Subversion of innate antiviral immunity by SARS-CoV-2: An example from the study of nonstructural protein 16.

Understanding the molecular basis of innate immune evasion by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an important consideration for designing the next wave of therapeutics. Here, we investigate the role of the nonstructural protein (NSP) 16 of SARS-CoV-2 in infection and pathogenesis. NSP16, a ribonucleoside 2’-O methyltransferase (MTase), catalyzes the transfer of a methyl group to mRNA as part of the capping process. Based on observations with other CoVs, we hypothesized that NSP16 2’-O MTase function protects SARS-CoV-2 from cap-sensing host restriction. Therefore, we engineered SARS-CoV-2 with a mutation that disrupts a conserved residue in the active site of NSP16. We subsequently show that this mutant is attenuated both in vitro and in vivo, using a hamster model of SARS-CoV-2 infection. Mechanistically, we confirm that the NSP16 mutant is more sensitive to type I interferon (IFN-I) in vitro. Furthermore, silencing IFIT1 or IFIT3, IFN-stimulated genes that sense a lack of 2’-O methylation, partially restores fitness to the NSP16 mutant. Conversely, overexpressing IFIT1 either alone or in combination with IFIT3 attenuates the NSP16 mutant relative to wild-type. Finally, we demonstrate that sinefungin, a MTase inhibitor that binds the catalytic site of NSP16, sensitizes wild-type SARS-CoV-2 to IFN-I treatment and attenuates viral replication in IFN-I competent cells. Overall, our findings highlight the importance of SARS-CoV-2 NSP16 to evading host innate immunity and suggest a possible target for future antiviral therapies.