Detrimental Effects of Donor Brain Death on Tolerance Induction May Be Eliminated by Delaying Mixed Chimerism in Non-Human Primates

Abstract

Induction of tolerance in combined heart/kidney transplantation in non-human primates is possible by applying a mixed chimerism-based conditioning protocol which induces indefinite tolerance. However, donor brain death negatively impacts tolerance induction, leading to a higher incidence of humoral as well as cellular rejection. Here, we investigated whether delaying conditioning and donor bone marrow transplantation (DBMT) until four months after organ transplantation would diminish the detrimental effects of donor brain death and permit tolerance induction despite donor brain death. Nine cynomolgus monkeys underwent combined heart/kidney transplantation using organs from donors rendered brain-dead 4 hours prior to organ procurement. Six recipients underwent nonmyeloablative conditioning that included total body irradiation, thymic irradiation, horse anti-thymocyte globulin, anti-CD154 mAb, CyA and DBMT on the day of solid organ transplantation, three animals were kept on triple immunosuppression for four months after combined heart/kidney transplantation before undergoing the identical conditioning protocol prior to DBMT. All drugs were stopped 29 days after DBMT. Three brain-dead donor animals rejected their allografts on day 127, 131 and 383, respectively. Two brain dead donor animals were euthanized without signs of rejection on day 30 and day 114 after transplantation due to sepsis and PTLD, respectively. One brain dead donor animal underwent elective euthanasia on day 400 after transplantation without signs of rejection. One animal in the delayed conditioning group developed no detectable donor chimerism and rejected its allografts on day 135, another delayed animal was euthanized for sepsis without signs of graft rejection on day 59 after DBMT. A third animal in the same group is still ongoing without histological signs of cellular or humoral allograft rejection on day 420 after DBMT. Donor brain death negatively impacts tolerance induction after mixed chimerism conditioning leading to a higher incidence of allograft rejection. Delaying recipient conditioning and DBMT might eliminate the detrimental effects of donor brain death and its associated pro-inflammatory side-effects, thus increasing the chance of long-term tolerance.