Abstract
The pathogenesis of radiation pneumonitis is not completely understood. The long latent period and involvement of unirradiated lung tissue may indicate an immune reaction in the injurious process of irradiated lung. To investigate the role of pulmonary macrophages in radiation pneumonitis, morphology and membrane antigen expression of pulmonary macrophages were studied in irradiated rat lung tissue following 4000R hemithoracic irradiation. Lungs were explanted at 2, 4, 6, 8, 16, and 28 weeks after irradiation. Cryosections of irradiated lung tissue were immunohistochemically studied, and alveolar macrophages in bronchoalveolar lavage fluid were also analyzed using monoclonal antibodies to rat major histocompatibility complex (MHC) antigens and macrophages with flow cytometry. Macrophage subpopulations were analyzed using a panel of monoclonal antibodies to MHC class I (HAM2), MHC class II (OX6) and macrophage differentiation antigens (ED1, ED2, ED3). Alveolar macrophages in bronchoalveolar lavage were morphologically studied by smear and flow cytometry of forward light scatter and 90° light scatter. At 2 weeks after irradiation, when histological changes did not appear, small lymphocyte-like macrophages and large foamy macrophages were observed in both the smear and histograms by flow cytometry. At 4, 6, and 8 weeks after irradiation, these new populations had markedly increased. However, at 16 and 28 weeks after irradiation, the size and shape of alveolar macrophages had returned to normal. In the expression of macrophage membrane antigens, an increase in the frequency of MHC class II+ cells in lavaged cells appeared at 2 weeks after irradiation, and became significant at 4 weeks. In cryosections at 4 weeks, MHC class II+ mononuclear cells were increased in both alveolar and peribronchovascular spaces, and ED1+ macrophages were increased in the alveolar space. Although only a few ED3+ macrophages were observed in normal lung, the frequency and the degree of antigen expression in ED3+ macrophages appeared increased. ED2+ macrophages did not increase. At 6 and 8 weeks, these changes became marked, and ED2+ macrophages increased in number, but the degree of the increase was not as marked as in the other cells labelled with ED1 and ED3. At 28 weeks after irradiation, the increased levels of these positive cells had recovered.
In conclusion, the changes in macrophage subpopulations consisting of changes of cell morphology and cell membrane antigen expression suggest that immunologic reactions such as activation of macrophages may play an important role in the process of radiation pneumonitis.
