Project Summary: The waning of immunity has a major impact on morbidity and mortality in the aged. Antigen-specific, acquired, and especially T-cellular is important in host defense against chronic intracellular pathogens and cancer. Infections with these microbes and skin cancers with high mutational burdens and neoepitope loads, are disproportionally high in the elderly. In this application, the investigative team uses an established tetramer and TCR toolkit for the study of T-cell responses to varicella zoster virus infections (VZV) in humans to determine the mechanisms of age-related susceptibility to shingles, and the mechanisms of action of a uniquely successful vaccine that retains efficacy in the elderly. Recently, it has been appreciated that host defense against many localized infections is mediated by special populations of tissue resident memory cells, abbreviated TRM. As murine and human studies have advanced, the lineage of TRM, TRM subsets, and the transcriptional and metabolic signature of TRM in various tissues have begun to come into focus. TRM are locally mobile, patrol tissue for antigen, and can proliferate upon antigen re-exposure, yet do not re-enter the blood and are out of reach of blood-based studies. Notably, studies of human antigen-specific TRM are rare, such that assays of overall TRM populations may overlook the complexity of TRM. Here, we use VZV infection and vaccination as related probes of TRM T-cells across the age spectrum. Aim 1 focuses on endogenous reactivation of VZV and uses biopsies of healed shingles and control skin to, for the first time, fully characterize helpful human virus-specific TRM at the single cell level. Aim 2 uses the new RZV vaccine for shingles prevention, which retains efficacy even in aged adults, to determine if the vaccine leads to TRM seeding in the skin, improvement in the pool of homing-committed cells in the blood, or both. Taken together, these studies will increase our understanding of age- related changes in vital effector T-cells and strategies that may be useful to overcome immune senescence.