AUTHOR=Wang Tiehui , Hu Yehfang , Wangkahart Eakapol , Liu Fuguo , Wang Alex , Zahran Eman , Maisey Kevin R. , Liu Min , Xu Qiaoqing , Imarai Mónica , Secombes Christopher J. 

TITLE=Interleukin (IL)-2 Is a Key Regulator of T Helper 1 and T Helper 2 Cytokine Expression in Fish: Functional Characterization of Two Divergent IL2 Paralogs in Salmonids

JOURNAL=Frontiers in Immunology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2018.01683

DOI=10.3389/fimmu.2018.01683

ISSN=1664-3224

ABSTRACT=<p>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, <italic>IL2</italic> orthologs have been identified with an additional divergent <italic>IL2-Like</italic> gene on the same locus present in several fish species. We report here two divergent <italic>IL2</italic> paralogs, <italic>IL2A</italic> and <italic>IL2B</italic>, in salmonids that originated from the whole genome duplication event in this fish lineage. The salmonid <italic>IL2</italic> paralogs 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 trout <italic>IL2</italic> paralogs 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 of <italic>CD4</italic> and <italic>CD8</italic>, suggesting that trout IL-2 isoforms are T cell growth/survival factors mainly expressed by activated T cells. The recombinant proteins for these two trout <italic>IL2</italic> paralogs have been produced in <italic>E. coli</italic> and possess shared but also distinct bioactivities. IL-2A, but not IL-2B, induced <italic>IL12P35A1</italic> and <italic>CXCR1</italic> expression in PBL. IL-2B had a stronger effect on upregulation of the T helper 1 (Th1) cytokine <italic>interferon-γ</italic> (<italic>IFNγ</italic>) and could sustain <italic>CD8α</italic> and <italic>CD8β</italic> expression levels. Nevertheless, both cytokines upregulated key Th1 (<italic>IFNγ1, IFNγ2, TNFα2</italic> and <italic>IL12</italic>) and T helper 2 (Th2) cytokines (<italic>IL4/13B1</italic> and <italic>IL4/13B2</italic>), cytokine and chemokine receptors and the antimicrobial peptide <italic>cathelicidin-1</italic> but had limited effects on T helper 17 cytokines and <italic>TGFβ1</italic> in 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 of <italic>IL2</italic> in vertebrates.</p>