Novel recombinant glycosylphosphatidylinositol (GPI)-anchored ADAMTS13 and variants for assessment of anti-ADAMTS13 autoantibodies in patients with thrombotic thrombocytopenic purpura

Mechanism underlying severe deficiency of plasma ADAMTS-13 activity in immune thrombotic thrombocytopenic purpura

BACKGROUND

Immune-mediated thrombotic thrombocytopenic purpura is caused by autoantibodies against ADAMTS-13, a plasma enzyme that cleaves von Willebrand factor. However, the mechanism resulting in severe deficiency of plasma ADAMTS-13 activity remains controversial.

OBJECTIVES

To determine the mechanism of autoantibody-mediated severe deficiency of plasma ADAMTS13 activity in immune-mediated thrombotic thrombocytopenic purpura.

METHODS

Fluorescence resonance energy transfer-VWF73 was used to determine plasma ADAMTS-13 activity. Enzyme-linked immunosorbent assay (ELISA) was used to determine anti-ADAMTS-13 immunoglobulin G. ELISA and capillary electrophoresis-based Western blotting were employed to assess plasma ADAMTS-13 antigen.

RESULTS

We showed that plasma ADAMTS-13 antigen levels varied substantially in the samples collected on admission despite all showing plasma ADAMTS-13 activity of <10 IU/dL (or <10% of normal level) using either ELISA or Western blotting. More severe deficiency of plasma ADAMTS-13 antigen (<10%) was detected in admission samples by ELISA than by capillary Western blotting. There was a significant but moderate correlation between plasma ADAMTS-13 activity and ADAMTS-13 antigen by either assay method, suggesting that severe deficiency of plasma ADAMTS-13 activity is not entirely associated with low levels of ADAMTS-13 antigen.

CONCLUSION

We conclude that severe deficiency of plasma ADAMTS-13 activity primarily resulted from antibody-mediated inhibition, but the accelerated clearance of plasma ADAMTS-13 antigen via immune complexes may also contribute significantly to severe deficiency of plasma ADAMTS-13 activity in a subset of patients with acute immune-mediated thrombotic thrombocytopenic purpura.

METALLOPROTEASE ADAMTS-13

Introduction. The signifi cance of ADAMTS-13 extends beyond its key role in the pathogenesis of thrombotic thrombocytopenic purpura (TTP); there is evidence of a relationship between a decrease in the ADAMTS-13 activity and thrombotic events in acute myocardial infarction and ischemic stroke.

Aim. To generalise available information on the structure and function of the metalloprotease ADAMTS-13.

General findings. The biological function of ADAMTS-13 consists in the cleavage of ultra-large von Willebrand factor (vWF) multimers. The fact that its defi ciency causes the development of TTP provides a basis for understanding the function of vWF–cleaving protease. ADAMTS-13 has a domain structure. The functional roles of most ADAMTS-13 domains, as well as the key role of the ADAMTS-13-vWF interaction in the regulation of haemostasis, are defi ned. The conformational activation of ADAMTS-13 by vWF constitutes an important aspect of its function. After getting into the bloodstream, ultra-large vWF multimers quickly adopt a closed conformation, which becomes very resistant to ADAMTS-13 proteolysis in the absence of shear stress. Ultra-large plasma vWF multimers regain their sensitivity to ADAMTS-13 after being exposed to high fl uid shear stress, which unfolds the central vWF-A2 domain. The unfolding of a vWF molecule under shear stress conditions reveals previously hidden exosites in domain A2, which gradually increase the binding affi nity between ADAMTS-13 and vWF. The mechanism underlying the production of autoantibodies against ADAMTS-13 is unknown and requires further study. The masking of cryptic epitopes in the closed conformation of ADAMTS-13 prevents the formation of autoantibodies. Early antigen recognition of ADAMTS-13 occurs through surface-exposed epitopes in the C-terminal domains. More detailed information on the mechanisms underlying the interaction between ADAMTS-13 and the vWF can improve the understanding of mechanisms involved in the regulation of the coagulation system.

Conflict of interest: the authors declare no confl ict of interest.

Financial disclosure: the study had no sponsorship.

Transfusion of Platelets Loaded With Recombinant ADAMTS13 (A Disintegrin and Metalloprotease With Thrombospondin Type 1 Repeats-13) Is Efficacious for Inhibiting Arterial Thrombosis Associated With Thrombotic Thrombocytopenic Purpura

Objective- ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats-13) cleaves VWF (von Willebrand factor). This process is essential for hemostasis. Severe deficiency of plasma ADAMTS13 activity, most commonly resulting from autoantibodies against ADAMTS13, causes thrombotic thrombocytopenic purpura. Therapeutic plasma exchange is the standard of care to date, which removes autoantibodies and replenishes ADAMTS13. However, such a therapy is often ineffective to raise plasma ADAMTS13 activity, and in-hospital mortality rate remains as high as 20%. Approach and Results- To overcome the inhibition by autoantibodies, we developed a novel approach by delivering rADAMTS13 (recombinant ADAMTS13 ) using platelets as vehicles. We show that both human and murine platelets can uptake rADAMTS13 ex vivo. The endocytosed rADAMTS13 within platelets remains intact, active, and is stored in α-granules. Under arterial shear (100 dyne/cm²), the rADAMTS13 in platelets is released and effectively inhibits platelet adhesion and aggregation on a collagen-coated surface in a concentration-dependent manner. Transfusion of rADAMTS13-loaded platelets into Adamts13^-/- mice dramatically reduces the rate of thrombus formation in the mesenteric arterioles after FeCl₃ injury. An ex vivo transfusion of rADAMTS13-loaded platelets to a reconstituted whole blood containing plasma from a patient with immune-mediated thrombotic thrombocytopenic purpura and the cellular components (eg, erythrocytes and leukocytes) from a healthy individual, as well as a fresh whole blood obtained from a patient with congenital or immune-mediated thrombotic thrombocytopenic purpura also dramatically reduces the rate of thrombus formation under arterial flow. Conclusions- Our results demonstrate that transfusion of rADAMTS13-loaded platelets may be a novel and potentially effective therapeutic approach for arterial thrombosis, associated with congenital and immune-mediated thrombotic thrombocytopenic purpura.

Human neutrophil peptides and complement factor Bb in pathogenesis of acquired thrombotic thrombocytopenic purpura

Acquired thrombotic thrombocytopenic purpura is primarily caused by the deficiency of plasma ADAMTS13 activity resulting from autoantibodies against ADAMTS13. However, ADAMTS13 deficiency alone is often not sufficient to cause acute thrombotic thrombocytopenic purpura. Infections or systemic inflammation may precede acute bursts of the disease, but the underlying mechanisms are not fully understood. Herein, 52 patients with acquired autoimmune thrombotic thrombocytopenic purpura and 30 blood donor controls were recruited for the study. The plasma levels of human neutrophil peptides 1-3 and complement activation fragments (i.e. Bb, iC3b, C4d, and sC5b-9) were determined by enzyme-linked immunosorbent assays. Univariate analyses were performed to determine the correlation between each biomarker and clinical outcomes. We found that the plasma levels of human neutrophil peptides 1-3 and Bb in patients with acute thrombotic thrombocytopenic purpura were significantly higher than those in the control (P<0.0001). The plasma levels of HNP1-3 correlated with the levels of plasma complement fragment Bb (rho=0.48, P=0.0004) and serum lactate dehydrogenase (rho=0.28, P=0.04); in addition, the plasma levels of Bb correlated with iC3b (rho=0.55, P<0.0001), sC5b-9 (rho=0.63, P<0.0001), serum creatinine (rho=0.42, p=0.0011), and lactate dehydrogenase (rho=0.40, P=0.0034), respectively. Moreover, the plasma levels of iC3b and sC5b-9 were correlated (rho=0.72, P<0.0001), despite no statistically significant difference of the two markers between thrombotic thrombocytopenic purpura patients and the control. We conclude that innate immunity, i.e. neutrophil and complement activation via the alternative pathway, may play a role in the pathogenesis of acute autoimmune thrombotic thrombocytopenic purpura, and a therapy targeted at these pathways may be considered in a subset of these patients.

Platelet-delivered ADAMTS13 inhibits arterial thrombosis and prevents thrombotic thrombocytopenic purpura in murine models

ADAMTS13 metalloprotease cleaves von Willebrand factor (VWF), thereby inhibiting platelet aggregation and arterial thrombosis. An inability to cleave ultralarge VWF resulting from hereditary or acquired deficiency of plasma ADAMTS13 activity leads to a potentially fatal syndrome, thrombotic thrombocytopenic purpura (TTP). Plasma exchange is the most effective initial therapy for TTP to date. Here, we report characterization of transgenic mice expressing recombinant human ADAMTS13 (rADAMTS13) in platelets and its efficacy in inhibiting arterial thrombosis and preventing hereditary and acquired antibody-mediated TTP in murine models. Western blotting and fluorescent resonance energy transfer assay detect full-length rADAMTS13 protein and its proteolytic activity, respectively, in transgenic (Adamts13(-/-)Plt(A13)), but not in wild-type and Adamts13(-/-), platelets. The expressed rADAMTS13 is released on stimulation with thrombin and collagen, but less with 2MesADP. Platelet-delivered rADAMTS13 is able to inhibit arterial thrombosis after vascular injury and prevent the onset and progression of Shigatoxin-2 or recombinant murine VWF-induced TTP syndrome in mice despite a lack of plasma ADAMTS13 activity resulting from the ADAMTS13 gene deletion or the antibody-mediated inhibition of plasma ADAMTS13 activity. These findings provide a proof of concept that platelet-delivered ADAMTS13 may be explored as a novel treatment of arterial thrombotic disorders, including hereditary and acquired TTP, in the presence of anti-ADAMTS13 autoantibodies.

Structure-function and regulation of ADAMTS-13 protease

ADAMTS-13, a plasma reprolysin-like metalloprotease, cleaves von Willebrand factor (VWF). Severe deficiency of plasma ADAMTS-13 activity results in thrombotic thrombocytopenic purpura (TTP), while mild to moderate deficiencies of plasma ADAMTS-13 activity are emerging risk factors for developing myocardial and cerebral infarction, pre-eclampsia, and malignant malaria. Moreover, Adamts13(-/-) mice develop more severe inflammatory responses, leading to increased ischemia/perfusion injury and formation of atherosclerosis. Structure-function studies demonstrate that the N-terminal portion of ADAMTS-13 (MDTCS) is necessary and sufficient for proteolytic cleavage of VWF under various conditions and attenuation of arterial/venous thrombosis after oxidative injury. The more distal portion of ADAMTS-13 (TSP1 2-8 repeats and CUB domains) may function as a disulfide bond reductase to prevent an elongation of ultra-large VWF strings on activated endothelial cells and inhibit platelet adhesion/aggregation on collagen surface under flow. Remarkably, the proteolytic cleavage of VWF by ADAMTS-13 is accelerated by FVIII and platelets under fluid shear stress. A disruption of the interactions between FVIII (or platelet glycoprotein 1bα) and VWF dramatically impairs ADAMTS-13-dependent proteolysis of VWF in vitro and in vivo. These results suggest that FVIII and platelets may be physiological cofactors regulating VWF proteolysis. Finally, the structure-function and autoantibody mapping studies allow us to identify an ADAMTS-13 variant with increased specific activity but reduced inhibition by autoantibodies in patients with acquired TTP. Together, these findings provide novel insight into the mechanism of VWF proteolysis and tools for the therapy of acquired TTP and perhaps other arterial thrombotic disorders.

Identification of epitopes on ADAMTS13 recognized by a panel of monoclonal antibodies with functional or non-functional effects on catalytic activity

INTRODUCTION

von Willebrand factor (VWF) cleavage by ADAMTS13 is mediated by multi-step interactions between their multi-domain structures. To clarify the relationship between inhibitory effects of monoclonal antibodies and epitopes on each ADAMTS13 domain, we analyzed how each ADAMTS13 domain contributes to catalyze VWF using a mouse anti-ADAMTS13 monoclonal antibody panel.

MATERIALS AND METHODS

FRETS-VWF73 assay was used to examine the effects of 14 anti-ADAMTS13 monoclonal antibodies on the catalytic activity of plasma ADAMTS13. Epitope mapping was performed using phage surface display. Libraries expressing peptide fragments of ADAMTS13 were screened with the monoclonal antibodies.

RESULTS

Eleven epitopes of 14 monoclonal antibodies were successfully defined. Three monoclonal antibodies recognizing metalloprotease or disintegrin-like domains strongly inhibited the catalytic activity and their epitopes were on Gln159-Asp166, Tyr 305-Glu327, and Asn308-Glu376. Five monoclonal antibodies recognizing TSP1-3 to -7 repeats showed weak inhibitory effects, and their epitopes were on Pro744-Ala806, Pro856-Cys864, Gln892-Gly940, Cys1007-Cys1072, and Gln1163-Asn1185. Four monoclonal antibodies recognizing the TSP1-1, TSP1-2, CUB1 or CUB2 domains had no inhibitory effects, and their epitopes, except that for TSP1-1, were Pro682-Cys742, Thr1200-Cys1213, and Gln1409-Glu1414. Two monoclonal antibodies recognizing cysteine-rich and spacer domains showed moderate inhibitory effects, but their epitopes were not determined.

CONCLUSIONS

We revealed the epitopes of 11 monoclonal anti-ADAMTS13 antibodies on each of the domains and clarified their association with inhibitory effects on VWF catalysis under static conditions. Catalytic activity correlated strongly with the epitopes on metalloprotease and disintegrin-like domains, weakly with those on TSP1-3 to -7 repeats, and negatively with those on TSP1-1, -2, and CUB domains.