NEONATALLY TOLERANT RATS ACTIVELY ELIMINATE DONOR-SPECIFIC LYMPHOCYTES DESPITE PERSISTENT CHIMERISM

Rats from the allotype-marked PVG-RT7(b) and PVG-RT1(u)-RT7(b) strains were injected at birth with semi-allogenic F-1 bone marrow (BM) cells from athymic nude rats (PVG-rnu/rnu x PVG-RT1(u)-rnu/rnu) to induce n eonatal tolerance. As adults 97% of the animals accepted donor-specifi c allogenic skin grafts and a majority (65%) of rats were chimeric/exp ressing the major histocompatibility complex class I and allogenic ski n grafts and a majority (65%) of rats were chimeric/expressing the maj or histocompatibility complex class I and allotype marker of the donor strain. Similar results obtained when PVG-RT1(u)-RT7(b) rats were inj ected at birth with fully allogenic PVG-rnu/rnu nude BM cells: as adul ts. 94% accepted donor-specific skin allografts and 76% of recipients were chimeric. Donor-derived CD4T cells. CD8T cells and B cells were f ound in low numbers (<2%) in peripheral blood of rats made tolerant by F-1 BM cells. A large proportion of T cells bore the phenotype of rec ent thymic emigrants. A large proportion of T cells bore the phenotype of recent thymic emigrants, suggesting that they were newly produced. All the evidence was consistent with clonal deletion tolerance. induc ed centrally within the thymus. The thymus was chimeric and thymocytes failed to respond in vitro to alloantigens of the donor-specific hapl otype:donor-specific skin allografts survived indefinitely on athymic nude recipients reconstituted with CD4(+) CD8(+) thymocytes or periphe ral CD4 T cells from tolerant animals. The chimeric state was interest ing. since the PVG and PVG-RT1(u) rat strains contain a natural killer (NK) cell system that rapidly eliminates (within 24 h) intravenously injected allogeneic or semi-allogenic lymphocytes - a phenomenon known as allogenic lymphocyte cytotoxicity or ALC. When neonatal tolerant r ats were tested. the ALC index (a measure of cell killing) was unchang ed in nonchimeric tolerant rats and significantly altered (reduced kil ling). but not abolished in chimeric animals. Hence, the injection of allogenic BM cells which induced specific tolerance in the T cell popu lation failed to tolerize the NK cell system. allowing the constant ki lling of newly produced donor-derived lymphocytes and putting at risk the very survival of the allogenic BM cells. This has interesting impl ications for clinical transplantation.