Cytosolic recognition of flagellin by mouse macrophages restricts Legionella pneumophila infection

Histone acetylation and flagellin are essential for Legionella pneumophila-induced cytokine expression

Legionella pneumophila causes severe pneumonia. Acetylation of histones is thought to be an important regulator of gene transcription, but its impact on L. pneumophila-induced expression of proinflammatory cytokines is unknown. L. pneumophila strain 130b induced the expression of the important chemoattractant IL-8 and genome-wide histone modifications in human lung epithelial A549 cells. We analyzed the IL-8-promoter and found that histone H4 was acetylated and H3 was phosphorylated at Ser(10) and acetylated at Lys(14), followed by transcription factor NF-kappaB. Recruitment of RNA polymerase II to the IL-8 promoter corresponded with increases in gene transcription. Histone modification and IL-8 release were dependent on p38 kinase and NF-kappaB pathways. Legionella-induced IL-8 expression was decreased by histone acetylase (HAT) inhibitor anacardic acid and enhanced by histone deacetylase (HDAC) inhibitor trichostatin A. After Legionella infection, HATs p300 and CREB-binding protein were time-dependently recruited to the IL-8 promoter, whereas HDAC1 and HDAC5 first decreased and later reappeared at the promoter. Legionella specifically induced expression of HDAC5 but not of other HDACs in lung epithelial cells, but knockdown of HDAC1 or 5 did not alter IL-8 release. Furthermore, Legionella-induced cytokine release, promoter-specific histone modifications, and RNA polymerase II recruitment were reduced in infection with flagellin-deletion mutants. Legionella-induced histone modification as well as HAT-/HDAC-dependent IL-8 release could also be shown in primary lung epithelial cells. In summary, histone acetylation seems to be important for the regulation of proinflammatory gene expression in L. pneumophila infected lung epithelial cells. These pathways may contribute to the host response in Legionnaires' disease.

Multiple Nod-like receptors activate caspase 1 during Listeria monocytogenes infection

Listeria monocytogenes escapes from the phagosome of macrophages and replicates within the cytosolic compartment. The macrophage responds to L. monocytogenes through detection pathways located on the cell surface (TLRs) and within the cytosol (Nod-like receptors) to promote inflammatory processes aimed at clearing the pathogen. Cytosolic L. monocytogenes activates caspase 1, resulting in post-translational processing of the cytokines IL-1beta and IL-18 as well as caspase 1-dependent cell death (pyroptosis). We demonstrate that the presence of L. monocytogenes within the cytosolic compartment induces caspase 1 activation through multiple Nod-like receptors, including Ipaf and Nalp3. Flagellin expression by cytosolic L. monocytogenes was detected through Ipaf in a dose-dependent manner. Concordantly, detection of flagellin promoted bacterial clearance in a murine infection model. Finally, we provide evidence that suggests cytosolic L. monocytogenes activates caspase 1 through a third pathway, which signals through the adaptor protein ASC. Thus, L. monocytogenes activates caspase 1 in macrophages via multiple pathways, all of which detect the presence of bacteria within the cytosol.

Mycobacterium tuberculosis prevents inflammasome activation

Mycobacterium tuberculosis (Mtb) parasitizes host macrophages and subverts host innate and adaptive immunity. Several cytokines elicited by Mtb are mediators of mycobacterial clearance or are involved in tuberculosis pathology. Surprisingly, interleukin-1beta (IL-1beta), a major proinflammatory cytokine, has not been implicated in host-Mtb interactions. IL-1beta is activated by processing upon assembly of the inflammasome, a specialized inflammatory caspase-activating protein complex. Here, we show that Mtb prevents inflammasome activation and IL-1beta processing. An Mtb gene, zmp1, which encodes a putative Zn(2+) metalloprotease, is required for this process. Infection of macrophages with zmp1-deleted Mtb triggered activation of the inflammasome, resulting in increased IL-1beta secretion, enhanced maturation of Mtb containing phagosomes, improved mycobacterial clearance by macrophages, and lower bacterial burden in the lungs of aerosol-infected mice. Thus, we uncovered a previously masked role for IL-1beta in the control of Mtb and a mycobacterial system that prevents inflammasome and, therefore, IL-1beta activation.

NLRs at the intersection of cell death and immunity

Inflammation is a crucial element of the host response to cellular insult. Pathogen-induced inflammation includes a molecular pathway which proceeds through activation of the protease caspase-1 to the release of the inflammatory cytokines interleukin-1 (IL-1) and IL-18. Importantly, pathogens may also induce forms of cell death that have inherently pro-inflammatory features. Here, we review recent evidence demonstrating that NLR (nucleotide-binding domain, leucine-rich repeat containing) family proteins serve as a common component of both caspase-1-activated apoptotic pathways and caspase-independent necrotic pathways. Parallels are drawn between NLR protein function and the activity of structurally similar proteins involved in cell death: the apoptotic mediator APAF1 (apoptotic-protease-activating factor 1) and the plant disease resistance NBS-LRR (nucleotide-binding site leucine-rich repeats) proteins.

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.

Forward genetic dissection of immunity to infection in the mouse

Forward genetics is an experimental approach in which gene mapping and positional cloning are used to elucidate the molecular mechanisms underlying phenotypic differences between two individuals for a given trait. This strategy has been highly successful for the study of inbred mouse strains that show differences in innate susceptibility to bacterial, parasitic, fungal, and viral infections. Over the past 20 years, these studies have led to the identification of a number of cell populations and critical biochemical pathways and proteins that are essential for the early detection of and response to invading pathogens. Strikingly, the macrophage is the point of convergence for many of these genetic studies. This has led to the identification of diverse pathways involved in extracellular and intracellular pathogen recognition, modification of the properties and content of phagosomes, transcriptional response, and signal transduction for activation of adaptive immune mechanisms. In models of viral infections, elegant genetic studies highlighted the pivotal role of natural killer cells in the detection and destruction of infected cells.

Inactivation of the type IV secretion system reduces the Th1 polarization of the immune response to Brucella abortus infection

The Brucella abortus type IV secretion system (T4SS), encoded by the virB operon, is essential for establishing persistent infection in the murine reticuloendothelial system. To gain insight into the in vivo interactions mediated by the T4SS, we compared host responses elicited by B. abortus with those of an isogenic mutant in the virB operon. Mice infected with the B. abortus virB mutant elicited smaller increases in serum levels of immunoglobulin G2a, gamma interferon (IFN-gamma), and interleukin-12p40 than did mice infected with wild-type B. abortus. Despite equal bacterial loads in the spleen, at 3 to 4 days postinfection, levels of IFN-gamma were higher in mice infected with wild-type B. abortus than in mice infected with the virB mutant, as shown by real-time PCR, intracellular cytokine staining, and cytokine levels. IFN-gamma-producing CD4(+) T cells were more abundant in spleens of mice infected with wild-type B. abortus than in virB mutant-infected mice. Similar numbers of IFN-gamma-secreting CD8(+) T cells were observed in the spleens of mice infected with B. abortus 2308 or a virB mutant. These results suggest that early differences in cytokine responses contribute to a stronger Th1 polarization of the immune response in mice infected with wild-type B. abortus than in mice infected with the virB mutant.

Inactivation of [Fe-S] metalloproteins mediates nitric oxide-dependent killing of Burkholderia mallei

BACKGROUND

Much remains to be known about the mechanisms by which O(2)-dependent host defenses mediate broad antimicrobial activity.

METHODOLOGY/PRINCIPAL FINDINGS

We show herein that reactive nitrogen species (RNS) generated by inducible nitric oxide (NO) synthase (iNOS) account for the anti-Burkholderia mallei activity of IFNgamma-primed macrophages. Inducible NOS-mediated intracellular killing may represent direct bactericidal activity, because B. mallei showed an exquisite sensitivity to NO generated chemically. Exposure of B. mallei to sublethal concentrations of NO upregulated transcription of [Fe-S] cluster repair genes, while damaging the enzymatic activity of the [Fe-S] protein aconitase. To test whether [Fe-S] clusters are critical targets for RNS-dependent killing of B. mallei, a mutation was constructed in the NO-induced, [Fe-S] cluster repair regulator iscR. Not only was the iscR mutant hypersusceptible to iNOS-mediated killing, but its aconitase pool was readily oxidized by NO donors as compared to wild-type controls. Although killed by authentic H(2)O(2), which also oxidizes [Fe-S] clusters, B. mallei appear to be resilient to NADPH oxidase-mediated cytotoxicity. The poor respiratory burst elicited by this bacterium likely explains why the NADPH oxidase is nonessential to the killing of B. mallei while it is still confined within phagosomes.

CONCLUSIONS/SIGNIFICANCE

Collectively, these findings have revealed a disparate role for NADPH oxidase and iNOS in the innate macrophage response against the strict aerobe B. mallei. To the best of our knowledge, this is the first instance in which disruption of [Fe-S] clusters is demonstrated as cause of the bactericidal activity of NO congeners.

A calculated response: control of inflammation by the innate immune system

Inflammation is a rapid yet coordinated response that can lead to the destruction of microbes and host tissue. Triggers capable of inducing an inflammatory response include tissue damage and infection by pathogenic and nonpathogenic microbes. Each of these triggers represents a qualitatively distinct stress to the host immune system, yet our understanding of whether they are interpreted as such remains poor. Accumulating evidence suggests that recognition of these distinct stimuli converges on many of the same receptors of the innate immune system. Here I provide an overview of these innate receptors and suggest that the innate immune system can interpret the context of an inflammatory trigger and direct inflammation accordingly.

Nod-like proteins in inflammation and disease

The field of innate immunity has undergone an enormous upheaval during the last decade. The discovery of different groups of proteins, called pattern recognition molecules (PRMs), which detect microbial components, so-called pathogen-associated molecular patterns (PAMPs) and trigger protective responses, had a huge impact on the understanding of innate immune responses. Among the PRMs, the intracellular Nod-like receptors (NLRs) have recently been identified as key mediators of inflammatory and immune responses. The NLR family is divided into subfamilies on the basis of their different signal transduction domains, and recent studies have highlighted the role of certain NLRs, including Nod1, Nod2, Nalp3, Ipaf and Naip5, in the detection of intracellular microbes and possibly 'danger signals'. In this review, we summarize the current knowledge on the function of these proteins in immunity and inflammation, with a focus on their participation in different disease pathologies.

Host cell processes that influence the intracellular survival of Legionella pneumophila

Key to the pathogenesis of intracellular pathogens is their ability to manipulate host cell processes, permitting the establishment of an intracellular replicative niche. In turn, the host cell deploys defence mechanisms that limit intracellular infection. The bacterial pathogen Legionella pneumophila, the aetiological agent of Legionnaire's Disease, has evolved virulence mechanisms that allow it to replicate within protozoa, its natural host. Many of these tactics also enable L. pneumophila's survival and replication inside macrophages within a membrane-bound compartment known as the Legionella-containing vacuole. One of the virulence factors indispensable for L. pneumophila's intracellular survival is a type IV secretion system, which translocates a large repertoire of bacterial effectors into the host cell. These effectors modulate multiple host cell processes and in particular, redirect trafficking of the L. pneumophila phagosome and mediate its conversion into an ER-derived organelle competent for intracellular bacterial replication. In this review, we discuss how L. pneumophila manipulates host cells, as well as host cell processes that either facilitate or impede its intracellular survival.

Anthrax lethal toxin and Salmonella elicit the common cell death pathway of caspase-1-dependent pyroptosis via distinct mechanisms

Caspase-1 cleaves the inactive IL-1beta and IL-18 precursors into active inflammatory cytokines. In Salmonella-infected macrophages, caspase-1 also mediates a pathway of proinflammatory programmed cell death termed "pyroptosis." We demonstrate active caspase-1 diffusely distributed in the cytoplasm and localized in discrete foci within macrophages responding to either Salmonella infection or intoxication by Bacillus anthracis lethal toxin (LT). Both stimuli triggered caspase-1-dependent lysis in macrophages and dendritic cells. Activation of caspase-1 by LT required binding, uptake, and endosome acidification to mediate translocation of lethal factor (LF) into the host cell cytosol. Catalytically active LF cleaved cytosolic substrates and activated caspase-1 by a mechanism involving proteasome activity and potassium efflux. LT activation of caspase-1 is known to require the inflammasome adapter Nalp1. In contrast, Salmonella infection activated caspase-1 through an independent pathway requiring the inflammasome adapter Ipaf. These distinct mechanisms of caspase-1 activation converged on a common pathway of caspase-1-dependent cell death featuring DNA cleavage, cytokine activation, and, ultimately, cell lysis resulting from the formation of membrane pores between 1.1 and 2.4 nm in diameter and pathological ion fluxes that can be blocked by glycine. These findings demonstrate that distinct activation pathways elicit the conserved cell death effector mechanism of caspase-1-mediated pyroptosis and support the notion that this pathway of proinflammatory programmed cell death is broadly relevant to cell death and inflammation invoked by diverse stimuli.

NLR, the nucleotide-binding domain leucine-rich repeat containing gene family

The NLR (nucleotide-binding domain leucine-rich repeat containing) family is found in plants and animals, and serves as crucial regulators of inflammatory and innate immune response, though its functions are likely to extend greatly beyond innate immunity, and even beyond the immune system. This review discusses recent findings regarding the function of NLR proteins in the control of IL-1, NF-kappaB, and host response to pathogens including distinct forms of cell death. The review also covers recent advances regarding the biochemical nature of NLRs, its regulation by intracellular nucleotides and extracellular ATP, by the chaperone protein HSP90, and the ubiquitin ligase-associated protein SGT1. Its role in inflammation is linked to the formation of biochemical complexes such as the inflammasome, and its roles in cell death might be linked to the proposed formation of pyroptosome and necrosome.

Pseudomonas aeruginosa activates caspase 1 through Ipaf

The innate immune system encodes cytosolic Nod-like receptors (NLRs), several of which activate caspase 1 processing and IL-1beta and IL-18 secretion. Macrophages respond to Salmonella typhimurium infection by activating caspase 1 through the NLR Ipaf. This activation is mediated by cytosolic flagellin through the activity of the virulence-associated type III secretion system (T3SS). We demonstrate here that Pseudomonas aeruginosa activates caspase 1 and induces IL-1beta secretion in infected macrophages. While live, virulent P. aeruginosa activate IL-1beta secretion through caspase 1 and Ipaf, strains that have mutations in the T3SS or in flagellin did not. Ipaf-dependent caspase 1 activation could be recapitulated by delivering P. aeruginosa flagellin to the macrophage cytosol. We examined the role of Naip5 in P. aeruginosa-induced caspase 1 activation by using A/J (Naip5-deficient) compared with C57BL/6 and BALB/c (Naip5-sufficient) macrophages and observed that A/J macrophages secrete IL-1beta in response to P. aeruginosa, S. typhimurium, and Listeria monocytogenes infection, as well as in response to cytosolic flagellin, but at slightly reduced levels. Thus, Ipaf-dependent detection of cytosolic flagellin is a conserved mechanism by which macrophages detect the presence of pathogens that use T3SS.

Exit strategies of intracellular pathogens

The exit of intracellular pathogens from host cells is an important step in the infectious cycle, but is poorly understood. It has recently emerged that microbial exit is a process that can be directed by organisms from within the cell, and is not simply a consequence of the physical or metabolic burden that is imposed on the host cell. This Review summarizes our current knowledge on the diverse mechanisms that are used by intracellular pathogens to exit cells. An integrated understanding of the diversity that exists for microbial exit pathways represents a new horizon in the study of host-pathogen interactions.

Macrophage activation redirects yersinia-infected host cell death from apoptosis to caspase-1-dependent pyroptosis

Infection of macrophages by Yersinia species results in YopJ-dependent apoptosis, and naïve macrophages are highly susceptible to this form of cell death. Previous studies have demonstrated that macrophages activated with lipopolysaccharide (LPS) prior to infection are resistant to YopJ-dependent cell death; we found this simultaneously renders macrophages susceptible to killing by YopJ⁻Yersinia pseudotuberculosis (Yptb). YopJ⁻Yptb-induced macrophage death was dependent on caspase-1 activation, resulting in rapid permeability to small molecules, followed by membrane breakdown and DNA damage, and accompanied by cleavage and release of proinflammatory interleukin-18. Induction of caspase-1-dependent death, or pyroptosis, required the bacterial type III translocon but none of its known translocated proteins. Wild-type Yptb infection also triggered pyroptosis: YopJ-dependent activation of proapoptotic caspase-3 was significantly delayed in activated macrophages and resulted in caspase-1-dependent pyroptosis. The transition to susceptibility was not limited to LPS activation; it was also seen in macrophages activated with other Toll-like receptor (TLR) ligands and intact nonviable bacteria. Yptb infection triggered macrophage activation and activation of caspase-1 in vivo. Y. pestis infection of activated macrophages also stimulated caspase-1 activation. These results indicate that host signaling triggered by TLR and other activating ligands during the course of Yersinia infection redirects both the mechanism of host cell death and the downstream consequences of death by shifting from noninflammatory apoptosis to inflammatory pyroptosis.

Pathogenic Yersinia are bacteria capable of interacting with host immune cells and inhibiting their function. Macrophages are potent antimicrobial immune cells that eliminate invading microbes, and represent a major target for Yersinia during infection. Yersinia triggers death of resting macrophages by apoptosis, a process thought to be advantageous for Yersinia growth during early stages of infection because important host cells are eliminated without perturbing the surrounding tissue. However, activated macrophages with enhanced antimicrobial activity play a crucial role in controlling Yersinia infection. To elucidate the mechanisms involved in successful defense against infection, the authors investigated the response of activated macrophages to Yersinia, which revealed induction of a proinflammatory cell death pathway termed pyroptosis. Unlike apoptosis, pyroptosis unleashes inflammatory mediators capable of enhancing immune responses and clearing bacteria. Macrophage activation and pyroptosis was observed in infected host tissue. Thus, regulating the mechanism of cell death is important for effective responses to Yersinia infection: activated macrophages resisting apoptosis are redirected to utilize pyroptosis, an inflammatory process facilitating host resistance.

Nod-like receptors: cytosolic watchdogs for immunity against pathogens

In mammals, tissue-specific sets of pattern-recognition molecules, including Nod-like receptors (NLR), enable concomitant and sequential detection of microbial-associated molecular patterns from both the extracellular and intracellular microenvironment. Repressing and de-repressing the cytosolic surveillance machinery contributes to vital immune homeostasis and protective responses within specific tissues. Conversely, defective biology of NLR drives the development of recurrent infectious, autoimmune and/or inflammatory diseases by failing to mount barrier functions against pathogens, to tolerate commensals, and/or to instruct the adaptive immune response against microbes. Better decoding microbial strategies that are evolved to circumvent NLR sensing will provide clues for the development of rational therapies aimed at curing and/or preventing common and emerging immunopathologies.

Pathogen subversion of cell-intrinsic innate immunity

The mammalian immune system has evolved under continuous selective pressure from a wide range of microorganisms that colonize and replicate in animal hosts. A complex set of signaling networks initiate both innate and adaptive immunity in response to the diverse pathogens that mammalian hosts encounter. In response, viral and microbial pathogens have developed or acquired sophisticated mechanisms to avoid, counteract and subvert sensors, signaling networks and a range of effector functions that constitute the host immune response. This balance of host response and pathogen countermeasures contributes to chronic infection in highly adapted pathogens that have coevolved with their host. In this review we outline some of the themes that are beginning to emerge in the mechanisms by which pathogens subvert the early innate immune response.

Toll-like receptor 5-deficient mice have dysregulated intestinal gene expression and nonspecific resistance to Salmonella-induced typhoid-like disease

The recognition of flagellin by Toll-like receptor 5 (TLR5) is the dominant means by which model intestinal epithelia activate proinflammatory gene expression in response to Salmonella enterica. The role of the flagellin-TLR5 interaction in vivo has been addressed primarily via studies that use flagellar mutants. Such studies suggest that host recognition of flagellin promotes rapid neutrophil recruitment that protects the host from this pathogen. However, these works do not directly address the role of TLR5 and are subject to the caveat that flagellar mutations may broadly affect Salmonella gene expression. Thus, we examined the role of the flagellin-TLR5 interaction via the use of TLR5-deficient (TLR5KO) mice. We utilized both the traditional model of murine Salmonella infection, wherein low-dose oral infection of mice with Salmonella enterica subsp. enterica serovar Typhimurium results in systemic typhoid-like disease, and a more recently characterized model in which mice are pretreated with streptomycin to result in gut-restricted acute enteritis. In the enteritis model, TLR5KO mice had more severe gut pathology, thus "phenocopying" previous results obtained with Salmonella mutants. In contrast, TLR5KO mice were resistant to Salmonella-induced typhoid-like disease. However, such resistance was not specific for flagellated serovar Typhimurium, but rather, TLR5KO mice were also resistant to challenges by flagellin-deficient serovar Typhimurium. Such resistance associated with elevations in the microbiota was ablated by antibiotic pretreatment and correlated with basal elevations in intestinal host defense gene expression. All together, these results indicate that the resistance of TLR5KO mice to Salmonella-induced typhoid-like illness resulted from alterations in their basal phenotype rather than from the lack of TLR5 ligation during the infection per se.

Genetic analysis of resistance to infections in mice: A/J meets C57BL/6J

Susceptibility to infectious diseases has long been known to have a genetic component in human populations. This genetic effect is often complex and difficult to study as it is further modified by environmental factors including the disease-causing pathogen itself. The laboratory mouse has proved a useful alternative to implement a genetic approach to study host defenses against infections. Our laboratory has used genetic analysis and positional cloning to characterize single and multi-gene effects regulating inter-strain differences in the susceptibility of A/J and C57BL/6J mice to infection with several bacterial and parasitic pathogens. This has led to the identification of several proteins including Nrampl (Slc11a1), Birc1e, Icsbp, C5a, and others that play critical roles in the antimicrobial defenses of macrophages against intracellular pathogens. The use of AcB/BcA recombinant congenic strains has further facilitated the characterization of single gene effects in complex traits such as susceptibility to malaria. The genetic identification of erythrocyte pyruvate kinase (Pklr) and myeloid pantetheinase enzymes (Vnn1/3) as key regulators of blood-stage parasitemia has suggested that cellular redox potential may be a key biochemical determinant of Plasmodium parasite replication. Expanding these types of studies to additional inbred strains and to emerging stocks of mutagenized mice will undoubtedly continue to unravel the molecular basis of host defense against infections.

Altered inflammatory responses in TLR5-deficient mice infected with Legionella pneumophila

Legionella pneumophila (Lp), an important cause of morbidity and mortality from pneumonia, infects alveolar macrophages (AMs) and is recognized by several TLRs as well as Birc1e (NAIP5) and IL-1 converting enzyme-protease activating factor. We examined the role of TLR5 during the murine response to aerosolized Lp infection. At 4 h after infection, Tlr5(-/-) mice had lower numbers of polymorphonuclear neutrophils (PMNs) in their broncho-alveolar lavage fluid in comparison to wild-type (WT) mice. At 24 and 72 h, the PMN recruitment was similar. WT mice infected with a flagellin-deficient strain (LpFlaA-) also showed an impaired early PMN response at 4 h compared with those infected with the WT strain. There was no consistent difference in bacterial counts at any of the time points when comparing the Tlr5(-/-) and WT mice. However, at 6 days after infection, the Tlr5(-/-) mice had increased leukocytic infiltrates in the alveolar and peribronchial interstitial spaces that were consistent with organizing pneumonia. We also examined the role of TLR5 during macrophage infection. In contrast to bone marrow-derived macrophages, AMs secreted TNF-alpha after stimulation with purified flagellin. In addition, WT, but not Tlr5(-/-), AMs produced TNF-alpha after stimulation with Lp. Live LpFlaA- did not induce TNF-alpha secretion in AM. These results suggested that AMs recognize Lp flagellin and that a majority of the Lp-induced TNF-alpha response is TLR5-mediated. Thus, TLR5 mediates recognition of Lp in AMs and performs a distinct role during the in vivo pulmonary immune response through regulation of early PMN recruitment and subsequent later development of pneumonia.

Orchestration of pathogen recognition by inflammasome diversity: Variations on a common theme

Innate immunity is a crucial part of the immune system, capable of mounting specific host responses against distinct pathogens. An integral component of the innate immune system is a network of pathogen recognition receptors, which are present at the surface of the cell or in the cytoplasm. Nucleotide oligomerization domain (Nod)-like receptors form the largest known family of intracellular innate immune sensors of microbes and danger signals that initiate early host responses. Some Nod-like receptors, such as NALP, NAIP and IPAF, form molecular machines termed inflammasomes, which are involved in the activation of inflammatory cytokines such as IL-1beta and IL-18. The diversity and the role of the different types of inflammasomes remain poorly defined. In this issue of the European Journal of Immunology, it is reported that the Gram-negative human pathogen Pseudonomas aeruginosa specifically activates an IPAF inflammasome. This finding, in combination with other recently published reports, reveals how different pathogens engage distinct types of inflammasomes, further highlighting the diversity and plasticity of inflammasomes activation.

Intracellular NOD-like receptors in host defense and disease

The innate immune system comprises several classes of pattern recognition receptors, including Toll-like receptors (TLRs), NOD-like receptors (NLRs), and RIG-1-like receptors (RLRs). TLRs recognize microbes on the cell surface and in endosomes, whereas NLRs and RLRs detect microbial components in the cytosol. Here we discuss the recent understanding in NLRs. Two NLRs, NOD1 and NOD2, sense the cytosolic presence of the peptidoglycan fragments meso-DAP and muramyl dipeptide, respectively, and drive the activation of mitogen-activated protein kinase (MAPK) and the transcription factor NF-kappaB. A different set of NLRs induces caspase-1 activation through the assembly of large protein complexes named inflammasomes. Genetic variations in several NLR members are associated with the development of inflammatory disorders. Further understanding of NLRs should provide new insights into the mechanisms of host defense and the pathogenesis of inflammatory diseases.

Immune recognition of Pseudomonas aeruginosa mediated by the IPAF/NLRC4 inflammasome

Pseudomonas aeruginosa is a Gram-negative bacterium that causes opportunistic infections in immunocompromised individuals. P. aeruginosa employs a type III secretion system to inject effector molecules into the cytoplasm of the host cell. This interaction with the host cell leads to inflammatory responses that eventually result in cell death. We show that infection of macrophages with P. aeruginosa results in activation of caspase-1 in an IPAF-dependent, but flagellin-independent, manner. Macrophages deficient in IPAF or caspase-1 were markedly resistant to P. aeruginosa-induced cell death and release of the proinflammatory cytokine interleukin (IL)-1beta. A subset of P. aeruginosa isolates express the effector molecule exoenzyme U (ExoU), which we demonstrate is capable of inhibiting caspase-1-driven proinflammatory cytokine production. This study shows a key role for IPAF and capase-1 in innate immune responses to the pathogen P. aeruginosa, and also demonstrates that virulent ExoU-expressing strains of P. aeruginosa can circumvent this innate immune response.

Toll like receptor-5: protecting the gut from enteric microbes

The intestine is normally colonized by a large and diverse commensal microbiota and is occasionally exposed to a variety of potential pathogens. In recent years, there has been substantial progress made in identifying molecular mechanisms that normally serve to protect the intestine from such enteric bacteria and which may go awry in chronic idiopathic inflammatory diseases of the gut. One specific molecular interaction that appears to play a key role in governing bacterial-intestinal interactions is that of the bacterial protein flagellin with toll-like receptor 5. This article reviews studies performed in vitro, in mice, and in humans that indicate an important role for the flagellin-TLR5 interaction in regulating both the innate and adaptive immune responses in the intestine.

The science of adjuvants

Adjuvants are substances that boost the immunogenicity of vaccines. However, most successful vaccines have been derived empirically and are capable of inducing robust T- and B-cell immunity without any adjuvant additives. Emerging evidence suggests that such live vaccines induce innate immune activation via a range of stimuli, including ligands specific for Toll-like receptors, which, in effect, serve as their own adjuvants. In contrast to these live vaccines, subunit vaccines need to be supplemented with adjuvants to boost their immunogenicity. However, there is a paucity of licensed adjuvants for clinical use and, thus, there is a critical need to develop safe and effective adjuvants. In this context, recent advances in innate immunity are beginning to offer new insights into how empiric vaccines and adjuvants mediate their efficacy. In this article, we review the latest progress and emerging concepts in adjuvant development, which includes novel findings in innate immune biology and their impact on vaccinology.

During infection of epithelial cells Salmonella enterica serovar Typhimurium undergoes a time-dependent transcriptional adaptation that results in simultaneous expression of three type 3 secretion systems

The biogenesis of the Salmonella-containing vacuole within mammalian cells has been intensively studied over recent years. However, the ability of Salmonella to sense and adapt to the intracellular environment of different types of host cells has received much less attention. To address this issue, we report the transcriptome of Salmonella enterica serovar Typhimurium SL1344 within epithelial cells and show comparisons with Salmonella gene expression inside macrophages. We report that S. Typhimurium expresses a characteristic intracellular transcriptomic signature in response to the environments it encounters within different cell types. The signature involves the upregulation of the mgtBC, pstACS and iro genes for magnesium, phosphate and iron uptake, and Salmonella pathogenicity island 2 (SPI2). Surprisingly, in addition to SPI2, the invasion-associated SPI1 pathogenicity island and the genes involved in flagellar biosynthesis were expressed inside epithelial cells at later stages of the infection, while they were constantly downregulated in macrophage-like cells. To our knowledge, this is the first report of the simultaneous transcription of all three Type Three Secretion Systems (T3SS) within an intracellular Salmonella population. We discovered that S. Typhimurium strain SL1344 was strongly cytotoxic to epithelial cells after 6 h of infection and hypothesize that the time-dependent changes in Salmonella gene expression within epithelial cells reflects the bacterial response to host cells that have been injured by the infection process.

Legionella pathogenicity: Genome structure, regulatory networks and the host cell response

Legionella spp. the causative agent of Legionnaires' disease is naturally found in fresh water where the bacteria parasitize intracellularly within protozoa. Upon aerosol formation via man-made water systems, Legionella can enter the human lung and cause a severe form of pneumonia. Here we review results from systematic comparative genome analysis of Legionella species with different pathogenic potentials. The complete genomes reveal that horizontal gene transfer has played an important role during the evolution of Legionella and indicate the importance of secretion machineries for the intracellular lifestyle of this pathogen. Moreover, we highlight recent findings on the in vivo transcriptional program of L. pneumophila and the regulatory networks involved in the biphasic life cycle. In order to understand how Legionella effectively subvert host cell functions for its own benefit the transcriptional host cell response upon infection of the model amoeba Dictyostelium discoideum was studied. The use of this model organism made it possible to develop a roadmap of host cell factors which significantly contribute to the uptake of L. pneumophila and the establishment of an ER-associated replicative vacuole.

Functional characterization of chicken TLR5 reveals species-specific recognition of flagellin

Mammalian Toll-like receptor 5 (TLR5) senses flagellin of several bacterial species and activates the innate immune system. The avian TLR repertoire exhibits considerable functional diversity compared to mammalian TLRs and evidence of a functional TLR5 in the avian species is lacking. In the present study we cloned and successfully expressed chicken TLR5 (chTLR5) in HeLa cells, as indicated by laser confocal microscopy. Infection of chTLR5 transfected cells with Salmonella enterica serovar Enteritidis activated NF-kappaB in a dose- and flagellin-dependent fashion. Similar NF-kappaB activation was observed with recombinant bacterial flagellin. Targeted mutagenesis of the proline residue at position 737 in the chTLR5-TIR domain was detrimental to chTLR5 function, confirming that the observed effects were conferred via chTLR5 and the MyD88 signaling pathway. Comparison of human, mouse and chicken TLR5 activation by flagellin of S. enterica serovar Typhimurium revealed that chTLR5 consistently yielded stronger responses than human but not mouse TLR5. This species-specific reactivity was not observed with flagellin of serovar Enteritidis. The species-specific TLR5 response was nullified after targeted mutagenesis of a single amino acid (Q89A) in serovar Typhimurium flagellin, while L415A and N100A substitutions had no effect. These results show that chickens express a functional TLR5 albeit with different flagellin sensing qualities compared to human TLR5. The finding that single amino acid substitutions in bacterial flagellin can alter the species-specific TLR5 response may influence the host range and susceptibility of infection.

Intracellular NOD-like receptors in innate immunity, infection and disease

The innate immune system comprises several classes of pattern-recognition receptors, including Toll-like receptors (TLRs) and nucleotide binding and oligomerization domain-like receptors (NLRs). TLRs recognize microbes on the cell surface and in endosomes, whereas NLRs sense microbial molecules in the cytosol. In this review, we focus on the role of NLRs in host defence against bacterial pathogens. Nod1 and Nod2 sense the cytosolic presence of molecules containing meso-diaminopimelic acid and muramyl dipeptide respectively, and drive the activation of mitogen-activated protein kinase and NF-kappaB. In contrast, Ipaf, Nalp1b and Cryopyrin/Nalp3 promote the assembly of inflammasomes that are required for the activation of caspase-1. Mutation in several NLR members, including NOD2 and Cryopyrin, is associated with the development of inflammatory disorders. Further understanding of NLRs should provide new insights into the mechanisms of host defence and the pathogenesis of inflammatory diseases.

Cytotoxicity in macrophages infected with rough Brucella mutants is type IV secretion system dependent

Smooth Brucella spp. inhibit macrophage apoptosis, whereas rough Brucella mutants induce macrophage oncotic and necrotic cell death. However, the mechanisms and genes responsible for Brucella cytotoxicity have not been identified. In the current study, a random mutagenesis approach was used to create a mutant bank consisting of 11,354 mutants by mariner transposon mutagenesis using Brucella melitensis rough mutant 16M delta manBA as the parental strain. Subsequent screening identified 56 mutants (0.49% of the mutant bank) that failed to cause macrophage cell death (release of 10% or less of the lactate dehydrogenase). The absence of cytotoxicity during infection with these mutants was independent of demonstrable defects in in vitro bacterial growth or uptake and survival in macrophages. Interrupted genes in 51 mutants were identified by DNA sequence analysis, and the mutations included interruptions in virB encoding the type IV secretion system (T4SS) (n = 36) and in vjbR encoding a LuxR-like regulatory element previously shown to be required for virB expression (n = 3), as well as additional mutations (n = 12), one of which also has predicted roles in virB expression. These results suggest that the T4SS is associated with Brucella cytotoxicity in macrophages. To verify this, deletion mutants were constructed in B. melitensis 16M by removing genes encoding phosphomannomutase/phosphomannoisomerase (delta manBA) and the T4SS (delta virB). As predicted, deletion of virB from 16M delta manBA and 16M resulted in a complete loss of cytotoxicity in rough strains, as well as the low level cytotoxicity observed with smooth strains at extreme multiplicities of infection (>1,000). Taken together, these results demonstrate that Brucella cytotoxicity in macrophages is T4SS dependent.

Restriction of Legionella pneumophila growth in macrophages requires the concerted action of cytokine and Naip5/Ipaf signalling pathways

Macrophages from the C57BL/6 (B6) mouse strain restrict intracellular growth of Legionella pneumophila, whereas A/J macrophages are highly permissive. The mechanism by which B6 macrophages restrict Legionella growth remains poorly understood, but is known to require the cytosolic microbe sensors Naip5 (Birc1e) and Ipaf. We hypothesized that Naip5 and Ipaf may act in partnership with other antimicrobial signalling pathways in macrophages. Indeed, we found that macrophages lacking either tumour necrosis factor (TNF)-alpha or type I interferon (IFN) signalling are permissive for growth of L. pneumophila, even in the presence of functional Naip5 and Ipaf alleles. Similarly, macrophages lacking Naip5 and/or Ipaf signalling were permissive even though we found that Naip5 or Ipaf were not required for induction of TNF-alpha and type I IFN. Therefore, our data suggest that the mechanism by which B6 macrophages restrict intracellular replication of L. pneumophila is more complex than previously appreciated, and involves the concerted action of cytokine and intracellular microbe sensor signalling pathways.

Injection of flagellin into the host cell cytosol by Salmonella enterica serotype Typhimurium

Bacterial flagellins are potent inducers of innate immunity. Three signaling pathways have been implicated in the sensing of flagellins; these involve toll-like receptor 5 (TLR5) and the cytosolic proteins Birc1e/Naip5 and Ipaf. Although the structural basis of TLR5-flagellin interaction is known, little is known about how flagellin enters the host cell cytosol to induce signaling via Birc1e/Naip5 and Ipaf. Here we demonstrate for the first time the translocation of bacterial flagellin into the cytosol of host macrophages by the vacuolar pathogen, Salmonella enterica serotype Typhimurium. Translocation of flagellin into the host cell cytosol was directly demonstrated using beta-lactamase reporter constructs. Flagellin translocation required the Salmonella Pathogenicity Island 1 Type III secretion system (SPI-1 T3SS) but not the flagellar T3SS.

Differential regulation of caspase-1 activation, pyroptosis, and autophagy via Ipaf and ASC in Shigella-infected macrophages

Shigella infection, the cause of bacillary dysentery, induces caspase-1 activation and cell death in macrophages, but the precise mechanisms of this activation remain poorly understood. We demonstrate here that caspase-1 activation and IL-1β processing induced by Shigella are mediated through Ipaf, a cytosolic pattern-recognition receptor of the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) family, and the adaptor protein apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (ASC). We also show that Ipaf was critical for pyroptosis, a specialized form of caspase-1-dependent cell death induced in macrophages by bacterial infection, whereas ASC was dispensable. Unlike that observed in Salmonella and Legionella, caspase-1 activation induced by Shigella infection was independent of flagellin. Notably, infection of macrophages with Shigella induced autophagy, which was dramatically increased by the absence of caspase-1 or Ipaf, but not ASC. Autophagy induced by Shigella required an intact bacterial type III secretion system but not VirG protein, a bacterial factor required for autophagy in epithelial-infected cells. Treatment of macrophages with 3-methyladenine, an inhibitor of autophagy, enhanced pyroptosis induced by Shigella infection, suggesting that autophagy protects infected macrophages from pyroptosis. Thus, Ipaf plays a critical role in caspase-1 activation induced by Shigella independently of flagellin. Furthermore, the absence of Ipaf or caspase-1, but not ASC, regulates pyroptosis and the induction of autophagy in Shigella-infected macrophages, providing a novel function for NLR proteins in bacterial–host interactions.

Shigella are bacterial pathogens that are the cause of bacillary dysentery known as shigellosis. A crucial aspect of the propensity of Shigella to cause diseases lies in its ability to invade the cytoplasm of epithelial cells as well as macrophages. The bacterial invasion of macrophages induces pyroptosis, the proinflammatory cell death associated with caspase-1 activation. Activated caspase-1 then cleaves and activates prointerleukin (proIL)-1β and proIL-18, which are proinflammatory cytokines involved in host inflammatory responses. However, the precise mechanisms of caspase-1 activation induced by Shigella infection remain poorly understood. Ipaf, a cytosolic pattern-recognition receptor of the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) family, is a crucial host factor that activates caspase-1 through the sensing of flagellin produced by some bacteria, such as Salmonella or Legionella. We discovered that Ipaf and the adaptor protein ASC are required for caspase-1 activation induced by non-flagellated Shigella infection. Thus, Ipaf and ASC mediate caspase-1 activation by sensing an unknown bacterial factor, but not flagellin. Autophagy, a cellular system for eliminating intracellular pathogens, was dramatically enhanced in Shigella-infected macrophages by the absence of caspase-1 or Ipaf, but not ASC. The inhibition of autophagy promoted Shigella-induced cell death, suggesting that autophagy protects infected macrophages from pyroptosis. This study provides evidence that in Shigella-infected macrophages, autophagy is inhibited by Ipaf and caspase-1, but positively regulated by ASC, providing a novel function for NLR proteins in bacterial–host interactions.

NLR proteins: integral members of innate immunity and mediators of inflammatory diseases

The innate immune system is the first line of defense against microorganisms and is conserved in plants and animals. The nucleotide-binding domain, leucine rich containing (NLR) protein family is a recent addition to the members of innate immunity effector molecules. These proteins are characterized by a central oligomerization domain, termed nucleotide-binding domain (NBD) and a protein interaction domain, leucine-rich repeats (LRRs) at the C terminus. It has been shown that NLR proteins are localized to the cytoplasm and recognize microbial products. To date, it is known that Nod1 and Nod2 detect bacterial cell wall components, whereas Ipaf and Naip detect bacterial flagellin, and NACHT/LRR/Pyrin 1 has been shown to detect anthrax lethal toxin. NLR proteins comprise a diverse protein family (over 20 in humans), indicating that NLRs have evolved to acquire specificity to various pathogenic microorganisms, thereby controlling host-pathogen interactions. Activation of NLR proteins results in inflammatory responses mediated by NF-kappaB, MAPK, or Caspase-1 activation, accompanied by subsequent secretion of proinflammatory cytokines. Mutations in several members of the NLR protein family have been linked to inflammatory diseases, suggesting these molecules play important roles in maintaining host-pathogen interactions and inflammatory responses. Therefore, understanding NLR signaling is important for the therapeutic intervention of various infectious and inflammatory diseases.

Signaling pathways downstream of pattern-recognition receptors and their cross talk

Pattern-recognition receptors (PRRs) initiate innate immunity through pathogen recognition. Serum PRRs opsonize pathogens for enhanced phagocytic clearance. Toll-like receptors (TLRs) initiate common NF-kappaB/AP-1 and distinct IRF3/7 pathways to coordinate innate immunity and to initiate adaptive immunity against diverse pathogens. Cytoplasmic caspase-recruiting domain (CARD) helicases, such as RIG-I/MDA5, mediate antiviral immunity by inducing the production of type I interferons via the adaptor IPS-1, whereas nucleotide-binding oligomerization domain (NOD)-like receptors mediate mainly antibacterial immunity by activating NF-kappaB or inflammasomes. Dectin-1 is important for antifungal immunity, promoting phagocytosis and activating NF-kappaB. Potentially harmful TLR signaling pathways can be negatively regulated by negative feedback mechanisms and also by anti-inflammatory factors such as TGFbeta, interleukin (IL)-10, and steroids. Many combinations of TLR-TLR and TLR-NOD modulate inflammatory responses. TLRs and NALP3 interplay to produce mature IL-1beta. Thus signaling pathways downstream of PRRs and their cross talk control immune responses in effective manners.

Proteasomes control caspase-1 activation in anthrax lethal toxin-mediated cell killing

Activation of caspase-1 through the inflammasome protein Nalp1b controls anthrax lethal toxin (LT)-induced necrosis in murine macrophages. In this study we analyzed physiological changes controlled by caspase-1 in LT-treated murine macrophages. The caspase-1 inhibitor Boc-D-cmk blocked caspase-1 activity and membrane impairment in LT-treated cells. To determine the relationship between caspase-1 activation and membrane integrity, we added Boc-D-cmk to J774A.1 macrophages at different time points following LT exposure. Remarkably, Boc-D-cmk rescued LT-treated macrophages, even when added at the peak of caspase-1 activation. Late addition of the caspase-1 inhibitor reversed the losses of plasma membrane integrity and metabolic activity in these cells. Similar results were obtained with the proteasome inhibitor MG132, one of the most potent inhibitors of LT toxicity. LT-treated macrophages displaying evidence of membrane impairment recovered upon the addition of MG132, mirroring the Boc-D-cmk response. Strikingly, late addition of proteasome inhibitors also abrogated caspase-1 activity in LT-treated macrophages. Proteasomal control of caspase-1 activity and membrane impairment, however, was restricted to LT-induced cytolysis, because proteasome inhibitors did not block caspase-1 activation and cell death triggered by lipopolysaccharide and nigericin. Our findings indicate that proteasome inhibitors do not target caspase-1 directly but instead control an upstream event in LT-treated macrophages leading to caspase-1 activation. Taken together, caspase-1-mediated necrosis appears to be tightly controlled and differentially regulated by proteasomes depending on the source of caspase-1 induction.

Genetics-squared: combining host and pathogen genetics in the analysis of innate immunity and bacterial virulence

The interaction of bacterial pathogens with their hosts' innate immune systems can be extremely complex and is often difficult to disentangle experimentally. Using mouse models of bacterial infections, several laboratories have successfully applied genetic approaches to identify novel host genes required for innate immune defense. In addition, a variety of creative bacterial genetic schemes have been developed to identify key bacterial genes involved in triggering or evading host immunity. In cases where both the host and pathogen are amenable to genetic manipulation, a combination of host and pathogen genetic approaches can be used. Focusing on bacterial infections of mice, this review summarizes the benefits and limitations of applying genetic analysis to the study of host-pathogen interactions. In particular, we consider how prokaryotic and eukaryotic genetic strategies can be combined, or "squared," to yield new insights in host-pathogen biology.

Regulatory molecules involved in inflammasome formation with special reference to a key mediator protein, ASC

The recent identification of cytosolic pattern recognition receptors (PRRs) with leucine-rich repeats, which recognize pathogen-associated molecular patterns (PAMPs), has been garnering considerable attention. Activated PRRs form molecular complexes called inflammasomes, consisting of related proteins that include procaspase 1[interleukin (IL) 1beta converting enzyme (ICE)]. Inflammasomes have been shown to facilitate molecular proximity, stimulate activation of procaspase 1, which consequently produces inflammatory cytokines IL-1beta and IL-18 and ultimately lead to the initiation of innate immunity. An adaptor protein, apoptosis-associated speck-like protein containing a CARD (ASC), which recruits PRRs carrying the pyrin homologous domain (PYD) and procaspase 1 through PYD and CARD, respectively, is responsible for the formation of some inflammasomes and also activation of procaspase 1. In this review, our main attention will be directed to PYD region analysis of ASC to understand the interaction between PYD-carrying PRRs and ASC. Taking into consideration the other aspects of the ASC gene in the proapoptotic ability and down-regulation by methylation, the biological function of ASC will be discussed in relation to the epigenetic aspects of infection, inflammation, and cancer.

Strategies that modulate inflammasomes: insights from host-pathogen interactions

The innate immune system is a dynamic and complex network for recognizing and responding to cellular insult or tissue damage after infection or injury. The primary effector mechanism of innate immunity is the generation of acute and chronic inflammatory responses through regulation of the processing and activation of proinflammatory caspases, particularly caspase 1, and cytokines, most notably IL-1beta and IL-18. Inflammasomes, cytosolic multi-protein complexes that function as molecular scaffolds for caspase activation, have recently emerged as the pivotal mechanism by which host innate immune and inflammatory responses are regulated. In this review, we investigate the mechanisms by which inflammasomes are modulated, both by endogenous host systems and by microbial pathogens.

TLR5 and Ipaf: dual sensors of bacterial flagellin in the innate immune system

The innate immune system precisely modulates the intensity of immune activation in response to infection. Flagellin is a microbe-associated molecular pattern that is present on both pathogenic and nonpathogenic bacteria. Macrophages and dendritic cells are able to determine the virulence of flagellated bacteria by sensing whether flagellin remains outside the mammalian cell, or if it gains access to the cytosol. Extracellular flagellin is detected by TLR5, which induces expression of proinflammatory cytokines, while flagellin within the cytosol of macrophages is detected through the Nod-like receptor (NLR) Ipaf, which activates caspase-1. In macrophages infected with Salmonella typhimurium or Legionella pneumophila, Ipaf becomes activated in response to flagellin that appears to be delivered to the cytosol via specific virulence factor transport systems (the SPI1 type III secretion system (T3SS) and the Dot/Icm type IV secretion system (T4SS), respectively). Thus, TLR5 responds more generally to flagellated bacteria, while Ipaf responds to bacteria that express both flagellin and virulence factors.

Mammalian NLR proteins; discriminating foe from friend

Eukaryotic organisms of the plant and animal kingdoms have developed evolutionarily conserved systems of defence against microbial pathogens. These systems depend on the specific recognition of microbial products or structures by molecules of the host innate immune system. The first mammalian molecules shown to be involved in innate immune recognition of, and defence against, microbial pathogens were the Toll-like receptors (TLRs). These proteins are predominantly but not exclusively located in the transmembrane region of host cells. Interestingly, mammalian hosts were subsequently found to also harbour cytosolic proteins with analogous structures and functions to plant defence molecules. The members of this protein family exhibit a tripartite domain structure and are characterized by a central nucleotide-binding oligomerization domain (NOD). Moreover, in common with TLRs, most NOD proteins possess a C-terminal leucine-rich repeat (LRR) domain, which is required for the sensing of microbial products and structures. Recently, the name 'nucleotide-binding domain and LRR' (NLR) was coined to describe this family of proteins. It is now clear that NLR proteins play key roles in the cytoplasmic recognition of whole bacteria or their products. Moreover, it has been demonstrated in animal studies that NLRs are important for host defence against bacterial infection. This review will particularly focus on two subfamilies of NLR proteins, the NODs and 'NALPs', which specifically recognize bacterial products, including cell wall peptidoglycan and flagellin. We will discuss the downstream signalling events and host cell responses to NLR recognition of such products, as well as the strategies that bacterial pathogens employ to trigger NLR signalling in host cells. Cytosolic recognition of microbial factors by NLR proteins appears to be one mechanism whereby the innate immune system is able to discriminate between pathogenic bacteria ('foe') and commensal ('friendly') members of the host microflora.

The Nod-like receptor family member Naip5/Birc1e restricts Legionella pneumophila growth independently of caspase-1 activation

Similar to Ipaf and caspase-1, the Nod-like receptor protein Naip5 restricts intracellular proliferation of Legionella pneumophila, the causative agent of a severe form of pneumonia known as Legionnaires' disease. Thus, Naip5 has been suggested to regulate Legionella replication inside macrophages through the activation of caspase-1. In this study, we show that cytosolic delivery of recombinant flagellin activated caspase-1 in A/J macrophages carrying a mutant Naip5 allele, and in C57BL/6 (B6) macrophages congenic for the mutant Naip5 allele (B6-Naip5(A/J)), but not in Ipaf(-/-) cells. In line with these results, A/J and B6-Naip5(A/J) macrophages induced high levels of caspase-1 activation and IL-1beta secretion when infected with Legionella. In addition, transgenic expression of a functional Naip5 allele in A/J macrophages did not alter Legionella-induced caspase-1 activation and IL-1beta secretion. Notably, defective Naip5 signaling renders B6-Naip5(A/J) macrophages permissive for Legionella proliferation despite normal caspase-1 activation. These results indicate that the restriction of intracellular Legionella replication is more complex than previously appreciated and requires both Ipaf-dependent caspase-1 activation as well as functional Naip5 signaling.

Activation of the inflammasome upon Francisella tularensis infection: interplay of innate immune pathways and virulence factors

Tularaemia is a zoonotic disease caused by the facultative intracellular bacterium Francisella tularensis. The virulence of this pathogen depends on its ability to escape into the cytosol of host cells. Pathogens are detected by the innate immune system's pattern recognition receptors which are activated in response to conserved microbial molecules (pathogen-associated molecular patterns). Cytosolic bacteria are sensed intracellularly, often leading to activation of the cysteine protease caspase-1 within a multimolecular complex called the inflammasome. Caspase-1 activation leads to both host cell death and release of pro-inflammatory cytokines in a process called pyroptosis. Here we review the pathway leading to, and the consequences of, inflammasome activation upon F. tularensis infection both in vitro and in vivo. Finally, we discuss recent data on how other innate immune pathways and F. tularensis virulence factors control the activation of the inflammasome during infection.

Blocking of the TLR5 Activation Domain Hampers Protective Potential of Flagellin DNA Vaccine

Flagellin is a key component of the flagella of many pathogens, including Pseudomonas aeruginosa. Flagellin is an attractive vaccine candidate because it is readily produced and manipulated as a recombinant protein and has intrinsic adjuvant activity mediated through TLR5. Although DNA vaccines encoding native Pseudomonas B-type (FliC) or A-type (FlaA) flagellin are strongly immunogenic, the resultant Ab response interferes with the interaction of homologous flagellin with TLR5. This reduces the ability of the host to clear homologous, but not heterologous, flagellin-expressing P. aeruginosa. To circumvent this problem, a DNA vaccine encoding a mutant FliC R90A flagellin was developed. The mutant Ag encoded by this vaccine was highly immunogenic, but its ability to interact with TLR5 was reduced by >100-fold. Vaccination with this flagellin mutant DNA vaccine induced cross-reactive Abs against both FliC and FlaA, but few Abs capable of interfering with TLR5 activation. The flagellin mutant DNA vaccine provided excellent protection against both FliC- and FlaA-expressing P. aeruginosa. These findings suggest that vaccines against flagellated pathogens should avoid inducing Abs against TLR5 and raise the possibility that flagellated bacteria evade host elimination by facilitating the production of Abs that reduce the host's ability to mount an innate immune response.

Vibrio vulnificus damages macrophages during the early phase of infection

Vibrio vulnificus is an estuarine bacterium that can cause primary septicemia as well as serious wound infections. Generally, clinical isolates have a high lethal effect compared with environmental isolates. However, little is known about the mechanisms by which V. vulnificus causes disease. In this study, we compared the pathogenicity of a clinical isolate, strain M2799, with that of an environmental isolate, strain JCM3731. The clinical isolate showed 100 times higher lethality in mice than the environmental isolate. In strain M2799-inoculated mice, the number of macrophages decreased significantly, whereas there was no appreciable change in the number of macrophages in strain JCM3731-inoculated mice. The clinical isolate showed high cytotoxic activity, especially to macrophages, compared with the environmental isolate in vitro. The growth of the clinical isolate was almost completely inhibited in the presence of macrophages. Moreover, the survival rate of the clinical isolate-inoculated mice increased by recruitment of macrophages. These results indicate that V. vulnificus infection progresses by damage to macrophages during the early phase of infection.