Influenza A Virus NS1 Inhibits RIPLET Activation of Duck RIG-I Signaling.

Retinoic acid-inducible gene I (RIG-I) is a crucial pattern recognition receptor for detecting viral RNA and initiating an immune response against influenza A viruses (IAVs). The activation of RIG-I in mammalian cells requires ubiquitination by two E3 ubiquitin ligases: TRIM25 and RIPLET. Using dual luciferase assays, we demonstrate that duck RIPLET enhances the activation of RIG-I, driving the IFN-β promoter activity in chicken DF-1 fibroblasts. qPCR analyses show that the co-expression of duck RIG-I and RIPLET significantly upregulates key immune genes and reduces viral RNA transcripts in DF-1 cells challenged with low pathogenic avian influenza (LPAI) H6N2. Co-immunoprecipitation and confocal microscopy studies suggest the interaction and confirm the colocalization of duck RIG-I and RIPLET in the cytoplasm. Further, we show that the non-structural protein 1 (NS1) of IAV, known for its role in immune evasion, suppression, and pathogenicity, from five different strains of IAV (PR8, BC500, CA431, D4AT, and VN1203) can all inhibit duck RIPLET activation of RIG-I, with NS1 from avian strains showing the greatest decrease in IFN-β promoter activity in chicken DF-1 cells. Overall, our research provides valuable insight into the E3 ubiquitin ligases required for RIG-I activation and susceptibility of this pathway to IAV interference across species.