Signaling pathways activated by SARS-CoV-2:
Knowing the intracellular molecules involved in activation of host immune response may help targeting them in designing therapeutic and vaccines strategies. This may have better effect than targeting the viral peptides themselves which are liable for viral mutations and evasion mechanisms(36)
Since there are many similarities between SARS-CoV-2 and SARS in structure and mode of infection, it is postulated that they share innate immunity methods of signaling pathways activation. Following the attachment of SARS-CoV-2 S protein to ACE2 receptors expressed on the host cells, the viral RNA is recognized through TLR 3, 7 and 8 and the cytosolic RNA receptors ; RIG-I (37).This recognition especially through TLR3, 7 and 8 initiates signaling pathways activation in monocytes such as IRF3 (IFN regulatory factor-3), nuclear factor κB (NF-κB), JAK (Janus kinase)/STAT (signal transducer and activator of transcription) leading to Interferon type I and other cytokines production which in turn lead to differentiation of T cells towards CD4 T helper cells(38).
The NF-κB/TNFα signaling pathway:
Nuclear factor Kappa Beta (NF-κB) is an important transcription factor which plays a significant role in regulating innate and adaptive immune responses. Various pathogens stimulate nuclear translocation of NF-κB which exists normally in the cytoplasm of cells (39). It, then, initiates the expression of genes whose products are necessary for the inflammatory response such as cytokines and chemokines (40). Although this activation is essential for an optimum immune response, it has been suggested to lead to exaggerated inflammatory response leading to lung injury and respiratory distress caused by respiratory viruses such as SARS (41). Furthermore, NF-κB activation has been implicated in enormous production of IL-6 and TNFα cytokines in murine macrophages after exposure to recombinant SARS S protein. This was postulated to be due SARS S protein related degradation of IκBα, a normal inhibitor of NF-κB, leading to augmented activation of NF-κB signaling pathway (42).
Another study confirms these findings; they found that certain therapies which are able to suppress NF-κB signaling pathway such as caffeic acid phenethyl ester and parthenolide led to decreased inflammatory process by inhibiting the expression of genes encoding inflammatory cytokines and chemokines as TNFα, CXCL2, and MCP-1 in the lungs of mice having SARS infection. This in turn helped in preventing disease progression in those mice and decreased their mortality rates after the infection(41) .
Similarly, it was reported that infection with SARS in macaques has led to increased translocation of NF-κB secondary to its activation especially in old ones more than the younger macaques leading to a strong inflammatory viral response(43).
All these data suggest that suppression of NF-κB could be an efficient way to escape the undesirable inflammatory process caused by SARS, however, designing therapies that aim to target this molecule specifically may be problematic and could instead affect the normal innate immunity process leading to exacerbation of infections. In addition, many viruses have the ability to successfully block this signaling pathway leading to in-effectivity of such therapy and highlights the hope of targeting its inflammatory products instead such as TNFα (44). Furthermore, Anti-TNFα biological treatments such as infliximab and adalimumab have been tried in treating a variety of autoimmune diseases such rheumatoid arthritis and psoriasis with a reasonable success rate which favors targeting this cytokine specifically among the other inflammatory cytokines which are involved in the cytokine storm (45). In fact, patients with active rheumatoid arthritis who were treated with such therapies had also diminished levels of other inflammatory cytokines including IL-6 and IL-1 leading to fast amelioration of their inflammatory conditions. Indeed, when an anti-TNFα is administrated in patients with active rheumatoid arthritis, it has been demonstrated to induce a rapid decrease of a broad spectrum of cytokines (e.g. IL-6 and IL-1), as well as of others acute-phase related proteins and vascular permeability factor (46)
Based on these data and on the high similarity between SARS-CoV 2 and SARS, monoclonal antibodies against TNFα were hypothesized to suppress the cytokine storm occurring in COVID-19 patients and decrease its possible consequences. A clinical trial study using Adalimumab as TNFα-inhibitor has been started in Chinese COVID-19 patients to investigate its efficacy and safety as well (8).
Moreover, it was found that TNFα-converting enzyme (TACE)-mediated shedding of ACE2 which is required for virus internalization into the host cells is enhanced by SARS S protein. This means that monoclonal antibodies against TNFα could exert their therapeutic effects through double hypotheses: prevention of viral entry and weakening of the inflammatory process and cytokine storm (47).
The IL-6/JAK/STAT signaling pathway:
IL-6 was found to be elevated significantly following SARS-CoV 2 infection where it was postulated to participate strongly in the cytokine storm in infected patients (48). IL-6 interacts with its receptors expressed on the immune cells such as glycoprotein 130 (gp 130) receptor and membrane bound IL-6 receptor as well as soluble receptor for gp130 leading to activation of the JAK/STAT signaling pathway (49) . Actually, there is bidirectional relationship between both IL-6 and JAK/STAT signaling pathway meaning that they could stimulate each other as activation of JAK/STAT signaling leads to more IL-6 secretion and vice-versa(50).
Many cell types are known to produce IL-6 such as macrophages, endothelial and smooth muscle cells and once produced, it stimulates the production of other cytokines especially MCP-1 which induces atherogenesis(51), increased expression of adhesion molecules(52) and proliferation of vascular smooth muscle cells (53). This may consequently explain the cardiovascular complications occurring in COVID-19 patients where high levels of IL-6 are detected (54).
The production of IL-6 is seen to be caused by angiotensin II, which is locally generated by inflamed vessels to bind to its receptor; angiotensin II receptor type 1 (AT1 receptor) leading to activation of JAK / STAT signaling. The augmented production of angiotensin II promotes more IL-6 secretion in AT1/JAK/STAT-dependent way, thus entering in a vicious circle of inflammatory response (55).
Remarkably, it was found that SARS S protein has an important role in decreasing the expression of ACE2 with a subsequent increase of angiotensin II (56). Similarly, it was postulated that SARS-CoV-2 could exert same actions and decrease ACE2 receptors expression leading to increased accumulation of angiotensin II and hence increased IL-6 secretion leading to cardiovascular complications and pulmonary damage (Fig. 2) (8). In addition, this inflammatory pathway has been implicated in the activation of NF-κB and ADAM pathways. Particularly, ADAM17 causes ACE2 cleavage thus inactivating it, and increasing angiotensin II, accordingly leading to hypertension and other cardiovascular pathologies (57).
It was demonstrated that the metalloprotease ADAM17 transforms the membrane form of IL-6 receptor α (IL-6Rα) to its soluble form (sIL-6Rα) with the subsequent activation of STAT3 which in turn activates NF-κB signaling. Therefore, SARS-CoV-2 could lead to activation of both NF-κB and STAT3 pathways promoting the enhanced production of IL-6 which is in turn implicated in more activation of NF-κB by STAT3 leading to a hyper-inflammatory response and may proceed to development of autoimmune diseases (58). It was proposed that the amplified production of IL-6 induces the secretion of many other inflammatory cytokines and chemokines leading to migration of lymphocytes and leucocytes to the site of inflammation and maintaining the IL-6 mediated inflammatory response at its highest level(Fig. 2) (59).