Origins of immunity: transcription factors and homologues of effector genes of the vertebrate immune system expressed in sea urchin coelomocytes

Echinoderms share common ancestry with the chordates within the deuterostom e clade. Molecular features that are shared between their immune systems an d that of mammals thus illuminate the basal genetic framework on which thes e immune systems have been constructed during evolution. The immune effecto r cells of sea urchins are the coelomocytes, whose primary function is prot ection against invasive marine pathogens; here we identify six genes expres sed in coelomocytes, homologues of which are also expressed in cells of the mammalian immune system. Three coelomocyte genes reported here encode tran scription factors. These are an NFKB homologue (SpNFKB); a GATA-2/3 homolog ue (SpGATAc); and a runt domain factor (SpRunt-1). All three of these coelo mocyte genes respond sharply to bacterial challenge: SpNFKB and SpRunt-1 ge nes are rapidly up-regulated, while transcripts of SpGATAc factor disappear within hours of injection of bacteria. Sham injection also activates SpNFK B and SpRunt, though with slower kinetics, but does not affect SpGATAc leve ls. Another gene, SpHS, encodes a protein related to the signal transductio n intermediate HS1 of lymphoid cells. Two other newly discovered genes, SpS RCR1 and SpSRCR5, encode proteins featuring SRCR repeats. These genes are m embers of a complex family of SRCR genes all expressed specifically in coel omocytes. The SRCR repeats most closely resemble those of mammalian macroph age scavenger receptors. Remarkably, each individual sea urchin expresses a specific pattern of SRCR genes. Our results imply some shared immune funct ions and more generally, a shared regulatory architecture which underlies i mmune system gene expression in all deuterostomes. We conclude that the ver tebrate immune system has evolved by inserting new genes into old gene regu latory networks dedicated to immunity.