Immunologic changes after loco-regional radiotherapy and fractionated total body irradiation (TBI) in mice.

The immunologic effects of fractionated irradiation to both hind limbs and the tail of adult (2.5-3 months old) male Balb/c mice were investigated. A dose of 34 Gy given in 17 fractions of 2 Gy, 1 fraction per day, 5 days per week, was delivered with a 60Co source. A significant decrease of the total splenocyte count (29% of control value) and of the PHA(phytohemagglutinin)-induced proliferation of T cells (22% of control value) was found immediately after irradiation. Both parameters normalized within 30 days after irradiation. Immediately after irradiation, the MLC (mixed lymphocyte culture) was supranormal (126% of control value), dropped to 45% 1 week later, and normalized within 1 month after radiotherapy. The NK (natural killer) activity was significantly decreased only the first week after loco-regional irradiation, while the LAK (lymphokine activated killer) activity was not altered at all. The percentage of goat-anti-mouse+ cells (mainly B lymphocytes) was not changed immediately after loco-regional irradiation, but rose to supranormal values (175% of control level) 3 months after irradiation. A persistent decrease of the percentage and the absolute numbers of the Lyt2+ cells (= CD8+ cells, suppressor/cytotoxic phenotype) was observed up to 3 months after irradiation, while the percentage of L3T4+ cells (= CD4+ cells, helper phenotype) remained normal for the total follow-up. No differences in allogeneic skin graft survival could be demonstrated between irradiated and control animals. The observed immunological effects could not be explained by the scatter irradiation to the whole body as total body irradiation (TBI) administered in a dose and dose rate similar to the scatter dose did not result in persistent immunologic changes. No dose-rate effect could be demonstrated in a low dose fractionated total body irradiation schedule. A total body irradiation similar to the scatter dose in humans did not result in significant immunologic changes.