Role of intrathymic rat class II+ cells in maintaining deletional tolerance in xenogeneic rat→mouse bone marrow chimeras

Background, Mixed xenogeneic bone marrow chimerism and tolerance can be induced in mice conditioned with a nonmyeloablative regimen followed by injection of T cell-depleted rat bone marrow cells. We hypothesized that, despite a gradual decline in rat hematopoiesis observed in these chimeras, as long as rat class II+ antigen-presenting cells remain in their thymi, tolerance will persist as a result of deletion of donor-reactive thymocytes. Methods. The level of chimerism and of mouse V beta 5 and V beta 11 T-cell deletion was followed over time. These results were correlated with the presence of rat class II+ cells in the thymus by immunohistochemistry and the presence of tolerance in long-term chimeras by in vivo and in vitro assays. Results. (1) Proliferation and cytotoxicity assays, as well as skin graft survival, demonstrated the presence of specific tolerance to host and to donor rat, with normal reactivity to third-party rat and mouse stimulators, even as late as 85 weeks after bone marrow transplantation, (2) The absence of mature V beta 5(+) and V beta 11(+) host T cells in the thymus and periphery was always associated with the presence of rat class II+ cells in the thymus, and incomplete deletion of T cells expressing these V beta families was observed in thymi in which rat class II+ cells were not detectable. Conclusions. Donor-specific T-cell tolerance is maintained during the period when donor-type reconstitution declines, and is most likely mediated by intrathymic clonal deletion of T cells that recognize antigens expressed on class II+ rat cells.