Role of Inducible Nitric Oxide Synthase and NADPH Oxidase in Early Control of Burkholderia pseudomallei Infection in Mice

Infection with the soil bacterium Burkholderia pseudomallei can result in a variety of clinical outcomes, including asymptomatic infection. The initial immune defense mechanisms which might contribute to the various outcomes after environmental contact with B. pseudomallei are largely unknown. We have previously shown that relatively resistant C57BL/6 mice can restrict bacterial B. pseudomallei growth more efficiently within 1 day after infection than highly susceptible BALB/c mice. By using this model, our study aimed to investigate the role of macrophage-mediated effector mechanisms during early B. pseudomallei infection. Depletion of macrophages revealed an essential role of these cells in the early control of infection in BALB/c and C57BL/6 mice. Strikingly, the comparison of the anti-B. pseudomallei activity of bone marrow-derived macrophages (BMM) from C57BL/6 and BALB/c mice revealed an enhanced bactericidal activity of C57BL/6 BMM, particularly after gamma interferon (IFN-γ) stimulation. In vitro experiments with C57BL/6 gp91phox⁻/⁻ BMM showed an impaired intracellular killing of B. pseudomallei compared to experiments with wild-type cells, although C57BL/6 gp91phox⁻/⁻ cells still exhibited substantial killing activity. The anti-B. pseudomallei activity of C57BL/6 iNOS⁻/⁻ BMM was not impaired. C57BL/6 gp91phox⁻/⁻ mice lacking a functional NADPH oxidase were more susceptible to infection, whereas C57BL/6 mice lacking inducible nitric oxide synthase (iNOS) did not show increased susceptibility but were slightly more resistant during the early phase of infection. Thus, our data suggest that IFN-γ-mediated but iNOS-independent anti-B. pseudomallei mechanisms of macrophages might contribute to the enhanced resistance of C57BL/6 mice compared to that of BALB/c mice in the early phase of infection.