Background: Tetrahydrobiopterin is a naturally occurring antioxidant and vitamin-like compound which exerts immunomodulatory activities in ischemia-reperfusion injury as well as in acute and chronic graft rejection models. While nitric oxide synthases have been shown to be involved in the attenuation of ischemia-reperfusion injury and allograft vasculopathy, the immunosuppressive mechanisms in acute rejection models are still unknown. Herein, we wanted to further dissect its immunosuppressive property.
Methods/Materials: A fully MHC mismatched (C3H/He to C57BL/6) mouse heterotopic heart transplantation model was used. Recipient mice were either treated with tetrahydrobiopterin (50mg/kg b.w.) three times a day or with Cyclosporine A (15mg/kg b.w.) every 24h. Syngeneic and untreated allograft recipients served as controls. We analysed graft survival as well as graft function by daily palpation and inspection. At day six days post-transplantation the organ was retrieved. The severity of acute rejection was assessed by histopathological analysis according to the International Society for Heart and Lung Transplantation score. Splenocytes were analyzed by flow cytometry.
Results: At day six the median graft functioning score was significantly higher in tetrahydrobiopterin treated compared to untreated animals (p < 0.05). It was comparable with Cyclosporine A treated animals (p = ns) as well as with syngeneic controls (p = ns). This result was reflected also by prolonged allograft survival in tetrahydrobiopterin and CsA treated recipients (p < 0.05). Histopathological analyses showed severe rejection in untreated allografts and absent to mild rejection in syngeneic controls. Cyclosporine A treated grafts displayed mild rejection (p < 0.05 compared to untreated grafts). Tetrahydrobiopterin treatment resulted in heterogeneous lymphocytic infiltrates varying between mild and severe (p = ns). Flow cytometry revealed in Cyclosporine A and tetrahydrobiopterin treated animals, comparable amounts of dendritic cells and NK cells. Tetrahydrobiopterin treated animals showed a substantial increase in cytotoxic T cells (p < 0.05), however, displaying lower CD28 expression (p < 0.05). Of note, mast cells and regulatory T cells were significantly increased in tetrahydrobiopterin treated animals compared to control and Cyclosporine A treated animals (p < 0.05).
Conclusion: Prolonged allograft survival by tetrahydrobiopterin is associated in this murine heart transplantation model with increased mobilization of mast cells and regulatory T-cells. These two cells are known to create a functional tolerogenic unit and might account for the immunosuppressive property of tetrahydrobiopterin in this acute rejection model.