Tolerance Induction by Myeloid Progenitor Cells Does Not Require Lethal Preconditioning or Hematopoietic Stem Cell Transplantation

Abstract

Purpose Modulation of the immune system using cellular therapy is seen as an increasingly attractive option to prevent solid organ graft rejection, while maintaining sufficient immune function to prevent many of the complications associated with general immunosuppression. We have previously demonstrated that murine myeloid progenitor cells (mMPC), cryopreserved after expansion in vitro, induce robust tolerance for matched organ grafts when preconditioning included lethal whole body irradiation and third party hematopoietic stem cell transplantation (HSC). In this study we evaluated tolerance induced by mMPC when non-lethal pre-conditioning is used. Methods BALB/c or C57Bl/6 mice were pre-conditioned with various combinations of depleting antibodies, chemotherapeutic agents and immunosuppressants as well as infusion of 3 to 6 million mMPC expanded from HSC in vitro using a 10-day expansion protocol. mMPC were injected i.v. one or two weeks after skin graft placement. Skin grafts were either matched or not matched to the mMPC. These experiments looked at survival following the treatment without HSC transplantation, engraftment by mMPC-derived cells and tolerance to mMPC-matched, but not unmatched, skin grafts. Results The pre-conditioning combinations tested were non-lethal, and did not require HSC rescue (30-day survival without HSC: 44/45 mice). Tolerance to mMPC-matched grafts was varied with the protocol used and ranged from rejection of all grafts, rejection of most grafts to tolerance for 90% of the grafts. Using a protocol combining anti-thymocyte serum, anti-CD3 antibodies, busulfan, and rapamycin over a two week period allowed mMPC to be infused as late as two weeks after the skin transplants. 40 out of 44 grafts tested using this protocol survived for more than 5 months. Skin grafts that were not mMPC matched (mismatched or no mMPC) were all rejected (0/18). mMPC engraftment followed similar kinetics to those seen following lethal irradiation. Conclusion mMPC can prevent rejection of a mMPC-matched transplanted organ without the need for lethal whole body irradiation or HSC transplantation. This, combined with the ability to administer mMPC at least two weeks after the organ transplant (skin graft), opens up the possibility to develop clinical protocols that use fully matched organ donor-derived mMPC.