key: cord-0979124-m9blfl6a
authors: Dong, Xiang; Ding, Mei; Zhang, Jinjin; Ogülür, Ismail; Pat, Yagiz; Akdis, Mübeccel; Gao, Yadong; Akdis, Cezmi A.
title: Involvement and therapeutic implications of airway epithelial barrier dysfunction in type 2 inflammation of asthma
date: 2022-03-05
journal: Chin Med J (Engl)
DOI: 10.1097/cm9.0000000000001983
sha: 2096eac38f96aa389f363b47388b6a17fa86fd61
doc_id: 979124
cord_uid: m9blfl6a

Type 2 inflammation is a complex immune response and primary mechanism for several common allergic diseases including allergic rhinitis, allergic asthma, atopic dermatitis, and chronic rhinosinusitis with nasal polyps. It is the predominant type of immune response against helminths to prevent their tissue infiltration and induce their expulsion. Recent studies suggest that epithelial barrier dysfunction contributes to the development of type 2 inflammation in asthma, which may partly explain the increasing prevalence of asthma in China and around the globe. The epithelial barrier hypothesis has recently been proposed and has received great interest from the scientific community. The development of leaky epithelial barriers leads to microbial dysbiosis and the translocation of bacteria to inter- and sub-epithelial areas and the development of epithelial tissue inflammation. Accordingly, preventing the impairment and promoting the restoration of a deteriorated airway epithelial barrier represents a promising strategy for the treatment of asthma. This review introduces the interaction between type 2 inflammation and the airway epithelial barrier in asthma, the structure and molecular composition of the airway epithelial barrier, and the assessment of epithelial barrier integrity. The role of airway epithelial barrier disruption in the pathogenesis of asthma will be discussed. In addition, the possible mechanisms underlying the airway epithelial barrier dysfunction induced by allergens and environmental pollutants, and current treatments to restore the airway epithelial barrier are reviewed.

Asthma is a common chronic inflammatory airway disease affecting all ages with an estimate of more than 300 million cases all around the world, varying widely between different countries. [1] In China, the prevalence of asthma in individuals older than 20 years was 4.2%, according to a recently published nationwide survey. [2] Noticeably, the ongoing increase in the prevalence of allergic asthma contributes to the growing number of asthma patients. [3] Type 2 inflammation has been described as the underlying immune responses driving allergic asthma. [4] Type 1 immunity is mainly regulated by CD4 + T helper 1 cells (Th1), which secrete interleukin (IL)-2, interferon-g and lymphotoxin-a. Th1 cells stimulate a type 1 immune response which is characterized by prominent phagocytotic activity. Type 2 inflammation originated as a response by the mucosal immunity against parasitic helminth infection that represents a very dedicated immune response to ameliorate the helminth burden in the tissues. [5] This type 2 cell-mediated immunity causes helminth expulsion or elimination, whilst simultaneously limits tissue injury, maintains tissue homeostasis, and contributes to regeneration and fibrosis. [6] [7] [8] Particularly, the expulsion response against helminth larvae represents all features of a fullblown type 2 immune response. An exciting series of molecular events to ensure the co-survival of the worm and the host are taking place. Löffler's pneumonia represents the basis for a type 2 immune response that was initially directed against Ascaris, hookworms, Toxocara and Schistosoma. [9, 10] The life cycle of Ascaris infection is depicted in Figure 1 . Similarly, an expulsion-like pathophysiology also occurs as an immune response to skin parasites, such as in scabies. [11] Type 2 immunity is associated with a wide range of allergic diseases such as allergic rhinitis (AR), allergic asthma, and atopic dermatitis (AD). [5] In asthma, airway type 2 inflammation is mediated by eosinophils, mast cells (MCs), basophils, CD4 + T helper 2 cells (Th2), group 2 innate lymphoid cells (ILC2) and immunoglobulin E (IgE)expressing memory B cells. [4] Type 2 immunity is mainly regulated by Th2 cells secreting IL-4, IL-5, and IL-13 and stimulating antibody production and eosinophilia. [4] Type 2 cytokines promote hallmark features of asthma with a type 2-high signature, such as eosinophilia, mucus hypersecretion, bronchial hyperresponsiveness (BHR), IgE production, and susceptibility to exacerbations. [12] Clinically, biological agents that target type 2 inflammation showed remarkable clinical efficacy in moderate to severe asthma. [13] Currently, five monoclonal antibodies against IgE (omalizumab), IL-5 (mepolizumab and reslizumab), IL-5 receptor a (benralizumab), and IL-4 receptor a (dupilumab) have been approved for the treatment of severe or refractory asthma, and function by blocking the type 2 inflammatory pathways. [14] Some potentially effective biologicals targeting upstream proinflammatory mediators, such as thymic stromal lymphopoietin (TSLP) and IL-33, are also under clinical trials. [15, 16] 

Epithelial barrier dysfunction has been demonstrated to participate in the development of allergic diseases. [17] Structural and functional disruption of the airway epithelial barrier was found in inflammatory and allergic respiratory diseases, i.e., asthma, AR, and chronic rhinosinusitis. [18] Studies showed that epithelial damage in allergic asthma was associated with tight junction (TJ) defects and decrease of adherence junctions. [19] [20] [21] The expression of TJ molecules, such as occludin and zonula occludens (ZO)-1, decreased in AR patients compared with healthy controls, which was associated with disease severity. [22] It is well-known that skin barrier dysfunction is a fundamental feature in AD. Filaggrin (FLG) loss-offunction gene mutations are the strongest known genetic risk factor for AD. [23] FLG deficiency is associated with impairment of keratinocyte differentiation, reduced inflammatory thresholds to irritants and haptens, and enhanced percutaneous microbial and allergen penetration. [24] [25] [26] In addition to FLG mutations, TJ barrier dysfunction has also been reported in AD. [27] Skin barrier dysfunction and AD is associated with an increased risk of food allergy and allergic asthma, and transcutaneous exposure of food or airborne allergens increases the risk of sensitization. [28] [29] [30] Moreover, skin barrier injury can induce intestinal MC expansion through skin-to-gut axis mediated by IL-33, IL-25, and ILCs. [31] Subsequently, degranulation of MCs causes increased intestinal permeability and leads to enhanced sensitization to food allergens in the intestinal tract. [31] Therefore, the dysfunction of the epithelial barrier in the airway, skin, and gut is closely associated with allergic diseases.

The "Epithelial Barrier Hypothesis" proposes that increased exposure to epithelial barrier damaging agents linked to industrialization, urbanization, and modern life underlies the rise in allergic, autoimmune, and other chronic conditions. [17, 32] It discusses whether the immune responses to dysbiotic microbiota that cross the damaged barrier are involved in the development of these diseases. [33] Almost two billion patients are affected with diseases which can be initiated or exacerbated with the exposure to epithelial barrier damaging agents. [34] The development of leaky epithelial barriers then leads to microbial dysbiosis and the translocation of bacteria to interepithelial and subepithelial areas and the development of tissue microinflammation [ Figure 2 ]. Studies on the epithelial barrier suggest that these processes underlie not only the development of allergy and autoimmune conditions in barrier-damaged tissues but also a wide range of diseases in which an immune response to commensal bacteria and opportunistic pathogens occurs [17] [ Figure 3 ].

The airway epithelium is a pseudostratified columnar structure composed of different types of cells. The predominant airway epithelial cells are ciliated epithelial cells, mucous-secreting goblet cells, airway basal cells, and club/clara cells; [35] and another three rare but specialized epithelial cells are neuroendocrine cells, solitary chemosensory cells, and ionocytes. [36, 37] Airway basal cells are stem-cell-like progenitor cells that can differentiate to ciliated cells, mucus-secreting goblet cells, or other specialized epithelial cells. [38] Basal cells anchor the epithelium to the basal membrane via hemidesmosomes. [39] Ciliated epithelial cells originate from basal cells and/or club cells and contain abundant cilia that are necessary for the mucociliary clearance. [35] Mucus-secreting goblet cells are secretory cells that contain vesicles with tightly packed mucin granules and surfactant proteins. [40] Ascaris infection occurs when their fertilized eggs are ingested. The eggs hatch in the intestine, and the larvae migrate to portal veins and then pass through the vena cava inferior, right heart, pulmonary artery, and enter the lungs. The size of the larvae ranges between 0.5 and 1 mm. The growing larvae of the worms cause an eosinophilic pneumonia with cough, as initially described by Löffler. As an essential mechanism of survival of the host, every single larva should be expulsed from the lungs, before they become adults. Because in the case of Ascaris, an adult is 15-20 cm long, and there is no space in the lungs for the adult worms to accommodate their substantially large size, which becomes a big threat to the survival of both the host and parasite. Accordingly, the larvae are fully expulsed from the lungs and swallowed, where they find sufficient space in the guts to become adults. Chinese Medical Journal 2022;135 (5) www.cmj.org Club cells, also called clara cells, are nonciliated secretory cells differentiated from basal cells in small airways, which can secrete a specific protein belonging to the secretoglobin family (secretoglobin family 1A member 1, SCGB1A1). [41] When the epithelium is injured, club cells are able to differentiate into ciliated and mucus-secreting goblet cells driven by the intercellular junctional protein E-cadherin. [42] Neuroendocrine cells are located at airway branch points with allergens and other harmful substances accumulating, contain dense granules of various neuropeptides, amines, and neurotransmitters regulated by the sympathetic and parasympathetic nervous system and serve as airway chemoreceptors. [43, 44] Solitary chemosensory cells contain an apical microvilli tuft, and the function and signal pathways of these cells are similar to intestinal tuft cells, which can regulate type 2 immunity and produce epithelial IL-25. [45, 46] The recently identified ionocytes account for only 1% of airway epithelial cells and lie in multiple levels of the respiratory tract. These cells originate from basal cells and highly express the cystic fibrosis transmembrane conductance regulator (CFTR). [36] The inhibition of CFTR has been found to reduce ZO-1 expression and epithelial differentiation, which implies that ionocytes play a role in regulating TJ assembly and epithelial barrier function. [47] The chemical and physical barriers form the airway epithelial barrier function. Most exogenous substances are trapped in the mucus layer and cleared away by ciliary movements. The production and maintenance of the airway mucus is precisely regulated. It has been found that the balance between Muc5AC and Muc5B, major mucins secreted by goblet cells, can influence mucus viscosity, the ciliary beating and subsequently the likelihood of environmental molecules coming into contact with the airway epithelial cells. [48] On the other hand, the coordinated interaction between neighboring epithelial cells via cell-cell adhesion complexes is of great importance for the physical barrier function, including TJ, adherence junction, desmosome and hemidesmosome [49] [ Figure 4 ]. These junctional structures not only build a physical barrier, but also play an important role in the regulation of epithelial permeability, cell proliferation and differentiation. [50] 

One of the direct implications of epithelial barrier damage is the increase in epithelial permeability leading to transepidermal water loss, which can be used as a measurable parameter for the assessment of epithelial barrier function. Although not available in routine clinical practice, some techniques can be used in research to evaluate epithelial permeability [ Table 1 ]. For example, histological examination via airway mucosal tissue biopsy and/or cytological examination of epithelial cells can provide specific analysis of junctional structure and proteins, albeit it is an invasive method. [49] In addition, early studies have reported that compounds with traceable radioisotopes, e.g. iodine-125 and technetium-99, can be used to assess the permeability of the airway epithelium. [51, 52] Mannitol, rarely metabolized and without radioactivity, was used in animal studies to evaluate the airway epithelial permeability. [53] However, a recent study showed no difference in serum mannitol levels between subjects with mild asthma and healthy controls after inhalation of mannitol. [54] Biomarkers for evaluating the epithelial barrier function are gaining research interest. One such potential biomarker of airway epithelial damage is club cell secretory protein-16 (CC16). [55] Studies have demonstrated that the levels of CC16 in serum and bronchoalveolar lavage fluid were elevated in subjects exposed to asbestos and ozone. [56, 57] Recently, zonulin, identified as pre-haptoglobin-2 (pre-HP2), was shown to modulate intercellular TJs and reversibly regulate epithelial permeability in the intestine. [58, 59] Studies in mice also indicated the involvement of zonulin in respiratory tract epithelial barrier damage. Rittirsch et al [60] reported that zonulin facilitated the development of acute lung injury (ALI) by enhancing albumin leak and complement activation. In addition, zonulin inhibitor was found to exhibit protective effects on influenza infection and mitigate pulmonary edema in ALI, [61] and might be a potential therapy for coronavirus disease 2019 according to a recent in silico analytic study. [62] It has to be noted here that by using electrical impedance spectroscopy, skin barrier integrity can be detected within 8 seconds in a robust and reliable manner. [63, 64] There is a current need for similar devices for the assessment of mucosal epithelia.

Many different exogeneous factors can open the skin and mucosal epithelial barriers. It must be emphasized that the substances mentioned in this review may cooperate in opening the barriers in a synergistic way together with epithelial inflammation. Airway epithelial barrier damage can be caused by a number of allergens, microbes, and environmental substances [ Figure 5 ]. Common aeroallergens, such as dust mites, pollens, and fungi, can disrupt the airway epithelium barrier. The cysteine proteinase allergen Der p1 from house dust mite (HDM), Dermatophagoides pteronyssinus, can directly cleave the TJ adhesion protein occludin. The disruption of intercellular TJs subsequently increases the permeability of the epithelial barrier and induces an immune response. [65] Saito et al [66] recently found that the amount of peroxisome proliferatoractivated receptor g coactivator-1 alpha (PGC-1a) and E-cadherin decreased significantly in HDM-stimulated cells. The HDM allergen disrupted the airway epithelial barrier function through the protease-activated receptor 2 (PAR2)/Toll-like receptor 4/PGC-1a pathway. Similarly, pollens often contain proteases, for example, serine proteases and metalloproteinases, which act on transmembrane adhesion proteins E-cadherin, claudin-1, and occludin, as well as the cytosolic complex ZO-1, and then damage intercellular TJs, the anchorage of columnar epithelial cells and the integrity of epithelial barrier. [67, 68] Proteases of Alternaria alternata can also induce the disruption of the airway epithelial barrier. [69] Importantly, increasing evidence indicates that exogenous noxious substances in the environment are risk factors for the airway epithelial barrier injury and leakiness, including cigarette smoke, [70, 71] diesel exhaust, [72] ozone, [73] particulate matter, [74, 75] nanoparticles, [76] microplastics, [77] detergents, surfactants, and proteolytic enzymes used in cleaning agents, [78] [79] [80] as well as emulsifiers in processed food. [81, 82] The skin epithelium is overwhelmingly exposed to toxic substances present in detergents and household cleaning products. [17] Increased use of detergents in general and the addition of surfactants to commercial detergents has significantly increased the daily exposure to tissue barrier-damaging substances. [83] An additional burden to the epithelial barrier was the introduction of proteolytic enzymes in washing powders in the mid-1960s to improve their cleaning efficiency. [84] Proteolytic enzymes derived from Bacillus subtilis have a direct disruptive effect on the airway epithelial barrier as observed in the development of asthma and rhinitis in employees of a detergent factory. [85, 86] On the other hand, certain strains of Bacillus subtilis can serve as probiotics, regulating TJ proteins (ZO-1) and reducing death of intestinal epithelial cells. [87] A systematic review of epidemiological studies showed an association between exposure to cleaning products and asthma in four cross-sectional, longitudinal, and casecontrol studies. [88] Occupational allergies and asthma in the detergent industry have significantly decreased by adopting extensive measures and development of best practice guidelines focusing on exposure control in production facilities. [89] There has been extensive research on replacing nonbiodegradable products with more environmentally friendly and safer alternatives. [90] However, daily exposure to tissue barrier damaging doses of detergents and household cleaners continues today with the addition of household and professional dishwashers.

Viruses, such as rhinoviruses [91] and coronaviruses, [92, 93] can disrupt TJs and epithelial barrier function, increasing epithelial permeability and viral invasion, and facilitating inflammatory reactions. A typical feature of chronic mucosal inflammation is the development of an immune response toward microbiome components or newly colonizing facultative pathogens, such as Staphylococcus aureus (S. aureus), moraxella, pneumococcus, hemophilus and Pseudomonas aeruginosa. [17, 94] S. aureus is the most abundant bacteria that colonize barrier damaged tissues in the skin and respiratory mucosa. Increased colonization of S. aureus in the nose of asthma patients and increased serum levels of IgE against S. aureus enterotoxins have been repeatedly reported. [95] [96] [97] [98] [99] Prevalence of antibodies against S. aureus components has been linked to asthma severity and exacerbations. [97] S. aureus can enhance the TJ barrier integrity in nasal tissue in healthy individuals but not in nasal polyps. [100] S. aureus has also been shown to be dominant in skin microbiome of patients with AD, suggesting a role of this pathogen in skin barrier dysfunction. [101] Role of Airway Epithelial Barrier Dysfunction in Type 2

The development of asthma and respiratory allergies is a complex interaction between genes, immune system, and the environment whereby the airway epithelial barrier function plays a key role. [18, 102] Airway epithelial damage leads to the loss of physical protection, facilitates the penetration of exogenous stimulants and allergens [103] and acts as an interface of innate and adaptive immunity. [104] Airway epithelial cells express pattern recognition receptors and detect environmental stimuli such as pathogens and allergens. [105] Epithelial barrier disruption has been the focus in understanding the pathogenesis of asthma with type 2 inflammation. [17] Aeroallergens, virus, bacteria, and environmental toxins can impair the epithelial barrier and promote airway epithelial cells to release alarmins IL-25, IL-33, and TSLP, as well as chemokines C-C motif chemokine ligand 2 (CCL2) and CCL20. [105] The alarmins can induce the differentiation of ILC2, which then releases type 2 cytokines IL-5 and IL-13. CCL2 and CCL20 can recruit immature dendritic cells (DCs) and monocytes, the precursors of DCs, to the lungs. [106, 107] Epithelial cytokines IL-25, IL-33, and TSLP favor the development of a proallergic DC phenotype. [108] Activated DCs act as antigen presenting cells and migrate to the draining lymph node where they induce the differentiation of naïve T cells to Th2 cells. The interactions between the airway epithelial cells, DCs and the regional lymph node provide a cytokine milieu for Th2 cell differentiation. [108] IL-4 produced by basophils, together with IL-4 and IL-21 produced by follicular helper T cells, promotes immunoglobulin class switch to IgE in B cells. Effector cells including MC, basophils, and eosinophils are activated, degranulate, and release inflammatory mediators upon being re-exposed to allergens. IL-4 and IL-13 are cardinal type 2 cytokines and central to many aspects of airway changes in asthma, e.g., directly participating in type 2 inflammation, disrupting the epithelial barrier function, acting on basement membrane, and promoting airway remodeling. Therefore, a vicious cycle composed of IL-4, IL-13, epithelial barrier impairment, and type 2 inflammation has been suggested in asthma. [106] In addition, airway epithelial barrier damage will also enhance permeability to foreign substances including allergens, [109] which are uptaken, processed, and presented by DCs and initiate adaptive immune responses. [110] A recent study showed that allergen-induced degranulation of MCs was only observed in those with injured nasal epithelia, and epithelial barrier dysfunction promoted transepithelial allergen passage, sensitization, and MC degranulation even in the absence of an inflammatory condition. [111] In return, MC mediators could rapidly increase epithelial permeability, which facilitated allergen penetration again. [112] Moreover, airway epithelial barrier function can maintain the balance of immunomodulation. Restoring the epithelial barrier integrity reduced inflammation in models of Th2-mediated respiratory inflammation. [113] In a mouse model, the activation of MCs was elevated when the epithelial barrier was disrupted. [50] It is speculated that nasal epithelial barrier dysfunction is one of the crucial risk factors in the inflammatory progression from upper to lower airways. [114] Thus, airway epithelial barrier dysfunction may represent a cardinal pathophysiological mechanism of type 2 immunity. The physical barrier injury, allergic sensitization, and immunological dysregulation resulted from airway epithelial barrier disruption and dysfunction participate in the pathogenesis of asthma and respiratory inflammatory diseases [ Figure 6 ].

Currently, the precise underlying mechanisms leading to airway epithelial barrier disruption in asthma are under extensive research. Different mechanisms may be involved for the various damaging factors of the epithelial barrier, such as allergens, bacteria, virus, particulate matter, and other environmental pollutants. [34, 115] Many allergens possess protease activity, which acts on protease-activated receptors (PARs) and induces airway epithelial barrier impairment. HDM allergens were reported to induce Chinese Medical Journal 2022;135(5) www.cmj.org airway epithelial barrier dysfunction via proteolytic activity. [65] However, another report showed that HDM-induced airway inflammation and hypersensitivity was dependent on allergen sensitization but not to serine/ cysteine protease activity, since HDM extract with the lowest serine protease activity still induced the most pronounced dysfunction of the epithelial barrier and CCL20 release in vitro. [116] Another study also demonstrated that inhalation of HDM allergens did not induce impairment of the airway epithelial barrier. [117] As nonallergic individuals mostly tolerate allergen exposure without or only with mild symptoms, mechanisms other than allergen-specific MC degranulation may have a relatively minor effect. It is suggested that HDM-induced PAR activation and epithelial barrier disruption depended on epidermal growth factor receptor (EGFR) signaling since EGFR inhibition reduced the HDM-triggered decrease in epithelial resistance and improved restoration of epithelial junctions. [118] Mitochondrial biogenesis and heat shock protein 90a have also been demonstrated to participate in HDM-induced airway epithelial barrier dysfunction with distinct signaling pathways. [66, 119] Aller-genic fungus A. alternata possesses serine protease activity and induces barrier disruption of airway epithelium in severe asthma patients. [69] German cockroach induced Ca 2+ release from intracellular Ca 2+ store by acting on PAR2 in the airway epithelium. [120] In addition, cockroach and HDM extracts also activated store-operated Ca 2+ entry and thus sustained intracellular Ca 2+ elevation in the airway epithelium, [121] which triggers proinflammatory cytokines release and airway epithelial barrier dysfunction. [122] Tumor necrosis factor (TNF)-a was shown to induce bronchial epithelial barrier dysfunction by activating Src-family kinase in severe asthma. [123] The impact of type 2 cytokine IL-13 on epithelial barrier dysfunction has been well-established in air-liquid interface (ALI) cultures of bronchial epithelial cells and mouse models of lung inflammation. [124, 125] IL-13, released both by ILC2 and Th2 cells, was shown to induce airway epithelial barrier disruption by targeting TJs in asthmatic patients. [124, 125] By contrast, another study demonstrated that IL-13 plays an important role in restoration of airway epithelial barrier via IL-13 receptor a2. [126] In addition to directly affecting TJ molecules in the epithelia, several programmed cell death processes have been suggested to contribute to airway epithelial barrier dysfunction. Both pyroptosis [127, 128] and apoptosis [129] have been demonstrated to play a possible role in the airway epithelial barrier dysfunction and airway inflammation. [130, 131] Similarly, ferroptosis and autophagy, [132] and their interactions [133] have also been suggested to contribute to airway epithelial barrier dysfunction in asthma. Particulate matter and respiratory syncytial virusinduced necroptosis of airway epithelial cells contribute to airway inflammation. [134, 135] However, the role of necroptosis in airway epithelial barrier impairment needs to be clarified further.

Epithelial barrier impairment is central to the pathogenesis of airway inflammation and may also be linked with severity and control of asthma, therefore, restoration of the barrier integrity may be a useful strategy in the treatment of asthma [ Figure 5 ]. Deoxyribonucleic acid containing repeated cytosine and guanine nucleotides linked with phosphate (CpG DNA) treatment exhibited a barrier healing capacity in vitro. [136] Reduced adrenomedullin expression in airway epithelial cells was observed in asthma patients, and supplementation with adrenomedullin could promote airway epithelial wound repair. [137] It is reported that Pim1 kinase activity is essential to maintaining airway epithelial integrity and protects against HDM-induced proinflammatory cytokine secretion from airway epithelium. [138] Inhibition of CpG methylation was found to improve the integrity of the bronchial epithelial barrier in asthma. [139] Parkinson's disease-associated gene could also protect against HDM-induced airway epithelial barrier impairment by attenuating epithelial cell pyroptosis. [127] Nitric oxide promoted airway epithelial wound repair through increasing the activity of matrix metalloproteinases 9. [140] As also shown in vitro in bronchial epithelial cells, [125] inhibition of histone deacetylase activity could restore nasal epithelial integrity and prevent the development of allergic airway inflammation in patients with AR. [141] Therefore, further studies are warranted to provide evidence of the potential use of histone deacetylase activity inhibitors to restore the bronchial epithelial integrity in asthma patients. Mechanical strain inhibited airway epithelial repair as demonstrated in in vitro cultured epithelial cells, [142] thus maintaining well-control of asthma may reduce mechanical strain induced by hyperinflation secondary to airflow limitation.

As to the currently available treatments for asthma, corticosteroid dexamethasone was able to restore the expression of E-cadherin and beta-and gamma-catenin that was inhibited by TNF-a, as demonstrated in primary human bronchial epithelial cells. [143] A few studies demonstrated a protective effect of long-acting betaagonists (LABA) on the airway epithelial barrier. [144, 145] Montelukast could suppress cysteinyl leukotriene-induced disruption of TJs and adherence junctions (AJs) in human airway epithelial cells. [146] Allergen-specific immunotherapy (AIT) was also able to restore airway epithelial integrity that was damaged in mice exposed to HDM component Der f through inhibition of IL-25 expression and endoplasmic reticulum stress. [147] The effect of biologicals, such as anti-IgE, anti-IL-5/R, and anti-IL-4Ra monoclonal antibodies on airway epithelial barrier dysfunction in asthma patients, is not fully understood. Short-chain fatty acids propionate and butyrate were also capable of restoring HDM-induced bronchial epithelial barrier dysfunction and have been suggested for the potential treatment of asthma. [148] Even though there is limited evidence on the potential of probiotics in restoring the airway epithelial barrier integrity, a study showed a decrease in airway epithelial permeability in both animal models and in vitro cultured bronchial epithelial cells. [149] Prospects It should be noted that the airway epithelial barrier integrity is dynamically regulated by disrupting and repairing factors, both of which may coexist simultaneously. To date, most studies focus only on disruption or restoration of the barrier. Studies aiming to elucidate the imbalance between disruption and repair under different exposomes and its impacts on type 2 inflammation with state-of-the-art techniques will be of great importance to the development of new diagnostic and therapeutic strategies for asthma. A better understanding of the epithelial barrier hypothesis is needed for the prevention, early intervention, and development of novel therapeutic approaches. [17] Possible strategies to reduce diseases associated with a disrupted epithelial barrier include: avoidance and dose control of all of the above-mentioned noxious substances; development of safer, less-toxic products; discovery of biomarkers for the identification of barrier leaky subjects; development of novel therapeutic approaches for restoration of the expression of tissuespecific barrier molecules; strengthening other components of the mucosal barrier; blocking bacterial translocation; avoiding the colonization of opportunistic pathogens; interventions through diet and microbiome, and many more novel approaches. In addition, an international network has been initiated together with the development of the European Academy of Allergy and Clinical Immunology guidelines on environmental health, and a working group to target epithelial barrier related research, education, and communication to outreach regulatory authorities has been recently taken off [150] [ Table 2 ].

In summary, future studies are warranted to understand:

(1) the imbalance between impairment and repair of the airway epithelial barrier; (2) the molecular components of different aeroallergens responsible for the induction of airway epithelial barrier damage; (3) exposomes including virus, bacteria, fungi, particulate matter, microplastics, and their interactions and contributions to airway epithelial barrier damage; (4) biomarkers of airway barrier dysfunction in asthma; (5) novel strategies to repair the airway epithelial barrier. Researches focusing on the interactions of airway epithelial barrier dysfunction and type 2 inflammation in the context of asthma will be helpful to find novel therapeutic targets for asthma. Adoption of state-of-art techniques such as single-cell sequencing, proteomics, airway organoids, Visium spatial imaging together with immunology and animal models will facilitate these studies.

The epithelial barriers of the skin, upper and lower airways, and gut mucosa have been severely impacted by the rapid change in the environment caused by industrialization, urbanization, and westernized lifestyle. The development of leaky epithelial barriers leads to the dysbiosis and translocation of microbiota to inter-and subepithelial areas, and the development of tissue microinflammation. Epithelial barrier dysfunction contributes to the development of type 2 inflammation in asthma, which then in turn aggravates barrier dysfunction. Allergens, bacteria, viruses, and environmental pollutants could cause epithelial barrier dysfunction by different mechanisms, such as proteases, Ca 2+ signaling and programmed death of airway epithelial cells. Most currently available treatments for asthma, such as corticosteroids, LABA, montelukast, and AIT, are able to restore airway epithelial integrity. The interplay between the epithelial barrier and type 2 inflammation in asthma, as well as therapies aimed at regulating this balance is a promising field to be further explored.

None. 

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Nanoparticles and danger signals: oral delivery vehicles as potential disruptors of intestinal barrier homeostasis

Impacts of polystyrene microplastic on the gut barrier, microbiota and metabolism of mice

Laundry detergents and detergent residue after rinsing directly disrupt tight junction barrier integrity in human bronchial epithelial cells

Anionic surfactants and commercial detergents decrease tight junction barrier integrity in human keratinocytes

Occupational asthma caused by cellulase and lipase in the detergent industry

Old Fashioned vs. ultra-processed-based current diets: possible implication in the increased susceptibility to type 1 diabetes and celiac disease in childhood

Impact of diet and the bacterial microbiome on the mucous barrier and immune disorders

Surface active agents; chemical types and applications

Laundry detergents: an overview

Pulmonary disease due to inhalation of derivatives of bacillus subtilis containing proteolytic enzyme

Occupational asthma and rhinitis due to detergent enzymes in healthcare

Probiotic bacillus subtilis CW14 reduces disruption of the epithelial barrier and toxicity of ochratoxin A to caco-2 cells

Asthma and rhinitis in cleaning workers: a systematic review of epidemiological studies

Control of occupational asthma and allergy in the detergent industry

Biodegradable chelating agents for industrial, domestic, and agricultural applications-a review

Rhinovirus disrupts the barrier function of polarized airway epithelial cells

The SARS coronavirus E protein interacts with PALS1 and alters tight junction formation and epithelial morphogenesis

SARS-CoV-2 envelope (E) protein interacts with PDZ-domain-2 of host tight junction protein ZO1

Association between mucosal barrier disruption by Pseudomonas aeruginosa exoproteins and asthma in patients with chronic rhinosinusitis

Staphylococcus aureus nasal colonization and asthma in adults: systematic review and meta-analysis

Total and specific IgE in nasal polyps is related

Sensitisation to staphylococcal enterotoxins and asthma severity: a longitudinal study in the EGEA cohort

Staphylococcus aureus enterotoxin sensitization is associated with allergic poly-sensitization and allergic multimorbidity in adolescents

IgE antibodies to Staphylococcus aureus. Prevalence in patients with atopic dermatitis

Staphylococcus aureus enhances the tight junction barrier integrity in healthy nasal tissue, but not in nasal polyps

Relations between epidermal barrier dysregulation and staphylococcus species-dominated microbiome dysbiosis in patients with atopic dermatitis

Epithelial barrier function and immunity in asthma

Epithelial dysfunction in chronic respiratory diseases, a shared endotype

Epithelium: at the interface of innate and adaptive immune responses

The airway epithelium in asthma

Type 2 immunity in the skin and lungs

Potential role of interleukin-25/ interleukin-33/thymic stromal lymphopoietin-fibrocyte axis in the pathogenesis of allergic airway diseases

Mechanisms of allergen immunotherapy for inhaled allergens and predictive biomarkers

Role of airway epithelial barrier dysfunction in pathogenesis of asthma

Dendritic cells and airway epithelial cells at the interface between innate and adaptive immune responses

Nasal epithelial barrier dysfunction increases sensitization and mast cell degranulation in the absence of allergic inflammation

Histamine and T helper cytokine-driven epithelial barrier dysfunction in allergic rhinitis

MP29-02 reduces nasal hyperreactivity and nasal mediators in patients with house dust mite-allergic rhinitis

European summit on the prevention and self-management of chronic respiratory diseases: report of the European union parliament summit

Biomarkers for diagnosis and prediction of therapy responses in allergic diseases and asthma

House dust mite-induced calcium signaling instigates epithelial barrier dysfunction and CCL20 production

The effects of inhaled house dust mite on airway barrier function and sensitivity to inhaled methacholine in mice

Epidermal growth factor receptor signalling contributes to house dust mite-induced epithelial barrier dysfunction

Extracellular heat shock protein 90a mediates HDM-induced bronchial epithelial barrier dysfunction by activating RhoA/MLC signaling

German cockroach extract activates protease-activated receptor 2 in human airway epithelial cells

Allergens stimulate store-operated calcium entry and cytokine production in airway epithelial cells

Store-operated Ca2+ channels in airway epithelial cell function and implications for asthma

TNF-(-mediated bronchial barrier disruption and regulation by src-family kinase activation

Type 2 innate lymphoid cells disrupt bronchial epithelial barrier integrity by targeting tight junctions through IL-13 in asthmatic patients

Regulation of bronchial epithelial barrier integrity by type 2 cytokines and histone deacetylases in asthmatic patients

IL-13 signaling through IL-13 receptor a2 mediates airway epithelial wound repair

PARK2 attenuates house dust mite-induced inflammatory reaction, pyroptosis and barrier dysfunction in BEAS-2B cells by ubiquitinating NLRP3

Der f1 induces pyroptosis in human bronchial epithelia via the NLRP3 inflammasome

Inhibition of the mitochondrial calcium uniporter prevents IL-13 and allergen-mediated airway epithelial apoptosis and loss of barrier function

Bronchial epithelial pyroptosis promotes airway inflammation in a murine model of toluene diisocyanate-induced asthma

GSDMB induces an asthma phenotype characterized by increased airway responsiveness and remodeling without lung inflammation

Airway epithelial regeneration requires autophagy and glucose metabolism

PEBP1 acts as a rheostat between prosurvival autophagy and ferroptotic death in asthmatic epithelial cells

Respiratory syncytial virus infection promotes necroptosis

Necroptosis contributes to urban particulate matter-induced airway epithelial injury

CpG-DNA enhances the tight junction integrity of the bronchial epithelial cell barrier

Suppression of adrenomedullin contributes to vascular leakage and altered epithelial repair during asthma

Pim1 kinase activity preserves airway epithelial integrity upon house dust mite exposure

Inhibition of CpG methylation improves the barrier integrity of bronchial epithelial cells in asthma

Nitric oxide promotes airway epithelial wound repair through enhanced activation of MMP-9

Blocking histone deacetylase activity as a novel target for epithelial barrier defects in patients with allergic rhinitis

Mechanical strain inhibits repair of airway epithelium in vitro

Modulation of cadherin and catenins expression by tumor necrosis factor-alpha and dexamethasone in human bronchial epithelial cells

PAR2 activation interrupts E-cadherin adhesion and compromises the airway epithelial barrier: protective effect of beta-agonists

Airway epithelial integrity is protected by a long-acting beta2-adrenergic receptor agonist

Epithelial folliculin enhances airway inflammation in aspirin-exacerbated respiratory disease

Allergy immunotherapy restores airway epithelial barrier dysfunction through suppressing IL-25 -induced endoplasmic reticulum stress in asthma

Butyrate and propionate restore the cytokine and house dust mite compromised barrier function of human bronchial airway epithelial cells

Probiotics for the airways: potential to improve epithelial and immune homeostasis

The defective bronchial epithelial barrier and epithelial barrier hypothesis in asthma. Global Atlas of Asthma

Involvement and therapeutic implications of airway epithelial barrier dysfunction in type 2 inflammation of asthma