Discovery and Functional Characterization of Cytotoxic Cells in Planarians

Research into functional cellular immune activity in invertebrates, specifically phagocytosis and cytotoxicity, remains an evolving field with ample room for exploration. Phagocytosis has garnered relatively more attention, while cytotoxicity, a fundamental aspect of immune defense, remains under-studied or indirectly addressed in some invertebrate species.

Planarians exhibit immune-related functions, including evidence of phagocytic activity directed towards bacteria and cellular components, and some hints of cytotoxic activity. This research delved into the cellular immune capabilities of planarians by means of flow cytometry-based functional immune assays, presenting the first evidence of cellular cytotoxic activity within such organisms. Furthermore, my work used live visualization to capture the dynamic interactions of cytotoxic cells with their targets, using fluorescence labeling of isolated cells and confocal microscopy. Remarkably, using functional immune assays of isolated cells of syngeneic, allogeneic, and xenogeneic species, I revealed that these immune responses did not exhibit specific cellular activity during encounters between allogeneic and xenogeneic cells, suggesting a lack of specificity in recognizing incompatible cells. This observation, combined with additional discoveries, strengthened my hypothesis that planarian cytotoxic cells belong to the level I cytotoxicity category. Furthermore, we successfully isolated a granular cell population enriched in cytotoxic cells that demonstrated enhanced target cell-killing capabilities and exhibited up-regulation of genes associated with cytotoxic activity. Several key characteristics align these planarian cytotoxic cells with neutrophil-like cells.

These findings shed light on the evolution of cellular immune lineages. Notably, these findings represent the first comprehensive characterization of early-stage cytotoxicity in this class of organisms.