Impaired mucus clearance exacerbates allergen-induced type 2 airway inflammation in juvenile mice

A scoping review: What are the cellular mechanisms that drive the allergic inflammatory response to fungal allergens in the lung epithelium?

Allergic airway disease (AAD) is a collective term for respiratory disorders that can be exacerbated upon exposure to airborne allergens. The contribution of fungal allergens to AAD has become well established over recent years. We conducted a comprehensive review of the literature using Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines to better understand the mechanisms involved in the allergic response to fungi in airway epithelia, identify knowledge gaps and make recommendations for future research. The search resulted in 61 studies for final analysis. Despite heterogeneity in the models and methods used, we identified major pathways involved in fungal allergy. These included the activation of protease-activated receptor 2, the EGFR pathway, adenosine triphosphate and purinergic receptor-dependent release of IL33, and oxidative stress, which drove mucin expression and goblet cell metaplasia, Th2 cytokine production, reduced barrier integrity, eosinophil recruitment, and airway hyperresponsiveness. However, there were several knowledge gaps and therefore we recommend future research should focus on the use of more physiologically relevant methods to directly compare key allergenic fungal species, clarify specific mechanisms of fungal allergy, and assess the fungal allergy in disease models. This will inform disease management and future interventions, ultimately reducing the burden of disease.

The Clinical Characteristics of Allergic Bronchopulmonary Mycosis Differ Among Pathogenic Fungi

Background

Allergic bronchopulmonary mycosis (ABPM) occurs with fungi, other than Aspergillus fumigatus. However, the clinical characteristics of ABPM caused by non-Aspergillus species are unspecified.

Methods

We retrospectively reviewed all patients with ABPM who visited to our hospital between April 2005 and December 2020. The causative fungi and clinical characteristics were analyzed. Patients were divided into the Aspergillus group and the non-Aspergillus group.

Results

Fourteen patients and five patients were included in the Aspergillus group and the non-Aspergillus group, respectively. Compared to the Aspergillus group, the non-Aspergillus group had a significantly low serum immunoglobulin E level and low forced vital capacity. In addition, the non-Aspergillus group had a lower rate of the requirement for oral corticosteroid treatment and a low frequency of recurrence.

Conclusion

Patients with non-Aspergillus ABPM had lower type 2 inflammation than did patients with allergic bronchopulmonary aspergillosis.

A novel thiol-saccharide mucolytic for the treatment of muco-obstructive lung diseases

BACKGROUND

Mucin disulfide cross-links mediate pathologic mucus formation in muco-obstructive lung diseases. MUC-031, a novel thiol-modified carbohydrate compound, cleaves disulfides to cause mucolysis. The aim of this study was to determine the mucolytic and therapeutic effects of MUC-031 in sputum from patients with cystic fibrosis (CF) and mice with muco-obstructive lung disease (βENaC-Tg mice).

METHODS

We compared the mucolytic efficacy of MUC-031 and existing mucolytics (N-acetylcysteine (NAC) and recombinant human deoxyribonuclease I (rhDNase)) using rheology to measure the elastic modulus (G') of CF sputum, and we tested effects of MUC-031 on airway mucus plugging, inflammation and survival in βENaC-Tg mice to determine its mucolytic efficacy in vivo.

RESULTS

In CF sputum, compared to the effects of rhDNase and NAC, MUC-031 caused a larger decrease in sputum G', was faster in decreasing sputum G' by 50% and caused mucolysis of a larger proportion of sputum samples within 15 min of drug addition. Compared to vehicle control, three treatments with MUC-031 in 1 day in adult βENaC-Tg mice decreased airway mucus content (16.8±3.2 versus 7.5±1.2 nL·mm-2, p<0.01) and bronchoalveolar lavage cells (73 833±6930 versus 47 679±7736 cells·mL-1, p<0.05). Twice-daily treatment with MUC-031 for 2 weeks also caused decreases in these outcomes in adult and neonatal βENaC-Tg mice and reduced mortality from 37% in vehicle-treated βENaC-Tg neonates to 21% in those treated with MUC-031 (p<0.05).

CONCLUSION

MUC-031 is a potent and fast-acting mucolytic that decreases airway mucus plugging, lessens airway inflammation and improves survival in βENaC-Tg mice. These data provide rationale for human trials of MUC-031 in muco-obstructive lung diseases.

The Pathology of Asthma: What Is Obstructing Our View?

Despite the advent of sophisticated and efficient new biologics to treat inflammation in asthma, the disease persists. Even following treatment, many patients still experience the well-known symptoms of wheezing, shortness of breath, and coughing. What are we missing? Here we examine the evidence that mucus plugs contribute to a substantial portion of disease, not only by physically obstructing the airways but also by perpetuating inflammation. In this way, mucus plugs may act as an immunogenic stimulus even in the absence of allergen or with the use of current therapeutics. The alterations of several parameters of mucus biology, driven by type 2 inflammation, result in sticky and tenacious sputum, which represents a potent threat, first due to the difficulties in expectoration and second by acting as a platform for viral, bacterial, or fungal colonization that allows exacerbations. Therefore, in this way, mucus plugs are an overlooked but critical feature of asthmatic airway disease.

Acupoint Catgut-Embedding Therapy Inhibits NF-κB/COX-2 Pathway in an Ovalbumin-Induced Mouse Model of Allergic Asthma

Allergic asthma is associated with T helper (Th) 2 cell-biased immune responses and characterized by the airway hyperresponsiveness (AHR). Studies have shown that the acupoint catgut-embedding therapy (ACE) has a therapeutic effect on allergic asthma. However, the relevant mechanism is poorly understood. In present study, female BALB/c mice were sensitized and challenged with ovalbumin (OVA) to establish a model of allergic asthma. AHR was evaluated by using airway resistance ( $R_L$ ) and lung dynamic compliance (Cdyn). Airway inflammation and mucus hypersecretion were observed by HE and PAS staining. Inflammatory cells were counted, and related cytokines in bronchoalveolar lavage fluid (BALF) were detected by enzyme-linked immunosorbent assay (ELISA). Pulmonary group 2 innate lymphoid cell (ILC2s) proportions were analyzed by flow cytometry. The expression of nuclear factor κB (NF-κB) and cyclooxygenase-2 (COX-2) was detected by immunostaining. Our results showed that OVA induction resulted in a significant increase in $R_L$ , accompanied by a significant decrease in Cdyn. The levels of interleukin- (IL-) 4, IL-13, OVA-specific IgE in BALF, and the percentage of ILC2 in the lungs were markedly increased accompanied by a significant decreased in interferon-γ (IFN-γ). Furthermore, the expressions of p-NF-κB p65 and COX-2 in airways were significantly upregulated. After ACE treatment, the indicators above were significantly reversed. In conclusion, ACE treatment inhibited the secretion of Th2 cytokines and the proliferation of ILC2s in the lungs, thereby dampening the inflammatory activity in allergic asthma. The underlying mechanism might be related to the inhibition of NF-κB/COX-2 pathway.

Age-Related Differences in Structure and Function of Nasal Epithelial Cultures From Healthy Children and Elderly People

The nasal epithelium represents the first line of defense against inhaled pathogens, allergens, and irritants and plays a key role in the pathogenesis of a spectrum of acute and chronic airways diseases. Despite age-dependent clinical phenotypes triggered by these noxious stimuli, little is known about how aging affects the structure and function of the airway epithelium that is crucial for lung homeostasis and host defense. The aim of this study was therefore to determine age-related differences in structural and functional properties of primary nasal epithelial cultures from healthy children and non-smoking elderly people. To achieve this goal, highly differentiated nasal epithelial cultures were established from nasal brushes at air–liquid interface and used to study epithelial cell type composition, mucin (MUC5AC and MUC5B) expression, and ion transport properties. Furthermore, we determined age-dependent molecular signatures using global proteomic analysis. We found lower numeric densities of ciliated cells and higher levels of MUC5AC expression in cultures from children vs. elderly people. Bioelectric studies showed no differences in basal ion transport properties, ENaC-mediated sodium absorption, or CFTR-mediated chloride transport, but detected decreased calcium-activated TMEM16A-mediated chloride secretory responses in cultures from children vs. elderly people. Proteome analysis identified distinct age-dependent molecular signatures associated with ciliation and mucin biosynthesis, as well as other pathways implicated in aging. Our data identified intrinsic, age-related differences in structure and function of the nasal epithelium and provide a basis for further studies on the role of these findings in age-dependent airways disease phenotypes observed with a spectrum of respiratory infections and other noxious stimuli.

Photobiomodulation Therapy Restores IL-10 Secretion in a Murine Model of Chronic Asthma: Relevance to the Population of CD4+CD25+Foxp3+ Cells in Lung

It is largely known that photobiomodulation (PBM) has beneficial effects on allergic pulmonary inflammation. Our previous study showed an anti-inflammatory effect of the PBM in an acute experimental model of asthma, and we see that this mechanism is partly dependent on IL-10. However, it remains unclear whether the activation of regulatory T cells is mediated by PBM in a chronic experimental model of asthma. In this sense, the objective of this study was to verify the anti-inflammatory role of the PBM in the pulmonary inflammatory response in a chronic experimental asthma model. The protocol used for asthma induction was the administration of OVA subcutaneously (days 0 and 14) and intranasally (3 times/week, for 5 weeks). On day 50, the animals were sacrificed for the evaluation of the different parameters. The PBM used was the diode, with a wavelength of 660 nm, a power of 100 mW, and 5 J for 50 s/point, in three different application points. Our results showed that PBM decreases macrophages, neutrophils, and lymphocytes in the bronchoalveolar lavage fluid (BALF). Moreover, PBM decreased the release of cytokines by the lung, mucus, and collagen in the airways and pulmonary mechanics. When we analyzed the percentage of Treg cells in the group irradiated with laser, we verified an increase in these cells, as well as the release of IL-10 in the BALF. Therefore, we conclude that the use of PBM therapy in chronic airway inflammation attenuated the inflammatory process, as well as the pulmonary functional and structural parameters, probably due to an increase in Treg cells.

Association of genetic variants of oxidative stress responsive kinase 1 (OXSR1) with asthma exacerbations in non-smoking asthmatics

BACKGROUND

Asthma exacerbation threatens patient's life. Several genetic studies have been conducted to determine the risk factors for asthma exacerbation, but this information is still lacking. We aimed to determine whether genetic variants of Oxidative Stress Responsive Kinase 1 (OXSR1), a gene with functions of salt transport, immune response, and oxidative stress, are associated with exacerbation of asthma.

METHODS

Clinical data were obtained from 1454 asthmatics and single nucleotide polymorphisms (SNPs) of OXSR1 were genotyped. Genetic associations with annual exacerbation rate were analyzed depending on smoking status.

RESULTS

Eleven SNPs were selected using Asian data in the International HapMap database. The common allele of rs1384006 C > T of OXSR1 showed a significantly higher annual exacerbation rate than the rare allele in non-smoking asthmatics (CC vs. CT vs. TT: 0.43 ± 0.04 vs. 0.28 ± 0.03 vs. 0.31 ± 0.09, P = 0.004, Pcorr = 0.039). The frequent exacerbators had a significantly higher frequency of the common allele of rs1384006 C > T than did the infrequent exacerbators (74.4% vs. 55.2%, P = 0.004, Pcorr = 0.038).

CONCLUSION

The common allele of rs1384006 C > T of OXSR1 was associated with the asthma exacerbation rate and a higher risk of being a frequent exacerbator, indicating that non-smoking asthmatics who carry common alleles may be vulnerable to asthma exacerbations.

Epigenetic reprogramming of airway macrophages promotes polarization and inflammation in muco-obstructive lung disease

Lung diseases, such as cystic fibrosis and COPD, are characterized by mucus obstruction and chronic airway inflammation, but their mechanistic link remains poorly understood. Here, we focus on the function of the mucostatic airway microenvironment on epigenetic reprogramming of airway macrophages (AM) and resulting transcriptomic and phenotypical changes. Using a mouse model of muco-obstructive lung disease (Scnn1b-transgenic), we identify epigenetically controlled, differentially regulated pathways and transcription factors involved in inflammatory responses and macrophage polarization. Functionally, AMs from Scnn1b-transgenic mice have reduced efferocytosis and phagocytosis, and excessive inflammatory responses upon lipopolysaccharide challenge, mediated through enhanced Irf1 function and expression. Ex vivo stimulation of wild-type AMs with native mucus impairs efferocytosis and phagocytosis capacities. In addition, mucus induces gene expression changes, comparable with those observed in AMs from Scnn1b-transgenic mice. Our data show that mucostasis induces epigenetic reprogramming of AMs, leading to changes favoring tissue damage and disease progression. Targeting these altered AMs may support therapeutic approaches in patients with muco-obstructive lung diseases.

IL-17A from innate and adaptive lymphocytes contributes to inflammation and damage in cystic fibrosis lung disease

BACKGROUND

Elevated levels of interleukin (IL)-17A were detected in the airways of patients with cystic fibrosis (CF), but its cellular sources and role in the pathogenesis of CF lung disease remain poorly understood. The aim of this study was to determine the sources of IL-17A and its role in airway inflammation and lung damage in CF.

METHODS

We performed flow cytometry to identify IL-17A-producing cells in lungs and peripheral blood from CF patients and β-epithelial Na⁺ channel transgenic (Scnn1b-Tg) mice with CF-like lung disease, and determined the effects of genetic deletion of Il17a and Rag1 on the pulmonary phenotype of Scnn1b-Tg mice.

RESULTS

T-helper 17 cells, CD3⁺CD8⁺ T-cells, γδ T-cells, invariant natural killer T-cells and innate lymphoid cells contribute to IL-17A secretion in lung tissue, lymph nodes and peripheral blood of patients with CF. Scnn1b-Tg mice displayed increased pulmonary expression of Il17a and elevated IL-17A-producing innate and adaptive lymphocytes with a major contribution by γδ T-cells. Lack of IL-17A, but not the recombination activating protein RAG1, reduced neutrophilic airway inflammation in Scnn1b-Tg mice. Genetic deletion of Il17a or Rag1 had no effect on mucus obstruction, but reduced structural lung damage and revealed an IL-17A-dependent macrophage activation in Scnn1b-Tg mice.

CONCLUSIONS

We identify innate and adaptive sources of IL-17A in CF lung disease. Our data demonstrate that IL-17A contributes to airway neutrophilia, macrophage activation and structural lung damage in CF-like lung disease in mice. These results suggest IL-17A as a novel target for anti-inflammatory therapy of CF lung disease.

Therapeutic Inhibition of Cathepsin S Reduces Inflammation and Mucus Plugging in Adult βENaC-Tg Mice

BACKGROUND

Elevated levels of the cysteine protease cathepsin S (CatS) are associated with chronic mucoobstructive lung diseases such as cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD). We have previously demonstrated that prophylactic treatment with a CatS inhibitor from birth reduces inflammation, mucus plugging, and lung tissue damage in juvenile β-epithelial Na+ channel-overexpressing transgenic (βENaC-Tg) mice with chronic inflammatory mucoobstructive lung disease. In this study, we build upon this work to examine the effects of therapeutic intervention with a CatS inhibitor in adult βENaC-Tg mice with established disease.

METHODS

βENaC-Tg mice and wild-type (WT) littermates were treated with a CatS inhibitor from 4 to 6 weeks of age, and CatS-/- βENaC-Tg mice were analysed at 6 weeks of age. Bronchoalveolar lavage (BAL) fluid inflammatory cell counts were quantified, and lung tissue destruction and mucus obstruction were analysed histologically.

RESULTS

At 6 weeks of age, βENaC-Tg mice developed significant airway inflammation, lung tissue damage, and mucus plugging when compared to WT mice. CatS-/- βENaC-Tg mice and βENaC-Tg mice receiving inhibitor had significantly reduced airway mononuclear and polymorphonuclear (PMN) cell counts as well as mucus plugging. However, in contrast to CatS-/- βENaC-Tg mice, therapeutic inhibition of CatS in βENaC-Tg mice had no effect on established emphysema-like lung tissue damage.

CONCLUSIONS

These results suggest that while early CatS targeting may be required to prevent the onset and progression of lung tissue damage, therapeutic CatS targeting effectively inhibited airway inflammation and mucus obstruction. These results indicate the important role CatS may play in the pathogenesis and progression of mucoobstructive lung disease.

Chronic rhinosinusitis with nasal polyps is associated with impaired TMEM16A-mediated epithelial chloride secretion

BACKGROUND

Chronic rhinosinusitis with nasal polyps (CRSwNP) is one of the most common chronic disorders with limited therapeutic options. However, the pathogenesis of CRSwNP remains poorly understood.

OBJECTIVE

We sought to determine the role of abnormalities in nasal epithelial ion transport in primary epithelial cultures and patients with CRSwNP.

METHODS

We studied epithelial ion transport and transcript levels of the Cl^- channels cystic fibrosis transmembrane conductance regulator and transmembrane protein 16A (TMEM16A) in human primary nasal epithelial cultures of patients with CRSwNP and healthy controls. Furthermore, we determined expression levels of proinflammatory cytokines that have been implicated in the regulation of epithelial ion channels (IL-1β, INF-γ, TNF-α, IL-13) and studied effects of the key T_H2 signaling molecule IL-13 in CRSwNP and control nasal epithelial cultures. Finally, we measured in vivo nasal potential difference to compare epithelial ion transport in patients with CRSwNP and controls.

RESULTS

Bioelectric studies demonstrated that Ca²⁺-activated Cl^- secretion was reduced in CRSwNP versus control nasal epithelial cultures. Transcript levels of IL-13 and the Ca²⁺-activated Cl^- channel TMEM16A were increased in CRSwNP cultures. Stimulation with IL-13 increased TMEM16A expression further and restored Ca²⁺-activated Cl^- secretion in CRSwNP cultures. Nasal potential difference measurements demonstrated reduced Ca²⁺-activated Cl^- transport in patients with CRSwNP versus controls.

CONCLUSIONS

This study demonstrates that TMEM16A-mediated Ca²⁺-activated Cl^- secretion is reduced in primary nasal epithelial cultures and nasal epithelia of patients with CRSwNP. Our data suggest that the Ca²⁺-activated Cl^- channel TMEM16A may be implicated in the pathogenesis and serve as a novel therapeutic target in patients with CRSwNP.

Lack of IL-1 Receptor Signaling Reduces Spontaneous Airway Eosinophilia in Juvenile Mice with Muco-Obstructive Lung Disease

Previous studies demonstrated spontaneous type 2 airway inflammation with eosinophilia in juvenile Scnn1b (sodium channel, non-voltage-gated 1, β-subunit)-transgenic (Scnn1b-Tg) mice with muco-obstructive lung disease. IL-1 receptor (IL-1R) signaling has been implicated in allergen-driven airway disease; however, its role in eosinophilic inflammation in muco-obstructive lung disease remains unknown. In this study, we examined the role of IL-1R signaling in the development of airway eosinophilia and type 2 inflammation in juvenile Scnn1b-Tg mice. We determined effects of genetic deletion of Il1r1 (IL-1 receptor type I) on eosinophil counts, transcript levels of key type 2 cytokines, markers of eosinophil activation and apoptosis, and tissue morphology in lungs of Scnn1b-Tg mice at different time points during neonatal development. Furthermore, we measured endothelial surface expression of intercellular adhesion molecule 1 (ICAM-1), an integrin involved in eosinophil transendothelial migration, and determined effects of eosinophil depletion using an anti-IL-5 antibody on lung morphology. Lack of IL-1R reduced airway eosinophilia and structural lung damage, but it did not reduce concentrations of type 2 cytokines and associated eosinophil activation in Scnn1b-Tg mice. Structural lung damage in Scnn1b-Tg mice was also reduced by eosinophil depletion. Lack of IL-1R was associated with reduced expression of ICAM-1 on lung endothelial cells and reduced eosinophil counts in lungs from Scnn1b-Tg mice. We conclude that IL-1R signaling is implicated in airway eosinophilia independent of type 2 cytokines in juvenile Scnn1b-Tg mice. Our data suggest that IL-1R signaling may be relevant in the pathogenesis of eosinophilic airway inflammation in muco-obstructive lung diseases, which may be mediated in part by ICAM-1-dependent transmigration of eosinophils into the lungs.

Emerging Concepts and Therapies for Mucoobstructive Lung Disease

A spectrum of intrapulmonary airway diseases, for example, cigarette smoke-induced bronchitis, cystic fibrosis, primary ciliary dyskinesia, and non-cystic fibrosis bronchiectasis, can be categorized as "mucoobstructive" airway diseases. A common theme for these diseases appears to be the failure to properly regulate mucus concentration, producing mucus hyperconcentration that slows mucus transport and, importantly, generates plaque/plug adhesion to airway surfaces. These mucus plaques/plugs generate long diffusion distances for oxygen, producing hypoxic niches within adherent airway mucus and subjacent epithelia. Data suggest that concentrated mucus plaques/plugs are proinflammatory, in part mediated by release of IL-1α from hypoxic cells. The infectious component of mucoobstructive diseases may be initiated by anaerobic bacteria that proliferate within the nutrient-rich hypoxic mucus environment. Anaerobes ultimately may condition mucus to provide the environment for a succession to classic airway pathogens, including Staphylococcus aureus, Haemophilus influenzae, and ultimately Pseudomonas aeruginosa. Novel therapies to treat mucoobstructive diseases focus on restoring mucus concentration. Strategies to rehydrate mucus range from the inhalation of osmotically active solutes, designed to draw water into airway surfaces, to strategies designed to manipulate the relative rates of sodium absorption versus chloride secretion to endogenously restore epithelial hydration. Similarly, strategies designed to reduce the mucin burden in the airways, either by reducing mucin production/secretion or by clearing accumulated mucus (e.g., reducing agents), are under development. Thus, the new insights into a unifying process, that is, mucus hyperconcentration, that drives a significant component of the pathogenesis of mucoobstructive diseases promise multiple new therapeutic strategies to aid patients with this syndrome.

Effect of Low-Level Laser Therapy (LLLT) in Pulmonary Inflammation in Asthma Induced by House Dust Mite (HDM): Dosimetry Study

Asthma is characterized by chronic inflammation in the airways. Several models have been proposed for the discovery of new therapies. Low-Level Laser Therapy (LLLT) is relatively new and effective, very low cost, with no side effects. However, there is still no consensus on the optimal dose to be used. In this sense, the objective of the present study was to evaluate the best dose in an experimental model of asthma induced by House Dust Mite (HDM). Balb/c mice received administration of 100 ug/animal HDM and LLLT applications (diode laser: 660 nm, 100 mW and four different energies 1J, 3J, 5J, and 7.5J) for 16 days. After 24 hours, we studied inflammatory, functional, and structural parameters. The results showed that LBI was able to modulate the pulmonary inflammation observed by reducing the number of cells in Bronchoalveolar Lavage Fluid (BALF) as well as reducing the percentage of neutrophils, eosinophils and T lymphocytes. On the other hand, LLLT increased the level of IL-10 and reduced levels of IL-4, IL-5 and IL-13 in BALF. LLLT was able to reduce the production of mucus, peribronchial eosinophils, collagen deposition, bronchoconstriction index, and bronchial and muscular thickening in the airways. We concluded that the use of LLLT in the treatment of chronic inflammation of the airways attenuated the inflammatory process and functional and structural parameters. We emphasize, in general, that the 1J and 3J laser presented better results. Thus, photobiomodulation may be considered a promising tool for the treatment of chronic pulmonary allergic inflammation observed in asthma.

Airway mucus, inflammation and remodeling: emerging links in the pathogenesis of chronic lung diseases

Airway mucus obstruction is a hallmark of many chronic lung diseases including rare genetic disorders such as cystic fibrosis (CF) and primary ciliary dyskinesia, as well as common lung diseases such as asthma and chronic obstructive pulmonary disease (COPD), which have emerged as a leading cause of morbidity and mortality worldwide. However, the role of excess airway mucus in the in vivo pathogenesis of these diseases remains poorly understood. The generation of mice with airway-specific overexpression of epithelial Na⁺ channels (ENaC), exhibiting airway surface dehydration (mucus hyperconcentration), impaired mucociliary clearance (MCC) and mucus plugging, led to a model of muco-obstructive lung disease that shares key features of CF and COPD. In this review, we summarize recent progress in the understanding of causes of impaired MCC and in vivo consequences of airway mucus obstruction that can be inferred from studies in βENaC-overexpressing mice. These studies confirm that mucus hyperconcentration on airway surfaces plays a critical role in the pathophysiology of impaired MCC, mucus adhesion and airway plugging that cause airflow obstruction and provide a nidus for bacterial infection. In addition, these studies support the emerging concept that excess airway mucus per se, probably via several mechanisms including hypoxic epithelial necrosis, retention of inhaled irritants or allergens, and potential immunomodulatory effects, is a potent trigger of chronic airway inflammation and associated lung damage, even in the absence of bacterial infection. Finally, these studies suggest that improvement of mucus clearance may be a promising therapeutic strategy for a spectrum of muco-obstructive lung diseases.