Key player in antiviral response. Induces expression of TLRs, including that of TLR3, TLR9 and TLR8a1, and that of cytosolic pattern recognition receptors, including DDX58/RIG-1, IFIH1/MDA5 and DHX58/LGP2. Also induces MX1 and its own expression. In the presence of intracellular IFNAR2 (iIFNAR2) and IFNAR1B, intracellular isoform 3 may mediate STAT1 and STAT2 phosphorylation and induction of EIF2AK2, MX1 and RSAD2. Isoform 1 and isoform 2 are expressed in several tissues, including gill, spleen, intestine, kidney and skin. In the RTG-2 fibroblastic cell line, isoform 1 is highly induced by polyinosine-polycytidylic acid (poly(I:C)), a synthetic analog of dsRNA, that binds TLR3. Isoforms 2 and 3 are also up-regulated by poly(I:C), but to a lesser extent. In vivo, all 3 isoforms are induced by poly(I:C) (in anterior kidney leukocytes) and by viral infection, such as that of viral hemorrhagic septicemia virus (VHSV) (isoform 1>isoform 3>>isoform 2) and infectious hematopoietic necrosis virus (IHNV). However, induction kinetics is isoform-specific. Also up-regulated in vivo by a DNA vaccine based on the IHNV glycoprotein. Salmonid type I IFNs are classified into 2 groups with 2 (group I) or 4 (group II) cysteines in the mature peptide. These groups can be further divided into 3 subgroups (group I IFN1-a, IFN1-d and IFN1-e and group II IFN1-b, IFN1-c and IFN1-f). Each subgroup can be represented by several genes (up to at least 7 for IFN1-e). Belongs to the alpha/beta interferon family.