ABSTRACT It has been believed that infection-fighting T cells primarily use glucose to fuel host defense against viruses and bacteria; however, using new in vivo metabolite profiling techniques, it has recently been found that glucose is not the primary fuel for mitochondrial energy (ATP) production in CD8+ T cells responding to infection in vivo, particularly in later phases of infection. This presents the immunometabolism community with a critical need to define and understand the metabolites that sustain infection-fighting CD8+ T cells. This knowledge is crucial for developing effective immuno-metabolic interventions that strengthen host defense and/or enhance immunization strategies. Preliminary data point to ketone bodies—a metabolite class enriched in the blood in response to infection, fasting, and certain diets—as a key fuel for CD8+ T cell effector function, with data suggesting that ketone body availability influences T cell-mediated adaptive immunity by regulating effector function (i.e., cytokine production) at the epigenetic level. However, the full impact of T cell-intrinsic ketone body metabolism on CD8+ T cell responses remains uncharacterized. OVERALL OBJECTIVE: to characterize the mechanisms by which T cell-intrinsic ketone body metabolism (“ketolysis”) impacts CD8+ T cell metabolism and function during immune responses to infection, and to determine the role of systemic ketone body metabolism in CD8+ T cell function and host defense. HYPOTHESIS: T cell-intrinsic ketone body metabolism is a non-redundant pathway supporting host defense by fueling and directing CD8+ T cell bioenergetics and effector function in vivo. SPECIFIC AIMS: (1) Characterize ketone body metabolism in CD8+ T cells and its impact on CD8+ T cell-mediated immune responses, (2) Deconstruct the mechanisms by which ketone body metabolism impacts T cell effector function, (3) Dissect the impact of systemic ketone body metabolism on CD8+ T cell responses. IMPACT: Upon completion, this proposed research will have determined how activated CD8+ T cells utilize ketone bodies, dissected the impact of ketone bodies on CD8+ T cell effector function (primary infection clearance) and memory responses (long-term immunity), and defined how systemic ketone body availability is controlled and impacts CD8+ T cell-mediated host defense. By defining the role of ketone body metabolism in protective CD8+ T cell-mediated immune responses, we will have laid the groundwork for defining dietary and pharmacological interventions to optimize endogenous host defenses to boost clearance of infections as well as enhance immunization strategies.