Heat-killed salmonella typhimurium (HKST) protects mice against radiation in TLR4-dependent manner

It is urgently required to develop novel safe and effective radioprotectors to alleviate radiation damages. Recently, several toll like receptors (TLRs), including TLR2, TLR4, TLR5, TLR9, have been proved to exert protective effects against ionizing radiation. Due to different tissue-distribution and distinct functions of TLRs, we hypothesized that co-activation of multiple TLRs simultaneously may produce extensive and stronger radioprotective effects. In this study, we found the co-agonist of TLR2, TLR4 and TLR5, heat-killed salmonella typhimurium (HKST) significantly inhibited radiation-induced cell apoptosis, increased cell survival and alleviated DNA damage. HKST also prolonged animal survival and protected radiosensitive tissues against radiation damages, such as bone marrow, spleen and testis. Decrease of CD4+ and CD8+ cells were also reversed by HKST treatment. By using TLR2 and TLR4 knockout mice, we found that most of radioprotective effects of HKST were abrogated in TLR4 knock out mice. And HKST failed to inhibited cell apoptosis in TLR5 knock down cells. In conclusion, we demonstrated that HKST effectively protected cells and radiosensitive tissues against radiation injury in a TLR4 biased mechanism, suggesting HKST as a potential radioprotector with low toxicity.