Interleukin (IL)-2 Is a Key Regulator of T Helper 1 and T Helper 2 Cytokine Expression in Fish: Functional Characterization of Two DivergentParalogs in Salmonids.

Mammalian interleukin (IL)-2 is a cytokine centrally involved in the differentiation and survival of CD4+ T helper subsets and CD4+ T regulatory cells and in activation of cytotoxic effector lymphocytes. In bony fish,orthologs have been identified with an additional divergentgene on the same locus present in several fish species. We report here two divergentparalogs,and, in salmonids that originated from the whole genome duplication event in this fish lineage. The salmonidparalogs differ not only in sequence but also in exon sizes. The IL-2 isoforms that are encoded have disparate pI values and may have evolved to preferentially bind specific IL-2 receptors. Rainbow troutparalogs are highly expressed in thymus, spleen, gills, kidney and intestine, important tissues/organs in fish T cell development and function. Their expression in peripheral blood leukocytes (PBL) is low constitutively but can be upregulated by the mixed leukocyte reaction, by the T cell mitogen phytohemagglutinin and by signal mimics of T cell activation (phorbol 12-myristate 13-acetate and calcium ionophore). Both trout IL-2 isoforms promoted PBL proliferation and sustained high-level expression ofand, suggesting that trout IL-2 isoforms are T cell growth/survival factors mainly expressed by activated T cells. The recombinant proteins for these two troutparalogs have been produced inand possess shared but also distinct bioactivities. IL-2A, but not IL-2B, inducedandexpression in PBL. IL-2B had a stronger effect on upregulation of the T helper 1 (Th1) cytokine() and could sustainandexpression levels. Nevertheless, both cytokines upregulated key Th1 (and) and T helper 2 (Th2) cytokines (and), cytokine and chemokine receptors and the antimicrobial peptidebut had limited effects on T helper 17 cytokines andin PBL. They could also enhance PBL phagocytosis. These results suggest, for the first time in fish, that IL-2 isoforms may have an important role in regulating Th1 and Th2 cell development, and innate and adaptive host defenses in fish, and shed light on lineage-specific expansion, evolution, and functional diversification ofin vertebrates.