Ionizing Radiation Induces Extracellular Trap Release from Macrophages

Macrophages are key innate immune cells in the host defense against pathogens. Ionizing radiation can impair macrophage functions such as phagocytosis and activate them, potentially exacerbating tissue injury. Macrophage extracellular traps (METs) are formed upon stimulation of macrophages with PAMPs or DAMPs. We hypothesized that macrophages exposed to ionizing radiation can release extracellular traps. Peritoneal macrophages were collected from C57BL/6 mice and subjected to 5 Gy radiation. We performed assays to detect METs, including the immunofluorescence of citrullination of histone H3 and cell-free DNA measurement in cell culture medium as well as cell death. The exposure of ionizing radiation killed a significant number of mouse peritoneal macrophages through pyroptosis, which was mediated by Gasdermin D (GSDMD). The onset of pyroptosis eventually caused METs by suicidal METosis via pyroptosis and vital METosis occurring in the cells surviving after exposure to radiation. We found that exposure of peritoneal macrophages to 5 Gy radiation significantly increased METosis, as revealed by increased levels of citrullinated histone H3 and an increased surface area of extracellular DNA surrounding the cells. We discovered that peptidyl arginine deiminase (PAD) 2 and 4 are required for peritoneal macrophages to generate extracellular traps in response to radiation exposure. Our data demonstrate that the ionizing radiation induces METs via the activation of GSDMD, and we confirmed the requirement of PADs for METosis after exposure to the ionizing radiation. Targeting METs may direct a new therapeutic strategy for mitigating radiation-induced tissue injury.