Involvement of Beta Interferon in Enhancing Inducible Nitric Oxide Synthase Production and Antimicrobial Activity of Burkholderia pseudomallei- Infected Macrophages

Abstract

Burkholderia pseudomallei is the causative agent of melioidosis, a life-threatening disease that affects both humans and animals. This bacterium is able to survive and multiply inside both phagocytic and nonphagocytic cells. We recently reported that mouse macrophages infected with B. pseudomallei fail to produce a significant level of inducible nitric oxide synthase (iNOS), a crucial enzyme needed for the cells to control the intracellular growth of this bacterium. In the present study, we extended our investigation to demonstrate that, unlike other gram-negative bacteria that have been investigated, B. pseudomallei only minimally activates beta interferon (IFN-␤) production; this minimal activation leads to a low level of interferon regulating factor 1 (IRF-1) in the macrophages, in parallel with poor iNOS expression. Adding exogenous IFN-␤ to the system could upregulate IRF-1 production, which in turn could enhance iNOS expression in the B. pseudomallei-infected macrophages and lead to suppression of the intracellular growth of this bacterium. Taken together, these results imply that the failure of macrophages to successfully control the growth and survival of intracellular B. pseudomallei is related, at least in part, to the defective production of IFN-␤, which modulates the ability of macrophages to synthesize iNOS.

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

“…Our previous findings are consistent with the prediction that B. pseudomallei escapes macrophage killing by interfering with IFN-␤ production, resulting in suppression of iNOS expression (2,29). In the present study, we showed that SARM depletion led to an increase in iNOS expression (Fig.…”

“…On the contrary, TRIF expression leads to the induction of IFN-␤, indicating that TRIF is an essential adaptor molecule for regulating IFN-␤ production (18,38). 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).…”

“…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).…”

“…We previously demonstrated that beta interferon (IFN-␤), an effector downstream of the MyD88-independent pathway, is essential for intracellular killing of B. pseudomallei (29). B. pseudomallei-infected macrophages fail to activate IFN-␤ production, leading to a reduced expression of a key enzyme, inducible nitric oxide synthase (iNOS), needed for antibacterial activity of the macrophages (29). Exogenous IFN-␤ can restore the ability of macrophages to activate iNOS expression and promote the killing of intracellular B. pseudomallei (28,29).…”

“…B. pseudomallei-infected macrophages fail to activate IFN-␤ production, leading to a reduced expression of a key enzyme, inducible nitric oxide synthase (iNOS), needed for antibacterial activity of the macrophages (29). Exogenous IFN-␤ can restore the ability of macrophages to activate iNOS expression and promote the killing of intracellular B. pseudomallei (28,29). The failure to stimulate IFN-␤ may be due to the fact that, unlike most other Gram-negative bacteria, B. pseudomallei fails to activate the MyD88-independent pathway, which is an essential signaling pathway of Toll-like receptor-4 (TLR4) (24).…”