ADAMTS13 autoantibodies in patients with thrombotic microangiopathies and other immunomediated diseases

IgG subclass distribution of anti-ADAMTS13 antibodies in patients with acquired thrombotic thrombocytopenic purpura

BACKGROUND

ADAMTS13-neutralizing IgG autoantibodies are the major cause of acquired thrombotic thrombocytopenic purpura (TTP).

OBJECTIVE

To analyze the IgG subclass distribution of anti-ADAMTS13 antibodies and a potential relationship between subclass distribution and disease prognosis.

METHODOLOGY

An enzyme-linked immunosorbent assay-based method was used to quantify the relative amounts of IgG subclasses of anti-ADAMTS13 antibodies in acquired TTP plasma.

RESULTS

IgG(4) (52/58, 90%) was the most prevalent IgG subclass in patients with acquired TTP, followed by IgG(1) (52%), IgG(2) (50%), and IgG(3) (33%). IgG(4) was found either alone (17/52) or with other IgG subclasses (35/52). IgG(4) was not detected in 10% of the patients. There was an inverse correlation between the frequency and abundance of IgG(4) and IgG(1) antibodies (P < 0.01). Patients with high IgG(4) levels and undetectable IgG(1) are more prone to relapse than patients with low IgG(4) levels and detectable IgG(1).

CONCLUSIONS

All IgG subclasses of anti-ADAMTS13 antibodies were detected in patients with acquired TTP, with IgG(4), followed by IgG(1), antibodies dominating the anti-ADAMTS13 immune response. Levels of IgG(4) could be useful for the identification of patients at risk of disease recurrence.

Characterization of conformation-sensitive antibodies to ADAMTS13, the von Willebrand cleavage protease

Background

The zinc metalloprotease ADAMTS13 is a multidomain protein that cleaves von Willebrand Factor (VWF) and is implicated in Thrombotic Thrombocytopenic Purpura (TTP) pathogenesis. Understanding the mechanism of this protein is an important goal. Conformation sensitive antibodies have been used to monitor protein conformation and to decipher the molecular mechanism of proteins as well as to distinguish functional and non-functional mutants.

Methodology/Principal Findings

We have characterized several antibodies against ADAMTS13, both monoclonal and polyclonal. We have used flow cytometry to estimate the binding of these antibodies to ADAMTS13 and demonstrate that antibodies raised against the TSP and disintegrin domains detect conformation changes in the ADAMTS13. Thus for example, increased binding of these antibodies was detected in the presence of the substrate (VWF), mainly at 37°C and not at 4°C. These antibodies could also detect differences between wild-type ADAMTS13 and the catalytically deficient mutant (P475S). The flow cytometry approach also allows us to estimate the reactivity of the antibody as well as its apparent affinity.

Conclusions/Significance

Our results suggest that these antibodies may serve as useful reagents to distinguish functional and non-functional ADAMTS13 and analyze conformational transitions to understand the catalytic mechanism.

Cardiac involvement in acute thrombotic thrombocytopenic purpura: association with troponin T and IgG antibodies to ADAMTS 13

INTRODUCTION

Evidence for cardiac involvement in thrombotic thrombocytopenic purpura (TTP) is uncommonly described.

METHODOLOGY

We retrospectively reviewed 41 patients assessing troponin T as a marker for cardiac involvement in acute TTP with clinical symptoms, electrocardiograms (ECG) and echocardiograms. A histopathological review of five patients who died of acute TTP was also undertaken.

RESULTS

In 54% (22/41) of patients, troponin T was >or=0.05microg L(-1) (normal range 0-0.01 microg L(-1)). Half (12/22) had cardiac symptoms and 8/22 with a raised troponin T reported chest pain. ECG changes were present in 62% of patients with a raised troponin T. Median anti-ADAMTS 13 IgG antibody was significantly higher (P=0.018) in patients with troponin T>or=0.05 microg L(-1) (58.5% (range 17-162%), compared with patients with troponin T<0.05 microg L(-1) (35%, range 9-134%). Patients who died had higher troponin T levels (median 0.305 microg L(-1)) and raised anti-ADAMTS 13 IgG (median 66.5%). On admission, there were no deaths in those with troponin T

CONCLUSION

Clinical symptoms, ECG changes and echocardiograms are poor predictors of cardiac disease in acute TTP. Troponin T is specific for cardiac muscle and a sensitive marker of myocardial damage. In TTP patients, raised levels (>or=0.05 microg L(-1)) signify myocardial necrosis associated with microvascular thrombi. Mortality and acute morbidity was associated with higher admission troponin T and raised IgG antibody (>67%) to ADAMTS 13.

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MESH Headings

• ADAM Proteins/immunology
• ADAMTS13 Protein
• Acute Disease
• Autoantibodies/blood
• Biomarkers/blood
• Heart Diseases/diagnosis
• Heart Diseases/mortality
• Humans
• Immunoglobulin G
• Morbidity
• Mortality
• Purpura, Thrombotic Thrombocytopenic/complications
• Purpura, Thrombotic Thrombocytopenic/epidemiology
• Purpura, Thrombotic Thrombocytopenic/mortality
• Retrospective Studies
• Troponin T/blood

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Grants Collapse

Affiliation(s)

C Hughes Department of Haematology, University College London Hospitals, London, UK
J R McEwan
I Longair
S Hughes
H Cohen
S Machin
M Scully

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Molecular basis of ADAMTS13 dysfunction in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a thrombotic microangiopathic disorder characterized by thrombocytopenia, hemolytic anemia, neurological and renal manifestations, and fever. It is associated with dysfunctional von Willebrand factor (VWF) proteolysis and the occurrence of VWF- and platelet-rich thrombi in the microcirculation of multiple organs, including the kidneys. Von Willebrand factor is a large glycoprotein that circulates in plasma as a series of multimers, and it plays a major role in primary hemostasis by inducing the formation of platelet plugs at sites of vascular injury and high-shear stress. Its activity is dependent on the sizes of the multimers, with ultra-large (UL) VWF multimers being biologically very potent. The ULVWF multimers are rapidly degraded upon their secretion from endothelial cells in normal individuals but not in the circulation of TTP patients, causing the formation of disseminated thrombi in the latter. The defective breakdown of VWF is attributed to a severely deficient activity of the VWF-cleaving protease ADAMTS13, a plasma metalloprotease synthesized in the liver, kidneys, and endothelium. This protease rapidly degrades VWF-platelet strings under flow by proteolytic cleavage of the VWF subunit, thereby regulating the size of the platelet thrombus. Congenital TTP occurs due to ADAMTS13 mutations, with the usual debut occurring during the first years of life, while acquired TTP is associated with auto-antibodies against ADAMTS13.

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

BACKGROUND

Autoantibodies directed towards ADAMTS13 are present in the majority of patients with acquired thrombotic thrombocytopenic purpura (TTP). Analysis of a set of antibodies derived from two patients with acquired TTP revealed frequent use of the VH1-69 heavy chain gene segment for the assembly of anti-ADAMTS13 antibodies.

OBJECTIVE

We explored the ability of two VH1-69 germline gene-encoded antibodies to inhibit the von Willebrand factor (VWF)-processing activity of ADAMTS13 under different experimental conditions. Furthermore, the presence of VH1-69 encoded anti-ADAMTS13 antibodies in 40 patients with acquired TTP was monitored using monoclonal antibody G8, which specifically reacts with an idiotype expressed on VH1-69 encoded antibodies.

METHODS AND RESULTS

Binding of the two VH1-69 encoded monoclonal antibodies was dependent on the presence of the spacer domain. Both antibodies inhibited ADAMTS13 activity under static conditions, as measured by cleavage of FRETS-VWF73 substrate and cleavage of VWF multimers. The recombinant antibodies were also capable of inhibiting the processing of UL-VWF strings on the surface of endothelial cells. G8-reactive antibodies directed towards ADAMTS13 were present in plasma of all patients containing anti ADAMTS13 antibodies.

CONCLUSIONS

These results suggest that VH1-69 derived antibodies directed towards ADAMTS13 develop in the majority of patients with acquired TTP.

Antiphospholipid antibodies: paradigm in transition

OBJECTIVES

This is a critical review of anti-phospholipid antibodies (aPL). Most prior reviews focus on the aPL syndrome (APS), a thrombotic condition often marked by neurological disturbance. We bring to attention recent evidence that aPL may be equally relevant to non-thrombotic autoimmune conditions, notably, multiple sclerosis and ITP.

ORGANIZATION

After a brief history, the recent proliferation of aPL target antigens is reviewed. The implication is that many more exist. Theories of aPL in thrombosis are then reviewed, concluding that all have merit but that aPL may have more diverse pathological consequences than now recognized. Next, conflicting results are explained by methodological differences. The lupus anticoagulant (LA) is then discussed. LA is the best predictor of thrombosis, but why this is true is not settled. Finally, aPL in non-thrombotic disorders is reviewed.

CONCLUSION

The current paradigm of aPL holds that they are important in thrombosis, but they may have much wider clinical significance, possibly of special interest in neurology.

Thrombotic thrombocytopenic purpura related to severe ADAMTS13 deficiency in children

Thrombotic thrombocytopenic purpura (TTP) related to a severely deficient activity of the von Willebrand factor cleaving protease, ADAMTS (A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats) 13, is a life-threatening event, the onset of which may occur as early as childhood. TTP is either inherited (Upshaw-Schulman syndrome) via ADAMTS13 gene mutations (neonatal onset) or acquired via anti-ADAMTS13 autoantibodies (childhood onset). TTP is due to platelet- and von-Willebrand-factor-rich thrombi of the microvasculature, inducing mechanical hemolytic anemia, consumption thrombocytopenia, and multivisceral ischemia. Clinical course consists of relapsing acute events triggered mostly by infections, associated icterus and hyperbilirubinemia, severe hemolytic anemia with schistocytosis and a negative Coombs test, severe thrombocytopenia, and sometimes symptoms related to visceral ischemia (renal failure, central nervous system vascular events, other organ failure). The recently available ADAMTS13 laboratory investigation combining measurement of ADAMTS13 activity in plasma, search for an ADAMTS13 circulating inhibitor, and anti-ADAMTS13 IgG and ADAMTS13 gene sequencing is a crucial addition to TTP diagnosis. Plasma exchanges are first-line treatment of acquired TTP, combined with steroids and immunosuppressive drugs. Curative treatment of acute events in Upshaw-Schulman syndrome relies on plasma infusions (provider of active ADAMTS13). Guidelines for preventive treatment of relapses are not clearly established but should associate plasmatherapy and caution to triggers of relapses. Therapeutic perspectives are focused on the development of concentrated plasma-derived ADAMTS13 or recombinant ADAMTS13.

ADAMTS13 activity and inhibitor

Thrombotic thrombocytopenic purpura (TTP) is often associated with acquired or congenital deficiency of the von Willebrand factor-cleaving metalloprotease, ADMATS13 (Lammle B et al., J Thromb Haemost 2005;3:1663-1675; Schneppenheim et al., Blood 2003;101:1845-1850). Although undetectable levels of enzyme activity (<10%) are diagnostic of inherited or acquired TTP in the correct clinical setting (absence is specific), not all patients diagnosed with TTP have severe protease deficiency, and it is therefore not recommended as an initial test for diagnosis (Copelovitch and Kaplan, Pediatr Nephrol, in press). Many prospective and retrospective studies have demonstrated that patients with severe protease deficiency have a higher likelihood of relapse, making it helpful as an indicator of recurrence. The short-term prognostic usefulness of ADAMTS13 testing during acute TTP warrants further investigation because of limited prospective studies (Ferrari S et al., Blood 2007;109:2815-2822; Peyvandi et al., Haematologica 2008;93:232-239).

Regional UK TTP registry: correlation with laboratory ADAMTS 13 analysis and clinical features

Thrombotic thrombocytopenic purpura (TTP) is an acute, rare, life-threatening disorder. This report presents the South East (SE) England registry for TTP, from April 2002 to December 2006, which included 176 patients and 236 acute episodes; 75% of patients were female and 25% were male, overall median age at presentation was 42 years. Mortality was 8.5%, most cases died before treatment was instigated. The main ethnic groups were Caucasian (64%) and Afro Caribbean (27%). Seventy-seven percent of cases were idiopathic, 5% were congenital and the remaining cases had a defined precipitant. Neurological features were the most prevalent, but cardiac involvement accounted for 42% of presenting features. The overall median number of plasma exchanges (PEXs) to remission was 15; between April 2002 and December 2003, the median number of PEXs was 19 and it was 12 between January 2004 and December 2006 (P < 0.0001). In the latter period, adjuvant therapies were reduced, but Rituximab was increased. ADAMTS 13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) activity was <10% in 74% and 95% of these cases had positive IgG antibodies to ADAMTS 13. Renal impairment and delayed normalisation of platelet count were the main differences between idiopathic and secondary TTP.

Pathogenesis of thrombotic microangiopathies

Profound thrombocytopenia and microangiopathic hemolytic anemia characterize thrombotic microangiopathy, which includes two major disorders: thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). TTP has at least three types: congenital or familial, idiopathic, and nonidiopathic. The congenital and idiopathic TTP syndromes are caused primarily by deficiency of ADAMTS13, owing to mutations in the ADAMTS13 gene or autoantibodies that inhibit ADAMTS13 activity. HUS is similar to TTP, but is associated with acute renal failure. Diarrhea-associated HUS accounts for more than 90% of cases and is usually caused by infection with Shiga-toxin-producing Escherichia coli (O157:H7). Diarrhea-negative HUS is associated with complement dysregulation in up to 50% of cases, caused by mutations in complement factor H, membrane cofactor protein, factor I or factor B, or by autoantibodies against factor H. The incomplete penetrance of mutations in either ADAMTS13 or complement regulatory genes suggests that precipitating events or triggers may be required to cause thrombotic microangiopathy in many patients.

Relationship between ADAMTS13 activity in clinical remission and the risk of TTP relapse

Idiopathic thrombotic thrombocytopenic purpura (TTP) is characterized by frequent recurrences. Effective screening for relapses will enable intervention prior to overt episodes of TTP. The present study used a modified assay to detect ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, 13) activity as low as 0.5%. This analytical improvement permits adequate measurement of ADAMTS13 activity levels in 97% of remission samples used for statistical modelling. ADAMTS13 activity and ADAMTS13 antibody (IgG) were measured in 157 serial samples prospectively collected from 24 TTP patients during periods of clinical remission. These patients were followed-up quarterly for an average of 23 months, during which time nine episodes of TTP relapse occurred among six patients. Finally, logistic regression modelling was used to define the relationship between ADAMTS13 activity levels (0.5-100%) and the probability of TTP relapses. Our data demonstrated that lower ADAMTS13 activity and younger age were significantly associated with higher risk of relapse in the 3 months after specimens were taken. In contrast, ADAMTS13 antibody IgG levels were not predictive of TTP relapses. Identification of low ADAMTS13 activity during clinical remission as a key risk factor for TTP relapses provides a new screening strategy to identify patients who may benefit from prophylactic therapy prior to disease relapses.

The clinical spectrum of antiphospholipid syndrome

Antiphospholipid syndrome (APS) is a disorder characterized by a wide variety of clinical manifestations. Virtually any organ system or tissue may be affected by the consequences of large- or small-vessel thrombosis. There is a broad spectrum of disease among individuals with antiphospholipid antibodies (aPL). Patients may exhibit clinical features suggesting APS but not fulfill the International Criteria for a "definite" diagnosis. Seronegative APS patients demonstrate typical idiopathic thromboses but aPL are not initially detected. Patients defined with definite APS demonstrate nearly identical sites of venous and arterial thrombosis, regardless of the presence or absence of systemic lupus erythematosus. Microangiopathic APS may present with isolated tissue and organ injury or as the overwhelming "thrombotic storm" observed in catastrophic APS.

The potential utility of B cell-directed biologic therapy in autoimmune diseases

Increasing awareness of the importance of aberrant B cell regulation in autoimmunity has driven the clinical development of novel B cell-directed biologic therapies with the potential to treat a range of autoimmune disorders. The first of these drugs-rituximab, a chimeric monoclonal antibody against the B cell-specific surface marker CD20-was recently approved for treating rheumatoid arthritis in patients with an inadequate response to other biologic therapies. The aim of this review is to discuss the potential use of rituximab in the management of other autoimmune disorders. Results from early phase clinical trials indicate that rituximab may provide clinical benefit in systemic lupus erythematosus, Sjögren's syndrome, vasculitis, and thrombocytopenic purpura. Numerous case reports and several small pilot studies have also been published reporting the use of rituximab in conditions such as myositis, antiphospholipid syndrome, Still's disease, and multiple sclerosis. In general, the results from these preliminary studies encourage further testing of rituximab therapy in formalized clinical trials. Based on results published to date, it is concluded that rituximab, together with other B cell-directed therapies currently under clinical development, is likely to provide an important new treatment option for a number of these difficult-to-treat autoimmune disorders.

Thrombotic thrombocytopenic purpura: a thrombotic disorder caused by ADAMTS13 deficiency

A serious disorder with characteristic microvascular thrombosis involving the brain and other organs, thrombotic thrombocytopenic purpura (TTP) typically presents with thrombocytopenia, hemolysis with schistocytes on blood smears, and mental changes or seizures. It may progress rapidly to a fatal end if the patient is not treated immediately with plasma. Recent advances have shown that TTP is caused by deficiency of a circulating, von Willebrand factor cleaving metalloprotease, ADAMTS13. This new knowledge will provide clues to improve the diagnosis and management of this intriguing disease.

Thrombotic microangiopathic hemolytic anemia in a patient with SLE: diagnostic difficulties

BACKGROUND

A 19-year-old woman with newly diagnosed systemic lupus erythematosus (SLE) presented with hemolytic anemia, thrombocytopenia, hypertension, tonic-clonic seizures, blurry vision, nephrotic syndrome and renal insufficiency.

INVESTIGATIONS

At a general hospital, the investigations included brain MRI, echocardiography, laboratory tests including measurement of the amount of protein excreted daily, platelet count, levels of lactate dehydrogenase, creatinine and anticardiolipin antibodies, direct Coombs' test, peripheral blood smear, and measurement of blood pressure. At a tertiary institution the investigations included physical examination, electroencephalography, brain MRI, magnetic resonance angiography, repetition of laboratory tests plus measurement of von Willebrand factor-cleaving protease activity, measurement of levels of antibodies to double-stranded DNA and platelets, and renal biopsy.

DIAGNOSIS

Thrombotic microangiopathic hemolytic anemia with a possible underlying diagnosis of malignant hypertension, antiphospholipid antibody syndrome, catastrophic antiphospholipid antibody syndrome, thrombotic thrombocytopenic purpura, or active SLE.

MANAGEMENT

At the general hospital, therapy included a single dose of intravenous cyclophosphamide 500 mg, eight daily plasma exchange treatments, three daily infusions of methylprednisolone 1 g followed by methylprednisolone 60 mg every 8 h, an infusion of rituximab 657 mg and ultrafiltration via hemodialysis. At the tertiary institution, therapy included an infusion of cyclophosphamide 650 mg, aspirin 81 mg daily, prednisone 40 mg daily, mycophenolate mofetil 750 mg twice daily, and aggressive management of hypertension.

Prognostic value of anti-ADAMTS 13 antibody features (Ig isotype, titer, and inhibitory effect) in a cohort of 35 adult French patients undergoing a first episode of thrombotic microangiopathy with undetectable ADAMTS 13 activity

To study both the pathophysiologic and the prognostic value of ADAMTS13 in thrombotic microangiopathies (TMAs), we enrolled a cohort of 35 adult patients combining a first acute episode of TMA, an undetectable (below 5%) ADAMTS13 activity in plasma, and no clinical background such as sepsis, cancer, HIV, and transplantation. All patients were treated by steroids and plasma exchange, and an 18-month follow-up was scheduled. Remission was obtained in 32 patients (91.4%), and 3 patients died (8.6%) after the first attack. At presentation, ADAMTS13 antigen was decreased in 32 patients (91.4%), an ADAMTS13 inhibitor was detectable in 31 patients (89%), and an anti-ADAMTS13 IgG/IgM/IgA was present in 33 patients (94%). The 3 decedent patients were characterized by the association of several anti-ADAMTS13 Ig isotypes, including very high IgA titers, while mortality was independent of the ADAMTS13 inhibitor titer. In survivors, ADAMTS13 activity in remission increased to levels above 15% in 19 patients (59%) but remained undetectable in 13 patients (41%). Six patients relapsed either once or twice (19%) during the follow-up. High levels of inhibitory anti-ADAMTS13 IgG at presentation were associated with the persistence of an undetectable ADAMTS13 activity in remission, the latter being predictive for relapses within an 18-month delay.

ADAMTS13 assays and ADAMTS13-deficient mice

PURPOSE OF REVIEW

Thrombotic thrombocytopenic purpura can be induced by acquired or congenital deficiency of the plasma von Willebrand factor-cleaving protease, ADAMTS13. Measurement of ADAMTS13 activity is important for the diagnosis and treatment of microangiopathies including thrombotic thrombocytopenic purpura. Phenotypic analysis of mice lacking the Adamts13 gene is valuable for understanding the pathogenesis of microangiopathies.

RECENT FINDINGS

The minimum substrate for ADAMTS13 activity was identified as 73 amino acid residues in the A2 domain of von Willebrand factor, called VWF73. Several new assays have been developed using this sequence. The VWF73-based assays are rapid, quantitative, and easy to handle, and are well correlated with the measures from previous assays. Mice lacking the Adamts13 gene were produced. The mice were viable and fertile. They showed a prothrombotic state but no symptoms of spontaneous thrombocytopenia, hemolytic anemia, or microvascular thrombosis were observed.

SUMMARY

VWF73-based ADAMTS13 assays will significantly facilitate the accurate diagnosis of microangiopathies and contribute to the improved clinical treatment of these diseases. Accumulated clinical information on patients with ADAMTS13 deficiency and mice lacking the Adamts13 gene indicates that additional environmental or genetic susceptibility factors are required to trigger thrombotic thrombocytopenic purpura.

The pathology of lupus nephritis

An international working group of clinicians and pathologists met in 2003 under the auspices of the International Society of Nephrology (ISN) and the Renal Pathology Society (RPS) to revise and update the 1982 and 1995 World Health Organization classification of lupus glomerulonephritis. This article compares and contrasts the ISN/RPS classification and the antecedent World Health Organization classifications. Although systemic lupus erythematosus is the prototypical systemic immune-complex disease, several non-immune-complex mechanisms of glomerular injury and dysfunction have been proposed, and this article summarizes the evidence supporting the pathogenic mechanisms of lupus vasculitis, glomerular capillary thrombosis, and lupus podocytopathy. The most significant and controversial feature of the ISN/RPS classification is the separation of diffuse glomerulonephritis into separate classes with either segmental (class IV-S) or global (class IV-G) lesions. Several groups have tested the prognostic significance of this separation, and this article discusses the implications of these studies for the ISN/RPS classification.

Structural and functional correlation of ADAMTS13

PURPOSE OF REVIEW

ADAMTS13 represents a landmark in a journey that began over 80 years ago with a single clinical case. Thrombotic thrombocytopenic purpura exemplifies how von Willebrand factor can be responsible for life-threatening thrombosis. This review summarizes recent progress on ADAMTS13, which prevents this deadly event.

RECENT FINDINGS

Recent advances are summarized in four main areas. First, the core ADAMTS13-binding site is contained in a short sequence in the A2 domain, but other domains affect this interaction. Mutations from thrombotic thrombocytopenic purpura and von Willebrand disease provide clues for the structural prerequisites and regulation of von Willebrand factor cleavage. Second, studies are unraveling the reasons why urea, BaCl2, and low ionic strength are required to cleave von Willebrand factor under static conditions. Third, studies on thrombotic thrombocytopenic purpura and ADAMTS13-knockout mice suggest that ADAMTS13 deficiency alone may not be sufficient to cause thrombotic thrombocytopenic purpura. Finally, ADAMTS13 could be an antithrombotic agent for thrombotic thrombocytopenic purpura and other thrombotic conditions.

SUMMARY

Study of ADAMTS13 has exploded since this metalloprotease was characterized. This knowledge reveals the nature of ADAMTS13's interaction with von Willebrand factor and the pathogenesis of clinical thrombotic thrombocytopenic purpura, especially in relation to ADAMTS13.

Remission in acute refractory and relapsing thrombotic thrombocytopenic purpura following rituximab is associated with a reduction in IgG antibodies to ADAMTS-13

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disorder and plasma exchange (PEX) remains the primary treatment modality. Twenty-five patients with acute refractory/relapsing idiopathic TTP received rituximab in conjunction with PEX because of progressive clinical disease or deterioration in laboratory parameters, despite intensive standard therapy. In relapsing TTP, rituximab was started if antibody to ADAMTS-13 (a disintegrin and metalloproteinase with thrombospondin motif-13) was demonstrated during previous episodes. All 25 patients attained complete clinical and laboratory remission in a median of 11 d after initiating rituximab. In 21 cases, ADAMTS-13 activity was within the normal range following rituximab. Inhibitors were detected in 24/25 patients by mixing studies and/or immunoglobulin G (IgG) antibodies to ADAMTS-13 pre-rituximab. There was no evidence of inhibitors and/or IgG activity <10% in 23/25 patients following rituximab. In acute refractory cases, the median number of PEX pre-rituximab and following the first rituximab infusion was 13 and 9, respectively. There have been no infectious complications, despite low CD 19 levels and no relapses. In patients with acute refractory/relapsing idiopathic TTP, rituximab appears to be a safe, effective, targeted therapy with a significant reduction in the requirement for PEX.

ADAMTS13 phenotype in plasma from normal individuals and patients with thrombotic thrombocytopenic purpura

The activity of ADAMTS13, the von Willebrand factor cleaving protease, is deficient in patients with thrombotic thrombocytopenic purpura (TTP). In the present study, the phenotype of ADAMTS13 in TTP and in normal plasma was demonstrated by immunoblotting. Normal plasma (n = 20) revealed a single band at 190 kD under reducing conditions using a polyclonal antibody, and a single band at 150 kD under non-reducing conditions using a monoclonal antibody. ADAMTS13 was not detected in the plasma from patients with congenital TTP (n = 5) by either antibody, whereas patients with acquired TTP (n = 2) presented the normal phenotype. Following immunoadsorption of immunoglobulins, the ADAMTS13 band was removed from the plasma of the patients with acquired TTP, but not from that of normal individuals. This indicates that ADAMTS13 is complexed with immunoglobulin in these patients. The lack of ADAMTS13 expression in the plasma from patients with hereditary TTP may indicate defective synthesis, impaired cellular secretion, or enhanced degradation in the circulation. This study differentiated between normal and TTP plasma, as well as between congenital and acquired TTP. This method may, therefore, be used as a complement in the diagnosis of TTP.

Intravenous gamma globulin for thrombotic microangiopathy of unknown etiology

We encountered the case of a 4-year-old boy with thrombotic microangiopathy (TMA) of unknown etiology. Verotoxin-induced hemolytic uremic syndrome (HUS), Streptococcus-pneumoniae-related HUS, factor H deficiency, drug-induced thrombotic thrombocytopenic purpura (TTP), and ADAMTS13 (von Willebrand factor-cleaving protease; a disintegrin-like and metalloprotease with thrombospondin type 1 repeats)-related TTP were excluded. His condition was refractory to anticoagulants and plasma exchange, and his clinical course was catastrophic, with central nervous system symptoms and progressive renal failure. However, factual treatment of intravenous gamma globulin (IVIG) ended the hemolysis and resulted in a rise in platelet count. He fully recovered except for end-stage renal failure, but he underwent a successful renal transplant after peritoneal dialysis. He has not suffered a relapse of TMA or an allograft rejection for 4 years. IVIG might be an option for some patients with TMA of unknown etiology refractory to conventional treatment.

[Acquired idiopathic thrombotic thrombocytopenic purpura: arguments for an autoimmune disease]

Thrombotic thrombocytopenic purpura (TTP) is a severe form of thrombotic microangiopathy (TMA) characterized by systemic platelet clumping, hemolytic anemia, and multiorgan failure. TTP results from a defect in ADAMTS13, a plasma enzyme specifically involved in the cleavage of highly hemostatic unusually large (UL) von Willebrand factor (vWF) multimers into smaller and less adhesive vWF forms. Failure to degrade these UL-vWF multimers leads to excessive platelet aggregation and capillary occlusion. ADAMTS13 deficiency is related to mutations of the encoding gene in hereditary TTP, whereas in acquired forms it results from autoantibodies that may alter the protein function. This latter finding strongly suggests that acquired idiopathic TTP corresponds to an autoimmune disease. Acquired idiopathic TTP appears to be associated with clinical features suggestive of autoimmunity in one third of cases. In two thirds, autoantibodies such as antinuclear antibodies may be observed. This review, based on an analysis of the literature and on French experience with TMA, focuses on the different autoimmune manifestations that may be observed in TTP, as well as the putative pathophysiological link between autoimmune manifestations and TTP.

TTP and ADAMTS13: When Is Testing Appropriate?

The last 10 years witnessed the publication of many studies on the pathophysiology of thrombotic thrombocytopenic purpura (TTP), a life-threatening disease characterized by microangiopathic hemolytic anemia, thrombocytopenia and multiorgan failure. The most important finding was the identification of a novel metalloprotease, named ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motives), that is involved in the regulation of the size of von Willebrand factor (VWF), a major modulator of platelet adhesion and aggregation in the microcirculation. Inherited or acquired deficiencies of ADAMTS13 impair VWF cleavage, leading in turn to the disseminated formation of platelet-rich thrombi in the micro-circulation and to symptoms of end-organ ischemia. By measuring ADAMTS13 in plasma, it has been clearly shown that patients with inherited TTP have severe ADAMTS13 deficiency. However, patients with acquired TTP present with clinical and laboratory heterogeneity, and there are unequivocal cases of acquired TTP with measurable plasma levels of ADAMTS13. This heterogeneity poses a challenge for understanding the pathogenesis of TTP and selecting appropriate therapies.

From humoral rejection to generalized thrombotic microangiopathy--role of acquired ADAMTS13 deficiency in a renal allograft recipient

A 9-year-old renal transplant recipient presented with elevated serum creatinine levels 4 years post-transplant renal biopsy revealed humoral rejection including lesions suggestive for thrombotic microangiopathy (TMA). He received methylprednisolone pulses followed by a normalization of serum creatinine. Two more steroid responsive acute rejection episodes occurred. Two months later he presented rapidly progressive life threatening symptoms including bilateral pyramidal syndrome and hemoptysis. Serum haptoglobin became undetectable at this time and platelet count decreased (70000/microl), suggesting TMA. Cerebral MRI revealed generalized ischemic white matter lesions. ADAMTS13 activity decreased to < 5%. Daily plasma exchanges (PE) resulted in immediate improvement. All attempts to discontinue PE were unsuccessful. Transplantectomy resulted in normalization of generalized symptoms, hemolysis and ADAMTS13 activity (110%). Multi-organ involvement has never been reported in acquired ADAMTS13 deficiency post-transplant. Rapid resolution after transplantectomy might suggest that renal TMA was responsible for acquired ADAMTS13 deficiency and thereby triggered the generalization of TMA lesions.

ADAMTS13 and microvascular thrombosis

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

Inhibitory autoantibodies against ADAMTS-13 in patients with thrombotic thrombocytopenic purpura bind ADAMTS-13 protease and may accelerate its clearance in vivo

BACKGROUND

Many patients with acquired thrombotic thrombocytopenic purpura (TTP) harbor autoantibodies that may bind and/or inhibit ADAMTS-13 proteolytic activity and accelerate its clearance in vivo.

METHODS

To test this hypothesis, we determined ADAMTS-13 activity and antigen levels in parallel plasma samples from patients clinically diagnosed with TTP. Collagen binding, GST-VWF73 and FRETS-VWF73 assays were used to determine ADAMTS-13 activity and to detect inhibitory autoantibodies. Enzyme-linked immunosorbent assay (ELISA) and immunoprecipitation plus Western blotting (IP/WB) were used to detect total anti-ADAMTS-13 IgG (inhibitory and non-inhibitory).

RESULTS

Among 40 patients with TTP (21 idiopathic and 19 non-idiopathic), inhibitory autoantibodies were detected (by FRETS-VWF73) in 52% of idiopathic and 0% of non-idiopathic TTP patients. In contrast, non-inhibitory IgG autoantibodies were detected in 29% of idiopathic and 50% of non-idiopathic TTP patients. The concentration of inhibitory IgG autoantibody in idiopathic TTP patients was significantly higher than that of non-inhibitory IgG in either idiopathic or non-idiopathic TTP patients. Idiopathic TTP patients demonstrated significantly reduced ADAMTS-13 activity compared with non-idiopathic patients, but only slightly lower ADAMTS-13 antigen levels. Interestingly, patients with inhibitory autoantibodies exhibited significantly lower ADAMTS-13 antigen levels than those with only non-inhibitory IgG autoantibodies or no autoantibody. Serial plasma exchanges increased levels of ADAMTS-13 activity and antigen concurrently in patients with inhibitory autoantibodies.

CONCLUSION

The identification of severe ADAMTS-13 deficiency and autoantibodies or inhibitors appears to be assay-dependent; the inhibitory IgG autoantibodies, in addition to binding and inhibiting ADAMTS-13 proteolytic activity, may accelerate ADAMTS-13 clearance in vivo.

Laboratory studies on pathophysiology of the catastrophic antiphospholipid syndrome

The 'catastrophic' variant of the antiphospholipid syndrome (APS) is characterized by a diffuse thrombotic microvasculopathy. In contrast to the classical APS, single venous or arterial medium-to-large blood vessel occlusions are uncommon. The mechanisms of catastrophic APS are not clearly understood. In addition, there are no studies on pathophysiologic mechanisms of catastrophic APS. The clinical manifestations of catastrophic APS probably depend on (a) the organs affected by the thrombotic events and extent of the thrombosis and (b) manifestations of the systemic inflammatory response syndrome which are presumed to be due to excessive cytokine release from affected and necrotic tissues. The evident relationship between APS and infection may enable us to explain the development of catastrophic APS using the sepsis model. This is because catastrophic APS is characterized by multiple microvascular thrombotic events, of rapid onset, and causing multiorgan failure, a picture suggestive of septic shock, in which, there is a massive, acute inflammatory response.

New subsets of the antiphospholipid syndrome in 2006: "PRE-APS" (probable APS) and microangiopathic antiphospholipid syndromes ("MAPS")

The concept of "probable" antiphospholipid syndrome (APS) is almost identical with several conditions which may presage the development of the APS with its major complications of large vessel thromboses resulting in deep vein occlusions in the lower limbs (DVT) particularly and strokes. These conditions comprising livedo reticularis, chorea, thrombocytopenia, fetal loss and valve lesions. These conditions, comprising livedo reticularis, chorea, thrombocytopenia, fetal loss and valve lesions may be followed, often years later by diagnosable APS. The issue whether these patients should be more aggressively treated on presentation in order to prevent the thrombotic complications. A new subset of the APS is proposed viz. microangiopathic antiphospholipid syndrome ("MAPS") comprising those patients presenting with thrombotic microangiopathy and demonstrable antiphospholipid antibodies who may share common although not identical provoking factors (e.g. infections, drugs), clinical manifestations and haematological manifestations (severe thrombocytopenia, hemolytic anaemia) and treatments viz. plasma exchange. Patients without large vessel occlusions may be included in the MAPS subset. These conditions include thrombotic thrombocytopenic purpura (TTP), hemolytic-uremic syndrome (HUS), and the HELLP syndrome. Patients with catastrophic antiphospholipid syndrome (CAPS) who do not demonstrate large vessel occlusions also fall into this group. Disseminated intravascular coagulation (DIC) has also been reported with demonstrable antiphospholipid antibodies and also manifests severe thrombocytopenia and small vessel occlusions. It may cause problems in differential diagnosis.

Molecular biology of ADAMTS13 and diagnostic utility of ADAMTS13 proteolytic activity and inhibitor assays

ADAMTS13, a reprolysin-like metalloprotease, limits platelet-rich thrombus formation in the small arteries by cleaving von Willebrand factor (vWF) at the Tyr1605-Met1606 peptide bond. Deficiency of plasma ADAMTS13 activity, due to either an inherited or an acquired etiology, may lead to a potentially lethal syndrome, thrombotic thrombocytopenic purpura (TTP). Molecular cloning and characterization of the ADAMTS13 gene have provided further insight into the structure-function relationships, biosynthesis, and regulation of the ADAMTS13 protease, in addition to understanding the pathogenesis of TTP and perhaps other thrombotic disorders. ADAMTS13 consists of a short propeptide, a typical reprolysin-like metalloprotease domain, followed by a disintegrin-like domain, first thrombospondin type 1 (TSP1) repeat, Cys-rich domain, and spacer domain. The carboxyl terminus of ADAMTS13 has seven more TSP1 repeats and two CUB domains. ADAMTS13 is synthesized mainly in hepatic stellate cells, but also in vascular endothelial cells. Recognition and cleavage of vWF require the proximal carboxyl terminal domains, but not the middle and distal carboxyl terminal domains. Cleavage of vWF appears to be modulated by shear force, binding to platelet or platelet glycoprotein-1balpha, heparin, inflammatory cytokine (interleukin-6), and chloride ion. At the site of thrombus formation, the ADAMTS13 may be inactivated by thrombin, plasmin, and factor Xa. Having a sensitive and specific assay for ADAMTS13 activity is not only critical to understand the basic biology of ADAMTS13 protease, but also to facilitate a more timely and accurate clinical diagnosis of TTP, and to initiate potentially life-saving plasma exchange therapy. Although many assays have been developed and tested for clinical applications, the fluorescent resonance energy transfer-vWF73 assay appears to be the simplest and most promising assay to date.

The molecular biology of thrombotic microangiopathy

Thrombotic microangiopathy, which includes thrombotic thrombocytopenic purpura (TTP), shiga-toxin-associated hemolytic uremic syndrome (Stx-HUS) and atypical HUS, is characterized by the development of hyaline thrombi in the microvasculature resulting in thrombocytopenia, microangiopathic hemolysis, and organ dysfunction. Renal failure is a predominant complication of both Stx-HUS and atypical HUS, whereas neurological complications are more prominent in TTP. Other disorders such as lupus or bone marrow transplantations may occasionally present with features of thrombotic microangiopathy. Recent studies have found autoimmune inhibitors or genetic mutations of a von Willebrand factor (VWF) cleaving metalloprotease ADAMTS13 in patients with TTP. In approximately 30-50% of patients with atypical HUS, mutations have been detected in complement factor H, membrane cofactor protein (CD46), or factor I. All three proteins are involved in the regulation of complement activation. Additionally, autoantibodies of factor H have been described in patients without genetic mutations. These advances illustrate that dysregulation of VWF homeostasis or complement activation owing to genetic or autoimmune mechanisms may lead to the syndrome of thrombotic microangiopathy.

Prevalence of secondary hematologic disorders in the antiphospholipid syndrome: impact on coagulation risk

Hematologic abnormalities may occur in antiphospholipid antibody syndrome. The most common of these abnormalities is thrombocytopenia. The mechanism by which low platelet counts occur is not known but studies continue to elucidate the potential role of autoantibodies and cell surface markers. The risk for thrombosis in patients with thrombocytopenia does not appear to be related to the presence of thrombocytopenia itself. Innovative treatment with anti-CD20 antibodies may be of benefit in reversing thrombocytopenia and treatment with splenectomy does not appear to increase the risk for subsequent thrombosis. Hemolytic anemia is associated with antiphospholipid antibodies as well, but the risk for thrombosis in patients with hemolytic anemia is not clearly elevated. Diagnostic features of disseminated intravascular coagulation may co-occur with the antiphospholipid syndrome and may be associated with a catastrophic clinical presentation.

Current concepts in thrombotic thrombocytopenic purpura

Recent advances have demonstrated that thrombotic thrombocytopenic purpura (TTP), characterized by widespread thrombosis in the arterioles and capillaries, is caused by deficiency of a circulating zinc metalloprotease, ADAMTS13. Two types of TTP are recognized: autoimmune TTP, caused by inhibitory antibodies of ADAMTS13, and hereditary TTP, caused by genetic mutations of ADAMTS13. This article reviews the characteristics and function of ADAMTS13, the mechanism by which ADAMTS13 deficiency may lead to thrombosis, and the causes of ADAMTS13 deficiency. It also discusses how the new knowledge may improve the diagnosis and treatment of this previously mysterious disorder.