title: Role of aryl hydrocarbon receptor in the induction of tolerogenic dendritic cells
title: Die Rolle des Aryl-Hydrocarbyl-Rezeptors bei der Induktion tolerogener dendritischer Zellen
Creators: name: Sadeghi Shermeh, Atefeh
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: Dendritic cells (DCs) are key components of the innate immune system, essential for bridging the innate and adaptive immunity. They present antigens to T cells to initiate adaptive responses and facilitate B cell activation in humoral immunity. Depending on factors such as the local microenvironment, cytokine milieu and specific signals from pathogens or host cells, DCs can exhibit either activatory or tolerogenic functions. Activatory DCs stimulate strong immune responses by activating effector T cells and producing pro-inflammatory cytokines, whereas tolerogenic DCs promote immune tolerance through the development of regulatory T cells and the secretion of anti-inflammatory cytokines. Aryl hydrocarbon receptor (AhR) is a transcription factor that is activated by ligand binding and significantly influences DC function. AhR activation in DCs can lead to either activatory or tolerogenic effects, influenced by factors such as ligand properties (size, concentration), organ-specific contexts and species-specific variations. Given that DCs are in constant contact with AhR ligands in their environment, it is crucial to investigate how AhR activation affects DC phenotype and function. The primary aim of this study was to investigate how AhR activation affects human monocyte-derived dendritic cells (hMoDCs) at a phenotypic and functional level. Using cell culture, qPCR (quantitative PCR) and flow cytometry, we investigated the effects of three AhR ligands - benzo[a]pyrene (BP), 6-formylindolo[3,2-b]carbazole (FICZ) and indoxyl-3-sulphate (I3S) - on hMoDCs. Our results showed that all three ligands activated AhR in hMoDCs, as indicated by increased expression of AhR-induced transcripts (CYP1A1, CYP1B1, AHRR). We then assessed changes in DC phenotype, including alterations in activation and co-stimulatory markers (CD25, CD80, CD83, CD86, MHCII). CD80 and CD83 were significantly downregulated, while CD25 was upregulated to varying degrees depending on the ligand, suggesting a trend towards a tolerogenic phenotype. Further investigation revealed increased levels of negative regulators associated with tolerogenic DCs, such as immunoglobulin-like transcripts (ILTs) and ectonucleotidases (CD39, CD73), particularly in BP-treated cells. Functionally, BP-treated DCs downregulated CCR7 expression and migration capacity, and reduced T cell proliferation while promoting regulatory T cell differentiation. Using CRISPR/Cas9 technology, we confirmed that these effects were AhR-dependent and in the absence of the AhR, neither expression of AhR-induced transcripts nor changes in DC surface markers are seen. In addition, we performed a murine study using the Cre-lox system to generate AhR knockout mice specifically in CD11c+ cells (AhRΔDC). Bone marrow-derived dendritic cells (BMDCs) from these mice were exposed to AhR ligands (BP, FICZ and I3S) and showed no changes in activation and co-stimulatory markers, except for downregulation of the DC marker CD11c compared to AhRfl/fl. Despite the induction of AhR transcripts by BP and FICZ in BMDCs, the intensity was lower compared to hMoDCs, which may be due to species-specific effects, especially in the case of tryptophan-derived metabolites. The low level of AhR induced transcripts in AhRfl/fl and AhRΔDC mice can be attributed to the presence of Ahrd allele in them. Analysis of immune cell distribution in AhRΔDC mice revealed an altered frequency of DCs, particularly reduced in lymphoid organs, while no differences were seen in the frequency of DCs in adipose tissue. Other immune cells examined also showed no changes in both lymphoid and non-lymphoid organs. In conclusion, this study highlights the critical role of AhR in DC function, which could open new therapeutic strategies, particularly in relation to autoimmune diseases. These results, particularly the comparison between the mouse and human systems, also highlight the importance of considering species-specific factors and ligand properties, when studying the role of AhR in DCs and how this may ultimately affect the immune response.