Proliferative response of bystander cells adjacent to cells, with incorporated radioactivity

Background: In a recent study, we showed that cells irradiated with γ-rays stimulate cell growth of unirradiated (bystander) cells, when the two populations are co-cultured as a mixture (Cytometry 2003;54A:1-7). Direct cell-to-cell contact appears to be a prerequisite for the proliferative response of the bystander cells (Cytometry, 2003 56A:71-80). The aim of the current work is to investigate the possible proliferative bystander effects caused by intracellular irradiation with incorporated radionuclides, specifically the short-range β particle emitter, tritium (³H). Methods: Subconfluent monolayers of rat liver epithelial cells (WB-F344) were incubated in the presence of (methyl-³H)thymidine (³HTdR) at concentrations ranging between 5.2 kBq/ml and 57.8 kBq/ml for 18 h. Radiolabeled cells, containing between 0.7 × 10^-3 Bq/cell and 8.8 × 10^-3 Bq/cell were mixed with unlabeled (i.e., bystander) cells in a ratio of 1:1 and cultured together for 24 h followed by an flow cytometry (FCM) study of their proliferation. In order to discriminate the two populations of co-cultured cells, one cell population (unlabeled bystander cells) was stained with carboxyfluorescein diacetate, succinimidyl ester (CFDA SE), which metabolizes intracellularly. The absorbed doses received by the radiolabeled cells that contained 0.7 × 10^-3, 2.5 × 10-3, and 8.8 × 10-3 Bq/cell were 0.14, 0.49, and 1.7 Gy, respectively. Results: Cells that were not treated with tritiated thymidine (unlabeled cells), in the presence of radiolabeled cells that received absorbed doses from 0.14-1.7 Gy, showed enhanced cell growth by approximately 9 to 10%. Conclusions: Cells labeled with ³HTdR can induce increased proliferation in neighboring unlabeled bystander cells. FCM provides an excellent basis for characterization of proliferative bystander effects in co-culture systems.