R. Jindal, R. Sucher, Y. Wang, M. Weinstock, R. Zanoun, T.W. Ng, K.K. Atsina, S. Schneeberger, G. Brandacher, X.X. Zheng, V.S. Gorantla, W.P.A. Lee
Division Of Plastic And Reconstructive Surgery, University of Pittsburgh, Pittsburgh/PA/UNITED STATES OF AMERICA

Body: Introduction: Regulation of alloimmune responses throughimmunomodulation rather than immunosuppression may be the key to tolerance in composite tissue allotransplantation (CTA). CD200 fusion protein is a novel agent shown to enhance regulatory T-cells(Tregs) and thereby counteracts anti-tolerogenic effects of cyclosporine-A (CsA). We hypothesize that upregulation of Tregs by CD200 will create a tolerogenic cellular and cytokine milieu that willprolong graft survival. Methods: Lewis rats received hind-limb transplants from Brown-Norways. Endpoints were graft survival >150 days or limb necrosis. T-cells (CD3/4/8/25/45R, FoxP3) werecharacterized by FACS. Skin biopsies were stained by H&E, and long-term survivors were challenged with donor and 3^rd party skin grafts. Results: Mean graft survival of groups 1(CsA), 2 (CsA+CD200), & 3 (ALS+CsA) were 39.8±2.8, 35.8±4.0, and 43.3±4.2 days. In group 4 (ALS+CsA+CD200), 50% of recipients rejected by day 43.5±4.7 while 50% ofgrafts survived long-term (p<0.05). FACS showed increased CD4⁺CD25⁺FoxP3⁺ Tregs in all CD200 animals vs. control by day 10 (p<0.05); however, byday 21 increased Tregs were present only in CD200 long-term survivors whereas CD200-treated rejectors had diminished to levels comparable to control animals. Histology at day 150 showed normal skinand muscle in long-term surviving grafts, with no signs of chronic rejection. Furthermore, these long-term animals accepted donor skin grafts transplanted on day 150, but acutely rejected 3rd party,implying donor specific tolerance. Conclusion: Our study provides the first evidence of a link between CD200 and donor-antigen specific tolerance in CTA through a sustained upregulation ofTregs over time. A likely mechanism of CD200 activity may be through IDO activation, which we continue to study.

Disclosure: All authors have declared no conflicts of interest.