Potentiation of chemically induced lung fibrosis by thorax irradiation

Intraperitoneal injection of butylated hydroxytoluene (BHT) causes epithelial cell death, followed 2-4 days later by extensive proliferation of type II alveolar cells in mouse lung. Five to 8 days after BHT, most dividing cells are capillary endothelial cells or interstitial cells. In animals that were exposed to 200 rad thorax irradiation immediately or 1 day after BHT, lung hydroxyproline was increased 2 weeks later. The response was dose dependent, and the interaction between BHT and thorax irradiation was synergistic. Light microscopy showed abnormal accumulation of collagen in the alveolar septa. Lung hydroxyproline was not increased in animals that were irradiated 6 days after BHT, compared to animals treated with BHT alone. We concluded that fibrosis develops if lung is damaged by a blood-borne agent and radiation to the thorax occurs at a time when it may compromise alveolar reepithelialization. Exposure to X-rays during proliferation of capillary endothelial cells or interstitial cells does not enhance development of fibrosis.