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Corticosteroids have previously been found to be protective against the mortality of radiation pneumonitis in mice, even when given well after lethal lung irradiation. We explored the possibility that this effect was due to their well-known anti-inflammatory actions by giving various nonsteroidal inhibitors of arachidonate metabolism to groups of mice that had received 19 Gy to the thorax (bilaterally). Treatments of four cyclooxygenase inhibitors, one lipoxygenase inhibitor, and one leukotriene receptor antagonist, given by various routes in various doses, were commenced 10 weeks after irradiation or sham irradiation and continued throughout the period when death from radiation pneumonitis occurs, 11-26 weeks after irradiation. Each of the treatments had the appropriate effect on arachidonate metabolism in the lungs as assessed by LTB4 and PGE2 levels in lung lavage fluid. The principal end point was mortality. The 5-lipoxygenase inhibitor diethylcarbamazine and the LTD4/LTE4 receptor antagonist LY 171883 markedly reduced mortality in dose-response fashion. The effects of cyclooxygenase inhibitors were divergent; piroxicam and ibuprofen were marginally protective, indomethacin in all doses accelerated mortality, and aspirin reduced mortality in a dose-response fashion. These results suggest that the protective effect of corticosteroids in radiation pneumonitis can be tentatively attributed to their anti-inflammatory actions, and that nonsteroidal anti-inflammatory agents, particularly those that affect lipoxygenase products, may offer equal or better protection than corticosteroids against mortality due to radiation pneumonitis.

We investigated the replicative activity of type 2 cells in the lungs of mice at various times from 3 to 22 weeks after 18 Gy of X rays to the thorax. No significant changes were found until 11 weeks after thoracic X irradiation. Thereafter the replicative index of type 2 cells was significantly elevated, rising four to sixfold above that of control, sham-irradiated mice. During the period when the replicative activity of type 2 cells was elevated, the breathing frequency increased and there was histologic evidence of the presence of radiation pneumonitis. The magnitude of each of these indices of pneumonitis correlated significantly with the type 2 cell replicative index, suggesting that type 2 cell replication is related to pneumonitis in extent as well as in chronology. How these changes relate to the pathogenesis of radiation pneumonitis is unclear.

We explored the protective effect of corticosteroids on the mortality of mice that received thoracic irradiation. Methylprednisolone, 100 mg/kg/week, given from 11 weeks after gamma irradiation of the thorax resulted in an increase in the LD50 (11-26 weeks) from 14.3 +/- 0.3 (mean +/- SE) Gy to 17.6 +/- 0.4 Gy, P less than 0.001, a protection factor of 1.2. Withdrawal of steroids at various times during the period of radiation pneumonitis resulted in accelerated mortality in the next 2-4 weeks, so that the cumulative mortality "caught up" with that of control animals by 4 weeks after steroid withdrawal. However, after the end of the usual period of pneumonitis withdrawal of steroids did not result in accelerated mortality, suggesting that the time when steroids are protective corresponds to the duration of pneumonitis. A smaller dose of steroids, 25 mg/kg/week, was found to be as protective as the larger dose used in the above experiments. The possibility that corticosteroids reduce mortality, even when given many weeks after radiation, may have important practical and theoretical implications.

Corticosteroid administration during radiation pneumonitis in mice markedly improves the physiologic abnormalities and decreases mortality, an effect that has been attributed to the stimulation of surfactant synthesis and secretion by type 2 alveolar epithelial cells. In the present experiments we explored the effects of corticosteroids on the replicative activity of type 2 cells of lethally irradiated lungs at the height of the radiation reaction. The labeling index of type 2 cells of irradiated mice was increased threefold above that of sham-irradiated controls. Corticosteroids given continuously from 10 weeks after thoracic irradiation further increased the type 2 cell labeling index another threefold above that of irradiated untreated mice. The enhanced reproductive activity of type 2 cells following thoracic irradiation is seen as a protective response that is augmented by corticosteroids, whose effect may be both to improve the physiology of the alveolar surface and to maintain the population of alveolar epithelial cells. The bearing of this result on the controversial role of the type 2 cell as a target in radiation pneumonitis is discussed.