Internalization of large particles by turbot (Scophthalmus maximus) IgM+ B cells mainly depends on macropinocytosis

Increasing evidence has demonstrated support for the endocytic capacities of teleost B cells. In the present study, the ability of turbot IgM+ B cells to ingest microspheres of different sizes and the corresponding internalization pathways were investigated. The results showed that IgM+ B cells exhibited relatively high endocytic capacities for 0.5 μm and 1 μm latex beads, and that different mechanisms were employed for IgM+ and IgM− cells to uptake 0.5 μm and 1 μm beads. For 0.5 μm beads, IgM+ B cells apparently employed macropinocytosis-dependent endocytic pathway, whereas IgM− cells utilized a different process involving both clathrin- and caveolae-mediated pathways. For the uptake of 1 μm beads, IgM+ cells relied mainly on macropinocytosis and partially on caveolae-mediated pathway, while IgM− cells utilized the routes similar to that of internalizing 0.5 μm beads. Consistently, the internalized microspheres were co-localized with high-molecular-mass dextran in IgM+ phagocytic cells. In addition to latex beads, IgM+ B cells could also ingest inactivated bacteria predominately through macropinocytosis and caveolae-mediated endocytosis. These results collectively indicated that macropinocytosis is principally responsible for particle uptake by turbot IgM+ B cells.