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T cell 'brakes' lost during human evolution

A significant difference between human and chimpanzee immune cells may provide clues in the search to understand the diverse array of human immune-related diseases. Researchers at the University of California, San Diego (UCSD) School of Medicine have uncovered a a specific type of molecule expressed on non-human primate T cells, but not human T cells. T cells are important orchestrators of the immune system.

In a study to be published on-line in advance of publication in Proceedings of the National Academy of Sciences the week of May 1-5, UCSD researchers report that unlike T cells from chimpanzees, bonobos, and gorillas (the "great apes" which are human's closest evolutionary relatives) human T cells lack expression of certain "Siglec" molecules. Siglecs are immune-dampening proteins that bind to sialic acids, the complex sugars found on the outside of cells. Siglec molecules seem to regulate T cell activation in chimpanzees by restricting the degree of signaling from the T cell receptor, which normally triggers the response of T cells in the immune system.

"Siglecs are like 'brakes' that can slow down the activation of an immune cell upon stimulation," said Ajit Varki, M.D., UCSD Professor of Medicine and Cellular and Molecular Medicine and co-director of UCSD Glycobiology Research and Training Center. "During human evolution, we seem to have shut off these brakes on our T cells, allowing them to become hyper-active."

Human T cells respond much more robustly than chimpanzee cells do, a disparity that could be explained by the absence of human T cell Siglecs. The explanation for this human-specific evolutionary loss of Siglecs is currently unknown. The UCSD scientists speculate that this may have been due to a selective pressure by a microbe that once drove human ancestors to require a high level of T cell activation. Another possibility is that this phenotype was secondarily acquired, during the adjustment to the human-speci
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Contact: Debra Kain
ddkain@ucsd.edu
619-543-6163
University of California - San Diego
1-May-2006


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