Regulatory T cells (Tregs) are formed through the expression of the transcription factor Foxp3 in T cells, resulting in the rewiring of the cell function into an immunosuppressive phenotype. Recently, it has been proposed that Tregs also have additional tissue-specific physiological roles when resident in different tissues. For example, tissue-specific Tregs residing in the muscle and adipose tissue possess immunological and non-immunological functions in these tissues, distinct from the generic Tregs in circulation. Currently, research into tissue-specific functions of Tregs, or any other migratory cell type, is limited by the available research tools. This project generated new synthetic biology tools for enriching tissue-resident cells and then applied these tools to the study of tissue-resident Tregs in the brain, lung, liver, kidney and pancreas, thus creating a comprehensive atlas of tissue-specific functions. We developed a new system for gene delivery of a Treg-specific survival factor in a tissue-specific manner, allowing this directed expansion and functional testing. We found a key role of brain-resident Tregs in preventing and repairing neurological damage. These studies were extended by systematic molecular, cellular and kinetic analysis using existing innovative methods established in the laboratory. Our tissue-specific gene delivery system will have a profound impact on immunology beyond the direct scope of the project, by providing a proof-of-principle approach for study tissue-specific immunology.