Neonatally tolerant rats actively eliminate donor‐specific lymphocytes despite persistent chimerism

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Abstract
Rats from the allotype‐marked PVG‐RT7b and PVG‐RT1u‐RT7b strains were injected at birth with semi‐allogeneic F1 bone marrow (BM) cells from athymic nude rats (PVG‐rnu/rnu x PVG‐RT1u‐rnu/rnu) to induce neonatal tolerance. As adults, 97% of the animals accepted donor‐specific allogeneic skin grafts and a majority (65%) of rats were chimeric, expressing the major histocompatibility complex class I and allotype marker of the donor strain. Similar results were obtained when PVG‐RT1u‐RT7b rats were injected at birth with fully allogeneic PVG‐rnu/rnu nude BM cells: as adults, 94% accepted donor‐specific skin allografts and 76% of recipients were chimeric. Donor‐derived CD4T cells, CD8T cells and B cells were found in low numbers (< 2%) in peripheral blood of rats made tolerant by F1 BM cells. 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, induced centrally within the thymus. The thymus was chimeric and thymocytes failed to respond in vitro to alloantigens of the donor‐specific haplotype; donor‐specific skin allografts survived indefinitely on athymic nude recipients reconstituted with CD4+CD8− thymocytes or peripheral CD4T cells from tolerant animals. The chimeric state was interesting, since the PVG and PVG‐RT1u rat strains contain a natural killer (NK) cell system that rapidly eliminates (within 24 h) intravenously injected allogeneic or semi‐allogeneic lymphocytes – a phenomenon known as allogeneic lymphocyte cytotoxicity or ALC. When neonatal tolerant rats were tested, the ALC index (a measure of cell killing) was unchanged in nonchimeric tolerant rats and significantly altered (reduced killing), but not abolished in chimeric animals. Hence, the injection of allogeneic BM cells which induced specific tolerance in the T cell population failed to tolerize the NK cell system, allowing the constant killing of newly produced donor‐derived lymphocytes and putting at risk the very survival of the allogeneic BM cells. This has interesting implications for clinical transplantation.