AHR and Autoimmunity: Perhaps most intriguing are recent studies demonstrating that the AhR is intimately involved in the development of T cell subsets, e.g., regulatory T cells (Treg) and IL-17-secreting Th17 cells, and that activation of the AhR in vivo enhances autoimmunity, presumably by up-regulating Th17 development and/or down-regulating Treg development [20-24].