3.1. Entry and Life Cycle of SARS-CoV-2 Virus
SARS-CoV-2 viral entry occurs via a host cell receptor known as angiotensin-converting enzyme 2 (ACE2), previously recognized as a SARS-CoV-2 receptor; a process mediated via its spike (S) protein, which consists of S1 and S2 subunits [
14]. ACE2 is a functional receptor that is highly expressed in pulmonary epithelial cells. This receptor is also present in cells of other vital organs, such as gastrointestinal tracts, heart, endothelial cells, and kidneys [
15]. During SARS-CoV-2 infection, the S1 protein binds to ACE2, leading to exposure of another cleavage site, called the ‘S2 site’ on S2 subunit, which is subsequently cleaved by host proteases [
16]. These host proteases, namely type II transmembrane serine protease (TMPRSS2) and lysosomal cathepsins are critical for S2 subunit activation, resulting in dissociation of S1 and drastic structural change in S2, exposing fusion peptide, promoting membrane fusion [
17]. S2 subunit activation at the plasma membrane surface is stimulated by TMPRSS2, while that in endosomes is induced by lysosomal cathepsin, which can also compensate for viral invasion into cells lacking TMPRSS2 [
17]. Following membrane fusion, the viral RNA genome is released into the cytosol of the host cell via fusion pore, inducing viral polymerase translation and RNA replication by host cell mechanisms, which subsequently leads to virion assembly and exocytosis of the newly synthesized virion, which can infect new host cells and continue the replication cycle [
18].
3.3. SARS-CoV-2-Induced Inflammation
Following viral infection alveolar epithelial cells undergo programmed cell death induced by pathogen-associated molecular patterns (PAMPs) such as viral RNA the subsequent release of damage-associated molecular patterns (DAMPs) such as RNA or histone, triggers activation of macrophages that lead to high cytokines secretion, particularly interleukin (IL)-6 and reactive oxygen species (ROS) [
23]. Circulation of IL-6 receptor complexes and cytokines indirectly activates a series of cells, including endothelial cells, platelets, and neutrophils, culminating in an exaggerated production of pro-inflammatory cytokines, which ultimately results in a cytokine storm. Cytokine storms occur due to significant overexpression of inflammatory cytokines that initiate lung inflammation and damage, followed by pneumonia, ARDS, septic shock, loss of respiratory functions, and ultimately death [
24]. The critical nuclear factor-κB (NF-ĸB) pathway is activated in a cytokine storm, releasing excessive inflammatory cytokines, such as IL-1β, IL-2, IL-7, IL-10, granulocyte-colony stimulating factor (G-CSF), Interferon-γ-inducible protein 10 (IP10), monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein (MIP) 1-A, and tumor necrosis factor (TNF)-α in severe COVID-19 patients and the resulting inflammation may lead to the development and progression of ARDS. COVID-19 patients admitted to the ICU had a higher concentration of these cytokines than healthy individuals, and has been reported in secondary hemophagocytic lymphohistiocytosis (HLH) associated with severe COVID-19 [
25,
26].
3.4. The Crosstalk Between Inflammation and Coagulation
Crosstalk between factor XIIa in the coagulation cascade and kallikrein and kinin system (KKS) is crucial in the cytokine storm and coagulopathy in COVID-19 patients, activated by SARS-CoV-2 RNA (
Figure 2) [
27]. Loss of inactivation of the pro-inflammatory
des-Arg9 bradykinin by ACE2 increases the production of inflammatory cytokines and oxidative stress by activating the kinin B1 receptor (KB1R) [
28]. Increased inflammatory cytokines in COVID-19 patients also induce TF and P-selectin in vascular endothelium cells, promoting vascular thrombosis and neutrophil extracellular traps (NETs) formation, respectively [
29]. Moreover, inhibition of anticoagulants by IL-1 elevates thrombin production and aggravates vascular thrombosis [
30]. Overexpression of P-selectin directly correlates with higher TF levels in monocytes, promoting DIC occurrence [
31]. This condition is common in severe COVID-19 patients, where D-dimer induces P-selectin and, subsequently, TF overexpression [
32]. Therefore, stimulating P-selectin production via D-dimer and high TF expression level is a mechanism for coagulopathy in severe COVID-19 patients.
Overexpression of both angiotensin II (AngII) and angiotensin II receptor type 1 (AT1R) can also lead to overproduction of TF by monocytes and vascularized endothelial cells [
33]. Activated platelets have also been shown to stimulate TF expression by monocytes, via P-selectin and integrin αIIb/β3 [
34], thus promoting a TF-dependent coagulation pathway. AngII/AT1R axis also inhibits the fibrinolytic pathway by increasing plasminogen activator inhibitor 1 (PAI-1) expression, further promoting a hypercoagulable state [
35]. Moreover, overexpression of AngII and AT1R leads to the upregulation of both intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1) in vascular endothelium, promoting leucocyte adhesion, and therefore associated with the severity of COVID-19 disease [
36,
37]. The resultant activation of endothelial cells is reflected by elevated levels of D-dimer, which is considered a biomarker for coagulopathy and associated with poor prognosis in COVID-19 patients. Although D-dimer represents the degradation of clots by the fibrinolytic pathway hence generated in the late stage of the coagulation pathway, the mechanisms of D-dimer elevation in COVID-19 patients are not fully understood.
Systemic inflammation in COVID-19 resulting from the severe elevation of inflammatory cytokines has been observed in association with the activation of coagulation cascades, thrombosis formation, and endothelial cell dysfunction [
38]. The resultant hyperinflammatory and hypercoagulability condition, characterized by cytokine storm and overstimulated coagulation cascade with impaired fibrinolytic system, respectively, is aptly coined as immuno-thrombosis [
37]. Generation of vascular microthrombi eventually leads to systemic DIC, followed eventually by multi-organ failure [
37]. In addition, the formation of platelet-neutrophil complexes (PNCs) and toxic NETs promote coagulopathy in several ways, including further cellular recruitment, activation of platelet and coagulation cascade pathways; and inhibition of anti-coagulant and fibrinolytic pathways which could confer detrimental effects in endothelial injury, lung damage, and thrombosis [
39,
40].
Disseminated intravascular coagulopathy is a condition where the formation of blood clots and thrombosis in vessels obstruct blood flow due to inflammation-mediated activation of the blood coagulation cascade [
41]. DIC, accompanied by fibrin formation in blood vessels, leads to organ dysfunction and increased patient mortality [
42]. Coagulopathy occurs when the balance between coagulation and anticoagulant factors is disturbed [
43]. On one hand, pro-inflammatory cytokines and CRP can induce blood coagulation cascade through TF/factor VIIa complex activation, primarily driven by the vascular endothelium cells. On the other hand, when these cells encounter pathogens or inflammatory cytokines, they lose their antithrombin (AT) properties, resulting in overall favorable conditions for coagulation [
44,
45]. In addition, hypoxic conditions due to lung damage in COVID-19 patients exacerbate the stimulation of TF production by the endothelium; thus, high TF levels are predictors of increased morbidity and mortality [
46].
The TF pathway activation by cytokines IL-1, IL-6, and TNF-α also downregulates anticoagulant factors such as AT, thrombomodulin (TM), and endothelial protein C receptor (EPCR), thus preventing fibrinolysis and promoting DIC [
47,
48]. Nevertheless, the relationship between inflammation and coagulation is not unidirectional. Coagulation can lead to inflammatory responses by protease-activated receptors (PARs) located on the endothelium of vessels and platelets. PARs bind to factor VIIa upon activation and increase inflammatory cytokines production, including IL-1β and TNF-α and ROS-induced oxidative stress [
49,
50]. Furthermore, TM/activated protein C (APC) and its EPCR are essential in eliminating coagulation factors, such as factor Va and VIIIa [
51]. The APC downregulation leads to endotheliopathy and vascular thrombosis[
52].
High levels of coagulation factors, such as von Willebrand factor (vWF), factor VIII, and soluble TM indicate endothelial damage in severe COVID-19 found in patients’ lung samples [
53]. The TNF-α and IL-1 cytokines play an important role in activating the endothelial damage biomarkers in this process, indicating the activation of coagulation cascades by inflammatory cytokines [
54]. In addition, downregulated AT levels have been reported in hospitalized COVID-19 patients, which led to increased ventilator requirement and mortality [
55]. Therefore, AT administration is recommended for critical COVID-19 patients hospitalized in the ICU, but discrepancies have been highlighted between studies [
56].
Coagulopathy caused by venous thromboembolism (VTE) events and unresponsiveness to prophylactic anticoagulants is common in severe COVID-19 patients in the ICU [
57]. This condition promotes heart and brain ischemia, which increases mortality among COVID-19 patients [
58]. D-dimer, which is a coagulopathy biomarker, exceeding 1000 ng/mL predicts VTE incidence and mortality in COVID-19 [
59]. Furthermore, the upregulation of TF, factor VIII, and vWF and the downregulation of anticoagulant factors, such as protein C and AT III, have been reported in COVID-19 cases [
60].