Tacrolimus-based partial conditioning produces stable mixed lymphohematopoietic chimerism and tolerance for cardiac allografts

Background-Thoracic organ transplantation remains limited by the reciprocal problems of rejection and the toxicities of nonspecific immunosuppression. Mixed bone marrow chimerism reliably produces donor-specific transplantation tolerance without immunosuppressive drugs. We have previously described a nonmyeloablative conditioning regimen based on recipient treatment with antilymphocyte serum, tacrolimus, and low-dose total-body irradiation that yields long-term multilineage allogeneic bone marrow chimerism in the rat. We have now investigated whether mixed bone marrow chimerism that arises from this partial conditioning strategy produces permanent acceptance of donor-specific cardiac allografts. Methods and Results-Mixed allogeneic chimeras (ACI-->WF) were prepared by treating Wistar Furth recipients with a single dose of antilymphocyte serum 5 days before bone marrow transplantation and tacrolimus 1 mg/kg/d from days -1 to 10, Five hundred cGy total-body irradiation was administered immediately before infusion of 1x10(8) donor (ACI) T-cell depleted marrow cells. All recipients were chimeric, with a mean level of donor chimerism=26.3+/-3.5%. Chimeras underwent heterotopic cardiac transplantation 4 weeks after bone marrow transplantation. All donor-specific (ACI) grafts were permanently accepted (follow-up, 230 to 360 days). Third-party grafts were rapidly rejected. Histology of long-surviving donor-specific grafts was without evidence of acute or chronic rejection. Second-set donor-specific skin grafts transplanted to chimeras 135 days after heart transplantation showed long-term survival (>130 days), whereas third-party skin grafts were rapidly rejected. Mixed lymphocyte reaction demonstrated in vitro donor-specific hyporeactivity. Conclusions-A tacrolimus-based nonmyeloablative recipient conditioning regimen produces mixed bone marrow chimerism and donor-specific tolerance to cardiac allografts in the rat.