The Th17 cell population and the immune homeostasis of the gastrointestinal tract.

Th17 cells are a recently discovered subset of CD4(+) T lymphocytes filling a hole in the repertoire of effector T cells. Th17 cells produce multiple cytokines, with pivotal impact on immune homeostasis, inflammation, and influencing a wide range of intestinal cell targets. The current issue of the International Reviews of Immunology is entirely dedicated to the various roles of Th17 T cells in the immune homeostasis and inflammation occurring in the gut. In addition to describing diverse Th17-mediated molecular pathways, a specific focus is being given to Th17 cell plasticity. This enables the Th17 cells to shift towards a Th1 profile, or to express IL-22, a protective cytokine in experimental colitis. Participation of microbiota-specific Th17 cells to normal immune homeostasis, and their role in the pathogenesis of inflammatory bowel disease (IBD), or of gluten specific-Th17 cells in celiac disease, are also being discussed. Neutralizing antibodies against IL-17A and IL-17F have commenced clinical testing in IBD. In conclusion, Th17 cells emerge as a key immune cell population and further elucidation of their roles and functional plasticity are warranted to support the discovery of novel therapies against IBD and other intestinal disorders.