IL-22-producing CD4+ T cells: middle-men between the immune system and its environment.

CD4(+) Th cell populations such as Th1, Th2, Th17 and regulatory T cells regulate immune responses by inducing (or inhibiting) proliferation, differentiation and activation of other immune cells. Recent findings have expanded the universe of CD4(+) T-cell subsets by identifying a cell population dedicated to the production of the cytokine IL-22. These so-called Th22 cells may mediate interactions of the immune system with stromal cells. Th22 cells have so far only been observed in humans and are present in inflamed tissues in some skin diseases and evidence suggests that these cells may play a role in the disease process. It is therefore of importance to learn the mechanisms of regulation of development and function of these cells. A paper in the current issue of the European Journal of Immunology indicates that the ligand-dependent transcription factor aryl hydrocarbon receptor is important for regulating IL-22 production of this new class of human Th cells.