Dose-dependent tolerance to ozone. I. Tracheobronchial epithelial reorganization in rats after 20 months' exposure

Two salient features of the pulmonary response to reactive oxidant air pollutants such as ozone are the heterogeneity of response by site within the respiratory tract and the development of tolerance to injury with repeated exposure. The purpose of this study was to establish whether the development of tolerance to long-term exposure is associated with changes in the tracheobronchial epithelium. Male F344-N rats were exposed to 0, 0.12, 0.5, or 1.0 ppm ozone 6 hours/day for 5 days/week for 20 months and killed 1 week post-exposure. Samples for light microscopic morphometry were obtained by microdissection from each infusion-fixed trachea and left lung lobe and included: 1) a cranial bronchus with short path length (generation no. 4 to 5) and small diameter; 2) a central bronchus with short path length (generation no. 4 to 5) and large diameter; and 3) a caudal bronchus with long path length (generation no. 10 to 12) and small diameter. In addition, three sites within the central acinus were examined from cranial, central, and caudal regions. These sites included terminal bronchiole, 0.5 to 1 mm proximal to terminal bronchiole and bronchiolarized alveolar duct. Intraepithelial mucin storage (AB/PAS-positive material quantified by image analysis) within the trachea decreased with dose. Mucin storage was unchanged in the central bronchus, increased threefold with dose in the caudal bronchus, and was six times higher in the cranial bronchus at 1.0 ppm ozone. Epithelial composition was unchanged in trachea or any bronchi; however, we noted a significant dose-dependent increase in nonciliated cell mass and volume fraction in terminal bronchioles in all three regions. There was also a significant increase in nonciliated cell mass in the bronchiolarized alveolar ducts. Bronchiolar nonciliated cells were identified greater than fourfold further into alveolar ducts in 1.0 ppm exposed than in 0 ppm animals. Nonciliated cells occurred almost 200 microns deeper into alveolar ducts in cranial regions than in caudal regions. We conclude: 1) that the development of tolerance to injury produced by long-term exposure to ozone involves changes in airway epithelium and 2) that these changes are site specific and involve alterations in both secretory activity and cellular composition.