Radiation-induced bystander effects: are they relevant for radiation protection of non-human biota?

This paper reviews our current knowledge of the mechanisms underlying the induction of bystander effects by low dose low LET ionizing radiation and discusses how they may be related to observed adaptive responses, low dose hyper-sensitivities or other effects of low dose exposures which are not correlated with dose in a simple relationship. Bystander effects appear to be the result of a generalized stress response in tissues or cells. They occur widely in many classes, including Crustacea, Molluscs, Pisces and Mammals and have been demonstrated following in vivo exposure to irradiation.. The signals may be produced by all exposed cells but the response appears to require a quoram to be expressed. The major response involving low LET radiation exposure discussed in the existing literature is a death response. This has many characteristics of apoptosis but is p53 independent. Whilst a death response might appear to be adverse, it is also possible that it could, at these low doses, be protective and remove damaged cells from the population. Since many cell populations carry damaged cells without being exposed to radiation (so called 'background damage'), it is possible that low doses exposures could cause removal of cells damaged by agents other than the test dose of radiation. This mechanism would lead to the production of 'U-shaped' dose response curves often observed in toxicology and radiobiology where a non-linear protective dose response precedes a linear or curvilinear high dose response. In this scenario, the level of 'adaptive' or beneficial response will be related to the background damage carried by the cell population. This model may be important when attempting to predict the consequences of mixed exposures involving radiation and other environmental stressors. (authors)