SARS-CoV-2 nsp12 attenuates type Ⅰ interferon production by inhibiting IRF3 nuclear translocation

SARS-CoV-2 is the pathogenic agent of COVID-19,which has evolved into a global pandemic.Compared with some other respiratory RNA viruses,SARS-CoV-2 is a poor inducer of type Ⅰ interferon(IFN).Here,we report that SARS-CoV-2 nsp12,the viral RNA-dependent RNA polymerase(RdRp),suppresses host antiviral responses.SARS-CoV-2 nsp12 attenuated Sendai virus(SeV)-or poly(I:C)-induced IFN-β promoter activation in a dose-dependent manner.It also inhibited IFN promoter activation triggered by RIG-I,MDA5,MAVS,and IRF3 overexpression.Nsp12 did not impair IRF3 phosphorylation but suppressed the nuclear translocation of IRF3.Mutational analyses suggested that this suppression was not dependent on the polymerase activity of nsp12.Given these findings,our study reveals that SARS-CoV-2 RdRp can antagonize host antiviral innate immunity and thus provides insights into viral pathogenesis.

Sensing of cytoplasmic chromatin by cGAS activates innate immune response in SARS-CoV-2 infection

The global coronavirus disease 2019(COVID-19)pandemic is caused by severe acute respiratory syndrome coronavirus 2(SARSCoV-2),a positive-sense RNA virus.How the host immune system senses and responds to SARS-CoV-2 infection remain largely unresolved.Here,we report that SARS-CoV-2 infection activates the innate immune response through the cytosolic DNA sensing cGAS-STING pathway.SARS-CoV-2 infection induces the cellular level of 2′3′-cGAMP associated with STING activation.cGAS recognizes chromatin DNA shuttled from the nucleus as a result of cell-to-cell fusion upon SARS-CoV-2 infection.We further demonstrate that the expression of spike protein from SARS-CoV-2 and ACE2 from host cells is sufficient to trigger cytoplasmic chromatin upon cell fusion.Furthermore,cytoplasmic chromatin-cGAS-STING pathway,but not MAVS-mediated viral RNA sensing pathway,contributes to interferon and pro-inflammatory gene expression upon cell fusion.Finally,we show that cGAS is required for host antiviral responses against SARS-CoV-2,and a STING-activating compound potently inhibits viral replication.Together,our study reported a previously unappreciated mechanism by which the host innate immune system responds to SARS-CoV-2 infection,mediated by cytoplasmic chromatin from the infected cells.Targeting the cytoplasmic chromatin-cGAS-STING pathway may offer novel therapeutic opportunities in treating COVID-19.In addition,these findings extend our knowledge in host defense against viral infection by showing that host cells’self-nucleic acids can be employed as a“danger signal”to alarm the immune system.