Monophosphoryl lipid A ameliorates radiation-induced lung injury by promoting the polarization of macrophages to the M1 phenotype

Background: Radiation-induced lung injury (RILI) often occurs during clinical chest radiotherapy and acute irradiation from accidental nuclear leakage. This study explored the role of monophosphoryl lipid A (MPLA) in RILI. Materials and Methods: The entire thoracic cavity of C57BL/6N mice was irradiated at 20 Gy with or without pre-intragastric administration of MPLA. HE staining, Masson trichrome staining, and TUNEL assay were used to assess lung tissue injury after treatment. The effect of irradiation on the proliferation of MLE-12 cells was analyzed using the Clonogenic assay. The effect of MPLA on the apoptosis of MLE-12 cells was analyzed using flow cytometry. Expression of gamma-H2AX and epithelial-mesenchymal transition (EMT) markers in MLE-12 cells was detected by immunofluorescence and Western blot, respectively. Results: MPLA attenuated early pneumonitis and late pulmonary fibrosis after thoracic irradiation and reversed radiation-induced EMT in C57 mice. MPLA further promoted proliferation and inhibited apoptosis of irradiated MLE-12 cells in vitro. Mechanistically, the radioprotective effect of MPLA was mediated by exosomes secreted by stimulated macrophages. Macrophage-derived exosomes modulated DNA damage in MLE-12 cells after irradiation. MPLA promoted the polarization of RAW 264.7 cells to the M1 phenotype. The exosomes secreted by M1 macrophages suppressed EMT in MLE-12 cells after irradiation. Conclusion: MPLA is a novel treatment strategy for RILI. Exosomes derived from macrophages are key to the radioprotective role of MPLA in RILI.