Sepsis is defined as a life-threatening organ dysfunction due to a dysregulated host response to infection [1]. It is the final common pathway to death from most infectious diseases worldwide, including bacterial and viral infections such as Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) [2]. The World Health Organization has listed sepsis as one of the global health priorities over the next few years, [3] and this is the most common cause of emergency admission to intensive care units (ICUs) in Europe ^[4][5][4,5].

Evidence has shown that the relationship between inflammation and coagulation, described by the term “thromboinflammation”, is critical in the pathogenesis of sepsis [6].

The same pathogenic mechanism has also been indicated as the most crucial, leading to elevated morbidity and mortality in patients affected by Coronavirus Disease-2019 (COVID-19), the complex clinical syndrome caused by the recently emerged Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) ^[7][8][9][7,8,9], which can lead to acute respiratory distress syndrome, sepsis, and multi-organ failure [10].

Although the precise mechanisms are not fully understood, inflammation through complement activation and cytokine release, platelet hyperactivity and apoptosis (thrombocytopathy), as well as coagulation abnormalities (coagulopathy) play critical roles in this complex scenario ^[10][11][10,11].

Extracellular vesicles (EVs) are membrane-derived vesicles released into extracellular space by all cell types, and they emerged as novel mediators of cell-to-cell and organ-to-organ crosstalk in many physiological and pathological conditions ^[12][13][12,13].

The diversity of EVs lies in different mechanisms of biogenesis, as well as in their different cellular origin [12]. EVs can be found in many biological fluids, such as plasma [14], saliva [15] and urine [16]. In addition, their ability to modulate inflammation has suggested EVs as potential novel biomarkers and next-generation biological therapeutics. Finally, there is growing interest in their ability to exchange functional components between cells. EVs may, indeed, communicate with cells and affect their phenotype or function via surface molecule-triggered uptake and intracellular signalling via cargo content [13].

The biological potential of EVs in the mechanism of sepsis- and COVID-19-related thromboinflammation is still only partially known; however, the rapid development of novel molecular platforms is contributing to envisage and develop theranostic applications of EVs in critical illness also.

2. Thromboinflammation in Sepsis and COVID-19: Differences and Overlaps

2.1. Thromboinflammation or Immunothrombosis?

In 2004, Tanguay and colleagues were the first to use the term “thromboinflammation” to indicate the platelet–leukocyte interaction mediated by P-selectin and P-selectin glycoprotein ligand 1 (PSGL-1) implicated in stent restenosis [17]. Inflammation-associated thrombosis, now known as thromboinflammation, occurs commonly in a broad range of human disorders. Thrombosis is well defined as an exaggerated hemostatic response, leading to the formation of an occlusive blood clot obstructing blood flow through the circulatory system. By comparison, inflammation is the term applied to the complex protective immune response to harmful stimuli, such as pathogens or damaged cells. Increasingly well-defined is the recognition that inflammation stimulates thrombosis, and in turn, thrombosis promotes inflammation [18]. In 2013, Engelmann and Massberg coined the term “immunothrombosis” to explain a tricky and mutual interaction, whereby, on the one hand, the activation of coagulation cascade triggers the immune system, cooperating with the identification, containment and destruction of pathogens [19], whereas, on the other hand, the innate immune cells promote the development of thrombi [20]. Today, thromboinflammation or immunothrombosis are considered interchangeable terms indicating dysregulation of the physiologic anti-thrombotic and anti-inflammatory functions of endothelial cells, which negatively influences hemostasis and favors thrombus deposition both in micro and macro-vasculature ^[18][21][18,21].

2.2. Cellular Mechanisms of Sepsis-Related Thrombosis

Sepsis is a life-threatening systemic illness associated with the invasion of the bloodstream by pathogens such as bacteria, viruses and fungi [1]. It is considered an extremely complex illness both on the cellular and on the molecular level, involving, among different pathophysiologic mechanisms, an imbalance in the inflammatory response, immune dysfunction, mitochondrial damage, coagulopathy, and immune network abnormalities, ultimately leading to multi-organ failure (MOF) and death ^[22][23][22,23]. It is known that septic patients are at high risk of developing thrombotic complications ranging from widespread microvascular involvement, such as disseminated intravascular coagulation (DIC), to venous thromboembolism, arising as deep vein thrombosis (DVT) or pulmonary embolism (PE). The development of these potentially fatal complications is believed to be triggered by a common final cascade of events characterized by serious injury to the microvasculature of affected tissues and organs and by the excessive activation of the coagulation system resulting in increased thrombus formation ^[6][24][25][6,24,25]. At the beginning of the infectious process, components of the bacterial cell wall such as pathogen-associated molecular patterns (PAMPs) are recognized by pattern recognition receptors (PRRs) present on the surface of endothelial cells, platelets, and leukocytes [26]. PRRs transduce signals leading to the release of inflammatory chemokines and cytokines, and of other inflammatory mediators that increase the expression of leukocyte-adhesion molecules [27]. As a consequence, the natural anticoagulant system on endothelial cells is altered and tissue factor (TF) production by monocytes and endothelial cells is increased. The relevance of TF in promoting thromboinflammation in sepsis has been confirmed using pharmacological TF inhibitors or through the inhibition of TF expression in mice exposed to endotoxin, which leads to diminished coagulation and inflammation and improved survival [28]. The location and grade of thromboinflammatory lesions are largely determined by the severity of damage to the endothelial cells (ECs) lining vessels. Usually, endothelium expresses both anti-inflammatory and anti-thrombotic substances, which antagonize leukocyte adhesion to and platelet accumulation [29]. Upon vascular injury, platelets are recruited to the site of damage. Their activation and adherence to the vessel wall lead to the release of platelet agonists, such as adenosine diphosphate (ADP), which further induce paracrine platelet activation and platelet aggregation [30]. Another crucial point is the activation of neutrophils that release nuclear DNA and granular protein, known as neutrophil extracellular traps (NETs) [31], with a strong antibacterial activity. Furthermore, platelets attached on the surface of neutrophils increase the formation of NETs that damage microcirculation, promote immunothrombosis, and lead to diffuse intravascular coagulation, since they facilitate the formation of thrombus acting as a scaffold [32].

2.3. COVID-19-Related Thromboinflammation

SARS-CoV-2 is an enveloped, single-stranded RNA virus [33] that uses the angiotensin-converting enzyme 2 (ACE2) receptor for internalization in the host cells, aided by transmembrane serine protease 2 (TMPRSS2) receptor [34]. ACE2 receptor is highly expressed in many human cells, including nasal and oral epithelial cells, endothelial cells, and myocardial cells [35]. SARS-CoV-2 infection may cause diffuse lung alveolar and endothelial cell damage with severe inflammation and increased vascular permeability leading to acute respiratory distress syndrome (ARDS). In addition to systemic inflammation and severe respiratory disease, COVID-19 is frequently complicated by the development of a hypercoagulable state and by the appearance of thrombotic events, which represent a primary cause of mortality in these patients ^{[7][8][11][36][37][38]}[7,8,11,36,37,38]. Histopathological findings, indeed, have reported pulmonary microthrombi in 57% of SARS-CoV-2 infection patients, compared with 24% of H1N1 influenza patients, a subtype of influenza A virus [39], with increased angiogenesis and pulmonary microthrombosis, respectively, three and nine times more prevalent in COVID-19 patients [40]. Although the appearance of a pro-thrombotic profile is now a well-recognized hallmark of COVID-19 ^{[36][40][41][42]}[36,40,41,42], the mechanisms underlying the development of thromboinflammation are not completely elucidated yet. In the process of SARS-CoV-2 infection progressing to a systemic and severe disease, multiple proinflammatory cytokines, which include interleukin (IL)-1, IL-6, tumor necrosis factor (TNF)-α, and chemokines, recruit more innate immune cells (such as macrophages, neutrophils and dendritic cells) to produce additional inflammatory cytokines, in a loop known as cytokine storm. This increases vascular dysfunction and thrombosis and favors the development of multiorgan failure ^[43][44][43,44]. Vascular dysfunction is primarily sustained by the abundant expression on endothelial cells of the ACE2 receptor, which leads to increased virus tropism in blood vessels [45]. In general, the vascular endothelium is considered the first responder of the host defense. Once homeostasis is disrupted by SARS-CoV-2 infection, endothelial cells lose their anti-thrombotic capacity as a consequence of glycocalyx damage [44]. In addition, on their surface they express procoagulants and proapoptotic factors, such as TF and phosphatidylserine (PS), thereby leading to the exposure of the basement membrane and the activation of the coagulation cascade ^[46][47][46,47]. SARS-CoV-2 also directly activates platelets and exacerbates the thromboinflammatory cascade by promoting platelet–neutrophil binding, which, in turn, increases the formation of NETs, activates additional inflammatory responses, and increases other prothrombotic pathways ^[48][49][48,49]. In this complex cellular and molecular interplay, one research group has recently demonstrated that thrombopoietin (THPO), a humoral growth factor involved in the proliferation and differentiation of megakaryocytes in the bone marrow, appears as an additional crucial candidate mediator of platelet hyper-activation and immunothrombosis/thromboinflammation in COVID-19 [50].

2.4. Do Sepsis and COVID-19 Overlap?

The complex inter-relationship and similarities between sepsis and COVID-19 are topics that have recently emerged with particular emphasis ^[51][52][53][51,52,53]. Data obtained in hospitalized COVID-19 patients has revealed that serum cytokine and chemokine levels are high in these patients, at levels comparable to those found in patients with sepsis ^[54][55][54,55]. Some researchers have also pointed out that severe and critically ill COVID-19 patients meet the diagnostic criteria for sepsis and septic shock according to the Sepsis-3 International Consensus, and, thus, recommend using the term ‘viral sepsis’ instead of the terms severe or critical illness because it is more appropriate ^[56][57][56,57]. Patil and co-authors have suggested that SARS-CoV-2 itself likely causes sepsis as a consequence of various mechanisms, which include immune-dysregulation, respiratory dysfunction leading to hypoxemia, and metabolic acidosis due to circulatory dysfunction [58]. Multiorgan failure seen in COVID-19 could also be explained by hypoxia and circulatory disorders that occur as a consequence of microvascular dysfunction, analogously to what is described in sepsis [59]. Finally, other authors have suggested that microvascular dysfunction may also contribute to hypoxia and subsequent organ failure by interrupting the blood flow to the lungs due to disseminated intravascular coagulation and micro-embolism [60].