Abstract
Most cancer patients receive radiotherapy during the course of their disease. Improvements in the therapeutic index have been based mainly on physical improvements in delivery, as radiosensitizer development to target tumor cells has yet to yield effective agents. Recent investigations have focused on the tumor stroma as a target for radiosensitization. Here, we report that depletion of tumor-associated macrophages (TAMvarphi) by systemic or local injection of the macrophage-depleting liposomal clodronate before radiotherapy can increase the antitumor effects of ionizing radiation (IR), either as a large single dose (20 Gy) or as a fractionated dose (2 Gy x 10). Coimplantation of tumor cells with bone marrow-derived macrophages (BMDMvarphi) increased tumor radioresistance. Studies using mice with germline deletions in tumor necrosis factor receptors 1 and 2 (TNFR1,2(-/-)) or TNFalpha (TNF(-/-)), or treatment of wild-type mice with a soluble TNF receptor fusion protein (Enbrel), revealed that radioresistance mediated by BMDMvarphi required intact TNFalpha signaling. Radiation exposure upregulated vascular endothelial growth factor (VEGF) in macrophages and VEGF-neutralizing antibodies enhanced the antitumor response to IR. Thus, the radioprotective effect of TNFalpha was mediated by TAM-produced VEGF. Our findings offer a mechanistic basis to target macrophage populations generally or TNFalpha-induced macrophage VEGF specifically as tractable strategies to improve the efficacy of radiotherapy.