Naïve T cells (TN) are produced in the thymus, but require peripheral mechanisms to maintain their homeostasis and function. The premise of this project is that, while thymic involution is a proximal cause of reduced TN numbers with aging, defects in peripheral maintenance mechanisms in secondary lymphoid organs (SLO) also significantly contribute to immunosenescence. Surprisingly, the nature and the causes of these defects that impair peripheral TN cell maintenance with aging remain largely unexplored. This project will test the hypothesis that defects in the peripheral maintenance of naïve T cells in the SLO must be understood and corrected to achieve lasting immune rejuvenation. This hypothesis is based on our recently published and preliminary findings that aging leads to disorganization of lymph node (LN) architecture resulting in a dysfunctional stromal cell compartment, including the fibroblastic reticular cell (FRC) network in the T cell zone. These defects consequently led to impairment in production and presentation of key cytokines essential for optimal cell-cell interactions critical for recruitment and maintenance of TN cells. In principle, the age-related changes/defects in TN cell maintenance could be due to changes/defects in TN cells themselves, or the SLO microenvironment or the circulatory factors surrounding TN and SLO . Therefore, we will examine how aging impairs maintenance of TN cells over the lifespan at the levels of: newly produced TN cells (SA1), stromal SLO elements (SA2) and the circulatory milieu (SA3). Once the defects are dissected, we will formulate interventions that improve peripheral T cell maintenance in aged organisms. These interventions will be tested by Core D, individually or combined with thymic rejuvenation treatments coming from P1-3, for the ability to improve protective immunity against infection. This project will also contribute to the generation of the Immune Aging Timeline, that will correlate age-related changes in thymus, LN and peripheral T cells in mice to those in humans, providing direct preclinical data that pave the way for human T cell rejuvenation in older adults.