TLR-4 pathway mediates the inflammatory response but not bacterial elimination in E. coli pneumonia

Regulation of immune responses to infection through interaction between stem cell-derived exosomes and toll-like receptors mediated by microRNA cargoes

Infectious diseases are among the factors that account for a significant proportion of disease-related deaths worldwide. The primary treatment approach to combat microbial infections is the use of antibiotics. However, the widespread use of these drugs over the past two decades has led to the emergence of resistant microbial species, making the control of microbial infections a serious challenge. One of the most important solutions in the field of combating infectious diseases is the regulation of the host's defense system. Toll-like receptors (TLRs) play a crucial role in the first primary defense against pathogens by identifying harmful endogenous molecules released from dying cells and damaged tissues as well as invading microbial agents. Therefore, they play an important role in communicating and regulating innate and adaptive immunity. Of course, excessive activation of TLRs can lead to disruption of immune homeostasis and increase the risk of inflammatory reactions. Targeting TLR signaling pathways has emerged as a new therapeutic approach for infectious diseases based on host-directed therapy (HDT). In recent years, stem cell-derived exosomes have received significant attention as factors regulating the immune system. The regulation effects of exosomes on the immune system are based on the HDT strategy, which is due to their cargoes. In general, the mechanism of action of stem cell-derived exosomes in HDT is by regulating and modulating immunity, promoting tissue regeneration, and reducing host toxicity. One of their most important cargoes is microRNAs, which have been shown to play a significant role in regulating immunity through TLRs. This review investigates the therapeutic properties of stem cell-derived exosomes in combating infections through the interaction between exosomal microRNAs and Toll-like receptors.

Using the Traditional Ex Vivo Whole Blood Model to Discriminate Bacteria by Their Inducible Host Responses

Whole blood models are rapid and versatile for determining immune responses to inflammatory and infectious stimuli, but they have not been used for bacterial discrimination. Staphylococcus aureus, S. epidermidis and Escherichia coli are the most common causes of invasive disease, and rapid testing strategies utilising host responses remain elusive. Currently, immune responses can only discriminate between bacterial 'domains' (fungi, bacteria and viruses), and very few studies can use immune responses to discriminate bacteria at the species and strain level. Here, whole blood was used to investigate the relationship between host responses and bacterial strains. Results confirmed unique temporal profiles for the 10 parameters studied: IL-6, MIP-1α, MIP-3α, IL-10, resistin, phagocytosis, S100A8, S100A8/A9, C5a and TF3. Pairwise analysis confirmed that IL-6, resistin, phagocytosis, C5a and S100A8/A9 could be used in a discrimination scheme to identify to the strain level. Linear discriminant analysis (LDA) confirmed that (i) IL-6, MIP-3α and TF3 could predict genera with 95% accuracy; (ii) IL-6, phagocytosis, resistin and TF3 could predict species at 90% accuracy and (iii) phagocytosis, S100A8 and IL-10 predicted strain at 40% accuracy. These data are important because they confirm the proof of concept that host biomarker panels could be used to identify bacterial pathogens.

Gut Microbiota, Inflammatory Bowel Disease, and Cancer: The Role of Guardians of Innate Immunity

Inflammatory bowel diseases (IBDs) are characterized by a persistent low-grade inflammation that leads to an increased risk of colorectal cancer (CRC) development. Several factors are implicated in this pathogenetic pathway, such as innate and adaptive immunity, gut microbiota, environment, and xenobiotics. At the gut mucosa level, a complex interplay between the immune system and gut microbiota occurs; a disequilibrium between these two factors leads to an alteration in the gut permeability, called 'leaky gut'. Subsequently, an activation of several inflammatory pathways and an alteration of gut microbiota composition with a proliferation of pro-inflammatory bacteria, known as 'pathobionts', take place, leading to a further increase in inflammation. This narrative review provides an overview on the principal Pattern Recognition Receptors (PRRs), including Toll-like receptors (TLRs) and NOD-like receptors (NLRs), focusing on their recognition mechanisms, signaling pathways, and contributions to immune responses. We also report the genetic polymorphisms of TLRs and dysregulation of NLR signaling pathways that can influence immune regulation and contribute to the development and progression of inflammatory disease and cancer.

BATF2 enhances proinflammatory cytokine responses in macrophages and improves early host defense against pulmonary Klebsiella pneumoniae infection

Basic leucine zipper transcription factor ATF-like 2 (BATF2) is a transcription factor that is emerging as an important regulator of the innate immune system. BATF2 is among the top upregulated genes in human alveolar macrophages treated with LPS, but the signaling pathways that induce BATF2 expression in response to Gram-negative stimuli are incompletely understood. In addition, the role of BATF2 in the host response to pulmonary infection with a Gram-negative pathogen like Klebsiella pneumoniae (Kp) is not known. We show that induction of Batf2 gene expression in macrophages in response to Kp in vitro requires TRIF and type I interferon (IFN) signaling, but not MyD88 signaling. Analysis of the impact of BATF2 deficiency on macrophage effector functions in vitro showed that BATF2 does not directly impact macrophage phagocytic uptake and intracellular killing of Kp. However, BATF2 markedly enhanced macrophage proinflammatory gene expression and Kp-induced cytokine responses. In vivo, Batf2 gene expression was elevated in lung tissue of wild-type (WT) mice 24 h after pulmonary Kp infection, and Kp-infected BATF2-deficient (Batf2-/-) mice displayed an increase in bacterial burden in the lung, spleen, and liver compared with WT mice. WT and Batf2-/- mice showed similar recruitment of leukocytes following infection, but in line with in vitro observations, proinflammatory cytokine levels in the alveolar space were reduced in Batf2-/- mice. Altogether, these results suggest that BATF2 enhances proinflammatory cytokine responses in macrophages in response to Kp and contributes to the early host defense against pulmonary Kp infection.NEW & NOTEWORTHY This study investigates the signaling pathways that mediate induction of BATF2 expression downstream of TLR4 and also the impact of BATF2 on the host defense against pulmonary Kp infection. We demonstrate that Kp-induced upregulation of BATF2 in macrophages requires TRIF and type I IFN signaling. We also show that BATF2 enhances Kp-induced macrophage cytokine responses and that BATF2 contributes to the early host defense against pulmonary Kp infection.

Phagocytosis is a primary determinant of pulmonary clearance of clinical Klebsiella pneumoniae isolates

Introduction

Klebsiella pneumoniae (Kp) is a common cause of hospital-acquired pneumonia. Although previous studies have suggested that evasion of phagocytic uptake is a virulence determinant of Kp, few studies have examined phagocytosis sensitivity in clinical Kp isolates.

Methods

We screened 19 clinical respiratory Kp isolates that were previously assessed for mucoviscosity for their sensitivity to macrophage phagocytic uptake, and evaluated phagocytosis as a functional correlate of in vivo Kp pathogenicity.

Results

The respiratory Kp isolates displayed heterogeneity in the susceptibility to macrophage phagocytic uptake, with 14 out of 19 Kp isolates displaying relative phagocytosis-sensitivity compared to the reference Kp strain ATCC 43816, and 5 out of 19 Kp isolates displaying relative phagocytosis-resistance. Intratracheal infection with the non-mucoviscous phagocytosis-sensitive isolate S17 resulted in a significantly lower bacterial burden compared to infection with the mucoviscous phagocytosis-resistant isolate W42. In addition, infection with S17 was associated with a reduced inflammatory response, including reduced bronchoalveolar lavage fluid (BAL) polymorphonuclear (PMN) cell count, and reduced BAL TNF, IL-1β, and IL-12p40 levels. Importantly, host control of infection with the phagocytosis-sensitive S17 isolate was impaired in alveolar macrophage (AM)-depleted mice, whereas AM-depletion had no significant impact on host defense against infection with the phagocytosis-resistant W42 isolate.

Conclusion

Altogether, these findings show that phagocytosis is a primary determinant of pulmonary clearance of clinical Kp isolates.

Diet-Induced Obesity Mice Execute Pulmonary Cell Apoptosis via Death Receptor and ER-Stress Pathways after E. coli Infection

Obesity has developed into a considerable health problem in the whole world. Escherichia coli (E. coli) can cause nosocomial pneumonia and induce cell apoptosis during injury and infection. Normal (lean) and diet-induced obesity mice (DIO, fed with high-fat diet) were chosen to perform nasal instillation with E. coli to establish a nonfatal acute pneumonia model. At 0 h, 12 h, 24 h, and 72 h postinfection, lung tissues were obtained to measure cell apoptosis. As shown in this study, both lean and DIO mice exhibited histopathological lesions of acute pneumonia and increased cell apoptosis in the lung infected with E. coli. Interestingly, the relative mRNA and protein expressions associated with either endoplasmic reticulum stress or death receptor apoptotic pathway were all dramatically increased in the DIO mice after infection, while only significant upregulation of death receptor apoptotic pathway in the lean mice at 72 h. These results indicated that the DIO mice executed excess cell apoptosis in the nonfatal acute pneumonia induced by E. coli infection through endoplasmic reticulum stress and death receptor apoptotic pathway.

Delayed Pulmonary Apoptosis of Diet-Induced Obesity Mice following Escherichia coli Infection through the Mitochondrial Apoptotic Pathway

Escherichia coli (E. coli) is one of pathogens causing nosocomial pneumonia and could induce pulmonary excessive apoptosis. Although much has been learned about metabolic diseases induced by obesity, the information linking bacterial pneumonia to obesity is limited. Accordingly, we investigated the apoptosis of normal (lean) and diet-induced obesity (DIO, fed a high-fat diet) mice after nasal instillation with E. coli. Lung tissues were obtained at 0 (preinfection), 12, 24, and 72 h after infection, and acute pulmonary inflammation was observed at 12 h. Elevated cell apoptosis and percentage of pulmonary cells depolarized with collapse of the mitochondrial transmembrane potential (Δψm) occurred in response to bacterial infection. The relative mRNA and protein expressions of Bax, caspase-3, and caspase-9 increased, but Bcl-2 decreased in the lung. Interestingly, the apoptotic percentage and most of apoptosis-associated factors mentioned above peaked at 12 or 24 h in the lean-E. coli group, while at 24 or 72 h in the DIO-E. coli group. Taken together, these findings indicated that the E. coli pneumonia caused excessive pulmonary apoptosis through the mitochondria-mediated pathway, and the apoptosis was delayed in the DIO mice with E. coli pneumonia.

Platelets inhibit apoptotic lung epithelial cell death and protect mice against infection-induced lung injury

Thrombocytopenia is associated with worse outcomes in patients with acute respiratory distress syndrome, which is most commonly caused by infection and marked by alveolar-capillary barrier disruption. However, the mechanisms by which platelets protect the lung alveolar-capillary barrier during infectious injury remain unclear. We found that natively thrombocytopenic Mpl ^-/- mice deficient in the thrombopoietin receptor sustain severe lung injury marked by alveolar barrier disruption and hemorrhagic pneumonia with early mortality following acute intrapulmonary Pseudomonas aeruginosa (PA) infection; barrier disruption was attenuated by platelet reconstitution. Although PA infection was associated with a brisk neutrophil influx, depletion of airspace neutrophils failed to substantially mitigate PA-triggered alveolar barrier disruption in Mpl ^-/- mice. Rather, PA cell-free supernatant was sufficient to induce lung epithelial cell apoptosis in vitro and in vivo and alveolar barrier disruption in both platelet-depleted mice and Mpl ^-/- mice in vivo. Cell-free supernatant from PA with genetic deletion of the type 2 secretion system, but not the type 3 secretion system, mitigated lung epithelial cell death in vitro and lung injury in Mpl ^-/- mice. Moreover, platelet releasates reduced poly (ADP ribose) polymerase cleavage and lung injury in Mpl ^-/- mice, and boiling of platelet releasates, but not apyrase treatment, abrogated PA supernatant-induced lung epithelial cell cytotoxicity in vitro. These findings indicate that while neutrophil airspace influx does not potentiate infectious lung injury in the thrombocytopenic host, platelets and their factors protect against severe pulmonary complications from pathogen-secreted virulence factors that promote host cell death even in the absence of overt infection.

Histopathological Changes Caused by Inflammation and Oxidative Stress in Diet-Induced-Obese Mouse following Experimental Lung Injury

Obesity has been identified as a risk factor for adverse outcomes of various diseases. However, information regarding the difference between the response of obese and normal subjects to pulmonary inflammation is limited. Mice were fed with the control or high-fat diet to establish the lean and diet-induced obese (DIO) mice. Escherichia coli was intranasally instilled to reproduce non-fatal acute pneumonia model. After infection, serum samples and lung tissues were obtained at 0, 12, 24, and 72 h. DIO mice exhibited increased serum triglyceride (TG) and total cholesterol (TC) contents as well as pulmonary resistin, IL-6, and leptin levels compared with lean mice. E. coli infection caused an acute suppurative inflammation in the lung with increased lung index and serum TG and TC contents; elevated pulmonary tumor necrosis factor-α, interleukin (IL)-1β, IL-6, IL-8, and leptin levels; and oxidative stress in mice. Interestingly, almost all the above-mentioned parameters peaked at 12 h after infection in the lean-E. coli group but after 12 h in the DIO-E. coli group. These results indicated that the DIO mice presented a delayed inflammatory response and oxidative stress in non-fatal acute pneumonia induced by E. coli infection.

15-epi-Lipoxin A4, Resolvin D2, and Resolvin D3 Induce NF-κB Regulators in Bacterial Pneumonia

Specialized proresolving mediators (SPMs) decrease NF-κB activity to prevent excessive tissue damage and promote the resolution of acute inflammation. Mechanisms for NF-κB regulation by SPMs remain to be determined. In this study, after LPS challenge, the SPMs 15-epi-lipoxin A₄ (15-epi-LXA₄), resolvin D1, resolvin D2, resolvin D3, and 17-epi-resolvin D1 were produced in vivo in murine lungs. In LPS-activated human bronchial epithelial cells, select SPMs increased expression of the NF-κB regulators A20 and single Ig IL-1R-related molecule (SIGIRR). Of interest, 15-epi-LXA₄ induced A20 and SIGIRR in an lipoxin A₄ receptor/formyl peptide receptor 2 (ALX/FPR2) receptor-dependent manner in epithelial cells and in murine pneumonia. This SPM regulated NF-κB-induced cytokines to decrease pathogen-mediated inflammation. In addition to dampening lung inflammation, surprisingly, 15-epi-LXA₄ also enhanced pathogen clearance with increased antimicrobial peptide expression. Taken together, to our knowledge these results are the first to identify endogenous agonists for A20 and SIGIRR expression to regulate NF-κB activity and to establish mechanisms for NF-κB regulation by SPMs for pneumonia resolution.

Thrombospondin-1 protects against pathogen-induced lung injury by limiting extracellular matrix proteolysis

Acute lung injury is characterized by excessive extracellular matrix proteolysis and neutrophilic inflammation. A major risk factor for lung injury is bacterial pneumonia. However, host factors that protect against pathogen-induced and host-sustained proteolytic injury following infection are poorly understood. Pseudomonas aeruginosa (PA) is a major cause of nosocomial pneumonia and secretes proteases to amplify tissue injury. We show that thrombospondin-1 (TSP-1), a matricellular glycoprotein released during inflammation, dose-dependently inhibits PA metalloendoprotease LasB, a virulence factor. TSP-1-deficient (Thbs1-/-) mice show reduced survival, impaired host defense, and increased lung permeability with exaggerated neutrophil activation following acute intrapulmonary PA infection. Administration of TSP-1 from platelets corrects the impaired host defense and aberrant injury in Thbs1-/- mice. Although TSP-1 is cleaved into 2 fragments by PA, TSP-1 substantially inhibits Pseudomonas elastolytic activity. Administration of LasB inhibitor, genetic disabling of the PA type II secretion system, or functional deletion of LasB improves host defense and neutrophilic inflammation in mice. Moreover, TSP-1 provides an additional line of defense by directly subduing host-derived proteolysis, with dose-dependent inhibition of neutrophil elastase from airway neutrophils of mechanically ventilated critically ill patients. Thus, a host matricellular protein provides dual levels of protection against pathogen-initiated and host-sustained proteolytic injury following microbial trigger.

CpG DNA analysis of bacterial STDs

BACKGROUND

Bacterial infections in the genital tract frequently result in morbidity through a variety of inflammation based symptoms. One component of the bacteria that may trigger host inflammatory response is their DNA. CpG motifs in this DNA are known targets for Toll-like receptor 9 (TLR9), which is a pathogen-recognition receptors focusing on CpG DNA. The activation of TLR9 induces the NF-κB inflammatory pathway. This study aims to provide a broad view of the inflammatory potential of CpG DNA motifs in bacteria related to genital diseases: C. trachomatis, E. coli, N. gonorrhoeae, G. vaginalis, H. ducreyi, L. crispatus, L. gasseri, M. hominis, M. genitalium, T. pallidum, and U. urealyticum.

METHODS

Publicly available genomic sequences of the bacterial species and strains have been analyzed in silico to produce a CpG index number. This CpG index number shows the relative inflammatory potential of the genome and has previously been used in a study by Lundberg et al. (2003). Higher CpG index values suggest a strong CpG induced inflammation potential during infection and vice versa.

RESULTS

The highest observed CpG index belongs to G. vaginalis with a value of 26,2, suggesting a strong pro-inflammatory potential when in contact with TLR9. The lowest index belongs to N. gonorrhoeae with a value of -79,5, suggesting a strong immunoinhibitory effect on TLR9 contact. Interestingly, Lactobacilli showed a mean CpG index value of 4,2, suggesting a weak inflammatory potential.

DISCUSSION

Our results show varying CpG index values between bacterial species. Comparison of CpG indices with the clinical course of several pathogens shows the CpG index helps clarify the clinical course of infection. However, we found no links between CpG index values and either obligate pathogenicity or facultative pathogenicity through bacterial vaginosis. Lactobacilli showed relatively low CpG indices which do suggest a lower inflammatory potential from these bacteria.

CONCLUSIONS

Our results show varying CpG index values between bacterial species, which may help clarify the clinical course of infection, and may help diagnosis.

Influence of lipopolysaccharide-binding protein on pulmonary inflammation in gram-negative pneumonia

Lipopolysaccharide-binding protein (LBP) is upregulated as part of the acute-phase response. Lipopolysaccharide-binding protein has a known multifunctional role in potentiating the recognition, clearance, and killing of gram-negative bacteria. In a Klebsiella pneumonia model, we previously demonstrated that LBP gene-deficient mice (LBP-/-) mice experience increased mortality when compared with wild-type (Wt) mice (98% vs. 59%). We hypothesize that LBP is essential to bacterial clearance from the lung, and its absence leads to alteration of the pulmonary inflammatory response to pneumonia. Twelve- to 16-week-old female C57Bl/6 Wt mice and age-matched LBP-/- mice were administered 1 × 10(3) colony-forming units of Klebsiella pneumoniae by intratracheal injection. Animals were euthanized at 6, 12, 24, or 36 h after inoculation. Lung tissue and bronchoalveolar lavage samples were obtained. Lung homogenate samples were assayed to determine quantitative bacterial load per whole lung, proinflammatory cytokine concentrations, myeloperoxidase activity, and assessment of pulmonary leukocyte populations. In vitro production of inflammatory mediators were also assayed after LPS stimulation of peritoneal macrophages isolated from Wt, Toll-like receptor 4 (TLR4)-deficient, and LBP-/- mice. The LBP-/- mice demonstrated significantly elevated levels of bacteria in the lung at 24 and 36 h when compared with Wt controls. The average lung levels of proinflammatory cytokines interleukin-1β (IL-1β), IL-6, keratinocyte-derived chemokine, and macrophage-inflammatory protein-2 were greater in the LBP mice and remained elevated longer when compared with those in the Wt mice. Myeloperoxidase activity, an indicator of neutrophil content, was significantly increased at time 36 h in the LBP mice. After in vitro stimulation of peritoneal macrophages with LPS, production of IL-1β, IL-6, IL-10, keratinocyte-derived chemokine, and macrophage-inflammatory protein-1α were suppressed in LBP and TLR4-deficient mice compared with that in Wt. Absence of a functional LBP-/- gene results in diminished clearance of gram-negative bacteria from the pulmonary system. Failure to recognize and clear gram-negative bacteria via the LBP/TLR4 axis results in an initial delayed inflammatory response. This delay in LBP-/- mice is followed by excessive amplification and prolonged elevation of proinflammatory mediators and neutrophil sequestration within the lungs.

Tylvalosin exhibits anti-inflammatory property and attenuates acute lung injury in different models possibly through suppression of NF-κB activation

Tylvalosin, a new broad-spectrum, third-generation macrolides, may exert a variety of pharmacological activities. Here, we report on its anti-oxidative and anti-inflammatory activity in RAW 264.7 macrophages and mouse treated with lipopolysaccharide (LPS) as well as piglet challenged with porcine reproductive and respiratory syndrome virus (PRRSV). Tylvalosin treatment markedly decreased IL-8, IL-6, IL-1β, PGE2, TNF-α and NO levels in vitro and in vivo. LPS and PRRSV-induced reactive oxygen species (ROS) production, and the lipid peroxidation in mice lung tissues reduced after tylvalosin treatments. In mouse acute lung injury model induced by LPS, tylvalosin administration significantly attenuated tissues injury, and reduced the inflammatory cells recruitment and activation. The evaluated phospholipase A2 (PLA2) activity and the increased expressions of cPLA2-IVA, p-cPLA2-IVA and sPLA2-IVE were lowered by tylvalosin. Consistent with the mouse results, tylvalosin pretreatment attenuated piglet lung scores with improved growth performance and normal rectal temperature in piglet model induced by PRRSV. Furthermore, tylvalosin attenuated the IκBα phosphorylation and degradation, and blocked the NF-κB p65 translocation. These results indicate that in addition to its direct antimicrobial effect, tylvalosin exhibits anti-inflammatory property and attenuates acute lung injury through suppression of NF-κB activation.

Effect of moxibustion treatment on expression of NF-κB and IκBα in peripheral blood monocytes of rats with Helicobacter pylori-associated gastritis

AIM: To explore the effect of moxibustion treatment on gastric inflammatory injury and expression of NF-κB and IκBα in peripheral blood monocytes of rats with Helicobacter pylori (H. pylori)-associated gastritis, to reveal the mechanisms underlying the protective effect of moxibustion treatment against gastric inflammatory injury.

METHODS: Fifty healthy rats were randomly divided into five groups, namely, a control group (A), a H. pylori model group (B), a moxibustion at acupoints group (C), a moxibustion at non-acupoints group (D) and an electro-acupuncture group (E). Gastritis was induced by oral gavage with live H. pylori. The expression of NF-κB and IκBα in peripheral blood monocytes was detected by Western blot.

RESULTS: Compared to group A, gastric mucosal inflammation score and expression of NF-κB in monocytes were significantly increased (0 ± 2.0 vs 2.5 ± 2.5, 0.54 ± 0.11/β-actin vs 0.36 ± 0.13/β-actin, both P < 0.01), and expression of IκBα in monocytes was significantly decreased in group B (0.21 ± 0.03/β-actin vs 0.65 ± 0.18/β-actin, P < 0.01). Compared to group B, gastric mucosal inflammation score and expression of NF-κB in monocytes were significantly decreased (2.5 ± 2.5 vs 0 ± 2.00, 0.36 ± 0.13/β-actin vs 0.50 ± 0.04/β-actin, both P < 0.01), and expression of IκBα in monocytes was significantly increased in group C (0.65 ± 0.18/β-actin vs 0.24 ± 0.06/β-actin, P < 0.01). Compared to group D, gastric mucosal inflammation score and expression of NF-κB in monocytes were significantly decreased (3.00 ± 2.5 vs 0 ± 2.00, 0.36 ± 0.12/β-actin vs 0.50 ± 0.04/β-actin, both P < 0.01), and expression of IκBα in monocytes was significantly increased in group C (0.64 ± 0.19/β-actin vs 0.24 ± 0.06/β-actin, P < 0.01). Compared to group E, gastric mucosal inflammation score and expression of NF-κB in monocytes were significantly decreased (3 ± 2.75 vs 0 ± 2.00, 0.35 ± 0.10/β-actin vs 0.50 ± 0.04/β-actin, both P < 0.01), and expression of IκBα in monocytes was significantly increased in group C (0.52 ± 0.17/β-actin vs 0.24 ± 0.06/β-actin, P < 0.01).

CONCLUSION: Moxibustion at acupoints can reduce H. pylori-induced gastric mucosal inflammatory injury possibly via mechanisms associated with reducing NF-κB expression in monocytes and decreasing the release of inflammatory cytokines.

Systemic CD14 inhibition attenuates organ inflammation in porcine Escherichia coli sepsis

Sepsis is an infection-induced systemic inflammatory response syndrome. Upstream recognition molecules, like CD14, play key roles in the pathogenesis. The aim of the present study was to investigate the effect of systemic CD14 inhibition on local inflammatory responses in organs from septic pigs. Pigs (n = 34) receiving Escherichia coli-bacteria or E. coli-lipopolysaccharide (LPS) were treated with an anti-CD14 monoclonal antibody or an isotype-matched control. Lungs, liver, spleen, and kidneys were examined for bacteria and inflammatory biomarkers. E. coli and LPS were found in large amounts in the lungs compared to the liver, spleen, and kidneys. Notably, the bacterial load did not predict the respective organ inflammatory response. There was a marked variation in biomarker induction in the organs and in the effect of anti-CD14. Generally, the spleen produced the most cytokines per weight unit, whereas the liver contributed the most to the total load. All cytokines were significantly inhibited in the spleen. Interleukin-6 (IL-6) was significantly inhibited in all organs, IL-1β and IP-10 were significantly inhibited in liver, spleen, and kidneys, and tumor necrosis factor, IL-8, and PAI-1 were inhibited only in the spleen. ICAM-1 and VCAM-1 was significantly inhibited in the kidneys. Systemic CD14-inhibition efficiently, though organ dependent, attenuated local inflammatory responses. Detailed knowledge on how the different organs respond to systemic inflammation in vivo, beyond the information gained by blood examination, is important for our understanding of the nature of systemic inflammation and is required for future mediator-directed therapy in sepsis. Inhibition of CD14 seems to be a good candidate for such treatment.

Lipopolysaccharide clearance, bacterial clearance, and systemic inflammatory responses are regulated by cell type-specific functions of TLR4 during sepsis

The morbidity associated with bacterial sepsis is the result of host immune responses to pathogens, which are dependent on pathogen recognition by pattern recognition receptors, such as TLR4. TLR4 is expressed on a range of cell types, yet the mechanisms by which cell-specific functions of TLR4 lead to an integrated sepsis response are poorly understood. To address this, we generated mice in which TLR4 was specifically deleted from myeloid cells (LysMTLR4KO) or hepatocytes (HCTLR4KO) and then determined survival, bacterial counts, host inflammatory responses, and organ injury in a model of cecal ligation and puncture (CLP), with or without antibiotics. LysM-TLR4 was required for phagocytosis and efficient bacterial clearance in the absence of antibiotics. Survival, the magnitude of the systemic and local inflammatory responses, and liver damage were associated with bacterial levels. HCTLR4 was required for efficient LPS clearance from the circulation, and deletion of HCTLR4 was associated with enhanced macrophage phagocytosis, lower bacterial levels, and improved survival in CLP without antibiotics. Antibiotic administration during CLP revealed an important role for hepatocyte LPS clearance in limiting sepsis-induced inflammation and organ injury. Our work defines cell type-selective roles for TLR4 in coordinating complex immune responses to bacterial sepsis and suggests that future strategies for modulating microbial molecule recognition should account for varying roles of pattern recognition receptors in multiple cell populations.

Macrophage migration inhibitory factor deficiency is associated with impaired killing of gram-negative bacteria by macrophages and increased susceptibility to Klebsiella pneumoniae sepsis

The cytokine macrophage migration inhibitory factor (MIF) is an important component of the early proinflammatory response of the innate immune system. However, the antimicrobial defense mechanisms mediated by MIF remain fairly mysterious. In the present study, we examined whether MIF controls bacterial uptake and clearance by professional phagocytes, using wild-type and MIF-deficient macrophages. MIF deficiency did not affect bacterial phagocytosis, but it strongly impaired the killing of gram-negative bacteria by macrophages and host defenses against gram-negative bacterial infection, as shown by increased mortality in a Klebsiella pneumonia model. Consistent with MIF's regulatory role of Toll-like 4 expression in macrophages, MIF-deficient cells stimulated with lipopolysaccharide or Escherichia coli exhibited reduced nuclear factor κB activity and tumor necrosis factor (TNF) production. Addition of recombinant MIF or TNF corrected the killing defect of MIF-deficient macrophages. Together, these data show that MIF is a key mediator of host responses against gram-negative bacteria, acting in part via a modulation of bacterial killing by macrophages.

Neutrophil extracellular traps directly induce epithelial and endothelial cell death: a predominant role of histones

Neutrophils play an important role in innate immunity by defending the host organism against invading microorganisms. Antimicrobial activity of neutrophils is mediated by release of antimicrobial peptides, phagocytosis as well as formation of neutrophil extracellular traps (NET). These structures are composed of DNA, histones and granular proteins such as neutrophil elastase and myeloperoxidase. This study focused on the influence of NET on the host cell functions, particularly on human alveolar epithelial cells as the major cells responsible for gas exchange in the lung. Upon direct interaction with epithelial and endothelial cells, NET induced cytotoxic effects in a dose-dependent manner, and digestion of DNA in NET did not change NET-mediated cytotoxicity. Pre-incubation of NET with antibodies against histones, with polysialic acid or with myeloperoxidase inhibitor but not with elastase inhibitor reduced NET-mediated cytotoxicity, suggesting that histones and myeloperoxidase are responsible for NET-mediated cytotoxicity. Although activated protein C (APC) did decrease the histone-induced cytotoxicity in a purified system, it did not change NET-induced cytotoxicity, indicating that histone-dependent cytotoxicity of NET is protected against APC degradation. Moreover, in LPS-induced acute lung injury mouse model, NET formation was documented in the lung tissue as well as in the bronchoalveolar lavage fluid. These data reveal the important role of protein components in NET, particularly histones, which may lead to host cell cytotoxicity and may be involved in lung tissue destruction.

Inhaled innate immune ligands to prevent pneumonia

Epithelial surfaces throughout the body continuously sample and respond to environmental stimuli. The accessibility of lung epithelium to inhaled therapies makes it possible to stimulate local antimicrobial defences with aerosolized innate immune ligands. This strategy has been shown to be effective in preclinical models, as delivery of innate immune ligands to the lungs of laboratory animals results in protection from subsequent challenge with microbial pathogens. Survival of the animal host in this setting correlates directly with killing of pathogens within the lungs, indicating the induction of a resistance mechanism. Resistance appears to be mediated primarily by activated epithelial cells rather than recruited leucocytes. Resistance reaches a peak within hours and persists for several days. Innate immune ligands can interact synergistically under some circumstances, and synergistic combinations of innate ligands delivered by aerosol are capable of inducing a high level of broad host resistance to bacteria, fungi and viruses. The induction of innate antimicrobial resistance within the lungs could have clinical applications in the prevention of lower respiratory tract infection in subjects transiently at high risk. These include cancer patients undergoing myeloablative chemotherapy, intubated patients being mechanically ventilated, vulnerable individuals during seasonal influenza epidemics, asthmatic subjects experiencing a respiratory viral infection, and healthy subjects exposed to virulent pathogens from a bioterror attack or emergent pandemic. In summary, stimulation of the lung epithelium to induce localized resistance to infection is a novel strategy whose clinical utility will be assessed in the near future.

Synergistic interactions of TLR2/6 and TLR9 induce a high level of resistance to lung infection in mice

Infectious pneumonias exact an unacceptable mortality burden worldwide. Efforts to protect populations from pneumonia have focused historically on antibiotic development and vaccine-enhanced adaptive immunity. However, we have reported recently that the lungs' innate defenses can be induced therapeutically by inhalation of a bacterial lysate that protects mice against otherwise lethal pneumonia. In this study, we tested in mice the hypothesis that TLRs are required for this antimicrobial phenomenon and found that resistance could not be induced in the absence of the TLR signaling adaptor protein MyD88. We then attempted to recapitulate the protection afforded by the bacterial lysate by stimulating the lung epithelium with aerosolized synthetic TLR ligands. Although most single or combination treatments yielded no protection, simultaneous treatment with ligands for TLR2/6 and TLR9 conferred robust, synergistic protection against virulent gram-positive and gram-negative pathogens. Protection was associated with rapid pathogen killing in the lungs, and pathogen killing could be induced from lung epithelial cells in isolation. Taken together, these data demonstrate the requirement for TLRs in inducible resistance against pneumonia, reveal a remarkable, unanticipated synergistic interaction of TLR2/6 and TLR9, reinforce the emerging evidence supporting the antimicrobial capacity of the lung epithelium, and may provide the basis for a novel clinical therapeutic that can protect patients against pneumonia during periods of peak vulnerability.

Inducible innate resistance of lung epithelium to infection

Most studies of innate immunity have focused on leukocytes such as neutrophils, macrophages, and natural killer cells. However, epithelial cells play key roles in innate defenses that include providing a mechanical barrier to microbial entry, signaling to leukocytes, and directly killing pathogens. Importantly, all these defenses are highly inducible in response to the sensing of microbial and host products. In healthy lungs, the level of innate immune epithelial function is low at baseline. This is indicated by low levels of spontaneous microbial killing and cytokine release, reflecting low constitutive stimulation in the nearly sterile lower respiratory tract when mucociliary clearance mechanisms are functioning effectively. This contrasts with the colon, where bacteria are continuously present and epithelial cells are constitutively activated. Although the surface area of the lungs presents a large target for microbial invasion, activated lung epithelial cells that are closely apposed to deposited pathogens are ideally positioned for microbial killing.

Effect of Toll-like receptor 4 inhibitor on LPS-induced lung injury

OBJECTIVE AND DESIGN

Toll-like receptor 4 (TLR4) plays important roles in the recognition of lipopolysaccharide (LPS) and the activation of inflammatory cascade. In this study, we evaluated the effect of TAK-242, a selective TLR4 signal transduction inhibitor, on acute lung injury (ALI).

MATERIALS AND METHODS

C57BL/6J mice were intravenously treated with TAK-242 15 min before the intratracheal administration of LPS or Pam3CSK4, a synthetic lipopeptide. Six hours after the challenge, bronchoalveolar lavage fluid was obtained for a differential cell count and the measurement of cytokine and myeloperoxidase levels. Lung permeability and nuclear factor-kappaB (NF-kappaB) DNA binding activity were also evaluated.

RESULTS

TAK-242 effectively attenuated the neutrophil accumulation and activation in the lungs, the increase in lung permeability, production of inflammatory mediators, and NF-kappaB DNA-binding activity induced by the LPS challenge. In contrast, TAK-242 did not suppress inflammatory changes induced by Pam3CSK4.

CONCLUSION

TAK-242 may be a promising therapeutic agent for ALI, especially injuries associated with pneumonia caused by Gram-negative bacteria.

Role of CD14 in lung inflammation and infection

This article is one of ten reviews selected from the Yearbook of Intensive Care and Emergency Medicine 2010 (Springer Verlag) and co-published as a series in Critical Care. Other articles in the series can be found online at http://ccforum.com/series/yearbook. Further information about the Yearbook of Intensive Care and Emergency Medicine is available from http://www.springer.com/series/2855.

Immune responses of airway neutrophils are impaired in asthma

Neutrophilic inflammation is a key effector arm of the innate immune response. Neutrophils may contribute significantly to airway inflammation in certain asthma subtypes. The objective of this study is to investigate the innate immune responses of isolated airway and circulating neutrophils in asthma. Participants with asthma (n = 17) and healthy volunteers (n = 11) underwent induced sputum and blood collection. Neutrophils were isolated from dispersed selected sputum and blood granulocytes using magnetic cell separation. Neutrophils were cultured with or without lipopolysaccharide (100 ng/mL) for 24 hours. Innate immune mediators were measured by enzyme-linked immunosorbent assay (ELISA) and real-time polymerase chain reaction (PCR). Airway neutrophils from participants with asthma spontaneously released lower levels of interleukin (IL)-8, IL-1beta and tumor necrosis factor-alpha proteins, and had lower levels of cytokine gene expression compared to healthy controls. Toll-like receptor 4 (TLR4) gene expression was significantly decreased in airway neutrophils from participants with asthma compared to healthy volunteers. Resting and lipopolysaccharide (LPS)-stimulated circulating neutrophils had lower levels of TLR2 and IL-1beta gene expression in asthma, but were otherwise similar to healthy controls. No differences were seen for matrix metalloproteinase (MMP)-9 release in asthma. Innate immune responses of airway neutrophil cells are impaired in asthmatic subjects on prophylactic therapy and may impact on susceptibility to, and severity of, airway infections.

Role of CD14 in Lung Inflammation and Infection

Toll-like receptors (TLR) on the surface of cells of the respiratory tract play an essential role in sensing the presence of microorganisms in the airways and lungs. These receptors trigger inflammatory responses, activate innate immune responses, and prime adaptive immune responses to eradicate invading microbes [1]. TLR are members of a family of pattern-recognition receptors, which recognize molecular structures of bacteria, viruses, fungi and protozoa (pathogen-associated molecular patterns or PAMPs), as well as endogenous structures and proteins released during inflammation (damage/danger-associated molecular patterns or DAMPs). To date, ten different TLR have been identified in humans and twelve in mice. TLR are expressed on all cells of the immune system, but also on parenchymal cells of many organs and tissues. The binding of a PAMP to a TLR results in cellular activation and initiates a variety of effector functions, including cytokine secretion, proliferation’ co-stimulation or phagocyte maturation. To facilitate microbial recognition and to amplify cellular responses, certain TLR require additional proteins, such as lipopolysaccharide (LPS) binding protein (LBP), CD14, CD36 and high mobility group box-l protein (HMGB-l). In this chapter, the role of CD14 as an accessory receptor for TLR in lung inflammation and infection is discussed. The central role of CD14 in the recognition of various PAMPs and amplification of immune and inflammatory responses in the lung is depicted in Fig. 1.

Fig. 1.

Role of CD14 in Lung Inflammation and Infection

Toll-like receptors (TLR) on the surface of cells of the respiratory tract play an essential role in sensing the presence of microorganisms in the airways and lungs. These receptors trigger inflammatory responses, activate innate immune responses, and prime adaptive immune responses to eradicate invading microbes [1]. TLR are members of a family of pattern-recognition receptors, which recognize molecular structures of bacteria, viruses, fungi and protozoa (pathogen-associated molecular patterns or PAMPs), as well as endogenous structures and proteins released during inflammation (damage/danger-associated molecular patterns or DAMPs). To date, ten different TLR have been identified in humans and twelve in mice. TLR are expressed on all cells of the immune system, but also on parenchymal cells of many organs and tissues. The binding of a PAMP to a TLR results in cellular activation and initiates a variety of effector functions, including cytokine secretion, proliferation, co-stimulation or phagocyte maturation. To facilitate microbial recognition and to amplify cellular responses, certain TLR require additional proteins, such as lipopolysaccharide (LPS) binding protein (LBP), CD14, CD36 and high mobility group box-1 protein (HMGB-1). In this chapter, the role of CD14 as an accessory receptor for TLR in lung inflammation and infection is discussed. The central role of CD14 in the recognition of various PAMPs and amplification of immune and inflammatory responses in the lung is depicted in Figure 1. Fig. 1.

Single immunoglobulin IL-1 receptor-related protein attenuates the lipopolysaccharide-induced inflammatory response in A549 cells

The lipopolysaccharide (LPS)-Toll-like receptor 4 (TLR4) signaling pathway in alveolar epithelial cells plays an important role in many pathologic processes such as acute lung injury (ALI). The single immunoglobulin IL-1 receptor-related protein (SIGIRR) is an inhibitor of LPS-TLR4 signaling, but its expression and function in alveolar epithelial cells are still unknown. In this study, we examined the expression of SIGIRR in normal human lung tissue using immunohistochemistry, reverse transcription-PCR (RT-PCR) and Western blot and found that SIGIRR was expressed in alveolar epithelial cells. Treatment of an alveolar epithelial cell line, A549, with LPS and we observed a downregulation of SIGIRR mRNA, which returned to normal levels 24h after LPS exposure. A549 cells were then transfected with a SIGIRR eukaryotic expression vector to over-express SIGIRR or, as a control, with an empty vector. Following LPS exposure, the transcriptional activity of NF-kappaB was measured using a dual-luciferase reporter assay system, and the concentration of IL-1beta, TNF-alpha and IL-6 was determined by ELISA, and cell proliferation was measured by MTT. In A549 cells that over-expressed SIGIRR, LPS treatment resulted in a significant decrease in the transcriptional activity of NF-kappaB and cell growth inhibition ratio, as well as lower levels of secreted IL-1beta, TNF-alpha and IL-6. In conclusion, SIGIRR in A549 cells inhibits the transcriptional activity of NF-kappaB and reduces the amount cytokines produced, protecting these cells from acute LPS-induced damage.

Differentiation of gram-negative bacterial aerosol exposure using detected markers in bronchial-alveolar lavage fluid

The identification of biosignatures of aerosol exposure to pathogens has the potential to provide useful diagnostic information. In particular, markers of exposure to different types of respiratory pathogens may yield diverse sets of markers that can be used to differentiate exposure. We examine a mouse model of aerosol exposure to known Gram negative bacterial pathogens, Francisella tularensis novicida and Pseudomonas aeruginosa. Mice were subjected to either a pathogen or control exposure and bronchial alveolar lavage fluid (BALF) was collected at four and twenty four hours post exposure. Small protein and peptide markers within the BALF were detected by matrix assisted laser desorption/ionization (MALDI) mass spectrometry (MS) and analyzed using both exploratory and predictive data analysis methods; principle component analysis and degree of association. The markers detected were successfully used to accurately identify the four hour exposed samples from the control samples. This report demonstrates the potential for small protein and peptide marker profiles to identify aerosol exposure in a short post-exposure time frame.

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 interactions of some commonly consumed drugs with mitochondrial adaptations to exercise

The importance of mitochondrial dysfunctions in the progression of chronic disease has been well established. Patients with chronic diseases are often prescribed a variety of medications, many of which have been shown to induce mitochondrial dysfunction. Exercise is a known stimulus for mitochondrial biogenesis and also recommended to patients as a lifestyle modification to supplement drug therapy. However, the potential interference of those drugs with mitochondrial adaptations to exercise has not been thoroughly investigated. This review provides a summary and discussion of known and potential interactions of commonly consumed drugs with exercise-induced mitochondrial adaptations.

Stimulated innate resistance of lung epithelium protects mice broadly against bacteria and fungi

Pneumonia is a serious problem worldwide. We recently demonstrated that innate defense mechanisms of the lung are highly inducible against pneumococcal pneumonia. To determine the breadth of protection conferred by stimulation of lung mucosal innate immunity, and to identify cells and signaling pathways activated by this treatment, mice were treated with an aerosolized bacterial lysate, then challenged with lethal doses of bacterial and fungal pathogens. Mice were highly protected against a broad array of Gram-positive, Gram-negative, and class A bioterror bacterial pathogens, and the fungal pathogen, Aspergillus fumigatus. Protection was associated with rapid pathogen killing within the lungs, and this effect was recapitulated in vitro using a respiratory epithelial cell line. Gene expression analysis of lung tissue showed marked activation of NF-kappaB, type I and II IFN, and antifungal Card9-Bcl10-Malt1 pathways. Cytokines were the most strongly induced genes, but the inflammatory cytokines TNF and IL-6 were not required for protection. Lung-expressed antimicrobial peptides were also highly up-regulated. Taken together, stimulated innate resistance appears to occur through the activation of multiple host defense signaling pathways in lung epithelial cells, inducing rapid pathogen killing, and conferring broad protection against virulent bacterial and fungal pathogens. Augmentation of innate antimicrobial defenses of the lungs might have therapeutic value for protection of patients with neutropenia or impaired adaptive immunity against opportunistic pneumonia, and for defense of immunocompetent subjects against a bioterror threat or epidemic respiratory infection.

Loss of red cell chemokine scavenging promotes transfusion-related lung inflammation

Red cell transfusions are associated with the development of acute lung injury in the critically ill. Recent evidence suggests that storage induced alterations of the red blood cell (RBC) collectively termed the "storage lesion" may be linked with adverse biologic consequences. Using a 2-event model of systemic endotoxemia followed by a secondary challenge of RBC transfusion, we investigated whether purified RBC concentrates from syngeneic C57BL/6 mice altered inflammatory responses in murine lungs. Transfusion of RBCs stored for 10 days increased neutrophil counts, macrophage inflammatory protein-2 (MIP-2) and chemokine (KC) concentrations in the airspaces, and lung microvascular permeability compared with transfusion of less than 1-day-old RBCs. Because RBCs have been shown to scavenge inflammatory chemokines through the blood group Duffy antigen, we investigated the expression and function of Duffy during storage. In banked human RBCs, both Duffy expression and chemokine scavenging function were reduced with increasing duration of storage. Transfusion of Duffy knockout RBCs into Duffy wild-type endotoxemic mice increased airspace neutrophils, inflammatory cytokine concentrations, and lung microvascular permeability compared with transfusion of Duffy wild-type RBCs. Thus, reduction in erythrocyte chemokine scavenging is one functional consequence of the storage lesion by which RBC transfusion can augment existing lung inflammation.

Pivotal Advance: Toll-like receptor regulation of scavenger receptor-A-mediated phagocytosis

Class-A scavenger receptors (SR-A) and TLR mediate early immune responses against pathogenic bacteria. SR-A and TLR molecules are expressed on phagocytes and interact with common ligands from Gram-negative and Gram-positive bacteria; however, the contribution of TLR activity to SR-A-mediated phagocytosis has not been assessed directly. Herein, we provide genetic and functional evidence that ligand- and TLR-specific stimuli synergize with SR-A to mediate bacterial phagocytosis. Although complete loss of SR-A (SR-A(-/-)) is known to impair bacterial clearance, here we identify the first deficiency attributable to SR-A heterozygosity: SR-A(+/-)TLR4(+/-) cells and mice are impaired significantly in the clearance of Gram-negative Escherichia coli. This phenotype is specific to the TLR signaling event, as SR-A(+/-)TLR4(+/-) cells are not deficient for the clearance of Gram-positive Staphylococcus aureus bacteria, which contain cell-surface TLR2 ligands but lack TLR4 ligands. We demonstrate that this is a global, phagocytic mechanism, regulated independently by multiple TLRs, as analogous to the SR-A(+/-)TLR4(+/-) deficit, SR-A(+/-)TLR2(+/-) cells are impaired for S. aureus uptake. In support of this, we show that SR-A(+/-)MyD88(+/-) cells recapitulate the phagocytosis defect observed in SR-A(+/-)TLR4(+/-) cells. These data identify for the first time that TLR-driven innate immune responses, via a MyD88 signaling mechanism, regulate SR-A-dependent phagocytosis of bacteria. These findings provide novel insights into how innate immune cells control SR-A-mediated trafficking and are the first demonstration that subtle changes in the expression of SR-A and TLRs can substantially affect host bacterial clearance.

No longer an innocent bystander: epithelial toll-like receptor signaling in the development of mucosal inflammation

Diseases of mucosal inflammation represent important causes of morbidity and mortality, and have led to intense research efforts to understand the factors that lead to their development. It is well accepted that a breakdown of the normally impermeant epithelial barrier of the intestine, the lung, and the kidney is associated with the development of inflammatory disease in these organs, yet significant controversy exists as to how this breakdown actually occurs, and how such a breakdown may lead to inflammation. In this regard, much work has focused upon the role of the epithelium as an “innocent bystander,” a target of a leukocyte-mediated inflammatory cascade that leads to its destruction in the mucosal inflammatory process. However, recent evidence from a variety of laboratories indicates that the epithelium is not merely a passive component in the steps that lead to mucosal inflammation, but is a central participant in the process. In addressing this controversy, we and others have determined that epithelial cells express Toll-like receptors (TLRs) of the innate immune system, and that activation of TLRs by endogenous and exogenous ligands may play a central role in determining the balance between a state of “mucosal homeostasis,” as is required for optimal organ function, and “mucosal injury,” leading to mucosal inflammation and barrier breakdown. In particular, activation of TLRs within intestinal epithelial cells leads to the development of cellular injury and impairment in mucosal repair in the pathogenesis of intestinal inflammation, while activation of TLRs in the lung and kidney may participate in the development of pneumonitis and nephritis respectively. Recent work in support of these concepts is extensively reviewed, while essential areas of further study that are required to determine the significance of epithelial TLR signaling during states of health and disease are outlined.

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.

CX3CL1 up-regulation is associated with recruitment of CX3CR1+ mononuclear phagocytes and T lymphocytes in the lungs during cigarette smoke-induced emphysema

CX3CR1 is expressed on monocytes, dendritic cells, macrophages, subsets of T lymphocytes, and natural killer cells and functions in diverse capacities such as leukocyte adhesion, migration, and cell survival on ligand binding. Expression of the CX3CL1 gene, whose expression product is the sole ligand for CX3CR1, is up-regulated in human lungs with chronic cigarette smoke-induced obstructive lung disease. At present, it is unknown whether CX3CL1 up-regulation is associated with the recruitment and accumulation of immune cells that express CX3CR1. We show that mice chronically exposed to cigarette smoke up-regulate CX3CL1 gene expression, which is associated with an influx of CX3CR1+ cells in the lungs. The increase in CX3CR1+ cells is primarily comprised of macrophages and T lymphocytes and is associated with the development of emphysema. In alveolar macrophages, cigarette smoke exposure increased the expression of both CX3CR1 and CX3CL1 genes. The inducibility of CX3CR1 expression was not solely dependent on a chronic stimulus because lipopolysaccharide up-regulated CX3CR1 in RAW264.7 cells in vitro and in mononuclear phagocytes in vivo. Our findings suggest a mechanism by which macrophages amplify and promote CX3CR1+ cell accumulation within the lungs during both acute and chronic inflammatory stress. We suggest that one function of the CX3CR1-CX3CL1 pathway is to recruit and sustain divergent immune cell populations implicated in the pathogenesis of cigarette smoke-induced emphysema.

Role of toll-like receptor 4 in acute neutrophilic lung inflammation induced by intratracheal bacterial products in mice

Background

Toll-like receptors (TLRs) represent a conserved family of innate immune recognition receptors. Among TLRs, TLR4 is important for the recognition of Gram-negative bacteria, whereas TLR2 recognizes cell wall constituents of Gram-positive microorganisms, such as peptidoglycan (PGN).

Methods

To evaluate the role of TLR4 in the pathogenesis of acute lung injury induced by Escherichia coli endotoxin (lipopolysaccharide; LPS) or PGN, we compared inflammatory cell accumulation in bronchoalveolar lavage (BAL) fluid and lung pathology between C3H/HeJ (TLR4 mutant) and wild-type C3H/HeN mice. The levels of proinflammatory cytokines and chemokines in plasma and BAL fluid and nuclear factor-κB (NF-κB) translocation in the lung were also evaluated.

Results

In C3H/HeJ mice, LPS-induced neutrophil emigration was significantly decreased compared with C3H/HeN mice, whereas PGN-induced neutrophil emigration did not differ. Differential cell count in BAL fluid revealed comparable neutrophil recruitment in the alveolar space. In TLR4 mutant mice, LPS-induced upregulation of tumor necrosis factor-alpha (TNF-α), KC, and CXCL10 in plasma and BAL fluid was attenuate, which was not different after PGN. NF-κB translocation in the lung was significantly decreased in C3H/HeJ compared with C3H/HeN mice, whereas PGN-induced NF-κB translocation was not different.

Conclusion

These results suggest that TLR4 mediates inflammatory cascade induced by Gram-negative bacteria that is locally administered.

Control of Pseudomonas aeruginosa in the lung requires the recognition of either lipopolysaccharide or flagellin

Acute lung infection due to Pseudomonas aeruginosa is an increasingly serious problem that results in high mortality especially in the compromised host. In this study, we set out to ascertain what components of the TLR system are most important for innate immunity to this microorganism. We previously demonstrated that TLR2,4-/- mice were not hypersusceptible to infection by a wild-type P. aeruginosa strain. However, we now find that mice lacking both TLR2 and TLR4 (TLR2,4-/- mice) are hypersusceptible to infection following challenge with a P. aeruginosa mutant devoid of flagellin production. We demonstrate that this hypersusceptibility is largely due to a lack of innate defense by the host that fails to control bacterial replication in the lung. Further evidence that a response to flagellin is a key factor in the failure of TLR2,4-/- mice to control the infection with the mutant strain was obtained by demonstrating that the intrapulmonary administration of flagellin over a 18 h period following infection, saved 100% of TLR2,4-/- mice from death. We conclude that the interactions of either TLR4 with LPS or TLR5 with flagellin can effectively defend the lung from P. aeruginosa infection and the absence of a response by both results in hypersusceptibility to this infection.

Depletion of resident alveolar macrophages does not prevent Fas-mediated lung injury in mice

Activation of the Fas/Fas ligand (FasL) system in the lungs results in a form of injury characterized by alveolar epithelial apoptosis and neutrophilic inflammation. Studies in vitro show that Fas activation induces apoptosis in alveolar epithelial cells and cytokine production in alveolar macrophages. The main goal of this study was to determine the contribution of alveolar macrophages to Fas-induced lung inflammation in mice, by depleting alveolar macrophages using clodronate-containing liposomes. Liposomes containing clodronate or PBS were instilled by intratracheal instillation. After 24 h, the mice received intratracheal instillations of the Fas-activating monoclonal antibody Jo2 or an isotype control antibody and were studied 18 h later. The Jo2 MAb induced increases in bronchoalveolar lavage fluid (BALF) total neutrophils, lung caspase-3 activity, and BALF total protein and worsened histological lung injury in the macrophage-depleted mice. Studies in vitro showed that Fas activation induced the release of the cytokine KC in a mouse lung epithelial cell line, MLE-12. These results suggest that the lung inflammatory response to Fas activation is not primarily dependent on resident alveolar macrophages and may instead depend on cytokine release by alveolar epithelial cells.

Pneumonia research to reduce childhood mortality in the developing world

Pneumonia is an illness, usually caused by infection, in which the lungs become inflamed and congested, reducing oxygen exchange and leading to cough and breathlessness. It affects individuals of all ages but occurs most frequently in children and the elderly. Among children, pneumonia is the most common cause of death worldwide. Historically, in developed countries, deaths from pneumonia have been reduced by improvements in living conditions, air quality, and nutrition. In the developing world today, many deaths from pneumonia are also preventable by immunization or access to simple, effective treatments. However, as we highlight here, there are critical gaps in our understanding of the epidemiology, etiology, and pathophysiology of pneumonia that, if filled, could accelerate the control of pneumonia and reduce early childhood mortality.

Stimulation of lung innate immunity protects against lethal pneumococcal pneumonia in mice

RATIONALE

The lungs are a common site of serious infection in both healthy and immunocompromised subjects, and the most likely route of delivery of a bioterror agent. Since the airway epithelium shows great structural plasticity in response to inflammatory stimuli, we hypothesized it might also show functional plasticity.

OBJECTIVES

To test the inducibility of lung defenses against bacterial challenge.

METHODS

Mice were treated with an aerosolized lysate of ultraviolet-killed nontypeable (unencapsulated) Haemophilus influenzae (NTHi), then challenged with a lethal dose of live Streptococcus pneumoniae (Spn) delivered by aerosol.

MEASUREMENTS AND MAIN RESULTS

Treatment with the NTHi lysate induced complete protection against challenge with a lethal dose of Spn if treatment preceded challenge by 4 to 24 hours. Lesser levels of protection occurred at shorter (83% at 2 h) and longer (83% at 48-72 h) intervals between treatment and challenge. There was also some protection when treatment was given 2 hours after challenge (survival increased from 14 to 57%), but not 24 hours after challenge. Protection did not depend on recruited neutrophils or resident mast cells and alveolar macrophages. Protection was specific to the airway route of infection, correlated in magnitude and time with rapid bacterial killing within the lungs, and was associated with increases of multiple antimicrobial polypeptides in lung lining fluid.

CONCLUSIONS

We infer that protection derives from stimulation of local innate immune mechanisms, and that activated lung epithelium is the most likely cellular effector of this response. Augmentation of innate antimicrobial defenses of the lungs might have therapeutic value.

Acute lung injury and cell death: how many ways can cells die?

Apoptosis has been considered as an underlying mechanism in acute lung injury/acute respiratory distress syndrome and multiorgan dysfunction syndrome. Recently, several alternative pathways for cell death (such as caspase-independent cell death, oncosis, and autophagy) have been discovered. Evidence of these pathways in the pathogenesis of acute lung injury has also come into light. In this article, we briefly introduce cell death pathways and then focus on studies related to lung injury. The different types of cell death that occur and the underlying mechanisms utilized depend on both experimental and clinical conditions. Lipopolysaccharide-induced acute lung injury is associated with apoptosis via Fas/Fas ligand mechanisms. Hyperoxia and ischemia-reperfusion injury generate reactive oxidative species, which induce complex cell death patterns composed of apoptosis, oncosis, and necrosis. Prolonged overexpression of inflammatory mediators results in increased production and activation of proteases, especially cathepsins. Activation and resistance to death of neutrophils also plays an important role in promoting parenchymal cell death. Knowledge of the coexisting multiple cell death pathways and awareness of the pharmacological inhibitors targeting different proteases critical to cell death may lead to the development of novel therapies for acute lung injury.

Sensitivity to Escherichia coli Nissle 1917 in mice is dependent on environment and genetic background

Escherichia coli Nissle 1917 (EcN) is a well-characterized probiotic bacterium. Although genomic comparisons of EcN with the uropathogenic E. coli strain CFT073 revealed high degrees of similarity, EcN is generally considered a non-pathogenic organism. However, as recent evidence suggests that EcN is capable of inducing inflammatory responses in host intestinal epithelial cells, we aimed to investigate potential pathogenic properties of EcN in an in vivo model using various germ-free (GF) mouse strains. With the exception of C3H/HeJZtm mice, which carry a defective toll-like receptor (TLR)4-allele, no lesions were obvious in mice of different strains orally inoculated with EcN for 1 week, although organ cultures (blood, lung, mesenteric lymph node, pancreas, spleen, liver and kidney) tested positive to various degrees. C3H/HeJZtm mice inoculated with EcN became clinically ill and the majority died or had to be euthanized. Organs of all gnotobiotic C3H/HeJZtm mice were positive for EcN by culture; major histological findings were moderate to severe pyogranulomatous serositis, typhlitis and pancreatitis. Histological findings were corroborated by highly elevated tumour necrosis factor (TNF) serum levels. Lesions were not detected in specified pathogen free maintained C3H/HeJZtm mice, GF C3H/HeJ mice lacking the interleukin-10 gene, or GF C3H/HeJZtm mice that were inoculated with E. coli K12 strain MG1655 as a control. In addition, mild histological lesions were detected in Ztm:NMRI mice 3 months after oral inoculation with EcN. This study shows that EcN is capable of displaying a virulent phenotype in GF C3H/HeJZtm mice. Whether this phenotype is linked to the bacterium's probiotic nature should be the focus of further studies.

Reducing the toll of inflammatory lung disease

Toll-like receptors (TLRs) are pivotal in human response to microbial stimuli. Their activation and signaling underpin much of the observed epidemiologic data generated by the hygiene hypothesis, and their contribution to infectious exacerbations of airways disease is likely to be highly important. Our growing knowledge in this field will have a significant impact on the understanding of the pathogenesis of inflammatory diseases, and TLR-based therapies are already in early clinical trials to modify atopic disease severity.

Reduced toll-like receptor 4 and substance P gene expression is associated with airway bacterial colonization in children

Neuro-immune interactions are increasingly relevant to human health and disease. The neuropeptide Substance P also has antibacterial activity and bears similarities to the innate immune antibacterial defensins. This suggests possible co-regulation of neuropeptide and innate immune mediators. In this study, non-bronchoscopic bronchoalveolar lavage (BAL) was performed on 69 children. BAL was examined for cellular profile, microbiology (bacteria, virus) and gene expression for TLRs 2, 3, 4; chemokine receptors (CCR3, CCR5, CXCR1); neurotrophins and neurokinin genes (TAC1, TAC3, CGRP, NGF). In children with bacterial colonization (n=10) there was an airway inflammatory response with increased BAL neutrophils, IL-8 protein, and CXCR1 expression. Substance P (TAC1) and TLR4 RNA expression were reduced in children with bacterial colonization. TLR3 mRNA was increased in 7.2% (n=5) children with rhinovirus, and there was a non-significant trend to increased TLR2. There is evidence for co-regulation of neurokinin (TAC1) and TLR4 gene expression in airway cells from children with airway bacterial colonization and their reduced expression may be associated with an impaired bacterial clearance.

The Duffy antigen modifies systemic and local tissue chemokine responses following lipopolysaccharide stimulation

The Duffy blood group Ag (dfy) binds selective CXC and CC chemokines at high affinity and is expressed on erythrocytes and endothelial cells. However, it does not transmit a signal via G proteins, as occurs with other seven-transmembrane receptors. We hypothesized that dfy functions as a chemokine reservoir and regulates inflammation by altering soluble chemokine concentrations in the blood and tissue compartments. We determined whether Duffy Ag "loss-of-function" phenotypes (human and murine) are associated with alterations in plasma chemokine concentrations during the innate inflammatory response to LPS. Plasma CXCL8 and CCL2 concentrations from humans homozygous for the GATA-1 box polymorphism, a dfy polymorphism that abrogates erythrocyte chemokine binding, were higher than in heterozygotes following LPS stimulation of their whole blood in vitro. Similarly, dfy(-/-) mice showed higher plasma MIP-2 concentrations than dfy(+/+) mice following LPS stimulation of whole blood in vitro. We then determined the relative contributions of erythrocyte and endothelial Duffy Ag in modifying chemokine concentrations and neutrophil recruitment in the lungs following intratracheal LPS administration in dfy(-/-) and dfy(+/+) mice reconstituted with dfy(-/-) or dfy(+/+) marrow. Mice lacking endothelial dfy expression had higher MIP-2 and keratinocyte chemoattractant concentrations in the airspaces. Mice lacking erythrocyte dfy had higher MIP-2 and keratinocyte chemoattractant concentrations in the lung tissue vascular space, but lower plasma chemokine concentrations associated with attenuated neutrophil recruitment into the airspaces. These data indicate that dfy alters soluble chemokine concentrations in blood and local tissue compartments and enhances systemic bioavailability of chemokines produced during local tissue inflammation.

IL-23-dependent IL-17 production is essential in neutrophil recruitment and activity in mouse lung defense against respiratory Mycoplasma pneumoniae infection

IL-23 induces IL-17 production in activated CD4+ T cells and participates in host defense against many encapsulated bacteria. However, whether the IL-23/IL-17 axis contributes to a Mycoplasma pneumoniae (Mp)-induced lung inflammation (e.g., neutrophils) has not been addressed. Using an acute respiratory Mp infection murine model, we found significantly up-regulated lung IL-23p19 mRNA in the early phase of infection (4h), and alveolar macrophages were an important cell source of Mp-induced IL-23. We further showed that Mp significantly increased IL-17 protein levels in bronchoalveolar lavage (BAL). Lung gene expression of IL-17, IL-17C and IL-17F was also markedly up-regulated by Mp in vivo. IL-17 and IL-17F were found to be derived mainly from lung CD4+ T cells, and were increased upon IL-23 stimulation in vitro. In vivo blocking of IL-23p19 alone or in combination with IL-23/IL-12p40 resulted in a significant reduction of Mp-induced IL-17 protein and IL-17/IL-17F mRNA expression, which was accompanied by a trend toward reduced lung neutrophil recruitment, BAL neutrophil activity, and Mp clearance. However, IL-23 neutralization had no effect on Mp-induced lung IL-17C mRNA expression. These results demonstrate that IL-17/IL-17F production is IL-23-dependent in an acute Mp infection, and contributes to neutrophil recruitment and activity in the lung defense against the infection.

Contribution of Toll-like receptors to the innate immune response to Gram-negative and Gram-positive bacteria

Innate recognition of bacteria is a key step in the activation of inflammation and coagulation, and it is dependent on pathogen-associated molecular pattern (PAMP) ligation to Toll-like receptors (TLRs) and CD14. The dominant receptors activated when cells encounter a whole bacterium, which express several PAMPs, are poorly defined. Herein, we have stimulated various human cells with prototypic Gram-negative and Gram-positive bacteria. Receptor-dependent responses to whole bacteria were assessed using both TLR-transfected cells and specific monoclonal antibodies against TLRs, MD-2, and CD14. Enterobacteria-activated leukocytes and endothelial cells in a TLR4/MD-2-dependent manner, most likely via lipopolysaccharide (LPS). TLR2 activation was observed with a high bacterial inoculum, and in epithelial cells expressing TLR2 but not TLR4. Pseudomonas aeruginosa stimulated cells by both TLR2 and TLR4/MD-2. Gram-positive bacteria activated cells only at high concentrations, in a partially TLR2-dependent but TLR4/MD-2-independent manner. Either TLR or CD14 neutralization blocked activation to all bacterial strains tested with the exception of some Gram-positive strains in whole blood in which partial inhibition was noted. This study identifies dominant TLRs involved in responses to whole bacteria. It also validates the concept that host cell activation by bacterial pathogens can be therapeutically reduced by anti-TLR4, -TLR2, and -CD14 mAbs.