Extracellular DNA traps in a ctenophore demonstrate conserved immune cell behaviors in a non-bilaterian

The formation of extracellular DNA traps (ETosis) is a first response mechanism by specific immune cells following exposure to microbes 1,2. Initially characterized in vertebrate neutrophils, cells capable of ETosis have been discovered recently in diverse non-vertebrate taxa 3 4-6. To assess the conservation of ETosis between evolutionarily distant non-vertebrate phyla, we observed and quantified ETosis using the model ctenophore Mnemiopsis leidyi and the oyster Crassostrea gigas. Here we report that ctenophores - thought to have diverged very early from the metazoan stem lineage 7-10- possess immune cell types capable of phagocytosis and ETosis. We demonstrate that both Mnemiopsis and Crassostrea immune cells undergo ETosis after exposure to diverse microbes and chemical agents that stimulate ion flux. Our results support ETosis as an evolutionarily ancient metazoan defense against pathogens.