Immunoregulatory Role of Nitric Oxide inLegionella pneumophila-Infected Macrophages

Abstract

Nitric oxide (NO) is an intercellular messenger molecule produced by a variety of cells, including macrophages. However, the role of NO in infection, especially its immunological role, is poorly understood. In the present study, the role of NO inLegionella pneumophila-infected macrophages was examined. Whereas infection of mouse macrophagesin vitrowithL. pneumophiladid not induce detectable NO, when the macrophages were primed with interferon-γ (IFN-γ), the treated macrophages markedly inhibited bacterial replication and produced a large amount of NO. Treatment with NO inhibitors, such as NG-monomethyl-l-arginine (l-MMA) or aminoguanidine, as well as culture in arginine-free medium, significantly inhibited NO production; however, the anti-L. pneumophilaactivity induced by IFN-γ was not diminished. Examination of cytokine levels inL. pneumophila-infected macrophages primed with IFN-γ revealed a moderate increase of interleukin-6 (IL-6) production; however, inhibition of NO byl-MMA markedly increased IL-6 production. Reconstitution of NO in theL. pneumophila-infected macrophages primed with IFN-γ and treated withl-MMA to inhibit endogenous NO production following addition of sodium nitroprusside reduced IL-6 production to normal levels. The levels of IL-6 mRNA inl-MMA-treated macrophages were the same as in nontreated macrophages, as demonstrated by quantitative RT-PCR. Thus, these results indicate that NO may regulate IL-6 production independently of its role in antimicrobial function inL. pneumophila-infected macrophages and their immunoregulation on IL-6 production may be due to a post-transcriptional mechanism.