Mixed Chimerism Achieved by a Nonlethal Conditioning Regimen Induces Donor-Specific Tolerance to Lung Allografts

BackgroundGraft 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.MethodsFour to 6-wk old ACI (RT1.Aa) and Wistar Furth (RT1.Au) 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 × 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.ResultsLong-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.Au) permanently accepted (>400 d) donor-specific lungs (RT1.Aa; n = 8), yet rapidly rejected (<8 d) third party hearts (RT1.Al; 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.ConclusionMixed chimerism achieved by a nonlethal conditioning regimen induced long-term donor-specific tolerance to lung allografts.