disease and declared a pandemic in March 2020 by WHO [1] , [2] . These three viruses belong to the same family of Coronaviridae, have zoonotic origin and share genomic similarities SARS-CoV-2 reveals 80% sequence distinctiveness with SARS-CoV genome and 50% with MERS causing virus [3] . Although the Coronaviridae family includes four genera: , ,  and ; but only  and  genera infect humans [4] . SARS-CoV-2 is actually a and IFN- etc [1] , [6] . Moreover, lymphopenia (low lymphocytes levels) and an elevated neutrophil-lymphocyte-ratio (NLR) has also been associated with seriousness of the infection in critical cases [7] . Hypertensive patients are prone to develop severe COVID- 19 infection, due to altered rennin-angiotensin system (RAS) and in turn activation of NLRP3 (NOD-like receptor 3) inflammasome [8] . Further, apart from underlying conditions of either hypertension, obesity and cardio-vascular diseases; gender seems to be a risk factor in fatal COVID-19 infections as well. Certain reports suggest male being more prone than females, and could be either due to protective estrogen properties in females or stronger immune response driven elevated cytokines levels in male, but requires additional study-based statistics [9] , [10] .
Our immune system comprises of both innate and acquired immune response, which corresponds via variable chemo-tactic factors called cytokines. Different cytokines vary in their molecular weights, structure, functions, cells involved in their secretion as well as target etc. The levels of these cytokines swing during different infections, resulting in inflammation and removal of pathogens. Innate immune response represents the first line of defence, which releases the cytokines to activate the acquired (adaptive) immune system, comprising of humoral (B-lymphocytes) and cell-mediated (T-lymphocytes: both CD4+ helper T cells (Th) and CD8+ cytotoxic T cells (Tc)) immunity.
During viral diseases including SARS-CoV-2, innate response directs infected cells to secrete interferons (IFNs) and several pro-inflammatory cytokines. The role of IFNs is to alarm the nearby, still uninfected cells for developing an antiviral stage and; proinflammatory cytokines increases blood flow near infection site and activate macrophage and other phagocytic cells for clearance of virus, as well as infested cells.
The cytokines also activate the CD4+ Th lymphocytes of adaptive responses, which further stimulate-B-lymphocytes for secreting specific neutralizing antibodies to counteract the virus and CD8+ Tc cells to initiate programmed cell death of viral laden cells. Thus, innate reaction exposes the virus and directs the acquired response to control infection, but success lies in a well-timed co-ordination between both immune responses. However occasionally, virus after encountering the innate response causes a delay in IFNs production [11] . This delay is unwarranted in controlling virus and results in an unimpeded increase in viral load. Exhaustive viral load renders the adaptive response incompetent in fighting the infection completely. The time loss in IFN production delay, bring along a storm of cytokines release (CRS) including IL-6. IL -8, IL-12, TNFα, IL-17, IFN-γ, IL-1β, MCP-1, IP-10, IL-10, C-reactive proteins (CRP), CXCL10, D- dimer and ferritin etc [6] , [12] , [13] . This consequently increases the severity of infection including inflammation and its associated complications. Further, a reduction in the counts of natural killer (NK) cells, B and T cells, monocytes, eosinophils and basophils (lymphopenia) is also reported in these critical patients, which results in a delay in viral clearance [7] , [13] . These indications in deceased patients suggest that an hyper-inflammatory response finally converts into an immune-suppressive condition and might play a critical role in deterioration of conditions [14] . Looking at these varied consequences, all contemplate a deregulated immune response, this review is an endeavour to comprehend and elucidate the progression of SARS-CoV-2 infection, which results in severity including CRS and lung injury (ARDS) manifestation.
Structure of SARS-CoV-2 and entry: SARS-CoV-2 is an enveloped virus with a positivesense single-stranded (Ss) RNA strand as genome ( Fig. 1 ) [4] . Its genome is already fully sequenced with 29,903 nucleotides, and is having one of the largest RNA genome [15] .
Although RNA viruses are known to have an unstable genome, contrastingly SARS-CoV-2 genome is reportedly stable [16] . The virus has 14 open reading frames (ORF) involved in structure, survival and virulence. Architecturally, SARS-CoV-2 structure comprises of four main proteins involving-Spike (S) glycoprotein, envelope (E) glycoprotein, membrane (M) glycoprotein, nucleocapsid (N) protein, and numerous additional proteins (Fig. 1 ). Spike proteins, S present on membranes, are transmembranous, form homo-trimers and give it a characteristic crown-like appearance.
High sequence similarity between both CoV and CoV-2, lead to the proposition that both viruses might utilize the same receptor, ACE2 (angiotensin converting enzyme 2), which is now recognized in separate studies (Fig 2) [17], [18] . The ACE2 is a membrane bound protein, present on the upper respiratory tract cells involving oesophagus, alveolar epithelial cells (primary viral target) and other ACE2 + cells present in kidney, heart and gastrointestinal tract etc [3] , [11] . Therefore, patients infected with the SARS-CoV-2 also encounter other ailments related to heart, kidneys and intestine etc [19] . MERS however utilizes a different membrane enzyme receptor, DPP4 (dipeptidyl peptidase-4) in the liver, gut, placenta, lung, and kidney cells etc [20] .
The virus entrance starts with virus inhalation through respiratory route, followed by binding of viral spike (S) protein on ACE2 receptor of alveolar cells. The attachment of S protein (having S1 and S2 domains) on to ACE2, lead to a change in receptor conformation and subsequently a host enzyme, TMPRSS2 (Transmembrane Serine Protease 2) primes/cleaves the viral S (spike) protein, at the S1/S2 site (Fig. 2) triggering viral internalization [21] . This signifies that S1 contains the receptor binding domain (RBD) and is responsible for host determination, while S2 mediates virus fusion in transmitting host cells [21] . Viral N protein is heavily phosphorylated and present adjoining RNA is involved in viral replication cycle, and influences host cell response to viral infections (Fig. 1 ). M protein controls the envelope structure and E protein is involved in the production and maturation of the virus (Fig. 1 ). Host membranes fusion with virus and internalization happen via receptor mediated endocytosis (Fig 2) [17] .
Hence, both ACE2 and TMPRSS2 are the major contributing factor in the viral entry.
Afterwards, SARS-CoV-2 releases its RNA in the cell. The positive nature of genomic RNA allows direct translation of non-structural proteins (nsp) encoding ORFs in first step. A ribosomal frameshift producing ORF1a and ORF1b, which is translated into two proteins-pp1a and pp1ab, having overlapping regions (Fig. 2) . Subsequently, nsp3 and nsp5 gets translated into two proteases, explicitly-Papain-Like Proteases (PLP) and a Mpro (chymotrypsin-like protease (3CLpro)), which cleave and process pp1a and pp1ab into proteins-1 to 11 and 12 to 16, respectively [22] . These nsp's are actually replicase proteins, essentially an assemblage of RNA-dependent RNA polymerase (RdRp)-along with a helicase, vital for polyprotein processing, and proficient viral replication [21] .
Later, negative-sense strand synthesis occurs as an intermediate product (Fig. 2) , tailed by its transcription to mRNAs and utilized for formation of various structural proteins.
Replicated genome binds to protein N, which further interacts with structural proteins forming daughter viral particles and leaving the cell through exocytosis (Fig. 2) . The daughter viral particles further infect nearby cells and viral dissemination commences.
The virus has an apparent symptomless gestation phase amongst 2-14 days, throughout which the viral transmission is still possible [2] . Initial (Innate) Immune Response against Virus: After internalization, the virus comes in contact with respiratory mucosal lining, patrolled by various warriors of innate response including dendritic cells (DCs) and macrophages etc [23] . However, the still evolving perception on SARS-CoV-2 progression suggests that unlike MERS and SARS-CoV-1 illness, where macrophages stand susceptible to virus, SARS-CoV-2 may not directly enter the macrophages. DC seems to be the primary cell of immune system that responds to pathogen and macrophages rather later gets involved in co-production of various cytokines [23] . Infested cells and DC respond to viral infection via IFNs (interferons) secretion, especially type-I IFN (α and β) that help achieve an antiviral state in nearby uninfected cells. It is not difficult to envision that inadequacy of an intact innate response (IFN production) actually predisposes humans to several fatal viruses' attack. In CoV-2 disease, IFNs are chiefly secreted by DC's cells, categorized into-the conventional DC (cDC) and plasmacytoid DC (pDC). The pDC distinctively traverses between blood vessels to the lymphatic system and back and initiates a robust type I IFN production [24] . cDC although contributes to IFNs production but not vigorous like pDC and rather secrete majority of pro-inflammatory cytokines. In these cells, various pDCs express TLR-7 and TLR-9 inside endosomes, are more restricted in their antigen uptake with lower levels of co-stimulatory and MHC (Major Histocompatibility Complex) molecules; and are called "professional" DC, in terms of secreting very large amounts type-I IFN (α and β both) in a short span (Fig. 3 ) [24] . Type-I IFNs translation is regulated by three transcription factors namely: c-jun/ATF-2, NF-κB and IFN regulatory factors (IRFs), via distinct mechanisms [7] . Pathogen unmethylated CpG motifs are recognized by TLR-9 and Ss viral RNA binds and activates TLR-7 of pDC, leading to instigation of a downstream adaptor protein MyD88 (Myeloid differentiation primary response protein 88), which recruit IRAK4 (Interleukin 1R-Associated Kinase) and IRAK1 (through their death domain (DD)), followed by phosphorylation and activation of IRAK1 (Fig. 3 ). IRAK1 further employs the phosphorylated IRF-3 and IRF-7, which move to the nucleus and produce type-I IFNs. IRF3/7 functions as either homodimers or heterodimers with each other. Though IRF-3 is constitutively expressed in different tissues (IFN- producers), but IRF-7 is expressed constitutively in pDC, B cells, monocytes; and is an ISG (Interferon Stimulated Gene) required for the expression of most IFN-α subtype, known as a "master regulator" among type-I IFNs [26] . This pathway in pDC is called classical pathway for type-I IFN production ( Fig. 3 ) [24] . All TLRs (except TLR3) of APC's (antigen presenting cells) can also initiate an IRFindependent pathway for type-I IFNs production, by activating transcription factor NF-B. Here, MyD88 rather interacts with its downstream effector, a ubiquitin ligase TRAF6 (TNF receptor-associated factor 6), and activates TAK-1 (Transforming Growth Factor- (TGF-)-Activating Kinase), thus promoting downstream activation of the IB Kinases (IKK): IKK and IKK. It is already known that the IKKs phosphorylate inhibitory IB, sequestering inactive NF-B in the cytosol. Thus IB phosphorylation, allow NF-B translocation to the nucleus, followed by induction of multiple genes encoding proinflammatory proteins and co-stimulatory molecules such as type-I IFN, IL-6 and TNFα etc in various cells (Fig. 3) .
The IFN-α secretion from pDC, is followed by activation the TLR-3 (endosomal; activated via dsRNA) in cDCs to initiate type-I IFN production, in a MyD88-independent manner (Fig. 3 ). TLR-3 employs adaptor protein TRIF (TIR-domain-containing adapterinducing interferon-β), further binding to TRAF-3 (TNF receptor-associated factors), to induce NF-B stimulation and expression of inflammatory cytokines IL-12, TNFα, IFN-γ, IL-1β, IL-6, IL-18, IL-33 and TGF-β etc (Fig. 3 ) [27] .
The cytoplasmic PRRs: RIG-1 (encode a helicase) and MDA-5, both contribute to type-I IFN production in a TLR independent mechanism to produce inflammatory cytokines like TNF, IL-1, IL-6 and IL-12 etc implicated in severity of CoV-2 infections (Fig. 3) .
Therefore, TLRs, MDA-5 and RIG-1, along with adaptor protein MyD88 are critical in innate immune response and release of type-I IFNs and chemokines, which in turn activate other cells including-macrophages, monocytes, NK cells, neutrophils and pulmonary epithelia etc. Hence, DC along with other cells of immune system provides both distinct as well as overlapping contributions in host defence against CoV-2 and keep on mounting a full response involving production of type-I/ type-III IFN and proinflammatory cytokines. These cells provide the first line of defence against viral entry and the resultant inflammatory cytokines employ monocytes, various granulocytes and lymphocytes from blood and furthermore activate the adaptive immune system [23] , [28] , [29] . However, lung natural killers (NK cells) do not exhibit the ACE2 receptor (like macrophages) and improbable to be directly infected by SARS-CoV-2 [23] . Even though antibodies might still stimulate NK cell (via Fc receptor) for cytokines release as well as lyses the infected cells. However, data suggest that NK cells might be more involved in cytokines production than relieving infection [23] .
It is reported that in recovered cases, within a few hrs of virus entry, both α and β IFNs (at first day of infection) are rapidly produced and an antiviral state is soon reached [23] . This antiviral stage involves activating enzymes that can cleave both viral RNA (by oligoadenylate synthase (OAS and RNAase L) and prevents translation of structural proteins involved in viral packaging, enabling them to contain viral multiplication. It is seen that the curve of type-I IFN production and increase in viral titre is largely superimposing each other in milder infections (Fig. 4A ) [30] . Actually, type-I IFN (both α and β) act as signalling molecules, are pleiotropic and produced from a large family of genes. They work as both autocrine and paracrine signalling cytokines, binding to both self and neighbouring nucleated cells (including macrophages, monocytes, Tlymphocytes), through their cognate receptor called IFNAR (interferon  receptor) and initiate the JAK/STAT signalling (Fig. 3) . Fundamentally, type-I IFN binds on their receptor called-IFNAR1 and IFNAR2, resulting in their dimerization, phosphorylation, and activating the Janus Kinase family members-JAK1 and TYK2. These phosphorylate and recruit STAT1/ STAT2, which collectively binds to IRF-9, and together these three (STAT1/STAT2/IRF9) form a trimeric transcription factor called ISGF3, which translocate to the nucleus and binds to ISRE on IFN-stimulated genes (ISGs) including OAS, IRF1, MX1, PKR, MDA-5, RIG-1, ZAP, ISG15, ADAR, viperin and multiple inflammatory genes like TNFα and IL-6 etc (Fig. 3) . Thus, a timely type-I IFN production, followed by JAK-STAT signalling lead to ISGs expression, which contain the virus multiplication and is instigate the adaptive response, whereas poor outcomes are often accompanied by persistent IL-6 and TNFα release by various cells, including infiltrating monocytes push several inflammatory reactions and monocyte-derived macrophages can rather amplify dysfunctional responses, leading to criticalness in patients [31] .
Independently, increased inflammation is also attained due to internalization of ACE2R in virus infected cells. It reduces ACE2 levels on cell surface and in turn increases Angiotensin II levels, which binds on AT1R to result increase in vasoconstriction, ROS levels and NF-B mediated inflammation [32] , [33] , [34] . Simultaneously, virus entry activates another class of PRR's called NOD-like receptors (NLRs) and triggers their homo-oligomerization, which is a multiprotein inflammasome complex called NLRP3 (Nod-like receptor family, pyrin domain-containing 3). It cleaves and activate Caspase-1, which process IL-1β and IL-18 and secreted out (Fig. 3) . IL-1β binds to IL-1R on macrophages and in an NF-B dependent manner bring secretion of IL-6, TNFα and IL-1 from them. IL-18 activates CD8+ T cells to secrete IFN-, which mediate macrophage and NK cells differentiation [34] . This severe inflammation can lead to cell death called pyroptosis [35] .
Therefore, it is mandatory to examine the different events that lead to both the delay in IFNs production and elevated inflammation, which essentially refers to viral strategies employed to avoid early recognition by the immune system and its subsequent hyperactivation, as discussed in the next section.
Evasion of Immune Response: Like many other positive-strand RNA viruses, SARS-CoV-2 has evolved different mechanisms to dodge the host responses to delay IFNs production and subsequent signalling, which finally contributes to the exuberant inflammatory response and critical lung immune-pathology (acute respiratory distress syndrome; ARDS) in certain patients [36] .
To begin with, in viral life cycle, the nsp's can induce formation of membranous compartments inside cytoplasm devoted to viral RNA synthesis, and let them be able to hide their PAMPs from detection by host PRRs proteins, RIG-1 and MDA5. (Fig. 2) . These compartments are actually double membrane vesicles (DMVs) (~100-300 nm) or viral replication organelles (RO), derived from membranes of the secretory pathway (Fig. 2) [36], [37] , [38] . Electron microscopic observations have confirmed that RO like structures exist and its membranes can establish contacts with ER and Golgi membranes for exchange. The viral encoded nsp3, nsp4 and nsp6 of SARS-CoV-2, are found to be critical in DMV biogenesis [36] , [39] .
In another strategy, the virus modifies its RNA like addition of 5'-cap on mRNA (m7GpppNm-RNA) catalysed by phosphatases and methyltransferase (MTase) to avoid their detection. In this approach, crystallization studies with SARS-CoV-2, nsp16 and nsp10 have shown that nsp16-nsp10 forms a heterodimer and bring about 2' Omethylation (MTase) of 5'-cap of viral encoded mRNA [40] , [41] . Thus, RNA viral appears akin to the host RNA and averts any PRRs (MDA5 and MAVS) detection [38] , [42] .
In addition to DMV's formation and RNA modification, the virus has other competent strategies to circumvent the immune response, and subsequently antagonize the IFNs response. Actually, the viral large genome contains many accessory proteins (nsp's), which are not critical for either duplication or discharge of viruses, but rather may have immune-evasion functions. Much of this information has come from studies of earlier spread of SARS-CoV in 2003. Virus can antagonize STAT1 activity by means of three separate mechanisms: first, nsp1, can inhibit phosphorylation of STAT1, without affecting phosphorylation of STAT2, JAK1 or TYK2 [43] . Second, viral ORF6 encodes accessory proteins, that impounds host karyopherins (nuclear import factors) to rough endoplasmic reticulum (RER)/Golgi membrane, required for bringing STAT1 in to nucleus and thus works as an IFN opponent [44] . Third, nsp3 encodes PLP, which process viral poly-peptides into efficient proteins during viral life cycle (mentioned earlier), but also antagonize innate response by deubiquitinating (DUB) and deISGylating host substrates involved in the removal of small regulatory polypeptides, ubiquitin and ISG15, respectively, from many target proteins. These DUB enzymes, catalyses the removal of Ub (Ubiquitin) moieties from tagged proteins (including IRF-3 and NF-B) and affect stability of regulatory proteins mediating intracellular sensing and signalling of viral infection in innate immune response [22] , [45] , [46] , [47] .
Additionally, PLP also inhibits the RIG-1 and TLR3 induction of the IFN-. Similarly, deISGylating activity involves removal of ISG-15 (interferon-stimulated gene-15) moieties from marked proteins. ISG15 is a small Ub-like peptide that is covalently attached to some proteins. ISG15 is an Ub-like transformer with pleiotropic outcomes, characteristically produced in host cell immune response [38] , [47] .
Mitochondria is also capable of responding to viral infection and produces proteins called mitochondrial antiviral signalling (MAVS) proteins that induce cell suicide or apoptosis. This starts with a RIG-1 dependent protein, which binds to MAVS. On one hand, MAVS interacts with ER resident protein STING (Stimulator of IFN Genes), which dimerizes and in turn activate IKK (Inducible IκB Kinase ) and NF-B signalling.
Additionally, MAVS also activates IRF-3, making it move to the nucleus and turning on production of type I-IFN. Conversely, viruses can antagonize the type-I IFN response mediated by PLP, which inhibits this signalling [46] .
Latest details indicate that ORF8 of SARS-CoV-2, can straightforwardly reduce MHC-I molecules surface countenance and interrupts antigen presentation, lowering the detection and eradication of virus-infected cells by Tc cells. Besides this, ORF8 can selectively target lysosomal degeneration by autophagy and in turn CTLs inefficiently eliminate the ORF8-expressing cells [48] .
Effective activation of NF-κB leads to a robust production of pro-inflammatory cytokines and chemokines. Therefore, targeting NF-B for delaying type-I IFNs production is another efficient strategy employed by viruses. NF-B remains inactive in cytoplasm, due to attached inhibitory molecule IB. SARS-CoV protein M inhibits phosphorylation and degradation of IB , thus halting NF-B relocation to nucleus [38] , [49] . Apart from this, M protein antagonizes the MAVS-mediated innate response against virus by inhibiting the phosphorylation of TBK1, IKKα/β, IRF3 and p65, which in turn disrupt nuclear localization of IRF3 thus inhibits type-I IFNs production [50] .
ORF3a of CoV-2 is found crucial in apoptosis (like CoV) and opposition of type-I interferon reactions in infested cells [51] . Besides them, ORF6, ORF8 and nucleocapsid protein also inhibit interferon signalling via different mechanisms [52] . Viral N protein rather interferes with phosphorylation of IRF3, followed by resettlement to nucleus [38] , [53] . Additionally, N protein modulates TGF-β signalling to obstruct apoptosis of SARS-CoV diseased host cells, with significant repercussions in tissue fibrosis [54] . SARS-CoV nsp1 can cause host mRNAs translation shutoff by attaching to cellular factors like 40S subunit of ribosomes [42] . Additionally, nsp1 protein causes host mRNA degradation by energising Xrn1 exonuclease and other proteins in cleavage machinery [55] .
Besides IFN's delay, emergent indications also suggest the role of SARS-CoV 3a protein into stimulation of the NLRP3 inflammasome, which in turn trigger caspases mediated IL-1β and IL-18 release, pyroptosis and multi-organ failure, leading to severity of infection [56] .
These vital strategies reported in various fatal cases lead to a possible delay in type-I IFNs production, which bargains an early viral constraint, leading to spike in viral load ( Fig. 4A & B) . This could be the probable reason behind asymptomatic individuals being effective carriers. However, after the initial delay in type-I IFNs production and simultaneous successful viral replications, finally IRF-3, 7 and NF-B mediate production of type-I IFNs, which activate the nearby cells, including macrophages and in turn helper T (Th) cells, to further secrete IFN- and pro-inflammatory cytokines (IL-6 and TNF) (Fig. 4B ) [1] , [2] . Developed DCs trigger T cells for their pivotal linkage of onset-increase, management and continuance of immune reactions, which also stimulates lung immunopathology with reduced survival and weakened antiviral protection [57] .
Adaptive (acquired) Immune Response: As discussed earlier, a deferral in IFNs secretion don't authorise the immune response to obstruct proliferation and viral load reaches at peak on day 10 (second week) in the upper respiratory tract (Fig. 4B ) [58] , [59] . However, IFNs level finally starts mounting and becomes an immune-modulator, recruiting macrophage, NK and other cells of adaptive response. This delay challenges the adaptive response with the mammoth assignment of clearing the viruses (Fig. 4B ).
This mismatch in the peaks of IFN secretion and viral load imparts a significant twist in disease progression, with induction of an overactive immune response, which soon gets completely de-regulated with secondary complications and fatal outcomes (Fig. 4A & B) [60]. It is known that acquired immunity involves action of both B-lymphocytes [7] . It's daunting to disclose that the antibody's half-life is found to be limited to a few weeks only and life-long immunity is not achievable [62] .
The Th cells secrete IFN- and IL-2 to activate CD8+ (Tc) cells, which recognize the viral infected cells and eliminate them by apoptosis. Nevertheless, in acute conditions, lymphopenia sets in, that too specifically biased against CD8+ Tc count (than other lymphocytes), causing a decrease in CD8+ population. It is possible that various mechanisms reported in separate studies either alone or cumulatively led to lymphopenia. One study reported that the reduction in CD8+ Tc cells is due to reduced IL-2/IL-2R and JAK1-STAT5 signalling [61] . Thus, it makes absolute sense that reduction CD8+ count comes concurrently with drop in IL-2 and IFN- levels in critical patients than severe and normal patients [61] . Apparently, premature cell death is also involved, mediated by excessive levels of cytokines like IL-6, IL-10 and TNF, which affect T cells population via affecting DC and neutrophils count [63] . High expression of pro-apoptotic molecules including caspases, FAS and TRAIL also lead to low population of Tc cells [63] . Another report document that soon after activation, CD8+ cells differentiation to express repressing receptors like PD1, TIM3, LAG3, CTLA4, NKG2A and CD39 resulting in Tc cell exhaustion [63] , [64] . Similar to CD8+ Tc cells, CD4+ Th cells also show fatigue due to activation markers like CD38 and HLA-DR [63] . Further, immunosuppressive Tregs cells affect immunological homeostasis by inhibiting and suppressing multiple immune cells, such as CD8+, CD4+ T cells, monocytes, NK cells, as well as B cells. Tregs compete with IL-2 in particular and restrict proliferation of Tlymphocytes to CD8+ and CD4+ T cells and also produce IL-10, IL-35 and TGF-β for inhibition of differentiation via unexplored mechanisms [65] . Tregs express inhibitory cytotoxic T lymphocyte antigen-4 (CTLA-4) and lymphocyte-activation gene-3 for deterring innate and acquired immune response. Conversely, IL-2 leads to raised apoptosis of Treg cells in mild cases [66] . Thus, if separate mechanisms contribute to lymphopenia, then its magnitude would be definitely be acute and extreme as Tlymphocytes clears about 80% of virus infected host cells. Reportedly, the memory T cells last for up to 6 years in COVID-19 patients [67] .
Besides, lymphocytopenia, the critical COVID-19 patients also show Th cells imbalance, which clearly show favouring a Th2 response. Th cells differentiate and polarise into more Th2 subset than Th1, as serum display increase in levels of Th2 cytokines (IL-4, IL-5 and IL-13) except IL-10 [68] . It is noteworthy as Th2 subset mediated response is pro-inflammatory, while Th1 response is rather crucial in eliminating virus and anti-inflammatory in nature. Allegedly, IL-6 induction of JAK-STAT signalling inhibits Th1 polarization and promotes Th2 formation (2), [63] . Interestingly, elevated levels of cytokines including IL-6 gained too much highlight in serious cases and weather cytokines could become biomarker for assessing criticalness is a matter deserving consideration. The increase in IL-10 levels correlates positively with IL-6 levels in ICU patients is actually a Th1 cytokine. Its rise could be substantiated as being antiinflammatory, IL-10 could be produced as a self-protective cytokine, attempting inhibition of multiple inflammatory molecules including IL-4, IL-5, and IL-13 [69] .
TNFα is also implicated in the intensive care patients and is secreted from NK, macrophages and CD8+ and CD4+ cells. It is a tumor-suppressive cytokine (involved in apoptosis) and acts via two TNF receptors: TNFR1, (Tumor necrosis factor receptor 1) present on all cells; and TNFR2, restricted to immune cells predominantly on regulatory T cells (Tregs) and can directly impact T cell biology [70] . TNFR2 actually lacks a death domain (DD) and once activated, recruits TRAF2 (TNFR-associated factor 2) and cIAP1-cIAP2 (cellular inhibitors of apoptosis proteins). This TRAF2-cIAP1-cIAP2 complex (inhibit caspases and other apoptosis-inducing factors) rather promotes cell proliferation and survival through PI3K/AKT signalling pathway [68] . Thus, it promotes survival of viral infested Tc cells as well. Further, this signalling activate NF-B and STAT5 to secrete IL-2 and TGF-/IL-10 respectively [68] . Furthermore, TNFα can also modulate T-cell dynamics by protecting immunosuppressive Tregs cells via similar mechanism.
Another mechanism contributes to TNF production. When virus enters through ACE2, it reduces the ACE2 levels on the target cells. Reduced ACE2 levels, increases the serum AngII levels and deregulates the rennin-angiotensin pathway. AngII is a vasoconstrictor and a pro-inflammatory cytokine (through AT1R). Its binding to AT1R, activates the NF-B and metalloprotease 17 (ADAM17), which generates mature form of TNF along with other ligands for EGFR (Epidermal Growth Factor Receptor). ADAM17 process mIL-6R (membrane form) to sIL-6R (soluble form) and with gp-130, it activates STAT3 and NF-B pathway (discussed later). Now both NF-B and STAT3 trigger IL-6 amplifier, which ultimately causes hyper-activation of NF-B and STAT3. This loop secretes IL-6 along with other pro-inflammatory cytokines, and increases COVID-19 mortality [7] , [71] . IL-6 implicated as a chief cytokine performer in SARS-CoV-2 infection and CRS, thus requires a deeper understanding.
IL-6 acts as both a pro-and anti-inflammatory molecule besides being embroiled in several viral infections [72] . In SARS-CoV infection, viral N protein triggers expression of IL-6 via attaching on NF-B controlling element on IL-6 promoter and deletion of Cterminal (in N protein) actually results in failure of IL-6 activation [14] . The infection of SARS-CoV virus lacking envelope (E) reduces IL-6 levels and increases host survival in animal studies [73] . Various cytokines like IL-1 and TNF also activate IL-6 expression [14] , [74] , [75] .
The IL-6 signalling is well studied and is pleiotropic (multi-effective) and produced from many cells including B and T lymphocytes, as well non-lymphocytic cells: macrophages, DC, monocytes, fibroblasts, vascular endothelial and mast cells etc [14] , [74] . Besides viral infection and immune response, it also plays a part in body temperature control, bone protection, and brain operation. It binds to its receptor IL-6R to activate STAT3 signalling pathway. IL-6R exists in two forms: one is membrane bound and the other one is soluble form (sIL-6R) [1] , [76] . Mere binding of IL-6 on IL-6R cannot initiate down-stream signalling as also requires a signal transducer glycoprotein 130 (gp-130) ( Fig. 5 (A) ) [76] . gp-130 is expressed on most of the cells whereas IL-6R is limited to hepatocytes, some leukocyte and epithelial cells only [72] . IL-6 mediates three pathways of signal transduction-(A) Classical (Cis) signalling pathway: IL-6 molecule binds to IL-6R, forming a complex, which further binds to gp-130 and initiate signalling.
Instigation of cis-signalling results in manifold effects on the acquired immune system (B and T lymphocytes) together with the innate immune response (including macrophages, neutrophils and NK cells), which contributes towards CRS. (B) Trans signalling pathway: Elevated IL-6 binds to circulating soluble IL-6R (sIL-6R) and composite IL-6/sIL-6R interacts with gp-130 (Fig. 5 (B) ) [1] , [72] . Thus, IL-6/sIL-6R mediated JAK-STAT3 signalling is stimulated in cells, which do not even express mIL-6R (like endothelial cells) and initiate CRS. This storm brings along release of VEGF (vascular endothelial growth factor), MCP-1, IL-8 and IL-6. VEGF causes increased vascular permeability and leakage, resulting in hypotension and ARDS. Besides this (C) Trans presentation also involves IL-6 uniting with mIL-6R present on immune cells, which forms a multiplex with gp-130 on T helper 17 (Th17) cells directing towards lowering T-cell signalling and ARDS (Fig. 5 (C) ) [74] . In addition, a natural soluble form of gp-130 (sgp-130) is found in the blood at concentrations close to 400 ng/ml [72] . receptors, connector proteins and protein kinases etc [1] , [19] , [74] .
In the initial stage of inflammation, IL-6 is expressed in a restricted region and later it progresses to the liver through the circulatory system, resulting in fast-moving production of a wide variety of acute phase proteins, like C-reactive protein (CRP), serum amyloid A (SAA) and fibrinogen etc. As earlier mentioned, IL-6 promotes differentiation of B-cells and production of antibodies. IL-6 also plays a major role in Th17 cells production and Treg. IL-6 along with IL-23, TNFα, and a few other cytokines ultimately lead to activation and differentiation of naive Th cells to Th17 [1] , [71] . grave concern is antibody-dependent enhancement (ADE), which involves binding of non-counteracting antibodies to freshly infecting virus, and rather encourage viral entry via FcR into host cells [77] .
Theoretically, different antiviral like Remdesivir and Lopinavir plus Ritonavir can be efficient in fighting the existing outbreak. However, given the complication of CRS, it would be inappropriate to annul the virus single-handedly with the antiviral without targeting CRS in therapy. The unexpected results due to use of immune-suppressants like corticosteroids remains quite contentious as they don't improve mortality and further delay viral clearance [11] . Lately there have been also apprehensions that steroids may intensify viral shedding and perhaps spearhead towards a greater mortality rate.
It is now clear that early detection and suitable management of this hyper-inflammation is vital in slashing the death of affected people with COVID-19. Redeployment of approved drugs is also popular as these medicines have established safety reports. signalling, whereas IL-6R antagonist can suppress trans presentation along with cis and trans signalling [11] . The antibody-based treatment is high-priced and has a limited range of outcomes. Hydroxychloroquine (or chloroquine), Baricitinib (JAK inhibitor) and Anakinra (IL-1R opponent ) were suggested as possible remedies for COVID-19 as well [13] .
Plasma therapy has also shown positive results in patients. It involves transferring plasma (having antibodies against virus) from recuperated patients of SARS-CoV-2 infection and may shrink death rate. Considering this, many researchers started pursuing the S protein as vaccine targets as well, including Sputnik-V. Although, reports suggest that humoral immunity may not be life lasting as rapid decay of IgG antibodies against S protein RBD was observed with half-life (t 1/2 ) close to 36 days [62] .
The other treatment option is around the understanding the delay in IFN production, culminating in release of pro-inflammatory cytokines. In China, besides safety precautions, health workers advised to inhale IFNs and this pre-treatment of cells with IFN counteracts development of SARS-CoV-2. It suggests that exogenic IFN may be adequate to overpower virus-mediated deferral of the IFN reaction. This is an indication that early diagnosis and management could be the best option in management of infection. 