Inhibition of caspase-1 or gasdermin-D enable caspase-8 activation in the Naip5/NLRC4/ASC inflammasome.

Legionella pneumophila is a Gram-negative, flagellated bacterium that survives in phagocytes and causes Legionnaires' disease. Upon infection of mammalian macrophages, cytosolic flagellin triggers the activation of Naip/NLRC4 inflammasome, which culminates in pyroptosis and restriction of bacterial replication. Although NLRC4 and caspase-1 participate in the same inflammasome, Nlrc4-/- mice and their macrophages are more permissive to L. pneumophila replication compared with Casp1/11-/-. This feature supports the existence of a pathway that is NLRC4-dependent and caspase-1/11-independent. Here, we demonstrate that caspase-8 is recruited to the Naip5/NLRC4/ASC inflammasome in response to flagellin-positive bacteria. Accordingly, caspase-8 is activated in Casp1/11-/- macrophages in a process dependent on flagellin, Naip5, NLRC4 and ASC. Silencing caspase-8 in Casp1/11-/- cells culminated in macrophages that were as susceptible as Nlrc4-/- for the restriction of L. pneumophila replication. Accordingly, macrophages and mice deficient in Asc/Casp1/11-/- were more susceptible than Casp1/11-/- and as susceptible as Nlrc4-/- for the restriction of infection. Mechanistically, we found that caspase-8 activation triggers gasdermin-D-independent pore formation and cell death. Interestingly, caspase-8 is recruited to the Naip5/NLRC4/ASC inflammasome in wild-type macrophages, but it is only activated when caspase-1 or gasdermin-D is inhibited. Our data suggest that caspase-8 activation in the Naip5/NLRC4/ASC inflammasome enable induction of cell death when caspase-1 or gasdermin-D is suppressed.

Murine Alveolar Macrophages Are Highly Susceptible to Replication of Coxiella burnetii Phase II In Vitro.

Coxiella burnetii is a Gram-negative bacterium that causes Q fever in humans. Q fever is an atypical pneumonia transmitted through inhalation of contaminated aerosols. In mammalian lungs, C. burnetii infects and replicates in several cell types, including alveolar macrophages (AMs). The innate immunity and signaling pathways operating during infection are still poorly understood, in part because of the lack of relevant host cell models for infection in vitro In the study described here, we investigated and characterized the infection of primary murine AMs by C. burnetii phase II in vitro Our data reveal that AMs show a pronounced M2 polarization and are highly permissive to C. burnetii multiplication in vitro Murine AMs present an increased susceptibility to infection in comparison to primary bone marrow-derived macrophages. AMs support more than 2 logs of bacterial replication during 12 days of infection in culture, similar to highly susceptible host cells, such as Vero and THP-1 cells. As a proof of principle that AMs are useful for investigation of C. burnetii replication, we performed experiments with AMs from Nos2(-/-) or Ifng(-/-) mice. In the absence of gamma interferon and nitric oxide synthase 2 (NOS2), AMs were significantly more permissive than wild-type cells. In contrast, AMs from Il4(-/-) mice were more restrictive to C. burnetii replication, supporting the importance of M2 polarization for the permissiveness of AMs to C. burnetii replication. Collectively, our data account for understanding the high susceptibility of alveolar macrophages to bacterial replication and support the use of AMs as a relevant model of C. burnetii growth in primary macrophages.

Inhibition of inflammasome activation by Coxiella burnetii type IV secretion system effector IcaA.

Coxiella burnetii is a highly infectious bacterium that promotes its own replication in macrophages by inhibiting several host cell responses. Here, we show that C. burnetii inhibits caspase-1 activation in primary mouse macrophages. By using co-infection experiments, we determine that the infection of macrophages with C. burnetii inhibits the caspase-11-mediated non-canonical activation of the NLRP3 inflammasome induced by subsequent infection with Escherichia coli or Legionella pneumophila. Genetic screening using flagellin mutants of L. pneumophila as a surrogate host, reveals a novel C. burnetii gene (IcaA) involved in the inhibition of caspase activation. Expression of IcaA in L. pneumophila inhibited the caspase-11 activation in macrophages. Moreover, icaA(-) mutants of C. burnetii failed to suppress the caspase-11-mediated inflammasome activation induced by L. pneumophila. Our data reveal IcaA as a novel C. burnetii effector protein that is secreted by the Dot/Icm type IV secretion system and interferes with the caspase-11-induced, non-canonical activation of the inflammasome.