Thymic involution constitutes a hallmark of an aging immune system. The thymus is comprised of thymic epithelial cells (TECs), fibroblasts and blood-borne stromal cells. Adoptive transfers of young and old hematopoietic stem cells into young vs old recipients has led to the conclusion that impaired thymopoiesis in the aged is largely the consequence of an aged thymic microenvironment. TEC differentiation, proliferation, and survival are controlled by cell intrinsic and extrinsic factors. Although much work has been focused on thymus development during embryogenesis, comparatively little is known about the mechanisms responsible for thymic involution. This especially pertains to TECs, which we hypothesize are the major impediment to thymopoiesis during aging. We now propose to fill this critical gap. Our specific aims are: Aim 1: To determine the key transcription factors & signaling pathways in TECs affected by aging. In aim 1A, we will interrogate physiological characteristics of aged vs young TEC subsets. In aim 1B, we hypothesize that key cell surface receptor and intracellular transcriptional pathways that regulate thymic development will be important for TEC maintenance and regeneration after genotoxic stress in aged mice. In aim 1C, we hypothesize that transcriptional regulation by micro-RNAs is an important component of TEC aging. Studies of loss-of-function in TECs alone will be performed in aims 1B and 1C. Aim 2: To test the hypothesis that genetic models of perturbations of the aging process will provide key insights as to the mechanisms responsible for TEC aging. We will identify common features affecting TECs between chronologically aged mice and those with premature aging and distinguishing features in a model of delayed thymic aging. As indicated, drugs that stimulate TECs as probes for assessing residual TEC function in aging.