Functional and phenotypic effects of AhR activation in inflammatory dendritic cells

Aryl hydrocarbon receptor (AhR) activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces immune suppression. Dendritic cells (DCs) are key antigen presenting cells governing T cell activation and differentiation. However, the consequences of AhR activation in DCs are not fully defined. We hypothesized that AhR activation alters DC differentiation and generates dysfunctional DCs. To test this hypothesis, inflammatory bone marrow-derived DCs (BMDCs) from C57Bl/6 mice were generated in the presence of vehicle or TCDD. TCDD decreased CD11c expression but increased MHC class II, CD86 and CD25 expression on the BMDCs. The effects of TCDD were strictly AhR-dependent but not exclusively DRE-mediated. Similar effects were observed with two natural AhR ligands, 6-formylindolo[3,2-b]carbazole (FICZ) and 2-(1H-Indol-3-ylcarbonyl)-4-thiazolecarboxylic acid (ITE). TCDD increased LPS- and CpG-induced IL-6 and TNF-α production by BMDCs but decreased their NO production. TCDD decreased CpG-induced IL-12p70 production by BMDCs but did not affect their secretion of IL-10. TCDD downregulated LPS- and CpG-induced NF-kB p65 levels and induced a trend towards upregulation of RelB levels in the BMDCs. AhR activation by TCDD modulated BMDC uptake of both soluble and particulate antigens. Induction of indoleamine-2,3-dioxygenase (IDO) and TGF-β3 has been implicated in the generation of regulatory T cells following AhR activation. TCDD increased IDO1, IDO2 and TGF-β3 mRNA levels in BMDCs as compared to vehicle. Despite the induction of regulatory mediators, TCDD-treated BMDCs failed to suppress antigen-specific T cell activation. Thus, AhR activation can directly alter the differentiation and innate functions of inflammatory DCs without affecting their ability to successfully interact with T cells.