The sea urchin immune system

Metchnikoff’s use of sea star larvae to observe encapsulation and phagocytosis, which was followed much later by allograft rejection kinetics, revealed that echinoderms had an innate immune system that was lacking of adaptive attributes. Larval sea urchins mount defenses in response to contact with microbes, which are mediated by phagocytic blastocoelar cells and pigment cells. In the adult, the coelomocytes mediate immune responses through phagocytosis and encapsulation of foreign particles in addition to degranulation of antimicrobial molecules. Molecular analysis of immune functions in the sea urchin has demonstrated a complement system that appears to have multiple alternative pathways and several activators of the lectin pathway, but may be missing the terminal pathway. Other genes and proteins involved in the sea urchin immunity include expanded sets of lectins, proteins with scavenger receptor cysteine-rich repeats, Toll-like receptors and associated signalling proteins. A vast array of proteins belonging to the 185/333 family are expressed in coelomocytes in response to lipopolysaccharide and show a surprising level of diversity. The sea urchin innate immune system has a number of large gene families with unexpected complexities and elevated levels of diversification.