Phenotypic and functional characterization of two coelomocyte subsets in Apostichopus japonicus

The hemocytes of invertebrates are composed of different cell subsets with different morphologies and structures. Different cell subsets have different immune functions, which play an important role in innate immune response against pathogens. However, the understanding of the classification of Apostichopus japonicus coelomocytes and the molecular basis of immune function of different cell subsets is very limited. In this study, two coelomocyte subpopulations of A. japonicus were isolated by Percoll density gradient centrifugation. They were identified from their morphological and structural characteristics, namely, spherical cells with a size of 10-12 μm spherical in shape and a large number of small granules inside; lymphocyte-like cells with a size of 4-5 μm spherical or oval in shape, and 1-3 filopodia. Functionally, the phagocytic capacity and lysosomal activity in spherical cells were significantly greater than those in lymphocyte-like cells. The results suggest that spherical cells may play a more critical role in the immune responses. Meanwhile, transcriptome sequencing analysis was performed to further clarify the functional differences between the two cell subsets. The data indicated significantly different gene expression patterns in them. Spherical cells tend to participate in immune defense, whereas lymphocyte-like cells tend to participate in energy metabolism. In addition, lymphocyte-like cells may convert oxidative phosphorylation to glycolysis by changing the manner of energy metabolism to quickly adapt to the energy demand of external stimuli. Spherical cells may respond to LPS stimulation through phagocytosis, and their response time is slower than that of lymphocyte-like cells. The expression of genes involved in endocytosis, phagocytosis, and lysosomal and humoral immunity in spherical cells was significantly higher than that in lymphocyte-like cells. These data provide valuable information for understanding the molecular basis of cellular and humoral immunity in A. japonicus.