Keywords COVID-19 - SARS-CoV-2 - cardiovascular disease - inflammation - thrombosis - RAAS
- ACE2
Introduction
The novel coronavirus disease 2019 (COVID-19) has rapidly progressed to a global pandemic
infecting over 23 million people in 188 countries by the middle of August 2020.[1 ] The basis underlying COVID-19 is infection with severe acute respiratory syndrome
coronavirus 2 (SARS-CoV-2), which originates from the Coronaviridae family of viruses
that are usually associated with respiratory infections.[2 ]
[3 ] Although the respiratory manifestations of COVID-19 are well documented,[4 ]
[5 ] recent studies have also observed cardiovascular complications in patients.[6 ]
[7 ] Viral infection is associated with increased inflammatory biomarkers including interleukin-6
(IL-6) and D-dimer,[8 ] which may influence severe cardiovascular clinical features such as thrombosis and
cardiac injury as observed in limited cohorts of COVID-19 patients.[9 ]
[10 ]
[11 ]
It is well established that outbreaks of acute respiratory infections such as influenza
may trigger an increase in coronary deaths due to myocardial infarction or stroke.[12 ]
[13 ] Previously, similar viral epidemics including severe acute respiratory syndrome
(SARS) reported common cardiovascular complications such as acute myocardial infarction
and increased susceptibility to thrombosis.[14 ]
[15 ] In the case of SARS-CoV-2, however, the risk of ischemic stroke was 7.5-fold higher
than that of influenza patients.[16 ] Furthermore, emerging evidence from the current COVID-19 pandemic suggests that
individuals with preexisting cardiovascular risk factors including heart failure,
hypertension, and diabetes may be more susceptible to severe infection.[4 ]
[17 ]
[18 ]
[19 ]
Although the interactions between COVID-19 and cardiovascular inflammation require
further investigation, this review will focus on the potential mechanisms by which
SARS-CoV-2 infects its host with a particular focus on vascular endothelial cell dysfunction.
Specifically, we seek to describe the immunoinflammatory mechanisms that may disproportionately
affect COVID-19 patients with underlying cardiovascular pathologies leading to their
hypercoagulable states and cardiac injury. Finally, we discuss promising therapeutic
options targeting the hyperinflammation associated with severe SARS-CoV-2 infection.
Mechanisms of Cellular Entry and Infection
Mechanisms of Cellular Entry and Infection
Viruses cause infections in hosts by entering the cells to exploit the cellular machinery
of the host to further replicate and spread from cell to cell. It has been established
that the SARS-CoV-2 uses the protein angiotensin-converting enzyme-2 (ACE2) efficiently,
even more so than the original SARS-CoV, to invade the host cells.[20 ]
[21 ]
[22 ] ACE2 is an extensively present cell surface enzyme. Li and colleagues recently analyzed
the expression of ACE2 across 31 human tissues using datasets provided from Genotype-Tissue
Expression (GTEx) and The Cancer Genome Atlas (TCGA). They found the highest expression
of the receptor in the small intestines, testes, kidneys, heart, thyroid, and adipose
tissue, whereas the lowest expression was observed in the blood, spleen, bone marrow,
blood vessels, and muscle.[23 ] Moderate expression levels were reported in the lungs, colon, liver, bladder, and
adrenal gland. Nevertheless, these findings do not specify cell-specific expression
of the receptor and remain to be further validated in protein levels. A study by Chen
et al examined the cellular expression of ACE2 in the human heart via single nuclear
transcriptome analysis and found that ACE2 expression was low in cardiomyocytes, whereas
it was high and specific to pericytes.[24 ] Moreover, another study by Nicin and colleagues using single nuclei RNA sequencing
likewise reported ACE2 expression particularly in pericytes.[25 ] They also reported the expression of the receptor in cardiomyocytes as well as mural
cells and lower levels of expression were also observed in fibroblasts, endothelial
cells, and leukocytes. Furthermore, cardiomyocyte expression of ACE2 was found to
be significantly increased in patients with heart disease. The extensive presence
of this receptor may be an explanation to the wide spectrum of symptoms and complications
of COVID-19, such as respiratory and gastrointestinal distress, loss of taste and
smell, and multiorgan dysfunction including cardiac and liver injury as well as renal
failure. ACE2 is a central regulator in the renin–angiotensin aldosterone system (RAAS),
a hormone system crucial for the maintenance of blood pressure as well as the fluid
and electrolyte homeostasis in the body ([Table 1 ]).[26 ] Imbalances in RAAS can lead to hypertension, and the components of this system are
known to further augment cardiovascular risk factors such as inflammation, thrombosis,
insulin resistance, and obesity ([Table 1 ]).[27 ] Therefore, the doorway receptor of SARS-CoV-2, ACE2, plays a pivotal role in cardiovascular
health and disease among other factors.
Table 1
The role of RAAS in cardiovascular comorbidities associated with severe COVID-19 infection
Pathology
Relevant role of RAAS
References
Hypertension
• RAAS is activated in response to renin released by kidneys with low blood supply
and it increases blood pressure via its vasoconstrictive hormone angiotensin II
[26 ]
[27 ]
[28 ]
• Zhong et al showed that angiotensin II infusion in ACE2-deficient mice leads to
hypertension as well as diastolic dysfunction
[55 ]
• In contrast to angiotensin II produced by ACE, angiotensin (1–7) produced by ACE2
acts as a vasodilator and reduces blood pressure
[26 ]
[27 ]
[28 ]
Insulin resistance
• RAAS is shown to enhance insulin resistance and thus type II diabetes in humans
via angiotensin II
[32 ]
[33 ]
• RAAS inhibition by losartan, an angiotensin receptor blocker, in patients showed
improved insulin resistance as well as glucose homeostasis
[148 ]
• Angiotensin (1–7)/MasR axis is shown to promote glucose uptake by rat skeletal muscle
in vivo and thereby improves insulin sensitivity
[43 ]
Obesity
• RAAS is activated in adipose tissue during obesity and promotes adipocyte growth
and inflammation
[36 ]
[39 ]
• Components of the RAAS were shown to be increased in obese patients
[34 ]
[35 ]
[38 ]
Endothelial dysfunction
• AT1 receptor in activated RAAS drives endothelial oxidative stress and adhesion molecule
expression via the NF-kB pathway, thus impairs endothelial function
[28 ]
[31 ]
• Angiotensin (1–7)/MasR axis promotes nitric oxide release
[44 ]
Inflammation
• Angiotensin II–AT1 axis promotes inflammation at the vascular wall via increased oxidative stress and
NF-kB-mediated adhesion molecule expression along with cytokine and chemokine release
[31 ]
[149 ]
• Angiotensin II supports endothelium–immune cell adhesion by stimulating endothelial
vascular cell adhesion molecule-2 via NF-kB
[149 ]
• ACE2/angiotensin (1–7) axis exerts anti-inflammatory and antifibrotic effects by
inhibiting the MAPK/NF-kB pathway
[45 ]
[46 ]
Abbreviations: ACE2, angiotensin-converting enzyme-2; AT1 , angiotensin II receptor type I; RAAS, renin–angiotensin aldosterone system.
RAAS is activated in response to renin released by kidneys in the events of low blood
supply and low sodium load. Circulating renin then cleaves its substrate angiotensinogen
produced by the liver, which produces the peptide hormone angiotensin I. Predominantly
occurring in the lungs, angiotensin I is further cleaved by ACE to produce angiotensin
II ([Table 1 ]).[28 ] Angiotensin II constricts blood vessels and increases blood pressure to replenish
the blood supply to the kidneys in addition to stimulating aldosterone synthesis in
the adrenal cortex for renal sodium reabsorption.[29 ] Consequently, RAAS activation leads to increased blood pressure and pharmacological
blockade of the RAAS via ACE inhibitors (ACEis) and angiotensin II receptor blockers
(ARBs) are used widely to treat hypertension in patients suffering from cardiovascular
disease (CVD).[27 ]
While the RAAS is fulfilling its aim in assisting the kidneys via the effects of angiotensin
II, its impact on the vasculature can introduce adverse cardiac outcomes such as left
ventricular hypertrophy due to hypertension.[30 ] This impact may be further detrimental especially in the case of present underlying
risks for CVDs including atherosclerosis. Angiotensin II and its receptor angiotensin
II receptor type I (AT1 ) promote inflammation at the vascular wall by several mechanisms including increased
oxidative stress via reactive oxygen species, NF-kB-mediated adhesion molecule expression,
and cytokine and chemokine release ([Table 1 ]).[31 ] These events vastly contribute to endothelial dysfunction and arterial leukocyte
recruitment, which are major drivers of atherosclerotic plaque development.[28 ] Moreover, RAAS has been shown to enhance insulin resistance. In the clinic, it could
be demonstrated that type II diabetes in humans may be dependent on actions of angiotensin
II as several studies have shown improved insulin resistance in patients treated with
ACEis as well as ARBs ([Table 1 ]).[32 ]
[33 ]
Furthermore, additional components of RAAS, such as aldosterone, renin, and angiotensinogen,
were shown to be elevated in the circulation of obese patients revealing a significant
link between RAAS and obesity ([Table 1 ]).[34 ]
[35 ]
[36 ]
[37 ]
[38 ] RAAS is also upregulated locally in adipose tissue during obesity, which links angiotensin
II to increased adipocyte growth and inflammation within the tissue.[39 ]
[40 ] In conclusion, activation of RAAS and thus its predominant effector hormone, angiotensin
II, introduces several deleterious consequences which are critical mechanisms driving
the pathophysiology of CVDs and its comorbidities.[41 ] The key switch antagonizing angiotensin II-driven effects of RAAS is the action
of ACE2. Although structurally homologous to ACE, the physiological function of ACE2
is actually to counterbalance the functions of ACE and to establish a vital equilibrium
in RAAS.[42 ] By hydrolyzing angiotensin II, ACE2 produces angiotensin (1–7) and ultimately diminishes
angiotensin II levels and function. Moreover, angiotensin (1–7) reduces blood pressure
by acting as a vasodilator in contrast to angiotensin II. Angiotensin (1–7) and its
receptor MAS1 oncogene (Mas) offer further cardioprotective effects such as reduced
insulin resistance, antithrombotic effects through nitric oxide release, and decreased
inflammation by NF-kB pathway blockade ([Table 1 ]).[43 ]
[44 ]
[45 ]
[46 ] Therefore, ACE2 is a crucial regulator of RAAS, overcoming its hostile side effects
and thereby supporting cardiac health.[47 ]
In spite of its extensively protective roles as mentioned above, ACE2 provides an
invasion pathway to SARS-CoV-2 via its extracellular domain that is recognized and
targeted by the virus to gain intracellular access.[48 ] The virus expresses a class I fusion protein, known as the Spike (S) protein, on
its envelope establishing its characteristic “crown-like” exterior hence its name
“corona.”[49 ] The S protein facilitates the engagement of the virus to the host cell via its subunit
S1, which possesses the binding region to the extracellular domain of ACE2.[50 ] Viral attachment is followed by fusion and internalization of the virus into the
target cell via the S protein subunit S2.[51 ] A crucial event enabling the S2 subunit-driven fusion is the priming of the S protein,
which is executed by the host transmembrane protease serine 2 (TMPRSS2). Notably,
TMPRSS2 is expressed in endothelial cells giving rise to their susceptibility as a
target cell. Confirming its role in viral entry, an inhibitor of this serine protease
involved in S protein priming can block cellular SARS-CoV-2 entry.[20 ]
[52 ] Internalization of the virus entails endocytosis of the virus presumably along with
its bound receptor ACE2. As a result, the virus entry eliminates ACE2 from the cell
surface and subsequently attenuates the receptor activity and its protective roles
through the angiotensin (1–7)–Mas pathway leading to unbalanced RAAS.[47 ]
[53 ] This is supported by the findings that SARS-CoV-infected mice displayed reduced
ACE2 levels in their lungs, which was likewise observed upon the recombinant SARS
S protein treatment.[54 ] Moreover, Zhong and colleagues showed that angiotensin II infusion in ACE2-deficient
mice led to hypertension, pathological hypertrophy, myocardial fibrosis, and diastolic
dysfunction. However, this phenotype was alleviated in wild-type mice with recombinant
human ACE2 ([Table 1 ]).[55 ] Therefore, in addition to the known pulmonary consequences of COVID-19-related inactivation
of ACE2 receptors such as the acute respiratory distress syndrome (ARDS), ACE2 inactivation
has great potential to also impair cardiovascular health in several ways.[56 ]
[57 ] Low ACE2 expression, due to various reasons such as older age, diabetes, or hypertension,
in patients may increase the severity of SARS-CoV-2 infection.[58 ] This notion is also in line with the epidemiological statistics revealing that significant
numbers of patients facing serious and even fatal manifestations of the COVID-19 consist
of elderly and CVD patients.[59 ]
SARS-CoV-2 and Endothelial Dysfunction
SARS-CoV-2 and Endothelial Dysfunction
As mentioned previously, SARS-CoV-2 can promote endothelial dysfunction by shifting
the balance in RAAS to the angiotensin II/AT1 axis, which elevates oxidative stress and inflammation. Endothelial dysfunction is
characterized by a decrease in nitric oxide levels as a consequence of impaired endothelial
nitric oxide synthase function. Nitric oxide is a vasodilator and its deficiency leads
to hypertension by constricting the blood vessels, and it can further elicit thrombosis
and vascular inflammation.[60 ]
[61 ]
[62 ] In addition to the RAAS-mediated effects, emerging evidence revealed that SARS-CoV-2
can also directly cause endothelial dysfunction by infecting endothelial cells. Varga
and colleagues showed accumulation of viral bodies in endothelial cells of several
organs, including the kidneys and small intestines, from COVID-19 patients, which
was accompanied by increased endothelial cell inflammation and apoptosis.[63 ] The authors also reported “lymphocytic endotheliitis in lung, heart, kidney, and
liver.”[63 ] Moreover, SARS-CoV-2 induces systemic inflammation in the host leading to significantly
increased levels of proinflammatory cytokines in the circulation, such as IL-6 and
tumor necrosis factor-α (TNF-α).[17 ] As the vascular endothelium forms a protective layer between the organs and the
circulatory system, endothelial cells are constantly exposed to various circulating
molecules. Therefore, in the event of SARS-CoV-2-induced cytokine release, endothelial
cells are primarily influenced by the potent effects of these inflammatory cytokines.
Increased adhesion molecule expression and chemoattractant release are critical processes
mediated by activated endothelial cells in response to inflammatory stimuli. These
events further augment inflammation of the vascular wall by promoting leukocyte recruitment.
In conclusion, SARS-CoV-2 can impair endothelial function by several mechanisms including
direct-viral-infection-induced endotheliitis and endothelial injury leading to shifts
in the angiotensin II/AT1 axis and host inflammatory response.
Furthermore, Chen et al point out that pericytes express high levels of ACE2, which
was especially increased in patients with basic heart failure leading to their evaluation
of pericytes as the “cardiac target cell of SARS-CoV-2.”[24 ] Additionally, the authors speculated that pericyte injury may lead to endothelial
dysfunction at the capillary level and might compromise the microcirculation. Further
complicating the issue, recent reports on detection of SARS-CoV-2 in the central nervous
system (CNS) of COVID-19 patients support the notion that severe illness may be due
to CNS involvement and neurological manifestations.[64 ] In the case of CNS involvement, it is clear that the blood–brain barrier and its
endothelium represent a unique setting compared with other endothelial cells in the
body due to its specific expression of enzymes and transport molecules.[65 ] Evidence from earlier SARS and MERS outbreaks suggest that SARS-CoV-2 likely invades
the CNS through ACE2 as it does with other tissues; however, additional molecules
including CD147 may also play a role in viral entry.[66 ]
Endothelial dysfunction is a common theme for numerous conditions known to be especially
disadvantageous for COVID-19 patients including CVDs and their comorbidities.[67 ] SARS-CoV-2-driven systemic endothelial cell injury raises the threat of multiple
organ failure, and patients who are already suffering from impaired endothelial function
due to underlying conditions, like CVDs, are at much higher risk for severe complications
of COVID-19. Accordingly, treatment strategies aimed at restoring endothelial function
in COVID-19 patients, such as tackling nitric oxide deficiency, should be implemented
strictly. For example, phosphodiesterase type 5 (PDE-5) inhibitors are used in the
treatment of erectile dysfunction with the aim of restoring NO-mediated erectile smooth
muscle relaxation, and the PDE-5 inhibitors, sildenafil and tadalafil, were shown
to improve endothelial function by increasing flow-mediated vasodilation in patients
with chronic heart failure and type 2 diabetes.[68 ]
[69 ] In addition to its endothelial-protective effects, nitric oxide is proven to protect
against the original SARS-CoV. Akerström et al showed that nitric oxide interferes
with S protein and ACE2-mediated viral fusion mechanism while also inhibiting viral
replication in the early stages.[70 ]
[71 ] Consumption of nitric oxide boosting foods, such as beetroot, may be beneficial
to improve the endothelial function and to limit thrombus formation as well as viral
infection.[72 ] In addition, experimental modalities for directly and specifically protecting endothelial
cells against damage-induced apoptosis, e.g., microRNA mimics, could be considered.[73 ]
Inflammation
While lung epithelial and vascular endothelial cell infection is the direct consequence
of SARS-CoV-2, viral infection can also elicit severe systemic inflammation that may
underlie the cardiovascular complications seen in COVID-19 patients. The severity
of SARS-CoV-2 infection has been associated with immune cell dysregulation together
with inflammatory cytokine storms ([Fig. 1 ]).[17 ]
[18 ]
Fig. 1 Summary of systemic effects of SARS-CoV-2 infection on endothelial cells, immune
cells, coagulation system, and cardiac inflammation. Viral infection first mediates
endothelial dysfunction with observed changes in the RAAS system as well as inflammation,
oxidative stress, upregulation of adhesion molecules for leukocyte recruitment, and
intravascular coagulation leading toward microthrombi in the lungs. Inflammatory cytokine
storms involving expression of macrophage IL-6 and TNF-α leading to hyperactivation
and increased apoptosis of lymphocytes characterize systemic inflammation in severe
COVID-19 patients. Ultimately, inflammation may be tied to both elevated levels of
thrombosis and cardiac injury as observed in markers such as the D-dimer and troponin.
Created with Biorender. CK, creatine kinase; IL-6, interleukin-6; NT-proBNP, NT-proB-type
natriuretic peptide; RAAS, renin–angiotensin aldosterone system; TNF-α: tumor necrosis
factor α.
Pathological analysis of lungs from patients with COVID-19 compared with patients
with influenza revealed similar total lymphocytic infiltration; however, CD4+ T cell
subsets were increased in COVID-19 patients while CD8+ T cell subsets were decreased.[74 ] More specifically, CD4+ T cells resembled proinflammatory CC-chemokine receptor
6 (CCR6 + ) T helper 17 (Th17) cells while CD8+ T cells harbored higher percentages
of cytotoxic granules.[75 ] A genome-wide association study from the Italian and Spanish epicenters observed
an association between SARS-CoV-2 infection with polymorphisms at chromosome 3p21,
which encodes a cluster genes for the ABO blood group as well as for chemokine receptors
including chemokine receptor 9 (CCR9) and C-X-C motif receptor 6 (CXCR6).[76 ] Interestingly, both chemokines control T cell migration, which may link them to
both Th17-mediated lung and atherosclerotic inflammations.[77 ]
[78 ]
[79 ]
Outside of the lungs, significant lymphopenia in the blood is associated with severe
infection.[80 ]
[81 ] Flow cytometric analysis revealed that T cells from COVID-19 patients were hyperactivated
with increased expression of human leukocyte antigen-DR isotope (HLA-DR) and CD38.[75 ] Furthermore, hyperactivated T cells from COVID-19 patients were shown to upregulate
CD25 and IL-2 expression while T regulatory-associated forehead box P3 (Foxp3) expression
was downregulated, which may lead to unregulated T cell proliferation in response
to SARS-CoV-2 infection.[82 ] Clinical characteristics of COVID-19 patients reported cytokine storms with increased
concentrations of several inflammatory cytokines including IL-2, IL-6, and TNF-α[17 ]. Overactivation of proinflammatory Th17 and high cytotoxicity of CD8+ cells may
help explain the severe lung injury presented in some COVID-19 patients. Although
this cytokine storm may be in part attributed to T cells, several reports have noted
a low level of interferon responses in COVID-19 patients suggesting SARS-CoV-2 has
more distinct transcriptional response compared with other respiratory viruses.[83 ]
[84 ]
Macrophages, on the other hand, represent another likely source of the cytokine storm.
The systemic cytokine profile observed in COVID-19 patients has been compared with
macrophage activation syndrome (MAS), which is typically characterized by uncontrolled
activation and expansion of both macrophages and T cells.[85 ]
[86 ] In addition to resident lung macrophages, proinflammatory monocyte-derived macrophages
appeared to be abundant in the bronchoalveolar fluid of COVID-19 patients. Interestingly,
RNA-sequencing (RNA-seq) analysis of those macrophages revealed an upregulation of
inflammatory cytokines including IL-1B and IL-6 as well as chemokine receptors such
as CCL2 and CCL3 in severe COVID infections, which may suggest recruitment of inflammatory
monocytic cells together with neutrophils.[87 ] The hyperactivation of macrophages with its subsequent cytokine profile may account
for the severe lymphopenia observed in COVID-19 patients as one study revealed increased
expression of the death receptor FAS on T cells that could mediate activation-induced
cell death.[88 ] Recently, severe COVID-19 has been characterized by a highly pronounced formation
and aggregation of neutrophil extracellular traps (NETs) inside microvessels, leading
to rapid occlusion, disturbed microcirculation, and organ damage. Neutrophil granulocytes
are strongly activated and adopt a low-density phenotype prone to spontaneously form
NETs, and accordingly markers of NET turnover are increased in COVID-19 and linked
to disease severity. This process could potentially be targeted by heparin ([Table 2 ]).[89 ]
Table 2
Potential therapeutics for treating the hyperinflammation observed in severe COVID-19
patients
Potential treatments
Targets and action
References
Anticoagulants
• Low-molecular-weight and unfractionated heparin as first line of treatment to prevent
thrombotic events through activation of antithrombin III
[112 ]
[113 ]
• Heparin may have additional antiviral and anti-inflammatory properties that prevent
viral entry into cells by displacing surface proteoglycans including the S protein
of SARS-CoV-2 as well as prevention of vascular-occluding neutrophil extracellular
traps
[89 ]
• Danaparoid, typically prescribed to patients with thrombocytopenia and venous thromboembolism,
may be a secondary option which inhibits factor Xa and thrombin
[112 ]
[113 ]
• Concentrated danaparoid dosage nebulized into the lungs may direct its effect toward
the lung, but no published reports exist for COVID-19 usage
[112 ]
RAAS inhibitors
• RAAS has been shown to drive inflammation through the angiotensin II–AT1 axis. Inhibitors of RAAS such as ACEis and ARBs interfere with the ACE2-driven angiotensin
II production and angiotensin II binding to its receptor, respectively. Therefore,
RAAS inhibitors may decrease RAAS-driven inflammation
[31 ]
[127 ]
Cytokine-blocking therapies
• Monoclonal antibody treatments targeting cytokines produced during the hyperinflammatory
state in COVID-19 patients have been previously shown to reduce risk in several diseases
including atherosclerosis
[102 ]
[103 ]
• The COVACTA trial which utilized tocilizumab to target the IL-6 receptor reported
that patient status and mortality were not improved after 4 weeks of treatment
[104 ]
[140 ]
• Ongoing clinical trials are testing the effectiveness of IL-1 inhibition through
the use of high-dose anakinra and canakinumab
[103 ]
[141 ]
[142 ]
Corticosteroids
• Systemic glucocorticoid treatment has been shown to reduce viral shedding in previous
SARS and MERS outbreaks on top of their known anti-inflammatory and immunosuppressive
effects
• The RECOVERY trial demonstrated a 6 mg daily dosage of dexamethasone reduced the
28-day mortality rate of patients receiving oxygen
[143 ]
[144 ]
Abbreviations: ACE2, angiotensin-converting enzyme-2; ACEi, ACE inhibitor; ARB, angiotensin
II receptor blockers; AT1 , angiotensin II receptor type I; IL, interleukin; MERS, Middle East respiratory syndrome;
RAAS, renin–angiotensin aldosterone system; SARS, severe acute respiratory syndrome.
However, several studies have also reported increased T cell exhaustion in severe
infections as noted by increased expression of programmed cell death protein 1 (PD-1)
on T cells from COVID-19 patients, which might be a consequence of T cell hyperactivation
that leads to lymphopenia.[81 ]
[90 ] Postmortem autopsies revealed that SARS-CoV-2 infection resulted in increased apoptosis
of T cells in lymph nodes and spleen, which may be mediated by direct infection though
lymphocytic ACE2 expression, which is still questionable.[75 ]
[88 ] In recovering COVID-19 patients, single cell (sc) RNA-seq and T cell receptor sequencing
(TCR-seq) revealed high levels of expression for inflammatory genes, but decreased
T cell expansion compared with healthy controls further suggesting that T cell exhaustion
plays an important role in SARS-CoV-2 infection.[91 ]
Importantly, inflammation from immune cells like T cells and macrophages plays a key
role in CVDs such as atherosclerosis. Due to this inflammation, COVID-19 patients
have a higher risk for cardiovascular manifestations including myocardial infarction
and stroke.[16 ]
[92 ] A link between acute infections and adverse cardiovascular events has been established,
but the cytokine storm observed in severe COVID-19 patients may heighten the risk.[93 ] Using hyperlipidemic mice models, previous research has established proatherogenic
roles for inflammatory cytokines within the cytokine storm such as IL-6 and TNF-α.[94 ]
[95 ] Both cytokines are actively produced by innate and adaptive immune cells, possibly
in response to initial complement cascades or innate immune cell inflammasome activation
and subsequent IL-1β production[96 ]
[97 ]
[98 ] leading to microvascular injury and thrombotic microangiopathy in some patients
with COVID-19 ([Fig. 1 ]). Inflammasome activation has been previously linked to pyroptosis of macrophages
and endothelial cells leading to massive thrombosis, which may be fundamental to understanding
the unusual thrombosis risks associated with COVID-19.[99 ]
[100 ]
IL-1β activates endothelial cells during vascular inflammation to upregulate adhesion
molecules allowing leukocytes to infiltrate and expand atherosclerotic lesions. Within
the plaque, IL-1β induces collagenase, metalloproteinase, and cytokine expression
leading to plaques that are more vulnerable to rupture.[101 ] Plaque rupture leads to the activation of platelets and thrombosis formation, which
may occlude the vessel lumen leading to potential cardiovascular complications. In
humans, the Canakinumab Anti-Inflammatory Thrombosis Outcomes Study (CANTOS) demonstrated
the ability of IL-1β inhibition to reduce adverse cardiac events.[102 ]
[103 ] Although most studies have not reported an increase in IL-1β levels, IL-1 receptor
blockade was associated with clinical improvement in COVID-19 patients without invasive
ventilation.[104 ] Similarly, IL-6 antagonism using tocilizumab has proven to be an effective treatment
for severe infection.[105 ] Considering several studies have reported hypercoagulable states in COVID-19 patients,
further studies investigating a link between SARS-CoV-2 infection, vascular inflammation,
and atherothrombosis are needed.
Thrombosis
In severe COVID-19 cases, patients develop a type of ARDS, which is characterized
by alveolar damage and fibrosis that may be due to the infiltration of immune cells
and cytokines as mentioned previously. Fibrin deposition may be a consequence of the
hyperactivation of macrophages and T cells during MAS, which leads to increased endothelial
cell damage and diffuse lung injury.[106 ] Supporting the inflammatory hypothesis for increased thrombosis, serum proteomic
analysis revealed that elevated IL-6 was a critical marker for upregulation of coagulation
markers including Factor 5, 7, and 10 in the most severe COVID-19 patients.[107 ] Thromboelastometry measures may be beneficial to distinguish the difference in hypercoagulability
of mild and severe cases as prolonged clot formation time and ThromboDynamic Index
were reported in critically ill patients needing invasive ventilation.[108 ] Ultimately, severe SARS-CoV-2 infection presents with pulmonary intravascular coagulation
that appears to be similar to disseminated intravascular coagulation.[106 ] Several studies observe consistent hematological parameters such as increased D-Dimer
with moderate thrombocytopenia that support an increase in thrombus formation as well
as the breakdown of fibrin products ([Fig. 1 ]).[4 ]
[9 ]
[109 ] Further complicating the issue, a small study comparing the clot lysis between control
samples and COVID-19 samples described impaired lysis pointing to fibrinolytic resistance
on top of the hypercoagulability during severe SARS-CoV-2 infection.[110 ]
When compared with influenza patients, COVID-19 patients had nine times as many alveolar
capillary microthrombi leading to significant capillary occlusion.[74 ] A series of autopsies found an interesting link between the increase in thrombosis
and ACE2 expression. Thrombotic microangiopathy was not observed in tissues not expressing
ACE2 such as vasculature of the kidneys; however, multiple thrombotic events were
discovered in ACE2-expressing lung and brain parenchymal capillaries.[111 ] Therapeutic anticoagulant treatment has been associated with decreased mortality
in COVID-19 patients highlighting thrombosis as a critical turning point in SARS-CoV-2
infection.[112 ] Specifically, low-molecular-weight and unfractionated heparin has been proposed
as the first line of treatment, which may possess both anti-inflammatory and antiviral
properties via disrupting viral interaction with ACE2 ([Table 2 ]).[113 ]
[114 ] More targeted anticoagulant therapies, including inhaled danaparoid, may allow a
directed approach to tailor treatment toward the thrombus-induced inflammation in
the lungs.[113 ]
While microthrombi contribute to the development of respiratory dysfunction, they
may also lead to multiorgan damage including cardiovascular complications such as
heart failure. Lung injury due to increased thrombosis may induce pulmonary hypertension,
which leads to observable increases in cardiac troponin, creatine kinase (CK), and
N-terminal pro-B type natriuretic peptide levels in critically ill COVID-19 patients.[6 ]
[11 ]
[18 ]
[115 ] Outcomes from an in-hospital study reported 32% of COVID-19 patients had heart failure.
However, the numbers were skewed toward nonsurvivors when comparing nonsurvivors (52%
heart failure) to survivors (12% heart failure), suggesting heart failure may correlate
with disease severity rather than infection itself.[8 ] Similarly, in a cohort of 799 COVID-19 patients, heart failure was the second most
common cause of death after ARDS.[116 ] The combination of right ventricular heart failure together with lung fibrosis might
contribute to decreased lung perfusion leading to a hypoxic state observed in severe
cases.[117 ] Interestingly, one postmortem study observed increased pulmonary angiogenesis in
COVID-19 patients, suggesting new vessel growth was necessary for accurate lung perfusion.[74 ] Furthermore, arterial or venous thrombosis accounted for 16.4% of COVID-19 hospital
readmissions in a cohort of 1,368 patients likely influencing ischemic conditions
in severe cases.[118 ] Preexisting cardiovascular comorbidities including hypertension and diabetes were
associated with COVID-19 case severity, which likely exacerbate heart failure and
other cardiac injuries.[4 ]
[17 ]
[19 ]
Cardiac Injury
Although heart failure represents one side of the cardiac injury involved in SARS-CoV-2
infection, several studies have also reported cardiac arrhythmia and myocarditis in
COVID-19 patients. A retrospective study identified the heart was the earliest damaged
tissue after the lungs following SARS-CoV-2 infection.[119 ] While most studies have not investigated specific arrhythmias, one report observed
arrhythmic complication in 16.7% of COVID-19 patients, making it the most common complication
after ARDS ([Fig. 1 ]).[18 ] Furthermore, arrhythmias may manifest in more severe cases as one report found arrhythmias
more often in patients admitted to an intensive care unit.[18 ] Thrombus-induced hypoxia or inflammation may in part explain the high prevalence
of arrhythmias in COVID-19 patients especially those with preexisting cardiovascular
risk factors.[120 ] However, elevated cardiac troponin and CK levels may indicate an underlying myocardial
inflammation considering irregular ventricular arrhythmias can be associated with
myocarditis.[121 ]
[122 ]
To date, there is limited clinical evidence of myocarditis with only a few case reports
in COVID-19 patients all with varying degrees of myocardial inflammation ([Fig. 1 ]).[6 ]
[7 ]
[123 ]
[124 ]
[125 ] However, the mechanism behind this cardiac injury in COVID-19 remains unclear particularly
as a primary or secondary effect of SARS-CoV-2 infection. Interestingly, the receptor
for viral entry, ACE2, is expressed in pericytes of the cardiovascular system, and
its expression appears to be upregulated in failing hearts.[24 ] Additionally, single-cell RNA sequencing revealed that cardiomyocytes also express
ACE2, which was upregulated in patients receiving ACEis for pre-existing cardiovascular
conditions.[25 ] However, COVID-19 patients present acute cardiac injury symptoms on average 15 days
after the onset of symptoms suggesting direct infection may not be the likely cause
of myocardial inflammation.[8 ]
[126 ] A secondary immune-mediated effect of SARS-CoV-2 might be the more likely explanation
considering the timeline of symptoms. Particularly, inflammatory hyperactivation,
as observed by the cytokine storm, and subsequent increase in inflammatory biomarkers
have been associated with myocardial damage and cardiac injury.[11 ]
[127 ] To distinguish the cardiac injury mechanisms at play in SARS-CoV-2 infection, further
investigation requires a systematic elevation of larger cohorts of severe COVID-19
cases as well as experimental work using both in vitro and in vivo models.
Outlook for Targeting SARS-CoV-2 Inflammation
Outlook for Targeting SARS-CoV-2 Inflammation
Hyperinflammation appears to be a common theme in the immunomodulatory, thrombotic,
and cardiovascular complications associated with SARS-CoV-2 infection. Therefore,
a variety of anti-inflammatory treatments have been purposed for severe COVID-19,
including RAAS inhibitors, cytokine-blocking therapies, and corticosteroids. However,
preliminary evidence for each therapy demonstrates both advantages and disadvantages
depending on their target.
Inhibitors of RAAS, such as ACEis and ARBs, are widely used to treat hypertension.
In the context of atherosclerosis, these inhibitors were also shown to be effective
in suppressing inflammation as well as oxidative stress ([Table 2 ]).[128 ] The use of RAAS blockers in COVID-19 patients, however, has caused a great dilemma
among health care workers, due to their probable impact on ACE2-SARS-CoV-2 dynamics.[129 ]
[130 ]
[131 ] Keidar and colleagues showed that mineralocorticoid receptor blockade via spironolactone,
aimed at hindering the activity of aldosterone, increased ACE2 expression and activity
in monocyte-derived macrophages collected from patients with congestive heart failure.[132 ] Another study showed that an ARB named telmisartan reduced ACE2 levels in the aorta
of spontaneous hypertensive rats.[133 ] Moreover, Ferrario and colleagues revealed that the treatment of Lewis rats with
angiotensin II receptor antagonist losartan increased cardiac messenger RNA (mRNA)
levels and activity of ACE2.[134 ] In view of that, RAAS inhibitor-related ACE2 upregulation has been hypothesized
by several scientists to increase the risk and incidence of SARS-CoV-2 infection as
there would be theoretically more doorways available for the virus entry.[135 ] Moreover, there is no scientific evidence to support the theoretical concern that
RAAS[136 ] blockers may increase the threat or severity of the SARS-CoV-2 infection. Meanwhile,
Milne and colleagues tested mRNA expression levels of ACE2 in human lung tissues upon
ACEi and ARB treatments and disclosed a decrease in ACE2 levels via ACEi treatment
whereas ARB treatment did not cause any differences.[137 ]
Of note, several studies investigating the association between the risk of SARS-CoV-2
infection and the use of RAAS inhibitors disclosed that RAAS inhibitors do not impose
an increased risk of viral infection. Mancia and colleagues reported no association
between the use of ACEi and ARB and COVID-19 in a case–control study in Lombardy,
Italy with a total of 6,272 cases and 30,759 matched controls.[138 ] Another study in New York City, United States evaluating the connection between
the likelihood of testing positive for COVID-19 as well as the severity of the disease
and the use of RAAS inhibitors among other treatments, such as β-blockers and calcium-channel
blockers, disclosed no association between any of these treatments and the risk of
infection as well as the disease severity.[139 ] Finally, another study by Mehra and colleagues with a database from 169 hospitals
in Asia, Europe, and North America reported that underlying CVDs are indeed associated
with an increased risk of death among the hospitalized COVID-19 patients, whereas
ACEi and ARB treatment was not associated with in-hospital death.[59 ]
It seems, regardless of its amount, the presence of ACE2 is sufficient to support
virus entry and a decrease of the receptor activity empowers the severity of the illness
due to the abolished protective roles of ACE2. In this case, targeting increased activity
of ACE2 may be of benefit rather than a disadvantage to restrict the impact of the
COVID-19, both for the pulmonary and cardiovascular systems.[140 ] Besides, COVID-19 has been shown to cause a strong inflammatory response in patients,
leading to a so-called cytokine storm, which also strongly contributes to an ARDS.[17 ] This rise in inflammation further feeds chronic inflammatory diseases, such as atherosclerosis,
and therefore worsens the patients' prognosis. The immunomodulatory benefit of the
ACE2–angiotensin (1–7)–Mas axis contrasting the proinflammatory role of RAAS is especially
advantageous with regard to the management of the inflammation and therefore the manifestations
of chronic inflammatory diseases.
The massive immune response observed during SARS-CoV-2 infection has prompted a search
for therapeutics primarily targeting the inflammatory cytokine storm. Unlike broad
immunosuppression, cytokine-blocking therapies such as those targeting IL-6 and IL-1β
likely should not dampen the host's response to the virus. While initial reports of
IL-6R antagonists were promising, the results from phase III of the COVACTA trial
recently announced that tocilizumab did not meet its primary endpoint of improved
clinical status of COVID-19 patients ([Table 2 ]).[104 ]
[141 ] Nevertheless, clinical trials are still pursuing the clinical relevance of other
cytokine-blocking therapies including IL-1β inhibition using anakinra and canakinumab
([Table 2 ]).[142 ]
[143 ]
A recent breakthrough heralded in a press release only at the time of submission may
provide further evidence of a more global role of excessive inflammation and the importance
of its control in COVID-19. The randomized controlled RECOVERY trial enrolled 2,100
patients who received a low dose of the corticosteroid dexamethasone for 10 days,
and compared them against 4,300 patients who received standard care. The results revealed
a striking effect of dexamethasone among critically ill patients on ventilators and
those receiving oxygen therapy, reducing their mortality by up to 30% ([Table 2 ]).[144 ] Dexamethasone is a type of glucocorticoid, which are known to exert potent anti-inflammatory
effects and are therefore used in the treatment of several autoimmune and inflammatory
diseases such as asthma and ulcerative colitis.[145 ] Therefore, glucocorticoids may be very useful in the treatment of heightened immune
response to COVID-19, including the cytokine storms. Moreover, dexamethasone treatment
might potentially offer further benefits to patients besides immunosuppression. Despite
the association of glucocorticoids with venous thromboembolism,[146 ] a study by van Giezen et al investigating hemostatic effects of dexamethasone on
rats showed a twofold decrease in arterial thrombosis and reduced platelet aggregation
with low-dose treatments (up to 1 mg/kg).[147 ] It is important to note, however, that higher doses of dexamethasone (from 1 mg/kg
onwards) yielded a decrease in fibrinolytic activity and counteracted the arterial
thrombosis. Further research is needed to explore such potential benefits of dexamethasone
as well as its dose-dependent effects.
Without a doubt, COVID-19 presents an immense challenge for the health care system
due to its wide-ranging impact on the health of diverse groups of patients. Although
severe COVID-19 cases typically present with similar thrombotic and inflammatory characteristics,
data describing the most representative biomarkers are still evolving. Therefore,
future treatments for thromboinflammation may need to be tailored to better fit the
patients' individual needs.[148 ] The safety of the drugs intended to treat COVID-19 patients should be carefully
considered, especially for those with underlying health problems, such as CVDs. Larger
studies investigating these drugs in the context of CVDs are needed to identify groups
of patients who are at higher risk for suffering from serious and even lethal consequences
of these treatments.
Conclusion
In this review, we aimed to highlight the immunoinflammatory mechanisms and subsequent
thrombohemostatic and cardiovascular effects of COVID-19 especially in patients with
underlying cardiovascular risk factors. Considering the novel nature of the virus,
our knowledge is still growing with regard to the systemic and local effects of SARS-CoV-2
infection. With this in mind, many questions remain unanswered about the primary and
secondary causes of the cardiovascular manifestations of COVID-19 patients. In the
upcoming months, systematic analyses of larger patient cohorts, in particular at a
genome-wide genetic level, are needed to dissect and explain differential predisposition
in different blood groups and ethnicities. Together with experimental work, researchers
may be able to shed more light on the identification of the underlying mechanisms
of inflammation, thrombosis, and cardiac injury in COVID-19 patients.[79 ] In the meantime, careful evaluation of new therapeutics for SARS-CoV-2 should highlight
their effects on the cardiovascular system as many studies have observed cardiovascular
complications ranging from ischemic stroke to myocarditis in severe cases especially
with hypertensive and diabetic patients. Considering we have seen several similar
coronaviruses in the past, careful and thorough research in SARS-CoV-2 will likely
improve our understanding of future coronaviruses.
