Radio-adappve response and bystander effects are known phenomena occurring in cells following exposure to ionizing radiaon (IR). In this study we examined possible radio-adaptaon of lymphocytes following bystander effects induced by CHO-K1 cells. Materials and Methods: Whole blood and CHO-K1 cells were cultured in RPMI-1640 complete medium. Cells were separately irradiated with various doses of gamma rays. A co-culture was set to examine the bystander effects induced by CHO-K1 cells on

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... es exposed later to a challenge dose of 4 Gy. Treated cells were exposed to cytochalasin-B to arrest cells in cytokinesis stage. Micronucleus (MN) as end point was scored in binucleate cells a2er staining in Giemsa. Results: The frequency of MN increased significantly with increasing dose of radiaon both in lymphocytes and CHO-K1 cells (p<0.001). Although, no significant difference was observed between control non-irradiated cells and those exposed to 0.2 Gy (p>0.05). Co-culture of the non-irradiated lymphocytes with pre-irradiated CHO-K1 cells significantly reduced the mean frequency of MN in lymphocytes irradiated with a dose of 4Gy (p<0.001). Conclusion: Results showed that bystander effects induced by gamma-irradiated CHO-K1 cells led to inducon of radio-adappve response in lymphocytes. The mechanism by which radio-adappve response is induced following bystander effect is not clearly known, however cellular signaling and genome instability induced in cells indirectly might be considered as possible triggering events for radio-adappve response.