Tolerance induction as a multi-step process.

Tolerant T cells are characterized by their partial or full resistance to activation by antigen. We investigated whether tolerant T cells were still receptive to further tolerogenic signals. T cells expressing a transgenic T cell receptor (TCR) specific for the major histocompatibility complex (MHC) class I molecule Kb were deleted in mice carrying Kb but not in mice expressing the mutant Kb-molecule Kbm1 [TCR (H-2bm1 x k) mice]. These T cells were tolerant in vivo but could be activated in vitro by the Kb antigen. This in vitro reactivity was abolished after the tolerant T cells encountered Kb-positive cells that had been intravenously injected. Furthermore, in TCR (H-2bm1 x k) mice expressing Kb only on hepatocytes, no T lymphocytes bearing the transgenic TCR could be found in the periphery, indicating that the additional contact with Kb on hepatocytes led to deletion of the tolerant T cells. These findings demonstrate that tolerance induction can be a multi-step process.