LAUSR

Purpose Our group has achieved cardiac allograft tolerance in non-human primates (NHPs) via hematopoietic cellular chimerism mediated through allogeneic bone marrow (BM) and kidney co-transplantation after non-myeloablative conditioning. This form of tolerance is believed to rely on the presentation of donor MHC molecules in the host's thymus and peripheral lymphoid tissues (MHC chimerism). Here, we investigated whether allogeneic exosomes instead of donor cells can be used to achieve a similar form of molecular chimerism (via MHC cross-dressing of recipient APCs) in nonhuman primates. Methods Exosomes were isolated through serial ultracentrifugation from donor BM cells expressing the MHC class I epitope H38. Three H38- cynomolgus macaque recipients underwent heart and kidney co-transplantation as part of 2 protocols. In the Simultaneous protocol (n=2), NHPs were conditioned with ATGAM and thymic irradiation, with or without total body irradiation followed by a single dose of exosomes on postoperative day 2. In the Delay protocol (n=1), the NHP received 2 months of triple drug immunosuppression, then was conditioned with ATGAM, aCD8, and ABT199 and received 2 doses of exosomes. All recipients underwent a short course of immunosuppression with a calcineurin inhibitor and anti-CD154 mAb after exosome infusion. Results Infusion of donor BM derived exosomes resulted in the presence of recipient peripheral blood leukocytes cross-dressed with donor H38 molecules in all animals. Flow cytometric analysis showed MHC class I chimerism restricted to the myeloid compartment. Tissues collected from the Simultaneous group showed H38+ cross-dressed cells present in the lymph nodes, thymus, and spleen. The first Simultaneous protocol NHP died of anemia on postoperative day 33 and the second was sacrificed on postoperative 96 day for cardiac allograft rejection. The Delay protocol NHP is currently 33 days post exosome infusion, showing good contractility of the cardiac allograft by palpation and ultrasound Conclusion Donor MHC chimerism in the form of MHC cross-dressing can be achieved in a NHP model using donor BM derived exosomes. These findings suggest that BM derived exosomes could be used in place of whole bone marrow in chimerism-induced allograft tolerance protocols. Exosome use could allow reduction of recipient conditioning while eliminating the risk of GVHD.