T helper cell populations: as flexible as the skin?

T helper cells can be defined by the cytokines they produce and are divided into Th1, Th2, Th17, T(FH) or regulatory T cells. Th17 cells have been shown to produce, in addition to IL-17, IL-22. In the current issue of the European Journal of Immunology, an article by Larsen et al. (Eur. J. Immunol. 2011. 41: 2596-2605) provides evidence that human T helper cells, like murine cells, can also express IL-22 in the absence of the other T helper cell signature cytokines. Moreover, they show that these IL-22-producing cells, namely Th22 cells, can be found in the skin of psoriasis patients, where they might contribute to the pathogenesis of this inflammatory skin disease. Finally, they show that, molecularly, Th22 cells are related to Th17 cells, and might therefore be derived from the latter. In this Commentary, the development of the pro-inflammatory T helper populations in the skin are discussed and a model that explains the development of Th22 cells found in the skin of psoriasis patients is proposed.