Background. We have previously reported the successful induction of mixed chimerism and long-term acceptance of renal allografts in MHC-mismatched nonhuman primates after non-myeloablative conditioning and donor bone marrow transplantation. In this study, we extended our regimen to cardiac allotransplantation and compared the immunological responses of heart and kidney allograft recipients. Methods. Five cynomolgus monkeys were conditioned with low-dose total body irradiation (1.5 Gy on days -6 and -5), supplemental thymic irradiation (7 Gy on day -1), antithymocyte globulin (50 mg/kg on days -2, -1, and 0), splenectomy (day 0), donor bone marrow transplantation (day 0), and a 4-week post-transplant course of cyclosporine. Heart allografts from MHC-mismatched donors were transplanted heterotopically on day 0. Results. Two monkeys failed to develop multilineage chimerism and rejected their allografts soon after cyclosporine was stopped (postoperative days [PODs] 43 and 56). Three monkeys developed multi-lineage chimerism, which persisted 20 to 43 days post-transplant by flow cytometric analysis and to POD 124 by polymerase chain reaction analysis. Allograft survival in these recipients was prolonged to 138, 428, and 509 days, and in vitro mixed leukocyte reaction and cell-mediated lympholysis (CML) assays demonstrated donor-specific hypo-responsiveness. However, in contrast to kidney allograft recipients, long-term heart allograft recipients eventually developed humoral and cellular immunity against the donor and rejected the grafts. At the time of rejection, 1.3% to 9.5% of donor coronary arteries exhibited intimal proliferation. Conclusions. The induction of transient mixed hematopoietic chimerism leads to long-term heart allograft survival in MHC disparate monkeys without chronic immunosuppression. However, unlike kidney allografts, full tolerance to cardiac allografts was not achieved. Organ-specific modifications of the preparative regimen may be necessary to prevent the chronic cellular and humoral immune responses elicited by cardiac allografts.
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