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.