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Background & Aims: Interleukin (IL)-17-producing CD4(+) helper T cells (Th17) mediate mucosal immunity and are involved in the pathogenesis of inflammatory bowel disease (IBD). They are believed to arise from the same precursor population as regulatory T (Treg) cells, but little is known about how these T-cell subsets interact under chronic inflammatory conditions. We studied Th17 and Treg cells isolated from intestinal lamina propria of patients with IBD to investigate their role in pathogenesis., Methods: FoxP3 expression (a marker of Treg cells) and IL-17 production were assessed in CD4(+) lamina propria lymphocytes isolated from IBD patients and healthy subjects. IL-17(+)FoxP3(+) and IL-17(+) CD4(+) T-cell clones were generated by limiting dilution. An in vitro suppression assay was performed to assess the functional capacity of derived T-cell clones., Results: IL-17(+)FoxP3(+) T cells were identified in inflamed intestinal mucosa of patients with Crohn disease (CD), but not in patients with ulcerative colitis (UC) or healthy controls. These cells shared phenotypic characteristics of Th17 and Treg cells, and showed potent suppressor activity in vitro. Transforming growth factor-β was necessary and sufficient to induce development of an IL-17(+) FoxP3(+) cell population in CD4(+) lamina propria lymphocytes derived from patients with UC., Conclusions: The inflammatory environment in the intestinal mucosa of patients with CD contributes to the generation of a distinct population of Treg cells that are FoxP3(+) and produce IL-17. These cells are likely to arise during differentiation of Th17 and Treg cells. Specific microenvironmental cues from tissues are likely to determine their commitment to either lineage and affect the balance between regulation and inflammation in the intestine., (Copyright © 2011 AGA Institute. Published by Elsevier Inc. All rights reserved.)