Effects of Transient Donor Chimerism on Rejection of MHC-Mismatched Vascularized Composite Allografts in Swine

Background: Despite encouraging outcomes in vascularized composite allograft (VCA) transplantation, the risks of chronic immunosuppression limit widespread applicability. It has been suggested that infusion of donor bone marrow along with the VCA may reduce the level of immunosuppression required to prevent clinical VCA rejection. However, no clear evidence has yet been presented to confirm the role of donor bone marrow in the prevention of rejection. In this study we investigated the immunologic effects of concurrent bone marrow transplantation in a large animal VCA model. Methods: MGH miniature swine (n=4) received a non-myeloablative conditioning regimen consisting of low-dose total body irradiation, T-cell depletion, a short course of Cyclosporine A, with or without varying doses of donor bone marrow cells in combination with a complete MHC-mismatched VCA. Animals were monitored daily for signs of rejection or graft versus host disease. Chimerism levels were assessed using flow cytometry and in vitro assays were performed to assess for donor-specific responses. Results: Transient chimerism was prolonged with increased bone marrow cell doses and total body irradiation. While animals that received BMC infusions did not have significantly prolonged VCA acceptance following cessation of immunosuppression compared to animals that received conditioning without BMCs, they demonstrated better early clinical outcomes and demonstrated donor-specific unresponsiveness during the presence of detectable chimerism. Conclusions: Detectable mixed chimerism following bone marrow transplantation and VCA mitigates donor-specific responses and acute rejection episodes, but does not appear to be sufficient for tolerance induction.