Virus–Platelet Associations

Of vascular defense, hemostasis, cancer, and platelet biology: an evolutionary perspective

We have established considerable expertise in studying the role of platelets in cancer biology. From this expertise, we were keen to recognize the numerous venous-, arterial-, microvascular-, and macrovascular thrombotic events and immunologic disorders are caused by severe, acute-respiratory-syndrome coronavirus 2 (SARS-CoV-2) infections. With this offering, we explore the evolutionary connections that place platelets at the center of hemostasis, immunity, and adaptive phylogeny. Coevolutionary changes have also occurred in vertebrate viruses and their vertebrate hosts that reflect their respective evolutionary interactions. As mammals adapted from aquatic to terrestrial life and the heavy blood loss associated with placentalization-based live birth, platelets evolved phylogenetically from thrombocytes toward higher megakaryocyte-blebbing-based production rates and the lack of nuclei. With no nuclei and robust RNA synthesis, this adaptation may have influenced viral replication to become less efficient after virus particles are engulfed. Human platelets express numerous receptors that bind viral particles, which developed from archetypal origins to initiate aggregation and exocytic-release of thrombo-, immuno-, angiogenic-, growth-, and repair-stimulatory granule contents. Whether by direct, evolutionary, selective pressure, or not, these responses may help to contain virus spread, attract immune cells for eradication, and stimulate angiogenesis, growth, and wound repair after viral damage. Because mammalian and marsupial platelets became smaller and more plate-like their biophysical properties improved in function, which facilitated distribution near vessel walls in fluid-shear fields. This adaptation increased the probability that platelets could then interact with and engulf shedding virus particles. Platelets also generate circulating microvesicles that increase membrane surface-area encounters and mark viral targets. In order to match virus-production rates, billions of platelets are generated and turned over per day to continually provide active defenses and adaptation to suppress the spectrum of evolving threats like SARS-CoV-2.

The association between micronutrients and the SARS-CoV-2-specific antibodies in convalescent patients

BACKGROUND

Various micronutrients play key roles in the immune responses to viral infection, antibody synthesis, and susceptibility to infection. This study aimed to investigate the role of micronutrients on the immune responses following SARS-CoV-2 infection.

METHODS

To evaluate humoral immunity following SARS-CoV-2 infection, the levels of SARS-CoV-2-specific IgM and IgG, as well as the concentrations of different micronutrients, were determined in 36 convalescent COVID-19 patients 60 days after infection. Furthermore, the correlation between biochemical and hematological parameters, clinical features, and the changes in adiposity with SARS-CoV-2 antibodies was evaluated.

RESULTS

Serum IgM and IgG antibodies were detected in 38.8% and 83.3% of recovered patients after 60 days of COVID-19 infection, respectively. The values of SARS-CoV-2-specific IgG were negatively correlated with the number of the platelet. Moreover, the values of SARS-CoV-2-specific IgM were positively correlated with LDH and the vitamin B₁₂ concentration. Furthermore, a gender-specific association of SARS-CoV-2-specific IgG and IgM with vitamins D as well as with B₉ and zinc was observed. A significant negative correlation was observed between the values of IgG with vitamin D in male participants and a positive correlation was detected between IgG values and B₉ in female participants. Moreover, IgM levels with serum zinc values in females were negatively correlated.

CONCLUSION

Our study suggests the potential role of micronutrients in gender-specific humoral immunity following SARS-CoV-2 infection. Further studies are required with a greater sample of subjects to substantiate the validity and robustness of our findings.

Thrombotic Thrombocytopenia after COVID-19 Vaccination: In Search of the Underlying Mechanism

The rollout of COVID-19 vaccines brings hope for successful pandemic mitigation and getting the transmission of SARS-CoV-2 under control. The vaccines authorized in Europe displayed a good safety profile in the clinical trials. However, during their post-authorization use, unusual thrombotic events associated with thrombocytopenia have rarely been reported for vector vaccines. This led to the temporary suspension of the AZD1222 vaccine (Oxford/AstraZeneca) in various European countries and the Ad26.COV2 vaccine (Janssen/Johnson&Johnson) in the United States, with regulatory bodies launching investigations into potential causal associations. The thromboembolic reactions were also rarely reported after mRNA vaccines. The exact cause of these adverse effects remains to be elucidated. The present paper outlines the hypotheses on the mechanisms behind the very rare thrombotic thrombocytopenia reported after the COVID-19 vaccination, along with currently existing evidence and future research prospects. The following are discussed: (i) the role of antibodies against platelet factor 4 (PF4), (ii) the direct interaction between adenoviral vector and platelets, (iii) the cross-reactivity of antibodies against SARS-CoV-2 spike protein with PF4, (iv) cross-reactivity of anti-adenovirus antibodies and PF4, (v) interaction between spike protein and platelets, (vi) the platelet expression of spike protein and subsequent immune response, and (vii) the platelet expression of other adenoviral proteins and subsequent reactions. It is also plausible that thrombotic thrombocytopenia after the COVID-19 vaccine is multifactorial. The elucidation of the causes of these adverse events is pivotal in taking precautionary measures and managing vaccine hesitancy. It needs to be stressed, however, that the reported cases are currently sporadic and that the benefits of COVID-19 vaccines vastly outweigh their potential risks.

Phytochemicals as Potential Therapeutics for SARS-CoV-2-Induced Cardiovascular Complications: Thrombosis and Platelet Perspective

After gaining entry through ACE2 aided by TMPRSS2, the SARS-CoV-2 causes serious complications of the cardiovascular system leading to myocarditis and other myocardial injuries apart from causing lung, kidney and brain dysfunctions. Here in this review, we are going to divulge the cellular and immunological mechanisms behind the cardiovascular, thrombotic and platelet impairments that are caused in COVID-19. In addition, we also propose the significance of various anti-platelet and anti-thrombotic phytochemicals in the treatment of COVID-19. The virus induces many immune-modulatory cytokines and chemokines which help in the intravascular coagulation and create a pro-thrombotic environment along with pulmonary embolism and thrombocytopenia. Different types of innate and adaptive immune cells and their granular contents regulate the pathophysiology of SARS-CoV-2 induced endothelial and platelet dysfunctions which correlate the involvement of platelets with myocardial injury and intravascular thrombi directly or indirectly. Hence, by exploiting the natural bioactive compounds from medicinal plants and inhibiting the platelet mediated thrombus formation can be beneficial for the treatment of SARS-CoV-2 infection.

Platelets: 'multiple choice' effectors in the immune response and their implication in COVID-19 thromboinflammatory process

Although platelets are traditionally recognized for their central role in hemostasis, the presence of chemotactic factors, chemokines, adhesion molecules, and costimulatory molecules in their granules and membranes indicates that they may play an immunomodulatory role in the immune response, flanking their capacity to trigger blood coagulation and inflammation. Indeed, platelets play a role not only in the innate immune response, through the expression of Toll‐like receptors (TLRs) and release of inflammatory cytokines, but also in the adaptive immune response, through expression of key costimulatory molecules and major histocompatibility complex (MHC) molecules capable to activate T cells. Moreover, platelets release huge amounts of extracellular vesicles capable to interact with multiple immune players. The function of platelets thus extends beyond aggregation and implies a multifaceted interplay between hemostasis, inflammation, and the immune response, leading to the amplification of the body's defense processes on one hand, but also potentially degenerating into life‐threatening pathological processes on the other. This narrative review summarizes the current knowledge and the most recent updates on platelet immune functions and interactions with infectious agents, with a particular focus on their involvement in COVID‐19, whose pathogenesis involves a dysregulation of hemostatic and immune processes in which platelets may be determinant causative agents.

Search for SARS-CoV-2 RNA in platelets from COVID-19 patients

The frequent finding of thrombocytopenia in patients with severe SARS-CoV-2 infection (COVID-19) and previous evidence that several viruses enter platelets suggest that SARS-CoV-2 might be internalized by platelets of COVID-19. Aim of our study was to assess the presence of SARS-CoV-2 RNA in platelets from hospitalized patients with aconfirmed diagnosis of COVID-19. RNA was extracted from platelets, leukocytes and serum from 24 COVID-19 patients and 3 healthy controls, real-time PCR and ddPCR for viral genes were carried out. SARS-CoV-2 RNA was not detected in any of the samples analyzed nor in healthy controls, by either RT-PCR or ddPCR, while RNA samples from nasopharyngeal swabs of COVID-19 patients were correctly identified. Viral RNA was not detected independently of viral load, of positive nasopharyngeal swabs, or viremia, the last detected in only one patient (4.1%). SARS-CoV-2 entry in platelets is not acommon phenomenon in COVID-19 patients, differently from other viral infections.

Antiviral anticoagulation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel envelope virus that causes coronavirus disease 2019 (COVID-19). Hallmarks of COVID-19 are a puzzling form of thrombophilia that has elevated D-dimer but only modest effects on other parameters of coagulopathy. This is combined with severe inflammation, often leading to acute respiratory distress and possible lethality. Coagulopathy and inflammation are interconnected by the transmembrane receptor, tissue factor (TF), which initiates blood clotting as a cofactor for factor VIIa (FVIIa)-mediated factor Xa (FXa) generation. TF also functions from within the nascent TF/FVIIa/FXa complex to trigger profound changes via protease-activated receptors (PARs) in many cell types, including SARS-CoV-2-trophic cells. Therefore, aberrant expression of TF may be the underlying basis of COVID-19 symptoms. Evidence suggests a correlation between infection with many virus types and development of clotting-related symptoms, ranging from heart disease to bleeding, depending on the virus. Since numerous cell types express TF and can act as sites for virus replication, a model envelope virus, herpes simplex virus type 1 (HSV1), has been used to investigate the uptake of TF into the envelope. Indeed, HSV1 and other viruses harbor surface TF antigen, which retains clotting and PAR signaling function. Strikingly, envelope TF is essential for HSV1 infection in mice, and the FXa-directed oral anticoagulant apixaban had remarkable antiviral efficacy. SARS-CoV-2 replicates in TF-bearing epithelial and endothelial cells and may stimulate and integrate host cell TF, like HSV1 and other known coagulopathic viruses. Combined with this possibility, the features of COVID-19 suggest that it is a TFopathy, and the TF/FVIIa/FXa complex is a feasible therapeutic target.

Platelets and Airway Diseases

The role of platelets as inflammatory cells is now well established. Given the peculiar characteristics of the lung circulation, with a broad capillary bed, platelets are especially involved with the physiology of the lungs and play a key role in a number of inflammatory lung disorders. The platelet precursors, megakaryocytes, are detected in the lung microcirculation; moreover platelets with their endothelium-protective and vascular reparative activities contribute to the lung capillary blood barrier integrity. Given the function of the lungs as first wall against pathogen invasion, platelets participate in immune defence of the normal lung. On the other hand, platelets may turn into effectors of the inflammatory reaction of the lungs to allergens, to infectious agents, to chemical agents and may contribute strongly to the perpetuation of chronic inflammatory reactions, largely by their ability to interact with other inflammatory cells and the endothelium. In this chapter we provide an overview of the role of platelets in several inflammatory lung disorders discussing the pathophysiologic bases of platelet involvement in these conditions and the experimental and clinical evidence for a role of platelets in lung diseases.