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Insight into evolution of adaptive immunity boosted by sea urchin genome sequencing

November 10, 2006

We're not close kin to the sea urchin, but genetically speaking we may have more in common than we think.

The decoding of the sea urchin genome featured in the Nov. 10 issue of the journal Science is accompanied by a companion article written by scientists from the University of Toronto, George Washington University, and the University of South Florida College of Medicine, including affiliates All Children's Hospital and H. Lee Moffitt Cancer Center.

In "Genomic Insights into the Immune System of the Sea Urchin," the authors, including USF geneticist Gary Litman, PhD, contend that the comprehensive analysis of the sea urchin genome has broad implications for understanding primitive host defense and the genetic underpinnings of immunity in vertebrates. An international team of researchers sequenced the entire genome of the purple sea urchin (Strongylocentrotus purpuratus), which like humans belongs to the evolutionary lineage known as deuterostomes.

The sea urchin has emerged as one of the leading models for analyzing genetic regulatory networks that control development. Sea urchins belong to one side of the deuterostome evolutionary split and a few invertebrates (termed protochordates) as well as vertebrates, including humans, belong to the other side of the split.

The investigators detected in the sea urchin an extraordinarily large number of genes that encode molecules involved in natural or innate immunity, the first line of defense against microorganisms. Innate immunity is preformed and directly inherited.

Surprisingly, the international team also found elements of the more customized adaptive immune system, which in vertebrates produces a complex arsenal of antibodies and T-cell receptors to fend off diverse pathogens and prevent repeated attacks. Adaptive immunity, which is not necessarily inherited and arises uniquely in each individual, was not seen until the emergence of vertebrates.

When the sea urchin genome was analyzed, it appeared to contain nearly all the various components that drive the genetic diversification of antibodies and T-cell antigen receptors - just not the actual receptors themselves, said Dr. Litman, the Hines Professor of Pediatrics at USF Health. So, while pieces of adaptive immunity were clearly present in the sea urchin, they were not yet interacting.

"Putting all the pieces of adaptive immunity together was clearly a late event in evolution," Dr. Litman said. "Such findings are particularly rare and ultimately will help us to better understand how complex genetic regulatory circuits are assembled from components that originally may have been dedicated to very different tasks.

"Innate immunity recognizes disease first and we're just beginning to understand how the adaptive immune system steps in to fight disease once the red flag is raised," he added. "This latest genome project may reveal important aspects of how our innate and adaptive immune systems interact. It may give us the best clue yet about how genes work together to keep us healthy.\\\

University of South Florida Health