Mixed allogeneic chimerism in the rat. Donor-specific transplantation tolerance without chronic rejection for primarily vascularized cardiac allografts.

Graft loss secondary to chronic rejection remains a major source of morbidity and mortality in solid organ transplantation. Mixed chimerism has been suggested as one potential approach to overcome this limitation. Until now, whether long-term tolerance for primarily vascularized allografts can be achieved with mixed chimerism has not been adequately assessed due to technical limitations in the mouse and the inability to establish a reliable model of mixed chimerism in the rat. We now report that stable multilineage mixed hematopoietic chimerism can be achieved following the transplantation of a mixture of T cell-depleted syngeneic and allogeneic bone marrow cells into myeloablated rat recipients using a number of MHC plus minor antigen-disparate donor and recipient strain combinations (F344+WF-->F344, F344+ACI-->F344, WF+F344-->WF, and WF+ACI-->WF). Ninety-one percent of animals engrafted with a level of lymphoid chimerism ranging between 12% and 93% (73.3 +/- 4.8%). Peripheral blood lymphocyte chimerism remained stable for up to 13 months after reconstitution. Multilineage chimerism for lymphoid (T and B cells) and myeloid (granulocyte and macrophage) lineages was present, which suggests that engraftment of the pluripotent rat stem cell had occurred. There was no clinical or histologic evidence of graft-versus-host disease. Donor-specific skin (mean survival time [MST] > or = 177 days) and primarily vascularized cardiac (MST > or = 213 days) grafts were accepted without evidence for acute or chronic rejection. In contrast, MHC-disparate third-party skin (MST = 14 days) and cardiac grafts (MST = 13 days) were rapidly rejected. The tolerance was systemic, since donor-specific tolerance was present in vitro as assessed by the mixed lymphocyte proliferation assay. These data suggest that mixed chimerism prevents graft loss secondary to chronic rejection in skin as well as primarily vascularized grafts. Furthermore, a rat model for mixed allogeneic chimerism may provide insight into the mechanisms involved in tolerance induction for a variety of allografts (lungs, small bowel, limb, etc.) not readily transplantable in mouse recipients.