Regulation and function of interferon regulatory factors of Atlantic salmon

Transcription factors of the interferon regulatory factor (IRF) family are major regulators of the early immune responses against viral infections. In particular, IRF1, IRF2, IRF3 and IRF7 of mammals are known to regulate the expression of type I interferons (IFNs), which constitute the obligate cytokines for antiviral defense. We therefore cloned the coding sequence of Atlantic salmon (As) IRF1, IRF2, IRF3 and IRF7B. Expression profiles were studied in Atlantic salmon TO cells after poly I:C (dsRNA) transfection, treatment with recombinant salmon IFNa1 and infection with infectious salmon anemia virus (ISAV). The main findings were that AsIRF1 was earliest up-regulated by all stimuli, while AsIRF3 and AsIRF7 had a similar activation profile induced at a slightly later time point. The ability to induce the Atlantic salmon IFNa1 promoter was measured in a luciferase reporter assay. The results showed that AsIRF1, AsIRF3 and AsIRF7B were able to induce the promoter in a dose-dependent manner. AsIRF2 repressed the promoter, while AsIRF7A and a splicing variant (AsIRF3D) lacking the interaction domain had almost no effect. Combination of AsIRF1 and AsIRF3 had a synergistic stimulatory effect on the promoter compared to each of the two IRFs alone. Overall, our findings suggest that AsIRF3 is the main regulator of salmon IFNa1 production along with AsIRF1, which is less potent. This confirms a similar role for salmon IRF3 as mammalian IRF3 to be one of the main IRFs eliciting salmon IFNa1 production. Surprisingly, AsIRF7A and AsIRF7B seemed to have a lesser role in salmon IFNa1 induction, which may indicate that these factors have a larger role in activating other IFN genes or interferon stimulatory genes of Atlantic salmon.