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).