Contribution of monocytes and macrophages to the local tissue inflammation and cytokine storm in COVID-19: Lessons from SARS and MERS, and potential therapeutic interventions

Abstract

The COVID-19-, SARS- and MERS-related coronaviruses share many genomic and structural similarities. However, the SARS-CoV-2 is less pathogenic than SARS-CoV and MERS-CoV. Despite some differences in the cytokine patterns, it seems that the cytokine storm plays a crucial role in the pathogenesis of COVID-19-, SARS- and MERS. Monocytes and macrophages may be infected by SARS-CoV-2 through ACE2-dependent and ACE2-independent pathways. SARS-CoV-2 can effectively suppress the anti-viral IFN response in monocytes and macrophages. Since macrophages and dendritic cells (DCs) act as antigen presenting cells (APCs), the infection of these cells by SARS-CoV-2 impairs the adaptive immune responses against the virus. Upon infection, monocytes migrate to the tissues where they become infected resident macrophages, allowing viruses to spread through all organs and tissues. The SARS-CoV-2-infected monocytes and macrophages can produce large amounts of numerous types of pro-inflammatory cytokines and chemokines, which contribute to local tissue inflammation and a dangerous systemic inflammatory response called cytokine storm. Both local tissue inflammation and the cytokine storm play a fundamental role in the development of COVID-19-related complications, such as acute respiratory distress syndrome (ARDS), which is a main cause of death in COVID-19 patients. Here, we describe the monocytes and macrophage responses during severe coronavirus infections, while highlighting potential therapeutic interventions to attenuate macrophage-related inflammatory reactions in possible approaches for COVID-19 treatment.

Keywords: COVID-19; Macrophages; Monocytes; Pathogenesis; SARS-CoV-2.

Graphical abstract

Fig. 1

Structure of SARS-CoV-2. SARS-Cov-2 belongs to the family of RNA viruses with other members including SARS-CoV and MERS-CoV. These viruses have characteristic crown-like-protrusions spike proteins (S) used to gain entry into the host cells and thereby inflict the respiratory disease COVID-19. ACE-2 is the host receptor mediating this process of internalization. Protease ACE-2 mediates the cleavage of spike protein which then releases an epitope that allows the subsequent fusion of the virus with the host cells. Inside the host, SARS-CoV-2 thrives in epithelial cells of lungs, kidneys, small intestines, and endothelial cells within arteries and veins. The viral genome is composed of a positive sense (+) RNA (~30 kb). The coronavirus group-2 also has hemagglutinin–acetylesterase (HE) glycoprotein that has an affinity to bind with sugar moieties on the cell membranes. The RNA-dependent-RNA polymerase can switch templates during the replication, a highly error-prone process. The virus envelope protein plays a central role in virus morphogenesis and assembly and its interaction with other viral proteins. The nucleocapsid (N), Spike (S), Envelope (E), and Membrane (M) structural proteins embedded into a lipid bilayer are the characteristic hallmarks of the SARS-CoV-2. For the virus to replicate into the host cells in inserts its RNA into the cells like monocytes and macrophages and takeovers the cellular machinery to produce new virions.

Fig. 2

SARS-CoV-2 intracellular signaling and potential targets for immunoevasion and host cells particularly macrophages elicit a variety of antiviral immunity genes shown above. The induction of TLR2 (via S protein), TLR3, TLR7, TLR8 (via virus-derived RNA following internalization process) leads to the expression of the pro-inflammatory mediators and IFNs through induction of the transcription factors NF-κB, IRF3, and IRF7. Viral-derived RNAs activate PKR- and OAS-related anti-viral pathways. The cytoplasmic sensors such as MAD-5 and RIG-1 also recognize various types of virus-derived RNAs and signal through adaptor MAVS/ISP-1 (located on our membrane of mitochondria). SARS-CoV-2 affects the cells of the innate immune system in particular monocytes, macrophages, and dendritic cells. These cells of the innate system play a crucial role in curbing the viral replication through the induction of Type-I IFNs assisted with the complement proteins and natural immunoglobulins against viral epitopes. These cellular responses (induction of proinflammatory mediators and IFNs Type-I, III) are tightly regulated by a series of intracellular signaling pathways elicited by surface receptors like TLRs, DC-SIGN, FcRs, and ACE-2 and TMPRSS2. Upon the viral entry into the host cells (via the clathrin-dependent internalization or ACE-2 mediated internalization) the spontaneous unloading of viral RNA is a subsequent step that follows. The viral RNA triggers the activation of intracellular RNA sensors like RIG-1 and MDA-5 each operating with distinct RNA conformations. RNA sensors then interact with MAVS that initiates Type-1 IFN signaling by activating the nuclear translocation of NF-κB and IRF3. The oligomeric RIG-1-CARD assembly and the polymeric formation of MAVS act as a signalosome for conducting the viral sensing signals further, which bifurcates into the activation of TRAF-2/6 to activate IKK complex and NF-κB activation. The other branch signals through TRAF-3 and activates the TANK/IKKγ/IKKε/TBK1 complex that acts as activators of IRF-3/7. Altogether the IRF-3/7 activation along with NF-κB drives the IFN and proinflammatory gene expression with the help of CREB-binding protein/p300 and transcription factors c-Jun and ATF-2. The IFN synthesized and secreted this way acts on the distant cells (paracrine) mode for spreading anti-viral immunity and in autocrine modes to fortify the intracellular viral clearance. SARS-CoV-2, however, attenuates these signaling pathways at various interception nodes. SARS-CoV proteins like ORF-9b may attenuate this antiviral response through targeting of MAVS by seizing poly (C)-binding protein 2 (PCBP2) and the HECT domain E3 ligase AIP4 to trigger the degradation of MAVS (not shown) along with TRAF-3 and TRAF-6. While ORF6 is reported to antagonize the STAT-1 function by sequestering its nuclear import factors. SARS-ORF-3b, ORF-6, and nucleocapsid protein function to antagonize interferon production. Besides this, the host mRNA destabilizing functions of NSP-1 are also reported. However, Nsp1 protein suppresses IFN-β mRNA accumulation without inhibiting IRF3 dimerization. Similarly, SARS-CoV NSP-15 inhibits MAVS-induced apoptosis sustaining the intracellular viral presence. SARS-CoV N protein can activate AP-1 but not the NF-κB signaling pathway. SARS-CoV proteins have been shown to inhibit the JAK-STAT pathway in the infected cell that responds to the Type-I IFNs secreted from bystander/neighboring cells. STAT-1-STAT-2-heterodimers combines with the IRF-9 to form the ISGF3 complex. This complex is crucial for the activation of genes harboring ISRE in their promoter regions. Viral protein ORF-6 blocks the nuclear import of ISGF3 by reducing the available import factor KPNB1 (Kβ1). Accordingly, various types of the IFNs are secreted from viral-infected cells that may induce various anti-viral restriction factors (such as OAS, PKR, viperin tetherin, IFITM, RNase L, GTPase, TRIM, ADAR1, APOBEC, and others) following binding to their receptors. Coronavirus-derived nonstructural protein also contributes to abrogate IFN expression or IFN-related signaling pathways. SARS-CoV-derived N protein can inactivate anti-virus restriction factor TRIM25 (RING-finger E3 ubiquitin ligase that controls RIG-I ubiquitination and IFN-β production). MAVS/ISP-1-related pathways may cause apoptosis through the induction of the caspases-mitochondria-mediated pathway.

Fig. 3

A. The steps in the life-cycle of SARS-CoV-2 are summarized along with the potential mechanism of entry into the host cells. Possible interventions using drugs and their targets during the SARS-CoV-2 life cycle are depicted. Specific signaling proteins that are targets of inhibition to suppress the host immunity are also depicted. The SARS-CoV-2 can infect its target cells through ACE2-dependent and ACE2-independent (including virus binding to cell surface molecules CD147, DC-SIGN, and L-SIGN; endocytosis of virions or virus-containing apoptotic bodies, and attachment of the virus-coated IgG to FcR). The SARS-CoV-2-related RNA molecules are recognized by intra-cellular PRRs including endosomal TLR3, TLR7 and TLR8; and cytoplasmic sensors including MDA5 and RIG-I. These PRR-mediated signaling pathways eventually may lead to the expression of Type I- and Type III interferons. However, coronaviruses interfere with IFN production through inactivating of the IRF-3. The binding of the SARS-CoV-2-related S protein to the surface TLR2 and the attachment of the virus-coated IgG to FcγRIIA lead to the expression of the pro-inflammatory cytokines and chemokines via induction of the NF-κB and MAPK-related pathways, respectively. The RNA transcription, translation, viral protein synthesis, viral assembling, viral budding to ER, viral transportation into Golgi vesicles, and exocytosis of infective virions are key steps in the cycle life of coronaviruses, that may be targeted by therapeutic agents. Key to the life cycle of SARS-CoV-2 inside the host cells (monocytes and macrophages along with other cell types): 1.a. Virus entry via ACE-2 mediated endocytosis; 1.b. virus entry through membrane fusion (following binding with ACE-2 and TMPRSS2); 2. release of the viral genome; 3. translation of viral polymerase protein; 4. RNA replication; 5. translation of viral structural proteins (S, M, E) via ER bound ribosomes and nucleocapsid (N) in the cytoplasm; 6. virion assembly at ERGIC; 7. formation of mature virion inside Golgi vesicle; 8. release of infective virions via exocytosis. The drugs (experimental/repurposed) that are currently prescribed the management of COVID-19 are mentioned above in green boxes. Each of these drugs/antibodies have specific intervention points where they either inhibit- the viral proteins or crucial process like viral entry, translation of viral proteins, assembly of new virions and viral budding, etc. thereby suppressing the multiplication of SARS-CoV-2. An extended list of such drugs is presented in Table 1. B. Involvement of monocytes/macrophages in the pathogenesis of SARS-CoV-2 the elicitation of mediators of the Cytokine storm is shown. The mild COVID-19 and moderate COVID-19 were associated with the effective expression of type I IFNs and ISGs in the lungs. Thus, an appropriate local IFN response in the respiratory system can control SARS-CoV-2 infection accompanied by mild and moderate forms of the disease. However, a lower proportion of the SARS-CoV-2-infected patients exhibit severe symptoms. It was proposed, when the viral load is high and the primary local IFN response is failed, the SARS-CoV-2 enters the blood from the lungs and attacks organs expressing high levels of ACE2. The SARS-CoV-2-infected monocytes and macrophages can produce large amounts of numerous types of pro-inflammatory cytokines and chemokines which contribute to the local tissue inflammation and cytokine storm. Both local tissue inflammation and cytokine storm play a key role in the development of COVID-19-related multi-organ failure which causes death in some COVID-19 patients.

Fig. 3

A. The steps in the life-cycle of SARS-CoV-2 are summarized along with the potential mechanism of entry into the host cells. Possible interventions using drugs and their targets during the SARS-CoV-2 life cycle are depicted. Specific signaling proteins that are targets of inhibition to suppress the host immunity are also depicted. The SARS-CoV-2 can infect its target cells through ACE2-dependent and ACE2-independent (including virus binding to cell surface molecules CD147, DC-SIGN, and L-SIGN; endocytosis of virions or virus-containing apoptotic bodies, and attachment of the virus-coated IgG to FcR). The SARS-CoV-2-related RNA molecules are recognized by intra-cellular PRRs including endosomal TLR3, TLR7 and TLR8; and cytoplasmic sensors including MDA5 and RIG-I. These PRR-mediated signaling pathways eventually may lead to the expression of Type I- and Type III interferons. However, coronaviruses interfere with IFN production through inactivating of the IRF-3. The binding of the SARS-CoV-2-related S protein to the surface TLR2 and the attachment of the virus-coated IgG to FcγRIIA lead to the expression of the pro-inflammatory cytokines and chemokines via induction of the NF-κB and MAPK-related pathways, respectively. The RNA transcription, translation, viral protein synthesis, viral assembling, viral budding to ER, viral transportation into Golgi vesicles, and exocytosis of infective virions are key steps in the cycle life of coronaviruses, that may be targeted by therapeutic agents. Key to the life cycle of SARS-CoV-2 inside the host cells (monocytes and macrophages along with other cell types): 1.a. Virus entry via ACE-2 mediated endocytosis; 1.b. virus entry through membrane fusion (following binding with ACE-2 and TMPRSS2); 2. release of the viral genome; 3. translation of viral polymerase protein; 4. RNA replication; 5. translation of viral structural proteins (S, M, E) via ER bound ribosomes and nucleocapsid (N) in the cytoplasm; 6. virion assembly at ERGIC; 7. formation of mature virion inside Golgi vesicle; 8. release of infective virions via exocytosis. The drugs (experimental/repurposed) that are currently prescribed the management of COVID-19 are mentioned above in green boxes. Each of these drugs/antibodies have specific intervention points where they either inhibit- the viral proteins or crucial process like viral entry, translation of viral proteins, assembly of new virions and viral budding, etc. thereby suppressing the multiplication of SARS-CoV-2. An extended list of such drugs is presented in Table 1. B. Involvement of monocytes/macrophages in the pathogenesis of SARS-CoV-2 the elicitation of mediators of the Cytokine storm is shown. The mild COVID-19 and moderate COVID-19 were associated with the effective expression of type I IFNs and ISGs in the lungs. Thus, an appropriate local IFN response in the respiratory system can control SARS-CoV-2 infection accompanied by mild and moderate forms of the disease. However, a lower proportion of the SARS-CoV-2-infected patients exhibit severe symptoms. It was proposed, when the viral load is high and the primary local IFN response is failed, the SARS-CoV-2 enters the blood from the lungs and attacks organs expressing high levels of ACE2. The SARS-CoV-2-infected monocytes and macrophages can produce large amounts of numerous types of pro-inflammatory cytokines and chemokines which contribute to the local tissue inflammation and cytokine storm. Both local tissue inflammation and cytokine storm play a key role in the development of COVID-19-related multi-organ failure which causes death in some COVID-19 patients.