SARS-CoV-2 ORF9b Antagonizes Type I and III Interferons by Targeting Multiple Components of RIG-I/MDA-5-MAVS, TLR3-TRIF, and cGAS-STING Signaling Pathways

Abstract Severe acute respiratory syndrome corona-virus 2 (SARS-CoV-2), the etiologic agent of the coronavirus disease 2019 (COVID-19), has a catastrophic effect on human health and society. Clinical findings indicated that the suppression of innate antiviral immunity, especially the type I and III interferon (IFN) production, contributes to the pathogenesis of COVID-19. However, how SARS-CoV-2 evades antiviral immunity still needs further investigations. Here, we reported that the open reading frame 9b (ORF9b) protein encoded by the SARS-CoV-2 genome inhibits the activation of type I and III IFN response by targeting multiple molecules of innate antiviral signaling pathways. SARS-CoV-2 ORF9b impaired the induction of type I and III IFNs by Sendai virus or the dsRNA mimic poly (I:C). SARS-CoV-2 ORF9b inhibits the activation of type I and III IFNs induced by the components of cytosolic dsRNA-sensing pathways of RIG-I/MDA5-MAVS signaling, including RIG-I, MDA-5, MAVS, TBK1, and IKKε rather than IRF3-5D, the active form of IRF3. SARS-CoV-2 ORF9b also suppressed the induction of type I and III IFNs by TRIF and STING, the adaptor protein of endosome RNA-sensing pathway of TLR3-TRIF signaling and the adaptor protein of cytosolic DNA-sensing pathway of cGAS-STING signaling, respectively. Mechanistically, SARS-CoV-2 ORF9b protein interacts with RIG-I, MDA-5, MAVS, TRIF, STING, TBK1, and prevents TBK1 phosphorylation, thus impeding the phosphorylation and nuclear trans-localization of IRF3 activation. Overexpression of SARS-CoV-2 ORF9b facilitates the replication of the vesicular stomatitis virus. Therefore, SARS-CoV-2 ORF9b negatively regulates antiviral immunity, thus, facilitate virus replication. This study contributes to our understanding of the molecular mechanism of how SARS-CoV-2 impaired antiviral immunity and providing an essential clue to the pathogenesis of COVID-19.