A rat heart allograft model, for induction of transplantation tolerance by a single dose of antithymocyte globulin, was studied. Experiments were made to transfer the tolerance to syngeneic recipients, either by spleen cells or sera obtained from tolerant rats. Different doses of tolerant spleen cells caused long-term surviving (LTS) allografts when transferred to sublethally irradiated recipients. Spleen cell transfer to untreated recipients resulted in prolonged allograft survival but no LTS grafts. The results indicate the presence of a cell-mediated suppressor mechanism in the tolerant animals.
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