The ongoing outbreak of a new coronavirus (2019-nCoV, or severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) has caused an epidemic of the acute respiratory syndrome known as coronavirus disease (COVID-19) in humans. SARS-CoV-2 rapidly spread to multiple regions of China and multiple other countries, posing a serious threat to public health. The spike (S) proteins of SARS-CoV-1 and SARS-CoV-2 may use the same host cellular receptor, angiotensin-converting enzyme 2 (ACE2), for entering host cells. The affinity between ACE2 and the SARS-CoV-2 S protein is much higher than that of ACE2 binding to the SARS-CoV S protein, explaining why SARS-CoV-2 seems to be more readily transmitted from human to human. Here, we report that ACE2 can be significantly upregulated after infection of various viruses, including SARS-CoV-1 and SARS-CoV-2, or by the stimulation with inflammatory cytokines such as interferons. We propose that SARS-CoV-2 may positively induce its cellular entry receptor, ACE2, to accelerate its replication and spread; high inflammatory cytokine levels increase ACE2 expression and act as high-risk factors for developing COVID-19, and the infection of other viruses may increase the risk of SARS-CoV-2 infection. Therefore, drugs targeting ACE2 may be developed for the future emerging infectious diseases caused by this cluster of coronaviruses.

As a highly pathogenic human coronavirus, SARS-CoV-2 has to counteract an intricate network of antiviral host responses to establish infection and spread. The nucleic acid-induced stress response is an essential component of antiviral defense and is closely related to antiviral innate immunity. However, whether SARS-CoV-2 regulates the stress response pathway to achieve immune evasion remains elusive. In this study, SARS-CoV-2 NSP5 and N protein were found to attenuate antiviral stress granule (avSG) formation. Moreover, NSP5 and N suppressed IFN expression induced by infection of Sendai virus or transfection of a synthetic mimic of dsRNA, poly (I:C), inhibiting TBK1 and IRF3 phosphorylation, and restraining the nuclear translocalization of IRF3. Furthermore, HEK293T cells with ectopic expression of NSP5 or N protein were less resistant to vesicular stomatitis virus infection. Mechanistically, NSP5 suppressed avSG formation and disrupted RIG-I–MAVS complex to attenuate the RIG-I–mediated antiviral immunity. In contrast to the multiple targets of NSP5, the N protein specifically targeted cofactors upstream of RIG-I. The N protein interacted with G3BP1 to prevent avSG formation and to keep the cofactors G3BP1 and PACT from activating RIG-I. Additionally, the N protein also affected the recognition of dsRNA by RIG-I. This study revealed the intimate correlation between SARS-CoV-2, the stress response, and innate antiviral immunity, shedding light on the pathogenic mechanism of COVID-19.

The coronavirus disease 2019 (COVID-19) caused by Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has quickly spread worldwide and has infected more than ten million individuals. One of the typical features of COVID-19 is that both type I and III interferon (IFN)-mediated antiviral immunity are suppressed. However, the molecular mechanism by which SARS-CoV-2 evades this antiviral immunity remains elusive. Here, we report that the SARS-CoV-2 membrane (M) protein inhibits the production of type I and III IFNs induced by the cytosolic dsRNA-sensing pathway of RIG-I/MDA-5-MAVS signaling. The SARS-CoV2 M protein also dampens type I and III IFN induction stimulated by Sendai virus infection or poly (I:C) transfection. Mechanistically, the SARS-CoV-2 M protein interacts with RIG-I, MAVS, and TBK1 and prevents the formation of a multi-protein complex containing RIG-I, MAVS, TRAF3, and TBK1, thus impeding IRF3 phosphorylation, nuclear translocation, and activation. Consequently, the ectopic expression of the SARS-CoV2 M protein facilitates the replication of vesicular stomatitis virus (VSV). Taken together, the SARS-CoV-2 M protein antagonizes type I and III IFN production by targeting RIG-I/MDA-5 signaling, which subsequently attenuates antiviral immunity and enhances viral replication. This study provides insight into the interpretation of the SARS-CoV-2-induced antiviral immune suppression and sheds light on the pathogenic mechanism of COVID-19.

Coronavirus possesses the largest RNA genome among all the RNA viruses. Its genome encodes about 29 proteins. Most of the viral proteins are non-structural proteins (NSP) except envelop (E), membrane (M), nucleocapsid (N) and Spike (S) proteins that constitute the viral nucleocapsid, envelop and surface. We have recently cloned all the 29 SARS-CoV-2 genes into vectors for their expressions in mammalian cells except NSP11 that has only 14 amino acids (aa). We are able to express all the 28 cloned SARS-CoV-2 genes in human cells to characterize their subcellular distributions. The proteins of SARS-CoV-2 are mostly cytoplasmic but some are both cytoplasmic and nuclear. Those punctate staining proteins were further investigated by immunofluorescent assay (IFA) using specific antibodies or by co-transfection with an organelle marker-expressing plasmid. As a result, we found that NSP15, ORF6, M and ORF7a are related to Golgi apparatus, and that ORF7b, ORF8 and ORF10 colocalize with endoplasmic reticulum (ER). Interestingly, ORF3a distributes in cell membrane, early endosome, endosome, late endosome and lysosome, which suggests that ORF3a might help the infected virus to usurp endosome and lysosome for viral use. Furthermore, we revealed that NSP13 colocalized with SC35, a protein standing for splicing compartments in the nucleus. Our studies for the first time visualized the subcellular locations of SARS-CoV-2 proteins and might provide novel insights into the viral proteins' biological functions.

A characteristic feature of COVID‐19, the disease caused by severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) infection, is the dysregulated immune response with impaired type I and III interferon (IFN) expression and an overwhelming inflammatory cytokine storm. RIG‐I‐like receptors (RLRs) and cGAS–STING signaling pathways are responsible for sensing viral infection and inducing IFN production to combat invading viruses. Multiple proteins of SARS‐CoV‐2 have been reported to modulate the RLR signaling pathways to achieve immune evasion. Although SARS‐CoV‐2 infection also activates the cGAS–STING signaling by stimulating micronuclei formation during the process of syncytia, whether SARS‐CoV‐2 modulates the cGAS–STING pathway requires further investigation. Here, we screened 29 SARS‐CoV‐2‐encoded viral proteins to explore the viral proteins that affect the cGAS–STING signaling pathway and found that SARS‐CoV‐2 open reading frame 10 (ORF10) targets STING to antagonize IFN activation. Overexpression of ORF10 inhibits cGAS–STING‐induced interferon regulatory factor 3 phosphorylation, translocation, and subsequent IFN induction. Mechanistically, ORF10 interacts with STING, attenuates the STING–TBK1 association, and impairs STING oligomerization and aggregation and STING‐mediated autophagy; ORF10 also prevents the endoplasmic reticulum (ER)‐to‐Golgi trafficking of STING by anchoring STING in the ER. Taken together, these findings suggest that SARS‐CoV‐2 ORF10 impairs the cGAS–STING signaling by blocking the translocation of STING and the interaction between STING and TBK1 to antagonize innate antiviral immunity.

Clinical benefit from immunotherapy of B7-H1/PD-1 checkpoint blockade indicates that it is important to understand the regulatory mechanism of B7-H1 expression in cancer cells. As an adaptive response to the endogenous antitumor immunity, B7-H1 expression is up-regulated in HCC cells. B7-H1 expression is induced mainly by IFN-γ released from tumor-infiltrating T cells in HCC. In addition, HCC is a prototype of inflammation-related cancer and TNF-α is a critical component of inflammatory microenvironment of HCC. In the present study, we asked whether TNF-α can promote the expression of B7-H1 induced by IFN-γ in HCC cells. We found that JAK/STAT1/IRF1 was the primary pathway responsible for induction of B7-H1 expression by IFN-γ in human HCC cell lines. TNF-α and IFN-γ synergistically induced the expression of B7-H1 in the HCC cells. Moreover, the mechanism of the synergy was that TNF-α enhanced IFN-γ signaling by upregulating the expression of IFN-γ receptors. Furthermore, B7-H1 expression induced synergistically by TNF-α and IFN-γ in murine HCC cells facilitated tumor growth in vivo. Our findings suggest that TNF-α may enhance the adaptive immune resistance mediated by IFN-γ-induced B7-H1 in HCC cells.

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