Identification of an Atlantic salmon IFN multigene cluster encoding three IFN subtypes with very different expression properties

A cluster of 11 interferon (IFN) genes were identified in the Atlantic salmon genome linked to the growth hormone 1 gene. The genes encode three different IFN subtypes; IFNa (two genes), IFNb (four genes) and IFNc (five genes), which show 22–32% amino acid sequence identity. Expression of the fish IFNs were studied in head kidney, leukocytes or TO cells after stimulation with the dsRNA poly I:C or the imidazoquinoline S-27609. In mammals, poly I:C induces IFN-β through the RIG-I/MDA5 or the TLR3 pathway, both of which are dependent on NF-kB. In contrast, S-27609 induces mammalian IFN-α in plasmacytoid dendritic cells through the TLR7 pathway independent of NF-κB. The presence of an NF-κB site in their promoters and their strong up-regulation by poly I:C, suggest that salmon IFNa1/IFNa2 are induced through similar pathways as IFN-β. In contrast, the apparent lack of NF-κB motif in the promoter and the strong upregulation by S-27609 in head kidney and leukocytes, suggest that IFNb genes are induced through a pathway similar to mammalian IFN-α. IFNc genes showed expression patterns different from both IFNa and IFNb. Taken together, salmon IFNa and IFNb are not orthologs of mammalian IFN-β and IFN-α, respectively, but appear to utilize similar induction pathways.