Abstract: Objective To investigate the effect of ionizing radiation on the expression of NKG2D ligand on the surface of oral squamous cell carcinoma cell line SCC25 and its cytotoxicity to tumor cells. Methods When SCC25 cells were cultured into logarithmic growth phase, they were randomly designed as control (without treatment) and experimental group (2 Gy ionizing radiation treatment) by drawing lots. Flow cytometry was used to detect the expressions of NKG2D ligands major histocompatibility complex class Ⅰ chainrelated molecule (MIC)A, MICB, UL16 binding protein (ULBP)1 on the surface of SCC25 in the control group and the experimental group cultured for 24 h. Real-time fluorescence quantification polymerase chain reaction (RT-PCR) was used to detect the changes of NKG2D ligand mRNA expression on SCC25 cell surface after 24 h culture in the experimental group and the control group. The cells were prepared and divided into blank control group (NC), 2 Gy ionizing radiation group (R), NK1 group (target ratio was 5∶1), NK2 group (target ratio was 20∶1), NK1+R group (target ratio was 5∶1, 2 Gy ionizing radiation), NK2+R group (target ratio was 20∶1, 2 Gy ionizing radiation). After each group was cultured for 24 h, the killing abilities of ionizing radiation and natural killer (NK) cells to oral squamous cell carcinoma SCC25 cells were detected by CCK8. Results Flow cytometry experiment showed that, among the NKG2D ligands, the MICA fluorescence values of experimental group and control group were respectively 21.04±0.39, 22.90±0.40 (t=2.465, P=0.069), MICB fluorescence values were 27.58±0.50, 29.83±1.05 (t=1.936, P=0.125), and ULBP1 fluorescence values were 21.04±0.40, 21.78±0.50 (t=1.154, P=0.313). This indicated that after ionizing radiation on SCC25, the NKG2D ligand MICA, MICB, ULBP1 expression increased slightly, but the differences were not statistically significant. RT-PCR indicated that mRNA expressions of MICB, ULBP1 were significantly different between the control group and the experimental group (t=18.334, P=0.000; t=6.381, P=0.008). The expressions of the experimental group were respectively 6.49, 1.64 times as those of the control group. The results of CCK8 showed that, there was a significant difference in cell killing ability among NK1 group, NK2 group and NC group (F=344.600, P=0.000), suggesting that NK cells could kill tumor cells, and the higher ratio of NK cells and SCC25, the stronger killing effect. The comparison between R group and NC group showed that the difference in cell killing ability was not statistically significant (P=0.567). NK1+R group and NK1 group were compared and the difference was not statistically significant (P=0.915). There was no significant difference between NK2+R group and NK2 group (P=0.678). This showed that the killing effect of ionizing radiation was weak. Conclusion  Ionizing radiation can increase the mRNA expression of NKG2D ligands MICB and ULBP1. This may provide a new way for tumor immunotherapy. The killing effect of ionizing radiation on cells is not obvious. It may be related to low radiation dose and only 24 h for cell culture.

Key words: Radiation, ionizing, Killer cells, natural, NKG2D ligands