Studies on kinetics of radio adaptive responses in BALB/c mice

Radio adaptive response is a phenomenon by which exposure to sub-lethal doses of ionizing radiation, known as conditioning or priming dose (PD), leads to increased resistance to a subsequent exposure to a higher dose, known as challenge dose (CD). The characterization of radio adaptive response exhibited by immune cells is of prime importance as it can directly influence various biological mechanisms like immune response against antigen, neoplastic growth and auto immunity. In this study, we have investigated radio adaptive response following whole body radiation of BALB/c mice. Mice were irradiated with 10 cGy PD, 2 Gy CD and priming and challenging dose (P+CD) with an interval of 4 hr. Unirradiated mice served as controls. Mice were sacrificed immediately; 24 h or 72 h after challenging dose, and single cell suspension of splenocytes were prepared. The cells were cultured in presence of concanavalin A (Con A) to assess apoptosis (propidium iodide staining), expression of early activation markers, CD71 and CD69 (flow cytometry), cytokine secretion (IL-2 and IFN-γ, ELISA) and proliferation (CFSE dilution and 3H thymidine incorporation). A three fold increase in apoptosis was observed in CD and P+CD groups compared to control or PD when mice were sacrificed immediately or 24 h after CD. Twenty four hours after CD, a significant increase was found in the expression of CD69 and CD71 in P+CD as compared to CD. Similar results were obtained in terms of proliferation response and secretion of IL-2 and IFN-γ, suggesting radio adaptive response. This was observed 72 h after CD also but not immediately after CD. This pattern of radio-adaptive response was obtained in six week old mice and was different in 4-6 month old mice. In conclusion, these results demonstrate the narrow window of radio adaptive response in splenocytes of BALB/c mice following exposure to whole body irradiation. (author)