A Platelet Factor 4-Dependent Platelet Activation Assay Facilitates Early Detection of Pathogenic Heparin-Induced Thrombocytopenia Antibodies

Abstract Background: Some recipients of ChAdOx1 nCoV-19 COVID-19 Vaccine AstraZeneca develop antibody-mediated vaccine-induced thrombotic thrombocytopenia (VITT), associated with cerebral venous and other unusual thrombosis resembling autoimmune heparin-induced thrombocytopenia. A prothrombotic predisposition is also observed in Covid-19. We explored whether antibodies against the SARS-CoV-2 spike protein induced by Covid-19 cross-react with platelet factor 4 (PF4/CXLC4), the protein targeted in both VITT and autoimmune heparin-induced thrombocytopenia.Methods: Immunogenic epitopes of PF4 and SARS-CoV-2 spike protein were compared via prediction tools and 3D modelling software (IMED, SIM, MacMYPOL). Sera from 222 PCR-confirmed Covid-19 patients from five European centers were tested by PF4/heparin ELISA, heparin-dependent and PF4-dependent platelet activation assays. Immunogenic reactivity of purified anti-PF4 and anti-PF4/heparin antibodies from patients with VITT were tested against recombinant SARS-CoV-2 spike protein. Results: Three motifs within the spike protein sequence share a potential immunogenic epitope with PF4. Nineteen of 222 (8.6%) Covid-19 patient sera tested positive in the IgG-specific PF4/heparin ELISA, none of which showed platelet activation in the heparin-dependent activation assay, including 10 (4.5%) of the 222 Covid-19 patients who developed thromboembolic complications. Purified anti-PF4 and anti-PF4/heparin antibodies from two VITT patients did not show cross-reactivity to recombinant SARS-CoV-2 spike protein. Conclusions: The antibody responses to PF4 in SARS-CoV-2 infection and after vaccination with COVID-19 Vaccine AstraZeneca differ. Antibodies against SARS-CoV-2 spike protein do not cross-react with PF4 or PF4/heparin complexes through molecular mimicry. These findings make it very unlikely that the intended vaccine-induced immune response against SARS-CoV-2 spike protein would itself induce VITT.

Download Full-text

Pathophysiology of Heparin-Induced Thrombocytopenia

Archives of Pathology & Laboratory Medicine ◽

10.5858/2000-124-1657-pohit ◽

2000 ◽

Vol 124 (11) ◽

pp. 1657-1666 ◽

Cited By ~ 4

Author(s):

Fabrizio Fabris ◽

Sarfraz Ahmad ◽

Giuseppe Cella ◽

Walter P. Jeske ◽

Jeanine M. Walenga ◽

...

Keyword(s):

Platelet Activation ◽

Serotonin Release ◽

Platelet Factor 4 ◽

Thrombin Inhibitors ◽

Platelet Factor ◽

Cellular Activation ◽

Heparin Induced Thrombocytopenia ◽

New Information ◽

Study Selection ◽

Release Assay

Abstract Objective.—This review of heparin-induced thrombocytopenia (HIT), the most frequent and dangerous side effect of heparin exposure, covers the epidemiology, pathophysiology, clinical presentation, diagnosis, and treatment of this disease syndrome. Data Sources and Study Selection.—Current consensus of opinion is given based on literature reports, as well as new information where available. A comprehensive analysis of the reasons for discrepancies in incidence numbers is given. The currently known mechanism is that HIT is mediated by an antibody to the complex of heparin–platelet factor 4, which binds to the Fc receptor on platelets. New evidence suggests a functional heterogeneity in the anti-heparin-platelet factor 4 antibodies generated to heparin, and a “superactive” heparin-platelet factor 4 antibody that does not require the presence of heparin to promote platelet activation or aggregation has been identified. Up-regulation of cell adhesion molecules and inflammatory markers, as well as preactivation of platelets/endothelial cells/leukocytes, are also considered to be related to the pathophysiology of HIT. Issues related to the specificity of currently available and new laboratory assays that support a clinical diagnosis are addressed in relation to the serotonin-release assay. Past experience with various anticoagulant treatments is reviewed with a focus on the recent successes of thrombin inhibitors and platelet GPIIb/IIIa inhibitors to combat the platelet activation and severe thrombotic episodes associated with HIT. Conclusions.—The pathophysiology of HIT is multifactorial. However, the primary factor in the mediation of the cellular activation is due to the generation of an antibody to the heparin-platelet factor 4 complex. This review is written as a reference for HIT research.

Download Full-text

Platelet Activation in Heparin-Induced Thrombocytopenia is Followed by Platelet Death via Complex Apoptotic and Non-Apoptotic Pathways

International Journal of Molecular Sciences ◽

10.3390/ijms21072556 ◽

2020 ◽

Vol 21 (7) ◽

pp. 2556

Author(s):

Elmira R. Mordakhanova ◽

Tatiana A. Nevzorova ◽

Gulnaz E. Synbulatova ◽

Lubica Rauova ◽

John W. Weisel ◽

...

Keyword(s):

Platelet Activation ◽

Immune Complexes ◽

Gel Filtration ◽

Human Platelets ◽

Platelet Factor 4 ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia ◽

Mitochondrial Membrane Depolarization ◽

Low Platelet Count ◽

Apoptotic Pathways

Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction characterized by thrombocytopenia and a high risk for venous or arterial thrombosis. HIT is caused by antibodies that recognize complexes of platelet factor 4 and heparin. The pathogenic mechanisms of this condition are not fully understood. In this study, we used flow cytometry, fluorimetry, and Western blot analysis to study the direct effects of pathogenic immune complexes containing platelet factor 4 on human platelets isolated by gel-filtration. HIT-like pathogenic immune complexes initially caused pronounced activation of platelets detected by an increased expression of phosphatidylserine and P-selectin. This activation was mediated either directly through the FcγRIIA receptors or indirectly via protease-activated receptor 1 (PAR1) receptors due to thrombin generated on or near the surface of activated platelets. The immune activation was later followed by the biochemical signs of cell death, such as mitochondrial membrane depolarization, up-regulation of Bax, down-regulation of Bcl-XL, and moderate activation of procaspase 3 and increased calpain activity. The results show that platelet activation under the action of HIT-like immune complexes is accompanied by their death through complex apoptotic and calpain-dependent non-apoptotic pathways that may underlie the low platelet count in HIT.

Download Full-text

Functional Microparticles Are up Regulated in Patients with Anti-Heparin/Platelet Factor 4 Antibodies: A Potential Mechanism of Thrombogenesis in the Heparin-Induced Thrombocytopenia Syndrome.

Blood ◽

10.1182/blood.v110.11.2105.2105 ◽

2007 ◽

Vol 110 (11) ◽

pp. 2105-2105

Author(s):

Josephine Cunanan ◽

Michelle Kujawski ◽

He Zhu ◽

Margaret Prechel ◽

Jeanine Walenga ◽

...

Keyword(s):

Platelet Activation ◽

Medical Center ◽

Annexin V ◽

Platelet Factor 4 ◽

Cellular Damage ◽

Clinical Samples ◽

Limiting Factor ◽

Therapeutic Effects ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia

Abstract Heparin-induced thrombocytopenia (HIT) is one of the most catastrophic adverse effects of heparin therapy, representing a complex syndrome involving immunopathologic and hemostatic disorders. Vascular and blood cellular damage results in the generation of microparticles (MP). These MP are formed from stress conditions/cellular disruption and apoptosis. Cellular MP mediated pathophysiologic responses include platelet activation, up regulation of adhesion molecules, monocyte activation, up regulation of tissue factor and endothelial dysfunction. Several methods based on flow cytometric and other immunologic probes have been used to measure MP in the HIT syndrome. Recently, a functional method based on the complexation of MP with annexin V promoting the generation of factor Xa and thrombin has become available (Hyphen Biomedical, Neuville-Oise, France). To validate the hypothesis that functional MP are elevated in the HIT syndrome, this method was utilized for the quantitation of MP in sera ELISA positive for anti-heparin/platelet factor 4 (HIT) antibodies. Specimens (n = 53) were selected from archived samples that had been referred to Loyola University Medical Center for the laboratory diagnosis of HIT by quantitating anti-heparin/PF4 antibodies by ELISA and by evaluating HIT antibody induced platelet activation using the 14C Serotonin Release Assay (SRA). All selected specimens were positive for HIT antibodies in the GTI PF4 Enhanced ELISA with a broad range of antibody titers (absorbance range of 0.4 – 2.5). Eleven of these specimens were positive in the SRA. In addition, serial samples from HIT patients treated with argatroban (from the ARG-911 clinical study) were included (n = 23). The normal samples represented control sera obtained from healthy human volunteers (n = 25) and processed in the same manner as the clinical samples. Test samples were added to microtiter plates coated with streptavidin and biotinylated annexin V. MP present in the test sample bound to annexin V via exposed surface phospholipids. Following incubation and washing steps, a FXa – FVa mixture containing calcium and prothrombin was added. The assay was optimized so that MP associated phospholipid was the limiting factor for the generation of thrombin. In normal non-HIT sera, the MP levels ranged 5.6 – 10.1 nM (6.1 ± 2.8 nM). The pre-treatment, baseline levels of circulating MP in the suspected HIT patients ranged from 4.2 – 26.8 nM (15.8 ± 7.3 nM). Interestingly, SRA positive/ELISA positive samples had relatively higher levels of MP (19.9 ± 7.7 nM; range 11.5 – 29.8 nM) than SRA negative/ELISA positive samples (14.2± 4.6; range 6.8–21.2). In the ARG-911 study, sequential blood samples exhibited MP levels at the baseline ranging from 8.2 – 38.6 nM (21.8 ± 10.8 nM), whereas after 3 days of argatroban treatment were reduced to 5.1 – 19.2 nM (12.6 ± 6.3). The results of these studies suggest that circulating functional MP are increased in patients with ELISA positive HIT antibodies. Anticoagulation with such direct thrombin agents as argatroban effectively decreases the circulating functional MP levels. Since the elevated MP levels may mediate thrombin and FXa generation, the therapeutic effects of these drugs in HIT may be related to the decreased activation of coagulation and related thrombogenic processes.

Download Full-text

Influence of Protein Tyrosine Phosphatase Polymorphisms on the Development of Antibodies to Platelet Factor 4 and the Risk of Heparin-Induced Thrombocytopenia.

Blood ◽

10.1182/blood.v116.21.3683.3683 ◽

2010 ◽

Vol 116 (21) ◽

pp. 3683-3683

Author(s):

Jerôme Rollin ◽

Claire Pouplard ◽

Dorothee Leroux ◽

Marc-Antoine May ◽

Yves Gruel

Keyword(s):

Immune Response ◽

Platelet Activation ◽

Conflicts Of Interest ◽

Critical Role ◽

Platelet Factor 4 ◽

Control Group ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia ◽

Protein Tyrosine ◽

Significant Difference

Abstract Abstract 3683 Introduction. Heparin-induced thrombocytopenia (HIT) results from an atypical immune response to platelet factor 4/heparin complexes (PF4/H), with rapid synthesis of platelet-activating IgG antibodies that activate platelets via FcgRIIa receptors. The reasons explaining why only a subset of patients treated with heparin develop IgG to PF4/H complexes, and why most patients who synthesize these antibodies do not develop HIT, have not been fully defined. The immune response in HIT involves both B and T cells, and protein tyrosine kinases (PTKs) and phosphatases (PTPs) are crucial for regulating antigen receptor-induced lymphocyte activation. Moreover, some PTPs such as CD148 and low-molecular-weight PTP (LMW-PTP) could also have a critical role in platelet activation. Dysregulation of the equilibrium between PTK and PTP function could therefore have pathologic consequences and influence the pathogenesis of HIT. Aim of the study. To investigate an association between polymorphisms affecting genes encoding 4 different PTPs i.e. CD45 (PTPRC), CD148 (PTPRJ), LYP (PTPN22) and LMW-PTP (ACP1) and the development of heparin-dependent antibodies to PF4 and HIT. Patients and methods. A cohort of 89 patients with definite HIT (positive PF4-specific ELISA and positive serotonin release assay) and two control groups were studied. The first control group (Abneg) consisted of 179 patients who had undergone cardiopulmonary bypass (CBP) with high doses of heparin and who did not develop Abs to PF4 post-operatively. The second control group (Abpos) consisted of 160 patients who had also undergone cardiac surgery with CPB and heparin, who had all developed significant levels of PF4-specific antibodies but without HIT. Genotypes of PTPRC 77C/G (rs17612648), PTPN22 1858C/T (rs2476601), PTPRJ 2965 C/G (rs4752904) and PTPRJ 1176 A/C (rs1566734) were studied by a PCR-HRM method using the LightCycler 480 (Roche). In addition, the ACP1 A, B, C alleles were defined by combining the analysis of T/C transition at codon 43 of exon 3 (rs11553742) and T/C transition at codon 41 of exon 4 (rs11553746). Results. The frequency of PTPRC 77G and PTPN22 1858T alleles was not different in HIT patients and controls, whether they had developed antibodies to PF4 or not. The third PTP gene analyzed was ACP1, in which three alleles (A, B and C) were previously associated with the synthesis of distinct active LMW-PTP isoforms exhibiting different catalytic properties. The percentage of subjects in our study carrying the AC, BB and BC genotypes was significantly higher in the HIT and the Abpos groups than in patients without antibodies to PF4 after CPB (Abneg). In addition, the ACP1 A allele was less frequent in patients with antibodies to PF4, whether they had developed HIT (25%) or not (27.5% in Abpos controls), than in Abneg subjects (37%). The AC, BB and BC genotypes (associated in Caucasians with the highest LMW-PTP enzyme activity) therefore appeared to increase the risk of antibody formation in heparin-treated patients (OR 1.8; 95% CI 1.2–2.6, p=0.004 after comparing Abpos + HIT vs. Abneg). We also evaluated 2 SNPs affecting PTPRJ encoding CD148. No significant difference was found concerning the 2965 C/G polymorphism, but the frequency of PTPRJ 1176 AC and CC genotypes was significantly lower in the HIT (17%) than in the Abneg and Abpos groups (35%, p=0.003 and 29.5%, p=0.041, respectively). The C allele therefore appeared to provide a significant protection from the risk of HIT (OR 0.52; 95%CI 0.29–0.94, p=0.041) in patients with antibodies to PF4. Discussion-Conclusion. Recent studies have demonstrated that CD148 is a positive regulator of platelet activation by maintaining a pool of active SFKs in platelets. This non-synonym PTPRJ 1176 A/C SNP is associated with a Q276P substitution inducing a torsional stress of a fibronectin domain that is critical for the activity of CD148 and may influence the pathogenic effects of HIT Abs. This study supports the hypothesis that PTPs such as LMW-PTP and CD148 influence the immune response to heparin and the risk of HIT in patients with antibodies to PF4. Disclosures: No relevant conflicts of interest to declare.

Download Full-text

Spontaneous Heparin-Induced Thrombocytopenia Syndrome: Two New Cases and a Proposal For Defining This Disorder

Blood ◽

10.1182/blood.v122.21.2328.2328 ◽

2013 ◽

Vol 122 (21) ◽

pp. 2328-2328 ◽

Cited By ~ 1

Author(s):

Theodore E. Warkentin ◽

Paul Andrew Basciano ◽

Richard A. Bernstein

Keyword(s):

Platelet Count ◽

Platelet Activation ◽

Research Funding ◽

Left Vertebral Artery ◽

Platelet Factor ◽

Shoulder Hemiarthroplasty ◽

Heparin Induced Thrombocytopenia ◽

Activation Assay ◽

Count Recovery ◽

Heparin Exposure

Abstract Introduction Heparin-induced thrombocytopenia (HIT) is a transient, autoimmune-like, prothrombotic disorder caused by heparin-dependent, platelet-activating IgG reactive against platelet factor 4/heparin (PF4/H). There is an emerging literature (Am J Med 2008;121:632-6. J Thromb Haemost 2008;6:1598-1600; Thromb Haemost 2013;109:669-75) pointing to rare instances of “spontaneous” HIT in patients without preceding heparin. We report 2 new cases and propose a definition for this controversial disorder. CASE #1. A 62-y.o. man presented with left middle cerebral artery stroke and thrombocytopenia (platelet count, 65×109/L). There was no previous history of thrombocytopenia, surgery, hospitalization, or heparin exposure. Clot extraction performed with heparin was complicated by further platelet count decline to 27 (nadir) and progressive thrombosis of the carotid artery. Aspirin was started, and the platelets recovered to >150 by day 13. CASE #2. A 54-y.o. female developed right leg swelling, left-upper extremity weakness/paresthesias, and thrombocytopenia (61×109/L) 15 days post-shoulder hemiarthroplasty; no intra-/postoperative heparin had been given. Brain MRI demonstrated acute infarct in the left posterior inferior cerebellar artery territory; angiography showed non-visualization of the left vertebral artery. Ultrasound revealed right lower-limb deep-vein thrombosis. Heparin treatment resulted in further platelet count fall to 37 (nadir). Treatment with argatroban, followed by fondaparinux, was associated with platelet count recovery to >150 by day 39. Methods Testing for HIT antibodies was performed by commercial EIA-IgG/A/M (Immucor GTI Diagnostics), in-house EIA-IgG (McMaster), and serotonin-release assay (SRA). Results Both patients’ sera (obtained before any heparin administration) tested strongly positive for HIT antibodies (Table), including strong platelet activation at 0.1 and 0.3 IU/mL heparin, as well as at 0 U/mL heparin, with no platelet activation at 100 IU/mL heparin: these serological features are characteristic of “delayed-onset HIT” (Ann Intern Med 2001;135:502-6). Antibody reactivity declined markedly by 2 to 4 weeks (including loss of platelet-activating properties at 0 IU/mL heparin), in keeping with the usual transience of HIT antibodies (N Engl J Med 2001;344:1286-92), and paralleling both patients’ platelet count recovery. Discussion These cases further support spontaneous HIT as an unusual explanation for acute arterial stroke and thrombocytopenia. One patient had preceding orthopedic surgery, an event previously reported with spontaneous HIT (Thromb Haemost 2013;109:669-75). The strong serum-dependent platelet activation at 0 IU/mL heparin helps to explain how thrombocytopenia and thrombosis can occur in a patient not receiving heparin. RECOMMENDATION. Based on the serological findings of these and previous cases, we propose that a definitive diagnosis of spontaneous HIT syndrome should be based upon all of the following criteria: thrombocytopenia, thrombosis, lack of proximate heparin exposure, strong-positive PF4-dependent immunoassay(s), and a strong-positive platelet activation assay featuring both heparin-dependent (e.g., high heparin neutralization) and heparin-independent platelet activation (at 0 IU/mL heparin). Disclosures: Warkentin: Pfizer Canada: Honoraria; Paringenix: Consultancy; Immucor GTI Diagnostics: Research Funding; WL Gore: Consultancy; GSK: Research Funding.

Download Full-text

Affinity purification of heparin-dependent antibodies to platelet factor 4 developed in heparin-induced thrombocytopenia: biological characteristics and effects on platelet activation

British Journal of Haematology ◽

10.1046/j.1365-2141.2000.02034.x ◽

2000 ◽

Vol 109 (2) ◽

pp. 336-341 ◽

Cited By ~ 49

Author(s):

J. Amiral ◽

C. Pouplard ◽

A. M. Vissac ◽

J. M. Walenga ◽

W. Jeske ◽

...

Keyword(s):

Platelet Activation ◽

Affinity Purification ◽

Platelet Factor 4 ◽

Biological Characteristics ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia

Download Full-text

A Prothrombotic Thrombocytopenic Disorder Resembling Heparin-Induced Thrombocytopenia Following Coronavirus-19 Vaccination

10.21203/rs.3.rs-362354/v2 ◽

2021 ◽

Author(s):

Andreas Greinacher ◽

Thomas Thiele ◽

Theodore E. Warkentin ◽

Karin Weisser ◽

Paul Kyrle ◽

...

Keyword(s):

Platelet Activation ◽

Adenoviral Vector ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia ◽

Post Vaccination ◽

Receptor Blocking ◽

Vein Thrombosis ◽

Activation Assay ◽

Laboratory Features ◽

High Concentrations

Abstract Background. Vaccines are important for managing the COVID-19 pandemic caused by SARS-CoV-2. However, following widespread vaccination using a recombinant adenoviral vector encoding the spike protein antigen of SARS-CoV-2 (AZD1222, AstraZeneca), reports have emerged of some vaccine recipients developing unusual thrombotic events and thrombocytopenia. We investigated whether such patients could have a prothrombotic disorder caused by platelet-activating antibodies directed against platelet factor 4 (PF4), as is known to be caused by heparin and sometimes other environmental triggers.Methods. We summarized the clinical and laboratory features of 9 patients in Germany and Austria who developed thrombosis and thrombocytopenia events following AZD1222 vaccination. Serum from four patients was used to test for anti-PF4/heparin antibodies, both by immunoassay and by platelet activation assays performed in the presence of heparin, PF4, or both.Results. The 9 patients (8 female; median age, 36 [range, 22—49) presented with thrombosis beginning 4 to 16 days post-vaccination: 7 patients had cerebral venous thrombosis (CVT), 1 had pulmonary embolism, and 1 had splanchnic vein thrombosis and CVT; 4 patients died. None had received heparin prior to symptom onset. All four patients tested strongly positive for anti-PF4/heparin antibodies by immunoassay; all 4 patients tested strongly positive in the platelet activation assay in the presence of PF4 independently of heparin. Platelet activation was inhibited by high concentrations of heparin, Fc receptor-blocking monoclonal antibody, and intravenous immunoglobulin.Conclusions. The AZD1222 vaccine is associated with development of a prothrombotic disorder that clinically resembles heparin-induced thrombocytopenia but which shows a different serological profile.

Download Full-text

Polyphosphate/platelet factor 4 complexes can mediate heparin-independent platelet activation in heparin-induced thrombocytopenia

Blood Advances ◽

10.1182/bloodadvances.2016000877 ◽

2016 ◽

Vol 1 (1) ◽

pp. 62-74 ◽

Cited By ~ 24

Author(s):

Douglas B. Cines ◽

Serge V. Yarovoi ◽

Sergei V. Zaitsev ◽

Tatiana Lebedeva ◽

Lubica Rauova ◽

...

Keyword(s):

Platelet Aggregation ◽

Cell Surface ◽

Chondroitin Sulfate ◽

Platelet Activation ◽

Platelet Factor 4 ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia ◽

Activated Platelets ◽

Key Points

Key Points Polyphosphates form antigenic complexes with PF4 that are recognized by HIT antibodies. Polyphosphate/PF4 complexes released by activated platelets can mediate platelet aggregation by HIT antibodies in the absence of heparin or cell-surface chondroitin sulfate.

Download Full-text

Characterization of New Monoclonal PF4-Specific Antibodies as Useful Tools for Studies on Typical and Autoimmune Heparin-Induced Thrombocytopenia

Thrombosis and Haemostasis ◽

10.1055/s-0040-1717078 ◽

2020 ◽

Author(s):

Caroline Vayne ◽

Thi-Huong Nguyen ◽

Jérôme Rollin ◽

Noémie Charuel ◽

Anne Poupon ◽

...

Keyword(s):

Platelet Activation ◽

Single Molecule ◽

Binding Sites ◽

Serotonin Release ◽

Titration Calorimetry ◽

Enzyme Linked Immunosorbent Assay ◽

Platelet Factor ◽

Heparin Induced Thrombocytopenia ◽

Activation Assay ◽

Group 3

Abstract Background Heparin-induced thrombocytopenia (HIT) is typically caused by platelet-activating immunoglobulin G (IgG) antibodies (Abs) against platelet factor 4 (PF4) complexed with heparin (H). Much less frequent “autoimmune” HIT is distinguished from typical HIT by platelet activation without heparin and the presence of both anti-PF4/H and anti-PF4 IgG. We developed three murine monoclonal anti-PF4 Abs with a human Fc-part, 1E12, 1C12, and 2E1, resembling autoimmune HIT Abs. Objectives To characterize 1E12, 1C12, and 2E1 in comparison to the heparin-dependent monoclonal anti-PF4/H Abs 5B9 and KKO, and polyclonal Abs from patients with typical HIT (group-2) and autoimmune HIT (group-3). Methods Interactions of Abs with PF4 and PF4/H were studied by enzyme-linked-immunosorbent assay, single-molecule force spectroscopy, isothermal titration calorimetry, and dynamic light scattering. Serotonin release assay and heparin-induced platelet activation assay were used to assess platelet activation. The binding sites of monoclonal Abs on PF4 were predicted in silico (MAbTope method). Results 1C12, 1E12, and 2E1 displayed higher affinity for PF4/H complexes than 5B9 and KKO, comparable to human group-3 Abs. Only 1C12, 1E12, 2E1, and group-3 Abs formed large complexes with native PF4, and activated platelets without heparin. The predicted binding sites of 1C12, 1E12, and 2E1 on PF4 differed from those of KKO and 5B9, but were close to each other. 2E1 exhibited unique bivalent binding, involving its antigen recognition site to PF4 and charge-dependent interactions with heparin. Conclusion 1C12, 1E12, and 2E1 are tools for studying the pathophysiology of autoimmune HIT. 2E1 provides evidence for a new binding mechanism of HIT Abs.

Download Full-text