Mixed chimerism achieved by a nonlethal conditioning regimen induces donor-specific tolerance to lung allografts

Background. Graft rejection and toxicity associated with chronic immunosuppressive therapy remain a major problem in lung transplantation (Tx). Mixed hematopoietic chimerism has been shown to produce long-lasting donor-specific transplant tolerance without immunosuppressive drugs in animal models; however, most conditioning regimens required to achieve mixed chimerism are too toxic for clinical use. The aim of this study was to develop a nonlethal conditioning regimen to induce tolerance to lung allografts. Methods. Four to 6-wk old ACI (RT1.A(a)) and Wistar Furth (RT1.A(u)) rats were used as organ donors and recipients, respectively. The recipient conditioning regimen included: 10 mg/animal antilymphocyte globulin (on day-5), 1 mg/kg/d tacrolimus (days 1 to 10), total body irradiation (500 cGy; day 0), and donor bone marrow (DBM) Tx (100 X 106 T-cell depleted cells on day 0 following irradiation). Six weeks after DBM Tx, chimeric animals received orthotopic left lung Tx. Graft survival was monitored by chest X-ray and histology. Results. Long-term DBM engraftment was observed: hematopoietic chimerism in the peripheral blood was 12.4 +/- 3.4%,36.7 +/- 14.1%, and 31.9 +/- 14.1% at 30 d, 6 mo, and 16 mo following DBM Tx, respectively. There was no graft versus host disease. Chimeric recipients (RT1.A(u)) permanently accepted (> 400 d) donor-specific lungs (RT1.A(a); n = 8), yet rapidly rejected (< 8 d) third party hearts (RT1.A(1); n = 5). Graft (lung) tolerant (> 150 d) chimeric recipients accepted secondary donor-specific heart grafts (> 150 d; n = 4) but rejected third party heart grafts (< 7 d; n = 3). Graft tolerant recipients demonstrated reduced (P < 0.05) in vitro donor-specific lymphoproliferative response and cytotoxicity, and no evidence of acute or chronic graft rejection. Conclusion. Mixed chimerism achieved by a nonlethal conditioning regimen induced long-term donor-specific tolerance to lung allografts. (c) 2008 Elsevier Inc. All rights reserved.