The results of this study confirm results published by others by showing that sublethal whole-body irradiation of mice bearing immunogenic tumors can result in complete tumor regression. The results show, in addition, that irradiation-induced tumor regression can be prevented by infusion, after irradiation, of Ly-1+,2-,L3T4+ suppressor T cells from the spleens of donors bearing an established tumor, but not by infusion of normal spleen cells. This evidence, plus the demonstration that irradiation fails to cause regression of tumors growing in immunocompetent mice, is consistent with the hypothesis that irradiation-induced regression is immunologically mediated, and that it depends on the ability of irradiation to preferentially eliminate suppressor T cells. By using passive transfer assays to measure the production of effector T cells and suppressor T cells against time of tumor growth, it was shown that irradiation of tumor-bearing mice on day 5 of tumor growth resulted in a failure to generate suppressor T cells on the one hand, and in a sustained production, effector T cells on the other. In other words, irradiation prevented the concomitant antitumor immune response from being downregulated by suppressor T cells. However, giving radiation on day 1 of tumor growth, in contrast to giving it 3-6 d later, caused immunodepression and enhancement of tumor growth. This is in keeping with published evidence showing that, whereas resting effector T cells are highly radiosensitive, antigen-activated effector T cells are relatively radioresistant. It is suggested that the radioresistance of activated effector T cells, coupled with the radiosensitivity of activated suppressor T cells, is the reason for the selectivity of ionizing radiation for suppressor T cells and why a tumor needs to be palpable to undergo regression in response to radiation therapy.

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