T H 17 cells are of pathologic relevance in autoimmune disorders and presumably also in allergy and asthma. Regulatory T (Treg) cells, in contrast, suppress inflammatory and allergen-driven responses. Despite these disparate functions, both T-cell subsets have been shown to be dependent on TGF-β for their development.The aim of the study was to analyze the differentiation and function of human T H 17 cells in comparison with other T H cell subsets.Naive human CD4 + T cells were differentiated in vitro, and gene expression was analyzed by means of quantitative real-time PCR, ELISA, and immunofluorescence. The function of T H cell subsets was assessed by monitoring the response of primary bronchial epithelial cells in coculture experiments.In vitro differentiated T H 17 cells differ from Treg and other T H cells in their potency to induce IL-6 and IL-1β expression in primary bronchial epithelial cells. TGF-β, IL-1β, IL-6, and IL-23 are necessary during T H 17 cell differentiation to acquire these functions, including IL-17 production. In contrast, TGF-β alone is necessary and sufficient to induce the transcription factor RORC2. This transcription factor, previously thought to be specific for T H 17 cells, is also expressed in Treg cells, CD25 + cells, cytotoxic T cells, and natural killer T cells.This study demonstrates mechanisms of differentiation to human T H 17 cells, a subset that effectively and uniquely modulates the function of primary bronchial epithelial cells.