Lethally irradiated F1 hybrid mice were given an i.v. injection of parental strain spleen cells. Six days later, their spleen cells were used as the effector cells to measure the in vitro cell-mediated cytotoxicity (CMC) of the parental cells. The treatment of the donors with hydrocortisone resulted in a marked decrease of the capacity of their spleen cells to produce a CMC reaction, whereas the treatment with antithymocyte serum (ATS) resulted in an almost complete loss of such activity. The mixing of spleen cells from hydrocortisone-treated parental donors with the spleen cells from ATS-treated parental donors before injection resulted in a synergistic amplification of the cytotoxic response. The anti-Thy-1 serum treatment of either spleen cell population abolished the synergism completely. These results indicate that cortico-resistant T cells act as precursors of cytotoxic lymphocytes and that ATS-resistant T cells produce an amplification of their reaction.

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This work was supported by grants from the Medical Research Council of Canada, the National Cancer Institute of Canada, and the Manitoba Heart Foundation.

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