Elucidating the Mechanistic Role of Microbiome-Derived Acetate in Sepsis-Induced Lung Injury - PROJECT SUMMARY Despite evidence that gut microbiota predict clinical outcomes in patients with sepsis, we understand little of how microbes mediate these outcomes. Among ICU patients with sepsis, gut microbiota composition predict disease trajectory, organ failure, and mortality; these findings have been recapitulated in animal models of sepsis and acute respiratory distress syndrome (ARDS). Empiric antibiotics with otherwise comparable antimicrobial coverage that differ in their effect on anaerobic gut commensals also differ in their associated risk of mortality and organ failure in sepsis and lung injury. Since we do not know the mechanisms by which commensal microbes mediate sepsis-induced lung injury, our therapeutic modulation of the microbiome is limited. One plausible pathway by which commensal gut microbiota affect extraintestinal organs, such as the lungs, is through the production and systemic release of bioactive metabolites. Microbiome-derived short-chain fatty acids (SCFA) are produced in the gut, enter the systemic circulation, and predict outcomes in critical illness. Using an in vivo model, I have found that in sepsis, acetate (the most abundant SCFA) decreases in the gut, increases in the blood, and correlates with disease severity. Acetate is produced by both microbes and host cells and the contribution of gut microbiota to the systemic surge in acetate during sepsis has not been adequately determined. Acetate is also found in the lungs where it has known biological effects; however, its role in sepsis-induced lung injury (S-ALI) is incompletely understood. The objective of this study will be to identify mechanisms by which the gut microbial metabolite acetate modulates S-ALI pathogenesis. To test my central hypothesis that concentrations of acetate in the blood and lungs are determined by gut microbiota and modulate the severity of septic lung injury, I will complete the following Specific Aims: (1) quantify the contribution of gut microbiota to acetate levels in the gut, blood, and lung, and (2) define how acetate modulates the severity of S-ALI. In Aim 1, I will experimentally manipulate gut microbiota and measure acetate in the gut, blood, and lung to identify correlations between gut bacterial taxa and acetate levels. I will use radiolabeled acetate to trace gut-derived acetate from the intestine to systemic and pulmonary compartments in sepsis. In Aim 2, I will augment systemic acetate in a model of S-ALI and measure changes in alveolar leak and lung histopathology. I will treat ex vivo precision-cut lung slices with acetate and measure changes in histopathology and inflammation in response to immune stimulation. Using complementary in vivo and ex vivo models, I will identify and interrogate biologically plausible mechanisms by which the bacterial metabolite, acetate, contributes to the pathogenesis of sepsis-induced lung injury. This proposed research and F30 award will be instrumental in my achievement of a PhD in microbiology, in furthering our understanding of host-microbiome interactions, and preparing me for a postdoctoral research-track clinical residency.
