Inhibition of Antiviral Innate Immunity by Birnavirus VP3 Protein via Blockage of Viral Double-Stranded RNA Binding to the Host

ChickenMDA5 (chMDA5), the sole known pattern recognition receptor for cytoplasmic viral RNA in chickens, initiates type I interferon (IFN) production. Infectious bursal disease virus (IBDV) evades host innate immunity, but the mechanism is unclear. We report here that IBDV inhibited antiviral innate immunity via the chMDA5-dependent signaling pathway. IBDV infection did not induce efficient type I interferon (IFN) production but antagonized the antiviral activity of beta interferon (IFN-) in DF-1 cells pretreated with IFN-/. Dual-luciferase assays and inducible expression systems demonstrated that IBDV protein VP3 significantly inhibited IFN- expression stimulated by naked IBDV genomic double-stranded RNA (dsRNA). The VP3 pro-tein competed strongly with chMDA5 to bind IBDV genomic dsRNA in vitro and in vivo, and VP3 from other birnaviruses also bound dsRNA. Site-directed mutagenesis confirmed that deletion of the VP3 dsRNA binding domain restored IFN- expression. Our data demonstrate that VP3 inhibits antiviral innate immunity by blocking binding of viral genomic dsRNA toMDA5. IMPORTANCE MDA5, a known pattern recognition receptor and cytoplasmic viral RNA sensor, plays a critical role in host antiviral innate im-munity. Many pathogens escape or inhibit the host antiviral immune response, but the mechanisms involved are unclear for most pathogens. We report here that birnaviruses inhibit host antiviral innate immunity via the MDA5-dependent signaling pathway. The antiviral innate immune system involving IFN- did not function effectively during birnavirus infection, and th