Radiation-induced adaptive response is not seen in cell lines showing a bystander effect but is seen in lines showing HRS/IRR response

Purpose: A previous study comparing the bystander effect and low-dose hypersensitivity found that they were inversely correlated. In the current study seven cell lines with established bystander effect and hyper-radiosensitivity/increased radioresistance (HRS/IRR) were further screened for the presence of an adaptive response. Materials and methods: Cell survival after exposure to direct radiation with or without a 0.1Gy priming dose, was determined using the colony forming assay for seven human cell lines (HaCAT, HPV-G, SW48, T98G, U373, HGL21 and HT29). Furthermore, the impact of the bystander effect on cell survival after exposure to irradiated cell culture medium was measured concurrently. Results: An adaptive response was induced in four cell lines (U373, T98G, HGL21 and HT29) causing increased cell survival. In agreement with previous publications, a bystander effect was induced in three cell lines (HPV-G, HaCAT and SW48); while no bystander effect was seen in U373, T98G, HGL21 and HT29. An adaptive response was detected in cell lines known to produce hypersensitive response, and was inversely correlated with the bystander effect. Conclusions: These results suggest that for the cell lines tested the ability to induce an adaptive response may be mutually exclusive to the bystander effect.