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Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is a rare and life-threatening blood disorder characterized by the formation of blood clots in small blood vessels. It is caused by antibodies targeting the A disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13 (ADAMTS13), which plays a role in cleaving von Willebrand factor. Most patients with iTTP have autoantibodies against specific domains of the ADAMTS13 protein, particularly the cysteine-rich and spacer domains. This study aimed to identify ADAMTS13 muteins that are resistant to autoantibodies and maintain their enzymatic activity. A panel of muteins was generated using rational and random mutagenesis methods and screened for autoantibody binding and ADAMTS13 activity. The selected muteins were assessed for pharmacodynamic biomarkers and pharmacokinetic profiles in the iTTP-mimic and wild-type mice, respectively. GC1126A was the most effective variant for escaping autoantibodies and had a longer half-life than the wild-type ADAMTS13 fragment (MDTCS). In the iTTP-mimic mouse model, GC1126A treatment significantly improved platelet counts, lactate dehydrogenase levels, and ADAMTS13 residual activity. In addition, GC1126A outperformed recombinant human wild-type ADAMTS13 (rh WT-ADAMTS13) and caplacizumab in terms of platelet recovery and sustained effectiveness. Results from the ex vivo study using plasma from patients with iTTP showed that GC1126A exhibited higher residual activity than rh WT-ADAMTS13, particularly in patients with high autoantibody titers. These findings suggest that GC1126A could be a promising new treatment option for patients with iTTP., Competing Interests: Competing interests: HK, GC, SK, YK, YL, CL, and HJN are inventors on a patent application regarding GC1126A. HK, GC, SK, JMP, YK, YL, CL, and HJN were employed by GC Biopharma Corp. The remaining authors declare no competing financial interests., (© 2025. The Author(s).)

Coronavirus disease-19 (COVID-19) vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare but serious clinical condition with high mortality rate in apparently healthy individuals without noticeable risk factors. VITT typically arises due to the administration of vaccines that possess recombinant adenoviral vectors, including ChAdOx1 nCov-19 (AstraZeneca) and Ad26 COV2.S (Johnson & Johnson/Janssen). Thrombosis frequently occurs at atypical sites, such as the cerebral or splanchnic circulations, in this particular pathological state. Similar to heparin-induced thrombotic thrombocytopenia (HITT), it seems that the cause of VITT is the misdirection of anti-platelet factor 4 antibodies (anti-PF4 Abs), an ancient antimicrobial mechanism. Anti-PF4 Abs in patients with VITT activates the coagulation system, leading to thrombosis. This process occurs through the stimulation of platelets (Plts) and neutrophils and subsequently release of neutrophil extracellular traps (NETs). Due to the potentially fatal consequences of VITT, early diagnosis is mandatory. In addition to thrombocytopenia, thrombosis, and the presence of anti-PF4 Abs, the day of symptoms onset and the elevation of D-dimer are also required for definitive diagnosis of VITT. The absence of one or more criteria can result in the exclusion of definitive VITT and lead to the diagnosis of probable, possible, or unlikely VITT., Competing Interests: Declarations. Ethical approval: This article does not contain any studies with human participants or animals performed by any of the authors. Conflict of Interest: The authors declare no competing interests., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare prothrombotic disorder with a unique clonality-restricted immunological profile. The study by Petito and Bury et al. provides insight into the role of HLA polymorphisms and an inherent predisposition to VITT. Commentary on: Petito et al. Association of human leucocyte antigen loci with vaccine-induced immune thrombotic thrombocytopenia: Potential role of the interaction between platelet factor 4-derived peptides and MHC-II. Br J Haematol 2025; 206:290-295., (© 2024 The Author(s). British Journal of Haematology published by British Society for Haematology and John Wiley & Sons Ltd.)

Background and Objectives: Thrombotic thrombocytopenic purpura (TTP) is a potentially fatal thrombotic microangiopathic disorder that can result from human immunodeficiency virus (HIV) infection. The pathogenesis involves a deficiency of the von Willebrand factor (vWF) cleaving protease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin motifs member 13) and the presence of anti-ADAMTS13 autoantibodies. However, there is insufficient information regarding the epitope specificity and reactivity of these autoantibodies. This study aimed to perform epitope-mapping analysis to provide novel insights into the specific epitopes on ADAMTS13 domains affected by autoantibodies., Materials and Methods: The study analysed 59 frozen citrate plasma samples from HIV-associated TTP patients in South Africa, measuring ADAMTS13 activity using Technozyme® ADAMTS13 activity test, total immunoglobulin (Ig) M and IgA antibodies levels using ELISA kit and purifying IgG antibodies using NAb™ Protein G spin columns. A synthetic ADAMTS13 peptide library was used for epitope mapping., Results: Overall, 90% of samples showed anti-ADAMTS13 IgG autoantibodies, with 64% of these antibodies being inhibitory, as revealed by mixing studies. Samples with ADAMTS13 antigen levels below 5% showed high anti-ADAMTS13 IgG autoantibody titres (≥50 IU/mL), whereas those with 5%-10% levels had low autoantibody titres (<50 IU/mL).The metalloprotease, cysteine-rich and spacer domains were 100% involved in binding anti-ADAMTS13 IgG antibodies, with 58% of samples containing antibodies binding to the C-terminal part of the ADAMTS13 disintegrin-like domain, indicating different pathogenic mechanisms., Conclusion: The metalloprotease, cysteine-rich and spacer domains are the primary targets for anti-ADAMTS13 IgG autoantibodies in patients with HIV-associated TTP. These findings suggest potential effects on the proteolytic activity of ADAMTS13, highlighting the complex nature of the pathogenic mechanisms involved., (© 2024 The Author(s). Vox Sanguinis published by John Wiley & Sons Ltd on behalf of International Society of Blood Transfusion.)

Thromboinflammation/immunothrombosis plays a role in several diseases including thrombotic thrombocytopenic purpura (TTP) and COVID-19. Unlike the extensive research that has been conducted on COVID-19 cytokine storms, the baseline and acute phase cytokine profiles of TTP are poorly characterized. Moreover, we compared the cytokine profiles of TTP and COVID-19 to identify the disease-specific/general characteristics of thromboinflammation/immunothrombosis. Plasma concentrations of 33 soluble mediators (SMs: cytokines, chemokines, soluble receptors, and growth factors) were measured by multiplex bead-based LEGENDplex™ immunoassay from 32 COVID-19 patients (32 non-vaccinated patients in three severity groups), 32 TTP patients (remission/acute phase pairs of 16 patients), and 15 control samples. Mainly, the levels of innate immunity-related SMs changed in both diseases. In TTP, ten SMs decreased in both remission and acute phases compared to the control, one decreased, and two increased only in the acute phase compared to remission, indicating mostly anti-inflammatory changes. In COVID-19, ten pro-inflammatory SMs increased, whereas one decreased with increasing severity compared to the control. In severe COVID-19, sixteen SMs exceeded acute TTP levels, with only one higher in TTP. PCA identified CXCL10, IL-1RA, and VEGF as the main discriminators among their cytokine profiles. The innate immune response is altered in both diseases. The cytokine profile of TTP suggests a distinct pathomechanism from COVID-19 and supports referring to TTP as thromboinflammatory rather than immunothrombotic, emphasizing thrombosis over inflammation as the driving force of the acute phase.

Thrombotic thrombocytopenic purpura (TTP) is one of the rare group disorders classified as thrombotic microangiopathies (TMAs). Approximately 90% of TTP developed immune-mediation by the formation of antibodies against the enzyme ADAMTS-13. The exact cause is unknown. To establish an association between human leukocyte antigen (HLA) and autoimmune basis, as susceptibility or protection against the disease, we contributed a study aiming to evaluate the role of HLA in immune-mediated TTP (iTTP). Considering epidemiological factors such as age, sex, ethnicity, and geographical origins, we contributed the study in our country, Turkey, which consist of a very heterogeneous population. Patients' data collection was retrospectively from electronic database on two University hospitals having big therapeutic apheresis service. Control arm was healthy people registered as stem cell donors matched in terms of age and sex. The frequency of HLA-DRB1 and HLA-DQB1 alleles between acquired TTP and the control group was compared using the chi-square method. Yates correction and logistic regression were performed on these results. A total of 75 iTTP patients and 150 healthy individuals enrolled to the study. HLA-DRB1∗11, HLA-DQB1∗03, HLA-DRB1∗11:01, HLA-DRB1∗14:01, HLA-DRB1∗13:05, HLA-DRB1∗11 + HLA-DQB1∗03 allele pair and HLA-DRB1∗15 + HLA- DQB1∗06 were proved to be susceptibility allele pairs for iTTP. HLA-DRB1∗15, HLA-DRB1∗01:01, HLA-DRB1∗07:01, HLA-DRB1∗13:01, HLA-DRB1∗14:54, HLA-DQB1∗05:01, HLA-DQB1∗02:02 and HLA-DRB1∗07 + HLA-DQB1∗02 allele pair were found to be protective against iTTP. Our findings support an association with iTTP across very heterogenous populations in Turkey., (Copyright © 2024 Wolters Kluwer Health, Inc. All rights reserved.)

Background: Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a complication of adenoviral-based vaccine against SARS-CoV-2 due to prothrombotic immunoglobulin (Ig) G antibodies to platelet factor 4 (PF4) and may be difficult to distinguish from heparin-induced thrombocytopenia (HIT) in patients treated with heparin., Objectives: We assessed the usefulness of competitive anti-PF4 enzyme immunoassays (EIAs) in this context., Methods: The ability of F(ab')2 fragments of 1E12, 1C12, and 2E1, 3 monoclonal anti-PF4 antibodies, to inhibit the binding of human VITT or HIT antibodies to PF4 was evaluated using EIAs. Alanine-scanning mutagenesis was performed to define the amino acids involved in the interactions between the monoclonal antibodies and PF4., Results: A strong inhibition of VITT IgG binding to PF4 was measured with 1E12 (median inhibition, 93%; n = 8), whereas it had no effect on the binding of HIT antibodies (median, 6%; n = 8). In contrast, 1C12 and 2E1 inhibited VITT (median, 74% and 76%, respectively) and HIT antibodies (median, 68% and 53%, respectively) binding to PF4. When a competitive anti-PF4 EIA was performed with 1E12 for 19 additional VITT samples, it strongly inhibited IgG binding to PF4, except for 1 patient, who had actually developed HIT according to the clinical history. Epitope mapping showed that 1E12 interacts with 5 key amino acids on PF4, of which 4 are also required for the binding of human VITT antibodies, thus explaining the competitive inhibition., Conclusion: A simple competitive anti-PF4 EIA with 1E12 could help confirm VITT diagnosis and distinguish it from HIT in patients when both diagnoses are possible., Competing Interests: Declaration of competing interests C.V. reports honorarium from Viatris. J.R. reports research grant from Stago. H.G. reports transport fees from Stago. F.M. reports speaker fees from Fresenius, Technoclone, and Werfen, all outside the submitted work. Y.G. reports research grant and symposium fees from Stago. C.P. reports research grant from Stago. All other authors of this paper have no conflicts of interest., (Copyright © 2024 International Society on Thrombosis and Haemostasis. Published by Elsevier Inc. All rights reserved.)

Background: Laboratory diagnosis of immune-mediated thrombotic thrombocytopenic purpura (iTTP) remains challenging when ADAMTS-13 activity ranges between 10% and 20%. To prevent misdiagnosis, open ADAMTS-13 conformation gained clinical attention as a novel biomarker, especially to diagnose acute iTTP in patients with diagnostic undecisive ADAMTS-13 activity. Plasma ADAMTS-13 conformation analysis corrects for ADAMTS-13 antigen, with both parameters being characterized in enzyme-linked immunosorbent assay (ELISA)-based reference assays requiring expert technicians., Objectives: To design ADAMTS-13 antigen and conformation assays on automated, easy-to-use fiber optic surface plasmon resonance (FO-SPR) technology to promote assay accessibility and diagnose challenging iTTP patients., Methods: ADAMTS-13 antigen and conformation assays were designed on FO-SPR technology. Plasma of 20 healthy donors and 20 acute iTTP patients were quantified, and data from FO-SPR and ELISA reference assays were compared., Results: Following assay design, both antigen and conformation FO-SPR assays were optimized and characterized, presenting strong analytical sensitivity (detection limit of 0.001 μg/mL) and repeatability (interassay variation of 14.4%). Comparative analysis suggested positive correlation (Spearman r of 0.92) and good agreement between FO-SPR and ELISA assays. As expected, FO-SPR assays showed a closed or open ADAMTS-13 conformation in healthy donors and acute iTTP patients, respectively., Conclusion: Both ADAMTS-13 antigen and conformation assays were transferred onto automated, easy-to-use FO-SPR technology, displaying potent analytical sensitivity and reproducibility. ADAMTS-13 antigen and conformation were determined for healthy donors and acute iTTP patients showing strong correlation with ELISA reference. Introducing FO-SPR technology in clinical context could support routine diagnosis of acute iTTP patients, notably when ADAMTS-13 activity fluctuates between 10% and 20%., Competing Interests: Declaration of competing interests J.L. is a member of the Board of Directors of FOx Biosystems. Q.B., C.D., T.B., E.T., G.K., B.S.J., A.V., P.C., C.T., S.F.D.M., and K.V. have no conflicts of interest to disclose., (Copyright © 2024 International Society on Thrombosis and Haemostasis. Published by Elsevier Inc. All rights reserved.)

Abstract: Previous studies have demonstrated that >38% of patients with immune-mediated thrombotic thrombocytopenic purpura in remission with activity >50% had an open ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13) conformation. We assessed ADAMTS13 conformation in remission (ADAMTS13 activity >60%), focusing on peak ADAMTS13 activity levels and longitudinal assessment in 420 samples across 157 patients. Fewer cases had an open conformation at peak ADAMTS13 activity than unselected remission samples with ADAMTS13 activity >60% (23% vs 43%). Patients with a closed ADAMTS13 conformation at peak ADAMTS13 activity had an eightfold lower relapse rate in the subsequent year (9% vs 46%) and a fivefold lower relapse rate within 2 years (23% vs 62%) compared with cases with an open conformation. Patients with an open conformation at peak ADAMTS13 activity required preemptive anti-CD20 treatment earlier than those with a closed conformation (median, 10 vs 25 months). Longitudinally, an open conformation was evident at, and often preceded relapse. When the conformation was already open before relapse, an increase in the conformation index at relapse was seen despite the undetectable anti-ADAMTS13 immunoglobulin G (IgG) antibody. In cases with detectable anti-ADAMTS13 IgG antibody, these became undetectable before achieving a closed conformation, highlighting the relapse risk even with undetectable anti-ADAMTS13 IgG antibody and the clinical utility of open/closed during monitoring. To our knowledge, this is the first study to show an association between relapse risk and ADAMTS13 conformation when activity levels are at a peak. The open conformation identifies antibody-mediated subclinical disease that is not detectable by the current ADAMTS13 testing., (© 2024 American Society of Hematology. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.)

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., (Copyright © 2024 Massachusetts Medical Society.)

Pregnancy is a potential trigger of acute thrombotic thrombocytopenic purpura (TTP). The management of pregnancy-associated immune-mediated TTP (iTTP) can be challenging, especially when it is refractory to standard treatment. Caplacizumab, a nanobody to von Willebrand factor (VWF) blocking its A1 domain, is a valuable new therapeutic option. Its use is, however, not approved during pregnancy and breastfeeding. We describe the successful off-label administration of caplacizumab during pregnancy and delivery in a patient with refractory iTTP. The favourable outcome without significant thrombotic or haemorrhagic complications indicates that caplacizumab may be an effective and safe treatment option in refractory iTTP during pregnancy., (© 2024 British Society for Haematology and John Wiley & Sons Ltd.)

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., Competing Interests: Declaration of competing interest X.L.Z. is a consultant for Alexion, Apollo, Sanofi, Takeda, Apollo, GC Biopharma, and Stago and a cofounder of Clotsolution., (Copyright © 2024 International Society on Thrombosis and Haemostasis. Published by Elsevier Inc. All rights reserved.)

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare but dangerous side effect of adenoviral-vectored COVID-19 vaccines. VITT had been linked to production of autoantibodies recognizing platelet factor 4 (PF4). Here, we characterize anti-PF4 antibodies obtained from a VITT patient's blood. Intact mass measurements indicate that a significant fraction of these antibodies represent a limited number of clones. MS analysis of large antibody fragments (the light chain and the Fc/2 and Fd fragments of the heavy chain) confirms the monoclonal nature of this component of the anti-PF4 antibodies repertoire and reveals the presence of a mature complex biantennary N -glycan within the Fd segment. Peptide mapping using two complementary proteases and LC-MS/MS was used to determine the amino acid sequence of the entire light chain and over 98% of the heavy chain (excluding a short N-terminal segment). The sequence analysis allows the monoclonal antibody to be assigned to the IgG2 subclass and verifies that the light chain belongs to the λ-type. Incorporation of enzymatic de- N -glycosylation into the peptide mapping routine allows the N -glycan in the Fab region of the antibody to be localized to the framework 3 region of the V H domain. This novel N -glycosylation site is the result of a single mutation within the germline sequence. Peptide mapping also provides information on lower-abundance (polyclonal) components of the anti-PF4 antibody ensemble, revealing the presence of all four subclasses (IgG1-IgG4) and both types of the light chain (λ and κ). This case study demonstrates the power of combining the intact, middle-down, and bottom-up MS approaches for meaningful characterization of ultralow quantities of pathogenic antibodies extracted directly from patients' blood.

Both qualitative and quantitative platelet abnormalities are common in patients with coronavirus disease 2019 (COVID-19) and they correlate with clinical severity and mortality. Activated platelets contribute to the prothrombotic state in COVID-19 patients. Several groups have shown immune-mediated activation of platelets in critically ill COVID-19 patients. Vaccine-induced immune thrombotic thrombocytopenia is an autoimmune condition characterized by thrombocytopenia and life-threatening thrombotic events in the arterial and venous circulation. Although the initial trigger has yet to be determined, activation of platelets by immune complexes through Fc gamma RIIA results in platelet consumption and thrombosis. A better understanding of platelet activation in COVID-19 as well as in vaccine-induced thrombotic complications will have therapeutic implications. In this review, we focused on the role of immune-mediated platelet activation in thrombotic complications during COVID-19 infection and vaccine-induced immune thrombotic thrombocytopenia., Competing Interests: TB has received research funding from CoaChrom Diagnostica GmbH, DFG, Robert Bosch GmbH, Stiftung Transfusionsmedizin und Immunhämatologie e.V.: Ergomed, Surrey, DRK Blutspendedienst, Deutsche Herzstiftung, Ministerium fuer Wissenschaft, Forschung und Kunst Baden-Wuerttemberg, has received lecture honoraria from Aspen Germany GmbH, Bayer Vital GmbH, Bristol-Myers Squibb GmbH & Co., Doctrina Med AG, Meet The Experts Academy UG, Schoechl medical education GmbH, Mattsee, Stago GmbH, Mitsubishi Tanabe Pharma GmbH, Novo Nordisk Pharma GmbH, has provided consulting services to: Terumo, has provided expert witness testimony relating to heparin induced thrombocytopenia (HIT) and non‐HIT thrombocytopenic and coagulopathic disorders. All of these are outside the current work. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Uzun, Pelzl, Singh and Bakchoul.)

Cardiovascular disease is a leading cause of death in survivors of immune-mediated thrombotic thrombocytopenic purpura (iTTP), but the epidemiology of major adverse cardiovascular events (MACE) in iTTP survivors is unknown. We evaluated the prevalence and risk factors for MACE, defined as the composite of non-fatal or fatal myocardial infarction (MI), stroke, and cardiac revascularization, during clinical remission in two large iTTP cohorts (Johns Hopkins University and Ohio State University). Of 181 patients followed for ≥ 3 months after recovery from acute iTTP, 28.6% had a MACE event over a median follow up of 7.6 years. Stroke was the most common type of MACE (18.2%), followed by non-fatal MI (6.6%), cardiac revascularization (4.9%) and fatal MI (0.6%). Compared to the general United States population, iTTP survivors were younger at first stroke in remission (males [56.5 years vs. 68.6 years, p = 0.031], females [49.7 years vs. 72.9 years, p <  0.001]) or MI in remission (males [56.5 years vs. 65.6 years, p <  0.001] and females [53.1 years vs. 72.0 years, p < 0.001]). Age (HR 1.03 [95% CI 1.002-1.054]), race (Black/Other vs. White) (HR 2.32 [95% CI 1.12-4.82]), and diabetes mellitus (HR 2.37 [95% CI 1.09-0.03]) were associated with MACE in a Cox regression model also adjusted for sex, hypertension, obesity, hyperlipidemia, chronic kidney disease, atrial fibrillation, autoimmune disease, and relapsing iTTP. Remission ADAMTS13 activity was not significantly associated with MACE. In conclusion, iTTP survivors experience high rates of MACE and may benefit from aggressively screening for and managing cardiovascular risk factors., (© 2021 Wiley Periodicals LLC.)

Immune thrombotic thrombocytopenic purpura (iTTP) is an autoimmune disorder of which the etiology is not fully understood. Autoantibodies targeting ADAMTS13 in iTTP patients have extensively been studied, the immunological mechanisms leading to the breach of tolerance remain to be uncovered. This review addresses the current knowledge on genetic factors associated with the development of iTTP and the interplay between the patient's immune system and environmental factors in the induction of autoimmunity against ADAMTS13. HLA-DRB1*11 has been identified as a risk factor for iTTP in the Caucasian population. Interestingly, HLA-DRB1*08:03 was recently identified as a risk factor in the Japanese population. Combined in vitro and in silico MHC class II peptide presentation approaches suggest that an ADAMTS13-derived peptide may bind to both HLA-DRB1*11 and HLA-DRB1*08:03 through different anchor-residues. It is apparent that iTTP is associated with the presence of infectious microorganisms, viruses being the most widely associated with development of iTTP. Infections may potentially lead to loss of tolerance resulting in the shift from immune homeostasis to autoimmunity. In the model we propose in this review, infections disrupt the epithelial barriers in the gut or lung, promoting exposure of antigen presenting cells in the mucosa-associated lymphoid tissue to the microorganisms. This may result in breach of tolerance through the presentation of microorganism-derived peptides that are homologous to ADAMTS13 on risk alleles for iTTP., Competing Interests: NG and JV are inventors on a patent-application describing novel compounds for the treatment of immune TTP. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Laghmouchi, Graça and Voorberg.)

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is associated with high titers of immunoglobulin G class antibodies directed against the cationic platelet chemokine platelet factor 4 (PF4). These antibodies activate platelets via FcγIIa receptors. VITT closely resembles heparin-induced thrombocytopenia. Inflammation and tissue trauma substantially increase the risk for forming pathogenic PF4 antibodies. We therefore propose the use of therapeutic plasma exchange as rescue therapy in VITT to deplete antibodies plus factors promoting inflammation such as excess cytokines in the circulation as well as extracellular vesicles derived from activated platelets., (Copyright © 2021 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Vaccination using the adenoviral vector COVID-19 vaccine ChAdOx1 nCoV-19 (AstraZeneca) has been associated with rare vaccine-induced immune thrombotic thrombocytopenia (VITT). Affected patients test strongly positive in platelet factor 4 (PF4)/polyanion enzyme immunoassays (EIAs), and serum-induced platelet activation is maximal in the presence of PF4. We determined the frequency of anti-PF4/polyanion antibodies in healthy vaccinees and assessed whether PF4/polyanion EIA+ sera exhibit platelet-activating properties after vaccination with ChAdOx1 nCoV-19 (n = 138) or BNT162b2 (BioNTech/Pfizer; n = 143). In total, 19 of 281 participants tested positive for anti-PF4/polyanion antibodies postvaccination (All: 6.8% [95% confidence interval (CI), 4.4-10.3]; BNT162b2: 5.6% [95% CI, 2.9-10.7]; ChAdOx1 nCoV-19: 8.0% [95% CI, 4.5% to 13.7%]). Optical densities were mostly low (between 0.5 and 1.0 units; reference range, <0.50), and none of the PF4/polyanion EIA+ samples induced platelet activation in the presence of PF4. We conclude that positive PF4/polyanion EIAs can occur after severe acute respiratory syndrome coronavirus 2 vaccination with both messenger RNA- and adenoviral vector-based vaccines, but many of these antibodies likely have minor (if any) clinical relevance. Accordingly, low-titer positive PF4/polyanion EIA results should be interpreted with caution when screening asymptomatic individuals after vaccination against COVID-19. Pathogenic platelet-activating antibodies that cause VITT do not occur commonly following vaccination., (© 2021 by The American Society of Hematology.)

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., (© 2021 by The American Society of Hematology.)

Objectives: Prevention and therapy of immunothrombosis remain crucial challenges in the management of coronavirus disease 2019, since the underlying mechanisms are incompletely understood. We hypothesized that endothelial damage may lead to substantially increased concentrations of von Willebrand factor with subsequent relative deficiency of a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13)., Design: Prospective controlled cross-over trial., Setting: Blood samples of patients with confirmed coronavirus disease 2019 and healthy controls were obtained in three German hospitals and analyzed in a German hemostaseologic laboratory., Patients: Seventy-five patients with confirmed coronavirus disease 2019 of mild to critical severity and 30 healthy controls., Measurements and Main Results: von Willebrand factor antigen, ADAMTS13, and von Willebrand factor multimer formation were analyzed. von Willebrand factor antigen was 4.1 times higher in COVID-19 patients compared with healthy controls (p < 0.0001), whereas ADAMTS13 activities were not significantly different (p = 0.18). The ADAMTS13/von Willebrand factor antigen ratio was significantly lower in COVID-19 than in the control group (24.4 ± 20.5 vs 82.0 ± 30.7; p < 0.0001). Fourteen patients (18.7%) undercut a critical ratio of 10 as described in thrombotic thrombocytopenic purpura. Gel analysis of multimers resembled a thrombotic thrombocytopenic purpura pattern with loss of the largest multimers in 75% and a smeary triplet pattern in 39% of the patients. The ADAMTS13/von Willebrand factor antigen ratio decreased continuously from mild to critical disease (analysis of variance p = 0.026). Furthermore, it differed significantly between surviving patients and those who died from COVID-19 (p = 0.001) yielding an area under the curve of 0.232 in receiver operating characteristic curve curve analysis., Conclusion: COVID-19 is associated with a substantial increase in von Willebrand factor levels, which can exceed the ADAMTS13 processing capacity resulting in the formation of large von Willebrand factor multimers indistinguishable from thrombotic thrombocytopenic purpura. The ADAMTS13/von Willebrand factor antigen ratio is an independent predictor of severity of disease and mortality. These findings provide a rationale to consider plasma exchange as a therapeutic option in COVID-19 and to include von Willebrand factor and ADAMTS13 in the diagnostic workup., Competing Interests: Dr. Witzke received funding from Amgen, Alexion, Astellas, Basilea, Biotest, Bristol-Myers Squibb, Correvio, Chiesi, Gilead, Hexal, and Janssen; he is supported by an unrestricted grant of the Rudolf-Ackermann-Stiftung (Stiftung für Klinische Infektiologie). Dr. Bachmann received support for article research from the National Institutes of Health and the Department of Defense. The remaining authors have disclosed that they do not have any potential conflicts of interest., (Copyright © 2021 by the Society of Critical Care Medicine and Wolters Kluwer Health, Inc. All Rights Reserved.)

IgG4 autoimmune diseases (IgG4-AID) are an emerging group of autoimmune diseases that are caused by pathogenic autoantibodies of the IgG4 subclass. It has only recently been appreciated, that members of this group share relevant immunobiological and therapeutic aspects even though different antigens, tissues and organs are affected: glomerulonephritis (kidney), pemphigus vulgaris (skin), thrombotic thrombocytopenic purpura (hematologic system) muscle-specific kinase (MuSK) in myasthenia gravis (peripheral nervous system) and autoimmune encephalitis (central nervous system) to give some examples. In all these diseases, patients' IgG4 subclass autoantibodies block protein-protein interactions instead of causing complement mediated tissue injury, patients respond favorably to rituximab and share a genetic predisposition: at least five HLA class II genes have been reported in individual studies to be associated with several different IgG4-AID. This suggests a role for the HLA class II region and specifically the DRβ1 chain for aberrant priming of autoreactive T-cells toward a chronic immune response skewed toward the production of IgG4 subclass autoantibodies. The aim of this review is to provide an update on findings arguing for a common pathogenic mechanism in IgG4-AID in general and to provide hypotheses about the role of distinct HLA haplotypes, T-cells and cytokines in IgG4-AID., Competing Interests: JT and ST have shares in the research and diagnostic laboratory Tzartos NeuroDiagnostics, Athens. FL discloses having received speaker honoraria from Grifols, Teva, Biogen, Bayer, Roche, Novartis, Fresenius, travel funding from Merck, Grifols, and Bayer and serving on advisory boards for Roche, Biogen, and Alexion. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Koneczny, Yilmaz, Lazaridis, Tzartos, Lenz, Tzartos, Tüzün and Leypoldt.)

Background: Checkpoint inhibitors enhance T-lymphocyte-mediated antitumor responses, resulting in increased survival for patients with neoplastic disease. However, a subset of patients receiving checkpoint inhibitor therapy may experience adverse complications that include the development of autoimmune conditions, such as thrombotic thrombocytopenic purpura (TTP). Given the potential etiologic differences of checkpoint inhibitor-related autoimmunity, TTP that develops in the presence of checkpoint inhibitors may be refractory to current treatment methods and therefore may require additional treatment and prognostic consideration., Case Report: Herein, we describe the unique clinical course of a patient who was treated with the combined checkpoint inhibitors nivolumab and ipilimumab for Stage IV malignant melanoma, who subsequently developed TTP. Unlike many patients with TTP, this patient failed to develop a sustained response to therapeutic plasma exchange. Additional use of steroids, anti-CD20, and plasma cell-targeting therapy (bortezomib) also failed to substantially reverse thrombocytopenia in a sustainable fashion. During this time, her melanoma progressed, and she ultimately succumbed., Conclusion: This case illustrates not only that TTP may be a potential complication of checkpoint inhibitor therapy, but also that TTP developing in this setting may result in an unpredictable response to commonly employed TTP treatment modalities. Ultimately, checkpoint inhibitor-related TTP may require distinct management approaches and prognostic considerations., (© 2020 AABB.)

Objectives: Acquired thrombotic thrombocytopenic purpura (aTTP) is a rare autoimmune disorder characterized by auto-antibodies to Willebrand factor (vWF) cleaving enzyme (ADAMTS13), resulting in unusually large vWF multimers that lead to platelet aggregation, microthrombi formation and microangiopathic hemolytic anemia. Hemolysis in aTTP is mechanical; thus, direct antiglobulin test (Coombs test) is usually negative. Multiple autoimmune conditions and various auto-antibodies have been described in the context of chronic myelomonocytic leukemia (CMML). In this paper, we describe the first case of CMML with auto-antibodies to ADAMTS13, presenting initially as plasmapheresis-refractory Coombs-positive aTTP. Results: Although our patient was not treated for CMML, a complete remission of aTTP was eventually achieved with rituximab. Conclusion; We propose that aTTP should be in the differential diagnosis of CMML patients with thrombocytopenia and anemia (Coombs positive or not) who develop signs of thrombotic microangiopathy. Further studies are much needed to decipher the immune-mediated processes in CMML.

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is a rare autoimmune disorder caused by neutralizing anti-ADAMTS13 autoantibodies. In white individuals, HLA allele DRB1*11 is a predisposing factor for iTTP, whereas DRB1*04 is a protective factor. However, the role of HLA in Asians is unclear. In this study, we analyzed 10 HLA loci using next-generation sequencing in 52 Japanese patients with iTTP, and the allele frequency in the iTTP group was compared with that in a Japanese control group. We identified the following HLA alleles as predisposing factors for iTTP in the Japanese population: DRB1*08:03 (odds ratio [OR], 3.06; corrected P [Pc] = .005), DRB3/4/5*blank (OR, 2.3; Pc = .007), DQA1*01:03 (OR, 2.25; Pc = .006), and DQB1*06:01 (OR,: 2.41; Pc = .003). The estimated haplotype consisting of these 4 alleles was significantly more frequent in the iTTP group than in the control group (30.8% vs 6.0%; Pc < .001). DRB1*15:01 and DRB5*01:01 were weak protective factors for iTTP (OR, 0.23; Pc = .076; and OR, 0.23, Pc = .034, respectively). On the other hand, DRB1*11 and DRB1*04 were not associated with iTTP in the Japanese. These findings indicated that predisposing and protective factors for iTTP differ between Japanese and white individuals. HLA-DR molecules encoded by DRB1*08:03 and DRB1*11:01 have different peptide-binding motifs, but interestingly, bound to the shared ADAMTS13 peptide in an in silico prediction model., (© 2020 by The American Society of Hematology.)

We herein report a patient with mixed connective tissue disease (MCTD) who had been stable for years but suddenly developed thrombotic thrombocytopenic purpura (TTP). The patient showed a clinical pentad of signs of TTP, low activity of ADAMTS13, and positivity of anti-ADAMTS13 antibodies. She did not respond to plasma exchange or steroid therapy and died five days after admission. An autopsy revealed microthrombi in the brain, heart, kidney, adrenal glands, esophageal submucosa, and bone marrow as well as diffuse alveolar hemorrhaging. Physicians should bear in mind that TTP can occur in MCTD patients regardless of disease activity.

Antibodies are specialized proteins generated by immune system for high specificity and affinity binding to target antigens. Because of their essential roles in immune system, antibodies have been successfully developed and engineered as biopharmaceuticals for treatment of various diseases. Analysis of antibody-protein interactions is always required to get detailed information on effectivity of such antibody-based therapeutics. Although physicochemical rules cannot be generalized for every antibody-protein interaction, there are some features which should be taken into account during antibody development and engineering efforts. In this chapter, physicochemical analysis of antibody paratope-protein epitope interactions will be discussed to highlight important characteristics. First, paratope and non-paratope regions of antibodies will be described and important roles of these regions on binding and biophysical features of antibodies will be discussed. Then, general features of epitope regions of protein antigens will be introduced along with several computational/experimental tools to identify them. Lastly, a rising star of antibody biopharmaceuticals, nanobodies, will be described to show importance of next-generation antibody fragment based biopharmaceuticals in drug development., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Immune thrombotic thrombocytopenic purpura (iTTP) is primarily caused by immunoglobulin G (IgG)-type autoantibodies that bind and inhibit plasma ADAMTS13 activity and/or accelerate its clearance from circulation. Approximately 50% of patients with iTTP who achieve initial clinical response to therapy experience recurrence (ie, exacerbation and/or relapse); however, a reliable biomarker that predicts such an event is currently lacking. The present study determines the role of longitudinal assessments of plasma ADAMTS13 biomarkers in predicting iTTP exacerbation/recurrence. Eighty-three unique iTTP patients with 97 episodes from the University of Alabama at Birmingham Medical Center between April 2006 and June 2019 were enrolled. Plasma levels of ADAMTS13 activity, antigen, and anti-ADAMTS13 IgG on admission showed no significant value in predicting iTTP exacerbation or recurrence. However, persistently low plasma ADAMTS13 activity (<10 U/dL; hazard ratio [HR], 4.4; 95% confidence interval [CI], 1.6-12.5; P = .005) or high anti-ADAMTS13 IgG (HR, 3.1; 95% CI, 1.2-7.8; P = .016) 3 to 7 days after the initiation of therapeutic plasma exchange was associated with an increased risk for exacerbation or recurrence. Furthermore, low plasma ADAMTS13 activity (<10 IU/dL; HR, 4.8; 95% CI, 1.8-12.8; P = .002) and low ADAMTS13 antigen (<25th percentile; HR, 3.3; 95% CI, 1.3-8.2; P = .01) or high anti-ADAMTS13 IgG (>75th percentile; HR, 2.6; 95% CI, 1.0-6.5; P = .047) at clinical response or remission was also predictive of exacerbation or recurrence. Our results suggest the potential need for a more aggressive approach to achieve biochemical remission (ie, normalization of plasma ADAMTS13 activity, ADAMTS13 antigen, and anti-ADAMTS13 IgG) in patients with iTTP to prevent the disease recurrence., (© 2019 by The American Society of Hematology.)

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disease related to the formation of microvascular thrombosis and subsequent organ failure. The disease is accompanied with microangiopathic haemolytic anaemia, consumptive thrombocytopenia and lies on a severe deficiency in ADAMTS13, the von Willebrand factor-cleaving protease. In the acquired, immune-mediated form, this deficiency is due to the production of autoantibodies directed against the enzyme. Therapeutic plasma exchange has been used empirically for decades and still represents the cornerstone of TTP treatment. However, a better understanding of pathophysiological mechanisms underlying the disease has led these last years to the development of highly effective targeted therapies that might in the future restraint the use of therapeutic plasma exchange., (Copyright © 2019 Elsevier Masson SAS. All rights reserved.)

Introduction: Immune thrombotic thrombocytopenic purpura (iTTP) is an immune-mediated deficiency in von Willebrand factor-cleaving protease ADAMTS13 allowing unrestrained adhesion of von Willebrand factor multimers to platelets and microthrombosis. Caplacizumab, an anti-von Willebrand factor humanized, bivalent single-domain nanobody preventing its binding to the platelet has been investigated and approved for use in the treatment of iTTP. Areas covered: The purpose of this article is to summarize the available clinical data on the efficacy and safety of caplacizumab in iTTP and to provide our opinion on the place of caplacizumab in current treatment regimens. Expert opinion: Caplacizumab is a new drug with a complementary mechanism of action with respect to the standard available therapeutics. It demonstrated efficacy in clinical trials through a faster platelet count normalization and protection of patients from exacerbations and refractoriness. Caplacizumab is well tolerated with minor bleeds as the most important side effect. The efficacy of caplacizumab now needs to be assessed in real-life but definitely, this drug opens hope for a significant improvement in iTTP prognosis at the very early, critical step of the disease.

Background: Thrombotic thrombocytopenic purpura (TTP) is an ultra-rare disorder in childhood and belongs to the microangiopathic hemolytic anemias (MAHA) and the thrombotic microangiopathies (TMA). In the acquired form, autoantibodies against ADAMTS13 inhibit cleaving of von Willebrand factor (vWF) multimers, thereby promoting their interaction with thrombocytes, causing TMA and MAHA. A recently introduced nanobody, caplacizumab, inhibits the binding of platelets to vWF., Case-Diagnosis/treatment: During a first episode, a 10-year-old girl was admitted for TTP. Plasma exchange (PE) and immunosuppressive therapy with corticosteroids and mycophenolate mofetil were initiated. The course was complicated by catheter-associated septicemia and a very slow hematological and clinical recovery. Platelet count became normal at day 40 after admission and treatment initiation. Three years later, the child presented again with TTP. During this second episode, caplacizumab was introduced together with PE and immunosuppressive therapy within 4 days after admission. With this regimen, platelet count normalized within 3 days of treatment, and PE treatment could be stopped after a total of 14 days. The child could be discharged and caplacizumab was continued on an outpatient basis until day 30 after initiation. Adverse events during the use of caplacizumab were not encountered., Conclusions: Caplacizumab treatment was safe and effective in a child with relapsing, autoantibody-mediated TTP. With respect to this potentially life-threatening condition, the add-on use of caplacizumab represents a novel option to reduce morbidity and mortality and improve quality of life in children and adolescents with TTP.

Acquired thrombotic thrombocytopenia purpura (aTTP) is caused by autoantibody-mediated severe deficiency of the von Willebrand factor (vWF) cleaving protease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13), with subsequent accumulation of ultra-large vWF-multimers that spontaneously form platelet-VWF complexes and microthrombi within the microcirculation. Therapeutic plasma exchange (TPE), by removing autoantibodies and excess ultra-large vWF multimers and replenishing ADAMTS13 activity, remains the urgent primary initial treatment. Although heterogeneity in treatment exists, most centers add upfront immunosuppression with steroids, and many also add upfront rituximab. Refractoriness, exacerbation and relapse are commonly treated with adjunct rituximab. Despite adjunct steroids and rituximab, TTP refractoriness, exacerbation, relapse, morbidity, and mortality remain problematic. Newer adjunct therapies include suppression of ADAMTS13 autoantibody production via plasma cell depletion, inhibition of vWF-platelet interaction, and replenishment of ADAMTS13 function with recombinant ADAMTS13 protein., (© 2019 AABB.)

A young Indian female visited hospital as a suspected case of thrombotic thrombocytopenic purpura (TTP) with relapsed thrombotic complications with low platelet counts, infarct in middle cerebral artery and thrombi in microvessels. We first confirmed the deficiency of ADAMTS13 metalloprotease in this patient showing improper cleavage of vWF multimers by her plasma unlike her parents and brother. Although patient had very less ADAMTS13 antigen in plasma, but it did not appear to be the cause of deficiency of the enzyme, because her father had similarly low antigen level and he never had prothrombotic complications. While investigating the genetic change in ADAMTS13, we observed four homozygous-SNPs (g.420T>C, g.1342C>G, g.1716G>A and g.2280T>C) in exon 5, 12, 15 and 19 respectively in patient and her father unlike the heterozygous form of same SNPs in mother and brother. Further to investigate the cause of ADAMTS13 deficiency, we observed an elevated level of antibody against ADAMTS13 in patient unlike her father and other family members. Our study therefore provides the molecular approach of diagnosis of TTP in this patient and also highlights the use of such techniques in India. More importantly, study provides the clue of alternate treatment such as immunosuppressant therapy to this patient., (Copyright © 2019 Elsevier Inc. All rights reserved.)

In autoantibody-mediated autoimmune diseases, autoantibody profiling allows patients to be stratified and links autoantibodies with disease severity and outcome. However, in immune-mediated thrombotic thrombocytopenic purpura (iTTP) patients, stratification according to antibody profiles and their clinical relevance has not been fully explored. We aimed to develop a new type of autoantibody profiling assay for iTTP based on the use of anti-idiotypic antibodies. Anti-idiotypic antibodies against 3 anti-spacer autoantibodies were generated in mice and were used to capture the respective anti-spacer idiotopes from 151 acute iTTP plasma samples. We next deciphered these anti-spacer idiotope profiles in iTTP patients and investigated whether these limited idiotope profiles could be linked with disease severity. We developed 3 anti-idiotypic antibodies that recognized particular idiotopes in the anti-spacer autoantibodies II-1, TTP73 or I-9, that are involved in ADAMTS13 binding; 35%, 24% and 42% of patients were positive for antibodies with the II-1, TTP73 and I-9 idiotopes, respectively. Stratifying patients according to the corresponding 8 anti-spacer idiotope profiles provided a new insight into the anti-spacer II-1, TTP73 and I-9 idiotope profiles in these patients. Finally, these limited idiotope profiles showed no association with disease severity. We successfully developed 3 anti-idiotypic antibodies that allowed us to determine the profiles of the anti-spacer II-1, TTP73 and I-9 idiotopes in iTTP patients. Increasing the number of patients and/or future development of additional anti-idiotypic antibodies against other anti-ADAMTS13 autoantibodies might allow idiotope profiles of clinical, prognostic value to be identified., (Copyright© 2019 Ferrata Storti Foundation.)

Background: Autoimmune thrombocytopenia in immune thrombocytopenic purpura (ITP), thrombotic thrombocytopenic purpura (TTP), and heparin-induced thrombocytopenia (HIT) is associated with immunologic degradation of platelets and reduced platelet counts in patients, leading to bleeding risk in patients. Considering the role of human leukocyte antigens (HLA) in the development of immune response, in this review, we examine the relationship between HLA and pathogenesis of the above-mentioned diseases., Methods: Relevant English-language literature was searched and retrieved from Google Scholar search engine and PubMed database (1979 to 2018). The following keywords were used: "Immune Thrombocytopenic purpura," "Thrombotic Thrombocytopenic Purpura," Human Leukocyte Antigen," and "Heparin-induced thrombocytopenia.", Results: In autoimmune thrombocytopenia, HLA molecule presents self-antigens or foreign antigens similar to self-antigens, provoking an immune response against platelets that results in the degradation of platelets in peripheral blood and possible bleeding in the patient. For example, HLA-DRB1 *11 presents the self-antigen and induces an immune response against ADAMTS13, which is associated with thrombocytopenia in TTP patients., Conclusions: HLA alleles can be used as prognostic biomarkers for immunologic disorders of platelet such as ITP, TTP, and HIT. Different DRB1 alleles enable the assessment of resistance to common ITP treatments as well as disease prognosis. Due to the genetic association between HLA-DR1 and HLA-DQ1 alleles and the role of HLA-DRB1 *11 in TTP, the HLA-DQB1 *02: 02 allele may also play a role in TTP pathogenesis.

Pentraxin-3 (PTX3) and C-reactive protein (CRP) have been shown to regulate complement activation in vitro , but their role has not been investigated in complement consumption in vivo . Thrombotic microangiopathies (TMA) are often accompanied by complement overactivation and consumption, therefore we analyzed the relation of the systemic pentraxin levels to the complement profile, laboratory parameters and clinical outcome of TMA patients. We determined the PTX3 and CRP levels, complement factor and activation product concentrations in blood samples of 171 subjects with the diagnosis of typical hemolytic uremic syndrome (STEC-HUS) ( N = 34), atypical HUS (aHUS) ( N = 44), secondary TMA ( N = 63), thrombotic thrombocytopenic purpura (TTP) ( N = 30) and 69 age-matched healthy individuals. Clinical data, blood count and chemistry were collected from medical records. To determine the in vitro effect of PTX3 on alternative pathway (AP) activation, sheep red blood cell-based hemolytic assay and AP activity ELISA were used. We found that PTX3 levels were elevated in the acute phase of STEC-HUS, aHUS and secondary TMA, whereas PTX3 elevation was exceptional is TTP. Conversely, a significantly higher median CRP was present in all patient groups compared to controls. PTX3, but not CRP was associated with signs of complement consumption in vivo , and PTX3 significantly decreased the AP hemolytic activity in vitro . Our results provide a detailed description of acute phase-TMA patients' complement profile linked to changes in the systemic pentraxin levels that may support further molecular studies on the function of PTX3 in disease pathogenesis and add to the laboratory assessment of complement consumption in TMA.

Thrombotic microangiopathy (TMA) refers to phenotypically similar disorders, including hemolytic uremic syndromes (HUS) and thrombotic thrombocytopenic purpura (TTP). This review explores the role of the influenza virus as trigger of HUS or TTP. We conducted a literature survey in PubMed and Google Scholar using HUS, TTP, TMA, and influenza as keywords, and extracted and analyzed reported epidemiological and clinical data. We identified 25 cases of influenza-associated TMA. Five additional cases were linked to influenza vaccination and analyzed separately. Influenza A was found in 83%, 10 out of 25 during the 2009 A(H1N1) pandemic. Two patients had bona fide TTP with ADAMTS13 activity <10%. Median age was 15 years (range 0.5-68 years), two thirds were male. Oligoanuria was documented in 81% and neurological involvement in 40% of patients. Serum C3 was reduced in 5 out of 14 patients (36%); Coombs test was negative in 7 out of 7 and elevated fibrin/fibrinogen degradation products were documented in 6 out of 8 patients. Pathogenic complement gene mutations were found in 7 out of 8 patients tested (C3, MCP, or MCP combined with CFB or clusterin). Twenty out of 24 patients recovered completely, but 3 died (12%). Ten of the surviving patients underwent plasma exchange (PLEX) therapy, 5 plasma infusions. Influenza-mediated HUS or TTP is rare. A sizable proportion of tested patients demonstrated mutations associated with alternative pathway of complement dysregulation that was uncovered by this infection. Further research is warranted targeting the roles of viral neuraminidase, enhanced virus-induced complement activation and/or ADAMTS13 antibodies, and rational treatment approaches.