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Restriction of Legionella pneumophila replication in macrophages requires concerted action of the transcriptional regulators Irf1 and Irf8 and nod-like receptors Naip5 and Nlrc4.

ABSTRACT: The unique permissiveness of A/J mouse macrophages for replication of Legionella pneumophila is caused by a deficiency in the Nod-like receptor (NLR) protein and intracellular sensor for L. pneumophila flagellin (Naip5). The signaling pathways and proteins activated by Naip5 sensing in macrophages were investigated. Transcript profiling of macrophages from susceptible A/J mice and from resistant A/J mice harboring a transgenic wild-type copy of Naip5 at 4 h following L. pneumophila infection suggested that two members of the Irf transcriptional regulator family, Irf1 and Irf8, are regulated in response to Naip5 sensing of L. pneumophila. We show that macrophages having defective alleles of either Irf1 (Irf1-/-) or its heterodimerization partner gene Irf8 (Irf8R294C) become permissive for L. pneumophila replication, indicating that both the Irf1 and Irf8 proteins are essential for macrophage defense against L. pneumophila. Moreover, macrophages doubly heterozygous (Naip5AJ/WT Irf8R294C/WT or Nlrc4-/+ Irf8R294C/WT) for combined loss-of-function mutations in Irf8 and in either Naip5 or Nlrc4 are highly susceptible to L. pneumophila, indicating that there is a strong genetic interaction between Irf8 and the NLR protein family in the macrophage response to L. pneumophila. Legionella-containing phagosomes (LCPs) formed in permissive Irf1-/- or Irf8R294C macrophages behave like LCPs formed in Naip5-insufficient and Nlrc4-deficient macrophages which fail to acidify. These results suggest that, in addition to Naip5 and Nlrc4, Irf1 and Irf8 play a critical role in the early response of macrophages to infection with L. pneumophila, including antagonizing the ability of L. pneumophila to block phagosome acidification. They also suggest that flagellin sensing by the NLR proteins Naip5 and Nlrc4 may be coupled to Irf1-Irf8-mediated transcriptional activation of key effector genes essential for macrophage resistance to L. pneumophila infection.

SUBMITTER: Fortier A

PROVIDER: S-EPMC2772531 | biostudies-literature | 2009 Nov

REPOSITORIES: biostudies-literature

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Restriction of Legionella pneumophila replication in macrophages requires concerted action of the transcriptional regulators Irf1 and Irf8 and nod-like receptors Naip5 and Nlrc4.

Fortier Anne A Doiron Karine K Saleh Maya M Grinstein Sergio S Gros Philippe P

Infection and immunity 20090831 11

The unique permissiveness of A/J mouse macrophages for replication of Legionella pneumophila is caused by a deficiency in the Nod-like receptor (NLR) protein and intracellular sensor for L. pneumophila flagellin (Naip5). The signaling pathways and proteins activated by Naip5 sensing in macrophages were investigated. Transcript profiling of macrophages from susceptible A/J mice and from resistant A/J mice harboring a transgenic wild-type copy of Naip5 at 4 h following L. pneumophila infection sug ...[more]

PMID: 19720760

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Project description:UnlabelledNucleotide-binding domain, leucine-rich repeat containing proteins (NLRs) activate caspase-1 in response to a variety of bacterium-derived signals in macrophages. NLR-mediated activation of caspase-1 by Legionella pneumophila occurs through both an NLRC4/NAIP5-dependent pathway and a pathway requiring the adapter protein Asc. Both pathways are needed for maximal activation of caspase-1 and for the release of the cytokines interleukin-1β (IL-1β) and IL-18. Asc is not required for caspase-1-dependent pore formation and cell death induced upon infection of macrophages by L. pneumophila. Here, temporal and spatial localization of caspase-1-dependent processes was examined to better define the roles of Asc and NLRC4 during infection. Imaging studies revealed that caspase-1 localized to a single punctate structure in infected cells containing Asc but not in cells lacking this adapter. Both endogenous Asc and ectopically produced NLRC4 tagged with green fluorescent protein (GFP) were found to localize to caspase-1 puncta following L. pneumophila infection, suggesting that NLRC4 and Asc coordinate signaling through this complex during caspase-1 activation. Formation of caspase-1-containing puncta correlated with caspase-1 processing, suggesting a role for the Asc/NLRC4/caspase-1 complex in caspase-1 cleavage. In cells deficient for Asc, NLRC4 did not assemble into discrete puncta, and pyroptosis occurred at an accelerated rate. These data indicate that Asc mediates integration of NLR components into caspase-1 processing platforms and that recruitment of NLR components into an Asc complex can dampen pyroptotic responses. Thus, a negative feedback role of complexes containing Asc may be important for regulating caspase-1-mediated responses during microbial infection.ImportanceCaspase-1 is a protease activated during infection that is central to the regulation of several innate immune pathways. Studies examining the macromolecular complexes containing this protein, known as inflammasomes, have provided insight into the regulation of this protease. This work demonstrates that the intracellular bacterium Legionella pneumophila induces formation of complexes containing caspase-1 by multiple mechanisms and illustrates that an adapter molecule called Asc integrates signals from multiple independent upstream caspase-1 activators in order to assemble a spatially distinct complex in the macrophage. There were caspase-1-associated activities such as cytokine processing and secretion that were controlled by Asc. Importantly, this work uncovered a new role for Asc in dampening a caspase-1-dependent cell death pathway called pyroptosis. These findings suggest that Asc plays a central role in controlling a distinct subset of caspase-1-dependent activities by both assembling complexes that are important for cytokine processing and suppressing processes that mediate pyroptosis.

Dataset Information

Restriction of Legionella pneumophila replication in macrophages requires concerted action of the transcriptional regulators Irf1 and Irf8 and nod-like receptors Naip5 and Nlrc4.

Publications

Restriction of Legionella pneumophila replication in macrophages requires concerted action of the transcriptional regulators Irf1 and Irf8 and nod-like receptors Naip5 and Nlrc4.

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