Role of Toll-like receptor 2 in recognition of Legionella pneumophila in a murine pneumonia model

Role of toll-like receptors and nod-like receptors in acute lung infection

The respiratory system exposed to microorganisms continuously, and the pathogenicity of these microbes not only contingent on their virulence factors, but also the host's immunity. A multifaceted innate immune mechanism exists in the respiratory tract to cope with microbial infections and to decrease tissue damage. The key cell types of the innate immune response are macrophages, neutrophils, dendritic cells, epithelial cells, and endothelial cells. Both the myeloid and structural cells of the respiratory system sense invading microorganisms through binding or activation of pathogen-associated molecular patterns (PAMPs) to pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and NOD-like receptors (NLRs). The recognition of microbes and subsequent activation of PRRs triggers a signaling cascade that leads to the activation of transcription factors, induction of cytokines/5chemokines, upregulation of cell adhesion molecules, recruitment of immune cells, and subsequent microbe clearance. Since numerous microbes resist antimicrobial agents and escape innate immune defenses, in the future, a comprehensive strategy consisting of newer vaccines and novel antimicrobials will be required to control microbial infections. This review summarizes key findings in the area of innate immune defense in response to acute microbial infections in the lung. Understanding the innate immune mechanisms is critical to design host-targeted immunotherapies to mitigate excessive inflammation while controlling microbial burden in tissues following lung infection.

Legionella pneumophila Rhizoferrin Promotes Bacterial Biofilm Formation and Growth within Amoebae and Macrophages

Previously, we showed that Legionella pneumophila secretes rhizoferrin, a polycarboxylate siderophore that promotes bacterial growth in iron-deplete media and the murine lung. Yet, past studies failed to identify a role for the rhizoferrin biosynthetic gene (lbtA) in L. pneumophila infection of host cells, suggesting the siderophore's importance was solely linked to extracellular survival. To test the possibility that rhizoferrin's relevance to intracellular infection was missed due to functional redundancy with the ferrous iron transport (FeoB) pathway, we characterized a new mutant lacking both lbtA and feoB. This mutant was highly impaired for growth on bacteriological media that were only modestly depleted of iron, confirming that rhizoferrin-mediated ferric iron uptake and FeoB-mediated ferrous iron uptake are critical for iron acquisition. The lbtA feoB mutant, but not its lbtA-containing complement, was also highly defective for biofilm formation on plastic surfaces, demonstrating a new role for the L. pneumophila siderophore in extracellular survival. Finally, the lbtA feoB mutant, but not its complement containing lbtA, proved to be greatly impaired for growth in Acanthamoeba castellanii, Vermamoeba vermiformis, and human U937 cell macrophages, revealing that rhizoferrin does promote intracellular infection by L. pneumophila. Moreover, the application of purified rhizoferrin triggered cytokine production from the U937 cells. Rhizoferrin-associated genes were fully conserved across the many sequenced strains of L. pneumophila examined but were variably present among strains from the other species of Legionella. Outside of Legionella, the closest match to the L. pneumophila rhizoferrin genes was in Aquicella siphonis, another facultative intracellular parasite of amoebae.

Metabolism Characteristics of Mycoplasma pneumoniae Infection in Asthmatic Children

PURPOSE

Studies have shown that Mycoplasma pneumoniae (Mp) infection can aggravate symptoms in asthmatics. However, the mechanism by which Mp infection exacerbates asthma remains unclear. Metabolomics can help identify the mechanism of Mp aggravating asthma in children, thereby providing more a potential target for improving clinical treatment programs. In this article, we analyzed the metabolic level of patients to explain how Mp aggravates asthma in children.

METHODS

We divided the subjects into the asthma, Mp infection, asthma combined with Mp infection and healthy groups. Patients' peripheral blood was collected for metabolic and interaction analysis. Cytokine levels were measured via serum and exhaled breath condensate (EBC).

RESULTS

A total of 150 participating subjects were divided into four groups after exclusion. We found out that there were different metabolic pathways between the healthy and disease groups. The major pathways of both asthma and asthma combined with Mp infection were valine, leucine and isoleucine biosynthesis; malate-aspartate shuttle was the main differential pathway for Mp infection. Moreover, even though three disease groups involved 81 metabolites at the same time, compared with asthma combined with Mp infection, 2 single disease groups still involved different amino acid pathways (phenylalanine, tyrosine and tryptophan biosynthesis; valine, leucine and isoleucine biosynthesis). Interaction analysis showed that Mp infection in asthmatic patients not only activated cytokines, but also activated Toll-like receptors (TLRs) 2 and 6. Finally, the levels of interleukin (IL)-4, IL-8, IL-13 and tumor necrosis factor-α in EBC with asthma combined with Mp infection were significantly higher than the 2 single disease groups.

CONCLUSIONS

Mp infection in asthmatic children can cause changes in the levels of various amino acids in the body, which were enriched in the pathways such as valine, leucine and isoleucine biosynthesis. Palmitic acid can activate TLR2, and iloprost reduces IL-10 levels, ultimately leading to the increased airway inflammation.

Legionella pneumophila: The Journey from the Environment to the Blood

An outbreak of a potentially fatal form of pneumonia in 1976 and in the annual convention of the American Legion was the first time that Legionella spp. was identified. Thereafter, the term Legionnaires’ disease (LD) was established. The infection in humans is transmitted by the inhalation of aerosols that contain the microorganisms that belong to the Legionellaceae family and the genus Legionella. The genus Legionella contains genetically heterogeneous species and serogroups. The Legionella pneumophila serogroup 1 (Lp1) is the most often detected strain in outbreaks of LD. The pathogenesis of LD infection initiates with the attachment of the bacterial cells to the host cells, and subsequent intracellular replication. Following invasion, Legionella spp. activates its virulence mechanisms: generation of specific compartments of Legionella-containing vacuole (LCV), and expression of genes that encode a type IV secretion system (T4SS) for the translocation of proteins. The ability of L. pneumophila to transmigrate across the lung’s epithelium barrier leads to bacteremia, spread, and invasion of many organs with subsequent manifestations, complications, and septic shock. The clinical manifestations of LD depend on the bacterial load in the aerosol, the virulence factors, and the immune status of the patient. The infection has two distinct forms: the non- pneumatic form or Pontiac fever, which is a milder febrile flu-like illness, and LD, a more severe form, which includes pneumonia. In addition, the extrapulmonary involvement of LD can include heart, brain, abdomen, and joints.

Human macrophages utilize a wide range of pathogen recognition receptors to recognize Legionella pneumophila, including Toll-Like Receptor 4 engaging Legionella lipopolysaccharide and the Toll-like Receptor 3 nucleic-acid sensor

Cytokines made by macrophages play a critical role in determining the course of Legionella pneumophila infection. Prior murine-based modeling indicated that this cytokine response is initiated upon recognition of L. pneumophila by a subset of Toll-like receptors, namely TLR2, TLR5, and TLR9. Through the use of shRNA/siRNA knockdowns and subsequently CRISPR/Cas9 knockouts (KO), we determined that TRIF, an adaptor downstream of endosomal TLR3 and TLR4, is required for full cytokine secretion by human primary and cell-line macrophages. By characterizing a further set of TLR KO's in human U937 cells, we discerned that, contrary to the viewpoint garnered from murine-based studies, TLR3 and TLR4 (along with TLR2 and TLR5) are in fact vital to the macrophage response in the early stages of L. pneumophila infection. This conclusion was bolstered by showing that i) chemical inhibitors of TLR3 and TLR4 dampen the cytokine output of primary human macrophages and ii) transfection of TLR3 and TLR4 into HEK cells conferred an ability to sense L. pneumophila. TLR3- and TLR4-dependent cytokines promoted migration of human HL-60 neutrophils across an epithelial layer, pointing to the biological importance for the newfound signaling pathway. The response of U937 cells to L. pneumophila LPS was dependent upon TLR4, a further contradiction to murine-based studies, which had concluded that TLR2 is the receptor for Legionella LPS. Given the role of TLR3 in sensing nucleic acid (i.e., dsRNA), we utilized newly-made KO U937 cells to document that DNA-sensing by cGAS-STING and DNA-PK are also needed for the response of human macrophages to L. pneumophila. Given the lack of attention given them in the bacterial field, C-type lectin receptors were similarly examined; but, they were not required. Overall, this study arguably represents the most extensive, single-characterization of Legionella-recognition receptors within human macrophages.

The Role of Innate Immunity in Pulmonary Infections

Innate immunity forms a protective line of defense in the early stages of pulmonary infection. The primary cellular players of the innate immunity against respiratory infections are alveolar macrophages (AMs), dendritic cells (DCs), neutrophils, natural killer (NK) cells, and innate lymphoid cells (ILCs). They recognize conserved structures of microorganisms through membrane-bound and intracellular receptors to initiate appropriate responses. In this review, we focus on the prominent roles of innate immune cells and summarize transmembrane and cytosolic pattern recognition receptor (PRR) signaling recognition mechanisms during pulmonary microbial infections. Understanding the mechanisms of PRR signal recognition during pulmonary pathogen infections will help us to understand pulmonary immunopathology and lay a foundation for the development of effective therapies to treat and/or prevent pulmonary infections.

Viewing Legionella pneumophila Pathogenesis through an Immunological Lens

Legionella pneumophila is the causative agent of the severe pneumonia Legionnaires' disease. L. pneumophila is ubiquitously found in freshwater environments, where it replicates within free-living protozoa. Aerosolization of contaminated water supplies allows the bacteria to be inhaled into the human lung, where L. pneumophila can be phagocytosed by alveolar macrophages and replicate intracellularly. The Dot/Icm type IV secretion system (T4SS) is one of the key virulence factors required for intracellular bacterial replication and subsequent disease. The Dot/Icm apparatus translocates more than 300 effector proteins into the host cell cytosol. These effectors interfere with a variety of cellular processes, thus enabling the bacterium to evade phagosome-lysosome fusion and establish an endoplasmic reticulum-derived Legionella-containing vacuole, which facilitates bacterial replication. In turn, the immune system has evolved numerous strategies to recognize intracellular bacteria such as L. pneumophila, leading to potent inflammatory responses that aid in eliminating infection. This review aims to provide an overview of L. pneumophila pathogenesis in the context of the host immune response.

Toll-like receptor 2 has a prominent but nonessential role in innate immunity to Staphylococcus aureus pneumonia

Staphylococcus aureus is an important cause of acute bacterial pneumonia. Toll‐like receptor 2 (TLR2) recognizes multiple components of the bacterial cell wall and activates innate immune responses to gram‐positive bacteria. We hypothesized that TLR2 would have an important role in pulmonary host defense against S. aureus. TLR null (TLR2^−/−) mice and wild type (WT) C57BL/6 controls were challenged with aerosolized S. aureus at a range of inocula for kinetic studies of cytokine and antimicrobial peptide expression, lung inflammation, bacterial killing by alveolar macrophages, and bacterial clearance. Survival was measured after intranasal infection. Pulmonary induction of most pro‐inflammatory cytokines was significantly blunted in TLR2^−/− mice 4 and 24 h after infection in comparison with WT controls. Bronchoalveolar concentrations of cathelicidin‐related antimicrobial peptide also were reduced in TLR2^−/− mice. Lung inflammation, measured by enumeration of bronchoalveolar neutrophils and scoring of histological sections, was significantly blunted in TLR2^−/− mice. Phagocytosis of S. aureus by alveolar macrophages in vivo after low‐dose infection was unimpaired, but viability of ingested bacteria was significantly greater in TLR2^−/− mice. Bacterial clearance from the lungs was slightly impaired in TLR2^−/− mice after low‐dose infection only; bacterial elimination from the lungs was slightly accelerated in the TLR2^−/− mice after high‐dose infection. Survival after high‐dose intranasal challenge was 50–60% in both groups. TLR2 has a significant role in early innate immune responses to S. aureus in the lungs but is not required for bacterial clearance and survival from S. aureus pneumonia.

Innate sensing and cell-autonomous resistance pathways in Legionella pneumophila infection

Legionella pneumophila is a facultative intracellular bacterium which can cause a severe pneumonia called Legionnaires' disease after inhalation of contaminated water droplets and replication in alveolar macrophages. The innate immune system is generally able to sense and -in most cases- control L. pneumophila infection. Comorbidities and genetic risk factors, however, can compromise the immune system and high infection doses might overwhelm its capacity, thereby enabling L. pneumophila to grow and disseminate inside the lung. The innate immune system mediates sensing of L. pneumophila by employing e.g. NOD-like receptors (NLRs), Toll-like receptors (TLRs), as well as the cGAS/STING pathway to stimulate death of infected macrophages as well as production of proinflammatory cytokines and interferons (IFNs). Control of pulmonary L. pneumophila infection is largely mediated by inflammasome-, TNFα- and IFN-dependent macrophage-intrinsic resistance mechanisms. This article summarizes the current knowledge of innate immune responses to L. pneumophila infection in general, and of macrophage-intrinsic defense mechanisms in particular.

The Type II Secretion System of Legionella pneumophila Dampens the MyD88 and Toll-Like Receptor 2 Signaling Pathway in Infected Human Macrophages

Previously, we reported that mutants of Legionella pneumophila lacking a type II secretion (T2S) system elicit higher levels of cytokines (e.g., interleukin-6 [IL-6]) following infection of U937 cells, a human macrophage-like cell line. We now show that this effect of T2S is also manifest upon infection of human THP-1 macrophages and peripheral blood monocytes but does not occur during infection of murine macrophages. Supporting the hypothesis that T2S acts to dampen the triggering of an innate immune response, we observed that the mitogen-activated protein kinase (MAPK) and nuclear transcription factor kappa B (NF-κB) pathways are more highly stimulated upon infection with the T2S mutant than upon infection with the wild type. By using short hairpin RNA to deplete proteins involved in specific pathogen-associated molecular pattern (PAMP) recognition pathways, we determined that the dampening effect of the T2S system was not dependent on nucleotide binding oligomerization domain (NOD)-like receptors (NLRs), retinoic acid-inducible protein I (RIG-I)-like receptors (RLRs), double-stranded RNA (dsRNA)-dependent protein kinase receptor (PKR), or TIR domain-containing adaptor inducing interferon beta (TRIF) signaling or an apoptosis-associated speck-like protein containing a CARD (ASC)- or caspase-4-dependent inflammasome. However, the dampening effect of T2S on IL-6 production was significantly reduced upon gene knockdown of myeloid differentiation primary response 88 (MyD88), TANK binding kinase 1 (TBK1), or Toll-like receptor 2 (TLR2). These data indicate that the L. pneumophila T2S system dampens the signaling of the TLR2 pathway in infected human macrophages. We also document the importance of PKR, TRIF, and TBK1 in cytokine secretion during L. pneumophila infection of macrophages.

Innate immunity against Legionella pneumophila during pulmonary infections in mice

Legionella pneumophila is an etiological agent of the severe pneumonia known as Legionnaires' disease (LD). This gram-negative bacterium is thought to replicate naturally in various freshwater amoebae, but also replicates in human alveolar macrophages. Inside host cells, legionella induce the production of non-endosomal replicative phagosomes by injecting effector proteins into the cytosol. Innate immune responses are first line defenses against legionella during early phases of infection, and distinguish between legionella and host cells using germline-encoded pattern recognition receptors such as Toll-like receptors , NOD-like receptors, and RIG-I-like receptors, which sense pathogen-associated molecular patterns that are absent in host cells. During pulmonary legionella infections, various inflammatory cells such as macrophages, neutrophils, natural killer (NK) cells, large mononuclear cells, B cells, and CD4+ and CD8+ T cells are recruited into infected lungs, and predominantly occupy interstitial areas to control legionella. During pulmonary legionella infections, the interplay between distinct cytokines and chemokines also modulates innate host responses to clear legionella from the lungs. Recognition by NK cell receptors triggers effector functions including secretion of cytokines and chemokines, and leads to lysis of target cells. Crosstalk between NK cells and dendritic cells, monocytes, and macrophages provides a major first-line defense against legionella infection, whereas activation of T and B cells resolves the infection and mounts legionella-specific memory in the host.

Toll-Like Receptor 2 Recognizes Orientia tsutsugamushi and Increases Susceptibility to Murine Experimental Scrub Typhus

Scrub typhus is a potentially lethal infection that is caused by the obligate intracellular bacterium Orientia tsutsugamushi The roles of Toll-like receptor 2 (TLR2) and TLR4 in innate recognition of O. tsutsugamushi have not been elucidated. By overexpression of TLR2 or TLR4 in HEK293 cells, we demonstrated that TLR2, but not TLR4, recognizes heat-stable compounds of O. tsutsugamushi that were sensitive to treatment with sodium hydroxide, hydrogen peroxide, and proteinase K. TLR2 was required for the secretion of tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) by dendritic cells. In an intradermal mouse infection model, TLR2-deficient mice did not show impaired control of bacterial growth or reduced survival. Moreover, after intraperitoneal infection, TLR2-deficient mice were even more resistant to lethal infection than C57BL/6 wild-type mice, which showed stronger symptoms and lower survival rates during the convalescent phase. Compared to the time of reduction of bacterial loads in TLR2-deficient mice, the reduction of bacterial loads in infected organs was accelerated in wild-type mice. The higher mortality of wild-type mice was associated with increased concentrations of serum alkaline phosphatase but not aspartate aminotransferase. The transcription of mRNA for TNF-α and IL-6 decreased more rapidly in peritoneum samples from wild-type mice than in those from TLR2-deficient mice and was therefore not a correlate of increased susceptibility. Thus, although TLR2 is an important mediator of the early inflammatory response, it is dispensable for protective immunity against O. tsutsugamushi Increased susceptibility to O. tsutsugamushi infection in TLR2-competent mice rather suggests a TLR2-related immunopathologic effect.

Guttridge D D, Amer AO. The Sphingosine-1-Phosphate Lyase (LegS2) Contributes to the Restriction of Legionella pneumophila in Murine Macrophages

L. pneumophila is the causative agent of Legionnaires’ disease, a human illness characterized by severe pneumonia. In contrast to those derived from humans, macrophages derived from most mouse strains restrict L. pneumophila replication. The restriction of L. pneumophila replication has been shown to require bacterial flagellin, a component of the type IV secretion system as well as the cytosolic NOD-like receptor (NLR) Nlrc4/ Ipaf. These events lead to caspase-1 activation which, in turn, activates caspase-7. Following caspase-7 activation, the phagosome-containing L. pneumophila fuses with the lysosome, resulting in the restriction of L. pneumophila growth. The LegS2 effector is injected by the type IV secretion system and functions as a sphingosine 1-phosphate lyase. It is homologous to the eukaryotic sphingosine lyase (SPL), an enzyme required in the terminal steps of sphingolipid metabolism. Herein, we show that mice Bone Marrow-Derived Macrophages (BMDMs) and human Monocyte-Derived Macrophages (hMDMs) are more permissive to L. pneumophila legS2 mutants than wild-type (WT) strains. This permissiveness to L. pneumophila legS2 is neither attributed to abolished caspase-1, caspase-7 or caspase-3 activation, nor due to the impairment of phagosome-lysosome fusion. Instead, an infection with the legS2 mutant resulted in the reduction of some inflammatory cytokines and their corresponding mRNA; this effect is mediated by the inhibition of the nuclear transcription factor kappa-B (NF-κB). Moreover, BMDMs infected with L. pneumophila legS2 mutant showed elongated mitochondria that resembles mitochondrial fusion. Therefore, the absence of LegS2 effector is associated with reduced NF-κB activation and atypical morphology of mitochondria.

Divergent functions of Toll-like receptors during bacterial lung infections

Lower respiratory tract infections caused by bacteria are a major cause of death in humans irrespective of sex, race, or geography. Indeed, accumulated data indicate greater mortality and morbidity due to these infections than cancer, malaria, or HIV infection. Successful recognition of, followed by an appropriate response to, bacterial pathogens in the lungs is crucial for effective pulmonary host defense. Although the early recruitment and activation of neutrophils in the lungs is key in the response against invading microbial pathogens, other sentinels, such as alveolar macrophages, epithelial cells, dendritic cells, and CD4(+) T cells, also contribute to the elimination of the bacterial burden. Pattern recognition receptors, such as Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain-like receptors, are important for recognizing and responding to microbes during pulmonary infections. However, bacterial pathogens have acquired crafty evasive strategies to circumvent the pattern recognition receptor response and thus establish infection. Increased understanding of the function of TLRs and evasive mechanisms used by pathogens during pulmonary infection will deepen our knowledge of immunopathogenesis and is crucial for developing effective therapeutic and/or prophylactic measures. This review summarizes current knowledge of the multiple roles of TLRs in bacterial lung infections and highlights the mechanisms used by pathogens to modulate or interfere with TLR signaling in the lungs.

Interleukin 1 receptor-driven neutrophil recruitment accounts to MyD88-dependent pulmonary clearance of legionella pneumophila infection in vivo

Legionella pneumophila, the etiological agent of Legionnaires' disease, triggers activation of multiple innate immune pathways that lead to the restriction of bacterial replication in vivo. Despite the critical role for MyD88 in infection clearance, the receptors and mechanisms responsible for MyD88-mediated pulmonary bacterial clearance are still unclear. Here, we used flagellin mutants of L. pneumophila, which bypass the NAIP5/NLRC4-mediated restriction of bacterial replication, to assess the receptors involved in MyD88-mediated pulmonary bacterial clearance. By systematically comparing pulmonary clearance of L. pneumophila in C57BL/6 MyD88(-/-), TLR2(-/-), TLR3(-/-), TLR4(-/-), TLR9(-/-), IL-1R(-/-), and IL-18(-/-) mice, we found that, while the knockout of a single Toll-like receptor or interleukin 18 resulted only in minor impairment of bacterial clearance, deficiency in the interleukin 1 (IL-1) receptor led to a significant impairment. IL-1/MyD88-mediated pulmonary bacterial clearance occurs via processes involving the recruitment of neutrophils. Collectively, our data contribute to the understanding of the effector mechanisms involved in MyD88-mediated pulmonary bacterial clearance.

Schistosoma japonicum infection induces macrophage polarization

The role of macrophages (Mφ) as the first line of host defense is well accepted. These cells play a central role in orchestrating crucial functions during schistosomal infection. Thus, understanding the functional diversity of these cells in the process of infection as well as the mechanisms underlying these events is crucial for developing disease control strategies. In this study, we adopted a Mφ polarization recognition system. M1 macrophage was characterized by expressing CD16/32, IL-12 and iNOS. M2 macrophage was characterized by expressing CD206, IL-10 and arg-1. In vivo (mouse peritoneal macrophages of different infection stages were obtained) and in vitro (different S. japonicum antigens were used to stimulate RAW264.7) were characterized by using the above mentioned system. NCA and ACA stimulated RAW264.7 express significantly higher levels of IL-12 while significantly higher levels of IL-10 were detected after soluble egg antigen (SEA) stimulation. The results showed that dramatic changes of antigen in the microenvironment before and after egg production led to macrophage polarization. Furthermore, through TLR blocking experiments, the TLR4 signaling pathway was found to play a role in the process of macrophage polarization toward M1. Our data suggest that macrophage polarization during S. japonicum infection had significant effects on host immune responses to S. japonicum.

A TLR6 polymorphism is associated with increased risk of Legionnaires' disease

Legionella pneumophila (Lp), the etiologic agent of Legionnaires’ Disease (LD), is an important cause of community-acquired and nosocomial pneumonia. However, the host immune and genetic determinants of human susceptibility to Lp are poorly understood. Here we show that both TLR6 and TLR1 cooperate with TLR2 to recognize Lp in transfected HEK293 cells. We also perform a human genetic association study of 14 candidate single nucleotide polymorphisms in Toll-like receptors (TLRs) 1, 2, and 6 in 98 LD cases and 268 controls from the Netherlands. No polymorphisms in TLR1 or TLR2 were associated with LD. A TLR6 polymorphism, 359T>C (rs5743808), was associated with an elevated risk of LD in genotypic and dominant (OR 5.83, p=7.9×10⁻⁵) models. The increased risk in persons with 359 TC or CC genotypes was further enhanced among smokers. In a multivariate model, 359T>C was associated with a higher risk of LD (OR 4.24, p=0.04), than any other variable, including age and smoking. Together, these data suggest that the human TLR6 variant, 359T>C, is an independent risk factor for LD.

IL-1α signaling initiates the inflammatory response to virulent Legionella pneumophila in vivo

Legionella pneumophila is an intracellular bacterial pathogen that is the cause of a severe pneumonia in humans called Legionnaires' disease. A key feature of L. pneumophila pathogenesis is the rapid influx of neutrophils into the lungs, which occurs in response to signaling via the IL-1R. Two distinct cytokines, IL-1α and IL-1β, can stimulate the type I IL-1R. IL-1β is produced upon activation of cytosolic sensors called inflammasomes that detect L. pneumophila in vitro and in vivo. Surprisingly, we find no essential role for IL-1β in neutrophil recruitment to the lungs in response to L. pneumophila. Instead, we show that IL-1α is a critical initiator of neutrophil recruitment to the lungs of L. pneumophila-infected mice. We find that neutrophil recruitment in response to virulent L. pneumophila requires the production of IL-1α specifically by hematopoietic cells. In contrast to IL-1β, the innate signaling pathways that lead to the production of IL-1α in response to L. pneumophila remain poorly defined. In particular, although we confirm a role for inflammasomes for initiation of IL-1β signaling in vivo, we find no essential role for inflammasomes in production of IL-1α. Instead, we propose that a novel host pathway, perhaps involving inhibition of host protein synthesis, is responsible for IL-1α production in response to virulent L. pneumophila. Our results establish IL-1α as a critical initiator of the inflammatory response to L. pneumophila in vivo and point to an important role for IL-1α in providing an alternative to inflammasome-mediated immune responses in vivo.

Effect of hochuekkito on alveolar macrophage inflammatory responses in hyperglycemic mice

Diabetes mellitus reduces immunological activity and increases susceptibility to various infections. Hochuekkito (TJ-41) has been reported to improve the weakened physical condition of various chronic diseases. BALB/c mice were divided into three groups; groups A and B were fed a standard diet, and group C, a TJ-41 diet. Two weeks after starting these diets, hyperglycemia was induced in groups B and C by injection with streptozotocin. Two weeks later, bronchoalveolar lavage was performed. Toll-like receptor (TLR) ligands (TLR2: peptidoglycan, PGN; TLR4: lipopolysaccharide, LPS; TLR5: flagellin, FLG) were used to stimulate alveolar macrophages (AMs), and TNF-α production was measured. Under hyperglycemic conditions and PGN or FLG stimulation, TNF-α production from AMs was significantly reduced in group B compared with group A. However, treatment with TJ-41 (group C) significantly improved the impaired production of TNF-α. These results suggest that, under hyperglycemic conditions, TJ-41 can improve the inflammatory responses of AMs with stimulation of TLR ligands.

The Role of TLR2 in Infection and Immunity

Toll-like receptors (TLRs) are recognition molecules for multiple pathogens, including bacteria, viruses, fungi, and parasites. TLR2 forms heterodimers with TLR1 and TLR6, which is the initial step in a cascade of events leading to significant innate immune responses, development of adaptive immunity to pathogens and protection from immune sequelae related to infection with these pathogens. This review will discuss the current status of TLR2 mediated immune responses by recognition of pathogen-associated molecular patterns (PAMPS) on these organisms. We will emphasize both canonical and non-canonical responses to TLR2 ligands with emphasis on whether the inflammation induced by these responses contributes to the disease state or to protection from diseases.

Impaired inflammatory responses to multiple toll-like receptor ligands in alveolar macrophages of streptozotocin-induced diabetic mice

OBJECTIVE

To investigate the effect of hyperglycemic state on the activation of alveolar macrophages (AMs) mediated via Toll-like receptors (TLRs) typically associated with bacterial infection.

METHODS

AMs obtained from normoglycemic control mice and streptozotocin-induced diabetic mice were stimulated ex vivo with the following: a TLR2 ligand, peptidoglycan (PGN); a TLR4 ligand, lipopolysaccharide (LPS); or a TLR5 ligand, flagellin (FLG). Cytokine production and mRNA expression were measured by ELISA and real-time PCR, respectively. TLR expression was assessed by real-time PCR and flow cytometry.

RESULTS

AMs from diabetic mice produced significantly less TNF-α after PGN or FLG stimulation, and less IL-6 after FLG stimulation, compared with AMs from control mice. The decrease in the production of these cytokines was associated with reduced mRNA expression of the corresponding cytokines. In contrast, production of TNF-α and IL-6 after LPS stimulation did not differ between groups. Furthermore, there was no substantial difference in the expression of TLR2, TLR4, and TLR5 in AMs between the groups. The increased JNK phosphorylation induced by PGN or FLG stimulation was downregulated in AMs from diabetic mice.

CONCLUSIONS

Hyperglycemic state impairs the reactivity of AMs to multiple TLR ligands. This effect might result from hyperglycemia-induced alteration of intracellular signaling and is unlikely due to the modulation of TLR expression.

Innate immunity to legionella pneumophila

Innate immune cells, such as macrophages, are highly adapted to rapidly recognize infections by distinct pathogens, including viruses, bacteria, fungi, and protozoa. This recognition is mediated by pattern recognition receptors (PRRs), which are found in host cell surface membranes and the host cell cytoplasm. PRRs include protein families such as the toll-like receptors, nod-like receptors, RIG-I-like receptors, and sensors of cytosolic DNA. The activation of these PRRs by pathogen-associated molecular patterns leads to transcriptional responses and specific forms of cell death. These processes effectively contribute to host resistance to infection either via cell-autonomous processes that lead to the intracellular restriction of microbial replication and/or by activating pathogen-specific adaptive immune responses. Legionella pneumophila, the causative agent of Legionnaires’ disease, is a Gram-negative bacterium that triggers responses by multiple PRRs. Here, we review a set of studies that have contributed to our specific understanding of the molecular mechanisms by which innate immune cells recognize and respond to L. pneumophila and the importance of these processes to the outcome of infection.

Legionella pneumophila type II secretion dampens the cytokine response of infected macrophages and epithelia

The type II secretion (T2S) system of Legionella pneumophila is required for the ability of the bacterium to grow within the lungs of A/J mice. By utilizing mutants lacking T2S (lsp), we now document that T2S promotes the intracellular infection of both multiple types of macrophages and lung epithelia. Following infection of macrophages, lsp mutants (but not a complemented mutant) elicited significantly higher levels of interleukin 6 (IL-6), tumor necrosis factor alpha (TNF-α), IL-10, IL-8, IL-1β, and MCP-1 within tissue culture supernatants. A similar result was obtained with infected lung epithelial cell lines and the lungs of infected A/J mice. Infection with a mutant specifically lacking the T2S-dependent ProA protease (but not a complemented proA mutant) resulted in partial elevation of cytokine levels. These data demonstrate that the T2S system of L. pneumophila dampens the cytokine/chemokine output of infected host cells. Upon quantitative reverse transcription (RT)-PCR analysis of infected host cells, an lspF mutant, but not the proA mutant, produced significantly higher levels of cytokine transcripts, implying that some T2S-dependent effectors dampen signal transduction and transcription but that others, such as ProA, act at a posttranscriptional step in cytokine expression. In summary, the impact of T2S on lung infection is a combination of at least three factors: the promotion of growth in macrophages, the facilitation of growth in epithelia, and the dampening of the chemokine and cytokine output from infected host cells. To our knowledge, these data are the first to identify a link between a T2S system and the modulation of immune factors following intracellular infection.

Toll-like receptor 2 regulates organic dust-induced airway inflammation

Organic dust exposure in agricultural environments results in significant airway inflammatory diseases. Gram-positive cell wall components are present in high concentrations in animal farming dusts, but their role in mediating dust-induced airway inflammation is not clear. This study investigated the role of Toll-like receptor (TLR) 2, a pattern recognition receptor for gram-positive cell wall products, in regulating swine facility organic dust extract (DE)-induced airway inflammation in mice. Isolated lung macrophages from TLR2 knockout mice demonstrated reduced TNF-α, IL-6, keratinocyte chemoattractant/CXCL1, but not macrophage inflammatory protein-2/CXCL2 expression, after DE stimulation ex vivo. Next, using an established mouse model of intranasal inhalation challenge, we analyzed bronchoalveolar lavage fluid and lung tissue in TLR2-deficient and wild-type (WT) mice after single and repetitive DE challenge. Neutrophil influx and select cytokines/chemokines were significantly lower in TLR2-deficient mice at 5 and 24 hours after single DE challenge. After daily exposure to DE for 2 weeks, there were significant reductions in total cellularity, neutrophil influx, and TNF-α, IL-6, CXCL1, but not CXCL2 expression, in TLR2-deficient mice as compared with WT animals. Lung pathology revealed that bronchiolar inflammation, but not alveolar inflammation, was reduced in TLR2-deficient mice after repetitive exposure. Airway hyperresponsiveness to methacholine after dust exposure was similar in both groups. Finally, airway inflammatory responses in WT mice after challenge with a TLR2 agonist, peptidoglycan, resembled DE-induced responses. Collectively, these results demonstrate that the TLR2 pathway is important in regulating swine facility organic dust-induced airway inflammation, which suggests the importance of TLR2 agonists in mediating large animal farming-induced airway inflammatory responses.

Nonhematopoietic cells are key players in innate control of bacterial airway infection

Airborne pathogens encounter several hurdles during host invasion, including alveolar macrophages (AMs) and airway epithelial cells (AECs) and their products. Although growing evidence indicates pathogen-sensing capacities of epithelial cells, the relative contribution of hematopoietic versus nonhematopoietic cells in the induction of an inflammatory response and their possible interplay is still poorly defined in vivo in the context of infections with pathogenic microorganisms. In this study, we show that nonhematopoietic cells, including AECs, are critical players in the inflammatory process induced upon airway infection with Legionella pneumophila, and that they are essential for control of bacterial infections. Lung parenchymal cells, including AECs, are not infected themselves by L. pneumophila in vivo but rather act as sensors and amplifiers of inflammatory cues delivered by L. pneumophila-infected AM. We identified AM-derived IL-1β as the critical mediator to induce chemokine production in nonhematopoietic cells in the lung, resulting in swift and robust recruitment of infection-controlling neutrophils into the airways. These data add a new level of complexity to the coordination of the innate immune response to L. pneumophila and illustrate how the cross talk between leukocytes and nonhematopoietic cells contributes to efficient host protection.

In vivo function of airway epithelial TLR2 in host defense against bacterial infection

Decreased Toll-like receptor 2 (TLR2) expression has been reported in patients with chronic obstructive pulmonary disease and in a murine asthma model, which may predispose the hosts to bacterial infections, leading to disease exacerbations. Since airway epithelial cells serve as the first line of respiratory mucosal defense, the present study aimed to reveal the role of airway epithelial TLR2 signaling to lung bacterial [i.e., Mycoplasma pneumoniae (Mp)] clearance. In vivo TLR2 gene transfer via intranasal inoculation of adenoviral vector was performed to reconstitute TLR2 expression in airway epithelium of TLR2(-/-) BALB/c mice, with or without ensuing Mp infection. TLR2 and lactotransferrin (LTF) expression in airway epithelial cells and lung Mp load were assessed. Adenovirus-mediated TLR2 gene transfer to airway epithelial cells of TLR2(-/-) mice reconstituted 30-40% TLR2 expression compared with TLR2(+/+) cells. Such airway epithelial TLR2 reconstitution in TLR2(-/-) mice significantly reduced lung Mp load (an appropriate 45% reduction), coupled with elevated LTF expression. LTF expression in mice was shown to be mainly dependent on TLR2 signaling in response to Mp infection. Exogenous human LTF protein dose-dependently decreased lung bacterial load in Mp-infected TLR2(-/-) mice. In addition, human LTF protein directly dose-dependently decreased Mp levels in vitro. These data indicate that reconstitution of airway epithelial TLR2 signaling in TLR2(-/-) mice significantly restores lung defense against bacteria (e.g., Mp) via increased lung antimicrobial protein LTF production. Our findings may offer a deliverable approach to attenuate bacterial infections in airways of asthma or chronic obstructive pulmonary disease patients with impaired TLR2 function.

Effects of the TLR2 agonists MALP-2 and Pam3Cys in isolated mouse lungs

Background

Gram-positive and Gram-negative bacteria are main causes of pneumonia or acute lung injury. They are recognized by the innate immune system via toll-like receptor-2 (TLR2) or TLR4, respectively. Among all organs, the lungs have the highest expression of TLR2 receptors, but little is known about the pulmonary consequences of their activation. Here we studied the effects of the TLR2/6 agonist MALP-2, the TLR2/1 agonist Pam₃Cys and the TLR4 agonist lipopolysaccharide (LPS) on pro-inflammatory responses in isolated lungs.

Methodology/Principal Findings

Isolated perfused mouse lungs were perfused for 60 min or 180 min with MALP-2 (25 ng/mL), Pam₃Cys (160 ng/mL) or LPS (1 µg/mL). We studied mediator release by enzyme linked immunosorbent assay (ELISA), the activation of mitogen activated protein kinase (MAPK) and AKT/protein kinase B by immunoblotting, and gene induction by quantitative polymerase chain reaction. All agonists activated the MAPK ERK1/2 and p38, but neither JNK or AKT kinase. The TLR ligands upregulated the inflammation related genes Tnf, Il1β, Il6, Il10, Il12, Ifng, Cxcl2 (MIP-2α) and Ptgs2. MALP-2 was more potent than Pam₃Cys in inducing Slpi, Cxcl10 (IP10) and Parg. Remarkable was the strong induction of Tnc by MALP2, which was not seen with Pam₃Cys or LPS. The growth factor related genes Areg and Hbegf were not affected. In addition, all three TLR agonists stimulated the release of IL-6, TNF, CXCL2 and CXCL10 protein from the lungs.

Conclusions/Significance

TLR2 and TLR4 activation leads to similar reactions in the lungs regarding MAPK activation, gene induction and mediator release. Several genes studied here have not yet been appreciated as targets of TLR2-activation in the lungs before, i.e., Slpi, tenascin C, Parg and Traf1. In addition, the MALP-2 dependent induction of Tnc may indicate the existence of TLR2/6-specific pathways.

Legionella pneumophila induces human beta defensin-3 in pulmonary cells

Background

Legionella pneumophila is an important causative agent of severe pneumonia in humans. Human alveolar epithelium and macrophages are effective barriers for inhaled microorganisms and actively participate in the initiation of innate host defense. The beta defensin-3 (hBD-3), an antimicrobial peptide is an important component of the innate immune response of the human lung. Therefore we hypothesize that hBD-3 might be important for immune defense towards L. pneumophila.

Methods

We investigated the effects of L. pneumophila and different TLR agonists on pulmonary cells in regard to hBD-3 expression by ELISA. Furthermore, siRNA-mediated inhibition of TLRs as well as chemical inhibition of potential downstream signaling molecules was used for functional analysis.

Results

L. pneumophila induced release of hBD-3 in pulmonary epithelium and alveolar macrophages. A similar response was observed when epithelial cells were treated with different TLR agonists. Inhibition of TLR2, TLR5, and TLR9 expression led to a decreased hBD-3 expression. Furthermore expression of hBD-3 was mediated through a JNK dependent activation of AP-1 (c-Jun) but appeared to be independent of NF-κB. Additionally, we demonstrate that hBD-3 elicited a strong antimicrobial effect on L. pneumophila replication.

Conclusions

Taken together, human pulmonary cells produce hBD-3 upon L. pneumophila infection via a TLR-JNK-AP-1-dependent pathway which may contribute to an efficient innate immune defense.

Legionella lipoprotein activates toll-like receptor 2 and induces cytokine production and expression of costimulatory molecules in peritoneal macrophages

Legionella bacterium, an intracellular pathogen of mononuclear phagocytes, causes acute fatal pneumonia, especially in patients with impaired cellular immune responses. Until recently, however, the toll-like receptor (TLR) engagement of bacterial proteins derived from Legionella is uncertain. We previously showed that a 19-kDa highly conserved peptidoglycan-associated lipoprotein (PAL) of Legionella pneumophila induced the PAL-specific B cell and T cell responses in mice. In this study, we observed that the rPAL antigen of L. pneumophila, as an effector molecule, activated murine macrophages via TLR2 and produced proinflammatory cytokines such as IL-6 and TNF-alpha. In both BALB/c and TLR4-deficient C3H/HeJ mice, pretreatment of macrophages with anti-TLR2 mAb showed severely impaired cytokine production in response to the rPAL. In addition, in vitro the rPAL treatment increased the cell surface expression of CD40, CD80, CD86 and MHC I/II molecules. We further showed that the synthetic CpG-oligodeoxynucleotides (CpG ODN) coadministered with the rPAL enhanced IL-12 and IL-6 production and expression of CD40, CD80 and MHC II compared to the rPAL treatment alone. In conclusions, these results indicate that Legionella PAL might activate macrophages via a TLR2-dependent mechanism which thus induce cytokine production and expression of costimulatory and MHC molecules.

The pattern recognition receptors Nod1 and Nod2 account for neutrophil recruitment to the lungs of mice infected with Legionella pneumophila

The intracellular bacterium Legionella pneumophila induces a severe form of pneumonia called Legionnaires diseases, which is characterized by a strong neutrophil (NE) infiltrate to the lungs of infected individuals. Although the participation of pattern recognition receptors, such as Toll-like receptors, was recently demonstrated, there is no information on the role of nod-like receptors (NLRs) for bacterial recognition in vivo and for NE recruitment to the lungs. Here, we employed a murine model of Legionnaires disease to evaluate host and bacterial factors involved in NE recruitment to the mice lungs. We found that L. pneumophila type four secretion system, known as Dot/Icm, was required for NE recruitment as dot/icm mutants fail to trigger NE recruitment in a process independent of bacterial multiplication. By using mice deficient for Nod1, Nod2, and Rip2, we found that these receptors accounted for NE recruitment to the lungs of infected mice. In addition, Rip2-dependent responses were important for cytokine production and bacterial clearance. Collectively, these studies show that Nod1, Nod2, and Rip2 account for generation of innate immune responses in vivo, which are important for NE recruitment and bacterial clearance in a murine model of Legionnaires diseases.

Toll-like receptor 2 (TLR2) plays a major role in innate resistance in the lung against murine Mycoplasma

Mycoplasma lipoproteins are recognized by Toll-like receptors (TLR), but TLRs' role in responses to infection are unknown. Mycoplasma pulmonis is a naturally occurring respiratory pathogen in mice. In the current study, we used TLR-transfected HEK cells and TLR2(-/-) bone marrow-derived dendritic cells to demonstrate TLR2-mediated events are important in the initial host-mycoplasma interactions promoting cytokine responses. As we found alveolar macrophages expressed TLR1, TLR2 and TLR6 mRNAs, a role for TLR2 in innate immune clearance in lungs was examined. Three days post-infection, TLR2(-/-) mice had higher M. pulmonis numbers in lungs, but not in nasal passages. However, TLR2(-/-) mice had higher lung cytokine levels, indicating TLR2-independent mechanisms are also involved in host responses. Thus, TLR2 plays a critical role in the ability of innate immunity to determine M. pulmonis numbers in the lung, and it is likely that early after respiratory infection that TLR2 recognition of M. pulmonis triggers initial cytokine responses of host cells.

Review: Toll-like receptors and NOD-like receptors in pulmonary antibacterial immunity

Lung diseases caused by bacteria are a leading cause of death in both immunocompromised and immunocompetent individuals as well as in children. Although neutrophil recruitment is critical to augment the host defence, excessive neutrophil accumulation results in life-threatening diseases, such as acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Therefore, it is important to modulate excessive neutrophil influx in ALI/ARDS to mitigate lung damage and mortality. A better understanding of the basic mechanisms underlying neutrophil influx is crucial to designing novel and innovative treatment strategies for ALI/ARDS. Recognition of bacteria in the lung is the critical first step leading to neutrophil influx. Pattern recognition receptors, such as Toll-like receptors and NOD-like receptors, play an important role in the recognition of bacterial pathogens. Understanding the molecular and cellular mechanisms associated with the recognition of bacterial pathogens by the host is critical for the development of effective therapeutic strategies to control parenchymal damage via modulating neutrophil accumulation in the lung.

Induction of human β-defensin-2 in pulmonary epithelial cells by Legionella pneumophila: involvement of TLR2 and TLR5, p38 MAPK, JNK, NF-κB, and AP-1

Legionella pneumophila is an important causative agent of severe pneumonia in humans. Human alveolar epithelium is an effective barrier for inhaled microorganisms and actively participates in the initiation of innate host defense. Induction of antimicrobial peptide human β-defensin-2 (hBD-2) by various stimuli in epithelial cells has been reported. However, the mechanisms by which bacterial infections enhance hBD-2 expression remain poorly understood. In this study, we investigated the effect of the pulmonary pathogen L. pneumophila on induction of hBD-2 in human pulmonary epithelial cells. Infection with L. pneumophila markedly increased hBD-2 production, and the response was attenuated in Toll-like receptor (TLR) 2 and TLR5 transient knockdown cells. Furthermore, pretreatment with SB-202190 (an inhibitor of p38 MAPK) and JNK II (an inhibitor of c-Jun NH(2)-terminal kinase), but not U0126 (an inhibitor of ERK), reduced L. pneumophila-induced hBD-2 release in A549 cells. L. pneumophila-induced hBD-2 liberation was mediated via recruitment of NF-κB and AP-1 to the hBD-2 gene promoter. Additionally, we showed that exo- and endogenous hBD-2 elicited a strong antimicrobial effect towards L. pneumophila. Together, these results suggest that L. pneumophila induces hBD-2 release in A549 cells, and the induction seems to be mediated through TLR2 and TLR5 as well as activation of p38 MAPK, JNK, NF-κB, and AP-1.

Mechanisms of neutrophil accumulation in the lungs against bacteria

Bacterial lung diseases are a major cause of morbidity and mortality both in immunocompromised and in immunocompetent individuals. Neutrophil accumulation, a pathological hallmark of bacterial diseases, is critical to host defense, but may also cause acute lung injury/acute respiratory distress syndrome. Toll-like receptors, nucleotide-binding oligomerization domain (NOD)-like receptors, transcription factors, cytokines, and chemokines play essential roles in neutrophil sequestration in the lungs. This review highlights our current understanding of the role of these molecules in the lungs during bacterial infection and their therapeutic potential. We also discuss emerging data on cholesterol and ethanol as environmentally modifiable factors that may impact neutrophil-mediated pulmonary innate host defense. Understanding the precise molecular mechanisms leading to neutrophil influx in the lungs during bacterial infection is critical for the development of more effective therapeutic and prophylactic strategies to control the excessive host response to infection.

Innate immune receptors on neutrophils and their role in chronic lung disease

Neutrophils, the prototypic cells of the innate immune system, are recruited to infected sites to protect the human body from invading pathogens. To accomplish this function, neutrophils sense pathogens and endogenous damage-associated molecules via innate immune receptors, such as Toll-like receptors (TLRs) and other pattern recognition receptors. This defence function is essential for the pulmonary microenvironment where the host is faced with millions of particles and pathogens inhaled daily. Chronic lung diseases, such as cystic fibrosis or chronic obstructive pulmonary disease are characterized by a neutrophil accumulation and chronic bacterial colonization of the airways. Consequently, insights into the role of TLRs on neutrophils in chronic lung diseases are of high relevance for further diagnostic and therapeutic approaches. Here we summarize and discuss recent advances in the expression, regulation and functional role of TLRs on neutrophils in chronic lung diseases.

The gut microbiota of toll-like receptor 2-deficient mice exhibits lineage-specific modifications

Mutations in toll-like receptors that mediate bacterial recognition by the mammalian innate immune system have the potential to substantially alter the composition of an individual's microbiota. Here we tested this hypothesis by comparing the intestinal microbiota of toll-like receptor 2-deficient mice, both young and middle aged, with that of wild-type mice of the same genetic background, housed together under specific pathogen-free conditions. Bacterial DNA was extracted from mouse caecal tissue samples, amplified using universal bacterial 16S ribosomal RNA gene primers, and cloned into a plasmid vector. Insert-containing colonies were picked for high-throughput sequencing, and sequence data were analysed yielding species-level phylogenetic data. Clone libraries were compared by phylogenetic composition analysis using UniFrac. While pairwise differences in phylogenetic population structure between mutant and wild-type mice were not statistically significant, anosim analysis did demonstrate a significant difference between toll-like receptor 2-deficient mice and their wild-type counterparts. The difference observed was probably due to a high level of colonization of the toll-like receptor 2-deficient mice by two distinct Helicobacter phylotypes that were totally absent from wild-type mice. Principal coordinate analysis clustering indicated that age is a weaker determinate than genotype and maternal heritage in the mouse caecal microbiota. The findings suggest that although mutations in toll-like receptors may cause increased susceptibility to specific opportunistic bacteria, they do not dramatically alter the phylogenetic structure of microbiota.

A novel role for neutrophils as critical activators of NK cells

Neutrophils are essential players in innate immune responses to bacterial infection. Despite the striking resistance of Legionella pneumophila (Lpn) to bactericidal neutrophil function, neutrophil granulocytes are important effectors in the resolution of legionellosis. Indeed, mice depleted of neutrophils were unable to clear Lpn due to a lack of the critical cytokine IFN-gamma, which is produced by NK cells. We demonstrate that this can be ascribed to a previously unappreciated role of neutrophils as major NK cell activators. In response to Lpn infection, neutrophils activate caspase-1 and produce mature IL-18, which is indispensable for the activation of NK cells. Furthermore, we show that the IL-12p70 response in Lpn-infected neutropenic mice is also severely reduced and that the Lpn-induced IFN-gamma production by NK cells is strictly dependent on IL-12. However, since dendritic cells, and not neutrophils, are the source of Lpn-induced IL-12, its paucity is a consequence of the absence of IFN-gamma produced by NK cells rather than the absence of neutrophils per se. Therefore, neutrophil-derived IL-18, in combination with dendritic cell-produced IL-12, triggers IFN-gamma synthesis in NK cells in Lpn-infected mice. We propose a novel central role for neutrophils as essential IL-18 producers and hence NK cell "helpers" in bacterial infection.

Importance of TLR2 in early innate immune response to acute pulmonary infection with Porphyromonas gingivalis in mice

The periodontal pathogen Porphyromonas gingivalis is implicated in certain systemic diseases including atherosclerosis and aspiration pneumonia. This organism induces innate responses predominantly through TLR2, which also mediates its ability to induce experimental periodontitis and accelerate atherosclerosis. Using a validated mouse model of intratracheal challenge, we investigated the role of TLR2 in the control of P. gingivalis acute pulmonary infection. TLR2-deficient mice elicited reduced proinflammatory or antimicrobial responses (KC, MIP-1alpha, TNF-alpha, IL-6, IL-12p70, and NO) in the lung and exhibited impaired clearance of P. gingivalis compared with normal controls. However, the influx of polymorphonuclear leukocytes into the lung and the numbers of resident alveolar macrophages (AM) were comparable between the two groups. TLR2 signaling was important for in vitro killing of P. gingivalis by polymorphonuclear leukocytes or AM and, moreover, the AM bactericidal activity required NO production. Strikingly, AM were more potent than peritoneal or splenic macrophages in P. gingivalis killing, attributed to diminished AM expression of complement receptor-3 (CR3), which is exploited by P. gingivalis to promote its survival. The selective expression of CR3 by tissue macrophages and the requirement of TLR2 inside-out signaling for CR3 exploitation by P. gingivalis suggest that the role of TLR2 in host protection may be contextual. Thus, although TLR2 may mediate destructive effects, as seen in models of experimental periodontitis and atherosclerosis, we have now shown that the same receptor confers protection against P. gingivalis in acute lung infection.

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.

Toll-like receptors: their roles in bacterial recognition and respiratory infections

Although respiratory infections cause significant morbidity and mortality throughout the world, the immunologic factors that mediate host susceptibility to these infections remain poorly understood. The lung contains a vast surface at the host-environment interface and acts as a crucial barrier to invading pathogens. The lung is equipped with specialized epithelial and hematopoietic cells, which express pattern recognition receptors that act as both sentinels and mediators of pulmonary innate immunity. Toll-like receptors (TLRs) mediate a particularly critical role in pathogen recognition and subsequent initiation of the host immune response. In this review, we will summarize current knowledge of TLRs and their bacterial ligands and explore their role in respiratory infections. Moreover, we will highlight recent advances in the role of TLRs in pulmonary infections from a human immunogenetics perspective.

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.

Activation of Toll-like receptors by Burkholderia pseudomallei

Background

Melioidosis, a lethal tropical infection that is endemic in southeast Asia and northern Australia, is caused by the saprophytic Gram-negative bacterium Burkholderia pseudomallei. Overall mortality approaches 40% yet little is known about mechanisms of host defense. Toll-like receptors (TLRs) are host transmembrane receptors that recognize conserved pathogen molecular patterns and induce an inflammatory response. The lipopolysaccharide (LPS) of Gram-negative bacteria is a potent inducer of the host innate immune system. TLR4, in association with MD-2, is the archetype receptor for LPS although B. pseudomallei LPS has been previously identified as a TLR2 agonist. We examined TLR signaling induced by B. pseudomallei, B. pseudomallei LPS, and B. pseudomallei lipid A using gain-of-function transfection assays of NF-κB activation and studies of TLR-deficient macrophages.

Results

In HEK293 cells transfected with murine or human TLRs, CD14, and MD-2, heat-killed B. pseudomallei activated TLR2 (in combination with TLR1 or TLR6) and TLR4. B. pseudomallei LPS and lipid A activated TLR4 and this TLR4-mediated signaling required MD-2. In TLR2^-/- macrophages, stimulation with heat-killed B. pseudomallei augmented TNF-α and MIP-2 production whereas in TLR4^-/- cells, TNF-α, MIP-2, and IL-10 production was reduced. Cytokine production by macrophages stimulated with B. pseudomallei LPS or lipid A was entirely dependent on TLR4 but was increased in the absence of TLR2. TLR adaptor molecule MyD88 strongly regulated TNF-α production in response to heat-killed B. pseudomallei.

Conclusion

B. pseudomallei activates TLR2 and TLR4. In the presence of MD-2, B. pseudomallei LPS and lipid A are TLR4 ligands. Although the macrophage cytokine response to B. pseudomallei LPS or lipid A is completely dependent on TLR4, in TLR2^-/- macrophages stimulated with B. pseudomallei, B. pseudomallei LPS or lipid A, cytokine production is augmented. Other MyD88-dependent signaling pathways may also be important in the host response to B. pseudomallei infection. These findings provide new insights into critical mechanisms of host defense in melioidosis.

Human and mouse macrophage-inducible C-type lectin (Mincle) bind Candida albicans

Candida albicans is a causative agent in mycoses of the skin, oral cavity, and gastrointestinal tract. Identification of receptors, and their respective ligands, that are engaged by immune cells when in contact with C. albicans is crucial for understanding inflammatory responses leading to invasive candidiasis. Mincle is a recently identified macrophage-expressed receptor that is important for host responses to C. albicans. The carbohydrate-recognition domain of human and mouse Mincle were expressed, purified under denaturing conditions, and successfully refolded. In addition to oligomers, there are isolatable monomeric and dimeric forms of the protein that occur under two different buffer solutions. The human and mouse homologues bound yeast extract, and the isolated dimeric and monomeric species also demonstrated the recognition of whole C. albicans yeast cells. The data are indicative of several functional states mediating the interaction of Mincle and yeast at the surface of the macrophage.

Toll-like receptor 9 regulates the lung macrophage phenotype and host immunity in murine pneumonia caused by Legionella pneumophila

Experiments were performed to determine the contribution of TLR9 to the generation of protective immunity against the intracellular respiratory bacterial pathogen Legionella pneumophila. In initial studies, we found that the intratracheal (i.t.) administration of L. pneumophila to mice deficient in TLR9 (TLR9(-/-)) resulted in significantly increased mortality, which was associated with an approximately 10-fold increase in the number of lung CFU compared to that of wild-type BALB/c mice. Intrapulmonary bacterial challenge in TLR9(-/-) mice resulted in the reduced accumulation of myeloid dendritic cells (DC) and activated CD4(+) T cells. Lung macrophages isolated from Legionella-infected TLR9(-/-) mice displayed the impaired internalization of bacteria and evidence of alternative rather than classical activation, as manifested by the markedly reduced expression of nitric oxide and type 1 cytokines, whereas the expression of Fizz-1 and arginase-1 was enhanced. The adoptive transfer of bone marrow-derived DC from syngeneic wild-type, but not TLR9(-/-), mice administered i.t. reconstituted anti-legionella immunity and restored the macrophage phenotype in TLR9(-/-) mice. Finally, the i.t., but not intraperitoneal, administration of the TLR9 agonist molecule CpG oligodeoxynucleotide stimulated protective immunity in Legionella-infected mice. In total, our findings indicate that TLR9 is required for effective innate immune responses against the intracellular bacterial pathogen L. pneumophila, and approaches to maximize TLR9-mediated responses may serve as a means to augment antibacterial immunity in pneumonia.