Vaccine-induced immune thrombotic thrombocytopenia: what do we know hitherto?

Vaccine-induced immune thrombotic thrombocytopenia (VITT), also known as thrombosis with thrombocytopenia syndrome, is a catastrophic and life-threatening reaction to coronavirus disease 2019 (COVID-19) vaccines, which occurs disproportionately in response to vaccination with non-replicating adenovirus vector (AV) vaccines. The mechanism of VITT is not well defined and it has not been resolved why cases of VITT are predominated by vaccination with AV vaccines. However, virtually all VITT patients have positive platelet-activating anti-platelet factor 4 (PF4) antibody titers. Subsequently, platelets are activated and depleted in an Fcγ-receptor IIa (FcγRIIa or CD32a)-dependent manner, but it is not clear why or how the anti-PF4 response is mounted. This review describes the pathogenesis of VITT and provides insight into possible mechanisms that prompt the formation of a PF4/polyanion complex, which drives VITT pathology, as an amalgam of current experimental data or hypotheses.

Multiple Electrode Aggregometry (Multiplate): Functional Assay for Vaccine-Induced (Immune) Thrombotic Thrombocytopenia (VITT).

Vaccine-induced immune thrombotic thrombocytopenia (VITT) was first described in 2021 and represents an adverse reaction to adenoviral vector COVID-19 vaccines AstraZeneca ChAdOx1 nCoV-19 (AZD1222) and Johnson & Johnson Ad26.COV2.S vaccine. VITT is a severe immune platelet activation syndrome with an incidence of 1-2 per 100,000 vaccinations. The features of VITT include thrombocytopenia and thrombosis within 4-42 days of first dose of vaccine. Affected individuals develop platelet-activating antibodies against platelet factor 4 (PF4). The International Society on Thrombosis and Haemostasis recommends both an antigen-binding assay (enzyme-linked immunosorbent assay, ELISA) and a functional platelet activation assay for the diagnostic workup of VITT. Here, the application of multiple electrode aggregometry (Multiplate) is presented as a functional assay for VITT.

Serotonin Release Assay: Functional Assay for Heparin- and Vaccine-Induced (Immune) Thrombotic Thrombocytopenia.

The serotonin release assay (SRA) has been the gold-standard assay for detection of heparin-dependent platelet-activating antibodies and integral for the diagnosis for heparin-induced thrombotic thrombocytopenia (HIT). In 2021, a thrombotic thrombocytopenic syndrome was reported after adenoviral vector COVID-19 vaccination. This vaccine-induced thrombotic thrombocytopenic syndrome (VITT) proved to be a severe immune platelet activation syndrome manifested by unusual thrombosis, thrombocytopenia, very elevated plasma D-dimer, and a high mortality even with aggressive therapy (anticoagulation and plasma exchange). While the platelet-activating antibodies in both HIT and VITT are directed toward platelet factor 4 (PF4), important differences have been found. These differences have required modifications to the SRA to improve detection of functional VITT antibodies. Functional platelet activation assays remain essential in the diagnostic workup of HIT and VITT. Here we detail the application of SRA for the assessment of HIT and VITT antibodies.

Heparin-Induced Thrombotic Thrombocytopenia (HITT) and Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT): Similar but Different.

Heparin-induced thrombocytopenia (HIT) represents an autoimmune process whereby antibodies are formed against heparin in complex with platelet factor 4 (PF4) after heparin administration. These antibodies can be detected by a variety of immunological assays, including ELISA (enzyme-linked immunosorbent assay) and by chemiluminescence on the AcuStar instrument. However, pathological HIT antibodies are those that activate platelets in a platelet activation assay and cause thrombosis in vivo. We would tend to call this condition heparin-induced thrombotic thrombocytopenia (HITT), although some workers instead use the truncated abbreviation HIT. Vaccine-induced (immune) thrombotic thrombocytopenia (VITT) instead reflects an autoimmune process whereby antibodies are formed against PF4 after administration of a vaccine, most notably adenovirus-based vaccines directed against COVID-19 (coronavirus disease 2019). Although both VITT and HITT reflect similar pathological processes, they have different origins and are detected in different ways. Most notable is that anti-PF4 antibodies in VITT can only be detected immunologically by ELISA assays, tending to be negative in rapid assays such as that using the AcuStar. Moreover, functional platelet activation assays otherwise used for HITT may need to be modified to detect platelet activation in VITT.

Coagulopathies after vaccination against SARS-CoV-2: The sole solution might lead to another problem

The common reported adverse impacts of COVID-19 vaccination include the injection site's local reaction followed by various non-specific flu-like symptoms. Nevertheless, uncommon cases of vaccine-induced immune thrombotic thrombocytopenia (VITT) and cerebral venous sinus thrombosis (CVST) following viral vector vaccines (ChAdOx1 nCoV-19 vaccine, Ad26.COV2 vaccine) have been reported. This literature review was performed using PubMed and Google Scholar databases using appropriate keywords and their combinations: SARS-CoV-2, adenovirus, spike protein, thrombosis, thrombocytopenia, vaccine-induced immune thrombotic thrombocytopenia (VITT), NF-kappaB, adenoviral vector, platelet factor 4 (PF4), COVID-19 Vaccine, AstraZeneca COVID vaccine, ChAdOx1 nCoV-19 COVID vaccine, AZD1222 COVID vaccine, coagulopathy. The s and titles of each article were assessed by authors for screening and inclusion English reports about post-vaccine CVST and VITT in humans were also collected. Some SARS-CoV-2 vaccines based on viral vector, mRNA, or inactivated SARS-CoV-2 virus have been accepted and are being pragmatic global. Nevertheless, the recent augmented statistics of normally very infrequent types of thrombosis associated with thrombocytopenia have been stated, predominantly in the context of the adenoviral vector vaccine ChAdOx1 nCoV-19 from Astra Zeneca. The numerical prevalence of these side effects seems to associate with this particular vaccine type, i.e., adenoviral vector-based vaccines, but the meticulous molecular mechanisms are still not clear. The present review summarizes the latest data and hypotheses for molecular and cellular mechanisms into one integrated hypothesis demonstrating that coagulopathies, including thromboses, thrombocytopenia, and other associated side effects, are correlated to an interaction of the two components in the COVID-19 vaccine.Copyright © 2022, Iranian Pediatric Hematology and Oncology Society. All rights reserved.

Platelet-activating functional assay resolution in vaccine-induced immune thrombotic thrombocytopenia: differential alignment to PF4 ELISA platforms.

Background: Anti-platelet factor 4 (PF4) antibodies in vaccine-induced immune thrombotic thrombocytopenia (VITT) appear to be transient, with discrepant persistence depending on the platform used for detection. Objectives: We aimed to report a longitudinal study of antibody persistence using 2 ELISA platforms and 2 platelet-activating functional assays in a clinical cohort of patients with VITT referred for follow-up testing. Methods: In total, 32 Australian patients with VITT or pre-VITT, confirmed by expert adjudication, with samples referred for clinical follow-up were included. Clinical follow-up assays, including Stago and Hyphen ELISAs, procoagulant platelet flow cytometry, and modified PF4-serotonin-release assay, were performed according to the pattern of reactivity for that patient at diagnosis. Results: The median follow-up was 24 weeks after diagnosis. A general decline in anti-PF4 antibody levels and platelet-activating capacity over time was observed with a more rapid median time to resolution of 16 weeks by functional assay vs 24 weeks by Stago ELISA. Decline in platelet-activating antibody levels detected by functional assays mirrored Stago ELISA titer but not Hyphen. However, 87% of patients received a documented second vaccination and 74% received an mRNA booster with no reported adverse events. Conclusion: Anti-PF4 antibodies persist longer than functional platelet-activating antibodies in VITT but do not warrant avoidance of subsequent vaccinations. Persistence detection is assay-dependent. Stago ELISA may be a surrogate where functional assays are unavailable for follow-up testing of confirmed patients with VITT.

Vaccine-induced immune thrombotic thrombocytopenia.

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is primarily a complication of adenoviral vector-based covid-19 vaccination. In VITT, thrombocytopenia and thrombosis mediated by anti-platelet factor 4 (PF4) antibodies can be severe, often characterized by thrombosis at unusual sites such as the cerebral venous sinus and splanchnic circulation. Like in heparin-induced thrombocytopenia (HIT) and spontaneous HIT, VITT antibodies recognize PF4-polyanion complexes and activate PF4-treated platelets but additionally bind to un-complexed PF4, a critical finding that could be leveraged for more specific detection of VITT. Intravenous immunoglobulin and non-heparin-based anticoagulation remain the mainstay of treatment. Second dose/boosters of mRNA covid-19 vaccines appear safe in patients with adenoviral vector-associated VITT. Emerging data is consistent with the possibility that ultra-rare cases of VITT may be seen in the setting of mRNA and virus-like particle (VLP) technology-based vaccinations and until more data is available, it is prudent to consider VITT in the differential diagnosis of all post-vaccine thrombosis and thrombocytopenia reactions.

A Case of Vaccine-Induced Thrombocytopenic Thrombosis Manifesting as Cerebral Venous Thrombosis and Intracerebral Bleed.

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare but detrimental syndrome that has been most commonly reported after the administration of vaccination for the prevention of viral infections. VITT often presents with thrombosis at unusual sites such as cerebral venous sinuses, portal, splanchnic or hepatic veins, in association with thrombocytopenia and elevated anti-platelet factor 4 (aPF-4) antibodies. We describe the case of a young male patient who developed thrombocytopenia, cerebral sinus venous thrombosis, and intracerebral bleed 12 days after receiving the Ad26.COV2.S (Janssen/Johnson&Johnson) COVID-19 vaccine.

Anti-PF4 antibodies after SARS-CoV-2 vaccines: Long term follow-up

Introduction A subgroup of anti-platelet factor 4 (PF4) antibodies can activate platelets via Fcgamma RIIA and cause thrombotic and thrombocytopenic diseases such as heparin-induced thrombocytopenia and vaccine-induced immune thrombotic thrombocytopenia (VITT). Nonpathological anti-PF4 antibodies are detected in 1-7 % of healthy blood donors and in 2-8 % of SARS-CoV-2 vaccinated individuals. In this study, we investigated the long-term course of anti- PF4 antibodies detected after the first SARS-CoV-2 vaccination in healthy subjects and in patients with VITT. Method Five healthy subjects (all female, median age (range): 40 years (29-62) ) who had anti-PF4 antibodies after the first vaccination with ChadOx1 nCov19 (Vaxzevria, AstraZeneca-Oxford) were included. None of the subjects developed VITT. Blood samples were collected as part of a longitudinal study (TuSeRe:exact) evaluating the immune response to SARS-CoV-2 vaccines among employees of an University Hospital. In addition, data from 4 patients with VITT (3 female, median age (range): 44 years (22 -62 years)) were included for long-term follow-up of anti-PF4 antibodies. Anti-PF4/heparin antibodies were measured using a commercially available ELISA assay (Zymutest HIA IgG, Hyphen BioMed, France). Platelet activation was tested with a modified heparin- induced platelet aggregation assay (HIPA). Results In the non-VITT group, the median (range) OD for IgG anti-PF4/heparin antibodies was 0.69 (0.60-1.83) after the first vaccination. Blood samples were available up to 16 months after the first vaccination (range: 5-16 months). Anti-PF4 antibody levels decreased in all subjects despite further vaccination. However, antibody levels returned to pre-vaccination levels in only one subject. In one subject who had received two doses of ChadOx1 nCov19, anti-PF4 antibodies remained above OD 1.0 at the last follow-up. All samples were negative in the modified HIPA assay. Patients with VITT received mRNA-based vaccine as second vaccination against SARS CoV2. No significant drop in platelet count or new thromboembolic complication was observed. Conclusion Nonpathological anti-PF4 antibodies can be detected even several months after the first vaccination. The clinical significance of these antibodies in case of subsequent exposure to a vector vaccine or heparin is not yet clear. Furthermore, subsequent vaccination seems safe in VITT patietns.

Subsequent vaccinations in VITT patients other than SARS-CoV2 vaccination

Introduction Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare, but severe side effect after vaccination with adenovirus vector-based vaccines (ChAdOx1 nCoV-19, AstraZeneca and Ad26.COV2.S, Johnson & Johnson/ Janssen) in which platelet activating anti-platelet factor 4 (PF4) antibodies cause thrombocytopenia and thrombosis at unusual sites. Patients and treating physicians are concerned about whether other vaccinations can also trigger thrombosis in patients with a history of VITT. We showed that VITT patients can safely receive their second and third vaccination against Covid-19 with an mRNA-based vaccine. [1] However, there is limited information on whether other vaccines than against Covid-19 could booster platelet activating anti-PF4 antibodies. Uncertainty increased after a report of VITT caused by human papilloma vaccination. [2] Method In our follow-up study of patients with laboratory confirmed VITT (EUPAS45098), an anti-PF4/heparin IgG enzyme immune assay (EIA) and a PF4-dependent platelet activation assay (PIPA) were performed at regular intervals and after each vaccination reported to us. Results Seventy-one VITT patients (43 female, median age at VITT diagnosis 48, range 18-80) were followed for a median of 56 weeks (range: 13-77 weeks). During the follow-up period, eight vaccinations other than against Covid-19 were reported: Six vaccinations against influenza (three Influvac, two Vaxigrip Tetra, one Influsplit Tetra) and two consecutive vaccinations against tick-borne encephalitis (TBE) in one patient. In six patients who received vaccination against influenza, all patients showed decreasing or stable EIA optical density (OD) levels. None of them showed a reactivation of platelet-activating anti- PF4-antibodies in the PIPA. The patient who was vaccinated against TBE twice showed stable EIA OD levels and remained negative in the PIPA throughout. No new thrombosis or recurrent thrombocytopenia were observed after any vac- cination. Five out of six patients still received therapeutic anticoagulation, one patient did not receive any anticoagulative drug (Fig. 1). Conclusion Similar to observations after consecutive mRNA-vaccinations against Covid-19 in VITT patients, vaccinations against influenza and TBE very unlikely reactivate platelet-activating anti-PF4-antibodies. Further follow up of the VITT patient cohort is performed to detect any new safety signal related to recurrence of VITT. (Table Presented).

New perspectives on the induction and acceleration of immune-associated thrombosis by PF4 and VWF.

Platelet factor 4 (PF4), also known as chemokine (C-X-C motif) ligand 4 (CXCL4), is a specific protein synthesized from platelet α particles. The combination of PF4 and heparin to form antigenic complexes is an important mechanism in the pathogenesis of heparin-induced thrombocytopenia (HIT), but vaccine-induced immune thrombotic thrombocytopenia (VITT) related to the COVID-19 vaccine makes PF4 a research hotspot again. Similar to HIT, vaccines, bacteria, and other non-heparin exposure, PF4 can interact with negatively charged polyanions to form immune complexes and participate in thrombosis. These anions include cell surface mucopolysaccharides, platelet polyphosphates, DNA from endothelial cells, or von Willebrand factor (VWF). Among them, PF4-VWF, as a new immune complex, may induce and promote the formation of immune-associated thrombosis and is expected to become a new target and therapeutic direction. For both HIT and VITT, there is no effective and targeted treatment except discontinuation of suspected drugs. The research and development of targeted drugs based on the mechanism of action have become an unmet clinical need. Here, this study systematically reviewed the characteristics and pathophysiological mechanisms of PF4 and VWF, elaborated the potential mechanism of action of PF4-VWF complex in immune-associated thrombosis, summarized the current status of new drug research and development for PF4 and VWF, and discussed the possibility of this complex as a potential biomarker for early immune-associated thrombosis events. Moreover, the key points of basic research and clinical evaluation are put forward in the study.

Platelet Activation and Cytokine Release of Interleukin-8 and Interferon-Gamma-Induced Protein 10 after ChAdOx1 nCoV-19 Coronavirus Vaccine Injection.

Coronavirus disease 2019 (COVID-19) vaccines are associated with serious thromboembolic or thrombocytopenic events including vaccine-induced immune thrombocytopenia and thrombosis and immune thrombocytopenia, particularly AZD1222/ChAdOx1. According to the proposed mechanism, COVID-19 vaccines stimulate inflammation and platelet activation. In this study, we analyzed the role of AZD1222/ChAdOx1 vaccines in the activation of platelets and the release of anti-PF4 antibodies and inflammatory cytokines in a cohort of healthy donors without vaccine-induced immune thrombotic thrombocytopenia (VITT). Forty-eight healthy volunteers were enrolled in this study. Blood samples were collected from peripheral blood at three time points: before vaccination and 1 and 7 days after vaccination. Compared with the prevaccination data, a decrease in the leukocyte and platelet counts was observed 1 day after vaccination, which recovered 7 days after injection. The percentage of activated GPIIb/IIIa complex (PAC-1) under high ADP or thrombin receptor-activating peptide stimulation increased 1 day after vaccination. Furthermore, interluekin-8 (IL-8) and interferon-gamma-induced protein 10 (IP-10) increased significantly. Additionally, platelet activation and inflammation, with the release of cytokines, were observed; however, none of the individuals developed VITT. Mild thrombocytopenia with platelet activation and inflammation with an elevation of IL-8 and IP-10 were observed after AZ vaccination.

Postmortem PF4 antibodies confirm a rare case of thrombosis thrombocytopenia syndrome associated with ChAdOx1 nCoV-19 anti-COVID vaccination.

We report a case of cerebral venous sinus thrombosis, bilateral adrenal hemorrhage, and thrombocytopenia in a 70-year-old man found dead. He had previously received the ChAdOx1 nCoV-19 vaccine (Vaxzevria®, AstraZeneca) 18 days before, and had since developed unspecific and undiagnosed characteristics of what proved to be a rare case of vaccine-associated thrombocytopenia with thrombosis syndrome (TTS). He was found dead 1 week after the beginning of symptoms (day 25 post-vaccine). Autopsy yielded venous hemorrhagic infarction with the presence of thrombi within dural venous sinuses, and extensive hemorrhagic necrosis of the central part of the adrenal glands. Antibodies against platelet factor 4 (PF4) were strongly positive in postmortem fluids, as measured with an enzyme-linked immunosorbent assay (ELISA). This difficult diagnosis is usually made during the patient's lifetime. After eliminating differential diagnoses, we concluded on a fatal case of vaccine-induced immune TTS with positive anti-PF4 antibodies in cadaveric blood, 3 weeks after ChAdOx1 nCoV-19 vaccination. Specific search for anti-PF4 antibodies in cadaveric blood appears therefore paramount to assess postmortem cases of TTS associated with anti-COVID vaccines.

Implication of Neutrophils Extracellular Traps in the Pathogenesis of SARS-CoV-2 pneumonia.

Peripheral blood polymorphonuclear neutrophils (PMNs) forming extracellular traps (NETs), as well as endothelial- and platelet-derived parameters, have been analyzed in patients with SARS-CoV-2 pneumonia, and their prognostic role has been evaluated. Eighty-seven consecutive patients hospitalized with SARS-CoV-2 pneumonia were prospectively selected. A sample of 30 healthy individuals served as the control group. Clinical and oxygenation (oxygen saturation to fraction of inspired oxygen ratio-SpO2/FiO2) characteristics and PMNs forming NETs, serum levels of myeloperoxidase, E-selectin, vascular cell adhesion molecule 1-VCAM1-vascular endothelial growth factor, P-selectin, platelet factor 4 and plasma concentrations of D-dimer were evaluated at hospital admission, at discharge and 14 days after discharge. Intensive care unit admission or death was the primary composite endpoint. Patients showed a higher number of PMNs forming NETs than healthy controls. The absolute number of PMNs forming NETs was inversely correlated with oxygen status (SpO2/FiO2) and positively with inflammatory (C-reactive protein, ferritin) markers and VCAM1. A decrease in, but not a normalization of NETs and endothelial-derived parameters was observed in patients who survived. In conclusion, the formation of NETs runs parallel to that of other inflammatory and endothelial activation markers, and is inverse to the oxygenation parameters, supporting a pathogenic role for PMNs in this entity.

Complementary Sets of Autoantibodies Induced by SARS-CoV-2, Adenovirus and Bacterial Antigens Cross-React with Human Blood Protein Antigens in COVID-19 Coagulopathies.

COVID-19 patients often develop coagulopathies including microclotting, thrombotic strokes or thrombocytopenia. Autoantibodies are present against blood-related proteins including cardiolipin (CL), serum albumin (SA), platelet factor 4 (PF4), beta 2 glycoprotein 1 (ß2GPI), phosphodiesterases (PDE), and coagulation factors such as Factor II, IX, X and von Willebrand factor (vWF). Different combinations of autoantibodies associate with different coagulopathies. Previous research revealed similarities between proteins with blood clotting functions and SARS-CoV-2 proteins, adenovirus, and bacterial proteins associated with moderate-to-severe COVID-19 infections. This study investigated whether polyclonal antibodies (mainly goat and rabbit) against these viruses and bacteria recognize human blood-related proteins. Antibodies against SARS-CoV-2 and adenovirus recognized vWF, PDE and PF4 and SARS-CoV-2 antibodies also recognized additional antigens. Most bacterial antibodies tested (group A streptococci [GAS], staphylococci, Escherichia coli [E. coli], Klebsiella pneumoniae, Clostridia, and Mycobacterium tuberculosis) cross-reacted with CL and PF4. while GAS antibodies also bound to F2, Factor VIII, Factor IX, and vWF, and E. coli antibodies to PDE. All cross-reactive interactions involved antibody-antigen binding constants smaller than 100 nM. Since most COVID-19 coagulopathy patients display autoantibodies against vWF, PDE and PF4 along with CL, combinations of viral and bacterial infections appear to be necessary to initiate their autoimmune coagulopathies.

Proteomics of fibrin amyloid microclots in long COVID/post-acute sequelae of COVID-19 (PASC) shows many entrapped pro-inflammatory molecules that may also contribute to a failed fibrinolytic system.

BACKGROUND: Post-acute sequelae of COVID-19 (PASC), also now known as long COVID, has become a major global health and economic burden. Previously, we provided evidence that there is a significant insoluble fibrin amyloid microclot load in the circulation of individuals with long COVID, and that these microclots entrap a substantial number of inflammatory molecules, including those that might prevent clot breakdown. Scientifically, the most challenging aspect of this debilitating condition is that traditional pathology tests such as a serum CRP (C-reactive protein) may not show any significant abnormal inflammatory markers, albeit these tests measure only the soluble inflammatory molecules. Elevated, or abnormal soluble biomarkers such as IL-6, D-Dimer or fibrinogen indicate an increased risk for thrombosis or a host immune response in COVID-19. The absence of biomarkers in standard pathology tests, result in a significant amount of confusion for patients and clinicians, as patients are extremely sick or even bed-ridden but with no regular identifiable reason for their disease. Biomarkers that are currently available cannot detect the molecules present in the microclots we identified and are therefore unable to confirm their presence or the mechanisms that drive their formation. METHODS: Here we analysed the protein content of double-digested microclots of 99 long COVID patients and 29 healthy controls. The patients suffering from long COVID reported their symptoms through a questionnaire completed by themselves or their attending physician. RESULTS: Our long COVID cohort's symptoms were found to be in line with global findings, where the most prevalent symptoms were constant fatigue (74%,) cognitive impairment (71%) and depression and anxiety (30%). Our most noteworthy findings were a reduced level of plasma Kallikrein compared to our controls, an increased level of platelet factor 4 (PF4) von Willebrand factor (VWF), and a marginally increased level of α-2 antiplasmin (α-2-AP). We also found a significant presence of antibodies entrapped inside these microclots. CONCLUSION: Our results confirm the presence of pro-inflammatory molecules that may also contribute to a failed fibrinolysis phenomenon, which could possibly explain why individuals with long COVID suffer from chronic fatigue, dyspnoea, or cognitive impairment. In addition, significant platelet hyperactivation was noted. Hyperactivation will result in the granular content of platelets being shed into the circulation, including PF4. Overall, our results provide further evidence of both a failed fibrinolytic system in long COVID/PASC and the entrapment of many proteins whose presence might otherwise go unrecorded. These findings might have significant implications for individuals with pre-existing comorbidities, including cardiovascular disease and type 2 diabetes.

Quantitative interpretation of PF4/heparin-EIA optical densities in predicting platelet-activating VITT antibodies.

BACKGROUND: Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a prothrombotic, heparin-induced thrombocytopenia (HIT)-mimicking, adverse reaction caused by platelet-activating anti-platelet factor 4 (PF4) antibodies that occurs rarely after adenovirus vector-based COVID-19 vaccination. Strength of PF4-dependent enzyme immunoassay (EIA) reactivity-judged by optical density (OD) measurements-strongly predicts platelet-activating properties of HIT antibodies in a functional test. Whether a similar relationship holds for VITT antibodies is unknown. OBJECTIVES: To evaluate probability for positive platelet activation testing for VITT antibodies based upon EIA OD reactivity; and to investigate simple approaches to minimize false-negative platelet activation testing for VITT. METHODS: All samples referred for VITT testing were systematically evaluated by semiquantitative in-house PF4/heparin-EIA (OD readings) and PF4-induced platelet activation (PIPA) testing within a cohort study. EIA-positive sera testing PIPA-negative were retested following 1/4 to 1/10 dilution. Logistic regression was performed to predict the probability of a positive PIPA per magnitude of EIA reactivity. RESULTS: Greater EIA ODs in sera from patients with suspected VITT correlated strongly with greater likelihood of PIPA reactivity. Of 61 sera (with OD values >1.0) testing negative in the PIPA, a high proportion (27/61, 44.3%) became PIPA positive when tested at 1/4 to 1/10 dilution. CONCLUSIONS: VITT serology resembles HIT in that greater EIA OD reactivity predicts higher probability of positive testing for platelet-activating antibodies. Unlike the situation with HIT antibodies, however, diluting putative VITT serum increases probability of a positive platelet activation assay, suggesting that optimal complex formation depends on the stoichiometric ratio of PF4 and anti-PF4 VITT antibodies.

Challenges and Strategies for Veno Venous (VV) Extracorporeal Membrane Oxygenation (ECMO) in COVID-19 Acute Respiratory Distress Syndrome (ARDS) During Second Wave of Pandemic: A Case Series

Case series: Extracorporeal membrane oxygenation (ECMO) use for severe acute respiratory distress syndrome due to coronavirus disease 2019 (COVID-19) patients has increased during the second wave of the pandemic. However, there are many complications associated with the management of ECMO in critically ill COVID- 19 patients. We report a case series of challenges and strategies for managing critically ill COVID-19 patients on ECMO support for severe ARDS. Seven COVID-19 patients required VV ECMO of which three were women and four were men of median age of 43 years. Among seven, three cases (42%) recovered. We experienced multiple challenges and complications in the management of the patients, being a non-ECMO centre with limited resources, in heavy workload during the second wave of the pandemic. All the patients required multiple invasive procedures like placement of invasive lines, frequent bronchoscopies for bronchial toileting. Displacement of both ECMO cannulas required repositioning under ultrasound guidance, four patients underwent percutaneous tracheostomy on ECMO. Three patients had ECMO-oxygenator failure that required the exchange of a new ECMO circuit. ACT was monitored for the management of anticoagulation. A challenging task is to achieve a balance between bleeding and thrombotic events, for which anticoagulation had to be stopped for the acceptable ACT, required transfusion of multiple blood products for correcting coagulopathy. One patient developed HIT antibodies and managed with bivalirudin for the management of anticoagulation which was challenging in titrating the drug dose and ACT. Two patients had an intracranial haemorrhage on ECMO support, managed conservatively despite anticoagulation. Pseudoaneurysm of femoral vein diagnosed and managed with ultrasound-guided thrombin injection. Four patients got decannulated from ECMO. One patient had unexplained severe haemolysis immediately after initiation of ECMO, unfortunately, he could not recover. Management of VV ECMO in resource-limited, non-ECMO centre in a pandemic is challenging. Mortality depends on various factors, despite expertise, advanced critical care management in COVID- 19 ARDS and ECMO. Increased use of VV ECMO during the second wave of pandemic reported significant changes in strategies for management of challenges, though further studies are still required for the best outcome.

Activated Platelets and Platelet-Derived Extracellular Vesicles Mediate COVID-19-Associated Immunothrombosis.

Activated platelets and platelet-derived extracellular vesicles (EVs) have emerged as central players in thromboembolic complications associated with severe coronavirus disease 2019 (COVID-19). Platelets bridge hemostatic, inflammatory, and immune responses by their ability to sense pathogens via various pattern recognition receptors, and they respond to infection through a diverse repertoire of mechanisms. Dysregulated platelet activation, however, can lead to immunothrombosis, a simultaneous overactivation of blood coagulation and the innate immune response. Mediators released by activated platelets in response to infection, such as antimicrobial peptides, high mobility group box 1 protein, platelet factor 4 (PF4), and PF4+ extracellular vesicles promote neutrophil activation, resulting in the release of neutrophil extracellular traps and histones. Many of the factors released during platelet and neutrophil activation are positively charged and interact with endogenous heparan sulfate or exogenously administered heparin via electrostatic interactions or via specific binding sites. Here, we review the current state of knowledge regarding the involvement of platelets and platelet-derived EVs in the pathogenesis of immunothrombosis, and we discuss the potential of extracorporeal therapies using adsorbents functionalized with heparin to deplete platelet-derived and neutrophil-derived mediators of immunothrombosis.

Vaccine-Induced Thrombotic Thrombocytopenia: A Case of Splanchnic Veins Thrombosis.

Vaccines have been vital in preventing and curbing the spread of SARS-CoV-2 infection. Adenoviral vector-based vaccines, namely the ChAdOx1-S vaccine (AstraZeneca, Cambridge, UK) and Ad26.COV2.S (Janssen; Johnson & Johnson, New Brunswick, NJ, USA), have been associated with a possibly fatal adverse event known as vaccine-induced thrombotic thrombocytopenia (VITT), wherein thrombi form in unusual sites, mainly the cerebral and splanchnic veins. With the female gender predominantly affected, patients present with headache, abdominal pain, neurological symptoms and fever. It is hypothesized that certain components of the vaccine, including the adenovirus vector, may trigger the formation of antibodies against platelet factor 4 (PF4). The antigen-antibody complexes that form thereafter then activate a cascade of reactions eventually leading to the consumptive coagulopathy. This pathogenesis closely resembles a well-understood complication of heparin, known as heparin-induced thrombocytopenia. The lab results in VITT are reflective of its proposed pathophysiology: low platelets, low fibrinogen and high D-dimer, in addition to elevated anti-PF4 titers are classic findings. Treatment mainly includes non-heparin anticoagulants, intravenous immune globulin (IVIG) and plasma exchange. There is some role for surgical intervention, such as mechanical thrombectomy. Mortality due to VITT is usually caused by cerebral hemorrhage. We describe a case of a 36-year-old female who presented with epigastric pain two weeks after receiving her first dose of the AstraZeneca vaccine, and upon workup, was subsequently found to have thrombosis of her right portal and right common iliac vein.