Phagocytes in the mesoglea of the scyphozoan Cassiopea sp

Cells specialized for the phagocytosis of pathogens and foreign particles, i.e. phagocytes, have been identified across diverse animal lineages. In addition to being widespread in Bilateria, they have also been reported in early-diverging groups such as cnidarians and ctenophores. Cnidarians are of particular interest for investigating the development of the innate immune system because of their close evolutionary proximity to bilaterians. Recent single cell sequencing studies have described specialized immune cells in many cnidarian lineages; however most functional work has focused on the Anthozoa, highlighting the need for a medusozoan model to gain a more comprehensive understanding of cnidarian immunity. The upside-down jellyfish Cassiopea sp., already a model for cnidarian symbiosis, is well-positioned to fill this gap. Here, we describe a selective dissociation technique that enables the live isolation of mesoglea from the Cassiopea polyp, which we have found to contain large numbers of symbiotic and non-symbiotic cells. We further show that amoebocytes, defined here as the non-symbiotic mesogleal cells, are phagocytically active against pHrodo heat-killed E. coli bioparticles, similarly to phagocytes found in anthozoans and ctenophores. By challenging these cells with the pharmacological agents cytochalasin D and nocodazole, we also demonstrate that this phagocytic activity is dependent upon actin polymerization, but not microtubule polymerization. The presence of mesenchymal phagocytes in a medusozoan supports the hypothesis that these cells represent an ancestral feature of cnidarians. The ability to isolate large numbers of viable amoebocytes without requiring advanced techniques positions Cassiopea as a powerful model for investigating cnidarian immunity.