Phagocytic oxidative burst generates reactive oxygen/nitrogen species (ROS/RNS) that are directly involved in killing bacterial pathogens, but can also cause tissue damage when produced in excess. Phagocytic cells utilize the NADPH oxidase to produce superoxide (O2 -), while nitric oxide (NO) is produced by the iNOS enzyme. Extracellular superoxide dismutase (ecSOD) is an anti-oxidant enzyme that converts O2 - into hydrogen peroxide (H2O2), thus protecting tissues from oxidative stress and immune-mediated inflammation. Although it is well established that neutrophils are required for the clearance of bacterial pathogens, it is not known how ecSOD impacts the function of neutrophils. Listeria monocytogenes (Lm) is an intracellular bacterium that is widely used as a model pathogen to study immune responses and host/pathogen interactions. Lm infection causes a very high mortality rate (~25%), and is a serious public health concern. Clinical infection with Lm can cause spontaneous abortions in women, and gastroenteritis, septicemia, endocarditis, and meningitis in immunocompromised individuals. Using novel congenic mice that express varying levels of ecSOD activity (ecSOD HI, ecSOD WT, and ecSOD KO), we have recently demonstrated that the presence of host ecSOD significantly decreases neutrophil function and host resistance to Lm infection. These novel observations have led to our central hypothesis: production of ecSOD by parenchymal cells inhibits bacterial killing by neutrophils, thus leading to reduced clearance of Lm. In vitro, ex vivo, and in vivo approaches will be utilized to test the following specific aims. Specific Aim 1: To determine how ecSOD inhibits protective neutrophil responses during Lm infection. Our working hypothesis is that phagocytosis of ecSOD by neutrophils results in decreased oxidative burst and bacterial killing during Lm infection. Specifi Aim 2: To determine whether ecSOD production by hematopoietic or parenchymal cells decreases the clearance of Lm. Our working hypothesis is that parenchymal cell production of ecSOD decreases resistance to Lm by inhibiting neutrophil responses. Upon completion of the aims of this small, yet conceptually innovative R03 grant, we will gain insight into the mechanism of ecSOD-mediated inhibition of neutrophil function during Lm infection. These studies will prompt translational research that leads to treatment options during bacterial infection and vaccination. Modulating host ecSOD activity could be beneficial during a myriad of infectious and inflammatory disease states involving neutrophils. Importantly, there are polymorphisms in the human ecSOD gene which lead to altered activity of the enzyme and impact disease susceptibility. Therefore, our studies, and future therapies resulting from them, may have a direct impact on human health and disease.