"It's obvious that T cells must be able to ignore ?or become 'tolerized' to ?normal intestinal tissue," states Turley, who is also an assistant professor of pathology at Harvard Medical School. "But it has been unclear how dendritic cells, which are extremely sensitive to microbial agents such as bacteria, teach T cells to resist attacking healthy intestinal cells."
In the new study, Turley and her colleagues found that, in fact, dendritic cells aren't essential in creating tolerance in T cells. Instead, and unexpectedly, tolerance is produced by stromal cells from nearby lymph nodes. Although they aren't classified as "professional antigen-presenters," as dendritic cells are, the stromal cells serve the same purpose: exhibiting normal-cell antigens to the immune system.
"Our study points to a previously unknown mechanism of immune system tolerance," Turley explains. "When you think of the conditions in the small intestine, with so many millions of bacteria cells and so much opportunity for dendritic cells to stimulate an immune attack, it's remarkable that intestinal tissue is so rarely the target of an immune attack. Our findings demonstrate that the immune system has features that remain to be discovered."