Echinodermata: The Complex Immune System in Echinoderms

The Echinodermata are an ancient phylum of benthic marine invertebrateswith a dispersal-stage planktonic larva. These animals have innate immune systems characterized initially by clearance of foreign particles, including microbes, from the body cavityof both larvae and adults, and allograft tissue rejection in adults. Immuneresponsiveness is mediated by a variety of adult coelomocytesand larval mesenchyme cells. Echinodermdiseases from a range of pathogens can lead to mass die-offs and impact aquaculture, but some individuals can recover. Genome sequences of several echinodermshave identified genes with immune function, including expanded families of Toll-like receptors, NOD-like receptors, and scavenger receptorswith cysteine-rich domains, plus signaling pathways and cytokines. The set of transcription factors that regulate proliferation and differentiation of the cellular immune system are conserved and indicate the ancestral origins of hematopoiesis. Both larval and adult echinodermsare in constant contact with potential pathogens in seawater, and they respond to infection by phagocytosis and encapsulation, and employ proteins that function in immune detection and response. Antipathogen responses include activation of the SpTransformer genes, a complement system, and the production of many types of antimicrobial peptides. Echinodermshave homologues of the recombinaseactivating genes plus all associated genes that function in vertebrates for immunoglobulin gene family rearrangement, although their gene targets are unknown. The echinodermimmune system has been characterized as unexpectedly complex, robust, and flexible. Many echinodermshave very long life-spans that correlate with an excellent capacity for cell damage repair. In many marine ecosystems, echinodermsare keystone predators and herbivores, and therefore are species that can serve as optimal sentinels of environmental health. Coelomocytescan be employed in sensor systems to test for the presence of marine pollutants. When Elie Metchnikoff inserted a rose prickle into a larval sea star and observed chemotaxis, phagocytosis, and encapsulation by the mesenchyme cells, he initiated not only the field of immunology but also that of comparative immunology, of which the echinodermshave been an important part.