Phenanthrene (Phe), a polycyclic aromatic hydrocarbon (PAH), is a major constituent of urban air pollution. There have been conflicting results regarding the role of other AhR ligands 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) and 6-formylindolo [3,2-b]carbazole (FICZ) in modifying regulatory T cell populations (Treg) or T helper (Th)17 differentiation, and the effects of Phe have been understudied. We hypothesized that different chemical entities of PAH induce Treg to become either Th2 or Th17 effector T cells through epigenetic modification of FOXP3. To determine specific effects on T cell populations by phenanthrene, primary human Treg were treated with Phe, TCDD, or FICZ and assessed for function, gene expression, and phenotype. Methylation of CpG sites within the FOXP3 locus reduced FOXP3 expression, leading to impaired Treg function and conversion of Treg into a CD4+CD25lo Th2 phenotype in Phe-treated cells. Conversely, TCDD treatment led to epigenetic modification of IL-17A and conversion of Treg to Th17 T cells. These findings present a mechanism by which exposure to AhR-ligands mediates human T cell responses and begins to elucidate the relationship between environmental exposures, immune modulation, and initiation of human disease. 1. Introduction The aryl hydrocarbon receptor (AhR) is a ligand-activated member of the basic-helix-loop-helix family of transcription factors that acts as a sensor for a wide variety of environmental air pollutants, including polycyclic aryl hydrocarbons (PAHs) [1–3]. Recent studies suggest that AhR signaling also has a role in development and immune modulation (reviewed in [4]). Ligand-activation of AhR plays a role in the differentiation of mouse Treg and Th17 cells with implications in autoimmune disease pathogenesis; however, there is controversy as to whether specific AhR-ligands regulate murine Treg versus T helper (Th)17 cell differentiation [5–10]. TCDD has been shown to induce Treg in mouse models [5, 9]. Additional complicating factors for understanding the mechanism of AhR-ligand-mediated effects on T cell immune function include the tissue and species specificity, in addition to the ligand-specific differences in the structure, function, and mechanism of action of the AhR itself [6]. Currently, there is limited information as to how AhR ligands modulate human T cells, the expression of AhR by different human T cell subsets, and the mechanisms by which AhR agonists change human T cell populations. A fuller understanding of these mechanistic pathways and effects of AhR-ligands on human T cells could
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