key: cord-0686359-awpwbcwt
authors: Iba, Toshiaki; Levy, Jerrold H.
title: The roles of platelets in COVID-19-associated coagulopathy and vaccine-induced immune thrombotic thrombocytopenia
date: 2021-08-27
journal: Trends Cardiovasc Med
DOI: 10.1016/j.tcm.2021.08.012
sha: 152c40a8f37dc54385bd52cbf62a38c224113716
doc_id: 686359
cord_uid: awpwbcwt

In coronavirus disease 2019 (COVID-19), multiple thromboinflammatory events contribute to the pathophysiology, including coagulation system activation, suppressed fibrinolysis, vascular endothelial cell injury, and prothrombotic alterations in immune cells such as macrophages and neutrophils. Although thrombocytopenia is not an initial presentation as an infectious coagulopathy, recent studies have demonstrated the vital role of platelets in COVID-19-associated coagulopathy SARS-CoV-2 and its spike protein have been known to directly or indirectly promote release of prothrombotic and inflammatory mediators that lead to COVID-19-associated coagulopathy. Although clinical features of vaccine-induced immune thrombotic thrombocytopenia include uncommon locations of thrombosis, including cerebral venous sinus, we speculate coronavirus spike-protein-initiated prothrombotic pathways are involved in the pathogenesis of vaccine-induced immune thrombotic thrombocytopenia, as current evidence suggests that the spike protein is the promotor and other cofactors such as perturbed immune response and inflammatory reaction enhance the production of anti-platelet factor 4 antibody.

Platelets are critical components of hemostasis but also participate in the host defense mechanisms of bacterial infections. Platelets are known to play vital roles as immune modifiers as well as the progenitor cells of clot formation in collaboration with the other immune cells and the coagulation system (1, 2) . In invertebrates, platelets have been shown to directly kill pathogens. Meanwhile, platelets induce neutrophil extracellular traps (NETs) release and indirectly dispose of pathogens in vertebrates (3, 4) . These responses to infection are elicited by the interaction between soluble mediators, including thrombin, damage-associated molecular patterns (DAMPs), and their membrane surface receptors (5) . Hemocytes, an ancestor cell that serves platelet functions in invertebrates have evolved to important modulators of host defense, inflammation, and hemostasis (3) . Platelets detect pathogens and deploy host-defense peptides such as platelet factor 4, a major platelet-derived CXC chemokine, to recruit and facilitate leukocyte responses in the context of the innate immune system (6) . Platelets also act as liaisons to promote T cell-B cell crosstalk, critical interactions required in adaptive immunity (7) . However, the extent to which platelets are the major response initially in the host defense against viral infection in humans has not been determined.

Microthrombosis, a prominent pathological feature of coronavirus disease 2019 (COVID- 19) , has been extensively documented by autopsy reports demonstrating extensive platelet-fibrin clot formation in the pulmonary microvasculature in 80-100% of lungs examined (8) . In this review, we discuss the roles of platelets in the pathogenesis of COVID-19-associated coagulopathy (CAC) and vaccine-induced immune thrombotic thrombocytopenia (VITT).

Although platelet counts are within the normal range in most patients who initially present with COVID-19, platelet activation plays a vital role in thrombosis development.

For instance, the cytokine storm described in COVID-19 stimulates platelets to express tissue factor on their surface and release procoagulant microvesicles (9) . As a result, increased levels of tissue factor-positive platelets and microvesicles are reported in COVID-19 patients (10) . The increase in von Willebrand factor, especially its ultra-high-molecular-weight form, occurs in the early stages of COVID-19 and platelet clumping occurs (11, 12) . The immunothrombus formed mainly by platelets and leukocytes is also a hallmark of intravascular clotting in COVID-19 (8, 13) . Manne et al. (14) reported both platelet aggregates and circulating platelet-neutrophil, -monocyte, and -T-cell aggregates increased significantly in COVID-19 patients. Virus-induced sensitization of platelets is observed with increased platelet responsiveness by thromboxane generation to low-dose agonist stimulation. Thrombin, a potent platelet agonist, also generated early in COVID-19 to further activate platelets. Platelets also release various cytokines that modulate inflammation (Interleukin [IL]-1α, IL-1β, IL-4, IL-10, IL-13, interferon-α, and interferon-γ), chemokines (monocyte chemoattractant protein 1), and growth factors (vascular endothelial growth factor) (15) .

The cytokine storm and macrophage activating syndrome play important roles in the pathogenesis of COVID-19. Excessive cytokines that are released are important in further orchestrating the systemic hemodynamic alterations and cardiovascular injury, and the mortality rate increases with higher cytokine levels (16) . In COVID-19, elevated levels of biomarkers for cardiac injuries, such as natriuretic peptides, troponins, myoglobin were observed along with the increased levels of C-reactive protein, IL-2, and IL-6. The possible mechanisms of cardiovascular injury include direct toxicity through the viral invasion to the cardiac myocytes, ACE2 receptor-mediated cardiovascular injury, microthrombosis, and cytokine release syndrome (17) . Among various cytokines, IL-17 attracted much attention recently. IL-17 is a multifunctional cytokine that shows a mixed immunopathological effect (from pro to opposite antiinflammatory) depending on the condition (18) . In COVID-19, IL-17 activates platelets and the severity of disease presentation was shown to positively correlate with levels of IL-17 and other T-helper-17 lymphocytes-producing pro-inflammatory cytokines such as IL-1, IL-6, IL-15 tumor necrosis factor, and interferon-γ (19) .

Although its efficacy is still inconclusive, the inhibitory effect of IL-17 has been examined in a pilot study (20) .

Except for D-dimer and fibrinogen levels, standard coagulation tests including prothrombin time and activated partial thromboplastin time are often normal. However, additional biomarkers of thrombin generation or activation, including thrombin-antithrombin complex (TAT) and prothrombin fragment 1.2 are markedly elevated in COVID-19 (21) . As mentioned, thrombin activation stimulates protease-activated receptor-1 (PAR1), a critical mechanism in platelet aggregation and upregulation of coagulation (22) . Although in most patients, platelet counts initially are normal, the count is lower in non-survivors than in survived patients with COVID-19, and platelet count less than 150,000 /μL is a predictor of worse outcome (23) .

Platelet size is an indicator of platelet recycling, and the mean platelet volume is known to correlate with platelet activation and increases in septic patients with thrombosis. Comer et al. (24) reported elevated mean platelet volume in COVID-19 higher levels in critically ill patients, while Zhang et al. (25) reported increased mean platelet volume correlated with platelet count decreases. Moreover, single-cell transcriptome analysis showed megakaryocyte progenitors increased up to 5% of CD34 + cells in all symptomatic COVID-19 patients, while such increase was absent in healthy or asymptomatic patients (26) .

Platelet susceptibility to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is still controversial, but Zhang et al. (27) reported platelets express angiotensin-converting enzyme 2 (ACE2), and the spike protein binding directly enhances platelet aggregation, α-granule secretion, and spike protein-induced thrombus formation ( Fig. 1) although Manne et al. (14) reported ACE2 was not detected by messenger RNA (mRNA) or protein levels in the platelets. The discussion on SARS-CoV-2 infection via ACE2 is still ongoing, and Campbell et al. (27) suggested that platelets may express different SARS-CoV-2 receptors.

In bacterial infections, microcirculatory thrombus is formed as a host defense mechanism that inhibits microbial dissemination. However, systemic microthrombosis impairs tissue circulation and leading to organ dysfunction (28) that is initially localized in the lung, platelet counts do not decrease. Nevertheless, platelets are functionally activated and contribute to thrombus formation, and thrombotic sequela. Platelet activation is also determined by additional factors that include P-selectin expression, platelet-leukocyte aggregate formation, and altered nitric oxide/prostacyclin synthesis (10, 29) . For further understanding of CAC, additional indicators of platelet function such as mean platelet volume, platelet factor 4, and P-selectin beyond platelet counts should be considered. It is also important to remind clinicians that the prothrombotic shift depends on the mutual effect between platelets and other factors including activated coagulation, vascular dysfunction, neutrophils, and the complement system (30, 31) .

Platelet factor 4, a CXC chemokine stored in α-granule of the platelet, is released from activated platelets during aggregation, and promotes further thrombogenesis via platelet clumping and NETs release (32, 33) . Platelet factor 4 is also known as a heparin-binding protein, and the physiological role of platelet factor 4 is to neutralize heparan sulfate on the endothelial surface, thereby inhibiting the local antithrombotic properties (34) . In addition, the positively charged platelet factor 4 binds negatively charged bacterial surfaces to promote opsonization. In this fashion, platelet factor 4 contributes to the host-defense against bacterial infection (24) . Platelets provide an essential role in bacteria-killing and immunothrombus formation in sepsis. Activated platelets release platelet factor 4 that stimulates platelets, macrophage, and neutrophils via the interaction with IgG and its receptor FcγRIIA on the cellular membrane (35) . platelet factor 4 is also known to bind polyanions charge-dependently and undergoes a conformational change. This change provokes the exposure of antigenic epitope that induces the production of anti-platelet factor 4/polyanions antibodies. As a result, antiplatelet factor 4/polyanion antibodies also helps to eliminate various pathogens from the host (36), but also can induce heparin-induced thrombocytopenia (HIT) without heparin, a response known as spontaneous HIT (37) . HIT is an immune complication of heparin therapy caused by antibodies to platelet factor 4/heparin conjugates (also known as HIT antibody), and spontaneous HIT can occur without prior heparin exposure. The clinical feature of HIT is represented by thrombosis with thrombocytopenia and the mortality exceeds 20% (38) .

Multiple platelet biomarkers are increased in COVID-19 patients (24, 39) , including platelet factor 4, soluble P-selectin, and thrombopoietin. Comer et al. reported agonist-induced ADP release from platelets was 30-to 90-fold higher in COVID-19 compared to controls and unrelated to platelet counts. The prevalence of IgG HIT antibodies in COVID-19 patients is high, and a prospective study demonstrated an incidence of 11% IgG HIT antibodies after hospitalization unrelated to a HIT diagnosis (40) . These observations suggest the SARS-CoV-2 can potentiate the platelet factor 4 release, and the circulating platelet factor 4-induced platelet activation may contribute to the pathogenesis of CAC.

In the early communication from the European Medicines Agency (https://www.ema.europa.eu/en/documents/prac-recommendation/signal-assessment-rep ort-embolic-thrombotic-events-smq-covid-19-vaccine-chadox1-s-recombinant-covid_en .pdf), 30 cases of thromboembolic events had been reported by March 11, 2021 among approximately 5.5 million ChAdOx1, an adenovirus-vectored vaccine, recipients. At that time, the thromboembolic events were rare, and the risk did not seem to increase beyond the expected rate (41) . However, soon after, the number of cases increased, and as of April 4, 2021, 222 cases with cerebral venous sinus thrombosis (CVST) were reported to the European drug safety database out of 34 million vaccinations with ChAdOx1 (estimated incidence of 1:150 000) (42) , and this thrombotic event has been recognized as rare but serious complication and named vaccine-induced immune The pathogenic HIT antibodies bind cellular FcγRIIA, a low-affinity receptor for the Fc-fragment of immunoglobulin G, on platelets and monocytes to upregulate aggregation and propagate inflammation (45) . However, since most of the VITT cases were not exposed to heparins, a similar mechanism with heparin-independent spontaneous HIT is suspected. In VITT, free DNA in the vector vaccine or DNA from other sources is assumed to bind platelet factor 4 and triggers the HIT antibody production (46) . McGonagle et al. (47) suggested viral DNA-platelet factor 4 interplay is a part of antiviral immunity, akin to the innate immunity for bacterial infection.

Another possible source of DNA is the exaggerated immune reaction of the leukocytes.

The inflammatory stimulus after vaccination can cause NETs formation with the DNA release from macrophages and neutrophils (48) . Other than above, hypersulfated glycosaminoglycan, chondroitin sulfate, is known to trigger spontaneous HIT after trauma and knee surgery (46) . Chondroitin sulfate, a major component of the glycocalyx on the endothelial cell that is easily injured and released into the circulation during inflammation, can be another candidate of the polyanion source (49). (Fig. 2) During the course of anti-platelet factor 4 antibody production, the involvement of spike protein is significant. Similar to SARS-CoV-2, the spike protein will stimulate the release of platelet factor 4 from platelets, activate immune cells to induce inflammatory reaction, and turn the antithrombotic properties of the vascular lumen toward the opposite side through the binding to ACE2 (50) . Therefore, although VITT was initially reported as the complication of the virus-vectored vaccine, it can be also found after the vaccination with mRNA vaccine (51-53).

As described previously, the spike protein of coronavirus upregulates the inflammatory response and injures the vascular endothelium by binding to ACE2 (54) . ACE2 converts angiotensin II to angiotensin 1-7, a molecule counterbalancing the vasoconstrictive, proinflammatory, and pro-coagulant effects of angiotensin II (55) . Therefore, decline of angiotensin 1-7 on the cellular membrane by COVID-19 infection, as well as loss of the endothelial anticoagulant effects, leads to microcirculatory disturbance by vasoconstriction, profound inflammation, and activated coagulation (56) . The vaccine-induced spike protein also may induce the similar reactions and may serve the underlying condition of thrombogenesis. Biancatelli et al. (57) reported the S1 subunit of SARS-CoV-2 spike protein alone could produce acute lung injury. So far, the amount of produced spike protein and the productivity difference between the vaccines have not been examined; however, the immunogenicity of the spike protein may not be the same among the vaccines. Kowarz et al. (58) reported the spike protein is immunogenic, and the immunogenicity of the spike protein generated by virus-vectored vaccine is more potent than that of mRNA vaccine. They also reported the virus-vectored vaccine can produce variant spike protein by splicing, and that may cause the intensified inflammation and hypercoagulation.

Although the immunogenicity is considered higher in virus-vectored vaccine, the induction of neutralizing antibody is more potent in mRNA vaccines, and neutralization level is reportedly highest in mRNA vaccines followed by convalescent serum, and the levels were lower in virus-vectored vaccines (59) . Thus, the production of anti-platelet (61) . These observations suggest that anti-platelet factor 4 antibody production is the consequence of the immune reaction evoked by the spike protein (60) . However, in many cases, the platelet activation test was negative, and the antibody is functionally inactive (62) . Therefore, anti-platelet factor 4 antibodies and other factors are necessary for VITT development.

The current evidences suggest spike-protein can trigger the production of anti-platelet factor 4 antibody, and the immunogenicity of vectored vaccine is more potent in terms of anti-platelet factor 4 antibody production, but other missing links should be found to explain the higher prevalence of VITT in the vectored vaccines.

The virus-vectored vaccine elicits poly clonal antibodies that make the virus neutralization possible driving other antibody-dependent effector functions as well as potent T cell responses (63) . The splicing occurs only in virus-vectored vaccines, and the immunogenicity of vectored vaccine to induce anti-platelet factor 4 antibody seems higher, however, VITT-like adverse events are also rarely found in mRNA vaccines. Whether spike protein-elicited thrombogenicity can explain the different prevalence of VITT still remains to be determined. As for the possibility of spike protein-induced thrombogenicity, SARS-CoV-2 is known to use ACE2 to infect lung epithelial cells and elicit intra-alveolar inflammation and peri-alveolar thrombogenicity (66) . It is generally accepted that SARS-CoV-2 attach to endothelial cells by binding to ACE2 expressed on the surface (67). However, some researchers doubt whether the spike protein of SARS-CoV-2 directly alters endothelial cell functions (68, 69) as the presence of ACE2 on endothelial cell membranes is uncertain. Nascimento Conde et al. (70) demonstrated that primary human endothelial cells lack ACE2 at both protein and RNA levels, and suggested SARS-CoV-2 is incapable of infect endothelial cells. If endothelial cell expresses ACE2, and the spike protein produced by the vectored vaccines upregulates the endothelial prothrombotic reaction more vigorously, the higher prevalence of VITT is understandable. The splicing variant can also be the breakthrough of this issue (51), but the presence of ACE2 on endothelium is the precondition of this theory.

Another possible explanation of the different VITT prevalence rates is the participation of specific components in the vectored vaccine that facilitate the formation of anti-platelet factor 4/polyanion antibody. Negatively charged extracellular DNA in the vector virus binds platelet factor 4 and DNA-platelet factor 4 engagement may lead to platelet factor 4-directed thrombosis (47) . In this case, the exaggerated inflammatory response induced by free DNA-Toll-like receptor on platelets may also be involved in the hyperinflammation and the release of platelet factor 4. Other than above, not only the humoral factors i.e., dendritic cell-B lymphocytes system but also the cellular immunity that includes dendritic cells, cytotoxic T cells, and helper T cells may contribute to the different prevalence in VITT (71) .

The perplexing mystery in VITT is why the thrombosis frequently occurs in cerebral venous sinus. According to the data prior to COVID-19 pandemic, thrombocytopenia was observed in 8.4% of total CVST, and heparin-induced thrombocytopenia was only 0.1% (72) . Consequently, even if the incidence of CVST in 2021 does not increase compared to anticipated risk, a strong connection should exist with vaccination. The incidence of CVST in COVID-19 was reportedly 0.02% (20/100,000, 95% CI: 4-60/100,000) however, these patients did not show thrombocytopenia and anti-platelet factor 4 antibodies in this report (73) Are there any specific backgrounds or comorbidities related to the risk of VITT?

Are there any genetic risks for the recipients? These additional questions also need to be answered.

SARS-CoV-2 can alter the host immune system, and the increased prevalence of autoantibodies such as antinuclear antibodies and antigen-specific autoantibodies in COVID-19 patients is reported (76) . The vaccination has been known to induce various autoimmune disorders like immune thrombotic thrombocytopenic purpura (ITP) (77) and Guillain-Barre syndrome (78) , and the pathogenic autoantibody generation due to the immunologic deregulation is highly suspected. The chimpanzee adenovirus-vectored Ebola vaccine is reported to cause thrombocytopenia without thrombosis however, the mechanism has not been resolved (79) . Regarding COVID-19 vaccine, ITP is listed as another autoimmune thrombocytopenic disease (80) . ITP is mediated by antiplatelet autoantibodies against membrane glycoprotein (GP) complexes such as GPIIb/IIIa and GPIb/IX (81) . However, the detection of these antibodies is not commonly performed, and the clinical diagnosis is made by excluding other thrombocytopenic diseases (82) .

Recently, a prospective cohort study surveyed the association between ChAdOx1 vaccination and ITP. The result reported the incidence was 1.13 (95%CI: 0.62-1.63) cases per 100,000 doses. The study concluded that the ChAdOx1 vaccination is associated with small increased risk of ITP with small increased risks arterial thromboembolic and hemorrhagic events (65) . Meanwhile, according to Vaccine Adverse Event Reporting System (VAERS), the prevalence of thrombocytopenia was higher and 8.0 per 100,000 doses for both mRNA and vectored vaccine (80) . Kuter (83) prospectively followed fifty-two consecutive chronic ITP patients after vaccination and reported a platelet count drop of 96% within 2-5 days after vaccination in 12% of the patients. The events could occur in both types of vaccines and the platelet counts recovered to more than 30,000 /μL after the treatment with corticosteroids and intravenous immunoglobulin (IVIG).

Other than ITP, cases are even rare but the complication of thrombotic thrombocytopenic purpura was reported after receiving either Ad26.COV2-S or mRNA-based anti-COVID-19 vaccine (84, 85) . Thus, when the patients demonstrate unexpected thrombocytopenia shortly after the vaccination, the possibility of these diseases should be considered.

SARS-CoV-2 infection induces a process known as immunothrombosis, in which activated neutrophils and monocytes interact with platelets and the coagulation systems, leading to intravascular clot formation from small to large vessels (86) . Therapeutic approach to mitigate immunothrombus may theoretically be useful, and anticoagulant and antiinflammatory agents have been proposed as therapeutic candidates. However, high-quality evidence does not exist to support the use of anticoagulants. An observational study demonstrated a better survival (Hazard ratio [HR]: 0.73, 95% CI: 0.66-0.81) in the patients treated with prophylactic dose of anticoagulants within 24 hours after admission compared to the patients treated without anticoagulation (87) . In contrast, therapeutic dose of anticoagulation with rivaroxaban or enoxaparin followed by rivaroxaban did not improve the clinical outcomes but increased bleeding compared with prophylactic anticoagulation (88) . A systematic review and meta-analysis revealed that the overall odds of mortality comparing anticoagulated to non-anticoagulated patients were similar. Of note was the mortality was lower with prophylactic dose In summary, the anticoagulation will be beneficial however, intensive anticoagulation may increase the bleeding risk and potentially harmful for the COVID-19 patients. The patients who may benefit from the intensive anticoagulation should be carefully selected.

Regarding the efficacy of antiinflammatory therapy, series of randomized controlled trials (RCTs) examined the effects of an anti-interleukin-6 receptor monoclonal antibody. In the early RCT, tocilizumab failed to prevent the intubation or death in moderately ill COVID-19 patients required oxygenation (91) . Next RCT performed in COVID-19 pneumonia patients treated without mechanical ventilation showed reduced progression to the composite outcome of mechanical ventilation or death (92) .

Following these, a large RCT examined the effect of tocilizumab in critically ill patients receiving organ support in ICU. The treatment with the tocilizumab and sarilumab improved outcomes including survival (93) . Most recent RCT was done in hospitalized patients with severe COVID-19 pneumonia, and the use of tocilizumab did not result in significantly better clinical status or lower mortality than placebo at 28 days (94) . Some other RCTs reported mixed results, and further study is necessary to obtain the definitive result.

Other than anti-interleukin-6 therapy, the effect of non-specific antiinflammatory therapy with corticosteroid have been reported. In RECOVERY trial, an open-label randomized trial performed in hospitalized patients with Covid-19, the use of dexamethasone resulted in lower 28-day mortality in the patients receiving oxygen or ventilated at randomization (95) . In another trial, the efficacy of 7-day fixed-dose of hydrocortisone or shock-dependent dosing of hydrocortisone was compared with no hydrocortisone. The results showed 93% and 80% probabilities of superiority with regard to the improvement in organ support-free days within 21 days; however, neither treatment strategy met prespecified criteria for statistical superiority (96) . While the definitive answer needs further investigation, the present evidence suggests that moderate to severely ill COVID-19 patients may benefit from corticosteroid therapy.

Since platelets contribute to the thrombotic and inflammatory response in sepsis, it is logical to think antiplatelet therapy might be beneficial. However, the reported efficacy of antiplatelet therapy in septic patients is inconsistent, and therapeutic approaches to inhibit platelet activation and microthrombosis formation need further investigation. Platelet-activating factor (PAF) is an endogenous proinflammatory mediator implicated in the pathogenesis of sepsis. The clinical effect of lexipafant, a PAF receptor antagonist, was examined in a small-size double-blind RCT but failed to show the improvement of survival (97) . Similarly, the effect of recombinant PAF acetylhydrolase that degrades PAF was examined in a phase III trial, but this agent also could not show a positive impact in terms of survival (98) . Much later, the effects of more common antiplatelet agents such as aspirin and P2Y 12 inhibitors, including clopidogrel, prasugrel, and ticagrelor were examined in an observational study, and the use of these agents was shown to associate with the reduced mortality in sepsis (OR: 0.82, 95% CI: 0.81-0.83) (99) . In contrast, in a prospective setting, 100 mg once daily of aspirin did not reduce the mortality in sepsis (HR: 1.08, 95% CI: 0. 82 In the event of major thrombotic events 4 to 30 days after COVID-19 vaccination, VITT should be considered, and the infusion of high-dose IVIG (1g/kg followed by a second dose 24 h later) in combination with non-heparin anticoagulants are recommended. The objective of immunoglobulin therapy is to neutralize the causative anti-platelet factor 4/polyanion antibody. Steroids or plasma exchange are also options to reduce the autoantibodies (103,104). Vayne et al. (105) reported platelet activation was suppressed by IV.3, a monoclonal antibody that binds FcγRIIA receptors and also by IdeS (IgG-degrading enzyme derived from Streptococcus pyogenes).

Thrombosis is a major pathological driver in COVID-19. Evolving evidence suggests that in addition to the activated leukocytes and derangement of antithrombotic property of endothelial cells, hyperactive platelets participate in thrombogenesis. The direct and indirect effects of SARS-CoV-2 spike protein on platelets stimulate the release of platelet factor 4. The spike protein also upregulates inflammation and coagulation through the binding to ACE2 on macrophages/monocytes, lung epithelial cells, and possibly vascular endothelial cells, reactions that lead to micro and macro circulatory clotting known as CAC. In this regard, CAC is not just a result of SARS-CoV-2 infection but the major facilitator of COVID-19. The virus vectored vaccines provide a certain amount of DNA that binds platelet factor 4 released from platelets. DNA-platelet factor 4 binding induces the antigenicity of platelet factor 4, and the newly generated anti-platelet factor 4/polyanion antibody causes VITT. Other than the adenovirus-vectored vaccine origin DNA, polyanion can be originated from NETs and endothelial glycocalyx. As for the treatment of CAC, there is no high-quality evidence that supports the use of antiplatelet therapy. For VITT, high-dose IVIG is recommended to neutralize anti-platelet factor 4 antibody-FcγRIIA binding. It may be rational to think that the thrombotic events in CAC and VITT occur not separately but rather some common mechanisms exist. Understanding the mechanism is extremely important to develop safer vaccines. 

Bacterial-platelet interactions: virulence meets host defense

The role of platelets in sepsis

Platelets in inflammation and infection

Platelet CXCL4 mediates neutrophil extracellular traps formation in ANCA-associated vasculitis

Recent advances in the research and management of sepsis-associated DIC

The beta-thromboglobulins and platelet factor 4: blood platelet-derived CXC chemokines with divergent roles in early neutrophil regulation

Platelets: at the nexus of antimicrobial defence

The Emerging Threat of (Micro)Thrombosis in COVID-19 and Its Therapeutic Implications

Potential role for tissue factor in the pathogenesis of hypercoagulability associated with in COVID-19

Platelet-leukocyte interactions in the pathogenesis of viral infections

Recombinant ADAMTS13 reduces abnormally up-regulated von Willebrand factor in plasma from patients with severe COVID-19

Morphologic changes in circulating blood cells of COVID-19 patients

Immunothrombosis and thromboinflammation in host defense and disease

Platelet gene expression and function in patients with COVID-19

Platelets promote thromboinflammation in SARS-CoV-2 pneumonia

Cytokine storm in COVID-19: pathogenesis and overview of anti-inflammatory agents used in treatment

Cardiac manifestations of COVID-19

Interleukin-17A (IL-17A), a key molecule of innate and adaptive immunity, and its potential involvement in COVID-19-related thrombotic and vascular mechanisms

COVID-19: a case for inhibiting IL-17?

Anti-IL-17 monoclonal antibodies in hospitalized patients with severe COVID-19: A pilot study

Retrospective analyses associate hemostasis activation biomarkers with poor outcomes in patients with COVID-19

Protease-activated receptor 1 is the primary mediator of thrombin-stimulated platelet procoagulant activity

Proteinase-activated receptor 1: A target for repurposing in the treatment of COVID-19?

COVID-19 induces a hyperactive phenotype in circulating platelets

SARS-CoV-2 binds platelet ACE2 to enhance thrombosis in COVID-19

Single-cell multi-omics analysis of the immune response in COVID-19

Is there a role for the ACE2 receptor in SARS-CoV-2 interactions with platelets?

Sepsis-Induced Coagulopathy and Disseminated Intravascular Coagulation

Platelet and endothelial qctivation as potential mechanisms behind the thrombotic complications of COVID-19 patients

COVID-19 and thrombosis: From bench to bedside

COVID-19 -A vascular disease

Synchronized integrin engagement and chemokine activation is crucial in neutrophil extracellular trap-mediated sterile inflammation

Platelet factor 4: a chemokine enigma

Platelet factor 4 neutralizes heparan sulfate-enhanced antithrombin inactivation of factor Xa by preventing interaction(s) of enzyme with polysaccharide

Platelets kill bacteria by bridging innate and adaptive immunity via platelet factor 4 and FcγRIIA

Bacterial killing by platelets: making sense of (H)IT

High-dose intravenous immunoglobulin for the treatment and prevention of heparin-induced thrombocytopenia: a review

Heparin-induced thrombocytopenia: a comprehensive clinical review

Platelet factor 4 binds to bacteria, [corrected] inducing antibodies cross-reacting with the major antigen in heparin-induced thrombocytopenia

Contrast between prevalence of HIT antibodies and confirmed HIT in hospitalized COVID-19 patients: a prospective study with clinical implications

Thromboembolism and the Oxford-AstraZeneca COVID-19 vaccine: side-effect or coincidence?

Eudravigilance -European database of suspected adverse reaction reports

Thrombotic Thrombocytopenia after ChAdOx1 nCov-19 Vaccination

Antibody epitopes in vaccine-induced immune thrombotic thrombocytopenia

Platelet transactivation by monocytes promotes thrombosis in heparin-induced thrombocytopenia

Spontaneous HIT syndrome: Knee replacement, infection, and parallels with vaccine-induced immune thrombotic thrombocytopenia

Mechanisms of immunothrombosis in vaccine-induced thrombotic thrombocytopenia (VITT) compared to natural SARS-CoV-2 infection

Pathological inflammation in patients with COVID-19: a key role for monocytes and macrophages

Recognizing Vaccine-Induced Immune Thrombotic Thrombocytopenia

SARS-CoV-2 vaccines: Lights and shadows

Cerebral Venous Thrombosis after BNT162b2 mRNA SARS-CoV-2 vaccine

The importance of recognizing cerebral venous thrombosis following anti-COVID-19 vaccination

Thrombosis with thrombocytopenia after the messenger RNA-1273 vaccine

Angiotensin-converting enzyme 2 (ACE2): COVID 19 gate way to multiple organ failure syndromes

COVID-19 infection, inflammation, and coagulopathy: missing pieces in the puzzle

The pivotal link between ACE2 deficiency and SARS-CoV-2 infection

The SARS-CoV-2 Spike Protein Subunit 1 induces COVID-19-like acute lung injury in Κ18-hACE2 transgenic mice and barrier dysfunction in human endothelial cells

Vaccine-Induced Covid-19 Mimicry" syndrome: splice reactions within the SARS-CoV-2 spike open reading frame result in spike protein variants that may cause thromboembolic events in patients immunized with vector-based vaccines

Neutralizing antibody levels are highly predictive of immune protection from symptomatic SARS-CoV-2 infection

Frequency of positive anti-platelet factor 4/polyanion antibody tests after COVID-19 vaccination with ChAdOx1 nCoV-19 and BNT162b2

The high frequency of anti-platelet factor 4/heparin antibodies in patients with COVID-19 is neither related to heparin treatment or to an increased incidence of thrombosis

COVID-19 patients often show high-titer non-platelet-activating anti-platelet factor 4/heparin IgG antibodies

Immunological mechanisms of vaccine-induced protection against COVID-19 in humans

Blood clots and bleeding events following BNT162b2 and ChAdOx1 nCoV-19 vaccine: An analysis of European data

First-dose ChAdOx1 and BNT162b2 COVID-19 vaccines and thrombocytopenic, thromboembolic and hemorrhagic events in Scotland

Single-cell RNA-seq data analysis on the receptor ACE2 expression reveals the potential risk of different human organs vulnerable to 2019-nCoV infection

Tissue distribution of ACE2 protein, the functional receptor for SARS coronavirus. A first step in understanding SARS pathogenesis

The vascular endothelium: the cornerstone of organ dysfunction in severe SARS-CoV-2 infection

COVID-19 vasculopathy: mounting evidence for an indirect mechanism of endothelial injury

Recombinant ACE2 expression is required for SARS-CoV-2 to infect primary human endothelial cells and induce inflammatory and procoagulative responses

SARS-CoV-2 immunity: review and applications to phase 3 vaccine candidates

Cerebrovascular events and outcomes in hospitalized patients with COVID-19: The SVIN COVID-19 multinational registry

Cerebral venous sinus thrombosis associated with COVID-19: a case series and literature review

The Altered anatomical distribution of ACE2 in the brain with Alzheimer's disease pathology

COVID-19-associated autoimmunity as a feature of acute respiratory failure

Recurrent idiopathic thrombotic thrombocytopenic purpura: a role for vaccination in disease relapse?

Acute thrombotic thrombocytopenic purpura after pneumococcal vaccination

Safety, reactogenicity, and immunogenicity of a chimpanzee adenovirus vectored Ebola vaccine in children in Africa: a randomised, observer-blind, placebo-controlled, phase 2 trial

Thrombocytopenia including immune thrombocytopenia after receipt of mRNA COVID-19 vaccines reported to the Vaccine Adverse Event Reporting System (VAERS)

Autoantibodies and autoantigens in chronic immune thrombocytopenic purpura

Evolution and utility of antiplatelet autoantibody testing in patients with immune thrombocytopenia

Exacerbation of immune thrombocytopenia following COVID-19 vaccination

Thrombotic thrombocytopenic purpura after Ad26.COV2-S vaccination

First report of a de novo iTTP episode associated with an mRNA-based anti-COVID-19 vaccination

Endothelial dysfunction and immunothrombosis as key pathogenic mechanisms in COVID-19

Early initiation of prophylactic anticoagulation for prevention of coronavirus disease 2019 mortality in patients admitted to hospital in the United States: cohort study

Therapeutic versus prophylactic anticoagulation for patients admitted to hospital with COVID-19 and elevated D-dimer concentration (ACTION): an open-label, multicentre, randomised, controlled trial

Anticoagulation and in-hospital mortality from coronavirus disease 2019: A systematic review and meta-analysis

Therapeutic Anticoagulation with Heparin in Critically Ill Patients with Covid-19

Efficacy of tocilizumab in patients hospitalized with Covid-19

Tocilizumab in Patients Hospitalized with Covid-19 Pneumonia

Interleukin-6 Receptor Antagonists in Critically Ill Patients with Covid-19

Tocilizumab in Hospitalized Patients with Severe Covid-19 Pneumonia

Dexamethasone in Hospitalized Patients with Covid-19

Effect of Hydrocortisone on Mortality and Organ Support in Patients With Severe COVID-19: The REMAP-CAP COVID-19 Corticosteroid Domain Randomized Clinical Trial

A double-blind placebo-controlled study of an infusion of lexipafant (Platelet-activating factor receptor antagonist) in patients with severe sepsis

Recombinant human platelet-activating factor acetylhydrolase for treatment of severe sepsis: results of a phase III, multicenter, randomized, double-blind, placebo-controlled, clinical trial

Association of prior antiplatelet agents with mortality in sepsis patients: a nationwide population-based cohort study

Effect of aspirin on deaths associated with sepsis in healthy older people (ANTISEPSIS): a randomised, double-blind, placebo-controlled primary prevention trial

Aspirin Use Is Associated With Decreased Mechanical Ventilation, Intensive Care Unit Admission, and In-Hospital Mortality in Hospitalized Patients With Coronavirus Disease

RECOVERY trial finds aspirin does not improve survival for patients hospitalised with COVID-19

SARS-CoV-2 Vaccine and Thrombosis: An Expert Consensus on Vaccine-Induced Immune Thrombotic Thrombocytopenia

Therapeutic Plasma Exchange in Vaccine-Induced Immune Thrombotic Thrombocytopenia

Immunoassays in Vaccine-Induced Thrombotic Thrombocytopenia