Split tolerance in spleen chimeras.

Transferring small doses of T cells to heavily irradiated F1 mice expressing isolated MHC class I or class II differences invariably leads to rapid death from graft-vs-host disease (GVHD). Paradoxically, GVHD is mild or absent when irradiated F1 mice are reconstituted with large doses of unseparated parental strain spleen cells. This applies when bulk populations of B6 spleen cells are transferred to irradiated class II-different (B6 x bm12)F1 mice or class I-different (B6 x bm1)F1 mice. In this study, we examined whether the donor T cells in long-term spleen chimeras become tolerant to host MHC Ags. On the basis of skin-allograft rejection and induction of GVHD on adoptive transfer, the results show that the donor T cells display strong tolerance to host antigens; this applies to CD4+ cells in class II-different chimeras and to CD8+ cells in class I-different chimeras. In marked contrast to the profound tolerance seen by these in vivo parameters, little or no tolerance is observed in standard in vitro assays. The results illustrate that typical in vitro tests for alloreactivity are an imprecise guide to physiologic tolerance of T cells in vivo.