 years apart, both pregnant and both died. At that time, I knew nothing about hereditary TTP (hTTP), the risks of pregnancy, or effective treatments. In 1991, a year after I moved to Oklahoma, therapeutic plasma exchange (TPE) was established as an effective treatment. With the availability of effective treatment, the number of patients presenting with suspected TTP soared. The diagnosis of TTP was imprecise. I worked with the Oklahoma Blood Institute (OBI) to understand the management of TTP. Because the OBI provided all TPE procedures for most of Oklahoma, we saw all consecutive patients within a defined geographic area who were identified at a uniform time early in the course of their TTP, without selection or referral bias. It was an inception cohort; this became the Oklahoma TTP Registry. In 2001, we began a very successful collaboration with the University of Bern, Switzerland, to measure ADAMTS13 activity in all of our patients. From our patients, we learned that acquired, autoimmune TTP (iTTP) is a chronic disease with risks for cognitive impairment and depression. Recognition in 2012 of three sisters with hTTP was reminiscent of the beginning of my story. hTTP has risks for multiple severe morbidities, beginning at birth and especially during pregnancy. Future management of both iTTP and hTTP will be more effective and more convenient.

The presentation, etiologies, pathophysiology, and treatment of pulmonary renal syndrome: A review of the literature

Pulmonary renal syndrome (PRS) is a constellation of different disorders that cause both rapidly progressive glomerulonephritis and diffuse alveolar hemorrhage. While antineutrophil cytoplasmic antibody associated vasculitis and anti-glomerular basement membrane disease are the predominant causes of PRS, numerous other mechanisms have been shown to cause this syndrome, including thrombotic microangiopathies, drug exposures, and infections, among others. This syndrome has high morbidity and mortality, and early diagnosis and treatment is imperative to improve outcomes. Treatment generally involves glucocorticoids and immunosuppressive agents, but treatment targeted to the underlying disorder can improve outcomes and mitigate side effects. Familiarity with the wide range of possible causes of PRS can aid the clinician in workup, diagnosis and early initiation of treatment. This review provides a summary of the clinical presentation, etiologies, pathophysiology, and treatment of PRS.

Mikroangiopathien in der Schwangerschaft

Das Wissen um die Differentialdiagnose der Mikroangiopathien ist auch für Geburtshelfer:innen wichtig. So ist die Kombination aus einer Thrombopenie und Hämolysezeichen wegweisend in der Erkennung der Erkrankung. Es sollten die Differenzialdiagnosen bekannt sein, da diese zu unterschiedlichen Therapienotwendigkeiten führen und Langzeitschäden, beispielsweise für die Niere, vermieden werden müssen. Die Differenzialdiagnosen der Thrombopenie stellen den Einstieg für die weitere Diagnostik dar. In der Kombination mit Hämolysezeichen und Veränderungen der Erythrozytenmorphologie (Fragmentozyten) liegen unterschiedliche Ursachen für die Mikroangiopathie vor. Eine thrombotisch-thrombozytopenische Purpura (TTP) kann während der gesamten Schwangerschaft, aber insbesondere im letzten Trimenon, die Ursache für solche Veränderungen sein. Bei dieser finden sich häufig gastrointestinale oder auch zusätzliche neurologische Symptome. Die Präeklampsie, insbesondere beim HELLP(„hemolysis, elevated liver enzymes, and a low platelet count“)-Syndrom, kann ebenfalls zu einer Mikroangiopathie führen, dieses i.d.R. mit entsprechender Erhöhung des Blutdrucks und insbesondere mit einer deutlichen Erhöhung der Lebertransaminasen, ebenfalls typischerweise im letzten Trimenon kombiniert. Wenn entsprechende Veränderungen nach der Geburt auftreten und diese neben der Hämolyse mit einem Nierenversagen verbunden sind, kann es sich hierbei um ein atypisches hämolytisch-urämisches Syndrom (aHUS) handeln, das spezifisch durch Antikörper therapiert werden sollte. Neben der typischen Gerinnungsaktivierung im Sinne einer Thrombosierung findet sich bei COVID-19 („corona virus disease“) auch das gesamte Bild einer entsprechenden Mikroangiopathie, zum Teil durch entsprechende Aktivierung des Gerinnungssystem, zum Teil durch eine Verstärkung der anderen Mikroangiopathien. Für alle Bereiche werden die Differenzialdiagnosen und mögliche Therapien skizziert.

Naturally Occurring Anti-Idiotypic Antibodies Portray a Largely Private Repertoire in Immune-Mediated Thrombotic Thrombocytopenic Purpura

Rare immune-mediated thrombotic thrombocytopenic purpura (iTTP) is a life-threatening disease resulting from a severe autoantibody-mediated ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motifs, member 13) deficiency. Acute iTTP episodes are medical emergencies, but when treated appropriately &amp;gt;95% of patients survive. However, at least half of survivors will eventually experience a relapse. How remission of an initial episode is achieved and factors contributing to reemergence of anti-ADAMTS13 Abs and a relapsing course are poorly understood. In acquired hemophilia and systemic lupus erythematosus, anti-idiotypic Abs counteracting and neutralizing pathogenic autoantibodies contribute to remission. We selected and amplified the splenic anti-idiotypic IgG&lt;sub&gt;1&lt;/sub&gt; Fab κ/λ repertoire of two relapsing iTTP patients on previously generated monoclonal inhibitory anti-ADAMTS13 Fabs by phage display to explore whether anti-idiotypic Abs have a role in iTTP. We obtained 27 single anti-idiotypic Fab clones, half of which had unique sequences, although both patients shared four H chain V region genes (V&lt;sub&gt;H&lt;/sub&gt;1-69*01, V&lt;sub&gt;H&lt;/sub&gt;3-15*01, V&lt;sub&gt;H&lt;/sub&gt;3-23*01, and V&lt;sub&gt;H&lt;/sub&gt;3-49*03). Anti-idiotypic Fab pools of both patients fully neutralized the inhibitor capacity of the monoclonal anti-ADAMTS13 Abs used for their selection. Preincubation of plasma samples of 22 unrelated iTTP patients stratified according to functional ADAMTS13 inhibitor titers (&amp;gt;2 Bethesda units/ml, or 1-2 Bethesda units/ml), with anti-idiotypic Fab pools neutralized functional ADAMTS13 inhibitors and restored ADAMTS13 activity in 18-45% of those cases. Taken together, we present evidence for the presence of an anti-idiotypic immune response in iTTP patients. The interindividual generalizability of this response is limited despite relatively uniform pathogenic anti-ADAMTS13 Abs recognizing a dominant epitope in the ADAMTS13 spacer domain.

Immune thrombotic thrombocytopenic purpura: Personalized therapy using ADAMTS-13 activity and autoantibodies

Recently, treatment of immune-mediated thrombotic thrombocytopenic purpura (ITTP) has changed with the advent of caplacizumab in clinical practice. The International Working Group (IWG) has recently integrated the ADAMTS-13 activity/autoantibody monitoring in consensus outcome definitions. We report three ITTP cases during the coronavirus disease 2019 pandemic, that received a systematic evaluation of ADAMTS-13 activity and autoantibodies. We describe how the introduction of caplacizumab and ADAMTS-13 monitoring could change the management of ITTP patients and discuss whether therapeutic choices should be based on the clinical response alone. ADAMTS-13 activity/antibodies were assessed every 5 days. Responses were evaluated according to updated IWG outcome definitions. These kinetics, rather than clinical remission, guided the therapy, allowing early and safe caplacizumab discontinuation and sensible administration of rituximab. Caplacizumab was cautiously discontinued after achieving ADAMTS-13 complete remission. These cases illustrate that prospective ADAMTS-13 evaluation and use of updated IWG definitions may improve real-life patients' management in the caplacizumab era.

Anti-ADAMTS13 autoantibody profiling in patients with immune-mediated thrombotic thrombocytopenic purpura

Anti-A Disintegrin and Metalloproteinase with a ThromboSpondin type 1 motif, member 13 (ADAMTS13) autoantibodies cause a severe ADAMTS13 deficiency in immune-mediated thrombotic thrombocytopenic purpura (iTTP). ADAMTS13 consists of a metalloprotease (M), a disintegrin-like (D) domain, 8 thrombospondin type 1 repeats (T1-T8), a cysteine-rich (C), a spacer (S), and 2 CUB domains (CUB1-2). We recently developed a high-throughput epitope mapping assay based on small, nonoverlapping ADAMTS13 fragments (M, DT, CS, T2-T5, T6-T8, CUB1-2). With this assay, we performed a comprehensive epitope mapping using 131 acute-phase samples and for the first time a large group of remission samples (n = 50). Next, samples were stratified according to their immunoprofiles, a field that is largely unexplored in iTTP. Three dominant immunoprofiles were found in acute-phase samples: profile 1: only anti-CS autoantibodies (26.7%); profile 2: both anti-CS and anti-CUB1-2 autoantibodies (12.2%); and profile 3: anti-DT, anti-CS, anti-T2-T5, anti-T6-T8, and anti-CUB1-2 autoantibodies (8.4%). Interestingly, profile 1 was the only dominant immunoprofile in remission samples (52.0%). Clinical data were available for a relatively small number of patients with acute iTTP (>68), and no correlation was found between immunoprofiles and disease severity. Nevertheless, profile 1 was linked with younger and anti-T2-T5 autoantibodies with older age and the absence of anti-CUB1-2 autoantibodies with cerebral involvement. In conclusion, identifying acute phase and remission immunoprofiles in iTTP revealed that anti-CS autoantibodies seem to persist or reappear during remission providing further support for the clinical development of a targeted anti-CS autoantibody therapy. A large cohort study with acute iTTP samples will validate possible links between immunoprofiles or anti-domain autoantibodies and clinical data.

Annual incidence and severity of acute episodes in hereditary thrombotic thrombocytopenic purpura

Hereditary thrombotic thrombocytopenic purpura (hTTP) is a rare thrombotic microangiopathy characterized by severe congenital ADAMTS13 deficiency and recurring acute episodes causing morbidity and premature death. Information on the annual incidence and severity of acute episodes in patients with hTTP is largely lacking. This study reports prospective data on 87 patients from the Hereditary TTP Registry (clinicaltrials.gov #NCT01257269) for survival, frequency, and severity of acute episodes from enrollment until December 2019. The 87 patients, followed up for a median of 4.2 years (range, 0.01-15 years), had a median age at overt disease onset and at clinical diagnosis of 4.6 years and 18 years (range, 0.0-70 years for both), respectively. Forty-three patients received regular plasma prophylaxis, whereas 22 did not, and treatment changed over time or was unknown in the remaining 22. Forty-three patients experienced 131 acute episodes, of which 91 (69%) occurred in patients receiving regular prophylaxis. This resulted in an annual incidence of acute episodes of 0.36 (95% confidence interval [CI], 0.29-0.44) with regular plasma treatment and of 0.41 (95% CI, 0.30-0.56) without regular plasma treatment. More than one-third of acute episodes (n = 51) were documented in children <10 years of age at enrollment and were often triggered by infections. Their annual incidence of acute episodes was significantly higher than in patients aged >40 years (1.18 [95% CI, 0.88-1.55] vs 0.14 [95% CI, 0.08-0.23]). The prophylactic plasma infusion regimens used were insufficient to prevent acute episodes in many patients. Such regimens are burdensome, and caregivers, patients, and their guardians are reluctant to start regular plasma infusions, from which children particularly would benefit.

Recognizing and managing hereditary and acquired thrombotic thrombocytopenic purpura in infants and children

We describe how infants and children with hereditary and acquired autoimmune thrombotic thrombocytopenic purpura (TTP) initially present and how they can be promptly diagnosed and effectively managed. These are uncommon disorders that are commonly misdiagnosed and can be rapidly fatal. TTP is caused by a severe deficiency of the plasma protease, A disintegrin and Metalloprotease with a ThromboSpondin type 1 motif, member 13 (ADAMTS13). Measurement of ADAMTS13 activity is becoming easily accessible. A common presentation of hereditary TTP is neonatal severe hemolysis and hyperbilirubinemia. However, the median age of diagnosis is not until 5.5 years. Plasma is effective treatment for exacerbations and for prophylaxis. Plasma may be replaced by recombinant ADAMTS13 when it becomes available. Acquired TTP is more frequent in older children, in whom it is more common in girls and is commonly associated with systemic lupus erythematosus. For acquired TTP, plasma exchange and immunosuppression are the current treatment for acute episodes; caplacizumab is now commonly used in adults and may replace plasma exchange.

A COVID-19 pregnant patient with thrombotic thrombocytopenic purpura: a case report

BACKGROUND

Pregnancy seems to increase the risk of thrombotic thrombocytopenic purpura (TTP) relapses and make the TTP more severe in any of the pregnancy trimesters, or even during the postpartum period.

CASE PRESENTATION

This study highlights details of treating a COVID-19 pregnant patient who survived. This 21-year addicted White woman was admitted at her 29th week and delivered a stillbirth. She was transferred to another hospital after showing signs of TTP, which was caused by a viral infection.

CONCLUSION

This viral infection caused fever and dyspnea, and the patient was tested positive for COVID-19 infection. A chest computed tomography scan showed diffuse multiple bilateral consolidations and interlobar septal thickening. She stayed at the Intensive Care Unit for 20 days and treated with plasmapheresis. As far as we know, this is the first report of a TTP pregnant patient with COVID-19 infection.

ADAMTS13 and von Willebrand factor assessment in steady state and acute vaso-occlusive crisis of sickle cell disease

BACKGROUND

Sickle cell disease (SCD) is characterized by vaso-occlusive crisis (VOC), acute chest syndrome (ACS) and multiorgan failure (MOF) complicated by thrombosis. Von Willebrand factor (VWF) is a strong marker of SCD-related endothelial injury.

OBJECTIVES

To decipher the role of VWF and its specific-cleaving metalloprotease, ADAMTS13, in the vaso-occlusive and thrombotic process of SCD.

PATIENTS/METHODS

We investigated the VWF antigen (Ag), ADAMTS13 activity, ADAMTS13 Ag and ADAMTS13 IgGs in a cohort of 65 patients with SCD prospectively enrolled in a 20-month period from three centers. Patients were divided into two groups: an asymptomatic group (n = 30) with treated or untreated SCD at steady state, and a VOC/ACS group (n = 35) with SCD with VOC/ACS requiring either medical management or intensive care management for MOF.

RESULTS AND CONCLUSIONS

VWF:Ag levels were increased (median, 167 IU/dL; interquartile range [IQR], 124 - 279), especially in patients with VOC SCD (227 IU/dL; IQR, 134-305; P = .04), and positively correlated with inflammatory markers (P < .02). Median ADAMTS13 activity was normal (70 IU/dL; IQR, 60-80), but 7 patients exhibited a partial deficiency between 25 and 45 IU/dL. ADAMTS13 activity/VWF:Ag ratio, however, did not change during VOC. Median ADAMTS13:Ag was slightly decreased (611 ng/mL; IQR, 504-703) with no significant difference between groups. Surprisingly, ADAMTS13 IgGs were detected in 33 (51%) of our patients. We conclude that, in SCD, VWF:Ag and nonrelevant ADAMTS13 IgGs may reflect the severity of the inflammatory vasculopathy enhancing vaso-occlusive and thrombotic complications.

The remarkable diversity of thrombotic thrombocytopenic purpura: a perspective

Understanding the autoimmune etiology of acquired thrombotic thrombocytopenic purpura (TTP) has provided precision for the diagnosis and a rationale for immunosuppressive treatment. These advances have also allowed recognition of the remarkable clinical diversities of patients' initial presentations and their long-term outcomes. These diversities are illustrated by the stories of patients from the Oklahoma TTP Registry. The initial presentation of TTP may be the discovery of unexpected severe thrombocytopenia in a patient with minimal or no symptoms. The patient may remain asymptomatic throughout treatment or may die suddenly before treatment can be started. ADAMTS13 activity may be reported as normal in a patient with characteristic clinical features of TTP, or the unexpected report of ADAMTS13 deficiency in a patient with another established disorder may lead to the discovery of TTP. ADAMTS13 activity during clinical remission is unpredictable. ADAMTS13 activity may recover and remain normal, it may remain severely deficient for many years, or it may become normal only many years after recovery. Our treatment of initial episodes and management of patients after recovery and during remission continue to change. The addition of rituximab to the treatment of acute episodes and preemptive rituximab for patients with severe ADAMTS13 deficiency during remission are reported to prevent relapse. Because TTP is uncommon, there are few data to guide these changes. Therefore our patients' stories are profoundly influential. Their stories are the foundation of our experience, and our experience is the guide for our decisions.

The International Hereditary Thrombotic Thrombocytopenic Purpura Registry: key findings at enrollment until 2017

Congenital thrombotic thrombocytopenic purpura is an autosomal recessive inherited disease with a clinically heterogeneous course and an incompletely understood genotype-phenotype correlation. In 2006, the Hereditary TTP Registry started recruitment for a study which aimed to improve the understanding of this ultra-rare disease. The objective of this study is to present characteristics of the cohort until the end of 2017 and to explore the relationship between overt disease onset and ADAMTS13 activity with emphasis on the recurring ADAMTS13 c.4143_4144dupA mutation. Diagnosis of congenital thrombotic thrombocytopenic purpura was confirmed by severely deficient ADAMTS13 activity (≤10% of normal) in the absence of a functional inhibitor and the presence of ADAMTS13 mutations on both alleles. By the end of 2017, 123 confirmed patients had been enrolled from Europe (n=55), Asia (n=52, 90% from Japan), the Americas (n=14), and Africa (n=2). First recognized disease manifestation occurred from around birth up to the age of 70 years. Of the 98 different ADAMTS13 mutations detected, c.4143_4144dupA (exon 29; p.Glu1382Argfs*6) was the most frequent mutation, present on 60 of 246 alleles. We found a larger proportion of compound heterozygous than homozygous carriers of ADAMTS13 c.4143_4144dupA with overt disease onset at < 3 months of age (50% vs. 37%), despite the fact that ADAMTS13 activity was <1% in 18 of 20 homozygous, but in only 8 of 14 compound heterozygous carriers. An evaluation of overt disease onset in all patients with an available sensitive ADAMTS13 activity assay (n=97) shows that residual ADAMTS13 activity is not the only determinant of age at first disease manifestation. Registered at clinicaltrials.gov identifier NCT01257269.

Clinical and laboratory diagnosis of TTP: an integrated approach

Thrombotic thrombocytopenia purpura (TTP) is a rare, life-threatening disease with an incidence of approximately 2 persons per million per year. It is characterized by severe deficiency of the von Willebrand cleaving protease, ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), leading to formation of platelet-rich thrombi in the microvasculature. Prompt initiation of appropriate therapy, particularly plasma exchange, may be life-saving. Diagnosis of TTP is challenging because of its diverse clinical manifestations, overlap in clinical presentation with other thrombotic microangiopathies, and limited availability of ADAMTS13 testing. Clinical prediction scores have been developed to estimate the pretest probability of severe ADAMTS13 deficiency and may be used as an adjunct to clinical judgment to guide initial management decisions. An ADAMTS13 activity level of less than 10% supports the diagnosis of TTP in appropriate clinical contexts, but many centers do not offer testing in-house and must send out the test to a reference laboratory with a turnaround time of several days. In such instances, initial management decisions must be made without the benefit of laboratory testing. In patients with TTP, inhibitor tests may be useful for distinguishing immune-mediated from congenital TTP. In this article, we review the epidemiology, natural history, and clinical presentation of TTP and laboratory assays for TTP including ADAMTS13 activity and inhibitor assays. We also describe an evidence-based approach to the evaluation of a patient with suspected TTP that integrates clinical and laboratory assessment.

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.

ADAMTS13 activity and genetic mutations in Japan

Thrombotic thrombocytopenic purpura (TTP), a life threatening disease, can be induced by congenital or acquired deficiency of plasma metalloprotease ADAMTS13. Since the publication of the first genetic analysis in patients with congenital ADAMTS13 deficiency in 2001, more than 100 genetic defects in the ADAMTS13 gene have been reported worldwide. Genetic analysis in patients with ADAMTS13 deficiency has greatly contributed to the understanding of the etiology of TTP. A rapid and quantitative assay method for the plasma ADAMTS13 activity was developed recently in 2005 and opened a new area of TTP research – namely genetic research using a general population to evaluate age and gender differences of ADAMTS13 activity as well as phenotype – genotype correlations of genetic polymorphisms and estimation of a homozygote or a compound heterozygote ADAMTS13 deficiencies. The Japanese general population study included 3616 individuals with an age between 30 – 80 years confirming other studies that while ADAMTS13 activity decreased with age, VWF antigen increased and VWF antigen levels are lowest in blood group O indviduals, whereas ADAMTS13 activity levels were not associated with the AB0 blood group. 25 polymorphisms with a minor allele frequency of more than 0.01 were found, among them 6 missense mutations and 19 synonymous mutations, except P475S missense polymorphisms that was only idenitified in an East Asian population, characterized by reduced ADAMTS13 activity. Prevalence of congenital ADAMTS13 deficiency in the Japanese population was estimated about one individual in 1.1 × 106 to be homozygote or compound heterozygote for ADAMTS13 deficiency. So far more than 40 mutations in Japanese congenital TTP patients were found, but R193W, Q449*, C754Afs*24 (c.2259delA) and C908Y were identified in more than four patients suggesting the precipitaion of these mutations in the Japanese population.

Acquired thrombotic thrombocytopenic purpura

The von Willebrand factor (VWF)-cleaving metalloprotease, ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motifs-13) is the only known target of the dysregulated immune response in acquired TTP. Autoantibodies to ADAMTS13 either neutralize its activity or accelerate its clearance, thereby causing a severe deficiency of ADAMTS13 in plasma. As a consequence, size regulation of VWF is impaired and the persistence of ultra-large VWF (ULVWF) multimers facilitates micro vascular platelet aggregation causing microangiopathic haemolytic anaemia and ischaemic organ damage. Autoimmune TTP although a rare disease with an annual incidence of 1.72 cases has a mortality rate of 20% even with adequate therapy.

We describe the mechanisms involved in ADAMTS13 autoimmunity with a focus on the role of B- and T-cells in the pathogenesis of this disorder. We discuss the potential translation of recent experimental findings into future therapeutic concepts for the treatment of acquired TTP.

FRETS-VWF73 rather than CBA assay reflects ADAMTS13 proteolytic activity in acquired thrombotic thrombocytopenic purpura patients

Collagen-binding activity (CBA) and FRETS-VWF73 assays are widely adopted methods for the measurement of the plasmatic activity of ADAMTS13, the von Willebrand factor (VWF) cleaving-protease. Accurately assessing the severe deficiency of ADAMTS13 is important in the management of thrombotic thrombocytopenic purpura (TTP). However, non-concordant results between the two assays have been reported in a small but relevant percentage of TTP cases. We investigated whether CBA or FRETS-VWF73 assay reflects ADAMTS13 proteolytic activity in acquired TTP patients with non-concordant measurements. Twenty plasma samples with non-concordant ADAMTS13 activity results, <10% using FRETS-VWF73 and ≥20% using CBA, and 11 samples with concordant results, <10% using either FRETS-VWF73 and CBA assays, were analysed. FRETS-VWF73 was performed in the presence of 1.5 M urea. ADAMTS13 activities were also measured under flow conditions and the VWF multimer pattern was defined in order to verify the presence of ultra-large VWF due to ADAMTS13 deficiency. In FRETS-VWF73 assay with 1.5 M urea, ADAMTS13 activity significantly increased in roughly 50% of the samples with non-concordant results, whereas it remained undetectable in all samples with concordant measurements. Under flow conditions, all tested samples showed reduced ADAMTS13 activity. Finally, samples with non-concordant results showed a ratio of high molecular weight VWF multimers higher than normal. Our results support the use of FRETS-VWF73 over CBA assay for the assessment of ADAMTS13 severe deficiency and indicate urea as one cause of the observed differences.

Weight loss reduces anti-ADAMTS13 autoantibodies and improves inflammatory and coagulative parameters in obese patients

Obese patients have been described at increased risk of thrombotic thrombocytopenic purpura, a disease caused by anti-ADAMTS13 autoantibodies. ADAMTS13 has a structure homology with the adipokine thrombospondin-1. We previously demonstrated an increased presence of anti-ADAMTS13 antibodies in obese patients. We aimed to study the changes induced by weight loss after bariatric surgery on some inflammatory and coagulative parameters and their link with anti-ADAMTS13 autoantibodies. We studied 100 obese patients before and after weight loss induced by bariatric surgery and 79 lean volunteers as controls. We measured anthropometric, metabolic and inflammatory parameters, thrombospondin-1, ADAMTS13 activity, anti-ADAMTS13 autoantibodies, Von Willebrand factor. At baseline, 13 % of patients was positive for anti-ADAMTS13 autoantibodies, while all controls were negative. Thrombospondin-1 levels were higher in obese subjects with than without antibodies, with a positive correlation between the two parameters. In multiple logistic regression analysis only thrombospondin-1 levels predicted positivity for anti-ADAMTS13 antibodies. After weight loss both anti-ADAMTS13 antibodies and thrombospondin-1 reduced significantly. Weight loss in obesity improves the inflammatory and coagulative profile, and in particular anti-ADAMTS13 autoantibodies, ADAMTS13 activity and thrombospondin-1.

Thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP; also known as Moschcowitz disease) is characterized by the concomitant occurrence of often severe thrombocytopenia, microangiopathic haemolytic anaemia and a variable degree of ischaemic organ damage, particularly affecting the brain, heart and kidneys. Acute TTP was almost universally fatal until the introduction of plasma therapy, which improved survival from <10% to 80-90%. However, patients who survive an acute episode are at high risk of relapse and of long-term morbidity. A timely diagnosis is vital but challenging, as TTP shares symptoms and clinical presentation with numerous conditions, including, for example, haemolytic uraemic syndrome and other thrombotic microangiopathies. The underlying pathophysiology is a severe deficiency of the activity of a disintegrin and metalloproteinase with thrombospondin motifs 13 (ADAMTS13), the protease that cleaves von Willebrand factor (vWF) multimeric strings. Ultra-large vWF strings remain uncleaved after endothelial cell secretion and anchorage, bind to platelets and form microthrombi, leading to the clinical manifestations of TTP. Congenital TTP (Upshaw-Schulman syndrome) is the result of homozygous or compound heterozygous mutations in ADAMTS13, whereas acquired TTP is an autoimmune disorder caused by circulating anti-ADAMTS13 autoantibodies, which inhibit the enzyme or increase its clearance. Consequently, immunosuppressive drugs, such as corticosteroids and often rituximab, supplement plasma exchange therapy in patients with acquired TTP.

Thrombotic thrombocytopenic purpura: diagnostic criteria, clinical features, and long-term outcomes from 1995 through 2015

Our objective was to describe new observations from the Oklahoma Thrombotic Thrombocytopenic Purpura (TTP) Registry experience (November 1995 through December 2015) on the diagnosis of TTP along with patients' clinical features and their outcomes. Among 363 patients with an initial episode of clinically suspected TTP, the diagnosis of TTP was supported by both ADAMTS13 activity <10% and clinical features in 78 patients (21%). ADAMTS13 activity was measured in all 363 patients by 2 methods: fluorescence resonance energy transfer (FRET) and immunoblotting (IB). Sixty patients had ADAMTS13 activity <10% by both methods, 15 had ADAMTS13 <10% only by FRET, and 3 had ADAMTS13 <10% only by IB. Five patients with ADAMTS13 activity <10% by 1 method had an alternative clinical diagnosis, not TTP. Two patients with characteristic clinical features of TTP (microangiopathic hemolytic anemia and thrombocytopenia, no alternative diagnosis) and multiple relapses initially had ADAMTS13 activity >10% by both measurements. ADAMTS13 inhibitor titers were not associated with presenting features or outcomes. Microangiopathic hemolytic anemia and thrombocytopenia were not severe in all patients. Forty-seven percent of patients had no or minor neurologic abnormalities; 95% had no or minor serum creatinine abnormalities. Ten patients (13%) died, 2 before completing 1 plasma exchange (PEX); 3 deaths were attributed to PEX complications. For patients presenting after we began using rituximab in some patients (December 2003), fewer PEX treatments were required and fewer relapses occurred. Patients with their first relapse presented with higher platelet counts and hematocrits and lower lactate dehydrogenase levels and required fewer PEX treatments compared with their initial episodes.

Thrombotic microangiopathy and human immunodeficiency virus in the era of eculizumab

Thrombotic microangiopathies (TMAs) include thrombotic thromobocytopenic purpura and hemolytic uremic syndrome (HUS). Among these conditions, atypical HUS is now recognized to be a disease of alternative complement pathway dysregulation. Eculizumab is a recombinant humanized monoclonal antibody that binds to the complement protein C5 and prevents the cleavage of C5 to C5a and C5b. Eculizumab has been used as a novel treatment for complement-mediated TMA. We present a case of a patient with human immunodeficiency virus infection who developed TMA and was successfully treated with eculizumab. The effect of long-term treatment with this new medication is unknown, and further studies are needed to establish guidelines in the management of this condition.

Caplacizumab for Acquired Thrombotic Thrombocytopenic Purpura

BACKGROUND

Acquired thrombotic thrombocytopenic purpura (TTP) is caused by aggregation of platelets on ultralarge von Willebrand factor multimers. This microvascular thrombosis causes multiorgan ischemia with potentially life-threatening complications. Daily plasma exchange and immunosuppressive therapies induce remission, but mortality and morbidity due to microthrombosis remain high.

METHODS

Caplacizumab, an anti-von Willebrand factor humanized single-variable-domain immunoglobulin (Nanobody), inhibits the interaction between ultralarge von Willebrand factor multimers and platelets. In this phase 2, controlled study, we randomly assigned patients with acquired TTP to subcutaneous caplacizumab (10 mg daily) or placebo during plasma exchange and for 30 days afterward. The primary end point was the time to a response, defined as confirmed normalization of the platelet count. Major secondary end points included exacerbations and relapses.

RESULTS

Seventy-five patients underwent randomization (36 were assigned to receive caplacizumab, and 39 to receive placebo). The time to a response was significantly reduced with caplacizumab as compared with placebo (39% reduction in median time, P=0.005). Three patients in the caplacizumab group had an exacerbation, as compared with 11 patients in the placebo group. Eight patients in the caplacizumab group had a relapse in the first month after stopping the study drug, of whom 7 had ADAMTS13 activity that remained below 10%, suggesting unresolved autoimmune activity. Bleeding-related adverse events, most of which were mild to moderate in severity, were more common with caplacizumab than with placebo (54% of patients vs. 38%). The frequencies of other adverse events were similar in the two groups. Two patients in the placebo group died, as compared with none in the caplacizumab group.

CONCLUSIONS

Caplacizumab induced a faster resolution of the acute TTP episode than did placebo. The platelet-protective effect of caplacizumab was maintained during the treatment period. Caplacizumab was associated with an increased tendency toward bleeding, as compared with placebo. (Funded by Ablynx; ClinicalTrials.gov number, NCT01151423.).

High prevalence of hereditary thrombotic thrombocytopenic purpura in central Norway: from clinical observation to evidence

Essentials The population prevalence of hereditary thrombotic thrombocytopenic purpura (TTP) is unknown. We studied the prevalence of hereditary TTP and population frequencies of two ADAMTS-13 mutations. A high frequency of hereditary TTP related to ADAMTS-13 mutation c.4143_4144dupA was found. Vicinity of ABO blood group and ADAMTS-13 loci may facilitate screening of ADAMTS-13 mutations.

SUMMARY

Background Hereditary thrombotic thrombocytopenic purpura (TTP) caused by ADAMTS-13 mutations is a rare, but serious condition. The prevalence is unknown, but it seems to be high in Norway. Objectives To identify all patients with hereditary TTP in central Norway and to investigate the prevalence of hereditary TTP and the population frequencies of two common ADAMTS-13 mutations. Patients/Methods Patients were identified in a cross-sectional study within the Central Norway Health Region by means of three different search strategies. Frequencies of ADAMTS-13 mutations, c.4143_4144dupA and c.3178 C>T (p.R1060W), were investigated in a population-based cohort (500 alleles) and in healthy blood donors (2104 alleles) by taking advantage of the close neighborhood of the ADAMTS-13 and ABO blood group gene loci. The observed prevalence of hereditary TTP was compared with the rates of ADAMTS-13 mutation carriers in different geographical regions. Results We identified 11 families with hereditary TTP in central Norway during the 10-year study period. The prevalence of hereditary TTP in central Norway was 16.7 × 10(-6) persons. The most prevalent mutation was c.4143_4144dupA, accounting for two-thirds of disease causing alleles among patients and having an allelic frequency of 0.33% in the central, 0.10% in the western, and 0.04% in the southeastern Norwegian population. The allelic frequency of c.3178 C>T (p.R1060W) in the population was even higher (0.3-1%), but this mutation was infrequent among patients, with no homozygous cases. Conclusions We found a high prevalence of hereditary TTP in central Norway and an apparently different penetrance of ADAMTS-13 mutations.

Efficacy and Safety of Rituximab for Refractory and Relapsing Thrombotic Thrombocytopenic Purpura: A Cohort of 10 Cases

OBJECTIVE

Idiopathic thrombotic thrombocytopenic purpura (TTP) is a life-threatening disorder mediated by autoantibodies directed against ADAMTS13. This provides a rationale for the use of rituximab in this disorder. We report our experience and the outcome of 10 cases of TTP (9 refractory and 1 relapsing) successfully treated with rituximab in combination with plasma exchange (PE) and other immunosuppressive treatments.

METHODS

The diagnosis of TTP was based on clinical criteria and supported by severe deficiency of ADAMTS13 activity and presence of inhibitors in seven cases. Rituximab was started after a median of 18.6 sessions of PE (range: 5–35) at the dose of 375 mg/m²/week for 4–8 weeks.

RESULTS

Complete remission was achieved in all patients after a median time of 14.4 days of the first dose (range: 6–30). After a median follow-up of 30 months (range: 8–78), eight patients were still in remission and two developed multiple relapses, treated again with the same therapy, and achieved complete responses; they are alive, and in complete remission after a follow-up of 12 and 16 months.

CONCLUSION

Rituximab appears to be a safe and effective therapy for refractory and relapsing TTP. However, longer follow-up is recommended to assess relapse and detect possible long-term side effects of this therapy.

Does corticosteroid treatment cause prolonged recovery and increased total bilirubin level in severe ADAMTS-13-deficient TTP patient?

A 41-year-old female patient complaining of fatigue, headache, mild confusion, and rush on her lower extremities was admitted to our emergency department. Laboratory tests revealed that he had anemia, thrombocytopenia, and increased levels of indirect bilirubin and lactic dehydrogenase (LDH) in blood tests. Direct and indirect Coombs tests were negative, and fragmented erythrocytes were observed in peripheral blood smears. The patient was diagnosed with thrombotic thrombocytopenic purpura (TTP). The best supportive care was provided. Therapeutic plasma exchange (TPE) and 1 mg/kg methylprednisolone treatments were administered. On the 10th day of treatment, LDH level and fragmented red blood cells in peripheral blood smear were decreased, but his direct and indirect bilirubin levels increased despite the fact that he was treated with 1 mg/kg methylprednisolone and TPE. The patient had severe ADAMTS-13 deficiency. After discontinued steroids treatment, his bilirubin level normalized within 4 days. On the 4th day after bilirubin level normalized, vincristine treatment was administered. TPE was also continued. There was no consensus about the optimal schedule for discontinuing plasmapheresis therapy, and also we observed total bilirubin level improvement with discontinued corticosteroid treatment. In this case, corticosteroid treatment was linked with the increase of total bilirubin level in severe ADAMTS-13-deficient TTP patient.

What's new in the diagnosis and pathophysiology of thrombotic thrombocytopenic purpura

Severe ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) deficiency causes thrombotic thrombocytopenic purpura (TTP), which is characterized by microangiopathic hemolytic anemia, thrombocytopenia, and the absence of oliguric or anuric renal failure. However, some patients with this constellation of findings do not have ADAMTS13 deficiency, and some patients with ADAMTS13 deficiency have renal failure or relatively normal blood counts. Consequently, many investigators and clinicians have incorporated severe ADAMTS13 deficiency into the case definition of TTP. This change has facilitated the timely initiation of treatment for patients with atypical clinical features who otherwise would not be recognized as having TTP. Conversely, excluding severe ADAMTS13 deficiency focuses attention on the diagnosis and treatment of other causes of thrombotic microangiopathy that require different treatment. The rapid return of ADAMTS13 data is important to make the best use of this information.

The splenic autoimmune response to ADAMTS13 in thrombotic thrombocytopenic purpura contains recurrent antigen-binding CDR3 motifs

The spleen harbors ADAMTS13-specific memory B cells following acute acquired TTP. The splenic anti-ADAMTS13 antibody repertoire is characterized by a set of unique and novel CDR3 motifs, 4 shared by 2 patients.

A novel flow-based assay reveals discrepancies in ADAMTS-13 inhibitor assessment as compared with a conventional clinical static assay

BACKGROUND

Several static Bethesda-type assays are routinely used to determine ADAMTS-13-neutralizing autoantibodies in acquired thrombotic thrombocytopenic purpura (TTP), but the inhibitory activity of these antibodies has not been thoroughly evaluated under the more physiologic condition of flow.

OBJECTIVES

We investigated whether ADAMTS-13 inhibitor assessment with the FRETS-VWF73 assay is predictive for evaluation under flow.

METHODS

Anti-ADAMTS-13 autoantibodies were purified from patients with acquired TTP by chromatography involving an ADAMTS-13 affinity matrix and/or protein G. ADAMTS-13 activity was measured with the FRETS-VWF73 assay and a novel flow assay determining the ADAMTS-13-mediated decrease in platelet aggregate surface coverage, caused by perfusion of a suspension containing platelets, erythrocytes and von Willebrand factor (VWF) over a surface coated with extracellular matrix components. The neutralizing activities of ADAMTS-13 inhibitors were compared under static conditions and under flow by use of the two assays.

RESULTS

The suitability of the flow-based ADAMTS-13 activity assay for quantification of ADAMTS-13 inhibitors could be demonstrated by reversibility of the ADAMTS-13-dependent decrease in surface coverage upon addition of goat ADAMTS-13 antiserum. Testing the neutralizing activity of purified autoantibodies from six patients in the flow assay according to their FRETS-VWF73-based inhibitor titers gave rise to vastly different inhibitory effects, indicating a discrepancy in inhibitor assessment between static and flow conditions.

CONCLUSIONS

Anti-ADAMTS-13 autoantibodies may show inhibitory properties in vivo that are not consistent with the ADAMTS-13 inhibitor levels determined in routine static assays, possibly because certain epitopes are selectively exposed under shear. Consequently, the course of disease and treatment efficacy may vary among TTP patients, despite common inhibitor titers.

Syndromes of thrombotic microangiopathy

This review article covers the diverse pathophysiological pathways that can lead to microangiopathic hemolytic anemia and a procoagulant state with or without damage to the kidneys and other organs.

Use of the ADAMTS13 activity assay improved the accuracy and efficiency of the diagnosis and treatment of suspected acquired thrombotic thrombocytopenic purpura

CONTEXT

Acquired thrombotic thrombocytopenic purpura (A-TTP) is a rare but significant disease requiring rapid diagnosis and treatment. The diagnosis is often difficult because of variability in the presence of specific clinical criteria. The primary etiology of A-TTP involves inhibitors directed against ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). Literature has shown that the ADAMTS13 activity assay is sensitive and specific for identifying cases of A-TTP, and application of this test as an on-site screening method has not been fully explored.

OBJECTIVE

Our objective is to determine if the ADAMTS13 activity assay can be used as a successful, on-site diagnostic modality to rapidly identify cases of A-TTP and prevent unnecessary use of prophylactic therapeutic plasma exchange.

DESIGN

A retrospective analysis was performed including 152 patients with clinically suspected A-TTP, screened using the ADAMTS13 activity assay. Results were correlated with potential therapeutic plasma exchange treatment for all cases highly suspicious for A-TTP and evaluated for unnecessary patient morbidity and financial cost.

RESULTS

The ADAMTS13 activity assay had an overall sensitivity and specificity of 100% and 99%, respectively. The positive predictive value was 91% and the negative predictive value was 100%. In 95% of the studies ordered, A-TTP was ruled out, leading to decreased patient morbidity and $1.7 million of potential treatment costs avoided.

CONCLUSION

Implementation of the fluorescence energy transfer-based ADAMTS13 activity assay as a point-of-care laboratory study decreased patient morbidity while also directing more efficient employment of therapeutic plasma exchange in cases of suspected A-TTP.

Persistence of circulating ADAMTS13-specific immune complexes in patients with acquired thrombotic thrombocytopenic purpura

Anti-ADAMTS13 autoantibodies are the main cause of acquired thrombotic thrombocytopenic purpura. Binding of these antibodies to ADAMTS13 eventually results in the formation of antigen-antibody immune complexes. Circulating ADAMTS13-specific immune complexes have been described in patients with acquired thrombotic thrombocytopenic purpura, although the prevalence and persistence of these immune complexes over time have hitherto remained elusive. Here, we analyzed a large cohort of patients with acquired thrombotic thrombocytopenic purpura for the presence of free and complexed anti-ADAMTS13 antibodies. In the acute phase (n=68), 100% of patients had free IgG antibodies and 97% had ADAMTS13-specific immune complexes. In remission (n=28), 75% of patients had free antibodies (mainly IgG) and 93% had ADAMTS13-specific immune complexes. Free antibodies were mainly of subclasses IgG1 and IgG4, whereas IgG4 was by far the most prevalent in ADAMTS13-specific immune complexes. Comparison of ADAMTS13 inhibitor and anti-ADAMTS13 IgG (total and subclasses) antibody titers in acute phase and in remission samples showed a statistically significant decrease in all parameters in remission. Although non-significant, a trend towards reduced or undetectable titers in remission was also observed for ADAMTS13-specific immune complexes of subclasses IgG1, IgG2 and IgG3. No such trend was discernible for IgG4; IgG4 immune complexes persisted over years, even in patients who had been treated with rituximab and who showed no features suggesting relapse.