A NONLETHAL CONDITIONING APPROACH TO ACHIEVE DURABLE MULTILINEAGE MIXED CHIMERISM AND TOLERANCE ACROSS MAJOR, MINOR, AND HEMATOPOIETIC HISTOCOMPATIBILITY BARRIERS

Reconstitution of lethally irradiated mice with a mixture of syngeneic and allogeneic (A + B-->A) bone marrow results in multilineage mixed allogeneic chimerism, donor-specific transplantation tolerance, superi or immunocompetence and resistance to graft-vs-host disease. However, the morbidity and mortality associated with lethal irradiation would b e a major limitation to the clinical application of chimerism to induc e tolerance for solid organ grafts or treat other nonmalignant hematol ogic diseases. We report here that durable multilineage mixed allogene ic chimerism and donor-specific transplantation tolerance for skin and primarily vascularized allografts can be achieved across multiple his tocompatibility barriers using a nonmyeloablative radiation-based appr oach. The percentage of B10 mouse recipients that engrafted directly c orrelated with the degree of disparity between donor and recipient and the dose of total body irradiation administered. Although the occurre nce of engraftment following conditioning with doses of total body irr adiation of greater than or equal to 600 cGy was similar for animals r eceiving bone marrow disparate at MHC or MHC, minor and hematopoietic (Hh-1) loci (67% vs 78%), the level of donor chimerism was significant ly less when multiple histocompatibility barriers were present (94.6 /- 3.8% vs 37.5 +/- 12.5%). Treatment of the recipient with cyclophosp hamide 2 days following allogeneic bone marrow transplantation reduced the dose of radiation sufficient for reliable engraftment to only 500 cGy of total body irradiation, regardless of MHC and Hh-1 disparity. Donor chimerism was stable and present in all lineages, with productio n of lymphoid (T and B cell), NK, and myeloid (erythrocyte, platelet, granulocyte, and macrophage) cells. Mixed chimeras exhibited donor-spe cific tolerance in vitro, as assessed by mixed lymphocyte culture (MLR ) and cytotoxicity (CML) assays, and in vivo to skin and primarily vas cularized cardiac allografts. The observation that engraftment and tol erance can he achieved across multiple histocompatibility barriers usi ng nonmyeloablative recipient conditioning may allow allogeneic bone m arrow transplantation to be applied to nonmalignant disease states in which lethal conditioning cannot be justified, including the induction of donor-specific tolerance for solid organ transplantation and the t reatment of hemoglobinopathies and enzyme deficiency states.