The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor regulating xenobiotic responses as well as physiological metabolism. Besides environmental polyaromatic chemicals, dietary AhR ligands and physiological metabolites activate the AhR. AhR signaling is negatively regulated by the AhR Repressor (AhRR) in a cell type-specific manner. We have shown that global AhR- as well as AhRR-deficient mice are protected from diet-induced obesity (DIO). Althoughmice lacking either AhR or AhRR in myeloid cells showed normal weight gain inDIO, deficiency of AhRR reduced hepatosteatosis as well as liver damage. Incontrast, mice with a myeloid cell-specific knockout of the AhR developedfibrosis in the er and gonadal adipose tissue. In the current fundingperiod, we aim to characterize the involvement of the AhR/AhRR signaling axisin the progression of diet-induced fibrosis. To this end, we will study thecrossalk between liver myeloid cells and hepatic stellate cells (HSC), knowne associated with the development of liver fibrosis. We will also performsingle-nuclei RNA-seq of non-parenchymal cells in the liver of conditional AhR-or AhRR-deficient mice in the presence or absence of dietary AhR ligandsto detect cell type specific changes in cellular programming. Spatialinformation about the affected cell types will be added using co-detection byindexing technology as well as spatial transcriptomics. Similar analyses willtended to the gonadal adipose tissue where severe fibrosis was alsoobserved in conditional AhR-deficient mice fed high fat diet. Mechanisticn the functional interaction of macrophages and HSC will be performedin vitro using 2D and 3D organoid culture systems. Using these approaches, weintend to define the AhRR as a potential target for prevention of liver damageand fibrosis.