Although the immune system is constantly patrolling for foreign invaders, it attacks neither the bacteria in the gut nor those intestinal cells exposed to the bacteria. Researchers at Dana-Farber Cancer Institute this year announced that a previously unrecognized population of cells in the lymph nodes signals the immune system to tolerate conditions that would normally prompt an attack.
Immunologists already knew that a population of cells called dendritic cells can teach T lymphocytes, also called T cells—key instigators of an immune response—how to react to a particular protein. When the dendritic cells are in a calm environment, they communicate tolerance to T cells. If the dendritic cells are in an environment with microbes or tissue inflammation, they tell the T cells to start an immune system attack. That paradigm left no explanation for how T cells learn to tolerate the conditions in the gut, where cells are constantly in the presence of bacteria.
The surprising finding, reports immunologist Shannon Turley in the January 2007 issue of Nature Immunology, is that dendritic cells share this job with another group of cells far removed from the intestine. Stromal cells that reside in the lymph nodes throughout the body manufacture proteins identical to those made by gut cells and use them to train T cells to ignore certain proteins. “No one would have thought that this sort of system would exist,” Turley says. “We had trained ourselves to think that dendritic cells could do it all, both tolerize and induce immunity to everything. That is probably not true. We probably need backup mechanisms.”
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