COVID-19 infection: an overview on cytokine storm and related interventions

At the end of 2019, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) originated in Wuhan, China. Since then, it has been rapidly spreading worldwide and becoming a public health concern worldwide. This disease leads to pneumonia infection named coronavirus disease 2019 (COVID-19), posing an enormous threat to global health [1]. COVID-19 can trigger a cytokine storm in pulmonary tissues through hyperactivation of the immune system and the uncontrolled release of cytokines [2]. The phrase “cytokine storm” is a descriptive term to encompass a variety of events that may ultimately result in multi-organ failure and death [3]. Cytokine storms can cause a severe clinical complication known as acute respiratory distress syndrome (ARDS). ARDS is induced by an excessive immune response rather than the viral load [4]. Growing evidence is being documented on the possible role of the pro-inflammatory cytokines in COVID-19 pathogenesis and related complications [5]. With such a reason in mind, management of the cytokine release syndrome (CRS) and preventing subsequent infections may be an intriguing approach for COVID-19 therapy. Accordingly, there is an imperative need to develop anti-inflammatory agents for COVID-19 patients who develop CRS. Identifying the underlying mechanisms can aid in developing therapeutic strategies and speed up recovery. Several clinical trials are ongoing to investigate novel supportive care and interventions to cure this infection. To date, no definite and effective therapeutic agents are available. However, some supportive therapeutic interventions and anti-viral agents with limited efficacy are in hand to decrease the outbreak of COVID-19 and mitigate its symptoms [6, 7]. In this regard, anti-inflammatory therapy in patients with COVID-19 may be a promising intervention for COVID-19-related pneumonia [8]. This article will address the current understanding of cytokines-induced alterations in COVID-19, focusing on interleukin (IL)-6, IL-1, IL-17, and tumor necrosis factor (TNF). Then, we will discuss therapeutic strategies to rescue affected patients with severe COVID-19.

It is believed that the severity of COVID-19 disease is linked to the virus-induced cytopathic effects and escape of the virus from the host immune system [9]. In patients suffering from COVID-19, the host immune system can cause a lethal inflammatory situation known as CRS [10]. As the name reveals, this is a phenomenon of an extreme inflammatory response, in which inflammatory cytokines are rapidly secreted in a massive amount in response to infective stimuli. Indeed, this unconstrained inflammatory cytokine storm is a severe status observed in patients requiring intensive care unit (ICU) admission [11]. CRS is one of the possible events for the progressive and severe forms of COVID-19 and its mortality. Defining clinical criteria for CRS is a challenging issue; however current studies propose a series of features such as clinical symptoms and laboratory findings to confirm this status [12, 13]. The underlying mechanisms responsible for the unrestrained release of inflammatory factors are still vague, but several hypotheses exist. The first one is related to virus replication, which leads to pyroptosis, a highly inflammatory form of lytic-programmed cell death (apoptosis). In COVID-19 patients, pyroptosis triggers the release of pro-inflammatory cytokines and affects macrophage and lymphocyte functions [14, 15], causing peripheral lymphopenia [16]. Growing evidence points to an alteration in innate immunity induced by interferon (INF)-1. INF-1 is a vital contributor to viral replication and promoting the adaptive immune systems. Indeed, COVID-19 influences the host’s innate immune response and weakens the function of INF-1 in response to infection [14, 17]. Following virus infection, macrophages, dendritic cells, and neutrophils start the immune response as the body's first-line defense. Consistent with this notion, the lung autopsies from patients who died of COVID-19 revealed a high infiltration of macrophages into the bronchial mucosa [18]. Besides, recent studies suggest that excessive production of some cytokines, such as IL-6 may be the leading cause of inflammatory response in COVID-19 [19]. The second hypothesis is associated with adaptive immunity and the production of neutralizing antibodies against the surface antigen of the virus. Several animal studies declared that immunoglobulin (Ig) Gs could bind to the S protein and trigger inflammatory cascades. This binding can accumulate pro-inflammatory macrophages and monocytes in the lungs through the release of IL-8 and monocyte chemoattractant protein (MCP)-1. The inflammatory reaction is mediated by the Fc receptor (FcR) interaction on the surface of monocytes/macrophages with the virus-anti-S-IgG complex. This notion is supported by the decreased level of pro-inflammatory cytokine following blockage of macrophage receptors [20, 21]. In addition, several pieces of evidence support that anti-viral IgGs are coincident with the onset of severe respiratory disease in COVID-19 patients [22].

As mentioned earlier, fatalities in the severe form of COVID-19 are strongly associated with CRS. Virtually all cells and tissues in the body can be influenced by cytokine storms. CRS is an acute and uncontrolled inflammatory response characterized by multi-organ dysfunction and diverse clinical manifestations such as fever [3]. With these backgrounds, a vast range of anti-inflammatory approaches is being developed to dampen CRS and save the life of affected patients.

Several pro-inflammatory cytokines can induce cell death in various cell types, leading to pathological conditions. In a study conducted on 416 patients affected with COVID-19, cardiac injury was present in about 19.7% during hospitalization of the patients [23]. There is a possibility that COVID-19 causes myocardial cell injury either directly by interacting with the angiotensin-converting enzyme-2 (ACE2) receptors or indirectly by other mechanisms [24].

In the case of ACE2 receptors, it is hypothesized that the COVID-19 first attacks several organs expressing ACE2 receptors, such as the heart, brain, vessels, liver, kidney, and, more importantly, lung [25]. Several lines of evidence declare that COVID-19 may cause brain injury via direct and indirect mechanisms. In this case, creating a hypercoagulable state leads to the occlusion of cerebral vessels and brain damage. Besides, cytokines may have neurotoxic effects on the brain and can disrupt the integrity of the blood–brain barrier (BBB), resulting in neurological manifestations [26]. These clinical effects are more common in patients with severe forms of COVID-19 disease [27].

The liver is another organ influenced by the cytokine storm of COVID-19. It has been reported that 14.8–53.1% of patients with COVID-19 had abnormal levels of serum aminotransferases [28]. Acute kidney injury is also prevalent in ICU admitted COVID-19 patients due to the activation of the innate and adaptive immune systems and inflammation caused by virus infiltration and cytopathic effects [29].

Importantly, ARDS seems to be the most serious complication of COVID-19, with a high mortality rate. In other words, ARDS is a consequence of CRS and leads to respiratory epithelium damage [30]. In these patients, the aberrant pathogenic T cells secrete higher levels of cytokines such as granulocyte–macrophage colony-stimulating factor (GM-CSF), resulting in an inflammatory storm and severe lung damage [31]. Relying on this concept and according to data from a meta-analysis of 38 studies involving 3062 patients with COVID-19, the incidence of ARDS cases was 19.5% and the fatality rate was 5.5% [32]. In addition to those mentioned above, cytokines have a critical role in other complications of COVID-19; for instance, IL-6 can reach the skin and induce skin lesions [33]. Collectively, the uncontrolled release of cytokines may cause multi-organ damage in COVID-19 patients.

ACE2 receptors are the main binding site for virus entry and subsequent viral replication (Fig. 1A). ACE2, as a transmembrane receptor, is mainly found in type II alveolar cells, bronchial epithelial cells, myocardial cells, oesophagus epithelial cells, liver cholangiocytes, neurons and glia, stomach, cholangiocytes, adipose tissue, pancreatic exocrine glands and islets, renal tubules, stomach epithelial cells, ileum, rectum and many other sites [34]. In this regard, Zou et al. provided evidence regarding the potential risk of different organs to COVID-19 infection. Some tissues are more vulnerable to COVID-19 due to higher expression of ACE2, such as the lower respiratory tract, lung, heart, ileum, oesophagus, kidney, and bladder. Since the liver and stomach have lower levels of ACE2-positive cells, they are at low risk for COVID-19 infection [35].

Following COVID-19 infection, patients experienced exuberant activation of T cells. T helper (Th) cells have a critical role in the adaptive immune system in viral infections. In this context, type 1 interferon production is inefficient and anti-viral response is impaired. Indeed, Th1 cells can mainly regulate the adaptive immune system via cytokine production, while cytotoxic T-lymphocytes (known as CD8⁺ T cells) act as specific mediators to destroy virus-infected cells. Converging evidence has revealed that COVID-19 infection affects T lymphocytes and decreases the numbers of CD4⁺ and CD8⁺ T cells and interferon-gamma (IFN-γ) levels. It is crucial to note that the reduction of T cells is correlated with the severity of COVID-19 in affected patients. In this condition, Th₂ responses are promoted and accompanied by overexpression of Th₂-derived cytokines such as IL-4, IL-5, and IL-13 [36]. It is noteworthy to highlight that an effective immune response against viral-mediated infections relies on the activation of cytotoxic T cells to clear virus-infected cells. As a result, promoting the number and function of T cells is a crucial point for successful recovery in affected patients [37].

Moreover, a recent study pointed out that 82.1% of COVID-19 patients had a low circulating lymphocyte count [38]. Similarly, in a retrospective study by Diao and colleagues from data of 522 COVID-19 patients, 499 cases were analyzed for lymphocyte count. The results disclosed that 75.75%, 75.95%, and 71.54% of ICU admitted patients had low levels in the count of total T (lower than 800/μL), CD4⁺ (lower than 300/μL), and CD8⁺ T (lower than 400/μL) cells, respectively. These findings were negatively associated with the survival rate in infected patients. Apart from reducing the T-cells number, T cells were functionally exhausted. In addition, a negative correlation was observed between T cell counts and cytokine levels of serum IL-10, IL-6, and TNF-α. However, it should be noted that ACE2, as a defined receptor of the COVID-19 virus, is absent on T cells (11), proposing that the reduced level of T counts in affected patients is likely not resulting from direct infection of T cells [39].

Based on recent studies, it was strikingly shown that the level of inflammatory cytokines is increased in COVID-19. An overview of the literature indicates that IL-6, IL-2, IL-7, IL-10, granulocyte colony-stimulating factor (G-CSF), IFN- γ, inducible protein (IP)-10, TNF-α, MCP-1, macrophage inflammatory protein (MIP)-1α play a crucial role in the pathogenesis of COVID-19 [40]. Besides, Liu et al. (2015) evaluated 48 cytokines in the blood plasma of COVID-19 patients. Compared to healthy subjects, 38 out of 48 cytokines were remarkably elevated in patients with COVID-19. In addition, there was a strong linear association between severe lung injury and the level of 15 cytokines including, IFN-γ, IFN-α2, IL-1ra, IL-2, 4, 7, 10, 12, and 17, as well as chemokines such as IP-10, macrophage colony-stimulating factor (M-CSF) and G-CSF. The levels of Th1, Th2, and Th17 cells were increased, too [41].

With this evidence, a great deal of attention has been paid to dampening signaling pathways of inflammatory cytokines aiming to reduce inflammatory responses and mortality in patients suffering from COVID-19. According to the literature, cytokines have a key role in regulating immunological and inflammatory profiles. Among the cytokines, IL-6 is known as a causative factor in the pathogenesis and severity of COVID-19 due to various pleiotropic functions. Therefore, continuous measurement of IL-6 level is suggested in affected subjects with COVID-19. Multiple clinical trials are ongoing to evaluate the benefit of cytokine blockade by corresponding inhibitors. Taken together, we concisely describe inflammatory markers responsible for CRS and possible therapeutic approaches in this regard.

IL-6 is a glycoprotein that can act as both pro-inflammatory and anti-inflammatory cytokines. It can be produced by stromal, almost all immune cells, and other cells such as endothelial cells, fibroblasts, keratinocytes, and tumor cells. It has been well established that IL-6 has a crucial role in the differentiation of B-cells and the production of antibodies. Other immunomodulatory roles of IL-6 are linked to the development of self-reactive pro-inflammatory CD4⁺ T-cell response, stimulation of cytotoxic T-lymphocyte activity, regulation of T-helper 17, and regulatory T-cell balance [42, 43]. Besides, some evidence instantiates more effects of IL-6 on immune and non-immune cells through acting in a hormone-like fashion. In addition to this, IL-6 plays a critical regulatory role in homeostasis, such as the acute-phase response and hematopoiesis [44].

Growing studies have indicated that elevated levels of IL-6 are associated with CRS [45]. In other words, the pathophysiological hallmark of COVID is closely associated with severe inflammatory responses; thereby, identifying the serum level of IL-6 may predict the progression of COVID-19 disease. According to a systematic review and meta-analyses of 10 cohort studies, including 1798 patients, elevated levels of IL6 were observed in patients with COVID-19 [46]. One study showed that the serum level of IL-6 went up to 517 ± 796 pg/mL in patients with severe acute respiratory syndrome. In contrast, in recovered patients, the level of this cytokine gradually decreases to 68.8 ± 25.9 pg/mL [47]. Another similar research found that the level of IL-6 was noticeably higher in 86.8% of hospitalized COVID-19 individuals with severe complications and 22.9% had more than a tenfold increment in IL-6 levels [45].

IL-6 can signal primarily through cis- and trans-signaling pathways, which is distinct in cellular distribution (Fig. 1B) [48]. IL-6 first binds to its receptor (IL-6R), and then the IL-6/sIL-6R complex associates with the signal-transducing membrane protein gp130 to initiate intracellular signaling. The gp130 is expressed by most cells in the body, while IL-6R expression is mainly restricted to immune cells such as neutrophils, hepatocytes, monocytes-macrophages, and specific lymphocytes [49]. In the era of molecular-based analysis, in the cis-activating pathway, IL-6 binds to the membrane-bound IL-6 receptor (mbIL-6R, which express in immune cells), building a complex. This signaling pathway can predominate at a lower level of IL-6, thereby mediating the anti-inflammatory effects [50]. In other words, upon binding IL-6 to its receptor, the complex interacts with gp130, resulting in dimerization and activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway. Due to the broad expression of gp130 in various effector cells, the higher level of IL-6 is associated with strong hyperactivity of the immune system. Relying on this, cis-activation of IL-6 causes several effects on immune systems in terms of the acquired (B and T cells) and innate (macrophages, neutrophils, and natural killer cells) systems, developing CRS [51]. The consequence of JAK/STAT3 activation leads to a systemic CRS and releases a variety of cytokines such as IL-8, IL-6, vascular endothelial growth factor (VEGF), MCP-1, and E-cadherin. Both VEGF and E-cadherin enhance the permeability and leakage of vascular endothelial cells, resulting in pathophysiological disturbances and lung dysfunction/failure [52].

On the contrary, the soluble form of the receptor (sIL-6R) is found in serum and synovial fluids and mediates the trans-signaling cascade. This provides stimulation of a variety of cells by IL-6. The soluble form of IL-6R is generated from mbIL-6R cleavage by a disintegrin and metalloprotease 17 (ADAM17) [53]. In a healthy status, classical (cis) signaling has a protective role and controls various metabolic processes. IL-6 trans-signaling pathway is positively correlated with disease-induced inflammation. Moreover, inducing the differentiation of monocytes into macrophages, recruiting immune cells, and restraining Treg cell activity are mediated by IL-6 trans-signaling pathway [54].

In addition to mentioned signaling pathways, a third mode called trans-presentation is present for IL-6 signaling (Fig. 1B). This is a juxtacrine signaling pathway in which dendritic cells and T cells are involved in IL-6 signaling and induce various events, including the generation of pathogenic Th17 cells, Treg cells suppression by IL-27, and priming Th17 cells in combination with transforming growth factor-beta 2 (TGF-β2). Following this process, phosphorylation and activation of the JAK/STAT3 pathway occurred [55]. Furthermore, other pathways such as rat sarcoma-rapidly accelerated fibrosarcoma (RAS-RAF) and phosphatidylinositol 3-kinase/protein kinase B (PI3KB) can be activated by IL-6-mediated effects [56].

Recent evidence has implied the possible role of the inflammatory cytokines in the pathology of severe COVID-19 [31]. Given the essential role of IL-6 as a key driver in inflammatory status, and based on international guidelines, finding a suitable and efficient approach to inhibit IL-6 signaling is in demand. Accordingly, applying neutralizing antibodies to target IL-6- mediated inflammatory responses may be a life-saving strategy for COVID-19 patients. Numerous investigations have been performed to alleviate IL-6-mediated inflammation, mainly in the advanced stage of the disease. Two main types of IL-6 inhibitors as monoclonal antibodies are available, targeting IL-6 (siltuximab) or the corresponding receptor (tocilizumab and sarilumab).

Growing evidence revealed the crucial role of IL-1 family members in inflammation. Among them, IL-1α and IL-1β have pro-inflammatory effects [86]. IL-1 as a pleiotropic cytokine has a substantial role in the pathogenesis of COVID-19. The higher levels of IL-1 can accumulate in the lungs of patients with COVID-19. IL-1 expresses in approximately all cell types, including epithelial cells, endothelial cells, and infiltrating myeloid cells within the damaged tissues, as observed in the lung of affected patients with COVID-19. Necrotic cell death in virus-mediated infection leads to the cell membrane rupture and subsequent release of IL-1α [87]. Both IL-1α and IL-1β bind to the biologically active receptor and inactive receptor. In order to activate the signaling pathway, the IL-1R must associate with the accessory protein [87]. In other words, IL-1α and IL-1β have the same pro-inflammatory effects through inflammatory-related signaling pathways. Along with this, several inflammatory-associated transcription factors are activated, for instance, NF-kB, activator protein-1, JNK, p38, mitogen-associated protein kinases (MAPKs), extracellular signal-regulated kinases (ERKs), and interferon-regulating genes [88].

IL-1α is expressed sufficiently by activated platelets, endothelial cells, and circulating monocytes during an inflammatory state [89]. Also, IL-1α is released from dead cells such as endothelial and epithelial cells, whereas IL-1β is released by immune cells, including neutrophils, macrophages, and infiltrating monocytes. Noteworthy, IL-I receptor antagonist (IL-1 Ra) can modulate and prevent excessive IL-1-mediated inflammatory responses via binding to IL-1 receptors. IL-1α and IL-1β are synthesized as precursor proteins; hence specific cellular proteases are needed to cleave and form mature cytokines for interacting with cell surface receptors. In addition to the mature form, the precursor of IL-1α is also active. However, the activity of this precursor is regulated by the decoy receptor through binding to IL-1α [90, 91].

It is well documented that the release of IL-1β mainly depends on the expression of the (NLR family pyrin domain containing 3) NLRP3 inflammasome, which controls the maturation of IL-1β. Published reports have suggested that NLRP3 inflammasome can recognize some RNA viruses [92]. As explained in previous sections, there is a life-threatening-associated cytokine storm in the advanced stage of COVID-19 disease, leading to diffuse alveolar epithelial and endothelial injury in the lung of affected patients [93]. Based on the IL-1-mediated inflammatory responses, it can be assumed that IL-1α-mediated inflammation is responsible for the development of COVID-19 pathogenesis. Once IL-1α is released from damaged epithelial cells, various pathological alterations are mediated through stimulation of several inflammatory cascades, sensing inflammatory myeloid cells, and activation of the inflammasome. Growing evidence showed that IL-1α plays a link between inflammatory reactions and the coagulation cascades. A local expression of IL-1α by endothelial cells promotes thrombosis by recruiting granulocytes to the lung[94]. Relying on this concept, the IL-1 blockade might avert thromboembolic events in COVID-19[95]. On the other hand, an IL-1 receptor antagonist (IL-1Ra) can limit inflammatory reactions and tissue damage during ARDS. Some clinical studies have declared that IL-1Ra is increased in the bronchoalveolar fluid in patients with ARDS, indicating the severe form of the disease [96]. With these backgrounds, overcoming IL-1-mediated hyper-inflammatory responses may have efficacy in patients with COVID-19. Accordingly, many ongoing clinical trials should be directed to evaluate the benefit of anti-inflammatory strategies in COVID-19.

It is most well-known that T helper (Th) cells have a critical role in the adaptive immune system during viral infections. Indeed, following the recognition of the virus by antigen-presenting cells (APCs), dendritic cells (DCs) initiate cytokine release and form a microenvironment to evoke T cell responses. Activation of Th17 cells can lead to the secretion of multiple inflammatory cytokines. It has been reported that naïve CD4⁺ T cells can differentiate into Th17 cells via activation of Th cells in the presence of IL-6, IL-1β, IL-23, and TGF-β [110]. According to current literature, alveolar macrophages such as APCs can release IL-6, IL-23, and many other cytokines in COVID-19. The binding of IL-6 and IL-23 to the corresponding receptors can cause polarization and maturation of naïve CD4⁺ T cells toward Th17 cells [111]. Th1 cells primarily regulate the adaptive immune response through cytokine production, and cytotoxic T-lymphocytes (CTLs), known as CD8⁺ T cells, kill virus-infected cells [112].

IL-17 is a member of pro-inflammatory cytokines secreted by Th17 cells, which has a crucial role in the recruitment of monocytes and neutrophils to the site of infection. The IL-17 family includes 6 members: IL-17A (known as IL-17) and related family members, including IL-17B, IL-17C, IL-17D, IL-17E (known as IL-25), and IL-17F. IL-17A is the most critical member of this family, known as IL-17 (Fig. 1D) [113]. IL17 can exacerbate inflammatory reactions by activating downstream cytokines, such as IL-1, IL-6, IL-8, TNF-α, and MCP-1 [114]. Moreover, IL-17 can induce secretion of G-CSF, GM-CSF, and various chemokine ligands, for instance, CXCL1, CXCL2, and CXCL8. IL-17A also induces IP-10, MIP-1α/β, and multiple matrix metalloproteinases (MMPs) [115]. In addition, there is a link between IL-17 and IL-6 mediated activity in viral infections. In this regard, Hou et al. reported that overproduction of IL-6 levels could enhance IL-17-producing Th17 cells. In this case, IL-17 and IL-6 synergistically promote viral persistence by hindering host defense [116]. Noteworthy, the increased Th17 cells and IL-17 responses were reported in the severe form of COVID-19 patients. Upregulation of this cytokine is almost associated with lung pathology and ARDS and can injure other organs such as the heart, liver, and kidney [117].

Several evidence reported that the severity of COVID-19 is strongly correlated with IL-17 -induced inflammation, beyond other pro-inflammatory cytokines. ICU admitted COVID-19 patients have elevated levels of Th17 cells and worsened clinical manifestations in comparison to non-ICU patients. This is possible due to the excessive production of IL-17A and other pro-inflammatory cytokines [30, 118]. Altogether, it is hypothesized that IL-17 blockade is an immunologically plausible approach to improve the aberrant immune responses in COVID-19 and hinder ARDS-associated mortality [119].

It has been well established that the JAK-STAT pathway mediates the differentiation of Th17 cells. The signals from IL-6 and IL-23 can cause TH17 cell polarization from naïve CD4⁺. Both IL-6 and IL-23 activate STAT3 through JAK2. Accordingly, targeting JAK with specific inhibitors could be a possible approach to restrict Th17 cells-induced hyperinflammation. Evidence instantiates that JAK2 inhibitors such as fedratinib could decrease IL-17 secretion in murine Th17 cells [120].

Also, another study found that Fedratinib has efficacy in reducing the expression level of IL-17 by murine Th17 cells. In addition to its effect on IL-17, fedratinib may also inhibit GM-CSF activity, as GM-CSF can interact with JAK2 to mediate signaling. It can be implied that fedratinib can prevent the production of Th17 signature cytokines. Thus, this intervention would be a promising approach to alleviate the deteriorating effects of Th17-related inflammatory responses in COVID-19 [121].

There are multiple antibodies against Th17, including anti-IL-17, and anti-IL-17R; however, these antibody-based therapies have limited beneficial effects. To date, three IL-17 blocking agents are available, including ixekizumab, secukinumab, and brodalumab. Ixekizumab and secukinumab are both monoclonal IgG1 antibodies that specifically target IL-17A, with a similar mechanism of action. Both have high efficacy, tolerability, and a good safety profile, without decreasing the lymphocyte count. In patients with severe COVID-19, the lymphocyte count was considerably reduced, so these two antagonists can be considered reasonable interventions in COVID-19 patients. Brodalumab is a recombinant human monoclonal antibody against anti-IL-17 receptor A (IL-17RA). This therapeutic option can completely block T-17- mediated pathways [119]. COVID-19 is characterized by an immune dysregulation rather than a viral load, which leads to abnormal production of pro-inflammatory cytokines by alveolar macrophages. In this case, ARDS is the most common outcome of the exuberant infiltration of inflammatory cells responsible for COVID-19 infection and subsequent secretion of pro-inflammatory cytokines [122].

TNF-α is also associated with bronchial hyperresponsiveness. TNF-α is linked with the reduction of airway calibre and enhanced neutrophilia in the epithelium of the respiratory tract. TNF-α can directly deteriorate the respiratory epithelium by producing inflammatory cytokines such as GM-CSF, IL-8, and intercellular adhesion molecules (ICAMs). Besides, TNF-α can induce the release of MMP-9 by neutrophils. All these events lead to irreversible alteration via pulmonary fibrosis [123].

TNF-α is detected in the blood and tissues of patients with COVID-19. It is well established that TNF-α has a fundamental role in almost acute inflammatory responses as an amplifier and even as a coordinator of inflammation. TNF-α is mainly produced by monocytes and macrophages. In addition, B-cells, T-cells, and fibroblasts can synthesize TNF-α. TNF-α exerts its activity by interacting and activating with two receptors, TNF receptor (TNFR) 1 and TNFR2 in order to transduce signals [124]. TNF is initially synthesized as a bioactive transmembrane precursor protein with a molecular weight of 26 kDa. To release the soluble active TNF protein (17 kDa), it can undergo proteolytic cleavage by the TNF-α-converting enzyme, ADAM17. TNFR1 signaling is activated by two forms of TNF receptors as soluble (sTNF) and transmembrane (tmTNF), while TNFR2 is activated by tmTNF [123].

Given the profound role of excessive TNF in the development, pathogenesis, and poor outcome of COVID-19, blockade of TNF offers a clinically effective intervention in this regard. Importantly, blocking of TNF- mediated inflammatory response leads to a rapid decline in the level of IL-6 and IL-1 in individuals with active inflammation. The data from inflammatory bowel disease (IBD) patients with COVID-19 revealed that among 116 patients who received anti-TNF therapy, 99 patients recovered without hospital admission. Besides, affected patients on anti-TNF treatment face a better outcome [125].

Five specific inhibitors against TNF were initially approved for clinical applications, including infliximab, adalimumab, golimumab, certolizumab, and etanercept. Etanercept is a TNF antagonist that binds to the members of the lymphotoxin (LT) family. Indeed, these anti-TNF agents have been utilized for many years in severe autoimmune and inflammatory disease cases. The mechanism of actions of these antagonists is based on binding to their cognate ligands such as sTNF or tmTNF, and for etanercept, by binding to LTα3 and LTα2β and blocking downstream cascades. In other words, all five antagonists obstruct interaction between TNF and corresponding receptors on expressed cells. More details about these antagonists are depicted in Table 1 [126]. In addition, it has been reported that IL-17A and TNF-α have a substantial role in lung damage of covid-19 disease with obesity [127].

Dysregulated acquired immune system, and hyperinflammatory innate immune responses may be responsible for cytokine storm in COVID-19. Herein, we discussed potential mechanisms behind the COVID-19-induced CRS and then summarized possible therapeutic approaches. Indeed, the CRS is closely associated with fatal outcomes in critically severe COVID-19 patients. The management of the cytokine storm by cytokine antagonists and immunomodulatory agents may improve the survival rate of the infected patients. In this regard, targeting inflammatory cytokines can benefit patients and enhance the therapeutic efficacy of anti-viral therapy in COVID-19 patients.

On the other hand, targeting one inflammatory signaling pathway might stimulate downstream compensatory immune responses due to the complexity of the inflammatory network. Therefore, balancing the risk versus benefit of anti-inflammatory drugs should be considered. The data presented here and the authors' perspective have identified that antibodies targeting inflammatory cytokines remain an attractive therapeutic approach; thereby, combination therapy of inflammatory inhibitors and other COVID-19 modalities may have a better impact than alone. Highly approved evidence is required to understand the underlying mechanism of cytokine storms in COVID-19. Further studies are needed to elucidate the importance of anti-inflammatory interventions to curb hyperinflammation.