TOLERANCE TO COMPOSITE TISSUE ALLOGRAFTS IS DEPENDENT ON THE ADMINISTRATION OF HEMATOPOIETIC STEM CELLS BUT NOT LONG-TERM ENGRAFTMENT

David Mathes^1,2, Jeff Chang^1,2, Scott Graves¹, Billana Hwang¹, Tiffany Butts¹, Rainer Storb¹.

¹Transplantation Biology Laboratory, Fred Hutchinson Cancer Research Center, Seattle, Washington, USA; ²Department of Surgery, Division of Plastic and Reconstructive Surgery, University of Washington, Seattle, Washington, USA.

Introduction: The use of composite tissue allografts (CTA) has become a clinical reality. However, the survival of these transplants is dependent on immunosuppression. The purpose of this experiment was to develop a large animal model for the simultaneous transplantation of hematopoietic stem cells (HSC) and CTA using our mixed chimerism protocol and to examine the role of the HSC infusion in our protocol.

Methods: 4 transplants were performed across a DLA matched, minor mismatch barrier. All dogs received 200 cGy of radiation and underwent a CTA transplant (Myocutaneous rectus flap) with intraoperative injection of the HSC. 2 dogs underwent the same protocol except without an HSC infusion. All dogs received post-grafting immunosuppression (35 days of cyclosporine and 28 days of Mycophenolate Mofetil). Animals were followed for donor cell chimerism in their blood and marrow. The allografts underwent biopsies and were followed clinically. All tolerant animals underwent both a donor and third-party skin graft. Finally, they were followed for levels of FoxP3, IL-10, GranzymeB expression in the C3+ cell populations derived from their peripheral blood, transplanted muscle and transplanted skin.

Results: All of the experimental animals demonstrated long-term tolerance to the CTA (497, 467, 465, and 400 days after transplant). Only three of the four dogs had detectable donor chimerism. One dog lost its chimerism at 10 weeks post-transplant but remained tolerant to the CTA. The expression of FoxP3 remained stable in the transplanted muscle and skin. The two dogs transplanted without an HSC infusion rejected their transplants at 45 days after the cessation of the post-grafting immunosuppression. All of the tolerant dogs accepted the donor skin graft and promptly rejected the third-party skin graft.

Conclusion: This study demonstrated simultaneous transplant of HSC and CTA is feasible and leads to tolerance to both the skin and muscle of the transplant. This tolerance induction appears to be dependent on the administration of HSC but not on the long-term engraftment of the HSC and the persistence of donor cell chimerism.