OVERALL- Abstract Infectious disease cancer and autoimmune disorders affect hundreds of millions of older adults. Theyreduce length and quality of life across the globe and inflict a massive economic burden on society as vividlyexemplified by the SARS-CoV-2 pandemic that has claimed 93.1% of its victims amongst those 50 years andolder and 74.4% in those 64 and older. Yet despite decades of research restoring protective immunity inolder adults has remained elusive. One critical factor contributing to age-related immune decline is a loss ofnave T (Tn) cell numbers and function and their rejuvenation is highly desirable in order to enhance protectiveimmunity and overall healthspan in older adults. The renewal of this T cell Rejuvenation Program Project is centered on two key questions: (1) why do Tncell numbers and function deteriorate with age; and (2) what can be done about it? The premise of the programis that Tn cell aging is multifactorial and that it can only be resolved by targeting multiple defects. Thymicinvolution and the resulting decline in T cell production is the earliest event leading to immunosenescence.This reduction is compounded by a decline in bone marrow function as well as by defects in Tn cellmaintenance and function in the periphery. These deficiencies combine to erode the ability of the older immunesystem to detect and eliminate infectious agents and neoplastic cells and to properly guard againstautoimmunity. In the first program period we strongly confirmed our initial hypotheses that lymphoid organstromal elements deteriorate earlier than previously thought and in a manner to decisively erode immunitywith aging. We will build on the synergy and success of the initial program period where discoveries in oneproject are critically informing science in others and continue to identify mechanistic reasons behind reducedcentral and peripheral lymphoid organ function with aging. We will then develop combined strategies toameliorate these defects to improve immune defense in the elderly. Our hypothesis is that mechanisticdissection of defects in both thymic production AND peripheral Tn cell maintenance is required to devise andtest effective interventions for T cell rejuvenation in the elderly. Three integrated projects led by experts in the field supported by four cutting-edge cores will test thishypothesis and achieve the following Program Goals: 1. Define mechanistic changes in primary and secondarylymphoid organ aging; 2. Determine the endogenous regenerative capacity of thymic and secondary lymphoidorgan stroma over the lifespan; 3. Relate the progression of murine thymus lymph node (LN) and T cell agingphenotypes to humans; and 4. Devise and test rejuvenation strategies to improve thymopoiesis T cell survivaland peripheral T cell maintenance and function so as to enhance protective immunity. Over this support period the above goals will provide a wealth of basic knowledge that will be translated topreclinical models and be poised for translation to older adults.