Interferon‐Gamma Reverses the Evasion of Birc1e/Naip5 Gene Mediated Murine Macrophage Immunity by Legionella pneumophila Mutant Lacking Flagellin

Abstract

Legionella pneumophila is the etiologic agent of Legionnaires' disease. This bacterium contains a single monopolar flagellum, of which the FlaA subunit is a major protein constituent The murine macrophage resistance against this bacterium is controlled by the Birc1e/Naip5 gene, which belongs to the NOD family. We evaluated the intracellular growth of the flaA mutant bacteria as well as another aflagellated fliA mutant, within bone marrow‐derived macrophages from mice with an intact (C57BL/6, BALB/c) or mutated (A/J) Birc1e/Naip5 gene. The flaA mutant L. pneumophila multiplied within C57BL/6 and BALB/c macrophages while the wild‐type strain did not Cell viability was not impaired until 3 days after infection when the flaA mutant bacteria replicated 102‐3‐fold in macrophages, implying that L. pneumophila inhibited host cell death during the early phase of intracellular replication. The addition of recombinant interferon‐gamma (IFN‐γ) to the infected macrophages restricted replication of the flaA mutant within macrophages; these treated cells also showed enhanced nitric oxide production, although inhibition of nitric oxide production did not affect the IFN‐γ induced inhibition of Legionella replication. These findings suggested that IFN‐γ activated macrophages to restrict the intracellular growth of the L. pneumophila flaA mutant by a NO independent pathway.

Interferons Direct an Effective Innate Response to Legionella pneumophila Infection

“…1, A and B and Refs. 13,26,27). Consistent with previous reports, IFN-␥ pretreatment rendered all three genotypes, but not IFNGR(Ϫ/Ϫ) BMMs, resistant to infection (Fig.…”

“…Both IFN-␥ and IFN-Is suppress L. pneumophila growth in murine macrophages (13,26,27). Our studies highlight a role for the IFN-I autocrine loop in the innate response to this bacterium.…”

“…1, A and D and Refs. 26,42,43). IFN-␣ also significantly suppressed L. pneumophila growth in all three genotypes, but not in IFNAR(Ϫ/Ϫ) macrophages (Fig.…”

“…Nitric Oxide Accounts for Phenotypic Differences between Type I and II IFNs-NO production has not been ascribed a significant role in the innate response to L. pneumophila infection (26,58). To determine whether NO might contribute to the noncanonical IFN-I response associated with L. pneumophila suppression, additional studies were carried out.…”

NAIP and Ipaf Control Legionella pneumophila Replication in Human Cells

“…On the other hand, full innate intracellular resistance to L. pneumophila (and certainly other intracellular bacteria) depends on many additional mechanisms such as types I and II IFN-as well as TNF-␣-mediated effects, and perhaps also on Nramp1, antimicrobial peptides, or immunity-related GTPases (13,15,(45)(46)(47)(48)(49)(50). It might thus be conceivable that in case of L. pneumophila infection, one of these last mentioned resistance mechanisms is less functional in human cells compared with mice resulting in an overall stronger replication of Legionella in human cells compared with, for example, C57BL/6 macrophages despite functional hNAIP and hIpaf.…”