Mechanisms in Induction of Immunological Tolerance
Final Report Abstract
In summary, not all proposed experiments of the grant application were conducted since key antibodies were not available in time. However, two other important projects were performed and most data was available at the end of the fellwoship. Some animals of these experiments are still in the postoperative observation period, delaying final publication of the results. In detail, erythropoetin alone does not seem to be the key hormone in vitro leading to allograft acceptance in heart alone transplantation by substituing the kidney. Although in vitro as well as murine data suggested a dose-dependent inhibitory effect of EPO on CD4+ and CD8+ T- cells, in vivo experiments in non-human primates did not lead to prolonged allograft survival or tolerance induction. Interestingly, one animal that accidentally received a high dose of anti-CD8 mAB on the day of transplant developed long-term allograft acceptance without signs of rejection, being the first NHP with a tolerant heart alone graft. To date, it was not possible to replicate this finding, given the high susceptibility of NHPs to infectious complications after receiving intravenous anti-CD8 mAB. Although this protocol is not clinically applicable yet, the success of finding a treatment regimen that may lead to allograft acceptance in heart alone grafts may lead to further mechanistic insights of tolerance induction. The results of this experiment will be published once all animals have finished their follow-up. The results of the protocol analyzing the influence of donor brain death on tolerance induction are extremely important for future steps toward clinical application of tolerance induction protocols. The conducted experiments showed a higher incidence of rejection and allograft loss in animals receiving combined heart/kidney transplantation from brain dead donors as compared to animals receiving combined heart/kidney transplantation from non-brain dead donors. One mechanistic explanation for these findings is the prolonged inflammatory state of recipients receiving organs from brain dead donors. Serum cytokine levels up to day 100 after transplantation are significantly higher in these recipients as compared to the control cohrt. Given that the current donor source in western countries especially for thoracic organs are almost exclusively brain dead donors, the here detected inferior results in these experiments impact clinical tolerance induction protocols largely. Further strategies need to be developed before translation of tolerance induction protocols to clinical application can be performed. Potential changes that are currently under investigation is delaying the conditioning and donor bone marrow transplantation until the inflammatory state is diminished following initial transplantation or using the newly introduced technique of ex vivo organ perfusion to reduce transfer of inflammatory signals from the brain dead donor. The results of this experiment will be published once the animals undergoing a delayed protocol have finished their follow -up period. The experiments focusing on translating an established tolerance induction protocol from kidney to lung transplantation revealed that organ specific differences obviate the direct transfer of treatment strategies between solid organs. The goal to use a tolerance induction protocol using only reagents that are currently clinically available in humans led to stable allograft acceptance in kidney transplanted non human primates, but failed to do so in primates receiving lung grafts. Similar to the previously described donor brain death model, lung recipients showed higher postoperative serum cytokine levels as compared to kidney recipients, indicating that the degree of inflammation after transplantation plays a key role for stable tolerance induction.
