Burkholderia pseudomallei interferes with inducible nitric oxide synthase (iNOS) production: A possible mechanism of evading macrophage killing

Burkholderia pseudomallei is a causative agent of melioidosis, a life threa tening disease which affects humans and animals in tropical and subtropical areas. This bacterium is known to survive and multiply inside cells such a s macrophages. The mechanism of host defense against this bacterium is stil l unknown. In this study, we demonstrated that B. pseudomallei exhibited un ique macrophage activation activity compared with Escherichia coli and Salm onella typhi. The mouse macrophage cell line (RAW 264.7) infected with B. p seudomallei at MOI of 0.1:1, 1:1 and 10:1 did not express a detectable leve l of inducible nitric oxide synthase (iNOS). Moreover, the B. pseudomallei infected cells released TNF-alpha only when they were infected with high MO I (10:1). Unlike the cells infected with B. pseudomallei, the cells infecte d with E. coli, and S. typhi expressed iNOS even at MOI of 0.1:1. These inf ected cells also released a significantly higher level of TNF-alpha at the low MOI ratio. The cells that were preactivated with IFN-gamma prior to bei ng infected with B. pseudomallei exhibited an enhanced production of iNOS a nd TNF-alpha release. The increased macrophage activation activity in the p resence of INF-gamma also correlated with the restriction of the intracellu lar bacteria survival. Moreover, IFN-gamma also prevented cell fusion and m ultinucleated cell formation induced by B. pseudomallei, a phenomenon recen tly described bg our group. Altogether, these results Indicate that interna lization of B. pseudomallei failed to trigger substantial macrophage activa tion, 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.