Effective antitumor immunity following elimination of suppressor T cell function.

An increased resistance to a transplantable tumor was demonstrable in Fischer (F344) strain rats after thymectomy and high doses of total body irradiation, followed by bone marrow cell repopulation if the animals were allowed to recover for 6 to 8 weeks. Further experiments demonstrated that removal of a subpopulation of T cells by lower doses of radiation plus thymectomy could (1) reduce recurrences after surgical excision, and (2) prolong survival of animals with established pulmonary metastases. It was found that thymectomy + 50 leads to 100 R total body irradiation could eliminate a T cell-mediated suppression of effective antitumor immunity. The removal of the suppressor cell component by lower doses of irradiation was dependent on pre-exposure of the animals to tumor antigen. These experiments indicate the specificity of suppressor T cells to tumor antigen and their exquisite sensitivity to radiation in appropriate circumstances.