Tea time for T cells.

Immunologists have a mystery on their hands. Immune cells called T cells circulate in the human bloodstream, but the roles of a portion of the cells, so-called gamma-delta T cells, remain murky. In the July IMMUNITY, scientists now report that these T cells respond to substances secreted by bacteria, as well as to molecules found in tea, wine, apples, and other edible plants.

"They recognize molecular structures that occur in bacteria and in plants that we eat every day," says study author Jack F. Bukowski of Brigham and Women's Hospital and Harvard Medical School, both in Boston.

Gamma-delta T cells make up nearly 5 percent of circulating T cells. Their distinguishing characteristic is that their receptors, surface proteins that bind to molecules and trigger the cells into action, consist of small proteins called gamma and delta chains, instead of the more common alpha and beta chains.

Studies of gamma-delta cells in mice suggest that these T cells fight off bacteria, such as those that cause tuberculosis, as well as viruses, but it has been unclear what those results mean for people. "The molecular structures recognized by human gamma-delta cells are much different than those recognized by the mouse cells. You can't necessarily extrapolate from mice to humans," says Bukowski.

He and his colleagues now report that human gamma-delta T cells proliferate and release an immune signal called interleukin 2 when they're exposed to alkylamines, a class of molecules secreted by bacteria. Alkylamines from green and black tea, red and white wine, and apples also stimulate the T cells. As a result, the investigators have just launched a study in people to examine whether drinking tea stimulates the activity of the gamma-delta T cells and bolsters overall immunity.