Particle and bacteria uptake by Japanese flounder (Paralichthys olivaceus) red blood cells: Size dependence and pathway specificity

Red blood cells (RBCs) are traditionally considered non-professional phagocytes functioning predominately in oxygen transport. In the present study, we examined the ability of Japanese flounder (Paralichthys olivaceus), a teleost species with important economic values, RBCs to uptake inorganic particles and bacteria in different size/form, as well as the involving endocytic pathways. We found that flounder RBCs exhibited relatively high uptake/attachment capacities for 0.1 mu m-1.0 mu m (diameter) latex beads, but not for 2.0 mu m beads. For the 0.1 beads, the uptake/attachment was executed through macropinocytosis and caveolae-mediated pathway, while for 0.5 mu m and 1.0 mu m beads, the uptake/attachment depended primarily on macropinocytosis and partially on the caveolin-mediated pathway. In addition to latex beads, flounder RBCs also exhibited an apparent capacity to engulf both live and inactivated bacteria. For live bacteria, the endocytosis was clathrin-mediated, while for inactivated bacteria, clathrin- as well as non-clathrin-mediated endocytosis were involved. Taken together, these results demonstrated that teleost RBCs possess particle uptake/attachment and bacteria phagocytosis capacities via different pathways that depend on the physical size and biological nature of the engulfed objects.