Alveolar macrophage proliferation in situ after thoracic irradiation of rats

Saline lavage was performed on rat lungs at weekly intervals for as long as 6 wk after thoracic irradiation (24 Gy). The number of alveolar macrophages recovered by lavage was significantly increased compared with that in control animals beginning at 4 wk after irradiation (p less than 0.05). Autoradiographic analysis of macrophages recovered demonstrated an increased labeling index compared with that in control animals beginning 3 wk after irradiation (p less than 0.05). Macrophage proliferation in vivo was also assessed by injecting rats with vincristine and evaluating macrophages recovered by lung lavage for arrested mitoses. The number of arrested mitoses noted was significantly increased in rats at 4 wk after irradiation compared with that in control animals (p less than 0.05). These data indicate that after high dose thoracic irradiation there is an expansion of the alveolar macrophage population that is due at least in part to increased local proliferation of alveolar macrophages.