Suppressor cells in transplantation tolerance. I. Analysis of the suppressor status of neonatally and adoptively tolerized rats.

The lymphocytes from neonatally tolerant rats which adoptively transfer tolerance to sublethally irradiated recipients do so by specifically suppressing the regeneration of alloreactivity which normally occurs after irradiation. Although tolerant cells will only partially suppress normal alloreactive cells when the two are mixed in near equivalent numbers, experiments in which the interval between injection of tolerant and normal cells into irradiated recipients was gradually extended, indicated that total suppression of normally alloreactive cells was achieved after 8 weeks of prior residence of tolerant cells in the adoptive host. Further evidence that tolerant cells would only suppress if present in excess of normal cells was obtained by reducing the tolerant cell population in tolerant donor rats by whole body irradiation. These animals then lost their ability to suppress normal alloreactive cells administered to them. The immune status of adoptively tolerized animals did not mimic that of the donors of the tolerant cells. Even where full tolerance, as measured by skin graft survival, failure to synthesize alloantibodies, and capacity to further transfer skin graft tolerance to secondary recipients, was evident the lymphocytes of these animals showed considerable graft-versus-host (GVH) reactivity. The persistence of tolerance through repeated adoptive transfers was correlated with the persistence of donor (chimeric) cells and the indicator skin graft on adoptive recipients only amplified tolerance expression where the inocula of tolerant cells given was weakly suppressive. Finally, removal of the minor population of chimeric cells from tolerant inocula using cytotoxic alloantisera abolished the capacity to transfer tolerance. These results imply an active role for chimeric cells which is best understood as an immune response involving proliferation driven by the idiotypes of the alloreceptors on host cells.