OZONE-INDUCED ACUTE TRACHEOBRONCHIAL EPITHELIAL INJURY - RELATIONSHIP TO GRANULOCYTE EMIGRATION IN THE LUNG

To investigate the relationship between granulocyte emigration and epithelial injury in specific airway generations of the tracheobronchial tree following short-term ozone exposure, we exposed rhesus monkeys for 8 h to 0.00 (controls) or 0.96 ppm ozone with post-exposure periods of 1, 12, 24, 72, and 168 h in filtered air before necropsy. There were five control and three exposed monkeys for each of the post-exposure times for a total of 20 monkeys. Neutrophils isolated from peripheral blood and labeled with In-111-tropolonate were infused in the cephalic vein in unanesthetized monkeys (except the 1-h group) 4 to 5 h before necropsy. The trachea and microdissected bronchi (fourth and ninth generations) and respiratory bronchioles (fifteenth generation) from the right upper lobe of each monkey were examined by electron microscopy. Labeled neutrophil influx into lung tissue and bronchoalveolar lavage fluid (BALF) was maximal at 12 h and returned to baseline by 24 h after exposure. This was in contrast to total neutrophils (labeled and unlabeled) in BALF, which were significantly elevated through 24 h after exposure but returned to baseline by 72 h. Lavage protein was significantly elevated at 24 h after exposure but was at control levels at all other times. Morphometric observations showed epithelial necrosis at 1 and 12 h in the trachea and bronchioles but continued to be observed in significant numbers at 24 h after exposure in bronchi. A significant increase in the labeling index of epithelial cells was observed at 12 h only in bronchi. Epithelial necrosis and repair was associated with the presence of granulocytes in the epithelium and interstitium of all airway levels. However, eosinophils were maximally increased in the epithelium and interstitium of bronchi at 24 h after exposure when epithelial necrosis was maximal in these airways and when lavage protein was significantly elevated. This study provides four novel observations in the evaluation of acute ozone-induced lung injury: (1) the sequence of epithelial injury and repair differed by airway generation, occurring earlier in the trachea and bronchioles than in bronchi; (2) there was a strong relationship between epithelial necrosis and the emigration and retention of neutrophils at all levels of the tracheobronchial tree; (3) there was a strong association between maximal epithelial necrosis, BALF protein levels, and granulocytes (especially eosinophils) in bronchi, but not the trachea and bronchioles; and (4) there was a strong relationship between increased mucin in BALF and loss of mucous granules in goblet cells of the tracheobronchial epithelium. We interpret these results as evidence for the contribution of granulocytes to injury and repair of the tracheobronchial epithelium after short-term ozone inhalation.