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].