Th17 cells in mucosal immunity and tissue inflammation.

T helper type 17 (Th17) cells are a distinct lineage of T cells that produce the effector molecules IL-17, IL-17F, IL-21, and IL-22. Th17 cells have been shown to have critical roles in autoimmunity and tissue inflammation. However, emerging evidence also shows these cells are critical regulators of host immunity against bacterial, fungal, and viral infections at mucosal surfaces. Moreover, these cells can be induced following vaccination and have been shown to be critical for vaccine efficacy against both extracellular and intracellular pathogens. In this issue, we summarize recent progress in our understanding of the function of Th17 cells and where these cells fit in protective immunity and immunopathology.