The Role of ecSOD in Phagosomal Containment of Listeria Monocytogenes

Extracellular superoxide dismutase (ecSOD) is a secreted enzyme associated with the extracellular matrix that plays a protective role during reactive oxygen species (ROS) mediated inflammatory responses. Listeria monocytogenes is an intracellular gram positive bacteria that causes listeriosis in infected individuals and animals. EcSOD has been shown to modulate the innate immune response to Listeria infection. With the use of ecSOD congenic mice (ecSOD HI, ecSOD WT, ecSOD KO), we have established that high ecSOD activity leads to decreased bacterial clearance despite an increase in neutrophil recruitment. Whereas, no ecSOD activity results in reduced neutrophil recruitment but effective bacterial clearance. Importantly, neutrophils in the livers of ecSOD KO mice show superior protective capabilities in comparison to neutrophils in ecSOD HI mice. To deduce the mechanism by which ecSOD contributes to neutrophil effector function during Listeria infection, we made use of different strains of GFP expressing Listeria monocytogenes. Neutrophils isolated from both the bone marrow and the liver of the ecSOD congenic mice were able to effectively phagocytose the bacteria. However, we observed that there was more bacterial escape from the phagosome of neutrophils isolated from the bone marrow of ecSOD KO mice in comparison to the ecSOD HI and WT mice. In contrast, there was no observable difference in the ability of neutrophils isolated from the livers of the ecSOD congenic mice to prevent Listeria escape from the phagosome. These data are contradictory to the previously reported protective nature of the ecSOD KO neutrophils and their importance for bacterial clearance from the livers of infected mice. The neutrophils used for the current in vitro studies were isolated from the tissues in the absence of infection. Neutrophils are generally not present in large numbers in tissues in the absence of infection, therefore, we inferred that the isolated neutrophils were not functionally active. In conclusion, our results suggest that our in vitro observations are not supportive of the previous in vivo data regarding the ability of ecSOD to modulate neutrophil function.