Background
Transplantation of vascularized composite allografts (VCA) offers the opportunity to restore form and function. Necessity of life-long immunosuppression limits clinical applicability. One promising method of inducing tolerance is the development of mixed chimerism. We previously demonstrated that a non-myeloablative stem cell transplant can lead to tolerance in a mismatched dog model. This protocol has been limited by graft-versus-host disease (GVHD). We observed several animals that lost their stem cell allograft but remained tolerant to the VCA. Animals that retained persistent donor cell chimerism inevitably developed GVHD. Our hypothesis was that our non-myeloablative haematopoietic stem cell transplant protocol could be used to induce tolerance to a recipient VCA without the need for persistent donor cell chimerism.
Methods
Haploidentical canine recipients received a non-myeloablative conditioning regimen of 350cGy TBI, mobilized donor stem cells (PBMC) and VCA transplantation followed by a short course of immunosuppression (MMF 56 days/Cyclosporine 70 days). Peripheral blood chimerism was evaluated by PCR weekly. VCA rejection was followed clinically and confirmed histologically after biopsies.
Results
All 5 animals tolerated the conditioning regimen. One dog rejected the PBMC at post-operative day (POD) 35 and rejected the VCA following the cessation of immunosuppression. One dog fully engrafted and converted to 100% donor chimerism with long-term tolerance to the VCA but developed GVHD. Three dogs demonstrated a prolonged period of transient chimerism (7 to 10 weeks post-transplant). They rejected their PBMCs after the cessation of immunosuppression without acute rejection of their VCAs. One was euthanized for persistent fevers at POD 147 with no sign of rejection. The remaining 2 had long-term acceptance of their VCA (>200 days) with no evidence of acute rejection. Later both demonstrated evidence of chronic rejection. Neither developed GVHD.
Conclusions
We demonstrate that our non-myeloablative protocol allows for selective rejection of donor stem cells and elimination of GVHD risks without acute rejection of the VCA transplant and that persistent donor chimerism can lead to GVHD.