Temporal response of leukocyte accumulation in the thoracic cavity after two types of surgical injury

To characterize pleural healing, we quantitated leukocyte accumulation in the pleural cavity and histological changes after two types of thoracic surgery. Rabbits underwent intercostal thoracotomy followed by abrasion of the parietal pleura and ligation and resection of the right middle lobe of the lung (group A), or only abrasion of the parietal pleura (group B). After surgery, the influx of leukocytes into the pleural exudate was characterized by an increase in the number of polymorphonuclear neutrophils (PMNs) followed by monocytes/macrophages. In group A, the total number of leukocytes reached maximum levels on days 5-7 after surgery, 80% of which were monocytes/ macrophages. In group B, the total number of leukocytes reached peak levels on postsurgical day 3, 85% of which were monocytes/macrophages. Histologically, we observed a relative delay in pleural healing in group A compared with group B. An inflammatory response including appearance of fibrinous exudates and infiltration of acute inflammatory cells occurred in group A on days 1-3 after surgery. On days 5-7, an increase in submesothelial connective tissue was seen. An increase in cellularity was observed in this layer (fibroplasia) and the wound surface was covered by macrophagelike cells. In group B, disappearance of fibrinous exudates and fibroplasia occurred by day 3. In both groups, these histological changes from inflammatory phase to proliferative phase occurred on the day when the number of monocytes/macrophages in the pleural cavity reached peak levels. These data demonstrate that different types of thoracic injury alter the kinetics of leukocyte accumulation in the pleural cavity and the healing process of parietal pleura, suggesting that macrophages that accumulate in the pleural cavity may be implicated in postsurgical repair.