[en] Objective: To explore the effects of B7-H3 down-regulation on cell cycle and apoptosis in lung cancer A549 cells. Method: Human lung cancer cell line A549 were cultured. siB7-H3 RNA which can specifically silence the expression of B7-H3 protein was transfected into A549 cells. ¹³⁷Cs γ-ray irradiation was used with a single dose of 4 Gy. Experiments included control group, siB7-H3 transfected group, irradiation group, and irradiation+siB7-H3 transfected group. After transfected with siB7-H3, Western blotting and quantitative real-time PCR assays were used to detect the expression of B7-H3 protein and mRNA in A549 cells. The cell cycle and apoptosis of A549 cells following 4 Gy irradiation were detected by flow cytometry. Results: The protein and mRNA expression of B7-H3 were significantly decreased in A549 cells transfected with siB7-H3 compared with the control group, and the differences between the two groups were statistically significant(t = -4.222, P = 0.013). siB7-H3 transfection induced significant effect on cell cycle with increase of G0/G1 phase arrest and reduction of S and G2/M phase population. A mild enhanced G0/G1 phase arrest and an obvious enhanced G2/M phase arrest of irradiation group were detected compared with the control group. An enhanced G0/G1 and G2/M phase arrest of irradiation+siB7-H3 transfected group were detected compared with the control group. Compared with the control group, the necrosis and apoptosis induction of the irradiated group significantly increased at 48 h after irradiation. However, No significant alterations of necrosis and apoptosis induction were observed between irradiation group and irradiation+siB7-H3 transfected group. Conclusions: Down-regulation of B7-H3 can significantly elevated the G0/G1 arrest in A549 cells following radiation. This conclusion indicated that the alteration of B7-H3 expression could play a key role in the regulation of the radiosensitivity of lung cancer via mediating the G0/G1 check point. (authors)