Infectious salmon anemia virus segment 7 ORF1 and segment 8 ORF2 proteins inhibit IRF mediated activation of the Atlantic salmon IFNa1 promoter

•ISAV proteins s7ORF1 inhibited IRF3 and IRF7B induced IFNa1 promoter activity.•ISAV protein s8ORF2 inhibited IRF1 and IRF3 induced IFNa1 promoter activity.•Immunoprecipitation data suggest that s7ORF1 and s8ORF2 can bind to IRF1, IRF3, IRF7A or IRF7B.•s7ORF1 and s8ORF2 did not inhibit activation of the IFNa1 promoter by the IFN promoter stimulating protein IPS-1.

Infectious salmon anemia virus (ISAV) is an orthomyxovirus, which may cause multisystemic disease and high mortality of Atlantic salmon (Salmo salar L). This suggests that ISAV encodes proteins that antagonize the type I interferon (IFN-I) system, which is of crucial importance in innate antiviral immunity. To find out how ISAV might inhibit IFN-I synthesis, we have here studied whether the two ISAV proteins s7ORF1 and s8ORF2 might interfere with activation of the IFNa1 promoter mediated by overexpression of interferon regulatory factors (IRFs) or by the IFN promoter activation protein IPS-1. The IRF tested were IRF1, IRF3, IRF7A and IRF7B. Promoter activation was measured using a luciferase reporter assay where Atlantic salmon TO cells were co-transfected with the IFNa1 promoter reporter plasmid together with an IRF plasmid and the s7ORF1 or the s8ORF2 construct or a control plasmid. The results showed that s7ORF1 significantly inhibited IRF3 and IRF7B induced IFN promoter activity, while s8ORF2 significantly inhibited IRF1 and IRF3 induced promoter activity. Neither s7ORF1 nor s8ORF2 inhibited IPS-1 mediated promoter activation. Immunoprecipitation data suggest that both s7ORF1 and s8ORF2 can bind to all four IRFs. Taken together, this study thus shows that the ISAV proteins s7ORF1 and s8ORF2 antagonizes IFN-I transcription activation mediated by the IRFs. As such this work provides further insight into the pathogenic properties of ISAV.