Molecular and functional characterization of IL-15 in rainbow trout Oncorhynchus mykiss: a potent inducer of IFN-gamma expression in spleen leukocytes.

IL-15 is a member of the common gamma-chain family of cytokines that possess a heterogeneous repertoire of activities on various cells of the immune system. We report here the first functional characterization of a fish IL-15 in rainbow trout. The trout IL-15 gene is 6-kb long and contains six exons and five introns that transcribe into a 1.2-kb mRNA containing seven out-of-frame AUG initiation codons and translate into a 193-aa peptide. Potential sites for transcriptional activators and repressors have been identified in the trout IL-15 gene. Like IL-15 from other species, trout IL-15 is closely linked to an INPP4B gene, but there is also a BCL10 gene located between the IL-15 and INPP4B genes. Three alternative splicing variants of the trout IL-15 gene have also been identified and their expression in vivo was studied. Trout IL-15 expression is present in all the tissues and cell lines studied. Recombinant trout IFN-gamma selectively increased IL-15 expression but had little effect on other cytokines such as IL-1 beta and IL-11. Recombinant trout IL-15 preferentially stimulated splenic leukocytes from healthy fish, where it induced a large increase in IFN-gamma expression, with little, if any, effect on IL-1 beta expression. This effect was quite long-lived, and was still apparent 24 h poststimulation. Although the exact cell types being affected have still to be determined, it is clear that once produced IL-15 will have a profound affect on the ability of the fish immune system to activate antimicrobial defenses and genes induced themselves by IFN-gamma.