Flow cytometric characterization of hemocytes of the solitary ascidian, Halocynthia roretzi

Internal defense of ascidians relies, at least partially, on cells circulating in body fluids and infiltrating in tissues, referred to as hemocytes, although structure and composition of ascidian hemocytes still remain unclear. In the present study, we investigated hemocyte types and their functions of the solitary ascidian Halocynthia roretzi using flow cytometry. Based on morphology, cellular activities and intracellular parameters from the flow cytometry, we identified eight hemocyte types including, three granulocytes (Gr-1, Gr-2, and Gr-3), 4 hyalinocytes (Hy-1, Hy-1′, Hy-2, and Hy-3) and lymphocyte-like (Ly-like) cells. The granulocyte Gr-1 accounted for 30% of the total circulating hemocytes and exhibited highest density of lysosomes and oxidative activity. Gr-1 was deeply involved in phagocytosis and degradation of foreign material. Hyalinocytes consist of two main populations, Hy-1 and Hy-2, and each accounted for 30% of the circulating hemocyte. Hy-1 displayed lysosomal content, an inducible oxidative activity, and no proteases, while Hy-2 expressed highest density of intracellular proteases, no lysosomes and a low oxidative activity. It was believed that Hy-2 may represent an important link between cellular and humoral immune reactions. Hy-1 did not show phagocytosis activity. Hy-3 and the Ly-like cells presented a similar profile except for their size and complexity, and Hy-3 may represent an intermediate differentiation/maturation step between Ly-like cells and other hemocyte populations. This first characterization of the hemocyte populations of H. roretzi provides a solid basis to investigate further their respective roles and functions in physiological and pathological contexts.