PROJECT SUMMARY Intestinal health relies on the homeostatic function of intestinal macrophages in controlling gut inflammation. Defects in establishing this macrophage function can lead to unresolved inflammation as seen in inflammatory bowel disease (IBD). Recent studies have highlighted the impact of tissue microenvironments on establishing macrophage tissue-specific functions. Of specific interest, metabolites produced by gut bacteria, such as short-chain fatty acids and secondary bile acids, exert profound immunomodulatory effects on macrophage functional polarization. They suppress the expression of pro-inflammatory cytokines and transform macrophages to anti-inflammatory phenotype. What remains lacking, however, is the knowledge of how macrophages sense bacterial metabolites and mediate their immunomodulatory effects, especially in the gut microenvironment. Cellular metabolism regulates macrophage functions. We have previously demonstrated that macrophage pro-inflammatory response can be regulated by controlling metabolic substrate uptake. We therefore propose that metabolite sensing in macrophages is mediated via coordinated expression of transport proteins, which transport specific metabolites across plasma membranes and allow them to integrate into intracellular metabolism or to be directly sensed by intracellular receptors. Our long-term goal is to identify transporter targets that promote bacterial metabolite sensing and macrophage homeostatic function in order to control intestinal inflammation. Our preliminary data have indicated that macrophages exhibit distinct transporter reprogramming during functional polarization. There, we identified that SLCO3A1, an organic anion transporter and a recently discovered IBD-associated gene, is specifically upregulated during macrophage pro-inflammatory activation. Overexpression of SLCO3A1 enhances bile acid uptake. Also, the expression of SLCO3A1 is specific for tissue-resident macrophages from both small and large intestines as compared to other tissue-resident macrophages. Based on these observations, the overall goal of this project is to define the role of macrophage SLCO3A1 in metabolite sensing and intestinal tissue homeostasis. Our central hypothesis is that the expression of SLCO3A1 in macrophages facilitates the sensing of bacterial metabolites that promotes macrophage homeostatic function and the prevention of IBD. This project will focus on the following specific aims: (1) Define the mechanism by which SLCO3A1 regulates bile acid sensing and its immunosuppressive effect in macrophages. (2) Determine the role of SLCO3A1 in intestinal macrophages and the development of IBD. Altogether, this project will elucidate the mechanisms by which SLCO3A1 regulates the sensing of bacterial metabolites and promotes the homeostatic function of macrophages in controlling intestinal inflammation. We believe that completion of this project will provide mechanistic insights into important principles th...