Preventing, delaying, or reversing aging-related thymic involution is a widely sought-after therapeutic goal with the potential to significantly improve immune function and health-span in aging humans. Thymic involution initiates in childhood, and is progressive thereafter, continuing to produce T cells that develop in a deteriorating microenvironment and raising the possibility that the process of involution itself may produce sub-functional T cells. Studies of involution that compare old with young mice investigate only the end stage of involution. The thymus is also highly sensitive to many kinds of stress. The very different life histories of mice in a controlled environment and humans living in the world could significantly influence the rate and degree of thymus involution, and the capacity for rebound. There is accumulating evidence that changes in the expression of the FOXN1 transcription factor may directly regulate initial involution. Our preliminary data show that the Atf3 stress response gene is suppressed by FOXN1, up-regulated in thymic epithelial cells (TEC) with aging, and may mediate some key aspects of thymic involution. These data directly implicate stress as a potentially important aspect of involution that is understudied in animal models, but may impact efforts to modulate involution in humans. This project is based on the premise that developing mouse models of thymic involution and immunosenesensce to include parameters that more accurately mimic the human condition will generate more relevant data for devising therapeutic strategies in humans. Based on this premise we propose to use novel computational approaches to generate a data-driven comparison of human and mouse thymic involution, to test whether a mouse strain with accelerated involution better mimics the effects of thymic involution on peripheral T cells in humans, and to investigate the effects of stress on age-associated involution. We will also test the hypothesis that the TEC response to repeated stress compromises the capacity for rebound with aging, and that the Atf3 stress response gene is up regulated with aging and promotes key aspects of thymus involution. These experiments will directly address the differences between mouse and human lifespan and life history, to develop and test new models for investigating thymic involution and its effects on peripheral T cell changes with aging. Together with Projects 3 and 4 and Cores A-C, the human-mouse comparisons will generate the Human-Mouse Timeline that will be not only a key resource to the Program, but a valuable reference for the community. This and other aspects of this Project will synergize with other Project outcomes and inform the design and interpretation of interventions testing in Core D.