Establishment of Chimerism in Rhesus Macaques through Nonmyeloablative Hematopoietic Stem Cell Transplant and Costimulation Blockade-Based Immunosuppression.

We established a strategy for producing high level hematopoietic chimerism after non-myeloablative conditioning 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 rapamycin 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. Mean peripheral blood peak donor chimerism was 81% with a median chimerism duration of 145 days. This prolonged chimerism occurred in the setting of full MHC disparity in unrelated transplant donors and recipients. Additional immune modulation strategies such as pre-transplant CD8 depletion, donor-specific transfusion, recipient thymectomy or peri-transplant deoxyspergualin treatment did not improve the level or durability of chimerism. 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. Nevertheless, this T cell costimulation blockade-based chimerism-induction regimen induced the longest-lived stem cell chimerism reported to date for non-human primates and thus represents an important platform upon which to evaluate emerging tolerance-induction strategies. Future nonmyeloablative transplant strategies based on immunomodulation via T cell costimulation blockade may have significant clinical relevence for nonmalignant hematologic diseases such as sickle cell disease and beta-thalassemia.