Eric B. Bell

Donor-specific blood transfusion (DST), designed to prolong allograft survival, sensitized recipients of the high-responder PVG-RT1^u strain, resulting in accelerated rejection of MHC-class I mismatched (PVG-R8) allografts. Rejection was found to be mediated by anti-MHC class I (A^a) alloantibody. By pretreating recipients 4 wk before grafting with cyclosporin A (CsA) daily (×7), combined with once weekly (×4) DST, rejection was prevented. The investigation explores the mechanism for this induced unresponsiveness. CD4 T cells purified from the thoracic duct of CsA/DST-pretreated RT1^u rats induced rejection when transferred to R8 heart-grafted RT1^u athymic nude recipients, indicating that CD4 T cells were not tolerized by the pretreatment. To determine whether B cells were affected, nude recipients were pretreated, in the absence of T cells, with CsA/DST (or CsA/third party blood) 4 wk before grafting. The subsequent transfer of normal CD4 T cells induced acute rejection of R8 cardiac allografts in third party- but not DST-pretreated recipients; prolonged allograft survival was reversed by the cotransfer of B cells with the CD4 T cells. Graft survival correlated with reduced production of anti-MHC class I (A^a) cytotoxic alloantibody. The results indicated that the combined pretransplant treatment of CsA and DST induced tolerance in allospecific B cells independently of T cells. The resulting suppression of allospecific cytotoxic Ab correlated with the survival of MHC class I mismatched allografts. The induction of B cell tolerance by CsA has important implications for clinical transplantation.

Rats from the allotype‐marked PVG‐RT7^b and PVG‐RT1^u‐RT7^b strains were injected at birth with semi‐allogeneic F₁ bone marrow (BM) cells from athymic nude rats (PVG‐rnu/rnu x PVG‐RT1^u‐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‐RT1^u‐RT7^b 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‐RT1^u 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.