Project Summary/Abstract Memory CD8 T cells with varying functional characteristics are generated after infection or immunization. We previously defined a population of T cells within the CD62Llo effector memory compartment that express high levels of effector molecules and continue to persist into the memory phase. Although over time in specific pathogen free mice LLECs tend to wane in number, they represent a substantial fraction of CD8 memory T cells in ‘dirty mice’ and dominate the secondary and tertiary memory pools. Importantly, our prior work showed that these ‘long-lived effector cells’ (LLEC) are the most robust memory T cells for mediating antigen-specific clearance of systemic viral and bacterial pathogens. Our recent RNA sequencing data indicates that LLEC may achieve this through unique expression of multiple NK cell-associated receptors as well as chemokine and trafficking molecules that may enforce their strict localization to the vasculature at the steady state. In this proposal we will determine: 1) if LLECs participate in tissue-initiated infections through either extravasation or from their position within the vasculature, 2) if NK cell receptors modulate LLEC function, and 3) if the LLEC subset uniquely thrives during inflammation for its persistence. Our proposal leverages recent transcriptional analysis along with innovative mouse models and technical approaches to understand the signals governing how circulating memory T cell populations mediate protective immunity. We predict our studies will expose novel considerations for generating robust memory T cell function, ultimately leading to improved vaccination and immunotherapy approaches.