Preoperative irradiation and donor splenocyte infusion induce tolerance in lung, but not heart allografts in a minipig model – role of passenger leukocytes

Objectives: Heterotopic retroperitoneal cardiac transplantation combines the advantages of monitoring allograft rejection by non invasive methods like palpation with facilitation of biopsying the transplant. Previously, our group developed a protocol for inducing stable tolerance in a lung transplantation model. In this study, we investigated if this established regimen would also work for inducing tolerance in a heterotopic cardiac transplantation model.

Methods: Heterotopic cardiac transplantation was performed in five, and left-sided lung transplantation in six adult female minipigs. Both groups received their grafts from gender and MHC mismatched donors. Furthermore, all recipients were administered non-myeloablative irradiation before transplantation, a donor splenocyte infusion on the day of transplantation and intravenous pharmacologic immunosuppression for 28 postoperative days (POD). Allograft survival was monitored by chest radiographs and bronchoscopy in lung-transplanted and by palpation, ultrasound examination and myocardial biopsies in heart-transplanted animals. Peripheral blood leukocyte chimerism was analyzed by Y-chromosomal quantitative real time PCR over time.

Results: All transplanted hearts were rejected within 100 postoperative days (median survival 70 days). In contrast, three animals of the lung transplanted group developed a stable tolerance surviving beyond POD 500, two minipigs rejected chronically on POD 239 and POD 360 and one animal (POD 150) is currently alive without signs of rejection. In both groups peripheral blood leukocyte chimerism peaked on the day of transplantation in a sample drawn 1 hour after reperfusion of the allograft. Importantly, the early chimerism level in the lung-transplanted group was significantly higher than in the heart-transplanted group (>7 days).

Conclusion: We conclude that lungs and hearts differ in their inherent potential to induce transplantation tolerance. When other components of the conditioning protocol like preoperative irradiation and perioperative splenocyte infusion are standardized, the variable of the transplanted organ determines the success of a tolerance induction protocol. Our data indicates an important role of the magnitude of early passenger leukocyte chimerism.