Burkholderia pseudomallei Interferes with Inducible Nitric Oxide Synthase (iNOS) Production: A Possible Mechanism of Evading Macrophage Killing

Abstract

Burkholderia pseudomallei is a causative agent of melioidosis, a life threatening disease which affects humans and animals in tropical and subtropical areas. This bacterium is known to survive and multiply inside cells such as macrophages. The mechanism of host defense against this bacterium is still unknown.In this study, we demonstrated that B. pseudomallei exhibited unique macrophage activation activity compared with Escherichia coli and Salmonella typhi, The mouse macrophage cell line (RAW 264.7) infected with B. pseudomallei at MOl of 0.1:1, 1:1 and 10:1 did not express a detectable level of inducible nitric oxide synthase (iNOS). Moreover, the B. pseudomallei infected cells released TNF-a only when they were infected with high MOl (10:1). Unlike the cells infected with B. pseudomallei, the cells infected with E. coli, and S. typhi expressed iNOS even at MOl of 0.1:1. These infected cells also released a significantly higher level of TNF-a at the low MOl ratio. The cells that were preactivated with IFN-oy prior to being infected with B. pseudomallei exhibited an enhanced production of iNOS and TNF-a release. The increased macrophage activation activity in the presence of IFN -oy also correlated with the restriction of the intracellular bacteria survival. Moreover, IFN-oy also prevented cell fusion and multinucleated cell formation induced by B. pseudomallei, a phenomenon recently described by our group. Altogether, these results indicate that internalization of B. pseudomallei failed to trigger substantial macrophage activation, a phenomenon which could prolong their survival inside the phagocytic cells and facilitate a direct cell to cell spreading of B. pseudomallei to neighboring cells.

Burkholderia pseudomallei-Induced Expression of a Negative Regulator, Sterile-α and Armadillo Motif-Containing Protein, in Mouse Macrophages: a Possible Mechanism for Suppression of the MyD88-Independent Pathway

“…3). However, intracellular suppression of B. pseudomallei observed in SARM-depleted macrophages was not as efficient as IFN-␥ treatment, as previously reported by our group (28,30). It is possible that regulation of SARM by B. pseudomallei may be only partially involved in inhibition of gene expression downstream of the MyD88-independent pathway.…”

“…Although the mechanism underlying macrophage activation by TLR signaling in B. pseudomallei infection has not been fully elucidated, we previously demonstrated that this bacterium fails to activate IFN-␤ production, leading to the suppression of iNOS expression and the failure to inhibit intracellular growth of B. pseudomallei (29). However, the addition of exogenous IFN-␤ or IFN-␥ could restore the macrophage's ability to activate iNOS expression and result in enhanced killing of intracellular B. pseudomallei (28)(29)(30). In contrast, an LPS mutant lacking the O-antigenic polysaccharide moiety is more susceptible to macrophage killing, because this mutant is able to activate IFN-␤ production (2).…”

“…Although macrophages play a critical role in innate immunity against a number of bacteria, B. pseudomallei can survive and replicate inside these phagocytes (30). We previously demonstrated that beta interferon (IFN-␤), an effector downstream of the MyD88-independent pathway, is essential for intracellular killing of B. pseudomallei (29).…”

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“…The results suggested that rpoS plays a critical role in resistance to hydrogen peroxide in B. pseudomallei. In addition, superoxide dismutase and catalase enzymes significantly inhibit macrophage bactericidal activity against B. pseudomallei in vitro suggesting the importance of antioxidant enzymes for the intracellular survival of this pathogen 4,5 . .…”

Suppression of Host Innate Immune Response by Burkholderia pseudomallei through the Virulence Factor TssM

“…B. pseudomallei has been shown to induce less IL-8 and IkBa degradation in A549 lung epithelial cells when compared with Salmonella (28). Similarly, it induces less inducible NO synthase and TNF-a in RAW264.7 macrophages when compared with other Gram-negative bacteria (29). The muted response was postulated to be because of a less stimulatory nature of B. pseudomallei LPS (30).…”