The five-year overall survival of gastric cancer patients is still low due to limited treatment response. Prognosis and therapy success are highly influenced by the tumor microenvironment and the kind of infiltrating immune cells found. Thus, the presence of regulatory T cells (Treg cells) is considered to favor tumor development and progression by dampening antitumor immunity. In contrast to that, also several cases have been reported where a positive disease outcome correlated with massive Treg infiltrations in the tumor. Different Treg cell subsets exist, which can explain discrepancies on the prognostic value of Treg cells and their influence on treatment response in gastric cancer. This T cell subset is already observed during Helicobacter pylori infection, which is a main risk factor for gastric cancer development. Even though a lot of evidence supports a crucial role for Treg cells in the progression and outcome of gastric disease, a clear characterization of these cells is still lacking. More importantly, identification of genetic programs related to Treg cell function that could be manipulated to enhance their functionality by reducing their immunosuppressive potential has not been addressed. Therefore, we propose to systematically characterize Treg cell subsets in human gastric normal and tumor tissue to identify transcriptional networks dictating functionality of these cells. Using cutting edge scRNA sequencing, ATAC-sequencing and by manipulating cells by CRISPR/Cas9 technology, we aim at generating T reg cells that will be tested for functionality in killing assays using human gastric tumor organoids. These approaches will allow us to develop further novel therapeutic strategies to treat gastric cancer patients and increase their survival. This project is conducted in collaboration with Prof. Schumann’s group (https://mikrobio.med.tum.de/en/home/research/schumann-group) and is supported by the German Cancer Aid (Krebshilfe 70114850).