Project Summary/Abstract Pancreatic cancer liver metastasis (PCLM) is present in 50% of patients diagnosed with pancreatic ductal adenocarcinoma (PDAC) and has a median survival of <6 months. Recent studies in mice found that modulating the gut microbiome can reduce PCLM tumor burden by augmenting anti-tumor immunity. The gut microbiome is constantly producing antigens the travel to the liver. This suggests that gut microbiome modulation induces PCLM anti-tumor immunity by altering the antigens presented in the liver, however the mechanism is unclear. Mucosal Associate Invariant T (MAIT) cells are an innate subset of T cells which can be classified into two phenotypes: an “anti-tumor” MAIT1 or a “pro-tumor” MAIT17 phenotype. MAIT cells closely interact with gut microbiome derived antigens through their activation mechanisms, and this can shift their phenotype. Direct activation leads to a MAIT17 phenotype, involving interactions with microbial riboflavin synthesis pathway- derived metabolite ligands presented on antigen presenting cells (APCs) via MHCI related protein (MR1). Indirect activation induces a MAIT1 phenotype by stimulating MAIT cell IL-12 and IL-18 receptors15. These cytokines are released by APCs upon stimulation by microbial-produced ligands. Consequently, the goal of this project is to harness the gut microbiome to modulate MAIT cell phenotypes towards treatment of PCLM. Preliminary data suggests that MR1 expression is upregulated in the PCLM tumor microenvironment (TME) which promotes a pro-tumor MAIT17 phenotype via direct MAIT cell activation. This suggests an abundance of gut microbiome derived MR1 ligands in the PCLM TME. In Aim 1, we hypothesize that gut microbiome dysregulation provides MR1 ligands to the PCLM TME which promotes PCLM via inducing a MAIT 17 phenotype. We will test this by 1) Correlating the abundance of MR1 ligands in the PCLM TME with the MAIT17 phenotype; 2) Determining the gut microbiome composition of mice with PCLM by shotgun metagenomic sequencings; 3) characterizing how a MAIT17 phenotype promotes PCLM tumor burden by single cell RNA sequencing. Further preliminary data shows that promoting a MAIT1 phenotype reduces PCLM tumor burden. Also, gut microbiome depletion by antibiotic treatment (ABX) promotes a MAIT1 phenotype, which likely promotes anti- tumor TH1 cells, in the liver. In Aim 2, we hypothesize that ABX reduces MR1 ligands in the PCLM TME which promotes a MAIT1 phenotype and anti-tumor immunity. We will test this by 1) Investigating the MAIT cell phenotype in PCLM+ABX mice and validating by fecal microbiome transfers; 2) Determining the gut microbiome composition following ABX; 3) Probing anti-tumor immunity in the PCLM+ABX TME by imaging mass cytometry. These studies will identify MAIT cells as the missing link between the gut microbiome and anti-tumor immunity and will affect how PCLM and other liver conditions are treated. This research plan and my outlined training plan are supported by e...