Cytosolic recognition of flagellin by mouse macrophages restricts Legionella pneumophila infection

Innate immune recognition of Yersinia pseudotuberculosis type III secretion

Specialized protein translocation systems are used by many bacterial pathogens to deliver effector proteins into host cells that interfere with normal cellular functions. How the host immune system recognizes and responds to this intrusive event is not understood. To address these questions, we determined the mammalian cellular response to the virulence-associated type III secretion system (T3SS) of the human pathogen Yersinia pseudotuberculosis. We found that macrophages devoid of Toll-like receptor (TLR) signaling regulate expression of 266 genes following recognition of the Y. pseudotuberculosis T3SS. This analysis revealed two temporally distinct responses that could be separated into activation of NFkappaB- and type I IFN-regulated genes. Extracellular bacteria were capable of triggering these signaling events, as inhibition of bacterial uptake had no effect on the ensuing innate immune response. The cytosolic peptidoglycan sensors Nod1 and Nod2 and the inflammasome component caspase-1 were not involved in NFkappaB activation following recognition of the Y. pseudotuberculosis T3SS. However, caspase-1 was required for secretion of the inflammatory cytokine IL-1beta in response to T3SS-positive Y. pseudotuberculosis. In order to characterize the bacterial requirements for induction of this novel TLR-, Nod1/2-, and caspase-1-independent response, we used Y. pseudotuberculosis strains lacking specific components of the T3SS. Formation of a functional T3SS pore was required, as bacteria expressing a secretion needle, but lacking the pore-forming proteins YopB or YopD, did not trigger these signaling events. However, nonspecific membrane disruption could not recapitulate the NFkappaB signaling triggered by Y. pseudotuberculosis expressing a functional T3SS pore. Although host cell recognition of the T3SS did not require known translocated substrates, the ensuing response could be modulated by effectors such as YopJ and YopT, as YopT amplified the response, while YopJ dampened it. Collectively, these data suggest that combined recognition of the T3SS pore and YopBD-mediated delivery of immune activating ligands into the host cytosol informs the host cell of pathogenic challenge. This leads to a unique, multifactorial response distinct from the canonical immune response to a bacterium lacking a T3SS.

Identification of host cytosolic sensors and bacterial factors regulating the type I interferon response to Legionella pneumophila

Legionella pneumophila is a gram-negative bacterial pathogen that replicates in host macrophages and causes a severe pneumonia called Legionnaires' Disease. The innate immune response to L. pneumophila remains poorly understood. Here we focused on identifying host and bacterial factors involved in the production of type I interferons (IFN) in response to L. pneumophila. It was previously suggested that the delivery of L. pneumophila DNA to the host cell cytosol is the primary signal that induces the type I IFN response. However, our data are not easily reconciled with this model. We provide genetic evidence that two RNA-sensing proteins, RIG-I and MDA5, participate in the IFN response to L. pneumophila. Importantly, these sensors do not seem to be required for the IFN response to L. pneumophila DNA, whereas we found that RIG-I was required for the response to L. pneumophila RNA. Thus, we hypothesize that bacterial RNA, or perhaps an induced host RNA, is the primary stimulus inducing the IFN response to L. pneumophila. Our study also identified a secreted effector protein, SdhA, as a key suppressor of the IFN response to L. pneumophila. Although viral suppressors of cytosolic RNA-sensing pathways have been previously identified, analogous bacterial factors have not been described. Thus, our results provide new insights into the molecular mechanisms by which an intracellular bacterial pathogen activates and also represses innate immune responses.

Initial detection of invading microorganisms is one of the primary tasks of the innate immune system. However, the molecular mechanisms by which pathogens are recognized remain incompletely understood. Here, we provide evidence that an immunosurveillance pathway (called the RIG-I/MDA5 pathway), thought primarily to detect viruses, is also involved in the innate immune response to an intracellular bacterial pathogen, Legionella pneumophila. In the response to viruses, the RIG-I/MDA5 immunosurveillance pathway has been shown to respond to viral RNA or DNA. We found that the RIG-I pathway was required for the response to L. pneumophila RNA, but was not required for the response to L. pneumophila DNA. Thus, one explanation of our results is that L. pneumophila RNA may access the host cell cytosol, where it triggers the RIG-I/MDA5 pathway. This is unexpected since bacteria have not previously been thought to translocate RNA into host cells. We also found that L. pneumophila encodes a secreted bacterial protein, SdhA, which suppresses the RIG-I/MDA5 pathway. Several viral repressors of the RIG-I/MDA5 pathway have been described, but bacterial repressors of RIG-I/MDA5 are not known. Thus, our study provides novel insights into the molecular mechanisms by which the immune system detects bacterial infection, and conversely, by which bacteria suppress innate immune responses.

Control of flagellar gene regulation in Legionella pneumophila and its relation to growth phase

The bacterial pathogen Legionella pneumophila responds to environmental changes by differentiation. At least two forms are well described: replicative bacteria are avirulent; in contrast, transmissive bacteria express virulence traits and flagella. Phenotypic analysis, Western blotting, and electron microscopy of mutants of the regulatory genes encoding RpoN, FleQ, FleR, and FliA demonstrated that flagellin expression is strongly repressed and that the mutants are nonflagellated in the transmissive phase. Transcriptome analyses elucidated that RpoN, together with FleQ, enhances transcription of 14 out of 31 flagellar class II genes, which code for the basal body, hook, and regulatory proteins. Unexpectedly, FleQ independent of RpoN enhances the transcription of fliA encoding sigma 28. Expression analysis of a fliA mutant showed that FliA activates three out of the five remaining flagellar class III genes and the flagellar class IV genes. Surprisingly, FleR does not induce but inhibits expression of at least 14 flagellar class III genes on the transcriptional level. Thus, we propose that flagellar class II genes are controlled by FleQ and RpoN, whereas the transcription of the class III gene fliA is controlled in a FleQ-dependent but RpoN-independent manner. However, RpoN and FleR might influence flagellin synthesis on a posttranscriptional level. In contrast to the commonly accepted view that enhancer-binding proteins such as FleQ always interact with RpoN to fullfill their regulatory functions, our results strongly indicate that FleQ regulates gene expression that is RpoN dependent and RpoN independent. Finally, FliA induces expression of flagellar class III and IV genes leading to the complete synthesis of the flagellum.

Mouse macrophages are permissive to motile Legionella species that fail to trigger pyroptosis

Legionella pneumophila, a motile opportunistic pathogen of humans, is restricted from replicating in the lungs of C57BL/6 mice. Resistance of mouse macrophages to L. pneumophila depends on recognition of cytosolic flagellin. Once detected by the NOD-like receptors Naip5 and Ipaf (Nlrc4), flagellin triggers pyroptosis, a proinflammatory cell death. In contrast, motile strains of L. parisiensis and L. tucsonensis replicate profusely within C57BL/6 macrophages, similar to flagellin-deficient L. pneumophila. To gain insight into how motile species escape innate defense mechanisms of mice, we compared their impacts on macrophages. L. parisiensis and L. tucsonensis do not induce proinflammatory cell death, as measured by lactate dehydrogenase (LDH) release and interleukin-1beta (IL-1beta) secretion. However, flagellin isolated from L. parisiensis and L. tucsonensis triggers cell death and IL-1beta secretion when transfected into the cytosol of macrophages. Neither strain displays three characteristics of the canonical L. pneumophila Dot/Icm type IV secretion system: sodium sensitivity, LAMP-1 evasion, and pore formation. Therefore, we postulate that when L. parisiensis and L. tucsonensis invade a mouse macrophage, flagellin is confined to the phagosome, protecting the bacteria from recognition by the cytosolic surveillance system and allowing Legionella to replicate. Despite their superior capacity to multiply in mouse macrophages, L. parisiensis and L. tucsonensis have been associated with only two cases of disease, both in renal transplant patients. These results point to the complexity of disease, a product of the pathogenic potential of the microbe, as defined in the laboratory, and the capacity of the host to mount a measured defense.

Transcriptional and post-transcriptional regulation of the GmaR antirepressor governs temperature-dependent control of flagellar motility in Listeria monocytogenes

Flagellar motility in Listeria monocytogenes (Lm) is restricted to temperatures below 37 degrees C due to the opposing activities of the MogR transcriptional repressor and the GmaR antirepressor. Previous studies have suggested that both the DegU response regulator and MogR regulate expression of GmaR. In this report, we further define the role of DegU for GmaR production and flagellar motility. We demonstrate that deletion of the receiver domain of DegU has no effect on flagellar motility in Lm. Using transcriptional reporter fusions, we determined that gmaR is cotranscribed within an operon initiating with fliN. Furthermore, the fliN-gmaR promoter (p(fliN-gmaR)) is transcriptionally activated by DegU and is also MogR-repressed. DNA affinity purification, gel mobility shift and footprinting analyses revealed that both DegU and MogR directly bind fliN-gmaR promoter region DNA and that the binding sites do not overlap. Quantitative analysis of gmaR transcripts in Delta mogR bacteria indicated that transcriptional activation of p(fliN-gmaR) by DegU is not inherently temperature-dependent. However, GmaR protein was not detectable at 37 degrees C in Delta mogR bacteria, indicating that a temperature-dependent, post-transcriptional mechanism limits GmaR production to temperatures below 37 degrees C. Our findings reveal that flagellar motility in Lm is governed by both temperature-dependent transcriptional and post-transcriptional regulation of the GmaR antirepressor.

Brucella abortus induces Irgm3 and Irga6 expression via type-I IFN by a MyD88-dependent pathway, without the requirement of TLR2, TLR4, TLR5 and TLR9

The innate immune system senses bacterial pathogens by pattern recognition receptors, such as the well-characterised Toll-like Receptors (TLR). The activation of TLR signalling cascades depends on several adaptor proteins, among which MyD88 plays a key role in triggering innate immune responses. Here, we show in murine macrophages that Brucella abortus triggers expression of the interferon-inducible resistance proteins (IRGs, p47 GTPases) via type-I IFN secretion at late time points, when Brucella has reached its replication niche. This induction requires the adaptor molecule MyD88 but does not involve the TLRs normally implicated in sensing Gram-negative bacteria, namely TLR2, TLR4, TLR5 and TLR9. Brucella mutants lacking the functional VirB type-IV secretion system were not capable of inducing Irgm3 and Irga6 expression, suggesting that the type-IV secretion system is part of the triggering of the activation process. Our data suggest that Brucella is recognized intracellularly by an unknown receptor, different from the conventional ones used for Gram-negative sensing, but one that nevertheless signals through MyD88.

Interferons direct an effective innate response to Legionella pneumophila infection

Legionella pneumophila remains an important opportunistic pathogen of human macrophages. Its more limited ability to replicate in murine macrophages has been attributed to redundant innate sensor systems that detect and effectively respond to this infection. The current studies evaluate the role of one of these innate response systems, the type I interferon (IFN-I) autocrine loop. The ability of L. pneumophila to induce IFN-I expression was found to be dependent on IRF-3, but not NF-kappaB. Secreted IFN-Is then in turn suppress the intracellular replication of L. pneumophila. Surprisingly, this suppression is mediated by a pathway that is independent of Stat1, Stat2, Stat3, but correlates with the polarization of macrophages toward the M1 or classically activated phenotype.

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

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.

Involvement of SPI-2-encoded SpiC in flagellum synthesis in Salmonella enterica serovar Typhimurium

BACKGROUND

SpiC encoded within Salmonella pathogenicity island 2 on the Salmonella enterica serovar Typhimurium chromosome is required for survival within macrophages and systemic infection in mice. Additionally, SpiC contributes to Salmonella-induced activation of the signal transduction pathways in macrophages by affecting the expression of FliC, a component of flagella filaments. Here, we show the contribution of SpiC in flagellum synthesis.

RESULTS

Quantitative RT-PCR shows that the expression levels of the class 3 fliD and motA genes that encode for the flagella cap and motor torque proteins, respectively, were lower for a spiC mutant strain than for the wild-type Salmonella. Further, this mutant had lower expression levels of the class 2 genes including the fliA gene encoding the flagellar-specific alternative sigma factor. We also found differences in flagella assembly between the wild-type strain and the spiC mutant. Many flagella filaments were observed on the bacterial surface of the wild-type strain, whereas the spiC mutant had only few flagella. The absence of spiC led to reduced expression of the FlhD protein, which functions as the master regulator in flagella gene expression, although no significant difference at the transcription level of the flhDC operon was observed between the wild-type strain and the spiC mutant.

CONCLUSION

The data show that SpiC is involved in flagella assembly by affecting the post-transcription expression of flhDC.

A Legionella type IV effector activates the NF-kappaB pathway by phosphorylating the IkappaB family of inhibitors

NF-kappaB is critical in innate immune defense responses against invading microbial pathogens. Legionella pneumophila infection of lung macrophages causes Legionnaire's disease with pneumonia symptoms. A set of NF-kappaB-controlled genes involved in inflammation and anti-apoptosis are up-regulated in macrophages upon L. pneumophila infection in a Legionella Dot/Icm type IV secretion system-dependent manner. Among approximately 100 Dot/Icm substrates screened, we identified LegK1 as the sole Legionella protein that harbors a highly potent NF-kappaB-stimulating activity. LegK1 does not affect MAPK and IFN pathways. Activation of the NF-kappaB pathway by LegK1 requires its eukaryotic-like Ser/Thr kinase activity and is independent of upstream components in the NF-kappaB pathway, including TRAFs, NIK, MEKK3, and TAK1. Cell-free reconstitution revealed that LegK1 stimulated NF-kappaB activation in the absence of IKKalpha and IKKbeta, and LegK1 efficiently phosphorylated IkappaBalpha on Ser-32 and Ser-36 both in vitro and in cells. LegK1 seems to mimic the host IKK as LegK1 also directly phosphorylated other IkappaB family of inhibitors including p100 in the noncanonical NF-kappaB pathway. Phosphorylation of p100 by LegK1 led to its maturation into p52. Thus, LegK1 is a bacterial effector that directly activates the host NF-kappaB signaling and likely plays important roles in modulating macrophage defense or inflammatory responses during L. pneumophila infection.

Inflammasomes: guardians of cytosolic sanctity

The innate immune system is critical in recognizing bacterial and viral infections to evoke a proper immune response. Certain members of the intracellular nucleotide-binding and oligomerization domain (NOD)-like receptor (NLR) family detect microbial components in the cytosol and trigger the assembly of large caspase-1-activating complexes termed inflammasomes. Autoproteolytic maturation of caspase-1 zymogens within these inflammasomes leads to maturation and secretion of the pro-inflammatory cytokines interleukin-1 beta (IL-1 beta) and IL-18. The NLR proteins ICE protease-activating factor (IPAF), NALP1b (NACHT domain-, leucine-rich repeat-, and PYD-containing protein 1b), and cryopyrin/NALP3 assemble caspase-1-activating inflammasomes in a stimulus-dependent manner. Bacterial flagellin is sensed by IPAF, whereas mouse NALP1b detects anthrax lethal toxin. Cryopyrin/NALP3 mediates caspase-1 activation in response to a wide variety of microbial components and in response to crystalline substances such as the endogenous danger signal uric acid. Genetic variations in Nalp1 and cryopyrin/Nalp3 are associated with autoinflammatory disorders and increased susceptibility to microbial infection. Further understanding of inflammasomes and their role in innate immunity should provide new insights into the mechanisms of host defense and the pathogenesis of autoimmune diseases.

Function of Nod-like receptors in microbial recognition and host defense

Nucleotide oligomerization domain (NOD)-like receptors (NLRs) are a specialized group of intracellular proteins that play a critical role in the regulation of the host innate immune response. NLRs act as scaffolding proteins that assemble signaling platforms that trigger nuclear factor-kappaB and mitogen-activated protein kinase signaling pathways and control the activation of inflammatory caspases. Importantly, mutations in several members of the NLR family have been linked to a variety of inflammatory diseases consistent with these molecules playing an important role in host-pathogen interactions and the inflammatory response. In this review, we focus on the role of Nod1 and Nod2 in host defense and in particular discuss recent finding regarding the role of Nlrc4, Nlpr1, and Nlrp3 inflammasomes in caspase-1 activation and subsequent release of proinflammatory cytokines such as interleukin-1 beta.

Patterns of pathogenesis: discrimination of pathogenic and nonpathogenic microbes by the innate immune system

The dominant conceptual framework for understanding innate immunity has been that host cells respond to evolutionarily conserved molecular features of pathogens called pathogen-associated molecular patterns (PAMPs). Here, we propose that PAMPs should be understood in the context of how they are naturally presented by pathogens. This can be experimentally challenging, since pathogens, almost by definition, bypass host defense. Nevertheless, in this review, we explore the idea that the immune system responds to PAMPs in the context of additional signals that derive from common "patterns of pathogenesis" employed by pathogens to infect, multiply within, and spread among their hosts.

Experimental Legionella longbeachae infection in intratracheally inoculated mice

This study established an experimental model of replicative Legionella longbeachae infection in A/J mice. The animals were infected by intratracheal inoculation of 10(3)-10(9) c.f.u. L. longbeachae serogroup 1 (USA clinical isolates D4968, D4969 and D4973). The inocula of 10(9), 10(8), 10(7) and 10(6) c.f.u. of all tested L. longbeachae serogroup 1 isolates were lethal for A/J mice. Inoculation of 10(5) c.f.u. L. longbeachae caused death in 90 % of the animals within 5 days, whilst inoculation of 10(4) c.f.u. caused sporadic death of mice. All animals that received 10(3) c.f.u. bacteria developed acute lower respiratory disease, but were able to clear Legionella from the lungs within 3 weeks. The kinetics of bacterial growth in the lungs was independent of inoculum size and reached a growth peak about 3 logarithms above the initial inoculum at 72 h after inoculation. The most prominent histological changes in the lungs were observed at 48-72 h after inoculation in the form of a focal, neutrophil-dominant, peribronchiolar infiltration. The inflammatory process did not progress towards the interstitial or alveolar spaces. Immunohistological analyses revealed L. longbeachae serogroup 1 during the early phase of infection near the bronchiolar epithelia and later co-localized with inflammatory cells. BALB/c and C57BL/6 mice strains were also susceptible to infection with all L. longbeachae serogroup 1 strains tested and very similar changes were observed in the lungs of infected animals. These results underline the infection potential of L. longbeachae serogroup 1, which is associated with high morbidity and lethality in mice.

Unleashing the therapeutic potential of NOD-like receptors

Nucleotide-binding and oligomerization domain (NOD)-like receptors (NLRs) are a family of intracellular sensors that have key roles in innate immunity and inflammation. Whereas some NLRs - including NOD1, NOD2, NAIP (NLR family, apoptosis inhibitory protein) and NLRC4 - detect conserved bacterial molecular signatures within the host cytosol, other members of this family sense 'danger signals', that is, xenocompounds or molecules that when recognized alert the immune system of hazardous environments, perhaps independently of a microbial trigger. In the past few years, remarkable progress has been made towards deciphering the role and the biology of NLRs, which has shown that these innate immune sensors have pivotal roles in providing immunity to infection, adjuvanticity and inflammation. Furthermore, several inflammatory disorders have been associated with mutations in human NLRgenes. Here, we discuss the effect that research on NLRs will have on vaccination, treatment of chronic inflammatory disorders and acute bacterial infections.

NLR-mediated control of inflammasome assembly in the host response against bacterial pathogens

The host response against diverse bacterial pathogens involves activation of specialized immune cells and elaboration of pro-inflammatory cytokines that help to coordinate appropriate host defense. Members of the interleukin-1 (IL-1) cytokine family, IL-1beta and IL-18, are central players in this process. Extracellular release of the mature, active form of these cytokines requires their processing by the cysteine protease caspase-1, which therefore serves as a key regulator of the inflammatory response. In addition to its role in secretion of pro-inflammatory cytokines, caspase-1 is also required for a form of cell death, recently termed pyroptosis, that occurs in macrophages infected by certain bacterial pathogens. Caspase-1 itself is synthesized as a pro-enzyme, which must first be activated by autocatalytic cleavage. This activation requires recruitment of caspase-1 into multiprotein complexes known as inflammasomes. The Nod-like receptor (NLR) family of cytosolic proteins play an important role in detecting inflammatory stimuli and subsequently mediate inflammasome assembly. A common feature of NLR proteins that trigger inflammasome assembly in response to bacterial infection is that they appear to sense membrane perturbation or delivery of bacterial components into the cytosol through bacterial pore-forming toxins or bacterial secretion systems. This review will discuss the recent developments regarding caspase-1 activation in response to bacterial infection, cross-talk between caspase-1 and other pathways involved in regulating cell death, and recent findings that a number of bacterial pathogens possess mechanisms to inhibit caspase-1 activation.

Rapid pathogen-induced apoptosis: a mechanism used by dendritic cells to limit intracellular replication of Legionella pneumophila

Dendritic cells (DCs) are specialized phagocytes that internalize exogenous antigens and microbes at peripheral sites, and then migrate to lymphatic organs to display foreign peptides to naïve T cells. There are several examples where DCs have been shown to be more efficient at restricting the intracellular replication of pathogens compared to macrophages, a property that could prevent DCs from enhancing pathogen dissemination. To understand DC responses to pathogens, we investigated the mechanisms by which mouse DCs are able to restrict replication of the intracellular pathogen Legionella pneumophila. We show that both DCs and macrophages have the ability to interfere with L. pneumophila replication through a cell death pathway mediated by caspase-1 and Naip5. L. pneumophila that avoided Naip5-dependent responses, however, showed robust replication in macrophages but remained unable to replicate in DCs. Apoptotic cell death mediated by caspase-3 was found to occur much earlier in DCs following infection by L. pneumophila compared to macrophages infected similarly. Eliminating the pro-apoptotic proteins Bax and Bak or overproducing the anti-apoptotic protein Bcl-2 were both found to restore L. pneumophila replication in DCs. Thus, DCs have a microbial response pathway that rapidly activates apoptosis to limit pathogen replication.

The immune system is designed to identify microbes that enter the body and elicit responses that prevent the replication and dissemination of these organisms. Dendritic cells play an important role in regulating host immunity to pathogens. Their phagocytic capacity enables DCs to internalize and destroy most microbes, and the ability of DCs to migrate to specialized lymphoid organs is important for inducing antigen-specific immunity. Here, we analyzed interactions between DCs and Legionella pneumophila, a bacterial pathogen that can subvert phagocytic host cell functions to create a vacuole that permits intracellular replication. We found that L. pneumophila infection rapidly induced DCs to commit cell death through apoptosis. Rapid apoptosis was not observed after infection of macrophages, which are the phagocytic cells that support L. pneumophila replication in the lungs of infected animals. Using cells derived from knockout mice, we found that DCs deficient in the proteins Bax and Bak, which are essential for induction of the apoptosis pathway, were unable to restrict the intracellular replication of L. pneumophila. Likewise, overproduction of Bcl-2, which is a negative regulator of apoptosis, resulted in DCs that were permissive for L. pneumophila replication. These data indicate DCs have the ability to rapidly undergo apoptosis when infected with a microbe capable of replicating intracellularly, and this response effectively prevents pathogen replication. We hypothesize that this response may be designed to interfere with the migration of infected DCs through the lymphatic system, which would prevent DCs from serving as a “Trojan Horse” that transports pathogenic microbes from peripheral sites to central organs.

A novel protein isoform of the multicopy human NAIP gene derives from intragenic Alu SINE promoters

The human neuronal apoptosis inhibitory protein (NAIP) gene is no longer principally considered a member of the Inhibitor of Apoptosis Protein (IAP) family, as its domain structure and functions in innate immunity also warrant inclusion in the Nod-Like Receptor (NLR) superfamily. NAIP is located in a region of copy number variation, with one full length and four partly deleted copies in the reference human genome. We demonstrate that several of the NAIP paralogues are expressed, and that novel transcripts arise from both internal and upstream transcription start sites. Remarkably, two internal start sites initiate within Alu short interspersed element (SINE) retrotransposons, and a third novel transcription start site exists within the final intron of the GUSBP1 gene, upstream of only two NAIP copies. One Alu functions alone as a promoter in transient assays, while the other likely combines with upstream L1 sequences to form a composite promoter. The novel transcripts encode shortened open reading frames and we show that corresponding proteins are translated in a number of cell lines and primary tissues, in some cases above the level of full length NAIP. Interestingly, some NAIP isoforms lack their caspase-sequestering motifs, suggesting that they have novel functions. Moreover, given that human and mouse NAIP have previously been shown to employ endogenous retroviral long terminal repeats as promoters, exaptation of Alu repeats as additional promoters provides a fascinating illustration of regulatory innovations adopted by a single gene.

NOD-like receptors: role in innate immunity and inflammatory disease

The NOD-like receptors (NLRs) are a specialized group of intracellular receptors that represent a key component of the host innate immune system. Since the discovery of the first NLR almost 10 years ago, the study of this special class of microbial sensors has burgeoned; consequently, a better understanding of the mechanism by which these receptors recognize microbes and other danger signals and of how they activate inflammatory signaling pathways has emerged. Moreover, in addition to their primary role in host defense against invading pathogens, their ability to regulate nuclear factor-kappa B (NF-kappaB) signaling, interleukin-1-beta (IL-1beta) production, and cell death indicates that they are crucial to the pathogenesis of a variety of inflammatory human diseases.

The inflammasomes: guardians of the body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1beta and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

Flagellin: key target of mucosal innate immunity

The mucosal immune system is charged with defending the host's vast interfaces with the outside world from the enormous and diverse group of microbes that colonizes these surfaces. A key means by which the mucosal immune system protects the host from such diverse microbes is using germ-line-encoded receptors that target structurally conserved motifs that mediate important bacterial functions. This review focuses on one embodiment of this notion, namely, the mucosal innate immune targeting of flagellin, the primary structural component of flagella, which afford bacteria the ability of directed locomotion. Specifically, we discuss the mechanisms by which flagellin is recognized by the innate immune system, their role in host defense, chronic inflammatory disease, and potential approaches to pharmacologically manipulate these pathways to benefit the host. Discussion will focus on the intestinal tract but will also incorporate key findings in other mucosal surfaces.

The Inflammasomes: Guardians of the Body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1β and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

The roles of TLRs, RLRs and NLRs in pathogen recognition

The mammalian innate immune system detects the presence of microbial infection through germ line-encoded pattern recognition receptors (PRRs). Toll-like receptors, retinoic acid-inducible gene-I-like receptors and nucleotide-binding oligomerization domain-like receptors serve as PRRs that recognize different but overlapping microbial components. They are expressed in different cellular compartments such as the cell surface, endosome, lysosome or cytoplasm and activate specific signaling pathways that lead to expression of genes that tailor immune responses to particular microbes. This review summarizes recent insights into pathogen sensing by these PRRs and their signaling pathways.

The Inflammasomes: Guardians of the Body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1β and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

The Inflammasomes: Guardians of the Body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1β and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

The Inflammasomes: Guardians of the Body

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and to eliminate them. The discovery of Toll-like receptors (TLRs) provided a class of membrane receptors that sense extracellular microbes and trigger antipathogen signaling cascades. More recently, intracellular microbial sensors have been identified, including NOD-like receptors (NLRs). Some of the NLRs also sense nonmicrobial danger signals and form large cytoplasmic complexes called inflammasomes that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines IL-1β and IL-18. The NALP3 inflammasome has been associated with several autoinflammatory conditions including gout. Likewise, the NALP3 inflammasome is a crucial element in the adjuvant effect of aluminum and can direct a humoral adaptive immune response. In this review, we discuss the role of NLRs, and in particular the inflammasomes, in the recognition of microbial and danger components and the role they play in health and disease.

Bacterial triggering of inflammation by intracellular sensors

Recognition of bacterial infection is the first key step to the initiation of an inflammatory response and host defense. Transmembrane proteins of the Toll-like receptor family have long been recognized as key detectors of the extracellular presence of pathogens. Recently, much research has identified a variety of intracellular detectors that also mediate innate immune responses following bacterial infection. These cannot only recognize bacteria that invade the cell cytoplasm, but also a variety of bacterial products that are introduced into cells by specialized secretion systems or are secreted toxins. This article will focus on these intracellular detectors and the bacterial components that they recognize. These detectors are particularly well adapted to recognize the presence of pathogenic bacteria as opposed to commensal organisms. Their growing importance suggests that targeting such intracellular pathways may be important in the future for manipulating the immune response to infection as an aid to augmenting host defense and providing more effective vaccines.

NLRP3/cryopyrin is necessary for interleukin-1beta (IL-1beta) release in response to hyaluronan, an endogenous trigger of inflammation in response to injury

Inflammation under sterile conditions is a key event in autoimmunity and following trauma. Hyaluronan, a glycosaminoglycan released from the extracellular matrix after injury, acts as an endogenous signal of trauma and can trigger chemokine release in injured tissue. Here, we investigated whether NLRP3/cryopyrin, a component of the inflammasome, participates in the inflammatory response to injury or the cytokine response to hyaluronan. Mice with a targeted deletion in cryopyrin showed a normal increase in Cxcl2 in response to sterile injuries but had decreased inflammation and release of interleukin-1beta (IL-1beta). Similarly, the addition of hyaluronan to macrophages derived from cryopyrin-deficient mice increased release of Cxcl2 but did not increase IL-1beta release. To define the mechanism of hyaluronan-mediated activation of cryopyrin, elements of the hyaluronan recognition process were studied in detail. IL-1beta release was inhibited in peritoneal macrophages derived from CD44-deficient mice, in an MH-S macrophage cell line treated with antibodies to CD44, or by inhibitors of lysosome function. The requirement for CD44 binding and hyaluronan internalization could be bypassed by intracellular administration of hyaluronan oligosaccharides (10-18-mer) in lipopolysaccharide-primed macrophages. Therefore, the action of CD44 and subsequent hyaluronan catabolism trigger the intracellular cryopyrin --> IL-1beta pathway. These findings support the hypothesis that hyaluronan works through IL-1beta and the cryopyrin system to signal sterile inflammation.

Induction of adaptive immunity by flagellin does not require robust activation of innate immunity

The ability of TLR agonists to promote adaptive immune responses is attributed to their ability to robustly activate innate immunity. However, it has been observed that, for adjuvants in actual use in research and vaccination, TLR signaling is dispensable for generating humoral immunity. Here, we examined the role of TLR5 and MyD88 in promoting innate and humoral immunity to flagellin using a prime/boost immunization regimen. We observed that eliminating TLR5 greatly reduced flagellin-induced cytokine production, except for IL-18, and ablated DC maturation but did not significantly impact flagellin's ability to promote humoral immunity. Elimination of MyD88, which will ablate signaling through TLR and IL-1beta/IL-18 generated by Nod-like receptors, reduced, but did not eliminate flagellin's promotion of humoral immunity. In contrast, loss of the innate immune receptor for profilin-like protein (PLP), TLR11, greatly reduced the ability of PLP to elicit humoral immunity. Together, these results indicate that, firstly, the degree of innate immune activation induced by TLR agonists may be in great excess of that needed to promote humoral immunity and, secondly, there is considerable redundancy in mechanisms that promote the humoral immune response upon innate immune recognition of flagellin. Thus, it should be possible to design innate immune activators that are highly effective vaccine adjuvants yet avoid the adverse events associated with systemic TLR activation.

Asc and Ipaf Inflammasomes direct distinct pathways for caspase-1 activation in response to Legionella pneumophila

Caspase-1 activation is a key feature of the innate immune response of macrophages elicited by pathogens and a variety of toxins. Here, we determined the requirement for different adapter proteins involved in regulating host processes mediated by caspase-1 after macrophage infection by Legionella pneumophila. The adapter protein Asc was found to be important for caspase-1 activation during L. pneumophila infection. Activation of caspase-1 through Asc did not require the flagellin-sensing pathway involving the host nucleotide-binding domain and leucine-rich repeat-containing protein Ipaf (NLRC4). Asc-dependent caspase-1 activation was inhibited by high extracellular potassium levels, whereas Ipaf-dependent activation was unaffected by potassium treatment. Activation of caspase-1 in macrophages occurred independently of Nalp3 and proteasome activity, suggesting that a previously uncharacterized mechanism for caspase-1 activation through Asc may be triggered by L. pneumophila. Rapid pore formation and pyroptosis induced by L. pneumophila required caspase-1, Ipaf, and bacterial flagellin but occurred independently of Asc. Equivalent levels of active interleukin-18 (IL-18) were detected in the lungs of mice infected with a flagellin-deficient strain of L. pneumophila and Asc-deficient mice infected with wild-type L. pneumophila. Active IL-18 was undetectable in the lungs of Asc-deficient mice infected with an L. pneumophila flagellin mutant, indicating independent roles for Ipaf and Asc in caspase-1-mediated processing and release of IL-18 in vivo. Ipaf-dependent activation of caspase-1 restricted bacterial replication in vivo, whereas Asc was dispensable for restriction of L. pneumophila replication in mice. Thus, L. pneumophila-mediated caspase-1 activation involves the coordinate activities of inflammasomes differentially regulated by Ipaf and Asc.

Sensing pathogens and danger signals by the inflammasome

The NLR (nucleotide-binding domain leucine-rich repeat containing) family of intracellular sensors is a crucial component of the innate immune system. A number of NLR family members can form multiprotein complexes, called inflammasomes, and are capable of activating the cysteine protease caspase-1 in response to a wide range of stimuli including both microbial and self-molecules. Caspase-1 activation leads to processing and secretion of the proinflammatory cytokines interleukin-1beta (IL-1beta) and IL-18, which play crucial roles in host defense to infectious insults. Dysregulation of the inflammasome has also been linked to a number of autoinflammatory and autoimmune disorders. Recent advances in the inflammasome field will be discussed in this review.

Pyroptosis: host cell death and inflammation

Eukaryotic cells can initiate several distinct programmes of self-destruction, and the nature of the cell death process (non-inflammatory or proinflammatory) instructs responses of neighbouring cells, which in turn dictates important systemic physiological outcomes. Pyroptosis, or caspase 1-dependent cell death, is inherently inflammatory, is triggered by various pathological stimuli, such as stroke, heart attack or cancer, and is crucial for controlling microbial infections. Pathogens have evolved mechanisms to inhibit pyroptosis, enhancing their ability to persist and cause disease. Ultimately, there is a competition between host and pathogen to regulate pyroptosis, and the outcome dictates life or death of the host.

Immune interventions of human diseases through toll-like receptors

Toll-like receptors (TLRs) are the immune sensors for infections, triggering robust innate immune activation followed by protective adaptive immunity against various infectious diseases. Recent evidence, however, has suggested that TLRs are involved in the pathogenesis of many diseases, including not only infectious diseases but also autoimmune diseases, allergy and atherosclerosis. Therefore, prophylactic or therapeutic application of TLR-based immune interventions should be potent, but their safety must be demonstrated using experimental animal models as well as human resources, including analysis of single nucleotide polymorphisms. Here, we focus on recent advances in understanding of the protective and pathogenic roles of TLRs in human diseases.

Elevated production of Legionella-specific immunoglobulin A in A/J mice is accompanied by T-helper 1-type polarization

Legionella pneumophila (Lpn) is a Gram-negative bacterium and an intracellular parasite that causes Legionnaires' disease. Secretion of immunoglobulin A (IgA) against Lpn on the mucosal surface of the upper respiratory tract may be important as a self-defense mechanism. A/J mice have been demonstrated to be permissive as to Lpn replication in macrophages due to a natural mutation in neuronal apoptosis inhibitory protein 5. We compared A/J and BALB/c mice as to IgA production after repeated intranasal immunization using a heat-killed Lpn in the presence of cholera toxin as a mucosal adjuvant. A/J mice secreted more Lpn-specific IgA in nasal washes than BALB/c mice. The Lpn-specific serum IgA level was also higher in A/J than BALB/c mice. Because both BALB/c and A/J mice are known to exhibit T-helper 2 (Th2)-biased immune responses, we examined whether the Lpn-specific IgA production is related to the stronger Th2 bias. There was no difference in IgG1 (Th2-controlled) while A/J mice produced more IgG2a (Th1-controlled), suggesting that the elevated IgA response was rather correlated with Th1-controlled isotype switching. Our results also suggest that A/J mice will be useful hosts for Lpn-specific IgA production such as for the preparation of IgA monoclonal antibodies.

Type IV secretion-dependent activation of host MAP kinases induces an increased proinflammatory cytokine response to Legionella pneumophila

The immune system must discriminate between pathogenic and nonpathogenic microbes in order to initiate an appropriate response. Toll-like receptors (TLRs) detect microbial components common to both pathogenic and nonpathogenic bacteria, whereas Nod-like receptors (NLRs) sense microbial components introduced into the host cytosol by the specialized secretion systems or pore-forming toxins of bacterial pathogens. The host signaling pathways that respond to bacterial secretion systems remain poorly understood. Infection with the pathogen Legionella pneumophila, which utilizes a type IV secretion system (T4SS), induced an increased proinflammatory cytokine response compared to avirulent bacteria in which the T4SS was inactivated. This enhanced response involved NF-κB activation by TLR signaling as well as Nod1 and Nod2 detection of type IV secretion. Furthermore, a TLR- and RIP2-independent pathway leading to p38 and SAPK/JNK MAPK activation was found to play an equally important role in the host response to virulent L. pneumophila. Activation of this MAPK pathway was T4SS-dependent and coordinated with TLR signaling to mount a robust proinflammatory cytokine response to virulent L. pneumophila. These findings define a previously uncharacterized host response to bacterial type IV secretion that activates MAPK signaling and demonstrate that coincident detection of multiple bacterial components enables immune discrimination between virulent and avirulent bacteria.

The host immune system senses bacterial infection by recognizing conserved bacterial components. The host can differentiate between virulent and avirulent bacteria by detecting the activity of bacterial secretion systems that inject effector proteins into cells. How the host responds to such bacterial secretion systems is not fully understood. Using the bacterial pathogen Legionella pneumophila and isogenic mutants that differ in defined virulence properties, a robust immune response to L. pneumophila and its type IV secretion system was identified. Using macrophages lacking various aspects of the innate immune system, it was found that this host response is comprised of signaling by extracellular and intracellular immune receptors, as well as host signaling triggered by the type IV secretion system. Through genomic and biochemical analyses of L. pneumophila–infected macrophages, type IV secretion was found to activate additional host signaling pathways distinct from known immune pathways. Our data indicate that coincident detection of multiple bacterial components is required for a robust immune response to bacterial infection and highlights a key host pathway triggered by bacterial type IV secretion that contributes to this immune response.

Inhibitor of apoptosis proteins in eukaryotic evolution and development: a model of thematic conservation

The past decade and a half has witnessed the discovery of a large, evolutionarily conserved family of cellular genes bearing homology to the prototype baculovirus Inhibitor of Apoptosis (IAP). The logical decision in the field to also refer to these cellular proteins as IAPs fails to do justice to this versatile group of factors that play a wide range of roles in eukaryotic development and homeostasis which include, but are not limited to, the regulation of programmed cell death. Here we describe the shared functional characteristics of several well-characterized IAPs whose defining motifs place them more in the category of multifunctional modular protein interaction domains.

Paradoxically high resistance of natural killer T (NKT) cell-deficient mice to Legionella pneumophila: another aspect of NKT cells for modulation of host responses

In the present study, we examined the roles of natural killer T (NKT) cells in host defence against Legionella pneumophila in a mouse model. The survival rate of NKT cell-deficient Jalpha281 knock-out (KO) mice was significantly higher than that of wild-type mice. There was no bacterial overgrowth in the lungs, but Jalpha281 KO mice showed enhanced pulmonary clearance at a later stage of infection, compared with their wild-type counterparts. The severity of lung injury in L. pneumophila-infected Jalpha281 KO mice was less, as indicated by lung permeability measurements, such as lung weight and bronchoalveolar lavage fluid albumin concentration. Recruitment of inflammatory cells in the lungs was approximately twofold greater in Jalpha281 KO mice on day 3. Interestingly, higher values of interleukin (IL)-1beta and IL-18, and increased caspase-1 activity were noted in the lungs of Jalpha281 KO mice from an early time point (6 h). Exogenous alpha-galactosylceramide, a ligand of NKT cells, induced IL-12 and gamma interferon at 6 h, but suppressed IL-1beta at later time points in wild-type, whereas no effects were evident in Jalpha281 KO mice, as expected. Systemic administration of heat-killed L. pneumophila, but not Escherichia coli LPS, reproduced exaggerated production of IL-1beta in the lungs of Jalpha281 KO mice. These results demonstrate that NKT cells play a role in host defence against L. pneumophila, which is characterized by enhanced lung injury and decreased accumulation of inflammatory cells in the lungs. The regulation of IL-1beta, IL-18 and caspase-1 may be associated with the modulating effect of host responses by NKT cells.

The Legionella pneumophila replication vacuole: making a cosy niche inside host cells

The pathogenesis of Legionella pneumophila is derived from its growth within lung macrophages after aerosols are inhaled from contaminated water sources. Interest in this bacterium stems from its ability to manipulate host cell vesicular-trafficking pathways and establish a membrane-bound replication vacuole, making it a model for intravacuolar pathogens. Establishment of the replication compartment requires a specialized translocation system that transports a large cadre of protein substrates across the vacuolar membrane. These substrates regulate vesicle traffic and survival pathways in the host cell. This Review focuses on the strategies that L. pneumophila uses to establish intracellular growth and evaluates why this microorganism has accumulated an unprecedented number of translocated substrates that are targeted at host cells.

A hemidominant Naip5 allele in mouse strain MOLF/Ei-derived macrophages restricts Legionella pneumophila intracellular growth

Mouse-derived macrophages have the unique ability to restrict or permit Legionella pneumophila intracellular growth. The common inbred mouse strain C57BL/6J (B6) restricts L. pneumophila growth, whereas macrophages derived from A/J mice allow >10(3)-fold bacterial growth within three days. This phenotypic difference was mapped to the mouse Naip5 allele. The B6 restrictive Naip5 allele is dominant, and six amino acid changes in its product were predicted to control permissiveness. By using the wild-derived mouse strain MOLF/Ei, we found that MOLF/Ei-derived macrophages also restrict L. pneumophila growth, yet the Naip5 protein is identical to the A/J Naip5 at the six-amino-acid signature. The MOLF/Ei restrictive trait, unlike that of B6-derived macrophages, was not dominant over the A/J trait. In spite of this phenotypic difference, the L. pneumophila growth restriction in MOLF/Ei macrophages was mapped to the Naip5 region as well, indicating that the originally predicted change in the A/J Naip5 allele may not be critical for restriction. In the product of the A/J Naip5 permissive allele, there are four unique amino acid changes that map to a NACHT-like domain. Similar misregulating mutations have been identified in the NACHT domains of Nod-like receptor (NLR) proteins. Therefore, one of these mutations may be critical for restriction of L. pneumophila intracellular growth, and this parallels results found with human NLR variants with defects in the innate immune response.

Interleukin-23 orchestrates mucosal responses to Salmonella enterica serotype Typhimurium in the intestine

Salmonella enterica serotype Typhimurium causes an acute inflammatory reaction in the ceca of streptomycin-pretreated mice that involves T-cell-dependent induction of gamma interferon (IFN-gamma), interleukin-22 (IL-22), and IL-17 expression (genes Ifn-gamma, Il-22, and Il-17, respectively). We investigated here the role of IL-23 in initiating these inflammatory responses using the streptomycin-pretreated mouse model. Compared to wild-type mice, the expression of IL-17 was abrogated, IL-22 expression was markedly reduced, but IFN-gamma expression was normal in the ceca of IL-23p19-deficient mice during serotype Typhimurium infection. IL-23p19-deficient mice also exhibited a markedly reduced expression of regenerating islet-derived 3 gamma, keratinocyte-derived cytokine, and reduced neutrophil recruitment into the cecal mucosa during infection. Analysis of CD3(+) lymphocytes in the intestinal mucosa by flow cytometry revealed that alphabeta T cells were the predominant cell type expressing the IL-23 receptor in naive mice. However, a marked increase in the number of IL-23 receptor-expressing gammadelta T cells was observed in the lamina propria during serotype Typhimurium infection. Compared to wild-type mice, gammadelta T-cell-receptor-deficient mice exhibited blunted expression of IL-17 during serotype Typhimurium infection, while IFN-gamma expression was normal. These data suggested that gammadelta T cells are a significant source, but not the sole source, of IL-17 in the acutely inflamed cecal mucosa of mice. Collectively, our results point to IL-23 as an important player in initiating a T-cell-dependent amplification of inflammatory responses in the intestinal mucosa during serotype Typhimurium infection.

The caspase-1 inflammasome: a pilot of innate immune responses

The inflammasome is a large multiprotein complex whose assembly leads to the activation of caspase-1, which promotes the maturation of proinflammatory cytokines interleukin-1beta (IL-1beta) and IL-18. Proteins encoded by the nucleotide-binding domain and leucine-rich repeat (NLR) containing gene family form the central components of inflammasomes and act as intracellular sensors to detect cytosolic microbial components and "danger" signals (such as ATP and toxins). The inflammasome not only plays a pivotal role in innate immune responses toward pathogens but also mediates the activity of aluminum adjuvants. Thus, the inflammasome and associated signaling pathways are attractive targets for new therapeutics and vaccines.

Engagement of TLR signaling as adjuvant: towards smarter vaccine and beyond

Toll like receptors (TLRs) are a family of conserved pattern recognition receptors that recognizes specific microbial patterns and allow the cell to distinguish between self and non-self materials. The very property of the TLRs to link innate and adaptive immunity offers a novel prospect to develop vaccines engaging TLR signaling. The presence of TLR ligands as adjuvant in conjunction with the vaccine is shown to increase the efficacy and response to the immunization with a particular antigen. For infectious as well as for noninfectious diseases, TLR activation have been used in both established and experimental vaccines. The choice of the TLR agonist to be used, the subsequent efficacy and the safety profile of the vaccine is thus a crucial step in vaccine development. Recent studies also suggest the involvement of other non-TLR immune receptors to control vaccine immunogenicity. Here we focus on the findings dealing with TLR ligands as adjuvant and discuss the importance of these studies to develop an optimal vaccine.

NLRC4/IPAF: a CARD carrying member of the NLR family

The NOD-like receptor (NLR) family of proteins is involved in the regulation of innate immune responses and cell death pathways. Recent findings show that the NLR family member NLRC4 (also known as IPAF) has important roles in innate immune responses to Gram-negative bacteria. Macrophages infected with Legionella pneumophila, Salmonella typhimurium, Shigella flexneri, or Pseudomonas aeruginosa activate caspase-1 in an NLRC4-dependent manner leading to macrophage cell death and the release of proinflammatory cytokines. This review will discuss these findings as well as the role of bacterial type III and type IV secretion systems and flagellin in NLRC4-mediated caspase-1 activation.

Multiple MyD88-dependent responses contribute to pulmonary clearance of Legionella pneumophila

MyD88-dependent signalling is important for secretion of early inflammatory cytokines and host protection in response to Legionella pneumophila infection. Although toll-like receptor (TLR)2 contributes to MyD88-dependent clearance of L. pneumophila, TLR-independent functions of MyD88 could also be important. To determine why MyD88 is critical for host protection to L. pneumophila, the contribution of multiple TLRs and IL-18 receptor (IL-18R)-dependent interferon-gamma (IFN-gamma) production in a mouse was examined. Mice deficient for TLR5 or TLR9, or deficient for TLR2 along with either TLR5 or TLR9, were competent for controlling bacterial replication and had no apparent defects in cytokine production compared with control mice. MyD88-dependent production of IFN-gamma in the lung was mediated primarily by natural killer cells and required IL-18R signalling. Reducing IFN-gamma levels did not greatly affect the kinetics of L. pneumophila replication or clearance in infected mice. Additionally, IFN-gamma-deficient mice did not have a susceptibility phenotype as severe as the MyD88-deficient mice and were able to control a pulmonary infection by L. pneumophila. Thus, MyD88-dependent innate immune responses induced by L. pneumophila involve both TLR-dependent responses and IL-18R-dependent production of IFN-gamma by natural killer cells, and these MyD88-dependent pathways can function independently to provide host protection against an intracellular pathogen.

XIAP regulates cytosol-specific innate immunity to Listeria infection

The inhibitor of apoptosis protein (IAP) family has been implicated in immune regulation, but the mechanisms by which IAP proteins contribute to immunity are incompletely understood. We show here that X-linked IAP (XIAP) is required for innate immune control of Listeria monocytogenes infection. Mice deficient in XIAP had a higher bacterial burden 48 h after infection than wild-type littermates, and exhibited substantially decreased survival. XIAP enhanced NF-kappaB activation upon L. monocytogenes infection of activated macrophages, and prolonged phosphorylation of Jun N-terminal kinase (JNK) specifically in response to cytosolic bacteria. Additionally, XIAP promoted maximal production of pro-inflammatory cytokines upon bacterial infection in vitro or in vivo, or in response to combined treatment with NOD2 and TLR2 ligands. Together, our data suggest that XIAP regulates innate immune responses to L. monocytogenes infection by potentiating synergy between Toll-like receptors (TLRs) and Nod-like receptors (NLRs) through activation of JNK- and NF-kappaB-dependent signaling.