Introduction:
Asthma is clinically characterized by coughing, shortness of breath and chest tightness and is usually caused by exposure to allergens and foreign pathogens. The asthmatic airway is chronically inflamed due to the activation and/or recruitment of a variety of tissue resident and infiltrating cells including eosinophils, mast cells, T lymphocytes, macrophages, airway epithelial cells (AECs), fibroblasts and airway smooth muscle cells. The AEC, which sits at the interface between the host and the external environment, is not only an efficient physical barrier but also represents the first line of defence against microorganisms, airborne irritants and allergens [1]. While asthma is an inflammatory disorder of the conducting airways, inflammation itself does not explain the origin(s) of this disease nor why the airways are so susceptible to a range of different environmental factors [2]. Placing the airway epithelium at the center of asthma origin, progression, and exacerbation is a paradigm shift away from considering T helper (Th)2-type inflammation as primary more toward defects in epithelial innate immunity and responses to injury [2].
The current diagnosis of asthma mainly depends on the patient’s clinical symptoms, lung function, bronchial challenge and variability in peak expiratory flow (PEF). However, some patients are not suitable for lung function tests because of pulmonary bullae, cardiac insufficiency or bronchodilator allergy [3]. Over the years, clinicians have defined several different phenotypes based on the patient’s symptoms, age of onset, severity of the disease, and the presence of other conditions, such as allergies, and also biochemical features including sputum or blood eosinophilia. Despite recognizing these phenotypes of asthma, the asthma management method recommended by the International Asthma Global Initiative (GINA) guidelines is still based on the severity of the disease, using a tiered treatment plan, which is to add drugs on the basis of asthma control. The development of the concept of precision medicine with the goal of individualized treatment has emphasized the need for improved biomarkers of asthma phenotypes, sub-phenotypes and endotypes.
Therefore, researchers have investigated the expression of numerous markers such as eosinophilic cationic protein, exhaled nitric oxide, 8-isoprostane, leukotrienes and periostin in sputum, exhaled breath condensate (EBC) and peripheral blood of asthma patients in an attempt to identify a suitable specific biomarker [4]. Most of these markers were pre-selected and aimed at monitoring asthma status and guiding medication by reflecting the level of airway inflammation and do not necessarily act as an early warning signal of AEC damage in the early stages of asthma. Epigenetic studies have confirmed that airway epithelial damage involves structural and functional changes and plays an important role in the pathogenesis of asthma [5]. The substances expressed and secreted by asthmatic AECs may provide resources for the study of biomarkers.
Structure and function of the airway epithelium:
At least ten epithelial cell lineages exist across the upper and lower airways and lung parenchyma [6]. Single cell RNA-sequencing analysis of bronchial biopsies from healthy subjects revealed that AECs consist of basal cells, club cells, ciliated cells, goblet cells, type 1 and type 2 alveolar cells, and rare but highly specialized cells (e.g. neuroendocrine cells, Tuft cells, microfold (M) cells), and the recently described ionocytes. The larger, proximal airways feature a pseudostratified columnar epithelium, in which all cells contact the basement membrane, while in smaller airways, the epithelium becomes columnar and cuboidal [7]. Viera-Braga and colleagues [6] used single-cell sequencing to map the cellular landscape of the lower airways and found an additional 4 cell states, including mucous ciliated cells, activated basal cells, cycling cells and serous cells from the submucosal glands, in asthma patients. Moreover, the airway wall tissue has increased numbers of goblet cells, intraepithelial mast cells, and pathogenic effector Th2 cells. However, analysis of the intercellular communication between healthy and asthmatic airway walls reveals a remarkable loss of structural cell communication and a concomitant increase in Th2 cell interactions.
1.1. Physical barrier function : The airway epithelium is the first physical barrier against inhaled harmful stimuli from the external environment. This barrier is composed of the airway surface fluid and cell-cell contacts between epithelial cells. Firstly, AECs form a complete barrier around the airway, and the structural and functional basis of the epithelial barrier correlates with the junctions between cells and the normal repair function of AECs [8]. These junctions involve tight junctions (TJs), adhesive junctions (AJs), and hemidesmosomes. TJs are composed of the transmembrane proteins zona occludens-1 (ZO-1), occludin, claudins and junction adhesion molecules (JAMs) and are the main regulators of epithelial permeability [9]. The TJs of the zonula occludens and AJs of the zonula adherens constitute a dense protein network that prevents the paracellular passage of essentially all molecules including water, ions and proteins, as well as of pathogens or other inhaled particulate matter [10].
1.2. Biochemical barrier system : The mucus layer formed by submucosal glands, epithelial cell secretions and tissue exudates contains immune factors such as anti-proteases, antioxidant factors, antibacterial peptides including defensins and cathelicidins and mucins that together exert antimicrobial activity against bacteria, fungi and certain viruses [11]. The inhaled particulate matter adheres to the mucus layer whilst the ciliated cells move rhythmically within the serous layer to enable mucociliary clearance. The proteins that make up mucus have a strong capacity to absorb water and can form a gel-like structure. Most mucin genes are constitutively expressed at low levels, however, under diverse pathological conditions their expression is rapidly and dramatically increased leading to significant hypersecretion of mucus and/or compositional changes and thus altering the physical properties of the mucus [12, 13].
1.3. Innate immune defense function : Furthermore, the airway epithelium is also a central participant in innate and adaptive immunity. AECs express many pattern recognition receptors (PRRs) (Figure 1) which rapidly detect and respond to internal or external environmental agents, pathogen-associated molecular patterns (PAMPs) found in microbes and damage-associated molecular patterns (DAMPs) released upon tissue damage, cell death or cellular stress [14]. PRRs include Toll-like receptors (TLRs), NOD-like receptors (NLRs), C-type lectins and protease-activated receptors. Upon recognition of PAMPs or DAMPs, PRRs activate downstream signaling pathways that promote the release of pro-inflammatory cytokines/chemokines including interleukin (IL)-6, IL-8, CCL20, CCL17, thymic stromal lymphopoietin (TSLP), IL-25, IL-33 and granulocyte-macrophage colony-stimulating factor (GM-CSF). These, in turn, attract and activate a wide range of cell types important in innate and adaptive immune responses [15].