en Radiation Biology Radiation Biology تحقيق بديع Original Research <p><strong>Background:</strong> Radio-adaptive response and bystander effects are known phenomena occurring in cells following exposure to ionizing radiation (IR). In this study we examined possible radio-adaptation of lymphocytes following bystander effects induced by CHO-K1 cells. <strong>Materials and Methods:</strong> 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 lymphocytes 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 after staining in Giemsa. <strong>Results:</strong> The frequency of MN increased significantly with increasing dose of radiation 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). <strong>Conclusion:</strong> Results showed that bystander effects induced by gamma-irradiated CHO-K1 cells led to induction of radio-adaptive response in lymphocytes. The mechanism by which radio-adaptive 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-adaptive response.</p> Bystander effect, radio-adaptive response, gamma rays, micronucleus assay 151 156 http://ijrr.com/browse.php?a_code=A-10-1-553&slc_lang=en&sid=1 Sh. Shirani 790031947532846009756 790031947532846009756 No School of Paramedicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran H. Mozdarani Mozdarah@modares.ac.ir 790031947532846009757 790031947532846009757 Yes Department of Medical Genetics, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran A. Mahmoodzadeh 790031947532846009758 790031947532846009758 No Novin Medical Radiation Institute, Tehran, Iran M. Salimi 790031947532846009759 790031947532846009759 No Department of Medical Genetics, Medical Biotechnology Institute, National Institute of Genetic Engineering and Biotechnology, Tehran, Iran