A nonlethal conditioning approach to achieve engraftment of xenogeneic rat bone marrow in mice and to induce donor-specific tolerance

BACKGROUND: The supply of solid organs for transplantation will never meet the growing demand. Xenotransplantation is considered to be a potential solution for the critical shortage of allografts. However, xenograft rejection is currently not controlled by conventional immunosuppressive agents. Bone marrow chimerism induces donor-specific tolerance without the requirement for chronic immunosuppressive therapy. The aim of this study was to develop a nonlethal recipient-conditioning approach to achieve mixed bone marrow chimerism and donor-specific tolerance. METHODS: C57BL/10SnJ mice were conditioned with total body irradiation followed by a single injection of cyclophosphamide on day +2. On day 0, mice were reconstituted with untreated bone marrow cells from Fischer 344 rats. Recipients were analyzed by flow cytometry for donor bone marrow engraftment and multilineage chimerism. Donor-specific tolerance was tested by skin grafting. RESULTS: One hundred percent of recipients engrafted after irradiation with 600 cGy total body irradiation, transplantation with 80 x 10(6) Fischer 344 bone marrow cells, and injection with 50 mg/kg cyclophosphamide intraperitoneally. Donor chimerism was detectable in all engrafted animals for up to 11 months. This conditioning was nonlethal, because conditioned untransplanted animals survived indefinitely. Mixed xenogeneic chimeras were tolerant to donor-specific skin grafts but rejected third-party (Wistar Furth) grafts as rapidly as naive C57BL/10SnJ mice. In contrast, animals that received less efficacious conditioning regimens and did not exhibit detectable chimerism showed prolonged graft survival, but delayed graft rejection occurred in all animals within 10 weeks. CONCLUSION: The induction of bone marrow chimerism and donor-specific tolerance after nonlethal conditioning might be useful to prevent the vigorous cellular and humoral rejection response to xenografts.