Costimulation blockade, busulfan, and bone marrow promote titratable macrochimerism, induce transplantation tolerance, and correct genetic hemoglobinopathies with minimal myelosuppression

Mixed hemopoietic chimerism has the potential to correct genetic hemologica l diseases (sickle cell anemia, thalassemia) and eliminate chronic immunosu ppressive therapy following organ transplantation. To date, most strategies require either recipient conditioning (gamma -irradiation, depletion of th e peripheral immune system) or administration of "mega" doses of bone marro w to facilitate reliable engraftment. Although encouraging, many issues rem ain that may restrict or prevent clinical application of such strategies. W e describe an alternative, nonirradiation based strategy using a single dos e of busulfan, costimulation blockade, and T cell-depleted donor bone marro w, which promotes titratable macrochimerism and a reshaping of the T cell r epertoire. Chimeras exhibit robust donor-specific tolerance, evidenced by a cceptance of fully allogeneic skin grafts and failure to generate donor-spe cific proliferative responses in an in vivo graft-versus-host disease model of alloreactivity. In this model, donor cell infusion and costimulation bl ockade without busulfan were insufficient for tolerance induction as donor- specific IFN-gamma -producing T cells re-emerged and skin grafts were rejec ted at similar to 100 days. When applied to a murine beta -thalassemia mode l, this approach allows for the normalization of hemologic parameters and r eplacement of the diseased red cell compartment. Such a protocol may allow for clinical application of mixed chimerism strategies in patients with end -stage organ disease or hemoglobinopathies.