The live attenuated yellow fever virus vaccine (YFV-17D) that confers life-long protective immunity is one of the most effective vaccines ever developed. The knowledge gained from understanding the molecular mechanisms of how this vaccine works will provide the platform to rationally develop new vaccines against emerging diseases including dengue virus that is now a major global epidemic. Project 1 (Ahmed/Wilson) will focus on immune memory, using a novel in vivo labeling technique in humans will measure the homeostasis and longevity of YFV-specific T cells, track the migration of YFV-17D-induced T cells to mucosal sites, and examine the relationship between CD4 T follicular helper cells (TFH) response and the type of antibody produced after both YFV-17D vaccination and dengue infection. Project 2 (Pulendran/Rice) will determine the molecular mechanisms by which the yellow fever vaccine strain YFV-17D and the pathogenic Asibi strain, as well as dengue virus, stimulate innate immunity to program adaptive immune responses. They will also examine a new paradigm of how the immune system can sense viruses by detecting stress signals in the environment, and modulate adaptive immunity. Project 3 (Goronzy) will examine signaling checkpoints that are important for CD4 T cell memory differentiation and that are compromised in the elderly and relate the epigenetic changes to these critical signaling checkpoints, in particular the AMPK and SIRT1 signaling pathways. A great limitation in human vaccine studies is the low frequency of antigen-specific cells and their inherent heterogeneity, which is amplified in the elderly who respond poorly to vaccination. The goal of the Technology Development Project (TDP) (Haining/Greenleaf) is to develop and apply sensitive assays to measure the transcriptional profiles and epigenetic state in rare cell populations or even single cells. This technology will be applied to each of the Research Projects. The human samples for the Research Projects and TDP will be provided by Core C (Mulligan/Yu/Chokephaibulkit) which is located at two sites: 1) The Emory Hope Clinic where volunteers will be enrolled into the YFV-17D studies. 2) Dengue Clinical Unit, at Siriraj Hospital, Bangkok, Thailand where dengue infected patients will be enrolled.