SUMMARY The fundamental role of neutrophils in host defense against pathogens is well established from animal models of infection and the increased susceptibility to infection in neutropenic patients. Despite this, many aspects of neutrophil biology remain underexplored. The metabolic programs needed to support antimicrobial neutrophil functions are poorly understood and the degree of plasticity these cells may achieve in disease states is not known. We have leveraged unique approaches to identify a novel association between increased infection risk in patients and a polymorphism in Cpt1a. Carnitine palmitoyltransferase 1a (Cpt1a) is required for mitochondrial metabolism of long chain fatty acids. Using a pharmacologic inhibitor of CPT1a and multiple murine models of bacterial pneumonia, we have demonstrated that CPT1a inhibition increased susceptibility to infection by decreasing neutrophil mobilization to, and activation at, the site of infection. We provide additional preliminary data to support a model whereby neutrophil mitochondria metabolize fatty acids to generate ATP, which is necessary for amplification of activating signals required for neutrophil activation, chemotaxis, and antibacterial functions. Based on these findings, we will test the overall hypothesis that fatty acid metabolism is an essential metabolic program for neutrophil trafficking and antibacterial defense in the lung. Specifically, we will test the sub-hypotheses that (i) fatty acid metabolism is an important mechanism for mitochondrial ATP production in neutrophils, (ii) fatty acid-derived mitochondrial ATP is required for amplification of neutrophil activating signals, and (iii) disruption of fatty acid metabolism impairs neutrophil activation, chemotaxis, antibacterial defenses, and neutrophilic lung inflammation. Specific Aim 1 will define the role of fatty acid metabolism on neutrophil energetics, migration, and effector functions. This work will establish the contribution of fatty acid metabolism to mitochondrial ATP production in neutrophils, define the role of fatty acid metabolism in neutrophil activation, and determine the importance of fatty acid metabolism for neutrophil antibacterial functions. Specific Aim 2 will determine the effect of inactivation of fatty acid metabolism on neutrophilic lung inflammation. This work will use multiple single-cell approaches to define neutrophil plasticity and activation heterogeneity in the infected lung microenvironment. Further, this Aim will determine the effects of fatty acid metabolism inhibition on neutrophilic lung inflammation. Completion of these Aims will establish fatty acid oxidation as an essential metabolic program to support antimicrobial neutrophil functions and add to the current prevailing paradigm that neutrophils rely solely on aerobic glycolysis as a metabolic program. Further, these studies will determine the functional consequences of neutrophil heterogeneity at the site of infection and establish neutrophil plasticity as a potential contributor to disease states. Finally, these aims will provide a mechanistic framework for the clinical association between a Cpt1a polymorphism existing in the human population and increased infection risk.