Establishment of Chimerism and Organ Transplant Tolerance in Laboratory Animals: Safety and Efficacy of Adaptation to Humans

Abstract

The definition of immune tolerance to allogeneic tissue and organ transplants in laboratory animals and humans continues to be the acceptance of the donor graft, rejection of third-party grafts, and specific unresponsiveness of recipient immune cells to the donor alloantigens in the absence of immunosuppressive treatments. Actively acquired tolerance was achieved in mice more than 60 years ago by the establishment of mixed chimerism in neonatal mice. Once established, mixed chimerism was self-perpetuating and allowed for acceptance of tissue transplants in adults. Successful establishment of tolerance in humans has now been reported in several clinical trials based on the development of chimerism after combined transplantation of hematopoietic cells and an organ from the same donor. This review examines the mechanisms of organ graft acceptance after establishment of mixed chimerism (allo-tolerance) or complete chimerism (self-tolerance), and compares the development of graft versus host disease (GVHD) and graft versus tumor (GVT) activity in complete and mixed chimerism. GVHD, GVT activity, and complete chimerism are also discussed in the context of bone marrow transplantation to treat hematologic malignancies. The roles of transient versus persistent mixed chimerism in the induction and maintenance of tolerance and organ graft acceptance in animal models and clinical studies are compared. Key differences in the stability of mixed chimeras and tolerance induction in MHC matched and mismatched rodents, large laboratory animals, and humans are examined to provide insights into the safety and efficacy of translation of results of animal models to clinical trials.

Keywords: animal models; chimerism; human trials; immune suppression; transplant tolerance.

Figure 1

Experimental Scheme for Establishment of Mixed Chimerism and Tolerance in Laboratory Animals and Patients After Organ Transplantation; Donor organ transplantation (heterotopic heart in mice, kidney in humans) is performed on day 0, and the first of 5 daily doses of ATG is given on day 0. On day 1 the first of 10 daily doses of TLI is given. Animals receive 5 doses during the first week and 5 doses during the second week posttransplant. Patients receive 4 doses during the first week and 6 during the second week. Donor hematopoietic cells (cryopreserved and thawed in humans, and fresh in mice) are infused immediately after the last dose of TLI. Serial tests of chimerism and organ graft function are performed thereafter.

Figure 2

Diagram of network of host and donor regulatory cell interactions after TLI based conditioning regimen and transplantation that are required for establishment of mixed chimerism and tolerance. TLI induced massive apoptosis of lymphocytes, and apoptotic bodies were engulfed by host CD8+DCs that interacted with host NKT cells such that both cell types became immunosuppressive/tolerogenic with NKT cell secretion of IL-4. Host NKT cell interacted with and activated host DCs, host Tregs, host MDSCs, and donor Tregs to upregulate production of immunosuppressive molecules including PDL-1, arginase-1, IDO, IL-10, and PD-1. Knockout or depletion of each of the latter cells or their secreted cytokines abrogated tolerance, and add back of each cell type restored tolerance (62, 63).

Figure 3

Schema for Establishment of Mixed Chimerism, and Complete Withdrawal of IS Drugs after HLA matched Kidney Transplantation. Kidney transplantation was performed on day 0, and the first of 5 doses of ATG was administered intra-operatively. The first of 10 doses of TLI was administered on day 1, and 3 additional daily doses were administered during the first week. Patients were discharged from the hospital on days 5 or 6 posttransplant, and received and 6 TLI doses in the clinic during the second week posttransplant. Cryopreserved and thawed donor cells were infused immediately after the completion of TLI. Donor cells were collected 2 months before kidney transplantation by apheresis from G-CSF mobilized blood, and CD34 cells were purified. The latter cells and a defined dose of T cells were cryopreserved until infusion after TLI. Prednisone was administered for 10 days, MMF was given for 30 days starting with the day of the donor cell infusion. A calcineurin inhibitor was administered for 6 months posttransplant at standard dosage, and then tapered to discontinuation during the first year posttransplant if chimerism persisted for at least 6 months, there was no evidence of GVHD, and there was no evidence of microscopic rejection on a protocol biopsy performed just before IS discontinuation.

Figure 4

Intrathymic Clonal Deletion: Newly generated donor and host T cells do not cause GVHD or graft rejection in mixed chimeras. Newly generated naïve donor T cells are clonally deleted against host alloantigens when T cell precursors interact with donor intrathymic DCs derived from residual host CD34 cells. Newly generated naïve recipient T cells are clonally deleted when T cell precursors interact with intrathymic DCs derived from injected donor CD34 cells.