Restriction Of Bacterial Replication Research Articles

Inflammasome-dependent Mechanisms Involved in Sensing and Restriction of Bacterial Replication.

Inflammasomes are multiprotein platforms assembled in the cytosol in response to pathogens and cell stress. Inflammasomes are recognized by their important role on defenses against bacterial infections and have been also implicated in a range of human inflammatory disorders. Intracellular sensors such as NLRP1, NLRP3, NLRC4, AIM2 and Pyrin induce assembly of inflammasomes, while caspase-11 induces the non-canonical pathway for activation of the NLRP3 inflammasome. The formation of the inflammasome leads to caspase-1 activation that triggers pyroptosis and activation of interleukin-1β (IL-1β) and IL-18. Pyroptotic cell death and cytokines production are involved in restriction of bacterial replication by limiting the replication niche of intracellular bacteria and by inducing inflammatory responses. In this review we focus on the mechanisms mediated by inflammasome activation that leads to inflammatory responses and restriction of bacterial infection.

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.

Primary Role for Toll-Like Receptor-Driven Tumor Necrosis Factor Rather than Cytosolic Immune Detection in Restricting Coxiella burnetii Phase II Replication within Mouse Macrophages.

Coxiella burnetii replicates within permissive host cells by employing a Dot/Icm type IV secretion system (T4SS) to translocate effector proteins that direct the formation of a parasitophorous vacuole. C57BL/6 mouse macrophages restrict the intracellular replication of the C. burnetii. Nine Mile phase II (NMII) strain. However, eliminating Toll-like receptor 2 (TLR2) permits bacterial replication, indicating that the restriction of bacterial replication is immune mediated. Here, we examined whether additional innate immune pathways are employed by C57BL/6 macrophages to sense and restrict NMII replication. In addition to the known role of TLR2 in detecting and restricting NMII infection, we found that TLR4 also contributes to cytokine responses but is not required to restrict bacterial replication. Furthermore, the TLR signaling adaptors MyD88 and Trif are required for cytokine responses and restricting bacterial replication. The C. burnetii NMII T4SS translocates bacterial products into C57BL/6 macrophages. However, there was little evidence of cytosolic immune sensing of NMII, as there was a lack of inflammasome activation, T4SS-dependent cytokine responses, and robust type I interferon (IFN) production, and these pathways were not required to restrict bacterial replication. Instead, endogenous tumor necrosis factor (TNF) produced upon TLR sensing of C. burnetii NMII was required to control bacterial replication. Therefore, our findings indicate a primary role for TNF produced upon immune detection of C. burnetii NMII by TLRs, rather than cytosolic PRRs, in enabling C57BL/6 macrophages to restrict bacterial replication.

A Caspase for Phagosomal-Lysosomal Fusion

Clearance of phagosomes containing pathogenic bacteria depends on caspase-11.

Emerging inflammasome effector mechanisms

Caspase 1 activation by inflammasome complexes in response to pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) induces the maturation and secretion of the pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18. Recent reports have begun to identify additional inflammasome effector mechanisms that proceed independently of IL-1β and IL-18. These include the induction of pyroptotic cell death, the restriction of bacterial replication, the activation of lipid metabolic pathways for cell repair and the secretion of DAMPs and leaderless cytokines. These non-canonical functions of caspase 1 illustrate the diverse mechanisms by which inflammasomes might contribute to innate immunity, repair responses and host defence.

NAIP and Ipaf Control Legionella pneumophila Replication in Human Cells

In mice, different alleles of the mNAIP5 (murine neuronal apoptosis inhibitory protein-5)/mBirc1e gene determine whether macrophages restrict or support intracellular replication of Legionella pneumophila, and whether a mouse is resistant or (moderately) susceptible to Legionella infection. In the resistant mice strains, the nucleotide-binding oligomerization domain (Nod)-like receptor (NLR) family member mNAIP5/mBirc1e, as well as the NLR protein mIpaf (murine ICE protease-activating factor), are involved in recognition of Legionella flagellin and in restriction of bacterial replication. Human macrophages and lung epithelial cells support L. pneumophila growth, and humans can develop severe pneumonia (Legionnaires disease) after Legionella infection. The role of human orthologs to mNAIP5/mBirc1e and mIpaf in this bacterial infection has not been elucidated. Herein we demonstrate that flagellin-deficient L. pneumophila replicate more efficiently in human THP-1 macrophages, primary monocyte-derived macrophages, and alveolar macrophages, and in A549 lung epithelial cells compared with wild-type bacteria. Additionally, we note expression of the mNAIP5 ortholog hNAIP in all cell types examined, and expression of hIpaf in human macrophages. Gene silencing of hNAIP or hIpaf in macrophages or of hNAIP in lung epithelial cells leads to an enhanced bacterial growth, and overexpression of both molecules strongly reduces Legionella replication. In contrast to experiments with wild-type L. pneumophila, hNAIP or hIpaf knock-down affects the (enhanced) replication of flagellin-deficient Legionella only marginally. In conclusion, hNAIP and hIpaf mediate innate intracellular defense against flagellated Legionella in human cells.