**5. Effects of AhR-ligands on the production of Th17/Treg subsets and autoimmunity**

Differentiation of Th17 cells depends on the presence of interleukin (IL)-6 and transforming growth factor (TGF)-beta, and it could be regulated by the activation of AhR [43]. The differentiation of Th17 cells could be enhanced by endogenous AhR agonists found normally in culture medium. The RPMI culture medium could support very low levels of Th17 polarization, because it lacks the presence of these ligands. In contrast, Iscove's modified Dulbecco's medium (IMDM) is known to be rich in aromatic amino acids, such as tryptophan, histidine, and phenylalanine, that were thought to be the precursors of endogenous AhR-ligands and therefore significantly increase the development of Th17 cells [43]. In addition, treating naïve CD4+ T cells with the AhR-ligand FICZ in Th17 cell polarizing conditions helps in skewing the differentiation of naive CD4+ T cells, *in vitro*, toward the development of the Th17 population, and as a result, significant amounts of IL-17*a*, IL-17*f,* and IL-22 cytokines will be secreted. In contrast, a significant reduction in the development of Th17 cells was observed in AhR knockout mice, suggesting that the development of the Th17 cells was AhR dependent [35].

The activation of AhR with different AhR-ligands can regulate Treg/Th17 balance in mice. A significant increase in Treg population was noticed when AhR is activated with TCDD. In addition, suppression in the severity of EAE disease by a TGF-β1-dependent mechanism [44] was seen. Moreover, C57Bl/6J mice carrying the *d* allele of the Ahr gene (Ahr*d* mice) were characterized by a reduced affinity of about 10–100-fold for AhR-ligands due to a mutation in its ligand-binding site, and treating Ahr*d* mice with (1 μg/mouse) TCDD, had no significant effect on the severity of EAE and the development of Treg cells [44]. In contrast, when AhR binds to FICZ, the activation of the receptor will interfere with the differentiation of Treg development, and cause a significant induction of the Th17 subset and worsen EAE disease which suggests that AhR regulates Treg/Th17 subset differentiation in a ligand-specific manner [44]. These data suggested that different AhR-ligands have different effects on the production of pro- or anti-inflammatory T helper cell subsets, by controlling the production of different cytokines in the surrounding environment.
