Involvement and Therapeutic Implications of Airway Epithelial Barrier Dysfunction in Type 2 Inflammation of Asthma
Type 2 inflammation is a complex immune response that plays a central role in the pathogenesis of several allergic diseases, including allergic rhinitis, allergic asthma, atopic dermatitis, and chronic rhinosinusitis with nasal polyps. This type of inflammation is primarily directed against parasitic helminths, aiming to prevent tissue infiltration and induce their expulsion. Recent research has highlighted the critical role of epithelial barrier dysfunction in the development of type 2 inflammation, particularly in asthma. This dysfunction may explain the increasing prevalence of asthma globally, including in China. The “epithelial barrier hypothesis” has gained significant attention, proposing that leaky epithelial barriers lead to microbial dysbiosis, bacterial translocation, and the development of tissue inflammation. Consequently, strategies aimed at preventing epithelial barrier impairment and promoting its restoration are emerging as promising therapeutic approaches for asthma.
Type 2 Inflammation and Its Role in Asthma
Asthma is a chronic inflammatory airway disease affecting over 300 million people worldwide, with significant variations in prevalence across different countries. In China, the prevalence of asthma in individuals over 20 years old is 4.2%, according to a recent nationwide survey. The increasing prevalence of allergic asthma has contributed to the growing number of asthma cases globally. Type 2 inflammation is the underlying immune response driving allergic asthma, characterized by the activation of eosinophils, mast cells, basophils, CD4+ T helper 2 (Th2) cells, group 2 innate lymphoid cells (ILC2), and immunoglobulin E (IgE)-expressing memory B cells.
Type 2 inflammation is regulated by Th2 cells, which secrete interleukin (IL)-4, IL-5, and IL-13. These cytokines promote hallmark features of asthma, including eosinophilia, mucus hypersecretion, bronchial hyperresponsiveness (BHR), IgE production, and susceptibility to exacerbations. Clinically, biological agents targeting type 2 inflammation, such as monoclonal antibodies against IgE (omalizumab), IL-5 (mepolizumab and reslizumab), IL-5 receptor α (benralizumab), and IL-4 receptor α (dupilumab), have shown remarkable efficacy in treating moderate to severe asthma. Additionally, potential biologics targeting upstream pro-inflammatory mediators like thymic stromal lymphopoietin (TSLP) and IL-33 are under clinical investigation.
Epithelial Barrier Dysfunction and Allergic Diseases
Epithelial barrier dysfunction has been implicated in the development of various allergic diseases. Structural and functional disruptions of the airway epithelial barrier are observed in inflammatory and allergic respiratory diseases, including asthma, allergic rhinitis, and chronic rhinosinusitis. In allergic asthma, epithelial damage is associated with defects in tight junctions (TJs) and a decrease in adherence junctions. For instance, the expression of TJ molecules such as occludin and zonula occludens (ZO)-1 is reduced in patients with allergic rhinitis compared to healthy controls, correlating with disease severity.
In atopic dermatitis (AD), skin barrier dysfunction is a fundamental feature, with filaggrin (FLG) loss-of-function mutations being the strongest known genetic risk factor. FLG deficiency impairs keratinocyte differentiation, reduces inflammatory thresholds to irritants and haptens, and enhances percutaneous microbial and allergen penetration. TJ barrier dysfunction has also been reported in AD, contributing to the increased risk of food allergy and allergic asthma. Moreover, skin barrier injury can induce intestinal mast cell expansion through the skin-to-gut axis, mediated by IL-33, IL-25, and ILCs, leading to increased intestinal permeability and enhanced sensitization to food allergens.
The “epithelial barrier hypothesis” suggests that increased exposure to epithelial barrier-damaging agents, linked to industrialization, urbanization, and modern lifestyles, underlies the rise in allergic, autoimmune, and other chronic conditions. Nearly two billion people are affected by diseases initiated or exacerbated by exposure to these agents. The development of leaky epithelial barriers leads to microbial dysbiosis, bacterial translocation, and tissue microinflammation, contributing to the pathogenesis of various diseases.
Cellular and Molecular Components of the Airway Epithelial Barrier
The airway epithelium is a pseudostratified columnar structure composed of various cell types, including ciliated epithelial cells, mucus-secreting goblet cells, airway basal cells, and club/clara cells. Additionally, rare specialized epithelial cells such as neuroendocrine cells, solitary chemosensory cells, and ionocytes are present. Airway basal cells are stem-cell-like progenitor cells that differentiate into ciliated cells, goblet cells, or other specialized epithelial cells. These cells anchor the epithelium to the basal membrane via hemidesmosomes.
Ciliated epithelial cells originate from basal cells and/or club cells and contain cilia essential for mucociliary clearance. Goblet cells secrete mucus, which traps exogenous substances and is cleared by ciliary movements. Club cells, also known as clara cells, are non-ciliated secretory cells that differentiate into ciliated and goblet cells upon injury. Neuroendocrine cells, located at airway branch points, contain dense granules of neuropeptides and serve as airway chemoreceptors. Solitary chemosensory cells, similar to intestinal tuft cells, regulate type 2 immunity and produce epithelial IL-25. Ionocytes, which account for only 1% of airway epithelial cells, highly express the cystic fibrosis transmembrane conductance regulator (CFTR) and play a role in regulating TJ assembly and epithelial barrier function.
The airway epithelial barrier consists of chemical and physical barriers. The mucus layer traps exogenous substances, while coordinated interactions between neighboring epithelial cells via cell-cell adhesion complexes, including TJs, adherence junctions, desmosomes, and hemidesmosomes, maintain the physical barrier. These junctional structures regulate epithelial permeability, cell proliferation, and differentiation.
Assessment of Epithelial Barrier Function
Assessing epithelial barrier function is crucial for understanding its role in disease pathogenesis. One direct implication of epithelial barrier damage is increased epithelial permeability, which can be measured by transepidermal water loss. Techniques such as histological examination of airway mucosal tissue biopsies and cytological examination of epithelial cells provide specific analysis of junctional structure and proteins, although these methods are invasive. Other techniques include permeability assays using radioisotopes like iodine-125 and technetium-99, and mannitol, which is rarely metabolized and lacks radioactivity.
Biomarkers for evaluating epithelial barrier function are gaining research interest. Club cell secretory protein-16 (CC16) is a potential biomarker of airway epithelial damage, with elevated levels observed in serum and bronchoalveolar lavage fluid in subjects exposed to asbestos and ozone. Zonulin, identified as pre-haptoglobin-2 (pre-HP2), modulates intercellular TJs and regulates epithelial permeability in the intestine. Studies in mice indicate that zonulin facilitates acute lung injury (ALI) by enhancing albumin leak and complement activation, and zonulin inhibitors may mitigate pulmonary edema in ALI and potentially treat coronavirus disease 2019 (COVID-19).
Electrical impedance spectroscopy is a non-invasive method for assessing skin barrier integrity, and similar devices for mucosal epithelia are needed. These techniques and biomarkers provide valuable tools for evaluating epithelial barrier function in research and clinical practice.
Common Allergens and Environmental Factors that Induce Airway Epithelial Barrier Dysfunction
Various exogenous factors can disrupt the airway epithelial barrier, including allergens, microbes, and environmental pollutants. Common aeroallergens such as dust mites, pollens, and fungi can impair the epithelial barrier. For example, the cysteine proteinase allergen Der p1 from house dust mite (HDM) cleaves the TJ adhesion protein occludin, increasing epithelial permeability and inducing an immune response. Pollens contain proteases that act on transmembrane adhesion proteins like E-cadherin, claudin-1, and occludin, damaging TJs and epithelial barrier integrity. Proteases from Alternaria alternata also induce airway epithelial barrier disruption.
Environmental pollutants, including cigarette smoke, diesel exhaust, ozone, particulate matter, nanoparticles, microplastics, detergents, surfactants, and proteolytic enzymes in cleaning agents, are significant risk factors for airway epithelial barrier injury. Cigarette smoke disrupts TJ components, while diesel exhaust particles modulate occludin expression in lung cells. Ozone exposure induces biphasic injury and inflammation in the respiratory barrier, controlled by IL-33. Particulate matter and nanoparticles impair airway epithelial barrier function, and microplastics impact gut barrier integrity, potentially affecting respiratory health.
Detergents and household cleaning products, particularly those containing surfactants and proteolytic enzymes, have significantly increased daily exposure to tissue barrier-damaging substances. Occupational allergies and asthma in the detergent industry have decreased with exposure control measures, but daily exposure to these agents continues to impact epithelial barrier integrity. Viruses, such as rhinoviruses and coronaviruses, disrupt TJs and increase epithelial permeability, facilitating viral invasion and inflammatory reactions. Chronic mucosal inflammation often involves an immune response toward microbiome components or newly colonizing facultative pathogens like Staphylococcus aureus (S. aureus), which is abundant in barrier-damaged tissues and associated with asthma severity and exacerbations.
Role of Airway Epithelial Barrier Dysfunction in Type 2 Inflammation of Asthma
Airway epithelial barrier dysfunction plays a pivotal role in the pathogenesis of asthma and respiratory allergies. Epithelial damage leads to the loss of physical protection, facilitating the penetration of exogenous stimulants and allergens. Airway epithelial cells express pattern recognition receptors that detect environmental stimuli, initiating innate and adaptive immune responses. Epithelial barrier disruption has been a focus in understanding the pathogenesis of asthma with type 2 inflammation.
Aeroallergens, viruses, bacteria, and environmental toxins impair the epithelial barrier, promoting the release of alarmins like IL-25, IL-33, and TSLP, as well as chemokines CCL2 and CCL20. These alarmins induce the differentiation of ILC2, which releases type 2 cytokines IL-5 and IL-13. CCL2 and CCL20 recruit immature dendritic cells (DCs) and monocytes to the lungs, favoring the development of a pro-allergic DC phenotype. Activated DCs migrate to draining lymph nodes, inducing the differentiation of naïve T cells into Th2 cells. IL-4, produced by basophils and follicular helper T cells, promotes immunoglobulin class switch to IgE in B cells. Effector cells, including mast cells, basophils, and eosinophils, are activated upon re-exposure to allergens, releasing inflammatory mediators.
IL-4 and IL-13 are central to many aspects of airway changes in asthma, including type 2 inflammation, epithelial barrier dysfunction, basement membrane effects, and airway remodeling. A vicious cycle of IL-4, IL-13, epithelial barrier impairment, and type 2 inflammation has been suggested in asthma. Epithelial barrier damage enhances permeability to foreign substances, which are uptaken, processed, and presented by DCs, initiating adaptive immune responses. Mast cell degranulation further increases epithelial permeability, facilitating allergen penetration and sensitization.
Airway epithelial barrier function maintains immunomodulation balance, and restoring barrier integrity reduces inflammation in Th2-mediated respiratory inflammation models. Nasal epithelial barrier dysfunction is a crucial risk factor in the inflammatory progression from upper to lower airways, highlighting the importance of epithelial barrier integrity in asthma pathogenesis.
Molecular Mechanisms Underlying the Disruption of Airway Epithelial Barriers
The precise mechanisms leading to airway epithelial barrier disruption in asthma are under extensive research. Allergens, bacteria, viruses, particulate matter, and environmental pollutants induce barrier dysfunction through various pathways. Many allergens possess protease activity, acting on protease-activated receptors (PARs) and impairing the epithelial barrier. HDM allergens, for example, induce airway epithelial barrier dysfunction via proteolytic activity, although allergen sensitization may play a more significant role than protease activity alone.
Mitochondrial biogenesis and heat shock protein 90α are involved in HDM-induced airway epithelial barrier dysfunction, with distinct signaling pathways. Allergenic fungi like Alternaria alternata possess serine protease activity, disrupting the airway epithelial barrier in severe asthma patients. German cockroach allergens activate PAR2, inducing Ca2+ release from intracellular stores and sustained intracellular Ca2+ elevation, triggering pro-inflammatory cytokine release and barrier dysfunction.
Tumor necrosis factor (TNF)-α induces bronchial epithelial barrier dysfunction by activating Src-family kinase in severe asthma. IL-13, released by ILC2 and Th2 cells, disrupts airway epithelial barrier integrity by targeting TJs in asthmatic patients. However, IL-13 also plays a role in barrier restoration through IL-13 receptor α2. Programmed cell death processes, including pyroptosis, apoptosis, ferroptosis, and autophagy, contribute to airway epithelial barrier dysfunction and inflammation. Particulate matter and respiratory syncytial virus-induced necroptosis of airway epithelial cells also contribute to airway inflammation.
Restoration of the Airway Epithelial Barrier
Restoring airway epithelial barrier integrity is a promising therapeutic strategy for asthma. Deoxyribonucleic acid containing repeated cytosine and guanine nucleotides linked with phosphate (CpG DNA) treatment exhibits barrier healing capacity in vitro. Adrenomedullin supplementation promotes airway epithelial wound repair, and Pim1 kinase activity maintains airway epithelial integrity, protecting against HDM-induced pro-inflammatory cytokine secretion. Inhibition of CpG methylation improves bronchial epithelial barrier integrity in asthma, and Parkinson’s disease-associated gene PARK2 attenuates HDM-induced airway epithelial barrier impairment by reducing epithelial cell pyroptosis.
Nitric oxide promotes airway epithelial wound repair by increasing matrix metalloproteinase 9 activity. Histone deacetylase inhibitors restore nasal epithelial integrity and prevent allergic airway inflammation in patients with allergic rhinitis, suggesting potential use in asthma. Mechanical strain inhibits airway epithelial repair, highlighting the importance of asthma control in reducing mechanical strain induced by hyperinflation.
Current asthma treatments, including corticosteroids, long-acting beta-agonists (LABA), montelukast, and allergen-specific immunotherapy (AIT), restore airway epithelial integrity. Dexamethasone restores E-cadherin and catenin expression inhibited by TNF-α, and LABAs protect the airway epithelial barrier. Montelukast suppresses cysteinyl leukotriene-induced disruption of TJs and adherence junctions in human airway epithelial cells. AIT restores airway epithelial integrity damaged by HDM component Der f through inhibition of IL-25 expression and endoplasmic reticulum stress.
Short-chain fatty acids propionate and butyrate restore HDM-induced bronchial epithelial barrier dysfunction, and probiotics decrease airway epithelial permeability in animal models and in vitro cultured bronchial epithelial cells. These findings highlight the potential of various therapeutic approaches in restoring airway epithelial barrier integrity and treating asthma.
Prospects
Future research should focus on understanding the imbalance between impairment and repair of the airway epithelial barrier, the molecular components of aeroallergens responsible for barrier damage, and the interactions of exposomes including viruses, bacteria, fungi, particulate matter, and microplastics. Biomarkers of airway barrier dysfunction in asthma and novel strategies to repair the epithelial barrier are also crucial areas of investigation. Single-cell sequencing, proteomics, airway organoids, and Visium spatial imaging, combined with immunology and animal models, will facilitate these studies.
The epithelial barrier hypothesis emphasizes the need for strategies to reduce diseases associated with a disrupted epithelial barrier, including avoidance of noxious substances, development of safer products, discovery of biomarkers, and novel therapeutic approaches. An international network and guidelines on environmental health, targeting epithelial barrier-related research, education, and communication, are essential for advancing this field.
Conclusions
Epithelial barrier dysfunction contributes significantly to the development of type 2 inflammation in asthma, driven by allergens, bacteria, viruses, and environmental pollutants. The interplay between the epithelial barrier and type 2 inflammation, along with therapies aimed at regulating this balance, represents a promising field for further exploration. Restoring airway epithelial barrier integrity through various therapeutic approaches offers potential benefits in treating asthma and improving patient outcomes.
doi.org/10.1097/CM9.0000000000001983
Was this helpful?
0 / 0