Macropinocytosis-dependent endocytosis of Japanese flounder IgM+ B cells and its regulation by CD22.

B cells in fish are proven to be endocytic and have a great contribution to innate immunity like phagocytosis. In this study, the endocytic capacity and the corresponding internalization pathways of IgM+ B cells in Japanese flounder (Paralichthys olivaceus) were investigated. The results showed that IgM+ B cells in peripheral blood leukocytes (PBL) and splenic leukocytes (SL) exhibited different abilities to ingest 0.5 μm and 1 μm latex beads through macropinocytosis-dependent endocytic pathway. Japanese flounder CD22 (PoCD22) co-stimulatory signals were identified to be essential for the innate immune responses in B cells. Most of IgM+ B cells and some IgM- cells were demonstrated to be PoCD22 positive. When PoCD22 was blocked by antibody, the endocytic activities and reactive oxygen species (ROS) activities of SL IgM+ B cells were significantly increased, while the endocytic and ROS activities of PBL IgM+ B cells were significant decreased. These results collectively suggest that Japanese flounder IgM+ B cells are able to employ macropinocytosis-dependent endocytic pathway, which is under the regulation of CD22.