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 minisatellites 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.

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In mammals, IL-21 is a common γ chain cytokine produced by activated CD4+ T cells and NKT cells that acts on multiple lineages of cells. Although IL-21 has also been discovered in birds, amphibians, and fish, to date, no functional studies have been reported for any nonmammalian IL-21 molecule. We have sequenced an IL-21 gene (tIL-21) in rainbow trout, which has a six-exon/five-intron structure, is expressed in immune tissues, and is induced by bacterial and viral infection and the T cell stimulant PHA. In contrast to mammals, calcium ionophore and PMA act synergistically to induce tIL-21. Recombinant tIL-21 (rtIL-21) induced a rapid and long-lasting (4–72 h) induction of expression of IFN-γ, IL-10, and IL-22, signature cytokines for Th1-, Th2-, and Th17-type responses, respectively, in head kidney leukocytes. However, rtIL-21 had little effects on the expression of other cytokines studied. rtIL-21 maintained the expression of CD8α, CD8β, and IgM at a late stage of stimulation when their expression was significantly decreased in controls and increased the expression of the Th cell markers CD4, T-bet, and GATA3. Intraperitoneal injection of rtIL-21 confirmed the in vitro bioactivity and increased the expression of IFN-γ, IL-10, IL-21, IL-22, CD8, and IgM. Inhibition experiments revealed that the activation of JAK/STAT3, Akt1/2, and PI3K pathways were responsible for rtIL-21 action. This study helps to clarify the role of IL-21 in lower vertebrates for the first time, to our knowledge, and suggests IL-21 is a likely key regulator of T and B cell function in fish.

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