X-box protein 1 (Xbp1), an essential transcription factor including an unstable form (Xbp1-u) and a stable form (Xbp1-s), plays an vital role in B cell activation and differentiation to plasma cells. In this study, we cloned and identified Xbp1-u gene from Nile tilapia (Oreochromis niloticus), containing 783 bp of nucleotide sequence encoding 260 amino acids. The deduced protein possesses a basic region leucine zipper domain (bZIP) and 26 ribonucleotides of OnXbp1-u transcript. Transcription analysis revealed OnXbp1-u and OnXbp1-s were widely distributed in all examined tissues, with a high expression in immune-related tissues. When stimulated with Streptococcus agalactiae in vivo, the expressions of OnXbp1-u and OnXbp1-s were significantly up-regulated in liver, spleen, head kidney, blood, skin and intestine. After in vitro challenge upon S.agalactiae, the similar up-regulations of OnXbp1-u and OnXbp1-s were also demonstrated in head kidney leukocytes. Moreover, the OnXbp1-u and OnXbp1-s could get involved in LPS-inducible B cell activation and (r)OnIL6-inducible B cell differentiation. Taken together, the results indicated that OnXbp1-u and OnXbp1-s might not only involved in the immune response against S. agalactiae challenge, but also in the B cell activation and differentiation in Nile tilapia.

X-box蛋白1（Xbp1）是必需的转录因子，包括不稳定形式（Xbp1-u）和稳定形式（Xbp1-s），在B细胞活化和向浆细胞的分化中起着至关重要的作用。在这项研究中，我们从尼罗罗非鱼（Oreochromis niloticus）克隆并鉴定了Xbp1-u基因，该基因包含783 bp的核苷酸序列，编码260个氨基酸。推导的蛋白质具有一个基本区域亮氨酸拉链结构域（bZIP）和OnXbp1-u转录本的26个核糖核苷酸。转录分析显示OnXbp1-u和OnXbp1-s广泛分布在所有检查的组织中，在免疫相关组织中高表达。当在体内用无乳链球菌刺激时在肝脏，脾脏，头部肾脏，血液，皮肤和肠道中，OnXbp1-u和OnXbp1-s的表达明显上调。后在体外攻击后无乳链球菌，对类似上规定OnXbp1-U和OnXbp1-S也表现在头肾白细胞。此外，OnXbp1-u和OnXbp1-s可能参与LPS诱导的B细胞活化和（r）OnIL6诱导的B细胞分化。两者合计，结果表明OnXbp1-u和OnXbp1-s可能不仅参与针对无乳链球菌的免疫反应 挑战，还涉及尼罗罗非鱼的B细胞活化和分化。