Early changes in lung perfusion, among other factors initiate, the development of hypoxia and chronic oxidative stress after irradiation. Tissue hypoxia is associated with a significant increase in the activation of macrophages and their continuous production of reactive oxygen species, stimulating the production of fibrogenic and angiogenic cytokines, and maintaining the development of chronic radiation-induced lung injury.

The mouse has been used extensively to model radiation injury to the lung, a major dose-limiting organ for radiotherapy. Substantial differences in the timing and sensitivity of this tissue between mouse strains have been reported, with some strains, including C57BL/6, being designated as "fibrosis-prone". Pleural effusions have also been reported to be a prominent problem in many mouse strains, but it remains unclear how this affects the lung function and survival of the standard C57BL/6 mouse. The purpose of this investigation was to re-evaluate this strain in comparison with C57L and CBA mice after whole-thorax irradiation at doses ranging from 10 to 15 Gy. Breathing rate measurements, micro-computerized tomography, lung tissue weight, pleural fluid weight and histopathology showed that the most prominent features were an early phase of pneumonitis (C57L and CBA) followed by a late incidence of massive pleural effusions (CBA and C57BL/6). A remarkable difference was seen between the C57 strains: The C57L mice were exquisitely sensitive to early pneumonitis at 3 to 4 months while C57BL/6 mice showed a delayed response, with most mice presenting with large accumulations of pleural fluid at 6 to 9 months. These results therefore caution against the routine use of C57BL/6 mice in radiation lung experiments because pleural effusions are rarely observed in patients as a consequence of radiotherapy. Future experiments designed to investigate genetic determinants of radiation lung damage should focus on the high sensitivity of the C57L strain (in comparison with CBA or C3H mice) and the possibility that they are more susceptible to pulmonary fibrosis as well as pneumonitis.

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The human lung is among the most sensitive and critical tissues of concern in localized and systemic radiation exposures, and it is a subject of active preclinical research for evaluating mitigating therapies within the radiation countermeasures program. Our previous study comparing C57BL/6, CBA and C57L mice after whole-thorax irradiation pointed to the problems of late pleural effusions that prevented the full development of lung injury in C57BL/6 mice and suggested that the CBA and C57L strains are more favorable for modeling lung injury in humans (Jackson et al., Radiat. Res. 173, 10–20, 2010). We extended these comparisons to include three other mouse strains (BALB/c, C57BR/J and A/J mice) irradiated with 10, 12.5 or 15 Gy. Most of these mice were unable to survive the first 6 months and presented with a mixture of lung injury and pleural effusions as determined from gross pathology, histology and micro-CT. The independent and varying development of compressive pleural effusions of ill-defined etiology represents a concern for these strains in that they may not satisfy the preclinical requirements for approval of medical countermeasures (e.g. radiation mitigators) for human use. Thus, among the various different mouse strains studied so far for these pathologies, only three (CBA, C3H and C57L) appear to be desirable in exhibiting an early wave of pulmonary dysfunction attributed exclusively to radiation pneumonitis and for further assessment of radioprotective and mitigating therapies. C57L mice are particularly relevant in that they show significant lung damage at lower radiation doses that are closer to what is predicted for humans.