Functional Characterization of Hexacorallia Phagocytic Cells

Phagocytosis is the cellular defense mechanism used to eliminate antigens derived from dysregulated or damaged cells, and microbial pathogens. Phagocytosis is a pillar of innate immunity, whereby foreign particles are engulfed and degraded in lysolitic vesicles. In hexacorallians, phagocytic mechanisms are poorly understood, though putative anthozoan phagocytic cells (amoebocytes) have been identified histologically. We identify and characterize phagocytes from the hexacorallian model and the sea anemone Nematostella vectensis. Using fluorescence-activated cell sorting and microscopy, we show that distinct populations of phagocytic cells engulf bacteria, fungal antigens, and beads. In addition to pathogenic antigens, we show that phagocytic cells engulf self, damaged cells. We show that target antigens localize to low pH phagolysosomes, and that degradation is occurring within them. Inhibiting actin filament rearrangement interferes with efficient particle phagocytosis but does not affect small molecule pinocytosis. These results improve our understanding of hexacorallian immune cell biology. Stony corals (subclass: Hexacorallia) contain symbiotic algae called Symbiodinaceae. During heat stress there is an induction of coral bleaching, a phenomenon in which the coral loses its symbiotic algae. We suggest that heat stress activates the coral immune system and leads to the expulsion of the symbiotic algae. Further research into coral cellular immune function under heat stress will help us understand the role of the immune system during coral bleaching.