Epithelial Dysfunction: A Unified Target in Airway Disease Management

The epithelial lining of the airways serves as the primary interface with the external environment, acting as a physical barrier against pathogens, allergens, and irritants. Beyond its barrier function, the epithelium actively participates in immune regulation, secreting cytokines, chemokines, and antimicrobial peptides that shape the inflammatory response. Disruption of this barrier and its immunomodulatory functions is now understood to be a critical initiating event in the pathogenesis of various airway diseases.¹

In allergic rhinitis and asthma, epithelial cells exposed to allergens can release alarmins such as IL-25, IL-33, and thymic stromal lymphopoietin (TSLP), which activate type 2 innate lymphoid cells (ILC2s) and drive T-helper 2 (Th2) inflammation.² Similarly, in chronic rhinosinusitis with nasal polyps, epithelial barrier defects are associated with increased susceptibility to microbial colonization and persistent inflammation.³ For COPD, chronic exposure to cigarette smoke or environmental pollutants directly damages the airway epithelium, leading to impaired mucociliary clearance, increased permeability, and a sustained inflammatory cascade that contributes to disease progression.⁴ These diseases collectively affect a significant portion of the global population, with asthma alone impacting over 300 million individuals worldwide, and COPD being a leading cause of morbidity and mortality. The prevalence of allergic rhinitis is also substantial, affecting 10-30% of adults and up to 40% of children. Understanding the common underlying epithelial dysfunction across these conditions offers a unified perspective for therapeutic development.

Evolving Therapeutic Strategies

Given the central role of epithelial dysfunction, therapeutic strategies are evolving beyond broad anti-inflammatory approaches to specifically target epithelial repair and restoration of barrier integrity. Preclinical studies have identified several pathways involved in epithelial repair, including those mediated by growth factors such as epidermal growth factor (EGF) and keratinocyte growth factor (KGF), and specific tight junction proteins.⁵

One area of focus involves agents that enhance tight junction formation. For example, certain corticosteroids have been shown to improve epithelial barrier function in vitro, independent of their anti-inflammatory effects.⁶ Furthermore, novel small molecules and biologics are under investigation that aim to directly modulate epithelial cell proliferation, differentiation, and repair processes. For instance, inhibitors of specific proteases, which can degrade tight junction proteins, are being explored as a means to prevent barrier disruption.⁷ These proteases, such as matrix metalloproteinases and serine proteases, are often upregulated in inflammatory airway conditions and contribute to tissue remodeling and barrier compromise. Targeting their activity could offer a direct mechanism to protect and restore epithelial integrity. Research in this area often involves in vitro models using primary human airway epithelial cells cultured at an air-liquid interface, allowing for the assessment of barrier function through transepithelial electrical resistance (TEER) measurements and permeability assays.

Another approach involves targeting the alarmins released by damaged epithelial cells. Biologics that neutralise TSLP, IL-25, or IL-33 have demonstrated efficacy in reducing type 2 inflammation in asthma and chronic rhinosinusitis. Tezepelumab, an anti-TSLP monoclonal antibody, has shown reductions in asthma exacerbations and improvements in lung function, indicating that upstream targeting of epithelial-derived mediators can have significant clinical benefits.⁸ While these agents primarily target the downstream consequences of epithelial activation, their success underscores the importance of the epithelium as the initial orchestrator of inflammation. These therapies are typically administered to patients with severe, uncontrolled forms of these diseases, often those who do not respond adequately to conventional treatments like inhaled corticosteroids and long-acting bronchodilators. Patient populations for these biologics are carefully selected based on clinical criteria and often biomarker profiles, such as elevated eosinophil counts or fractional exhaled nitric oxide (FeNO), which indicate underlying type 2 inflammation.

Limitations in current understanding include the precise mechanisms by which various environmental factors induce epithelial damage and the heterogeneity of epithelial responses across different airway diseases. Future research will need to delineate specific epithelial endotypes that may respond differentially to targeted therapies. Furthermore, developing non-invasive biomarkers of epithelial barrier integrity and function will be crucial for identifying patients most likely to benefit from these evolving treatments and for monitoring therapeutic response. The long-term efficacy and safety of epithelial-targeted therapies also require further investigation in larger clinical trials. A deeper understanding of the genetic predispositions and epigenetic modifications that influence epithelial susceptibility and repair capacity is also needed. Current diagnostic tools often rely on indirect measures of inflammation or lung function, rather than direct assessments of epithelial health. Therefore, the development of novel imaging techniques or molecular assays capable of quantifying epithelial barrier integrity in vivo would represent a significant advancement, enabling more precise patient stratification and personalized treatment approaches.