Infectious pneumonias exact an unacceptable mortality burden worldwide. Efforts to protect populations from pneumonia have historically focused on antibiotic development and vaccine-enhanced adaptive immunity. However, we have recently reported that the lungs’ innate defenses can be therapeutically induced by inhalation of a bacterial lysate that protects mice against otherwise lethal pneumonia. Here, we tested in mice the hypothesis that Toll-like receptors (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. While 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.
Secretory epithelial cells of the proximal airways synthesize and secrete gel-forming polymeric mucins. The secreted mucins adsorb water to form mucus that is propelled by neighboring ciliated cells, providing a mobile barrier which removes inhaled particles and pathogens from the lungs. Several features of the intracellular trafficking of mucins make the airway secretory cell an interesting comparator for the cell biology of regulated exocytosis. Polymeric mucins are exceedingly large molecules (up to 3 × 106 Da per monomer) whose folding and initial polymerization in the ER requires the protein disulfide isomerase Agr2. In the Golgi, mucins further polymerize to form chains and possibly branched networks comprising more than 20 monomers. The large size of mucin polymers imposes constraints on their packaging into transport vesicles along the secretory pathway. Sugar side chains account for >70% of the mass of mucins, and their attachment to the protein core by O-glycosylation occurs in the Golgi. Mature polymeric mucins are stored in large secretory granules ∼1 μm in diameter. These are translocated to the apical membrane to be positioned for exocytosis by cooperative interactions among myristoylated alanine-rich C kinase substrate, cysteine string protein, heat shock protein 70, and the cytoskeleton. Mucin granules undergo exocytic fusion with the plasma membrane at a low basal rate and a high stimulated rate. Both rates are mediated by a regulated exocytic mechanism as indicated by phenotypes in both basal and stimulated secretion in mice lacking Munc13-2, a sensor of the second messengers calcium and diacylglycerol (DAG). Basal secretion is induced by low levels of activation of P2Y2 purinergic and A3 adenosine receptors by extracellular ATP released in paracrine fashion and its metabolite adenosine. Stimulated secretion is induced by high levels of the same ligands, and possibly by inflammatory mediators as well. Activated receptors are coupled to phospholipase C by Gq, resulting in the generation of DAG and of IP3 that releases calcium from apical ER. Stimulated secretion requires activation of the low affinity calcium sensor Synaptotagmin-2, while a corresponding high affinity calcium sensor in basal secretion is not known. The core exocytic machinery is comprised of the SNARE proteins VAMP8, SNAP23, and an unknown Syntaxin protein, together with the scaffolding protein Munc18b. Common and distinct features of this exocytic system in comparison to neuroendocrine cells and neurons are highlighted.
secretion; exocytosis; mucin; mucus; MARCKS; Munc18; Munc13; synaptotagmin
Airway mucin secretion and MC (mast cell) degranulation must be tightly controlled for homoeostasis of the lungs and immune system respectively. We found the exocytic protein Munc18b to be highly expressed in mouse airway epithelial cells and MCs, and localized to the apical pole of airway secretory cells. To address its functions, we created a mouse with a severely hypomorphic Munc18b allele such that protein expression in heterozygotes was reduced by ~50%. Homozygous mutant mice were not viable, but heterozygotes showed a ~50% reduction in stimulated release of mucin from epithelial cells and granule contents from MCs. The defect in MCs affected only regulated secretion and not constitutive or transporter-mediated secretion. The severity of passive cutaneous anaphylaxis was also reduced by ~50%, showing that reduction of Munc18b expression results in an attenuation of physiological responses dependent on MC degranulation. The Munc18b promoter is controlled by INR (initiator), Sp1 (specificity protein 1), Ets, CRE (cAMP-response element), GRE (glucocorticoid-response element), GATA and E-box elements in airway epithelial cells; however, protein levels did not change during mucous metaplasia induced by allergic inflammation. Taken together, the results of the present study identify Munc18b as an essential gene that is a limiting component of the exocytic machinery of epithelial cells and MCs.
exocytosis; mast cell; mucin; mucus; Munc18; secretion; AB-PAS, Alcian Blue/periodic acid/Schiff reagent; bHLH, basic helix–loop–helix; CCSP, Clara cell secretory protein; Clca3, chloride channel, calcium-activated, family member 3; CRE, cAMP-response element; DNP, 2,4-dinitrophenol; FBS, fetal bovine serum; FcϵRIα, high-affinity IgE receptor, α subunit; FRT, flippase recognition target; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GRE, glucocorticoid-response element; HA, haemagglutinin; HSA, human serum albumin; HRP, horseradish peroxidase; IL-3, interleukin-3; INR, initiator; ISH, in situ hybridization; MC, mast cell; mBMMC, mouse bone-marrow-derived MC; mClca3, mouse Clca3; MFI, mean fluorescent intensity; mtCC, mouse transformed Clara cell; NK, natural killer; OCT, optimal cutting temperature compound; PAFS, periodic acid/fluorescent Schiff reagent; PBST, PBS containing 0.05% Tween 20; PGD2, prostaglandin D2; PGK, phosphoglucokinase; SCF, stem cell factor; SM, Sec1/Munc18; SNAP, soluble N-ethylmaleimide-sensitive factor-attachment protein; SNARE, SNAP receptor; Stxbp2, syntaxin-binding protein 2; TK, thymidine kinase; TNFα, tumour necrosis factor α; WT, wild-type; YFP, yellow fluorescent protein
Lower respiratory tract infections caused by influenza A continue to exact unacceptable worldwide mortality, and recent epidemics have emphasized the importance of preventative and containment strategies. We have previously reported that induction of the lungs' intrinsic defenses by aerosolized treatments can protect mice against otherwise lethal challenges with influenza A virus. More recently, we identified a combination of Toll like receptor (TLR) agonists that can be aerosolized to protect mice against bacterial pneumonia. Here, we tested whether this combination of synthetic TLR agonists could enhance the survival of mice infected with influenza A/HK/8/68 (H3N2) or A/California/04/2009 (H1N1) influenza A viruses. We report that the TLR treatment enhanced survival whether given before or after the infectious challenge, and that protection tended to correlate with reductions in viral titer 4 d after infection. Surprisingly, protection was not associated with induction of interferon gene expression. Together, these studies suggest that synergistic TLR interactions can protect against influenza virus infections by mechanisms that may provide the basis for novel therapeutics.
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-κB, 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.
innate immunity; pneumonia; immunocompromised host; lung epithelium
Recent studies have demonstrated that K-ras mutations in lung epithelial cells elicit inflammation that promotes carcinogenesis in mice (intrinsic inflammation). The finding that patients with chronic obstructive pulmonary disease (COPD), an inflammatory disease of the lung, have an increased risk of lung cancer after controlling for smoking suggests a further link between lung cancer and extrinsic inflammation. Besides exposure to cigarette smoke, it is thought that airway inflammation in COPD is caused by bacterial colonization, particularly with non-typeable Hemophilus influenzae (NTHi). Previously, we have shown that NTHi-induced COPD-like airway inflammation promotes lung cancer in an airway conditional K-ras-induced mouse model. To further test the role of inflammation in cancer promotion, we administered the natural anti-inflammatory agent, curcumin, 1% in diet before and during weekly NTHi exposure. This significantly reduced the number of visible lung tumors in the absence of NTHi exposure by 85% and in the presence of NTHi exposures by 53%. Mechanistically, curcumin markedly suppressed NTHi-induced increased levels of the neutrophil chemoattractant keratinocyte-derived chemokine by 80% and neutrophils by 87% in bronchoalveolar lavage fluid. In vitro studies of murine K-ras-induced lung adenocarcinoma cell lines (LKR-10 and LKR-13) indicated direct anti-tumoral effects of curcumin by reducing cell viability, colony formation and inducing apoptosis. We conclude that curcumin suppresses the progression of K-ras-induced lung cancer in mice by inhibiting intrinsic and extrinsic inflammation and by direct anti-tumoral effects. These findings suggest that curcumin could be used to protract the premalignant phase and inhibit lung cancer progression in high-risk COPD patients.
Lower respiratory tract infections continue to exact unacceptable worldwide mortality, often because the infecting pathogen cannot be identified. The respiratory epithelia provide protection from pneumonias through organism-specific generation of antimicrobial products, offering potential insight into the identity of infecting pathogens. This study assesses the capacity of the host gene expression response to infection to predict the presence and identity of lower respiratory pathogens without reliance on culture data.
Mice were inhalationally challenged with S. pneumoniae, P. aeruginosa, A. fumigatus or saline prior to whole genome gene expression microarray analysis of their pulmonary parenchyma. Characteristic gene expression patterns for each condition were identified, allowing the derivation of prediction rules for each pathogen. After confirming the predictive capacity of gene expression data in blinded challenges, a computerized algorithm was devised to predict the infectious conditions of subsequent subjects.
We observed robust, pathogen-specific gene expression patterns as early as 2 h after infection. Use of an algorithmic decision tree revealed 94.4% diagnostic accuracy when discerning the presence of bacterial infection. The model subsequently differentiated between bacterial pathogens with 71.4% accuracy and between non-bacterial conditions with 70.0% accuracy, both far exceeding the expected diagnostic yield of standard culture-based bronchoscopy with bronchoalveolar lavage.
These data substantiate the specificity of the pulmonary innate immune response and support the feasibility of a gene expression-based clinical tool for pneumonia diagnosis.
Purpose of review
Airway mucus plugging has long been recognized as a principal cause of death in asthma. However, molecular mechanisms of mucin overproduction and secretion have not been understood until recently. These mechanisms are reviewed together with ongoing investigations relating them to lung pathophysiology.
Of the five secreted gel-forming mucins in mammals, only MUC5AC and MUC5B are produced in significant quantities in intrapulmonary airways. MUC5B is the principal gel-forming mucin at baseline in small airways of humans and mice, and therefore likely performs most homeostatic clearance functions. MUC5AC is the principal gel-forming mucin upregulated in airway inflammation and is under negative control by forkhead box a2 and positive control by hypoxia inducible factor-1. Mucin secretion is regulated separately from production, principally by extracellular triphosphate nucleotides that bind P2Y2 receptors on the lumenal surface of airway secretory cells, generating intracellular second messengers that activate the exocytic proteins, Munc13-2 and synaptotagmin-2.
Markedly upregulated production of MUC5AC together with stimulated secretion leads to airflow obstruction in asthma. As MUC5B appears to mediate homeostatic functions, it may be possible to selectively inhibit MUC5AC production without impairing airway function. The precise roles of mucin hypersecretion in asthma symptoms such as dyspnea and cough and in physiologic phenomena such as airway hyperresponsiveness remain to be defined.
airway; asthma; mucin; mucous; mucus
Protective host responses to respiratory pathogens are typically characterized by inflammation. However, lung inflammation is not always protective and it may even become deleterious to the host. We have recently reported substantial protection against Streptococcus pneumoniae (pneumococcal) pneumonia by induction of a robust inflammatory innate immune response to an inhaled bacterial lysate. Conversely, the allergic inflammation associated with asthma has been proposed to promote susceptibility to pneumococcal disease. This study sought to determine whether preexisting allergic lung inflammation influences the progression of pneumococcal pneumonia or reduces the inducibilty of protective innate immunity against bacteria.
To compare the effect of different inflammatory and secretory stimuli on defense against pneumonia, intraperitoneally ovalbumin-sensitized mice were challenged with inhaled pneumococci following exposure to various inhaled combinations of ovalbumin, ATP, and/or a bacterial lysate. Thus, allergic inflammation, mucin degranulation and/or stimulated innate resistance were induced prior to the infectious challenge. Pathogen killing was evaluated by assessing bacterial CFUs of lung homogenates immediately after infection, the inflammatory response to the different conditions was evaluated by measurement of cell counts of bronchoalveolar lavage fluid 18 hours after challenge, and mouse survival was assessed after seven days.
We found no differences in survival of mice with and without allergic inflammation, nor did the induction of mucin degranulation alter survival. As we have found previously, mice treated with the bacterial lysate demonstrated substantially increased survival at seven days, and this was not altered by the presence of allergic inflammation or mucin degranulation. Allergic inflammation was associated with predominantly eosinophilic infiltration, whereas the lysate-induced response was primarily neutrophilic. The presence of allergic inflammation did not significantly alter the neutrophilic response to the lysate, and did not affect the induced bacterial killing within the lungs.
These results suggest that allergic airway inflammation neither promotes nor inhibits progression of pneumococcal lung infection in mice, nor does it influence the successful induction of stimulated innate resistance to bacteria.
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.
innate immunity; pneumonia; immunocompromised host; lung epithelium
Nontypeable Haemophilus influenzae (NTHi) commonly colonizes the lower airways of patients with chronic obstructive pulmonary disease (COPD). Whether it contributes to COPD progression is unknown. Here, we determined which aspects of the COPD phenotype can be induced by repetitive exposure to NTHi products. Mice were exposed weekly to an aerosolized NTHi lysate, and inflammation was evaluated by measurement of cells and cytokines in bronchoalveolar lavage fluid (BALF) and immunohistochemical staining; structural changes were evaluated histochemically by periodic acid fluorescent Schiff's reagent, Masson's trichrome, and Picrosirius red staining; mucin gene expression was measured by quantitative RT-PCR; and the role of TNF-α was examined by transgenic airway overexpression and use of an inhibitory antibody. NTHi lysate induced rapid activation of NF-κB in airway cells and increases of inflammatory cytokines and neutrophils in BALF. Repetitive exposure induced infiltration of macrophages, CD8+ T cells, and B cells around airways and blood vessels, and collagen deposition in airway and alveolar walls, but airway mucin staining and gel-forming mucin transcripts were not increased. Transgenic overexpression of TNF-α caused BALF neutrophilia and inflammatory cell infiltration around airways, but not fibrosis, and TNF-α neutralization did not reduce BALF neutrophilia in response to NTHi lysate. In conclusion, NTHi products elicit airway inflammation in mice with a cellular and cytokine profile similar to that in COPD, and cause airway wall fibrosis but not mucous metaplasia. TNF-α is neither required for inflammatory cell recruitment nor sufficient for airway fibrosis. Colonization by NTHi may contribute to the pathogenesis of small airways disease in patients with COPD.
pulmonary disease, chronic obstructive; Haemophilus influenzae; bronchiolitis; inflammation; fibrosis
Single-dose administration of beta-adrenoceptor agonists produces bronchodilation and inhibits airway hyperresponsiveness (AHR), and is the standard treatment for the acute relief of asthma. However, chronic repetitive administration of beta-adrenoceptor agonists may increase AHR, airway inflammation, and risk of death. Based upon the paradigm shift that occurred with the use of beta-blockers in congestive heart failure, we previously determined that chronic administration of beta-blockers decreased AHR in a murine model of asthma. To elucidate the mechanisms for the beneficial effects of beta-blockers, we examined the effects of chronic administration of several beta-adrenoceptor ligands in a murine model of allergic asthma. Administration of beta-blockers resulted in a reduction in total cell counts, eosinophils, and the cytokines IL-13, IL-10, IL-5, and TGF-β1 in bronchoalveolar lavage, and attenuated epithelial mucin content and morphologic changes. The differences in mucin content also occurred if the beta-blockers were administered only during the ovalbumin challenge phase, but administration of beta-blockers for 7 days was not as effective as administration for 28 days. These results indicate that in a murine model of asthma, chronic administration of beta-blockers reduces inflammation and mucous metaplasia, cardinal features of asthma that may contribute to airflow obstruction and AHR. Similar to heart failure, our results provide a second disease model in which beta-blockers producing an acutely detrimental effect may provide a therapeutically beneficial effect with chronic administration.
beta-blockers; beta-adrenoceptor; asthma; mucin; airway inflammation
Inhalational anthrax is initiated by the entry of Bacillus anthracis spores into the lung. A critical early event in the establishment of an infection is the dissemination of spores from the lung. Using in vitro cell culture assays, we previously demonstrated that B. anthracis spores are capable of entering into epithelial cells of the lung and crossing a barrier of lung epithelial cells without apparent disruption of the barrier integrity, suggesting a novel portal for spores to disseminate from the lung. However, in vivo evidence for spore uptake by epithelial cells has been lacking. Here, using a mouse model, we present evidence that B. anthracis spores are taken up by lung epithelial cells in vivo soon after spores are delivered into the lung. Immunofluorescence staining of thin sections of lungs from spore-challenged BALB/c mice revealed that spores were associated with the epithelial surfaces in the airway and the alveoli at 2 and 4 h postinoculation. Confocal analysis further indicated that some of the associated spores were surrounded by F-actin, demonstrating intracellular localization. These observations were further confirmed and substantiated by a quantitative method that first isolated lung cells from spore-challenged mice and then stained these cells with antibodies specific for epithelial cells and spores. The results showed that substantial amounts of spores were taken up by lung epithelial cells in vivo. These data, combined with those in our previous reports, provided powerful evidence that the lung epithelia were directly targeted by B. anthracis spores at early stages of infection.
Influenza pneumonia causes high mortality every year, and pandemic episodes kill millions of people. Influenza-related mortality has been variously ascribed to an ineffective host response that fails to limit viral replication, an excessive host inflammatory response that results in lung injury and impairment of gas exchange, or to bacterial superinfection. We sought to determine whether lung inflammation promoted or impaired host survival in influenza pneumonia.
Methods and Findings
To distinguish among these possible causes of influenza-related death, we induced robust lung inflammation by exposing mice to an aerosolized bacterial lysate prior to challenge with live virus. The treatment induced expression of the inflammatory cytokines IL-6 and TNF in bronchoalveolar lavage fluid 8- and 40-fold greater, respectively, than that caused by lethal influenza infection. Yet, this augmented inflammation was associated with striking resistance to host mortality (0% vs 90% survival, p = 0.0001) and reduced viral titers (p = 0.004). Bacterial superinfection of virus infected lungs was not observed. When mice were repeatedly exposed to the bacterial lysate, as would be clinically desirable during an influenza epidemic, there was no tachyphylaxis of the induced viral resistance. When the bacterial lysate was administered after the viral challenge, there was still some mortality benefit, and when ribavirin was added to the aerosolized bacterial lysate, host survival was synergistically improved (0% vs 93.3% survival, p<0.0001).
Together, these data indicate that innate immune resistance to influenza can be effectively stimulated, and suggest that ineffective rather than excessive inflammation is the major cause of mortality in influenza pneumonia.
Mucus hypersecretion contributes to morbidity and mortality in many obstructive lung diseases. Gel-forming mucins are the chief glycoprotein components of airway mucus, and elevated expression of these during mucous metaplasia precedes the hypersecretory phenotype. Five orthologous genes (MUC2, MUC5AC, MUC5B, MUC6, and MUC19) encode the mammalian gel-forming mucin family, and several have been implicated in asthma, cystic fibrosis, and chronic obstructive pulmonary disease pathologies. However, in the absence of a comprehensive analysis, their relative contributions remain unclear. Here, we assess the expression of the entire gel-forming mucin gene family in allergic mouse airways and show that Muc5ac is the predominant gel-forming mucin induced. We previously showed that the induction of mucous metaplasia in ovalbumin-sensitized and -challenged mouse lungs occurs within bronchial Clara cells. The temporal induction and localization of Muc5ac transcripts correlate with the induced expression and localization of mucin glycoproteins in bronchial airways. To better understand the tight regulation of Muc5ac expression, we analyzed all available 5′-flanking sequences of mammalian MUC5AC orthologs and identified evolutionarily conserved regions within domains proximal to the mRNA coding region. Analysis of luciferase reporter gene activity in a mouse transformed Clara cell line demonstrates that this region possesses strong promoter activity and harbors multiple conserved transcription factor–binding motifs. In particular, SMAD4 and HIF-1α bind to the promoter, and mutation of their recognition motifs abolishes promoter function. In conclusion, Muc5ac expression is the central event in antigen-induced mucous metaplasia, and phylogenetically conserved 5′ noncoding domains control its regulation.
mucin; metaplasia; airway; lung; epithelium