Congenital Heart Surgeons' Society

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Graft-infiltrating Regulatory T Cells Are Associated With Long Term Cardiac Xenograft Survival
Kristopher B. Deatrick, MD, Avneesh Singh, PhD, Sunjay Kaushal, MD PhD, Bartley Griffith, MD, Muhammad Mohiuddin, MD.
University of Maryland, Baltimore, MD, USA.

Objective(s): Neonates and infants in need of cardiac transplantation may benefit from genetically engineered (GE) xenografts, yet control of the immune response remains challenging. T-(CD4+CD25HiFoxP3+) cells (Treg) are a subset of CD4+ T cells important for immunologic tolerance and are increased in the blood of cardiac xenograft recipients. In this study, we examined the presence of FoxP3+ Tregs in xenograft-infiltrating lymphocytes (GILS).
Methods: 8-12 week old GE pig hearts were heterotopically transplanted in 3-year-old specific pathogen free baboons (n=6). All GE pigs had alpha 1-3 galactosidase knockout, and combinations of transgenic expression of human complement inhibitory protein (CD46), thrombomodulin (TBM), endothelial protein C receptor (EPCR), tissue factor pathway inhibitor (TFPI), decay accelerating factor (DAF) and CD47. Immunosuppression was targeted T and B cell depletion and conventional anti-rejection agents. Immunophenotyping was performed on GILS from explanted xenografts.
Results: Following xenotransplantation, WBC and lymphocyte counts were reduced but recovered to a normal level. 3/6 cardiac xenografts were explanted after 115 days. GILS were isolated and CD4+CD25HiFoxP3+Tregs were identified. Long term survivors had an increased percentage of FoxP3+ Treg cells compared to the GILS in cardiac xenografts with earlier graft failure. Tregs were not identified in GILS from recipients with early graft failure. The presence of Tregs was associated with improved graft function.
Conclusions: FoxP3+ Tregs are associated with increased cardiac xenograft survival and function, and may regulate donor-reactive T cells. This suggests a TReg-dependent mechanism of survival for xenografts, and supports future experiments investigating this mechanism to bring xenotransplantation to clinical reality.