Rigorous biocontainment prevents laboratory mice from being infected with natural pathogens and allows researchers to perform animal experiments in a reliable and reproducible manner. However, such specific pathogen-free (SPF) mice do not necessily model immune responses that occur in nature, and this may contribute to the often-reported disconnect between pre-clinical research and human clinical trials. Athough many key advances in biomedical research have been made possible by the study of SPF mice, there is now compelling evidence that microbial or viral experience can greatly impact immune pathophysiology. In past two decades, regulatory T (Treg) cells have emerged as a dedicated immune population crucial for the negative regulation of immune responses. Considering that Treg cell homeostasis and function can be greatly influenced by unique signals present in different tissue environments, it is conceiable that the microbial exposure outside the current laboratory setting could heavily impact Treg cells and their ability not only to establish immunological tolerance against ‘‘self’’ or innocuous foreign antigens, but also to keep in check effector immune responses to pathogens. To gain a better understanding of Treg cell-mediated immune regulation in a more physiological setting, we have worked closely with the Animal Care Program at UC San Diego to establish a new protocol to generate, maintain, and extensively characterize a stable repository of physiological microbe-exposed environmentally conditioned (PC) animals. By taking this approach, two well-establsihed mouse models that affords Treg cell isolation and Treg cell-specific ablation will be first adapted to the “dirty” condition. Next, through performing extensive comparative immune phenotype analysis and single-cell sequencing studies of Treg cells isolated from both lymphoid and non-lymphoid tissues in SPF mice and PC mice, the impact of physiological microbial exposure on Treg cell biology will be determined. Finally, we will examine the suppressor function of Treg cells from PC mice to maintain immune homeostasis and to control ongoing inflammation in comparison with their SPF counterparts. Together, our study will not only further extend our fundamental knowledge of Treg cell-dependent immune regulation but also provide critical insights into the possibilities and limitations of targeting Treg cells to treat a wide array of human immunological diseases. .