Bile acid metabolites enhance expression of cathelicidin antimicrobial peptide in airway epithelium through activation of the TGR5-ERK1/2 pathway.

Signals for the maintenance of epithelial homeostasis are provided in part by commensal bacteria metabolites, that promote tissue homeostasis in the gut and remote organs as microbiota metabolites enter the bloodstream. In our study, we investigated the effects of bile acid metabolites, 3-oxolithocholic acid (3-oxoLCA), alloisolithocholic acid (AILCA) and isolithocholic acid (ILCA) produced from lithocholic acid (LCA) by microbiota, on the regulation of innate immune responses connected to the expression of host defense peptide cathelicidin in lung epithelial cells. The bile acid metabolites enhanced expression of cathelicidin at low concentrations in human bronchial epithelial cell line BCi-NS1.1 and primary bronchial/tracheal cells (HBEpC), indicating physiological relevance for modulation of innate immunity in airway epithelium by bile acid metabolites. Our study concentrated on deciphering signaling pathways regulating expression of human cathelicidin, revealing that LCA and 3-oxoLCA activate the surface G protein-coupled bile acid receptor 1 (TGR5, Takeda-G-protein-receptor-5)-extracellular signal-regulated kinase (ERK1/2) cascade, rather than the nuclear receptors, aryl hydrocarbon receptor, farnesoid X receptor and vitamin D3 receptor in bronchial epithelium. Overall, our study provides new insights into the modulation of innate immune responses by microbiota bile acid metabolites in the gut-lung axis, highlighting the differences in epithelial responses between different tissues.

Modulation of innate immunity in airway epithelium for host-directed therapy.

Innate immunity of the mucosal surfaces provides the first-line defense from invading pathogens and pollutants conferring protection from the external environment. Innate immune system of the airway epithelium consists of several components including the mucus layer, mucociliary clearance of beating cilia, production of host defense peptides, epithelial barrier integrity provided by tight and adherens junctions, pathogen recognition receptors, receptors for chemokines and cytokines, production of reactive oxygen species, and autophagy. Therefore, multiple components interplay with each other for efficient protection from pathogens that still can subvert host innate immune defenses. Hence, the modulation of innate immune responses with different inducers to boost host endogenous front-line defenses in the lung epithelium to fend off pathogens and to enhance epithelial innate immune responses in the immunocompromised individuals is of interest for host-directed therapy. Herein, we reviewed possibilities of modulation innate immune responses in the airway epithelium for host-directed therapy presenting an alternative approach to standard antibiotics.

Aroylated phenylenediamine HO53 modulates innate immunity, histone acetylation and metabolism.

In the current context of antibiotic resistance, the need to find alternative treatment strategies is urgent. Our research aimed to use synthetized aroylated phenylenediamines (APDs) to induce the expression of cathelicidin antimicrobial peptide gene (CAMP) to minimize the necessity of antibiotic use during infection. One of these compounds, HO53, showed promising results in inducing CAMP expression in bronchial epithelium cells (BCi-NS1.1 hereafter BCi). Thus, to decipher the cellular effects of HO53 on BCi cells, we performed RNA sequencing (RNAseq) analysis after 4, 8 and 24 h treatment of HO53. The number of differentially expressed transcripts pointed out an epigenetic modulation. Yet, the chemical structure and in silico modeling indicated HO53 as a histone deacetylase (HDAC) inhibitor. When exposed to a histone acetyl transferase (HAT) inhibitor, BCi cells showed a decreased expression of CAMP. Inversely, when treated with a specific HDAC3 inhibitor (RGFP996), BCi cells showed an increased expression of CAMP, indicating acetylation status in cells as determinant for the induction of the expression of the gene CAMP expression. Interestingly, a combination treatment with both HO53 and HDAC3 inhibitor RGFP966 leads to a further increase of CAMP expression. Moreover, HDAC3 inhibition by RGFP966 leads to increased expression of STAT3 and HIF1A, both previously demonstrated to be involved in pathways regulating CAMP expression. Importantly, HIF1α is considered as a master regulator in metabolism. A significant number of genes of metabolic enzymes were detected in our RNAseq data with enhanced expression conveying a shift toward enhanced glycolysis. Overall, we are demonstrating that HO53 might have a translational value against infections in the future through a mechanism leading to innate immunity strengthening involving HDAC inhibition and shifting the cells towards an immunometabolism, which further favors innate immunity activation.

Host-Directed Therapy as a Novel Treatment Strategy to Overcome Tuberculosis: Targeting Immune Modulation.

Tuberculosis (TB) is one of the leading causes of mortality and morbidity, particularly in developing countries, presenting a major threat to the public health. The currently recommended long term treatment regimen with multiple antibiotics is associated with poor patient compliance, which in turn, may contribute to the emergence of multi-drug resistant TB (MDR-TB). The low global treatment efficacy of MDR-TB has highlighted the necessity to develop novel treatment options. Host-directed therapy (HDT) together with current standard anti-TB treatments, has gained considerable interest, as HDT targets novel host immune mechanisms. These immune mechanisms would otherwise bypass the antibiotic bactericidal targets to kill Mycobacterium tuberculosis (Mtb), which may be mutated to cause antibiotic resistance. Additionally, host-directed therapies against TB have been shown to be associated with reduced lung pathology and improved disease outcome, most likely via the modulation of host immune responses. This review will provide an update of host-directed therapies and their mechanism(s) of action against Mycobacterium tuberculosis.

Cyclic mechanical stretch down-regulates cathelicidin antimicrobial peptide expression and activates a pro-inflammatory response in human bronchial epithelial cells.

Mechanical ventilation (MV) of patients can cause damage to bronchoalveolar epithelium, leading to a sterile inflammatory response, infection and in severe cases sepsis. Limited knowledge is available on the effects of MV on the innate immune defense system in the human lung. In this study, we demonstrate that cyclic stretch of the human bronchial epithelial cell lines VA10 and BCi NS 1.1 leads to down-regulation of cathelicidin antimicrobial peptide (CAMP) gene expression. We show that treatment of VA10 cells with vitamin D3 and/or 4-phenyl butyric acid counteracted cyclic stretch mediated down-regulation of CAMP mRNA and protein expression (LL-37). Further, we observed an increase in pro-inflammatory responses in the VA10 cell line subjected to cyclic stretch. The mRNA expression of the genes encoding pro-inflammatory cytokines IL-8 and IL-1ß was increased after cyclic stretching, where as a decrease in gene expression of chemokines IP-10 and RANTES was observed. Cyclic stretch enhanced oxidative stress in the VA10 cells. The mRNA expression of toll-like receptor (TLR) 3, TLR5 and TLR8 was reduced, while the gene expression of TLR2 was increased in VA10 cells after cyclic stretch. In conclusion, our in vitro results indicate that cyclic stretch may differentially modulate innate immunity by down-regulation of antimicrobial peptide expression and increase in pro-inflammatory responses.

Conditions associated with the cystic fibrosis defect promote chronic Pseudomonas aeruginosa infection.

RATIONALE: Progress has been made in understanding how the cystic fibrosis (CF) basic defect produces lung infection susceptibility. However, it remains unclear why CF exclusively leads to chronic infections that are noninvasive and highly resistant to eradication. Although biofilm formation has been suggested as a mechanism, recent work raises questions about the role of biofilms in CF. OBJECTIVES: To learn how airway conditions attributed to CF transmembrane regulator dysfunction could lead to chronic infection, and to determine if biofilm-inhibiting genetic adaptations that are common in CF isolates affect the capacity of Pseudomonas aeruginosa to develop chronic infection phenotypes. METHODS: We studied P. aeruginosa isolates grown in agar and mucus gels containing sputum from patients with CF and measured their susceptibility to killing by antibiotics and host defenses. We also measured the invasive virulence of P. aeruginosa grown in sputum gels using airway epithelial cells and a murine infection model. MEASUREMENTS AND MAIN RESULTS: We found that conditions likely to result from increased mucus density, hyperinflammation, and defective bacterial killing could all cause P. aeruginosa to grow in bacterial aggregates. Aggregated growth markedly increased the resistance of bacteria to killing by host defenses and antibiotics, and reduced their invasiveness. In addition, we found that biofilm-inhibiting mutations do not impede aggregate formation in gel growth environments. CONCLUSIONS: Our findings suggest that conditions associated with several CF pathogenesis hypotheses could cause the noninvasive and resistant infection phenotype, independently of the bacterial functions needed for biofilm formation.

Battle and balance at mucosal surfaces--the story of Shigella and antimicrobial peptides.

Shigella is a major cause of morbidity and mortality for children in many developing countries. Emergence of antibiotic-resistance among Shigella demands the development of effective medicines. Antimicrobial peptides (AMPs) are expressed in phagocytes and at epithelial surfaces and are important effector molecules of innate immunity. We have found that pathogens are able to turn off the endogenous expression of AMPs, resulting in serious infections such as shigellosis. A therapeutic rationale to prevent microbial invasion would be to strengthen the epithelial line of defence through enhancing AMP expression. We have identified several inducers of AMP-production, including butyrate, phenylbutyrate and vitamin D, which have been investigated in animal models of shigellosis as well as in clinical trials. We believe that the conceptual framework presented here can be applied to additional clinical entities and that this novel approach can be an alternative or complement to traditional antibiotics in the future.

Azithromycin maintains airway epithelial integrity during Pseudomonas aeruginosa infection.

Tight junctions (TJs) play a key role in maintaining bronchial epithelial integrity, including apical-basolateral polarity and paracellular trafficking. Patients with chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF) often suffer from chronic infections by the opportunistic Gram-negative bacterium Pseudomonas aeruginosa, which produces multiple virulence factors, including rhamnolipids. The macrolide antibiotic azithromycin (azm) has been shown to improve lung function in patients with CF without reducing the bacterial count within the lung. However, the mechanism of this effect is still debated. It has previously been shown that azm increased transepithelial electrical resistance (TER) in a bronchial epithelial cell line. In this study we used an air-liquid interface model of human airway epithelia and measured TER, changes in TJ expression and architecture after exposure to live P. aeruginosa PAO1, and PAO1-Deltarhl which is a PAO1 mutant lacking rhlA and rhlB, which encode key enzymes for rhamnolipid production. In addition, the cells were challenged with bacterial culture medium conditioned by these strains, purified rhamnolipids, or synthetic 3O-C(12)-HSL. Virulence factors secreted by P. aeruginosa reduced TER and caused TJ rearrangement in the bronchial epithelium, exposing the epithelium to further bacterial infiltration. Pretreatment of the bronchial epithelium with azm attenuated this effect and facilitated epithelial recovery. These data suggest that azm protects the bronchial epithelium during P. aeruginosa infection independent of antimicrobial activity, and could explain in part the beneficial results seen in clinical trials of patients with CF.

Bacterial DNA indicated as an important inducer of fish cathelicidins.

Cathelicidins are antimicrobial peptides indicated as important in the control of the natural microflora as well as in the fight against bacterial invasion in mammals. Little is known about cathelicidins in fish and here the Chinook salmon (Oncorhynchus tshawytscha) embryo cell line (CHSE-214) was used as a model system to study the expression of cathelicidins due to fish pathogenic bacteria. The cDNA of cathelicidin from CHSE-214 cells (csCath) was cloned and shown to be closely related to gene 2 of both rainbow trout and Atlantic salmon. The deducted amino acid sequence showed highest sequence identity to rtCath2 with 95% and 72% for the cathelin and the antibacterial part, respectively. Cathelicidin gene expression was studied and various Gram positive and Gram negative bacteria caused the upregulation of the gene (csCath). Bacterial DNA and protein were shown important for the induction of cathelicidin expression in these cells. LPS (Escherichia coli) also causes the upregulation of cathelicidins, but digestion of the LPS with DNase I before incubation of the cells, totally abolished the upregulation of cathelicidin and suggests DNA contamination in the LPS to be the trigger for this effect. These results could explain the limited responsiveness of fish cells towards pure LPS and confirm previous suggestions that fish cells are less sensitive to LPS than mammalian cells.

Differentiation potential of a basal epithelial cell line established from human bronchial explant.

Due to the cellular complexity of the airway epithelium, it is important to carefully define bronchial cell lines that capture the phenotypic traits of a particular cell type. We describe the characterization of a human bronchial epithelial cell line, VA10. It was established by transfection of primary bronchial epithelial cells with retroviral constructs containing the E6 and E7 oncogenes from HPV16. The cell line has been cultured for over 2 yr, a total of 60 passages. Although prolonged culture resulted in increased chromosomal instability, no major phenotypic drift in marker expression was observed. The cells expressed cytokeratins 5, 13, 14, and 17 suggesting a basal-like phenotype. This was further supported by the expression of alpha6beta4 integrins and the basal cell-associated transcription factor p63. The VA10 cell line generated high transepithelial electrical resistance in suspended and air-liquid interface culture, indicating functionally active tight junction (TJ) complexes. Immunocytochemistry showed the typical reticular structures of occludin and TJ-associated F-actin. VA10 produced pseudostratified layer in air-liquid interface culture with expression of p63 restricted to the basal layer. Furthermore, VA10 produced round colonies when cultured in laminin-rich reconstituted basement membrane, and immunostaining of claudin-1 and the basolateral marker beta4 integrin revealed colonies that generated polarization as expected in vivo. These data indicate that VA10 epithelia have the potential to model the bronchial epithelium in vivo and may be useful to study epithelial regeneration and repair and the effect of chemicals and potential drug candidates on TJ molecules in airway epithelia.

Novel effects of azithromycin on tight junction proteins in human airway epithelia.

The macrolide antibiotic azithromycin improves lung function and prognosis among patients with cystic fibrosis or diffuse panbronchiolitis, independently of bacterial eradication. Anti-inflammatory effects have been implicated, but data from in vivo studies are scarce, and the link between abnormal electrolyte content in airway surface liquid and bronchial infections remains uncertain. In the present study, we treated human airway epithelia on filter supports with azithromycin and monitored transepithelial electrical resistance. We found that azithromycin increased transepithelial electrical resistance of airway epithelia in a dose-dependent manner. Immunocytochemistry and Western blotting showed that addition of azithromycin changed the locations of proteins in cell cultures and induced processing of the tight junction proteins claudin-1 and claudin-4, occludin, and junctional adhesion molecule-A. These effects were reversible, and no effect was seen when cells were treated with penicillin or erythromycin. The data indicate that azithromycin increases the transepithelial electrical resistance of human airway epithelia by changing the processing of tight junction proteins. The results are novel and may help explain the beneficial effects of azithromycin in patients with cystic fibrosis, diffuse panbronchiolitis, and community-acquired pneumonia.

Isolation and identification of antimicrobial components from the epidermal mucus of Atlantic cod (Gadus morhua).

The epidermal mucus of fish species has been found to contain antimicrobial proteins and peptides, which is of interest in regard to fish immunity. An acidic extract from the epidermal mucus of the Atlantic cod (Gadus morhua) was found to exhibit antimicrobial activity against Bacillus megaterium, Escherichia coli and Candida albicans. This activity varied significantly when salt was added to the antimicrobial assay, and was eliminated by pepsin digestion. No lysozyme activity was detected in the extract. By using weak cationic exchange chromatography together with reversed-phase chromatography, and monitoring the antimicrobial activity, we have isolated four cationic proteins from the mucus extract. Using N-terminal and C-terminal amino acid sequence analysis, together with MS, the antimicrobial proteins were identified as histone H2B (13 565 Da), ribosomal protein L40 (6397 Da), ribosomal protein L36A (12 340 Da) and ribosomal protein L35 (14 215 Da). The broad spectra of antimicrobial activities in the cod mucus and the characterization of four antimicrobial polypeptides suggest that mucus compounds contribute to the innate host defence of cod.

Antimicrobial peptides in the first line defence of human colon mucosa.

Antimicrobial peptides and proteins are effector molecules in the protection of epithelial surfaces. We have evaluated the presence of antimicrobial peptides/proteins that can participate in human colonic defence against microbes. A peptide/protein extract of normal human colon mucosa was found to be active against Gram-positive bacteria, Gram-negative bacteria, and fungi. Four polypeptides with antimicrobial activity were isolated from this material and they were identified by N-terminal amino acid sequence analysis as ubiquicidin, histone H2B, eosinophil cationic protein, and phospholipase A(2) (PLA(2)). Using immunodetection and mass spectrometry, LL-37, HNP1-3, and HBD-1 were also identified. Combined, these results indicate that the colon mucosa is protected by a complex mixture of polypeptides, able to kill invading microbes and working in synergy as a barrier against bacterial invasion.