THE BONE COMPONENT OF CTA GIVES RISE TO DONOR HSCS WHICH MIGRATE TO RECIPIENT THYMUS AND DIFFERENTIATE TOWARDS A MATURE T CELL PHENOTYPE

Robert Sucher^1,2,3, Cheng-Hung Lin^1,3, Dong Zhang¹, Wensheng Zhang¹, Vijay Gorantla¹, Stefan Schneeberger^1,2,3, W.P. Andrew Lee^1,3, Fadi Lakkis¹, Gerald Brandacher^1,2,3, Xin Xiao Zheng¹.

¹Department of Plastic and Reconstructive Surgery, UPMC, Pittsburgh, PA, USA; ²Department of Visceral, Transplant and Thoracic Surgery, D. Swarovski Research Laboratory, Innsbruck Medical University, Innsbruck, Austria; ³Department of Plastic and Reconstructive Surgery, Johns Hopkins University School of Medicine, Baltimore, MD, USA.

Introduction: Composite Tissue Allotransplantation (CTA) is immunologically unique in that it represents the only type of graft to include a vascularized functional bone marrow component. Here we studied if the bone component of a composite tissue graft represents the source of HSCs that differentiate in the thymus and thereby reconstitute a functional immune system (CD3+ T cells in peripheral blood/lymphoid organs) in immunodeficient B6/SCID recipients rather than originate from donor mature passenger T cells that expand in the host.

Material and Methods: B6 (WT/nude) murine composite tissue grafts (osteomyocutaneous or myocutaneous) were transplanted heterotopically to B6 (WT/scid) recipients using a non-suture cuff technique for revascularization. Flow cytometry of peripheral blood (CD3, CD19) was performed at pod 7,14,21,28, and 56. In addition, histopathology (H&E) and immunohistochemistry of tissues was performed at indicated time points. To assess immunocompetence, allogeneic skin grafts (Balb/c) were transplanted to either naïve B6/nude, naïve B6/scid or B6/scid mice that prior received a B6/nude CTA.

Results: The surgical success rate was 85% in all groups. As expected no CD3⁺ cells and no rejection of skin allografts were detected in B6/nude and B6/scid controls. B6/scid mice that received B6/nude osteomyocutaneous flaps demonstrated B and T cell immunity from pod 7 and 21 respectively. The percentage of CD3⁺ and CD19⁺ cells within peripheral blood mononuclear cells steadily increased to 57.7% and 17.1% respectively at pod 56. Allogeneic skin allografts were rejected 2 weeks after transplantation. However, no B and T cell reconstitution was observed in B6/scid mice receiving B6/nude myocutaneous flaps (without bone component).

Conclusion: The vascularized bone marrow component of CTA provides an effective source of HSCs to restore immunocompetence in T- and B- cell deficient mice. This might also contribute to chimerism induction and maintenance after CTA and facilitate the clinically observed immunoprivilege of CTAs.