Evidence for pronounced bystander effects caused by nonuniform distributions of radioactivity using a novel three-dimensional tissue culture model

A new in vitro multicellular cluster model has been developed to assess the impact of nonuniform distributions of radioactivity on the biological resp onse of mammalian cells, and the relative importance of bystander effects c ompared to conventional radiation effects. Chinese hamster V79 cells are la beled with tritiated thymidine ([H-3]dThd), mixed with unlabeled V79 cells, and centrifuged gently to form multicellular clusters about 1.6 mm in diam eter. The short range of the H-3 beta particles effectively allows only sel f-irradiation of labeled cells and no cross-irradiation of unlabeled cells, The percentage of labeled cells is controlled precisely by varying the numb er of labeled cells mixed with unlabeled cells prior to assembling the clus ter, The clusters are assembled in the absence or presence of 100 mu M lind ane, a chemical that is known to inhibit formation of gap junctions. After the clusters are maintained at 10.5 degrees C for 72 h, the cells are dispe rsed and plated for colony formation. In the case of 100% labeling, the sur vival of cells in the cluster depends exponentially (SF = e(-A/1.8)) on the cluster activity A (in kBq), and lindane has no effect on the response. ii two-component exponential response is obtained for 50% labeling in the abs ence of lindane (SF = 0.33 e(-A/0.81) + 0.67 e(-A/11.8)), and lindane has a marked effect on the response (SF = 0.33 e(-A/1.6) + 0.67 e(-A/41.6) ). Th ese data suggest that bystander effects play an important role in the biolo gical response of V79 cells when the H-3 is localized in the cell nucleus a nd distributed nonuniformly among the cells. In contrast, bystander effects cannot be detected above traditional radiation effects (i.e. direct + indi rect) when the H-3 is localized in the cell nucleus and distributed uniform ly among the cells. These results indicate that this multicellular cluster model is well suited for studying the effects of nonuniform distributions o f radioactivity, including bystander and "hot-particle" effects. Furthermor e, these results suggest that by-stander effects may play an important role in the prediction of the biological effects of radiopharmaceuticals used i n medical diagnosis and treatment. (C) 1999 by Radiation Research Society.