Mechanisms for and Effects of Airway Epithelial Damage in Asthma

The airway epithelium plays a crucial role in protecting the underlying connective tissue (CT) from noxious agents. Damage and shedding of the epithelium are observed in the airways of asthma, cystic fibrosis and rhinitis patients. The aim of this study was to investigate the mechanisms by which epithelial damage occurs, and the consequences of such damage for the inflammatory process in the airways. In this study, cultured normal human bronchial epithelial cells, excised rat tracheae, and cultured murine mast cells were used as model systems. Metabolic alterations, morphological changes and cell-cell contact stabilities were investigated. The T-helper (Th)-1 cytokines, tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ), and interleukin (IL)-1β were found to be pro-inflammatory, leading to major morphological changes, inhibitions in desmosome formation, and accelerated cell death. The Th2 cytokines, IL-4, IL-5, and IL-13 were found to cause no changes in cell death, nitric oxide levels and desmosome formation but instead an increase in proliferation, therefore were anti-inflammatory in this respect. Increasing the osmolarity of the airway surface liquid (ASL) altered the integrity of the tight junction (TJ) and allowed a 4-kDa compound to penetrate the epithelial layer and access the CT. This effect was reversible if the ASL was returned to 295 mOsm. Intentionally breaking the TJ with EGTA and subsequent osmolar changes in ASL demonstrated the importance of TJ and the fragility of the CT under hyperosmolar stress, leading to a disrupted CT with larger capillaries and altered elemental ion content and epithelial denudation. Hydrocortisone was shown to downregulate IL-4-induced IL-6 upregulation in murine mast cells. Furthermore, incubating mast cells with hydrocortisone lead to a new subpopulation that was morphologically unique, that displayed new cell surface markers (CD44 and CD61) and that lacked CD54. These changes modify the interactions of mast cells with surrounding cells in the CT and epithelium. In conclusion, the balance between pro- and anti-inflammatory cytokines and ASL osmolarity may influence the role of the airway epithelium as a barrier. The pharmacological use of hyperosmolarity to disrupt TJ reversibly may help facilitate the delivery of drugs through the airway epithelium