It is unsurprising that platelet-neutrophil interactions are greatly increased during inflammatory responses (16–19). For the most part, soluble mediators initiate these interactions, which directly activate these cells. Platelets and neutrophils co-incubated with septic patient plasma induced platelet adhesion to neutrophils mediated by the TLR4 receptor (20, 21).

Neutrophils are the most abundant subset of leukocytes in arterial thrombi from patients with myocardial infarction (22). Activated neutrophils express adhesion molecules belonging to the selectin and integrin families promoting platelets and the endothelium binding (23). As well, activated platelets express adhesion molecules on their surface membrane, such as P-selectin that mediates binding of platelets to its main receptor on neutrophils, P-selectin-glycoprotein-ligand-1 or PSGL-1 (24). Indeed, neutrophils and activated platelets can recruit each other to inflamed or injured tissues, thereby causing thrombo-inflammation (25). In this regard, we found that platelets can modulate neutrophil adhesion to the injured arterial wall and that both elements influence the degree of post-injury vasoconstriction in in vivo porcine model involving arterial injury by angioplasty (26). More recently, we have revealed that platelet activation and binding to neutrophils enhance the secretion of platelet MMP-2 via an adhesive interaction between P-selectin and PSGL-1, which contributes to increase platelet-neutrophil aggregation (27). Inhibition of platelet-leukocyte binding, using a recombinant PSGL-1 reduced restenosis (28) and prevented in in-stent restenosis via reduction of thrombo-inflammatory reactions (29).

Neutrophil extracellular traps (NETs) are composed of DNA, histones, and antimicrobial peptides, and they are produced as part of an antimicrobial mechanism, which is affected by immune/immune-related cells during NETosis (30). Indeed, NETosis appears to be associated with many inflammatory disorders, including infections, cancers, endothelial dysfunction, atherosclerosis, thrombosis, and ischemia (31, 32).

Neutrophil extracellular traps contribute to thrombosis through direct and indirect mechanisms. Although the vast majority of studies use NET components rather than intact NETs, the role of intact versus NET component in activating coagulation is controversial (33). In addition to their ability to promote thrombin formation, NETs were also known for providing a scaffold for pro-coagulant molecules such as VWF, fibrinogen, FXII, and tissue factor, as well as pro-coagulant extracellular vesicles TF-bearing EVs for instance (34–38).

Platelets can induce dysregulation of NET, resulting in tissue damage, hypercoagulability, and thrombosis (34). In addition, NETosis is well documented in its role in the pathogenesis of sepsis and ARDS, causing vascular tissue damage and spreading microthrombi that eventually cause multiorgan failure and death (39, 40).

The studies on NETosis provide growing evidence that NETosis is connected to inflammation, atherosclerosis, and atherothrombosis, as well as poor prognoses of ischemic/reperfusion injuries (41).

Mouse studies have examined the effects of NETosis and platelet aggregation on outcome after ischemia/reperfusion (42). Then, Cf-DNAs trigger DNA-platelet and DNA-platelet-granulocyte colonization. This form of NET and platelet aggravation leads to NETosis.

There are no doubts that NETs play a significant role in thrombosis and hemostasis (Figure 1). Both arterial and venous thrombosis are affected by NETs, and NETs are implicated in stenosis that regulate thrombosis in different ways (43, 44). Additionally, preventing the formation of NETs can reduce thrombogenicity, which could be useful in preventing thrombosis (44). A thorough understanding of how new therapeutic options targeting NETs affect thrombosis will require both preclinical and clinical trials.

In view of the critical role for platelets in thrombosis, studying platelet function may provide novel biomarkers for arterial thrombosis (45). There are numerous platelet function tests available in clinical practice, but most commonly, aggregometry-based tests are carried out (46). The aggregometry method, however, provides information on platelet functionality in the presence of exogenous and soluble agonists, which does not represent in vivo platelet activation. Thus, so far results from platelet function tests in vitro have limited value as biomarkers of arterial thrombosis.

A biomolecule or metabolite associated with activated platelets may be able to provide information about arterial thrombosis (47). Platelet-derived extracellular vesicles (PEV) are among these biomarkers, covering platelet microparticles/microvesicles and exosomes (48).

A number of benefits attributed to platelets are likely mediated by platelet-derived extracellular vesicles (PEVs), which are small vesicles released from activated platelets. Indeed, the release of platelet α-granules have been considered as small vesicles, as revealed by electron microscopic analyses (49). Also known as microparticles, this release of vesicles from the platelet plasma membrane occurs as a result of the extrusion of the platelet cytomembrane structures (50). Later studies using electron microscopy confirmed the presence of the two types of PEVs: small vesicles with diameters of 80–200 nm, and larger vesicles with diameters of 400–600 nm, which retained procoagulant activity mediated by factor V-like activity and tissue factor (51).

Platelet-derived extracellular vesicle can serve as biomarkers for autoimmune diseases, cancer, cardiovascular diseases, and infectious diseases (52–55). Rheumatoid arthritis patients have PEV detected in their synovial fluid (56), and increased levels of circulating PEV correlate with disease activity (57). Additionally, in murine models of atherosclerosis and autoimmune arthritis, PEV concentrations have been found to be increased in lymph (52, 58). As a result of systemic lupus erythematosus (SLE), PEV levels in blood can be increased. Higher levels of PEV have been associated with declining kidney function (59). Recently, an increase in PEV circulating in blood of COVID-19 patients has been observed (55, 60, 61).

Aside from transporting and producing inflammation-promoting mediators such as prostaglandins and leukotrienes, the PEV is also the transporter and producer of many lipid mediators (62).

A wide variety of bacteria can interact with platelets, including the Staphylococci family, Neisseria gonorrhea, Porphyromonas gingivalis, and Helicobacter pylori (76–78). GPIIb/IIIa, FcγRIIa, and IgG receptors on platelets help them adhere and aggregate around bacteria, as well as fibrinogen and fibronectin (79, 80). TLR1, 2, 4, 6, and 9 within platelets enable them to bind bacteria, which will either cause platelets to secrete thrombocidins (antibacterial proteins within platelet α-granule, including thrombocidin 1 and 2) or aggregate around bacteria to trap them for elimination by phagocytes (20, 81–84). Upon interacting with bacteria, platelets release antimicrobial compounds that are contained within their granules. Alpha toxin released by S. Aureus mediates the release of β-defensin from platelets, which is responsible for NET formation (85). Platelets binding to Gram-negative bacteria releases a PF4 with exposed heparin-like epitopes, increasing antibody binding to the surface of bacteria and possibly leading to neutrophil opsonization and phagocytosis (86, 87). Some Gram-negative bacteria, such as Yersinia pestis, are insensitive to mammalian TLR4s, making this mechanism crucial (88, 89). Based on in vivo experiments with another Gram-negative bacteria, Porphyromonas gingivalis, it has been shown that during infection, platelets interact with neutrophils forming heterotypic aggregates in a TLR2-dependant fashion and that TLR2 promotes the aggregation of platelets (90). These study findings indicate that platelets can trigger platelet thrombotic pathway and/or inflammatory pathway activation upon recognizing bacterial components (90).

On the other hand, platelets interact with various types of viruses and their phenotype may vary depending on the type of viral infection (91). Thrombocytopenia and even thrombosis can accompany viral infections (92). Viruses can be divided into those that have either DNA or RNA genomes. Furthermore, RNA viruses can be divided into double-stranded and single-stranded viruses. Several DNA viruses, such as the herpes simplex virus type 1 (HSV1), cytomegalovirus (CMV) and vaccinia, have been identified associated with platelets. However, it is unknown if these viruses can be internalized by platelets (93–95). In contrast, RNA viruses such as HIV, hepatitis C virus (HCV), dengue, influenza, CVB, and EMCV have smaller sizes and are easily internalized by platelets (96–101).

There was recently evidence that SARS-CoV-2, a single-stranded RNA virus, can increase platelet activity and formation of platelet-monocyte aggregates facilitated by TF expression on monocytes (55, 102–105). Koupenova et al. (106) reported that SARS-CoV-2 promotes programmed cell death in platelets in another aspect of platelet-SARS-CoV-2 interaction. According to this study, RNA sequence analysis for SARS-CoV-2 shown by ARTIC v3 sequencing, transmission electron microscopy and immunofluorescence showed that SARS-CoV-2 virions internalized when attached to microparticles. As a consequence of such internalization, apoptosis, necroptosis, and EV release occur, which contribute to impaired immunity and thrombosis (106).

Several studies suggest that platelets may associate with SARS-CoV-2 RNA molecules, and that this event may be more likely to occur in older patients (55, 105, 107). However, Bury et al. did not detect viral RNA in platelets from COVID-19 patients (108). The disparity could be due to the size of the cohort which is significantly larger in the studies that detected traces of viral RNA in platelets of some COVID-19 patients.