Background: In our previous study, we have demonstrated that vascularized composite tissue allografts treated with combined a b -T cell receptor (a b -TCR) monoclonal antibody (mAb) and Cyclosporine-A (CsA) protocol for 7 days can develop robust tolerance across MHC barrier. In this study, we investigated the role of host thymus in the induction of tolerance in rat hind-limb allograft transplantation model.

Material and Methods: Five-week old, either euthymic or thymectomized, Lewis-Brown-Norway (LBN, donors, RT1^l+n, F1) and Lewis (LEW, recipient, RT1^l) rats were involved in this study. Fifty-six hind-limb allotransplantations from LBN to LEW were performed. Isograft and allograft rejection controls received no treatment. In euthymic and thymectomized experimental groups combined a b -TCR/CsA protocol was applied. All transplants were evaluated clinically, immunologically and histologically. Mixed lymphocyte reaction (MLR) and skin grafting were performed for the evaluation of the donor specific tolerance. Flow cytometry (FC) analysis was applied for assessment of the efficacy of the immunosuppressive treatment and the presence of donor specific, hematopoietic chimerism.

Results: Combined treatment with a b -TCR/CsA protocol in euthymic rats resulted in long-term transplant survival (>100 days) and extended the survival of the transplants to the thymectomized rats up to 51 days, post-transplant (p<0.001). Mixed lymphocyte reaction (MLR) showed donor specific tolerance in the PBL of euthymic long-term survivors at day 100 post-transplant. FC revealed stable, donor specific chimerism in the peripheral blood of the tolerated euthymic recipients (12-18% of RT1ⁿ positive cells, 100 days post-transplant) and transient chimerism in the peripheral blood of thymectomized limb allograft recipients (8-11% of RT1ⁿ positive cells) at day 21 after transplantation.

Conclusion: In this study, presence of thymus facilitated the induction of donor specific tolerance under a b -TCR/CsA protocol in the composite tissue allograft model. Tolerogenic activity of the thymus resulted in stable hematopoietic chimerism within the limb recipients.