B lymphocyte-induced maturation protein 1 (Blimp-1), a transcription factor containing zinc finger , is required and sufficient to trigger terminal differentiation of B cells in mammals. The Blimp-1 ( OnBlimp-1 ) from Nile tilapia ( Oreochromis niloticus ) was identified and characterized its expression pattern during B cell activation and maturation. The cDNA of OnBlimp-1 open reading frame is 2547 bp encoding a protein of 848 amino acids and the predicted molecular weight is 93.36 kDa. OnBlimp-1 contains a SET domain and five ZnF_C2H2 domains, which shares high homology with that of other species. OnBlimp-1 transcription was detected in all examined tissues with high expression in the spleen (SPL). Analysis of sorted lymphocyte populations , including IgM + and IgM − cells from peripheral blood (PBL), SPL and anterior kidney (AK), indicated that the OnBlimp-1 transcription was highly expressed in the IgM + B cells. Upon LPS stimulation, OnBlimp-1 expression was up-regulated in tissues of PBL, SPL and AK significantly. The expression of OnBlimp-1 , as well as the secreted IgM , was significantly up-regulated in the SPL and AK leukocytes stimulated with anti-OnIgM monoclonal antibody and LPS in vitro , respectively. Above results suggest that OnBlimp-1, a cytokine regulating the terminal differentiation of activated B cells to antibody-secreting cells, is likely to play important roles in B cell activation and maturation in Nile tilapia.
Teleost B cells have phagocytic activities for ingesting particulate antigens, such as bacteria, in addition to the functional secretion of immunoglobulins (Igs). In the present study, the phagocytic activities of IgM+ B cells under various differentiational conditions residing in peripheral blood leukocytes were investigated in a teleost fish Nile tilapia (Oreochromis niloticus). The IgM+ B cells were recognized as IgMlo or IgMhi subsets based on their membrane IgM (mIgM) levels. The mIgM, secreted IgM (sIgM), major histocompatibility complex class II and reactive oxygen species were detected. Expressions of transcription factors (Pax5 and Blimp-1) and B cell signaling molecules (CD79a, CD79b, BLNK, and LYN) suggested that IgMlo B cells were resembling as plasma-like cells and IgMhi resembling as naïve/mature B cells, respectively. Analysis of phagocytic activities demonstrated that both IgMlo and IgMhi B cells have a similar phagocytic ability (phagocytosis percentage); however, the phagocytic capacity [phagocytic index and the mean fluorescence intensity (MFI)] of IgMhi B cells was significantly higher than that of IgMlo B cells. Taken together, the results indicated that B cell differentiation may cause the decrease of phagocytic capacity but not phagocytic ability of phagocytic IgM+ B cells in teleost. The finding may provide an evolutionary evidence for understanding the greater specialization of the B cell in more sophisticated adaptive humoral immunity, by decreasing phagocytic activity in order to contribute its function more specifically into antibody-secreting.
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