Objective:To investigate whether irradiated U251 glioma cells can induce bystander effects in unexposed neural stem cells (NSCs) thus affecting its proliferation, stemness and differentiation.Methods:The cells were divided into NSCs group, NSCs+ U251 group (co-cultured with U251) and NSCs+ IR U251 group (co-cultured with 10 Gy irradiated U251). Glioma cells and NSCs were co-cultured in a transwell insert set. Cell counting and neurosphere diameter measuring were carried out to evaluate the proliferation and neurosphere formation ability of NSCs. Immunofluorescence assay was performed to detect the expression of Nestin protein to evaluate the stemness maintenance of NSCs, and to measure the expression levels of Tuj1 and GFAP proteins, the number of neuronal dendrites, synaptic length, the number of glial protrusions, as well as the length of glial protrusions.Results:The number of NSCs cultured with irradiated U251 cells was obviously smaller than that of NSCs cultured with sham-irradiated U251 cells ( t=2.52, P<0.05). The neurosphere formation ability of NSCs and the percentage of Nestin positive NSCs after co-culture with irradiated U251 cells significantly reduced in comparison with those after co-culture with sham-irradiated U251 cells ( t=-3.50, P<0.05). The percentages and the extent of NSCs differentiating into neuronal cells and glial cells( t=6.09, P<0.05)decreased obviously after co-culture with irradiated U251 cells in comparison with those after co-culture with sham-irradiated U251 cells. Conclusions:Irradiated glioma cells can significantly inhibit the proliferation, stemness and differentiation of unexposed NSCs due to bystander effect.