Cell-cell contact during gamma irradiation is not required to induce a bystander effect in normal human keratinocytes: evidence for release during irradiation of a signal controlling survival into the medium.

Killing of unirradiated cells by medium from cultures of irradiated cells implies the release of a cytotoxic substance by the irradiated cells. The finding of the gamma-ray-induced cytotoxic effect exclusively in epithelial cells and not in fibroblasts suggested that tissue architecture or cell communication might be important. Normal human keratinocytes and fibroblasts and radiosensitive carcinoma cells were irradiated as single cells, …