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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Tumour necrosis factor‐α (TNF‐α) is a multifunctional cytokine involved in immune system homeostasis, antimicrobial defence, regulation of apoptosis, cell proliferation and differentiation. Although the pro‐inflammatory property of TNF‐α has been made new progress, detailed research on host defence against bacterial infection and inducing apoptosis remains to be revealed in early vertebrates. Here, we reported the TNF‐α homologue (ToTNF‐α) from pufferfish (Takifugu obscurus). The open reading frame (ORF) of ToTNF‐α was 753 bp, encoding a protein of 250 aa contained the TNF family signature and conserved cysteine residues. The mRNA expression of ToTNF‐α had a wide range of tested tissues, with the highest expression in the skin. After Aeromonas hydrophila infection, the mRNA expression of ToTNF‐α was significantly up‐regulated both in vivo and in vitro experiments. After stimulation by recombinant protein of ToTNF‐α ((r)ToTNF‐α), the relative expressions of endogenous TNF‐α, caspase 8, caspase 3, p53, and Bax inhibitor‐1 in head kidney leucocytes were all notably up‐regulated. These results showed that ToTNF‐α might induce apoptosis depend on pro‐ and anti‐apoptotic proteins at mRNA level. Moreover, flow cytometry analysis indicated that the (r)ToTNF‐α can induce apoptosis of head kidney leucocytes. Taken together, these characteristics suggest that ToTNF‐α can participate in immune response against A. hydrophila and induce apoptosis at mRNA and cellular level, which will help to understand the mechanism of apoptosis and immune response in teleost fish.