The NS1 protein of influenza B virus binds 5′-triphosphorylated dsRNA to suppress RIG-I activation and the antiviral innate immune response

Influenza A and B viruses overcome the host antiviral response to cause a contagious and often severe human respiratory disease. Here, integrative structural biology and biochemistry studies on non-structural protein 1 of influenza B virus (NS1B) reveal a previously unrecognized viral mechanism for innate immune evasion. Conserved basic groups of its C-terminal domain (NS1B-CTD) bind 5′-triphosphorylated double-stranded RNA (5′ppp-dsRNA), the primary pathogen-associated feature that activates the host retinoic acid-inducible gene I protein (RIG-I) to initiate interferon synthesis and the cellular antiviral response. Like RIG-I, NS1B-CTD preferentially binds blunt-end 5′ppp-dsRNA. NS1B-CTD also competes with RIG-I for binding 5′ppp-dsRNA, and thus suppresses activation of RIG-I′s ATPase activity. Although the NS1B N-terminal domain also binds dsRNA, it utilizes a different binding mode and lacks 5′ppp-dsRNA end preferences. In cells infected with wild-type influenza B virus, RIG-I activation is inhibited. In contrast, RIG-I activation and the resulting phosphorylation of transcription factor IRF-3 are not inhibited in cells infected with a mutant virus encoding NS1B with a R208A substitution it its CTD that eliminates its 5′ppp-dsRNA binding activity. These results reveal a novel mechanism in which NS1B binds 5′ppp-dsRNA to inhibit the RIG-I antiviral response during influenza B virus infection, and open the door to new avenues for antiviral drug discovery. ### Competing Interest Statement GTM is a founder and consultant of Nexomics Biosciences, Inc. This does not represent a conflict of interest for this study.