Interleukin (IL) −2, a member of the four α-helical cytokine family, has broad regulatory roles in mediating vertebrate immune response. In mammals, IL-2 and IL-15 share a common evolutionary origin and possess overlapping but distinct functions. IL-2 and IL-15 bind to distinct private receptors for signaling. However, fish appear to possess a single IL-15Rα like gene whilst lack additional gene(s) coding for IL-2Rα. Whether the IL-2 and IL-15 interact with the same receptor in fish and how their functions and receptors have evolved are not fully understood. In this study, homologues of IL-2 and IL-2/15Rα were sequenced from a teleost species, grass carp (Ctenopharyngodon idella), and the crystal structure of IL-2 was determined. The grass carp IL-2 (termed CiIL-2) displayed a classical cytokine structure consisting of four helical bundles which shares significant similarity with human IL-15. The key amino acids involved in the interface interaction of IL-2/15 and their receptors are well conserved. The CiIL-2 has been shown to bind the IL-2/15Rα like homologue with an affinity of 2.45 nM, supporting the notion that fish IL-2 and IL-15 may share a single common private receptor for exerting functions. Syntenic analysis suggests that the IL-2Rα of tetrapods has evolved from an IL-15Rα like homologue, in which a second sushi domain (D2) in the extracellular region has been duplicated to facilitate the specific interaction with IL-2. The CiIL-2 was predominantly expressed in lymphocyte-rich tissues such as the spleen, kidney and thymus, and could be induced by PHA and IL-21. In vivo challenge with grass carp reovirus and Flavobacterium columnare also resulted in upregulation of CiIL-2 expression. The recombinant CiIL-2 was shown to activate expression of STAT5b, IL-1β, IL-22 and IFN-γ, and to promote the proliferation of the primary cell cultures from head kidney leucocytes. Our results shed lights into the co-evolution of IL-2 and its private receptor, and the functional divergence of IL-2 and IL-15 during evolution.

白细胞介素 (IL) -2 是四个 α-螺旋细胞因子家族的成员，在介导脊椎动物免疫反应中具有广泛的调节作用。在哺乳动物中，IL-2 和 IL-15 具有共同的进化起源并具有重叠但不同的功能。IL-2 和 IL-15 与不同的私人信号受体结合。然而，鱼似乎拥有单一的IL-15Rα 样基因，而缺乏额外的编码IL-2Rα 的基因。IL-2 和 IL-15 是否与鱼类中的相同受体相互作用以及它们的功能和受体如何进化尚不完全清楚。在这项研究中，IL-2和IL-2/15Rα的同源物从硬骨鱼种草鱼（Ctenopharyngodon idella)，并确定了 IL-2 的晶体结构。草鱼 IL-2（称为Ci IL-2）显示出由四个螺旋束组成的经典细胞因子结构，与人类 IL-15 具有显着相似性。参与 IL-2/15 及其受体界面相互作用的关键氨基酸非常保守。所述次IL-2已经显示出与2.45 nM的亲和力结合IL-2 /15Rα像同源物，支撑鱼IL-2和IL-15可以共享单个公共专用受体用于施加功能的概念。Syntenic 分析表明，四足动物的 IL-2Rα 是从类似 IL-15Rα 的同源物进化而来的，其中细胞外区域中的第二个寿司结构域 (D2) 被复制以促进与 IL-2 的特异性相互作用。该词IL-2主要在脾脏、肾脏和胸腺等富含淋巴细胞的组织中表达，并可被PHA和IL-21诱导。草鱼呼肠孤病毒和黄杆菌柱的体内攻击也导致Ci IL-2表达上调。重组Ci IL-2 显示激活STAT5b、IL-1β、IL-22和IFN-γ 的表达，并促进来自头肾白细胞的原代细胞培养物的增殖。我们的研究结果揭示了 IL-2 及其私有受体的共同进化，以及进化过程中 IL-2 和 IL-15 的功能差异。