Previous studies on patients with severe COVID-19 have demonstrated elevated levels of pro-inflammatory cytokines (IL-1, IL-2, IL-6, IL-8, IL-10, IL-17 and TNF-α) (18,19). Elevated levels of cytokines cause inflammation and hypercoagulability in patients with COVID-19, which may be ascribed to several reasons. First, cytokines interact with the coagulation system. Cytokines, such as IL-1, IL-6 and TNF-α facilitate the release of tissue factor (TF), which activates the extrinsic coagulation pathway. Moreover, they promote the expression of PAI-1, which results in the inhibition of the fibrinolysis system (Fig. 1) (20). Beyond that, the levels of TF pathway inhibitors and antithrombin are decreased in an inflammatory environment, which causes the activation of the coagulation system (21). It should be noted that the activation of thrombin can induce the overproduction of pro-inflammatory cytokines through PAR-1, while FXa can activate PAR-1 and PAR-2 to enhance the inflammatory response (22). Second, cytokines interact with NETs. Previous studies have indicated that cytokines promote NET formation, which triggers the extrinsic and intrinsic coagulation pathways, resulting in thrombin generation. Correspondingly, NETs also promote the release of inflammatory cytokines to cause cytokine storms (23). Third, when promoted by IL-1β, IL-8 and TNF-α, blood cells (erythrocytes, leukocytes, and lymphocytes) expose phosphatidylserine (PS) to their outer membrane, which leads to a hypercoagulable state (24). In summary, it is inferred that the elevated levels of cytokines in COVID-19 patients and the subsequent activation of the coagulation system leads to AIS.

Varga et al found viral particles in endothelial cells of renal tissue, intestinal tissue, and lung tissue of patients with COVID-19, suggesting that SARS-CoV-2 invades endothelial cells in multiple tissues and organs (25). Similarly, Leisman et al found that SARS-CoV-2 can invade endothelial cells through the angiotensin-converting enzyme 2 (ACE2) receptor, leading to tissue damage and cytokine disruption (26). Additionally, cytokine storms of COVID-19 can also damage endothelial cells. Endothelial cells express TF and PS under the stimulation of cytokines or NETs (27). The former initiates the extrinsic pathway of coagulation, while the latter can provide a catalytic surface for coagulation factors and promote intrinsic and extrinsic FXa and thrombin production. In addition, damaged endothelial cells can express VWF, which interacts with platelets to promote thrombosis. Damaged endothelial cells also release PAI-1, which inhibits the fibrinolytic system and results in thrombosis. Previous studies (28,29) have demonstrated that damaged endothelial cells are closely related to AIS. More importantly, brain neurons, endothelial cells and vascular smooth muscle cells express the ACE2 receptor, which enables SARS-CoV-2 to cross the blood-brain barrier, leading to damage of the central nervous system (28,29). In summary, SARS-CoV-2 may cause damage to brain endothelial cells and induce local hypercoagulability to promote cerebral thrombosis.

The majority of patients with severe COVID-19 have abnormal laboratory data, including low platelet and lymphocyte counts and increased levels of neutrophils, D-dimer and C-reactive protein (30). Among these, a low platelet count has attracted increasing attention. Xu et al hypothesized that there were 3 main reasons for platelet reduction: First, viruses may cause decreased platelet synthesis; second, viruses may lead to increased platelet destruction; and third, viruses may contribute to thrombosis, which results in platelet consumption (31). Although platelet counts are reduced in patients with COVID-19, platelets are actually activated as reduced platelets are used to form microthrombi, which may adhere to the endothelium of blood vessels damaged by the virus (32). Thus, it is reasonable to believe that activated platelets promote hypercoagulability. On the one hand, activated platelets release microparticles (MPs), VWF and PAI to promote coagulation (27), and promote the generation of NETs that participate in the formation of hypercoagulability (33). Moreover, platelet activation is a prominent feature of AIS. Therefore, platelet activation in patients with COVID-19 may promote hypercoagulability and induce AIS.

Under inflammatory conditions, neutrophils catch bacteria by extruding neutrophil extracellular traps (NETs), which are composed of DNA, histones and other active proteins (34). However, NETs are associated with hypercoagulable states of disease and even promote thrombosis (Fig. 1) (35). Patients with COVID-19 are in a state of inflammation, which is advantageous for the generation of NETs. Some patients with COVID-19 already have elevated neutrophil counts (14). Moreover, the inflammatory response triggered by excessive NET formation is directly related to the destruction of surrounding tissues and microthrombosis and plays an important role in organ damage. The above 3 points have been proven to be essential causes of multiorgan failure in patients with severe COVID-19 (36). Zuo et al found that the plasma levels of cell free DNA (cf-DNA), myeloperoxidase (MPO)-DNA and citrullinated histone H3 (CitH3) were significantly elevated in patients with COVID-19, thereby confirming the presence of NETs in these patients (37). Their study also found that plasma cf-DNA levels were positively associated with plasma D-dimer levels, suggesting that NETs were associated with hypercoagulability in patients (37). Previous research has demonstrated the presence of NETs in thrombi and the peripheral blood of patients with AIS, and the NET content in peripheral blood is associated with the hypercoagulability of patients with AIS (33). Based on the above-mentioned studies, it can be hypothesized that NETs in patients with COVID-19 lead to a hypercoagulable state that induces AIS (Fig. 2).

MPs are vesicles (100-1,000 nm) in diameter derived from the shedding of the cellular membrane of activated or apoptotic cells. Circulating MPs are produced by a variety of cells, such as erythrocytes, granulocytes, monocytes, platelets, and endothelial cells (38). In recent years, studies have found that MPs induce hypercoagulability in a number of diseases, such as sepsis, stroke and nephrotic syndrome (39,40). The expression of phosphatidylserine (PS) is the underlying cause of MP-induced coagulation. Cell membrane PS can provide a catalytic surface for coagulation factors, promoting intrinsic and extrinsic FXa and thrombin production (Fig. 1) (41). In addition, Wang et al found that neutrophil extracellular trap-MP complexes can promote the intrinsic pathway of coagulation in a sepsis mouse model; they subsequently found that neutrophil extracellular trap-MP complexes can trigger inflammation by mediating neutrophil aggregation through HMGB1-TLR2/TLR4 signaling (42,43). Damaged endothelial cells and epithelial cells are also found to release MPs in a number of respiratory diseases, such as chronic obstructive pulmonary disease, asthma, pulmonary fibrosis and pulmonary hypertension (44). However, whether MPs play roles in COVID-19 has not yet been determined. Previous research has indicated that the systemic inflammatory response syndrome leads to the activation or apoptosis of blood cells throughout the body, resulting in the production of MPs (45). Systemic inflammatory response syndrome is the main cause of multiple organ failure in patients with severe COVID-19 (19). Therefore, there is reason to believe that MPs play a procoagulant role in patients with severe COVID-19.

The complement system plays a major role in regulating the immune system to protect against viruses. However, inadequate complement activation may also cause a systemic inflammatory response that leads to tissue damage. In addition, complement activation is also associated with coagulation and microthrombosis (Fig. 1). Complement activation contributes to membrane attack complex formation, which drives neutrophil activation and endothelial damage (46). Cugno et al demonstrated that the plasma levels of C5a and sC5b-9 were significantly higher in patients with COVID-19 compared with healthy subjects, suggesting the activation of the complement system in patients with COVID-19 (47). Magro et al suggested that complement system activation was associated with thrombosis in patients with severe COVID-19 (48). Complement activation may be caused by the following 2 mechanisms (49): i) The antigen-antibody complexes formed by antibodies combined with viral antigens initiate complement activation; and ii) viral invasion results in damage to endothelial cells, which leads to the activation of the complement system. Strategies targeting the complement system have been exert protective effects against brain ischemia-reperfusion injury, indirectly reflecting that complement activation plays an important role in the development of AIS (50). Therefore, it is feasible to believe that the activation of the complement system in COVID-19 contributes to hypercoagulability, which induces AIS.

ARDS, the most common symptom of patients with severe COVID-19, contributes to hypoxia in the entire body, playing an essential role in regulating thrombosis by triggering a number of molecular signaling pathways (51). Platelets are more likely to be activated in low oxygen environments (52). Moreover, hypoxia prompts the generation of hypoxia inducible factor (HIF)-1 and HIF-2, leading to the activation of the coagulation system and impairment of the fibrinolysis system. More precisely, HIF-1α increases the expression of TF, which activates extrinsic blood coagulation, while HIF-1 and HIF-2 promote the expression of PAI-1, which impairs fibrinolysis (Fig. 1) (53). Additionally, HIF-1α induces the formation of NETs (54). NET formation released by activated neutrophils promotes a hypercoagulable state in the circulation.

It has previously been reported that bacteria or viruses can induce antiphospholipid antibodies. A recent article in the New England Journal of Medicine demonstrated that antiphospholipid antibody in patients with COVID-19 was associated with hypercoagulability and the onset of AIS (55). Although only 3 patients were discussed in that article, sufficient attention should be paid. Subsequently, Beyrouti et al also found that patients with COVID-19 with AIS had positive antiphospholipid antibodies, which were suspected to be related to the onset of stroke (14). It has been demonstrated that antiphospholipid antibodies initiate coagulation by activating the TF signaling pathway (56). It has also been proven that positive antiphospholipid antibodies are a risk factor for AIS (57). Therefore, it is considered that the antiphospholipid antibody in COVID-19 activates the coagulation system and contributes to the onset of AIS. However, there are no drugs that target antiphospholipid antibodies in COVID-19. Further research into this matter is required in the future.

Currently, traditional antithrombotic treatments for COVID-19 with coagulopathy include anticoagulant therapy and thrombolytic therapy. Heparin, an anticoagulant and anti-inflammatory drug, has been used in the treatment of the hypercoagulable state of COVID-19. Tang et al demonstrated that COVID-19-induced mortality was reduced by heparin treatment (58). Compared with heparin, low molecular weight heparin (LMWH) has a long half-life, is associated with less bleeding and there is no need for routine coagulation monitoring. Oudkerk et al suggested that patients with COVID-19 should be treated with prophylactic LMWH as soon as they are admitted to the hospital (59). Klok et al found that despite the prevention of systemic thrombosis with a low dose of LMWH, a number of patients with COVID-19 still developed thrombotic events; therefore, a high dose of LMWH was recommended for anticoagulation in patients with COVID-19 admitted to the ICU (60). However, has not been determined which dose of LMWH should be used and this remains controversial. A previous study found that 31.6% of subjects supported an intermediate dose for moderate or severe COVID-19, while 5.2% of subjects supported a therapeutic dose, and the rest supported a prophylactic dose (61). New oral anticoagulants were also recommended during the COVID-19 pandemic due to their safety, convenience, and strong anticoagulant effects (62). SARS-CoV-2 infection not only activates the coagulation system but also inhibits the fibrinolytic system. Tissue plasminogen activator (tPA), a thrombolytic drug for AIS, has been reported to be effective for COVID-19 patients complicated with ARDS (63). However, the risk of bleeding should also be evaluated. In addition, chloroquine, which has been widely used in the treatment of COVID-19, can also play an antithrombotic role by inhibiting NETs and interfering with platelet aggregation (64). Statins can prevent the virus from infecting cells and inhibit the inflammatory response and coagulation activation, so they could be used to treat thrombosis in patients with COVID-19 (65). Further information on the antithrombotic therapy of COVID-19 is presented in Table III. In addition, some recommendations for antithrombotic therapy are presented in Table IV. Although anticoagulant therapy and thrombolytic therapy have been shown to be effective, a number of patients with COVID-19 develop thrombotic complications. Therefore, it is necessary to explore novel methods of anti-thrombotic therapy. Below, antithrombotic therapy targeting different mechanisms for coagulation is discussed.

Considering the crucial role of cytokines in leading to tissue damage and a hypercoagulable state, targeting cytokines in patients with COVID-19 has become an inevitable trend. It has been proven that IL-6 initiates the inflammatory response in patients with COVID-19. Tocilizumab, sarilumab and siltuximab, as inhibitors of IL-6 and its receptor, have been approved by the Food and Drug Administration for the treatment of rheumatic and lymphoproliferative diseases. Importantly, tocilizumab has been approved for the treatment of patients with COVID-19 with elevated levels of IL-6 (66,67). IL-17, which acts upstream of IL-1β, IL-6 and TNF-α, should attract increasing attention. Secukinumab, ixekizumab and brodalumab, as inhibitors of IL-17 and its receptor, have been shown to be effective in the treatment of psoriasis (18); however, their effects on COVID-19 need to be further investigated.

Preventing viruses from entering cells has become a novel method for the treatment of COVID-19. Human recombinant soluble angiotensin-converting enzyme 2 (hrsACE2), currently being studied, can competitively bind to viruses to inhibit viruses from being able to invade into cells (68). Fortunately, hrsACE2 has been approved for the treatment of ARDS (68-70). Given the damage exerted by cytokines to endothelial cells, anti-inflammatory treatment such as with tocilizumab may also protect endothelial cells (18,66). Additionally, colchicine, azithromycin and famotidine have also been proven to reduce endothelial injury (71).

Previous studies have demonstrated that aspirin is effective in patients with pneumonia, particularly in those with severe pneumonia complicated by ARDS or sepsis (72). In animal experiments, the combination of clopidogrel and oseltamivir has been shown to effectively inhibit the inflammatory response induced by influenza pneumonia and can thus improve the survival rate of mice (73). Furthermore, aspirin and clopidogrel can efficiently prevent the occurrence of cardiovascular and cerebrovascular diseases.

The targeting of NETs for the treatment of inflammatory diseases and thrombotic diseases has improved. Thus, there is reason to believe that therapies that target NETs are of vital importance for COVID-19 treatment. Currently, there are 3 main mechanisms against NETs: The inhibition of NET formation, the direct degradation of NETs and the inhibition of upstream regulatory molecules of NETs. The histone inhibitor, activated protein C and the neutrophil elastase inhibitor, sivelestat, have been shown to reduce hypercoagulability in AIS by inhibiting the generation of NETs (33). BWA3, an anti-histone H4 antibody, can effectively reduce infarct size in t-MACO mice (74). Recombinant DNase I (dornase alfa) can directly degrade NETs in the airway of patients with cystic fibrosis, relieving patient symptoms (75). In addition, recombinant DNase I has been shown to improve the efficiency of tPA thrombolysis in vitro (76) and reduced hypercoagulability in patients with AIS (33). IL-1β and HMGB1 have been reported to mediate the generation of NETs in various diseases. Targeting IL-1β or HMGB1 can inhibit the generation of NETs, thereby improving multiorgan functions (77,78).

Lactadherin can inhibit the expression of PS on MPs both in vivo and in vitro and has been shown to effectively prevent hypercoagulability in disease (39,40). Tiotropium is a muscarinic antagonist that inhibits the release of MPs from bronchial epithelial and endothelial cells, thereby reducing the risk of acute exacerbations of chronic obstructive pulmonary disease (79). Pirfenidone has been proven effective for the treatment of idiopathic fibrosis by inhibiting the p38-mediated formation of procoagulant MPs (80). However, whether lactadherin, tiotropium and pirfenidone can be used in the treatment of patients with severe COVID-19 warrants further investigation.

The inhibition of the complement system may be an underlying treatment for patients with severe COVID-19 with coagulopathy. In a virus-infected mouse model, the inhibition of C3 or C5 was shown to reduce the inflammatory response by attenuating the release of cytokines (81). The C5 inhibitor eculizumab may treat COVID-19 by hampering the amplification of the complement cascade (47). The C3 inhibitor, AMY-101, which can effectively inhibit the production of C3a and C5a and inhibit the release of IL-6, is currently undergoing clinical trials for the treatment of periodontal disease (NCT03694444) (82).

Mechanical ventilation is a key treatment for patients with severe COVID-19 with ARDS. Hyperbaric oxygen therapy has been shown to improve neurological function in patients with stroke (83). Extracorporeal membrane oxygenation has been used in the treatment of COVID-19-related ARDS (84). However, whether oxygen therapy or extracorporeal membrane oxygenation is effective for COVID-19 patients with AIS needs to be studied further.

SARS-CoV-2-induced AIS encompasses 3 main stages: An early stage that involves controllable levels of cytokines with mild symptoms, a middle stage that is characterized by the activation of the coagulation system and occurrence of AIS, and an advanced stage that involves the progression to massive cerebral infarction. The present review summarized the pathophysiology, symptoms and laboratory tests at different stages of SARS-CoV-2-induced AIS. More importantly, antithrombotic strategies are suggested for different stages of SARS-CoV-2-induced AIS (Fig. 3).