Regulatory Role of Fc Receptor in mIgM+ B Lymphocyte Phagocytosis in Flounder (Paralichthys olivaceus)
Yanbo Hao, Xiaoqian Tang, Jing Xing, Xiuzhen Sheng, Heng Chi, Wenbin Zhan
Abstract
Fc receptor (FcR) is an important opsonin receptor on the surface of immune cells, playing an important role in antibody-dependent cell-mediated immunity. Our previous work found that the FcR of flounder showed a marked expression response in phagocytizing IgM + B cell, which suggested that FcR might participate in regulating Ig-opsonized phagocytosis. In this paper, in order to elucidate the potential role of FcR in mediating phagocytosis of IgM + B cell, flounder anti- E. tarda serum was prepared and complement-inactivated for the use of E. tarda opsonization, and the sera of healthy flounder were used as control. Flow cytometric analysis showed that the phagocytosis rates of antiserum-opsonized E. tarda in peripheral blood mIgM + B lymphocytes were significantly higher than the control group, and higher phagocytosis rates of mIgM + B lymphocyte could be detected with an increasing incubation time ranging from 1 to 5 h. The phagocytosis rates of antiserum-opsonized E. tarda by mIgM + B lymphocyte for an incubation time of 1, 3 or 5 h were 51.1, 63.0, and 77.5% respectively, which were significantly higher than the phagocytosis rates in the control groups with 40.2, 50.9, and 63.8%, respectively. While the Fc fragment of IgM on the surface of opsonized E. tarda was blocked by rabbit anti-flounder IgM polyclonal antibodies, phagocytosis rates of mIgM + B lymphocyte decreased significantly compared with the unblocked group. Moreover, the proportion of mIgM + B lymphocytes with higher intracellular reactive oxygen species (ROS) levels rose to 32.1% from the control level of 23.0% after phagocytosis of antiserum-opsonized E. tarda . FcγRII and Syk were found to be significantly upregulated, while FcγRIII was significantly downregulated in the mIgM + B lymphocytes post phagocytosis. Furthermore, when FcγRII of mIgM + B lymphocytes was blocked by the prepared antibodies, their phagocytosis rate of antiserum-opsonized E. tarda was 39.0%, which was significantly lower than the unblocked group of 54.0%. These results demonstrate that FcR plays a critical role in mediating phagocytosis and bactericidal activity of mIgM + B lymphocytes, which would facilitate an improved understanding of the regulatory roles of FcR in phagocytosis of teleost B lymphocytes.