Mechanisms of IFN-γ and ceftazidime interaction for synergistic killing of Burkholderia
Date
2014
Authors
Mosovsky, Kara, author
Dow, Steven, advisor
Schenkel, Alan, committee member
Schweizer, Herbert, committee member
Callan, Robert, committee member
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Abstract
Burkholderia pseudomallei is a Gram negative, facultative intracellular pathogen which infects both phagocytes and non-phagocytes and causes severe acute infections in humans and animals. Due to its inherent resistance to many classes of antibiotics, new therapies are needed which can supplement or substitute for conventional treatments in order to combat this emerging infectious disease. We have previously shown that interferon (IFN)-γ can interact with the conventionally administered antibiotic, ceftazidime, to synergistically control intracellular bacteria burden of Burkholderia infected macrophages. The goal of the studies presented here was to determine the mechanism by which IFN-γ and ceftazidime exert their synergistic effect. After investigating several potential mediators of immuno-antimicrobial synergy, we showed that IFN-γ stimulation of macrophages led to increased generation of reactive oxygen species (ROS), which led us to hypothesize that IFN-γ induced ROS may interact with ceftazidime to control intracellular bacterial burden. We next found that ROS scavenging antioxidants such as N-acetylcysteine (NAC) and reduced glutathione (GSH) were capable of reversing the IFN-γ and ceftazidime synergistic effect, while the ROS-inducing drug buthionine sulfoximine (BSO) could not only potentiate the synergy, but could completely substitute for IFN-γ to synergize with ceftazidime and control intracellular bacterial burden. These results were consistent with a ROS interaction with ceftazidime. We further showed that IFN-γ prevented vacuolar escape and actin polymerization, a finding which was recapitulated with BSO. Taken together, these results suggested that generation of IFN-γ induced ROS responses synergized with ceftazidime to enhance control of intracellular bacterial burden. IFN-γ induced ROS was also responsible for preventing vacuolar escape and therefore may have limited intracellular replication and spread of infection. In the second half of our study we identified and then investigated the separate and compartmentalized contributions of IFN-γ and ceftazidime to the overall synergistic effect. We determined that ceftazidime alone controlled extracellular killing in our macrophage infection model while IFN-γ alone controlled the killing of Burkholderia in the intracellular compartment. We confirmed a role for IFN-γ induced ROS responses to kill intracellular bacteria and control intracellular replication, though we also conclude that other IFN-γ-dependent and ROS-independent pathways are at play. Overall we suggest a new model to describe the dynamics of the classically used macrophage infection model. We suggest that both intracellular and extracellular control of bacteria is required for the overall synergistic effect we see with combination of IFN-γ and ceftazidime. Together our studies have implications for the use of IFN-γ, or other ROS-inducing drugs, as non-specific antibiotic potentiating agents for enhanced clearance of bacterial pathogens.
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Subject
Burkholderia
macrophage
pathogenesis
reactive oxygen species
ceftazidime
IFN-gamma