Effect of selected splenic irradiation on growth of meth A-fibrosarcoma in mice and partial characterization of splenic effector and suppressor cell populations.

Meth A-fibrosarcoma bearing BALB/c mice were subjected to selected splenic irradiation (2.0-4.0 Gy) on days 7 and 14 of tumor growth. Tumor growth was recorded by serial measurement. Irradiation given on day 7 caused regression of tumor, but irradiation given on day 14 did not show tumor regression. Antitumor activity in the Winn assay was detected in spleen cells 3 days after irradiation, but was not detected 7 days after. The cell surface phenotypes were analyzed on days 3, 7 and 14 of splenic irradiation using monoclonal antibodies (anti-Thy1.2 antibody, anti-Lyt1 antibody, anti-Lyt2 antibody, anti-L3T4 antibody) by flow cytometry. Thy 1.2, Lyt1, and L3T4 cells were increased on day 3 of splenic irradiation, but were not on days 7 and 14. Lyt2-cells did not show increase on days 3, 7 and 14. It was possibly suggested that selected splenic irradiation induced tumor regression was caused by the ability of irradiation to preferentially eliminate suppressor T cells, thereby allowing effector T-cells to become relatively dominant.