Identification and bioactivities of IFN-γ in rainbow trout Oncorhynchus mykiss: The first Th1-type cytokine characterized functionally in fish

IFN-γ is one of the key cytokines in defining Th1 immune responses. In this study, an IFN-γ homologue has been identified in rainbow trout Oncorhynchus mykiss, and its biological activities have been characterized. The trout IFN-γ cDNA is 1034 bp in length and translates into a 180-aa protein. The first intron of the trout IFN-γ gene contains highly polymorphic GACA mini-satellites and 44-bp DNA repeats, giving rise to at least six alleles. IFN-γ is structurally conserved among vertebrates, and a signature motif has been identified. A nuclear localization sequence known to be crucial for IFN-γ biological activities is also present in the C-terminal region of the trout IFN-γ. The IFN-γ expression was induced in head kidney leukocytes by stimulation with PHA or poly(I:C) and in kidney and spleen of fish injected with poly(I:C). rIFN-γ produced in Escherichia coli significantly stimulated gene expression of IFN-γ-inducible protein 10 (γIP-10), MHC class II β-chain, and STAT1, and enhanced respiratory burst activity in macrophages. Deletion of 29-aa residues from the C terminus containing the nuclear localization sequence motif resulted in loss of activity with respect to induction of γIP-10 in RTS-11 cells. Moreover, IFN-γ-induced γIP-10 expression was completely abolished by the protein kinase C inhibitor staurosporine, and partially reduced by U0126, a specific inhibitor for ERKs. Taken together, the present study has demonstrated for the first time a functional IFN-γ homologue in a fish species, strongly suggesting a conserved Th1 immune response is most likely present in lower vertebrates.