EXTRACELLULAR SUPEROXIDE DISMUTASE ENHANCES NEUTROPHIL RECRUITMENT TO THE LIVER BY MODULATING THE EXTRACELLULAR ENVIRONMENT

Listeria monocytogenes (LM) infection represents one of the leading causes of death from foodborne infection, especially in immunocompromised individuals, and can cause spontaneous abortion in pregnant women; however, it is commonly used as a model to study the host immune response against infection. Our lab utilizes three groups of mice with varying levels of extracellular superoxide dismutase (ecSOD) activity (high, wild-type, and none) to determine the impact that this antioxidant enzyme has on the host immune response during LM infection. We have previously shown that ecSOD activity leads to decreased innate immune response against LM. Our current data shows that ecSOD activity protects the extracellular matrix from degradation, and leads to increased neutrophil recruitment; however, these neutrophils display inhibited function and therefore do not provide adequate protection against LM. This research is relevant due to the potential use of ecSOD inhibitors to enhance immune responses during bacterial infection. In addition, these findings help clarify the impact of the oxidative environment on the immune response and how antioxidants contribute to this dynamic. Purpose (a): Listeria monocytogenes (LM) is an intracellular foodborne pathogen that causes severe disease in immunocompromised individuals, spontaneous abortion in pregnant women, and results in ~25% mortality rate in infected individuals. Extracellular superoxide dismutase (ecSOD) converts superoxide into hydrogen peroxide in the extracellular milieu and protects against oxidative stress. We have previously shown that ecSOD activity inhibits innate immune responses during LM infection leading to increased bacterial burden; however, it is unclear whether ecSOD activity affects neutrophil recruitment and function in a cell-intrinsic manner or by modulating the extracellular environment. Methods (b): Congenic mice with high ecSOD activity (ecSOD HI), wild type ecSOD activity (ecSOD WT), or lacking ecSOD (ecSOD KO), on the C57Bl/6 background were used to perform adoptive transfer experiments after intravenous infection with ~10,000 wild-type LM (WTLM). Either isolated neutrophils or labeled whole bone marrow cells were transferred from ecSOD HI or ecSOD KO mice into ecSOD WT mice and then flow cytometry analysis was performed and colony forming units (CFUs) were calculated. Concentrations of hyaluronan and lymphotoxin alpha were determined by ELISA. Results (c): Whole bone marrow cell transfers indicated that there was no difference in recruitment of neutrophils transferred from ecSOD HI or ecSOD KO mice to the liver when the neutrophils were all in the same environment (ecSOD WT mice). In addition, neutrophils isolated from ecSOD HI or ecSOD KO mice showed no difference in their ability to protect against LM infection, as shown by equivalent CFUs, when in comparable environments (ecSOD WT mice). Analysis of hyaluronan concentrations – a component of the extracellular matrix (ECM) – indicated that ecSOD activity protects the ECM from degradation. Conclusions (d): We observed from adoptive transfer experiments that ecSOD activity does not affect neutrophil recruitment or function in a cell-intrinsic manner. Additionally, we determined that ecSOD activity protects the ECM, which is important for neutrophil trafficking. Overall, we concluded that ecSOD activity enhances neutrophil recruitment yet decreases their function by modulating the extracellular environment.