scholarly journals To Clot or Not to Clot? Ad is the Question - Insights on Mechanisms Related to Vaccine Induced Thrombotic Thrombocytopenia

2021 ◽  
Author(s):  
Maha Othman ◽  
Alexander T. Baker ◽  
Elena Gupalo ◽  
Abdelrahman Elsebaie ◽  
Carly M. Bliss ◽  
...  
Keyword(s):  
Platelet Activation ◽  
Severe Thrombocytopenia ◽  
Host Proteins ◽  
Platelet Factor ◽  
Heparin Induced Thrombocytopenia ◽  
Autoimmune Condition ◽  
Global Concern ◽  
Mechanistic Basis ◽  
The Uk

Vaccine-induced immune thrombotic thrombocytopenia (VITT), or thrombotic thrombocytopenic syndrome (TTS), has caused global concern. VITT is characterized by thrombosis and thrombocytopenia following COVID-19 vaccinations with the AstraZeneca ChAdOx1 nCov-19 and the Janssen Ad26.COV2.S vaccines. The clinical features of VITT include thrombosis, typically cerebral venous thrombosis, and severe thrombocytopenia developing 5 to 24 days following first dose of vaccine, with elevated D-dimer, and antibodies specific to platelet factor 4 (PF4), signifying platelet activation. As of June 1, 2021, over 1.93 billion COVID-19 vaccine doses had been administered worldwide. Currently, 467 VITT cases (0.000024%) have been reported across the UK, Europe, Canada and Australia. Clinically, VITT presents similarly to a rare autoimmune condition called “spontaneous/autoimmune heparin Induced Thrombocytopenia” (HIT) without prior heparin exposure. Guidance on diagnosis and management of VITT has been reported but the pathogenic mechanism of VITT is not fully elucidated. A definite causal relationship with the vaccine material is yet to be confirmed. To date, it is established that IgG antibodies recognizing PF4 activate platelets through FcγRIIA, however it remains unclear what triggers production of these antibodies. The fact that VITT, has only been described in association with adenoviral vector-based DNA virus vaccines, but not mRNA/lipid-based vaccines, raises the likelihood that the syndrome is somehow linked to the vector or other constituents in the vaccine preparation. Here, we propose and discuss potential mechanisms in relation to adenovirus induction of VITT. We discuss adenovirus immunogenicity and interactions with platelets and other host proteins, the role of PF4 and platelet activation. Whilst confirming a single mechanism underpinning VITT is challenging, we provide insights and clues into areas warranting investigation into the mechanistic basis of VITT, highlighting the unanswered questions. Further research is required to help solidify a pathogenic model for this condition.

Pathogenic Role of Surface Platelet Factor 4 Complexes in Heparin-Induced Thrombocytopenia: Diagnostic and Therapeutic Implications.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 55-55 ◽  
Author(s):  
Lubica Rauova ◽  
Li Zhai ◽  
M. Anna Kowalska ◽  
Gowthami M. Arepally ◽  
Douglas B. Cines ◽  
...  
Keyword(s):  
Platelet Factor 4 ◽  
Thrombin Inhibitors ◽  
Current Therapy ◽  
Severe Thrombocytopenia ◽  
Pathogenic Role ◽  
Platelet Factor ◽  
Platelet Surface ◽  
Heparin Induced Thrombocytopenia

Abstract Heparin-induced thrombocytopenia (HIT) is caused by antibodies that recognize complexes between high molecular weight heparin and Platelet Factor 4 (PF4). Current therapy with direct thrombin inhibitors is not effective in all cases, likely because it acts downstream of antibody-induced platelet activation. More directed therapies to the underlying pathology in HIT may be more effective. Heparin and PF4 only bind HIT antibodies over a narrow molar ratio of reactants at which ultralarge soluble complexes are formed. We asked whether similar complexes form between PF4 and endogenous platelet glycosaminoglycans (GAG) and their pathogenic role in experimental HIT. Platelet surface GAG:PF4 complexes are indeed antigenic over a narrow molar range of reactants. Heparin is not required for either HIT-IgG or a HIT-like monoclonal antibody KKO to bind to PF4 on human or mouse platelet surfaces in vitro, but enhances antigenicity when very high levels of surface PF4 are present. Antigenicity is maximal at a PF4 concentration of 50 μg/mL (well within the range that can be achieved within a thrombus) and ~25 μg/mL heparin (~0.5 U/mL, which is within the therapeutic range) optimally enhances antigenicity when surface PF4 levels were increased 4-fold. Using transgenic mice lines each with platelets expressing a different level of hPF4, ranging from 0.5 – 6 X’s human platelet levels and all expressing FcRγIIA, were given KKO. The different lines developed thrombocytopenia proportional in severity and duration to hPF4 expression. A standard subcutaneous (sq) heparininzing dose (20 U/kg, sq daily) prolonged the duration of severe thrombocytopenia in high hPF4 expressing mice. We reasoned that altering the ratio of PF4 to GAG in either direction would alter antigenicity and could block the development of thrombocytopenia. In accordance with this concept, both high concentrations of anionic heparin (100 U/kg, sq daily) and cationic protamine sulfate (2 mg/kg, sq daily) decreased KKO binding in vitro and prevented KKO-induced thrombocytopenia in vivo as a demonstration of successful therapeutic intervention. These studies affirm a central role of surface GAG:PF4 complexes in the development of HIT, suggest ways to identify patients at high risk to develop HIT even prior to heparin exposure, and offers a new and rationale therapeutic paradigm based on disrupting surface antigen formation.

scholarly journals Heparin-induced thrombocytopenia: research and clinical updates

Hematology ◽  
2016 ◽  
Vol 2016 (1) ◽  
pp. 262-268 ◽  
Author(s):  
Oluwatoyosi Onwuemene ◽  
Gowthami M. Arepally
Keyword(s):  
Limited Data ◽  
Platelet Factor ◽  
Heparin Induced Thrombocytopenia ◽  
Diagnostic Strategies ◽  
Immune Mediated ◽  
Emerging Therapies ◽  
Current Perspective ◽  
Clinical Advances ◽  
A Current

Abstract Heparin-induced thrombocytopenia (HIT) remains an important diagnosis to consider in hospitalized patients developing thrombocytopenia. HIT is an immune-mediated prothrombotic disorder caused by antibodies to platelet factor 4 (PF4) and heparin. Recent basic scientific studies have advanced our understanding of disease pathogenesis through studies of the PF4/heparin structure, immune mechanisms, and cellular basis of thrombosis. Clinical advances have also occurred in areas of HIT prevention, description of disease variants, and diagnostic strategies. Emerging anticoagulants with the potential to change HIT treatment are evolving, although with limited data. This review will provide a current perspective on HIT pathogenesis, disease features, diagnostic strategies, and role of emerging therapies for the management of HIT.

Heparin-induced thrombocytopenia: when a low platelet count is a mandate for anticoagulation

Hematology ◽  
2009 ◽  
Vol 2009 (1) ◽  
pp. 225-232 ◽  
Author(s):  
Thomas L. Ortel
Keyword(s):  
Platelet Count ◽  
Heparin Therapy ◽  
Severe Thrombocytopenia ◽  
Drug Induced ◽  
Platelet Factor ◽  
Heparin Induced Thrombocytopenia ◽  
Immune Mediated ◽  
Number Of Patients ◽  
Increased Risk ◽  
Antibody Levels

Abstract Heparin-induced thrombocytopenia (HIT) is an immune-mediated disorder caused by the development of antibodies to platelet factor 4 (PF4) and heparin. The thrombocytopenia is typically moderate, with a median platelet count nadir of ~50 to 60 × 109 platelets/L. Severe thrombocytopenia has been described in patients with HIT, and in these patients antibody levels are high and severe clinical outcomes have been reported (eg, disseminated intravascular coagulation with microvascular thrombosis). The timing of the thrombocytopenia in relation to the initiation of heparin therapy is critically important, with the platelet count beginning to drop within 5 to 10 days of starting heparin. A more rapid drop in the platelet count can occur in patients who have been recently exposed to heparin (within the preceding 3 months), due to preformed anti-heparin/PF4 antibodies. A delayed form of HIT has also been described that develops within days or weeks after the heparin has been discontinued. In contrast to other drug-induced thrombocytopenias, HIT is characterized by an increased risk for thromboembolic complications, primarily venous thromboembolism. Heparin and all heparin-containing products should be discontinued and an alternative, non-heparin anticoagulant initiated. Alternative agents that have been used effectively in patients with HIT include lepirudin, argatroban, bivalirudin, and danaparoid, although the last agent is not available in North America. Fondaparinux has been used in a small number of patients with HIT and generally appears to be safe. Warfarin therapy should not be initiated until the platelet count has recovered and the patient is systemically anticoagulated, and vitamin K should be administered to patients receiving warfarin at the time of diagnosis of HIT.

Pathophysiology of Heparin-Induced Thrombocytopenia

2000 ◽  
Vol 124 (11) ◽  
pp. 1657-1666 ◽  
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.

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

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.

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 ◽  
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.

Heparin-induced thrombocytopenia: new evidence for the dynamic binding of purified anti-PF4–heparin antibodies to platelets and the resultant platelet activation

Blood ◽  
2000 ◽  
Vol 96 (1) ◽  
pp. 182-187 ◽  
Author(s):  
Peter M. Newman ◽  
Beng H. Chong
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Surface Expression ◽  
Dynamic Binding ◽  
Platelet Factor ◽  
Heparin Induced Thrombocytopenia ◽  
Activated Platelets ◽  
Igg Binding ◽  
Iodine 125 ◽  
First Time

Immune heparin-induced thrombocytopenia (HIT) is associated with antibodies directed against a complex of platelet factor 4 (PF4) and heparin. We were able to affinity purify anti-PF4–heparin IgG (HIT IgG) from the plasma of 2 patients with HIT. Under conditions that were more physiological and sensitive than those in previous studies, we observed that this HIT IgG caused platelet aggregation on the addition of heparin. Platelets activated with HIT IgG increased their release and surface expression of PF4. We quantitated, for the first time, the binding of affinity-purified HIT iodine 125–IgG to platelets as they activated in a plasma milieu. Binding of the HIT IgG was dependent on heparin and required some degree of platelet activation. Blocking the platelet FcγRII with the monoclonal antibody IV.3 did not prevent HIT IgG binding to activated platelets. We concluded that anti-PF4–heparin IgG is the component in these HIT plasmas that induces platelet aggregation. The Fab region of HIT IgG binds to PF4–heparin on the surface of activated platelets. We propose that only then does the Fc portion of the bound IgG further activate the same or adjacent platelets through the Fc receptor. Our data support a dynamic model of platelet activation in which released PF4 enhances further antibody binding and more release.

Ultralarge complexes of PF4 and heparin are central to the pathogenesis of heparin-induced thrombocytopenia

Blood ◽  
2005 ◽  
Vol 105 (1) ◽  
pp. 131-138 ◽  
Author(s):  
Lubica Rauova ◽  
Mortimer Poncz ◽  
Steven E. McKenzie ◽  
Michael P. Reilly ◽  
Gowthami Arepally ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Platelet Activation ◽  
Low Molecular Weight Heparin ◽  
Heparin Therapy ◽  
Low Molecular Weight ◽  
Dependent Manner ◽  
Fondaparinux Sodium ◽  
Platelet Factor ◽  
Heparin Induced Thrombocytopenia ◽  
Autoantibody Formation

Abstract Heparin-induced thrombocytopenia and thrombosis (HITT) is a severe complication of heparin therapy caused by antibodies to complexes between unfractionated heparin (UFH) and platelet factor 4 (PF4) that form over a narrow molar range of reactants and initiate antibody-induced platelet activation. We observed that UFH and tetrameric PF4 formed ultralarge (> 670 kDa) complexes (ULCs) only over a narrow molar range with an optimal ratio of PF4 to heparin of approximately 1:1. These ULCs were stable and visible by electron microscopy, but they could be dissociated into smaller complexes upon addition of heparin. ULCs formed inefficiently when PF4 was incubated with low-molecular-weight heparin, and none formed with the pentasaccharide fondaparinux sodium. In addition, mutation studies showed that formation of ULCs depended on the presence of PF4 tetramers. The ULCs were more reactive as determined by their capacity to bind to a HITT-like monoclonal antibody and showed greater capacity to promote platelet activation in an antibody- and FcγRIIA-dependent manner than were the smaller complexes. The capacity of PF4 to form ULCs composed of multiple PF4 tetramers arrayed in a lattice with several molecules of UFH may play a fundamental role in autoantibody formation, antibody-dependent platelet activation, and the propensity for thrombosis in patients with HITT.

scholarly journals Refractory vaccine-induced thrombotic thrombocytopenia (VITT) managed with delayed therapeutic plasma exchange (TPE)

2021 ◽  
Author(s):  
Ajay Major ◽  
Timothy Carll ◽  
Clarence W. Chan ◽  
Chancey Christenson ◽  
Geoffrey D. Wool ◽  
...  
Keyword(s):  
Plasma Exchange ◽  
Alcoholic Cirrhosis ◽  
Therapeutic Plasma Exchange ◽  
High Dose ◽  
Severe Thrombocytopenia ◽  
Platelet Factor ◽  
Heparin Induced Thrombocytopenia ◽  
Sustained Improvement ◽  
Vein Thrombosis ◽  
Hematologic Disorder

Abstract Vaccine-induced thrombotic thrombocytopenia (VITT) is a newly-described hematologic disorder which presents as acute thrombocytopenia and thrombosis after administration of adenovirus-based vaccines against COVID-19. Due to positive assays for antibodies against platelet factor 4 (PF4), VITT is managed similarly to autoimmune heparin-induced thrombocytopenia (HIT) with intravenous immunoglobulin (IVIG) and non-heparinoid anticoagulation. We describe a case of VITT in a 50-year-old man with antecedent alcoholic cirrhosis who presented with platelets of 7 × 103/µL and portal vein thrombosis 21 days following administration of the Ad26.COV2.S COVID-19 vaccine. The patient developed progressive thrombosis and persistent severe thrombocytopenia despite IVIG, rituximab and high-dose steroids and had persistent anti-PF4 antibodies over 30 days after his initial presentation. As such, delayed therapeutic plasma exchange (TPE) was pursued as salvage therapy, with a rapid and sustained improvement in his platelet count. Our case serves as proof-of-concept of the efficacy of TPE in VITT.

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