Xbp1-u and Xbp1-s from Nile Tilapia (oreochromis Niloticus): Transcriptional Profiling Upon Streptococcus Agalactiae Infection and the Potential Role in B Cell Activation and Differentiation.

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.