ID: 67: Influenza A virus NS1 inhibits interferon-mediated signaling independently of its block of general host gene expression

Abstract

Type I interferons (IFNs) act as the first line of defense against viral infections. Upon secretion and receptor binding, they initiate a signaling cascade that eventually leads to the establishment of an antiviral state owing to the production of IFN-induced effector proteins. The type I IFN signaling cascade involves phosphorylation of signal transducer and activator of transcription 1 (STAT1) and STAT2. Phosphorylated STATs heterodimerize and translocate to the nucleus, where they activate transcription of IFN-responsive genes. Previous studies have shown that influenza A virus (IAV) infection causes disruption of IFN-mediated signaling. Using a luciferase-based reporter assay, we found that overexpression of avian IAV non-structural protein 1 (NS1) causes a dramatic reduction of IFN-induced gene expression. However, STAT1 phosphorylation was not affected. Instead, immunofluorescence data demonstrated that NS1 interferes with IFN-induced nuclear translocation of STAT proteins. We then assessed whether NS1 proteins from multiple IAV strains differ in their ability to suppress the signaling events in response to IFN. Intriguingly, most human and avian NS1 proteins which failed to inhibit general host gene expression remained effective IFN signaling antagonists. In addition, mutation of NS1 residues essential for interaction with CPSF30 and subsequent block in host mRNA maturation restored general gene expression, while still interfering with IFN-mediated signaling. Conversely, disruption of a conserved putative protein–protein interaction motif partially restored IFN signaling reporter activity. Taken together, we show that IAVs have evolved multiple strategies to inhibit IFN-mediated signaling, relying on both general and specific suppression of host gene expression.