Modifying ADAMTS13 to modulate binding of pathogenic autoantibodies of patients with acquired thrombotic thrombocytopenic purpura

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

ABSTRACT

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

Anti-ADAMTS13 Autoantibodies in Immune-Mediated Thrombotic Thrombocytopenic Purpura

Autoantibodies to ADAMTS13 are at the center of pathology of the immune-mediated thrombotic thrombocytopenic purpura. These autoantibodies can be either inhibitory (enzymatic function) or non-inhibitory, resulting in protein depletion. Under normal physiologic conditions, antibodies are generated in response to foreign antigens, which can include infectious agents; however, these antibodies may at times cross-react with self-epitopes. This is one of the possible mechanisms mediating formation of anti-ADAMTS13 autoantibodies. The process known as "antigenic mimicry" may be responsible for the development of these autoantibodies that recognize and bind cryptic epitopes in ADAMTS13, disrupting its enzymatic function over ultra large von Willebrand factor multimers, forming the seeds for platelet activation and microthrombi formation. In particular, specific amino acid sequences in ADAMTS13 may lead to conformational structures recognized by autoantibodies. Generation of these antibodies may occur more frequently among patients with a genetic predisposition. Conformational changes in ADAMTS13 between open and closed states can also constitute the critical change driving either interactions with autoantibodies or their generation. Nowadays, there is a growing understanding of the role that autoantibodies play in ADAMTS13 pathology. This knowledge, especially of functional qualitative differences among antibodies and the ADAMTS13 sequence specificity of such antibodies, may make possible the development of targeted therapeutic agents to treat the disease. This review aims to present what is known of autoantibodies against ADAMTS13 and how their structure and function result in disease.

Vaccine Molecule Design Based on Phage Display and Computational Modeling against Rhabdovirus

Rhabdoviruses with rich species lead a variety of high lethality and rapid transmission diseases to plants and animals around the globe. Vaccination is one of the most effective approaches to prevent and control virus disease. However, the key antigenic epitopes of glycoprotein being used for vaccine development are unclear. In this study, fish-derived Abs are employed for a Micropterus salmoides rhabdovirus (MSRV) vaccine design by phage display and bioinformatics analysis. We constructed an anti-MSRV phage Ab library to screen Abs for glycoprotein segment 2 (G2) (G129-266). Four M13-phage-displayed Abs (Ab-5, Ab-7, Ab-8 and Ab-30) exhibited strong specificity to target Ag, and Ab-7 had the highest affinity with MSRV. Ab-7 (300 μg/ml) significantly increased grass carp ovary cell viability to 83.40% and significantly decreased the titer of MSRV. Molecular docking results showed that the key region of Ag-Ab interaction was located in 10ESQEFTTLTSH20 of G2. G2Ser11 and G2Gln12 were replaced with alanine, respectively, and molecular docking results showed that the Ag-Ab was nonbinding (ΔG > 0). Then, the peptide vaccine KLH-G210-20 was immunized to M. salmoides via i.p. injection. ELISA result showed that the serum Ab potency level increased significantly (p < 0.01). More importantly, the challenge test demonstrated that the peptide vaccine elicited robust protection against MSRV invasion, and the relative percentage survival reached 62.07%. Overall, this study proposed an approach for screening key epitope by combining phage display technology and bioinformatics tools to provide a reliable theoretical reference for the prevention and control of viral diseases.

Novel ADAMTS13 mutations in a patient with congenital thrombotic thrombocytopenic purpura

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

Impact of N-glycan mediated shielding of ADAMTS-13 on the binding of pathogenic antibodies in immune thrombotic thrombocytopenic purpura

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is a rare thrombotic disorder, with 1.5 to 6.0 cases per million per year. The majority of patients with TTP develop inhibitory autoantibodies that predominantly target the spacer domain of ADAMTS-13. ADAMTS-13 is responsible for cleaving von Willebrand factor (VWF) multimers, thereby regulating platelet adhesion at sites of high-vascular shear stress. Inhibition and/or clearance of ADAMTS-13 by pathogenic autoantibodies results in accumulation of VWF multimers that promotes the formation of platelet-rich microthrombi. Previously, we have shown that insertion of a single N-glycan (NGLY) in the spacer domain prevents the binding of antispacer domain antibodies.

OBJECTIVES

To explore whether NGLY mediated shielding of the ADAMTS-13 spacer domain effectively prevents binding of pathogenic antispacer autoantibodies in patients with immune-mediated TTP (iTTP).

METHODS

We screened 5 NGLY-ADAMTS-13 variants (NGLY3, NGLY7, NGLY8, NGLY3+7, and NGLY3+8) for binding of autoantibodies and for their activity in the presence and absence of 50 samples derived from patients with iTTP.

RESULTS

NGLY variants showed greatly reduced antibody binding, down to 27% of wild-type (wt) ADAMTS-13 binding. Moreover, NGLY variants of ADAMTS-13 remained more active in FRETS-VWF73 assay in the presence of the plasma samples from these 50 patients with acute phase iTTP when compared with wtADAMTS-13. On average, wtADAMTS-13 activity was reduced to 37% of regular levels in the presence of plasma, while NGLY3 and NGLY3+7 remained 69% and 81% active, respectively.

CONCLUSION

These results reinforce our previous findings that NGLYs shield ADAMTS-13 from antibody binding and hence restore ADAMTS-13 activity in the presence of autoantibodies.

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

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

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

BACKGROUND

Recombinant human ADAMTS13 (rADAMTS13) is a key protein in fundamental research for investigating its mode of action and the pathophysiology of thrombotic thrombocytopenic purpura (TTP). However, the expression of rADAMTS13 is quite low in mammalian cells, which makes the production of the protein time-consuming and labor-intensive.

OBJECTIVES

We aimed at increasing the yield of rADAMTS13 by (1) using a more optimal signal peptide (SP) and (2) constructing an N-terminal fusion protein of ADAMTS13 with human serum albumin domain 1 (AD1-ADAMTS13).

METHODS

Six SPs were investigated to select the most optimal SP. Expression plasmids containing the most optimal SP and ADAMTS13 cDNA or the fusion construct AD1-ADAMTS13 were generated and transiently transfected into CHOEBNALT85 cell-line. Expression levels of rADAMTS13 in expression medium were analyzed and compared with the expression level of rADAMTS13 with native SP (nat-SP).

RESULTS

Expression of rADAMTS13 with coagulation factor VII (FVII) SP was 3-fold higher (16.00 μg/ml) compared with the expression with nat-SP (5.03 μg/ml). The highest yields were obtained with AD1-ADAMTS13 protein with a 15-fold higher concentration (78.22 μg/ml) compared with the expression with nat-SP. The rADAMTS13 expressed with FVII-SP retained its activity (104.0%) to cleave von Willebrand factor, whereas AD1-ADAMTS13 demonstrated even higher activity (144.3%).

CONCLUSION

We succeeded in generating expression vectors that yield (1) rADAMTS13 at higher levels because of more optimal FVII-SP and (2) high levels of AD1-ADAMTS13 N-terminal fusion protein. The highest expression levels were obtained with AD1-ADAMTS13 N-terminal fusion protein, which is paving the way for highly efficient protein production.

ADAMTS13 conformations and mechanism of inhibition in immune thrombotic thrombocytopenic purpura

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

Basic science research opportunities in thrombosis and hemostasis: Communication from the SSC of the ISTH

Bleeding and thrombosis are major clinical problems with high morbidity and mortality. Treatment modalities for these diseases have improved in recent years, but there are many clinical questions remaining and a need to advance diagnosis, management, and therapeutic options. Basic research plays a fundamental role in understanding normal and disease processes, yet this sector has observed a steady decline in funding prospects thereby hindering support for studies of mechanisms of disease and therapeutic development opportunities. With the financial constraints faced by basic scientists, the ISTH organized a basic science task force (BSTF), comprising Scientific and Standardization Committee subcommittee chairs and co-chairs, to identify research opportunities for basic science in hemostasis and thrombosis. The goal of the BSTF was to develop a set of recommended priorities to build support in the thrombosis and hemostasis community and to inform ISTH basic science programs and policy making. The BSTF identified three principal opportunity areas that were of significant overarching relevance: mechanisms causing bleeding, innate immunity and thrombosis, and venous thrombosis. Within these, five fundamental research areas were highlighted: blood rheology, platelet biogenesis, cellular contributions to thrombosis and hemostasis, structure-function protein analyses, and visualization of hemostasis. This position paper discusses the importance and relevance of these opportunities and research areas, and the rationale for their inclusion. These findings have implications for the future of fundamental research in thrombosis and hemostasis to make transformative scientific discoveries and tackle key clinical questions. This will permit better understanding, prevention, diagnosis, and treatment of hemostatic and thrombotic conditions.

TTP: From empiricism for an enigmatic disease to targeted molecular therapies

The 100th anniversary of the first description of Thrombotic Thrombocytopenic Purpura (TTP) as a disease by Dr. Eli Moschcowitz approaches. For many decades, TTP remained mostly a mysterious fatal condition, where diagnosis was often post-mortem. Initially a pentad of symptoms was identified, a pattern that later revealed to be fallible. Sporadic observations led to empiric interventions that allowed for the first impactful breakthrough in TTP treatment, almost 70 years after its first description: the introduction of plasma exchange and infusions as treatments. The main body of knowledge within the field was gathered in the latest three decades: patient registries were set and proved crucial for advancements; the general mechanisms of disease have been described; the diagnosis was refined; new treatments and biomarkers with improvements on prognosis and management were introduced. Further changes and improvements are expected in the upcoming decades. In this review, we provide a brief historic overview of TTP, as an illustrative example of the success of translational medicine enabling to rapidly shift from a management largely based on empiricism to targeted therapies and personalized medicine, for the benefit of patients. Current management options and present and future perspectives in this still evolving field are summarized.

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.

Best practices and recommendations for drug regimens and plasma exchange for immune thrombotic thrombocytopenic purpura

INTRODUCTION

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening thrombotic microangiopathy characterized by microangiopathic hemolytic anemia, thrombocytopenia, and organ injury. TTP pathophysiology is based on a severe ADAMTS13 deficiency, and is a medical emergency with fatal outcome if appropriate treatment is not initiated promptly.

AREAS COVERED

Authors will review the best options currently available to minimize mortality, prevent relapses, and obtain the best clinical response in patients with immune TTP (iTTP). Available bibliography about iTTP treatment has been searched in Library's MEDLINE/PubMed database from January 1990 until April 2021.

EXPERT OPINION

The generalized use of plasma exchange marked a paradigm in the management of iTTP. In recent years, strenuous efforts have been done for a better understanding of the pathophysiology of this disease, improve diagnosis, optimize treatment, reduce mortality, and prevent recurrences. The administration of front-line rituximab and, more recently, the availability of caplacizumab, the first targeted therapy for iTTP, have been steps toward a further reduction in early mortality and for the prevention of relapses.

N-glycan-mediated shielding of ADAMTS13 prevents binding of pathogenic autoantibodies in immune-mediated TTP

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is an autoimmune disorder caused by the development of autoantibodies targeting different domains of ADAMTS13. Profiling studies have shown that residues R568, F592, R660, Y661, and Y665 within exosite-3 of the spacer domain provide an immunodominant region of ADAMTS13 for pathogenic autoantibodies that develop in patients with iTTP. Modification of these 5 core residues with the goal of reducing autoantibody binding revealed a significant tradeoff between autoantibody resistance and proteolytic activity. Here, we employed structural bioinformatics to identify a larger epitope landscape on the ADAMTS13 spacer domain. Models of spacer-antibody complexes predicted that residues R568, L591, F592, K608, M609, R636, L637, R639, R660, Y661, Y665, and L668 contribute to an expanded epitope within the spacer domain. Based on bioinformatics-guided predictions, we designed a panel of N-glycan insertions in this expanded epitope to reduce the binding of spacer domain autoantibodies. One N-glycan variant (NGLY3-ADAMTS13, containing a K608N substitution) showed strongly reduced reactivity with TTP patient sera (28%) as compared with WT-ADAMTS13 (100%). Insertion of an N-glycan at amino acid position 608 did not interfere with processing of von Willebrand factor, positioning the resulting NGLY3-ADAMTS13 variant as a potential novel therapeutic option for treatment of iTTP.

A critical evaluation of caplacizumab for the treatment of acquired thrombotic thrombocytopenic purpura

Introduction: Acquired thrombotic thrombocytopenic purpura (aTTP) is a thrombotic microangiopathy caused by inhibitory autoantibodies against ADAMTS13 protein. Until recently, the combination of plasma exchange (PEX) and immunosuppression has been the standard front-line treatment in this disorder. However, aTTP-related mortality, refractoriness, and relapse are still a matter of concern. Areas covered: The better understanding of the pathophysiological mechanisms of aTTP has allowed substantial improvements in the diagnosis and treatment of this disease. Recently, the novel anti-VWF nanobody caplacizumab has been approved for acute episodes of aTTP. Caplacizumab is capable to block the adhesion of platelets to VWF, therefore inhibiting microthrombi formation in the ADAMTS13-deficient circulation. In this review, the characteristics of caplacizumab together with the available data of its efficacy and safety in the clinical setting will be analyzed. Besides, the current scenario of aTTP treatment will be provided, including the role of other innovative drugs. Expert opinion: With no doubt, caplacizumab is going to change the way we treat aTTP. In combination with standard treatment, caplacizumab can help to significantly reduce aTTP-related mortality and morbidity and could spare potential long-term consequences by minimizing the risk of exacerbation.