The interaction of immune and epithelial cells is necessary to maintain tissue homeostasis (1). The immune response is dependent upon the co-ordinated response and activation of pattern recognition receptors (PRRs) and damage-associated molecular patterns (DAMPs) that cause immune cell activation and recruitment (1, 2). In the healthy airway, there is an effective activation and resolution of the immune cell associated repair mechanisms that are induced by and in response to epithelial injury (1). The interaction of immune cells and the airway epithelium means there is a continuous influence of the epithelial cells on the immune cells and vice versa. As discussed previously, the airway epithelium is the first line defence against any foreign particles which are recognised by the PRRs and DAMPs. The main mechanism of defence activated by the PRRs, and DAMPs are epithelial alarmin release. Epithelial alarmins including IL-25, IL-33, TSLP and HMGB and chemokine release including CCL17, CCL-11 and CCL22, activates and recruits immune cells, both directly and indirectly (1–3).

Asthmatic airways show increased numbers of basal cells with an impaired proliferation and differentiation capacity, goblet cell hyperplasia and an imbalance in the production of mucins, with increased MUC5AC and reduced MUC5B ( Figure 2 ) (15, 17, 18, 22, 48–50). These two mucins have different roles within the airways: MUC5B functions in normal mucus mucociliary clearance whereas MUC5AC exacerbates airway hyperresponsiveness and mucus plugging (51). Endobronchial biopsies from patients with asthma, imaged with scanning electron microscopy, also show reduced ciliation and increased areas of squamous epithelium (52). Functionally, ciliated cells sampled from patients with severe asthma show dyskinetic movement, slower ciliary beat frequency and microtubule defects (53). Club cell markers (CCSP) are also reduced in bronchial alveolar lavage fluid taken from patients with asthma (54). CCSP has also been shown to increase following allergen challenge in patients with allergic asthma, suggesting acute leakage due to club cell damage in the small airway epithelium (55).

Abnormal cell differentiation may contribute to these abnormalities. EGFR, which is increased in the asthmatic epithelium, is a driver of abnormal cell differentiation and epithelial dysfunction (56–58). Furthermore, IL-13, elevated in allergic asthma, inhibits ciliated cell differentiation and causes reduced ciliary beat frequency and abnormal distribution of basal bodies, whilst enhancing goblet cell differentiation (17, 49). Specifically, IL-13 upregulates SPDEF and MUC5AC expression, whilst downregulating FOXJ1 and other cilia-related transcription factors via activation of JAK/STAT and Notch signalling and changes in histone methylation (59–61).

In asthma, epithelial defects in the adherens, tight and gap junctions, as well as altered permeability and integrity, are present from the nose through to the lower airways ( Figure 1 ). Junctional complex disruption is thought to occur through hyperphosphorylation and decreased expression of complex genes and proteins in asthmatic patients because of deeper penetration and increased immune response to particles (1, 17, 62). The disrupted and leaky barrier in asthma is well-documented and a positive feedback loop of IL-4 and IL-13 contributes to this phenotype seen in patients with asthma. Reduced expression of the barrier forming claudins -1, -3, -4 and -18 is associated with increased IL-13 production in asthma, from mild-moderate to severe asthma (14, 15, 33). Reduced ZO-1 and E-cadherin expression are evident in asthma where they cause denudation and detachment of ciliated epithelial cells (1, 14, 15, 62).

Ciliated epithelial cell detachment is paired with loss of cell-cell contacts and reduced integrity (12, 14, 15, 63, 64). This leads to pro-inflammatory cytokine release and recruitment of immune cells, which induce apoptosis and further damage the epithelial layer (12, 14, 15, 63, 64). Apoptosis and autophagy occur in the non-asthmatic airway epithelium and are crucial for airway epithelial homeostasis (21, 64). These mechanisms regulate the number of proliferating epithelial cells and remove damaged cells without the induction of a chronic inflammatory response (21, 64). However, in asthma there is increased apoptosis of ciliated epithelial cells leading to epithelial shedding and barrier dysfunction, which increases with disease severity.

Environmental pathogenic factors, including allergens, air pollutants and respiratory infections, and the associated immune response, cause airway epithelial injury ( Figure 2 ). Allergen proteases, such as Der p1 in house dust mites, induce inflammation and disrupt the epithelial barrier either directly by cleaving tight junction proteins, or indirectly by activating protease activated receptor (PAR)-2 (14, 65, 66). PAR2-dependent signalling promotes epithelial junction disassembly through ZO-1 and occludin degradation, and EGFR-dependent redistribution of E-cadherin (67). Furthermore, ATP released from the injured epithelium may also promote loss of epithelial integrity through activation of purinergic receptors (68). EGFR activation by allergens also promotes EMT in airway epithelial cells (69). Allergens promote oxidative stress, which triggers epithelial cell death and inflammation through oxidative damage of DNA and mitochondria (70–74). Mitochondrial dysfunction has been shown to cause ROS-mediated disruption of barrier function in intestinal epithelium, whilst reduced E-cadherin expression and loss of barrier function are associated with attenuated mitochondrial biogenesis in airway epithelial cells (75, 76). Viral infections and inhaled pollutants can also cause epithelial injury by inducing cell death and disruption of tight junctions (77–80).

The asthmatic airway immune response also leads to loss of epithelial integrity. Mast cell degranulation, IL-5 and direct contact with eosinophils, leads to disruption of tight junctions and increased permeability (17, 37, 81). Furthermore, the T2 cytokines IL-4 and IL-13 inhibit the expression of the key tight and adherens junction genes ZO-1, E-cadherin and occludin, and drive goblet cell differentiation from basal cells ( Figure 2 ) (13, 82).

Susceptibility to epithelial injury by environmental irritants, and abnormal repair mechanisms, are key factors in asthma pathogenesis. House dust mites, extracts from the allergenic fungus Alternaria Alternata and cigarette smoke were shown to induce greater barrier dysfunction in air-liquid-interface (ALI) cultures of asthmatic airway epithelial cells compared to those of healthy subjects (68, 83, 84). Inadequate antioxidant and antiviral mechanisms in asthmatic airways may also confer epithelial cells more susceptibility to oxidative and viral-induced damage (72, 85, 86). Epithelial cells from patients with asthma are also abnormally slow at repairing injury in in vitro models. This is possibly caused by asynchronous mitosis of epithelial cells and is associated with increased production of TGF-β which drives EMT and ECM production (87–89).

Taken together, the data suggests that there is a vicious cycle of epithelial cell injury and immune activation, which in conjunction with increased susceptibility and abnormal repair mechanisms promote an abnormal airway epithelium in asthma (13, 16, 37).

In the asthmatic airway, alarmin and chemokine release induced cytokine activation is a well-recognised disease driving mechanism. The release of alarmins primes the Th2 CD4 T-cell response through maturing dendritic cells (DCs) via CCL17 (42). DCs in asthma mount an uncontrolled Th2 response and acts to enhance ILC2, basophil and mast cell functionality (90). The epithelium response to injury enhances the Th2 release of pro-inflammatory cytokines such as IL-4 from mast cells and basophils increasing levels of IgE, IL-5 for the activation and maturation of eosinophils in the bone marrow and IL-13 that induces hyper-responsive smooth muscle contraction, goblet cell hyperplasia and the activation of macrophages (90).

The interaction between the epithelium and immune cells can influence many different parts of the airway through alarmin release; in the innate immune system, DCs cause increased activation of Th2 cells, basophils have increased histamine degranulation and ILC2 cells release increased pro-inflammatory cytokines (17). In the epithelial and endothelial cells, a feedback loop is induced, the epithelium becomes leakier due to cleaved junctions, smooth muscle cells become hyper-responsive and fibroblasts release increase CCL5, GM-CSF and CXCL8 (17).

There is evidence of increased numbers of basal cells in the airways of patients with COPD, which however show compromised capacity to regenerate and differentiate (94). EGFR is expressed in basal cells and can be activated by EGF produced by ciliated cells in response cigarette smoke and inflammatory mediators (5, 6). EGFR is increased in smokers and is associated with increased mucous and squamous metaplasia via Notch signalling (3).

Samples taken at bronchoscopy from people who smoke have shown a loss of cilia, shorter cilia and ultrastructural defects in the axoneme (8, 9, 90, 95–97). Gene expression of dynein arms (DNAH5, DNAH9 and DNAH11) that drive cilia beating, and intraflagellar transport proteins (IFT43, IFT 57, IFT144 and IFT172), which determine cilia length and development, have been shown to be reduced in smokers. Within COPD, similar features of loss of cilia, shorter cilia and slower cilia are seen when compared to healthy controls and healthy smokers (8, 98). Although inhaled therapies for COPD, β₂-agonists and muscarinic antagonists, can increase ciliary movement these effects are blunted by high levels of inflammation (99, 100).

Goblet cells are increased in smokers and patients with COPD, which is thought to lead to an increase in baseline mucus production (7). Although submucosal gland density does not appear to be increased in COPD, gland inflammation is increased and may contribute to increased mucus release in response to infectious and non-infectious triggers (10). The primary mucins, MUC5AC and MUC5B are both increased in COPD (101–104). Furthermore, club cells are reduced in the small airway epithelium of patients with COPD (105). CCSP is reduced in smokers and patients with COPD and has been shown to correlate with FEV₁ (106, 107).

In COPD, there is a known and recognised defect in the epithelial barrier permeability and integrity in response to stressors ( Figure 1 ). In epithelial cells from patients with COPD, reduced expression of occludins, claudins and JAM-1 is seen. Oxidative stress leads to abnormal phosphorylation of occludin and redistribution of occludins, ZO-1, E-cadherin and β-catenin throughout the junction (79). Increased permeability of the epithelium is seen in response to cigarette smoke and associated chronic inflammation, with a reduction in staining of ZO-1 (108, 109). Many viruses including rhinovirus also reduce ZO-1, and further lead to reduced epithelial resistance and increased permeability in ALI cultures (110), Epithelial cells exposed to TNFα and IFN-γ also show reduced ZO-1 and JAM staining and attenuated barrier function (111). This process of inflammation-induced loss of tight and adherens junctions predisposes the epithelium to further infection and exacerbates the persistent chronic inflammatory state seen in COPD.

The reticular basement membrane in COPD is thicker than in healthy controls but as not thick as seen in patients with asthma (112, 113). However, fragmented reticular basement membrane is seen in COPD and healthy smokers (114). In COPD, however, repeated trauma leads to persistent EMT activation through TGF-β activation and further basement membrane fragmentation (115, 116). EMT markers have been shown be increased in COPD and to correlate with the severity of airflow obstruction (117, 118).

Chronic inflammation is a key factor in COPD pathogenesis resulting from repeated insults and oxidative stress, which cause cell death, senescence and abnormal cell differentiation and function. While there are many risk factors, cigarette smoke is the most common in COPD. Cigarette smoke contains reactive oxygen species (ROS) which can be directly cytotoxic to epithelial cells and trigger an acute inflammatory response, which is further exacerbated by bacterial and viral infections. Epithelial cells release pro-inflammatory cytokines CXCL8, G-CSF, LTB4 and MCP-1 resulting in the recruitment of monocytes, macrophages and neutrophils which in turn release IL-6 and TNFα (4). The inflammatory response and oxidative stress lead to necrosis and apoptosis of epithelial cells. In response to this trauma, cells adjacent to the area de-differentiate to cover the denuded area, forming a flattened area of squamous epithelium to protect against future damage (119). In health, if toxic exposure to the epithelium ceases, basal cells and CD45- stem cells differentiate for form a pseudostratified epithelium (120, 121). However, in COPD, persistent exposures and chronic inflammation prevent the formation of structurally and functionally normal epithelium ( Figure 3 ).

In the COPD epithelium, the chronic inflammatory process occurs due to ongoing oxidative stress (usually from cigarette smoke), persistent activation of inflammatory cells through cell damage and impaired processes to fight infection. Toll-like receptors on epithelial cells are directly stimulated by cigarette smoke activating a cascade of inflammatory mediators, further tissue damage and release of antigenic material (122, 123). Antigens are also taken up by dendritic cells (which are increased in the COPD epithelium) and are presented to CD8+ T cells (124, 125). CD8+ T cells are increased in COPD and numbers inversely correlated with FEV₁ (126). These cells are cytotoxic to epithelial cells and release proteolytic enzymes including matrix metalloproteases (MMPs) which can drive emphysema, a clinical feature of COPD. Phagocytes, in particular neutrophils and macrophages, are altered in COPD. Neutrophils and macrophages engulf microbes and apoptotic cells. Neutrophils and macrophages are increased in the epithelium and are associated with chronic bronchitis, more severe airflow obstruction and faster lung function decline (127–129). Despite being increased in number, they show impaired phagocytic ability which predispose to recurrent infection (130–132). Neutrophils and macrophages also release reactive oxygen species, pro-inflammatory cytokines, growth factors and MMPs, which propagate chronic inflammation, emphysema, and airway remodelling processes, such as EMT, fibroblast activation and airway smooth muscle proliferation (1) ( Figure 3 ).

The continued interaction of the immune cells and epithelium (39, 133) in COPD is facilitated by the proximity of DCs to the airway epithelial cells. In COPD, DCs have been identified as upregulating the expression of chemokines CXCL10, CCL-20 and CCL-10 that cause the activation of T-cells, predominantly a Th1-pro inflammatory profile (39, 133). Epithelial alarmin release of IL-33 has also been identified as being upregulated in COPD; this has been shown to cause an increase in neutrophil and macrophage infiltration and activation. This drives a positive feedback loop on both epithelial cells and immune cell upregulation of IL-6 and CXCL8 release (39). The up-regulation of HGMB1 on epithelial cells in COPD stimulates increased IL-1b production and release from activated macrophages (39, 133).

ROS, from cigarette smoke or the inflammatory response, drive cell death, senescence, abnormal cell-cell connections and abnormal remodelling (134, 135). Epithelial cells have antioxidant mechanisms to mitigate the ill effects of ROS but these are impaired in COPD which renders these cells more susceptible to oxidative damage (72, 136–138). Epithelial cells in COPD, have a dysregulated redox state which promotes airway inflammation and remodelling by regulating the activity of ROS-dependent kinases, phosphatases, transcription factors and epigenetic effectors, and by drivingTGF- β expression and EGFR activation (139–141).

DNA damage occurs in response to oxidative stress and is associated with nucleotide base oxidation, and single and double-strand DNA breaks in COPD (142, 143). In addition to the increased insults, DNA repair processes are defective in patients with COPD (143, 144). These along with telomere damage, as a result of oxidative stress, lead to upregulation of the cell cycle inhibitor p16^INK4 and mammalian target of rapamycin (mTOR)-mediated signalling to trigger premature senescence in COPD airway epithelial cells (145–149). Although acute senescence may play a role in damage repair, prolonged senescence may lead to impaired epithelial regeneration and damage repair by limiting the proliferative capacity of progenitor cells. Furthermore, although in cell cycle arrest, senescent cells remain metabolically active and secrete a multitude of proteases, cytokines, chemokines and growth factors, such as TGF-β (148). Therefore, accumulation of senescent cells may exacerbate lung inflammation and epithelial injury, and drive emphysema and fibrosis (150–155).

Mitochondria are major sites of oxidative damage in COPD epithelial cells. Cigarette smoke alters mitochondrial morphology and respiration in normal bronchial epithelial cells, whilst bronchial epithelial cells from COPD patients show abnormal mitochondrial morphology, possibly as a result of prolonged exposure to oxidative stress (156, 157). Mitochondrial dysfunction leads to increased production of ROS, which in turn cause further mitochondrial damage. This creates a vicious cycle of oxidative stress and mitochondrial dysfunction that drives pathology. Specifically, mitochondrial ROS promote apoptosis, senescence and inflammatory mediator release in airway epithelial cells, as well as impaired cellular function manifesting as reduced mucociliary clearance (158–161). Mitochondrial dysfunction also triggers ROS-dependent activation of necroptosis, a lytic form of cell death, that is accompanied with the release of immunogenic damage-associated molecular patterns (DAMPs). DAMPs, which include mitochondrial molecules such as ATP and cardiolipin, promote further inflammatory mediator release and result in increased MUC5AC expression in the airway epithelium (162–164).

Autophagy is a cellular process that is pivotal in the adaptation to oxidative stress. In this process, damaged molecules and organelles are engulfed into double-membrane bound vesicles called autophagosomes and fuse with lysosomes leading to their degradation. Reduced autophagic activity in COPD lungs, resulting from prolonged oxidative stress and ageing, may lead to insufficient removal of damaged cellular components and premature airway epithelial cell senescence (165–168). Specifically, impaired autophagic removal of mitochondria (mitophagy) has been reported in the airway epithelium of COPD patients, where it leads to accumulation of damaged mitochondria and epithelial cell senescence through ROS-mediated DNA damage (161, 169–171). However, other studies show excessive autophagic activity in COPD, leading to airway epithelial cell apoptosis, necroptosis and inflammatory mediator release (172–174). Cigarette smoke-induced autophagy has also been shown to cause degradation of ciliary proteins, and to promotes MUC5AC expression through activation of c-Jun N-terminal kinase (JNK) and activating protein (AP)-1 (175, 176). Therefore, dysregulated autophagy leads to the abnormal adaptation of airway epithelial cells to injury and contributes to airway remodelling.

Predisposing factors combined with exposures both in early life and adulthood likely contribute to epithelial pathogenesis seen in COPD. Gene polymorphisms, and epigenetic changes including DNA methylation, histone modifications and non-coding RNAs, may play a role in susceptibility and abnormal repair following epithelial injury in COPD (152, 177).

DNA methylation at gene promoters, a modification associated with reduced gene expression, is altered by cigarette smoking in airway epithelial cells (178–180). In ALI cultures derived from COPD bronchial epithelial cells, DNA hypomethylation at the SPDEF gene promoter leads to increased MUC5AC expression (178). In cigarette smoke extract-exposed COPD airway epithelial cells, gene promoter hypomethylation triggers increased expression of the aryl hydrocarbon receptor repressor (AHRR), an inhibitor of the cytoprotective transcription factor aryl hydrocarbon receptor (AHR). This leads to inhibition of AHR expression and increases susceptibility to CS-induced epithelial cell apoptosis and necroptosis (180). Conversely, reduced expression of the antioxidant transcription factor nuclear factor E2-related factor 2 (Nrf2) due to DNA hypermethylation in COPD, may increase the susceptibility of bronchial epithelial cells to oxidative stress-dependent cell death (181).

Acetylation of lysine residues on histones, which is catalysed by histone acetyltransferases and removed by histone deacetylases, is usually associated with open chromatin configuration and active gene transcription. Histone methylation on lysine and arginine residues, regulated by histone methyltransferases and demethylases, can activate or inhibit gene expression depending on the site and number of methyl groups added (182). An imbalance between histone acetylation and deacetylation, favouring acetylation, has also been observed in COPD lungs and has been associated with increased inflammatory gene expression (167, 179). Reduced expression of histone acetyltransferase binding to ORC1 (HBO1) in COPD leads to downregulation of anti-apoptotic protein Bcl-2, sensitising airway epithelial cells to oxidative stress-mediated apoptosis (183). Furthermore, reduced expression of the arginine methyltransferase coactivator-associated arginine methyltransferase 1 (CARM1) in COPD bronchial epithelial cells is associated with increased senescence, and impaired club cell regeneration and epithelial repair, possibly through altered histone methylation of cell cycle and differentiation genes (184, 185).

miRNAs are small non-coding RNAs that regulate gene expression through the induction of mRNA degradation and inhibition of translation. A number of miRNAs have been shown to be abnormally expressed in COPD airways where they are involved in airway epithelial inflammation, mucous hypersecretion, EMT and cellular senescence (152, 186–188).

Some of the gene polymorphisms identified by genome-wide association studies to be associated with COPD may increase susceptibility to epithelial injury. A gene variant of FAM13A, a protein expressed mainly in club cells and alveolar type 2 cells, is associated increased gene expression and reduced lung function in COPD. FAM13A is an inhibitor of Wnt signalling, a regulator of epithelial differentiation, therefore its upregulation may contribute to impaired club cell differentiation in COPD airways (189). Furthermore, FAM13A promotes fatty acid oxidation and mitochondrial respiration by driving the expression of carnitine palmitoyltransferase 1A (CPT1A), contributing to ROS-mediated bronchial epithelial apoptosis (190). However, FAM13A inhibits TGF-β-mediated EMT in airway epithelial cells, indicating it may also have protective effects against airway remodelling (191). Variants of the iron-regulatory protein 2 (IRP2) have also been associated with COPD susceptibility. IRP2 is found in airway epithelial cells, predominantly at the cilial surface. Increased IRP2 expression in COPD causes mitochondrial dysfunction due to iron overload, leading to increased airway epithelial cell death, inflammation and impaired mucociliary clearance (192, 193).