We added spleen cells from adult BALB/c mice treated with total lymphoid irradiation (TLI) to the mixed leukocyte reaction (MLR) using a variety of responder and stimulator cells. The spleen cells nonspecifically suppressed the uptake of [3H]-thymidine and the generation of cytolytic cells regardless of the responder-stimulator combination used. We also examined the effect of the spleen cells on the generation of antigen-nonspecific and antigen-specific suppressor cells in the MLR. The experimental results suggest that the spleen cells from TLI-treated mice inhibit the generation of nonspecific suppressor cells, but do not inhibit the generation of antigen-specific suppressor cells. Thus, alloantigenic stimulation of normal responder cells in vitro in the presence of spleen cells from TLI-treated mice generates large numbers of antigen-specific suppressor cells, but few cytolytic cells or nonspecific suppressor cells. Similar nonspecific inhibition of the MLR was observed with neonatal spleen cells. This in vitro system provides a regulatory model for the induction and maintenance of tolerance in vivo, in which adult mice given TLI or neonatal mice accept allogeneic bone marrow transplants without graft-vs.-host disease.

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Spleen cells from newborn BALB/c mice were added to the mixed leukocyte reaction (MLR) between a variety of responder and stimulator cells. The newborn cells nonspecifically suppressed the uptake of (3H)-thymidine and the generation of cytolytic cells regardless of the responder-stimulator combination used. Suppressor cell activity fell rapidly during the first 4 days after birth, and could not be detected by day 20. Newborn spleen cells inhibited the generation of nonspecific suppressor cells during the MLR but did not inhibit the generation of antigen-specific suppressor cells. Thus, newborn spleen cells exhibit a pattern of regulation of the MLR similar to that reported previously for spleen cells from adult mice given total lymphoid irradiation (TLI). These regulatory interactions provide a model that explains the ease of induction of transplantation tolerance in vivo in newborn mice and in TLI-treated adult mice.

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