Interactions of Apoptotic Cells with Macrophages in Radiation-Induced Bystander Signaling

Nontargeted effects that result in ongoing cellular and tissue damage show genotype-dependency in murine models with CBA/Ca, but not C57BL/6, exhibiting sensitivity to induced genomic instability. In vivo, radiation exposure is associated with genotype-dependent macrophage activation, and these cells are a source of bystander signaling involving cytokines and reactive oxygen and nitrogen species. The mechanisms responsible for macrophage activation and production of damaging bystander signals after irradiation are unclear. Macrophages from CBA/Ca exhibit an M1 (proinflammatory) phenotype compared to the M2 (anti-inflammatory) phenotype of C57BL/6 macrophages. Using the murine RAW264.7 macrophage-like cell line, we show that the ability of macrophages to interact with apoptotic cells and their responses to interaction varies significantly according to macrophage phenotype. Nonstimulated and M2 macrophages induce anti-inflammatory markers arginase and TGFß after engulfment of apoptotic cells. In contrast, M1 macrophages do not induce anti-inflammatory responses, but express the proinflammatory markers NOS2, IL-6, TNFa, superoxide and NO, able to contribute to a damaging microenvironment. Macrophages stimulated with both inflammatory and anti-inflammatory agents prior to exposure to apoptotic cells induce a mixed response. The results indicate a complex cross-talk between macrophages and apoptotic cells and demonstrate that phagocytic clearance of apoptotic cells induced by genotoxic stress can produce microenvironmental responses consistent with the induction of a chromosomal instability phenotype in sensitive CBA/Ca mice with M1 macrophage activation, but not in resistant C57BL/6 mice with M2 macrophage activation. Modulation of macrophage phenotypes may represent a novel approach for reducing the nontargeted effects of radiation.