Recent data suggest that activation of aryl hydrocarbon receptor （AhR） by its high-affinity ligand 2,3,7,8-tetrachlorodihenzo-p-dioxin （TCDD） results in expansion of regulatory T （Treg） cells and suppresses the development of autoimmune and allergic diseases in several models. Treg cells have been increasingly documented to suppress allograft rejection and even to establish stable long-term graft acceptance. However, the involvement of TCDD in the regulation of solid organ transplantation rejec- tion is largely unknown. Here, we examined whether activation of AhR with TCDD altered cardiac al- lograft rejection in an allogeneic heart transplant model. Recipient C57BL/6 （H-2b） mice were adminis- trated with a single intraperitoneal injection of TCDD, and the murine cardiac transplant models from BALB/c （H-2d） to C57BL/6 （H-2b） were built 24 h later. The complete cessation of cardiac contractility was defined as the observation endpoint. The effect of TCDD on T-cell proliferation was assessed by mixed lymphocyte reaction （MLR）. Histological and immunohistochemical analyses were performed to estimate the severity of rejection. The phenotype and cytokine profile of lymphocytes were analyzed by flow cytometry and enzyme-linked immunosorbent assay （ELISA）. Activation of AhR remarkably pro- longed the survival of cardiac allografts to more than 20 days. In vitro, TCDD ugregulated the fre- quency of CD4＋CD25＋Foxp3＋ Treg cells and suppressed the proliferation of T lymphocytes. In vivo, the prolonged survival time was associated with increased number of Treg cells in allografls and spleens Furthermore, the secretion of interferon-3, （IFN-3,） and interleukin-17 （IL-17） was reduced to less than 50% of that of the PBS treatment control group by TCDD treatment, whereas IL-10 was elevated to 10-fold of that of the PBS treatment control group. Collectively, our data indicate that activation of AhR with a single dose of TCDD significantly prolonged the survival of fully allogeneic card
