Bone marrow transplantation (BMT) has considerable potential for the treatment of malignancies, hemoglobinopathies, and autoimmune diseases, as well as the induction of transplantation allograft tolerance. Toxicities associated with standard preparative regimens for bone marrow transplantation, however, make this approach unacceptable for all but the most severe of these clinical situations. Here, we demonstrate that stable mixed hematopoietic cell chimerism and donor-specific tolerance can be established in miniature swine, using a relatively mild, non-myeloablative preparative regimen. We conditioned recipient swine with whole-body and thymic irradiation, and we depleted their T-cells by CD3 immunotoxin-treatment. Infusion of either bone marrow cells or cytokine-mobilized peripheral blood stem cells from leukocyte antigen-matched animals resulted in stable mixed chimerism, as detected by flow cytometry in the peripheral blood, thymus, and bone marrow, without any clinical evidence of graft-versus-host disease (GvHD). Long-term acceptance of donor skin and consistent rejection of third-party skin indicated that the recipients had developed donor-specific tolerance.

Posttransplantation lymphoproliferative disease (PTLD) is a major complication of current clinical transplantation regimens. The lack of a reproducible large-animal model of PTLD has limited progress in understanding the pathogenesis of and in developing therapy for this clinically important disease. This study found a high incidence of PTLD in miniature swine undergoing allogeneic hematopoietic stem cell transplantation and characterized this disease in swine. Two days before allogeneic peripheral blood stem cell transplantation, miniature swine were conditioned with thymic irradiation and in vivo T-cell depletion. Animals received cyclosporine daily beginning 1 day before transplantation and continuing for 30 to 60 days. Flow cytometry and histologic examination were performed to determine the cell type involved in lymphoproliferation. Polymerase chain reaction was developed to detect and determine the level of porcine gammaherpesvirus in involved lymph node tissue. PTLD in swine is morphologically and histologically similar to that observed in human allograft recipients. Nine of 21 animals developed a B-cell lymphoproliferation involving peripheral blood (9 of 9), tonsils, and lymph nodes (7 of 9) from 21 to 48 days after transplantation. Six of 9 animals died of IntroductionOur laboratory has been successful in developing protocols for establishing mixed chimerism and tolerance across major histocompatibility complex (MHC) barriers in miniature swine without the use of whole-body irradiation (WBI). 1 In one of our protocols developed for this purpose we have observed a high incidence of posttransplantation lymphoproliferative disorder (PTLD). Because of the importance of PTLD clinically, we have attempted to characterize this phenomenon in the miniature swine model.The development of lymphoid neoplasms in allograft recipients receiving immunosuppressive therapy has been recognized as a major complication of solid organ and bone marrow transplantation for over 30 years. 2,3 PTLD and acquired immunodeficiency syndrome (AIDS)-associated B-cell lymphoma are serious and often lethal complications of immunosuppression. The majority of neoplasms involved in PTLD, including those lacking surface immunoglobulin expression, are of B-cell origin. 4,5 A strong correlation has been reported between B-cell neoplasms developing in immunosuppressed patients and the presence of the Blymphotropic gammaherpesvirus Epstein-Barr virus (EBV). 6 In humans, PTLD is thought to represent a spectrum of EBV-driven lymphoid proliferations ranging in histologic appearance from a reactive polymorphic expansion of EBV-infected lymphocytes to monoclonal B-cell lymphomas. 7 Studies of patients who developed PTLD have implicated several risk factors, including T-cell depletion and the degree of immunosuppression; however, the pathogenesis of PTLD is not completely understood. 8,9 The lack of a reproducible large-animal model of PTLD has limited progress in understanding the pathogenesis of and in developing therapy for this clinically import...

These data indicate that this nonmyelosuppressive protocol can induce stable chimerism and robust tolerance even in animals conditioned without thymic irradiation. However, the data also demonstrate that macrochimerism does not always correlate with tolerance. Lack of in vitro antidonor immune responses in chimeric animals is an important predictor of renal allograft acceptance in this model.

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