Antibacterial Properties of Isolated Amoebocytes From the Sea Anemone Actinia equina

The antimicrobial defenses of anthozoans were investigated in vitro by extracting amoebocytes from the mesenteric filaments of the beadlet anemone, Actinia equina, and testing for their ability to phagocytose and kill the gram-negative bacterium Psychrobacter immobilis. Only the hyaline amoebocytes exhibited phagocytosis in vitro, with about 40% seen to ingest one or more bacteria over 45 min. Mixed cultures of viable amoebocytes were further found to produce O2- ions and other reactive oxygen species (ROS) after stimulation with phorbol myristate acetate or lipopolysaccharide. Co-incubation of viable amoebocytes with P. immobilis for 3 h in vitro resulted in reduced growth of the bacterium compared to saline-incubated bacteria, but because the growth of P. immobilis was also impaired by lysed control amoebocytes, the contribution made to bacterial killing by ROS could not be evaluated. Instead, as confirmed by additional experiments using lysate supernatants of the amoebocytes, it appears that the cells contain soluble bactericidal factors. The nature of these agents is at present unknown, although preliminary tests indicate that killing is not mediated by lysozyme.