Ribosomal protein S6 kinase beta-1 (S6K1) is a serine/threonine kinase downstream of the mechanistic target of rapamycin (mTOR) pathway, and plays crucial roles in immune regulation. Although remarkable progress has been achieved with a mouse model, how S6K1 regulates adaptive immunity is largely unknown in early vertebrates. In this study, we identified an S6K1 from Nile tilapia Oreochromis niloticus (OnS6K1), and further investigated its potential regulatory role on the adaptive immunity of this fish species. Both sequence and structure of OnS6K1 were highly conserved with its homologs from other vertebrates and invertebrates. OnS6K1 was widely expressed in immune tissues, and with a relative higher expression level in the liver, spleen and head kidney. At the adaptive immune stage of Nile tilapia that infected with Aeromonas hydrophila, mRNA expression of OnS6K1 and its downstream effector S6 was significantly up-regulated in spleen lymphocytes. Meanwhile, their phosphorylation level was also enhanced during this process, suggesting that S6K1/S6 axis participated in the primary response of anti-bacterial adaptive immunity in Nile tilapia. Furthermore, after spleen lymphocytes were activated by the T cell-specific mitogen PHA or lymphocytes agonist PMA in vitro, mRNA and phosphorylation levels of S6K1 were elevated, and phosphorylation of S6 was also enhanced. Once S6K1 activity was blocked by a specific inhibitor, both mRNA and phosphorylation levels of S6 were severely impaired. More importantly, blockade of S6K1/S6 axis reduced the expression of T cell activation marker IFN-γ and CD122 in PHA-activated spleen lymphocytes, indicating the essential role of S6K1/S6 axis in regulating T cell activation of Nile tilapia. Together, our study suggests that S6K1 and its effector S6 regulate lymphocyte activation of Nile tilapia, and in turn promote lymphocyte-mediated adaptive immunity. This study enriched the mechanism of adaptive immune response in teleost and provided useful clues to understand the evolution of adaptive immune system.

核糖体蛋白S6激酶beta-1（S6K1）是雷帕霉素（mTOR）通路的机械靶标下游的丝氨酸/苏氨酸激酶，在免疫调节中起关键作用。尽管在小鼠模型上已经取得了显着进展，但在早期脊椎动物中很大程度上未知S6K1如何调节适应性免疫。在这项研究中，我们从尼罗罗非鱼Oreochromis niloticus中鉴定出S6K1（OnS6K1），并进一步研究了其对这种鱼类适应性免疫的潜在调节作用。OnS6K1的序列和结构与其他脊椎动物和无脊椎动物的同系物高度保守。OnS6K1在免疫组织中广泛表达，并且在肝，脾和头肾中具有相对较高的表达水平。尼罗罗非鱼适应性免疫阶段感染了嗜水气单胞菌，脾脏淋巴细胞中OnS6K1及其下游效应子S6的mRNA表达明显上调。同时，它们的磷酸化水平在此过程中也得到增强，表明S6K1 / S6轴参与了尼罗罗非鱼抗菌适应性免疫的主要反应。此外，在脾细胞被T细胞特异性的有丝分裂原PHA或淋巴细胞激动剂PMA激活后，S6K1的mRNA和磷酸化水平升高，S6的磷酸化也增强。一旦S6K1的活性被特定的抑制剂阻断，S6的mRNA和磷酸化水平都将严重受损。更重要的是，封锁S6K1 / S6轴可降低PHA激活的脾淋巴细胞中T细胞激活标记IFN-γ和CD122的表达，表明S6K1 / S6轴在调节尼罗罗非鱼T细胞活化中的重要作用。总之，我们的研究表明S6K1及其效应物S6调节尼罗罗非鱼的淋巴细胞活化，进而促进淋巴细胞介导的适应性免疫。这项研究丰富了硬骨鱼适应性免疫反应的机制，并为理解适应性免疫系统的发展提供了有用的线索。