A strategy for producing high-level hematopoietic chimerism after non-myeloablative conditioning has been established in the rhesus macaque. This strategy relies on hematopoietic stem cell transplantation after induction with a non-myeloablative dose of busulfan and blockade of the IL2-receptor in the setting of mTOR inhibition with sirolimus and combined CD28/CD154 costimulation blockade. Hematopoietic stem cells derived from bone marrow and leukopheresis products both were found to be successful in inducing high-level chimerism. When transplants were performed between animals that were totally unmatched at the MHC, mean peripheral blood peak donor chimerism was 81% with a median chimerism duration of 145 days. Additional immune modulation strategies, such as pre-transplant CD8 depletion, donor-specific transfusion, recipient thymectomy or peritransplant deoxyspergualin treatment did not improve the level or durability of chimerism in the setting of these MHC-unmatched transplants. Recipient immunologic assessment suggested that chimerism occurred amidst donor-specific down-regulation of alloreactive T cells, and the reappearance of vigorous T-mediated alloreactivity accompanied rejection of the transplants. Furthermore, viral reactivation constituted a significant transplant-related toxicity and may have negatively impacted the ability to achieve indefinite survival of transplanted stem cells. In order to address the dual complications of lack of indefinite chimerism and viral reactivation, we sought to increase the level of MHC matching between donor and recipient transplant pairs. This necessitated a large-scale analysis of an NIH-sponsored rhesus macaque colony in order to determine the familial relationships and MHC-disparity between potential transplant pairs. We have analyzed over 500 animals from this colony, which has resulted in the first primate transplant series between donors and recipients with known degrees of MHC disparity. Our results suggest that viral reactivation is vastly improved when transplant occurs with even one shared MHC haplotype, but that the current immunomodulation strategy is insufficient to achieve immune tolerance in these haplo-identical transplants. However, we have observed long-term mixed chimerism in fully MHC matched transplant pairs that is stable even years after withdrawl of costimulation blockade-based immunomodulation. These results suggest that costimulation blockade may be an important addition to current immunomodulation strategies for producing stable engraftment after nonmyeloablative pre-transplant preparation, with potential application to non-malignant hematologic diseases and genetic disorders.
Disclosure: No relevant conflicts of interest to declare.