X-ray-induced radioresistance against high-LET radiations from accelerated heavy ions in mice.

Induction of an adaptive response by priming X rays in combination with challenge irradiations from high-LET accelerated heavy ions was attempted in young adult female C57BL/6J Jms mice using 30-day survival after the challenge irradiations as an index. Three kinds of accelerated heavy ions from monoenergetic beams of carbon, silicon and iron ions with LETs of about 15, 55 and 200 keV/μm, respectively, were examined. A priming low dose of 0.50 Gy X rays in combination with a challenging dose of 7.50 Gy was used in the animals serving as a positive control group to confirm the successful induction of an adaptive response. The priming low dose of 0.50 Gy X rays was also used in combination with accelerated heavy ions. The priming low dose of X rays significantly reduced the mortality from the high challenge doses of carbon or silicon particles but not from iron particles. These results indicate that an adaptive response could be induced by priming low-LET X rays in combination with subsequent challenge high-LET irradiations from certain kinds of accelerated heavy ions, and successful induction of an adaptive response would possibly be an event related to the LET and/or the type of heavy ions. This is the first time that the existence of an adaptive response induced by low-LET X rays against high-LET whole-body irradiation in mice has been demonstrated. These findings would provide new insight into the radiation-induced adaptive response in vivo.