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Immune regulatory CD4 ϩ CD25 ϩ T cells play a crucial role in inducing and maintaining allograft tolerance in experimental models of transplantation (Tx). In humans, the effect of Tx and immunosuppression on the function and homeostasis of CD4 ϩ CD25 ϩ regulatory T cells (Tregs) is not well characterized. In this study, the frequency of Tregs in liver transplant recipients was determined based on flow cytometric analysis of CD4, CD25, CD45RO, and cytotoxic T lymphocyte antigen (CTLA)-4 markers, and the suppressor activity of Tregs was assessed in a mixed-leukocyte reaction. A link between Tregs, acute rejection, and immune-suppressive treatment was investigated. Liver transplant recipients had significantly higher Treg levels in peripheral blood pre-Tx than healthy controls. After Tx, a significant drop in the Treg fraction was observed. This reduction of circulating Tregs was transient and was associated with immunosuppression. In recipients who did not develop rejection, a relative recovery of Treg levels was seen within the first year after Tx. Recipients who experienced an episode of steroid-treated acute rejection, however, had sustained low Treg levels. The suppressive activities of CD4 ϩ CD25 ϩ Tregs from rejectors, nonrejectors, and healthy controls on proliferation and interferon (IFN)-␥ production were indistinguishable. In conclusion, the percentage of CD4 ϩ CD25 ϩ CD45RO ϩ CTLA-4 ϩ quadruple-positive Tregs in peripheral blood decreases significantly after liver Tx. Treatment with methylprednisolone during Tx and for acute rejection is associated with low circulating Tregs. Despite these quantitative differences between rejectors and nonrejectors, the suppressive quality of Transplant tolerance, the ultimate goal in solid organ transplantation (Tx), occurs more often after Tx of the liver compared to other organs. Cessation of immunesuppressive therapy without allograft rejection has been reported to be successful in a considerable proportion of liver transplant recipients. 1,2 The exact mechanisms involved in achieving transplant tolerance remain unknown, although animal models suggest a possible role for regulatory T cells (Tregs). 3,4 In vitro studies of a distinct subset of Tregs expressing CD4, the ␣-chain of the IL-2 receptor (CD25), and the transcription factor Foxp3 showed that these cells do not proliferate upon stimulation, but instead suppress activation of effector T cells in a cell-contact-dependent manner. [5][6][7] Transfer of CD4 ϩ CD25 ϩ Tregs from animals with long-term surviving allografts to naive recipients prevents the development of allograft rejection. 8 The suppressive capacity of CD4 ϩ CD25 ϩ Tregs is not only restricted to foreign antigen-driven T-cell responses, but also entails autoreactive T-cell responses, thereby preventing the development of autoimmune diseases and maintaining peripheral tolerance to selfantigens. 9,10 In vivo studies on the mechanism of Treg-mediated suppression have shown a functional role for cytotoxic T lymphocyte antigen 4 (CTLA-4, CD152), which i...

Conversion to MMF increases the percentage and CD25 expression of CD4+FOXP3+ cells indicating that MMF therapy can overturn the repressive effect of CNI on circulating Treg levels and therefore may promote Treg-mediated suppression of alloreactivity.

Organ transplantation (Tx) results in a transfer of donor leukocytes from the graft to the recipient, which can lead to chimerism and may promote tolerance. It remains unclear whether this tolerance involves donor-derived regulatory T cells (Tregs). In this study, we examined the presence and allosuppressive activity of CD4+CD25+Foxp3+ Tregs in perfusates of human liver grafts and monitored the cells presence in the circulation of recipients after liver Tx. Vascular perfusions of 22 liver grafts were performed with University of Wisconsin preservation and albumin solutions. Flow cytometric analysis revealed that perfusate T cells had high LFA-1 integrin expression and had a reversed CD4 to CD8 ratio compared with control blood of healthy individuals. These findings indicate that perfusate cells are of liver origin and not derived from residual donor blood. Further characterization of perfusate mononuclear cells showed an increased proportion of CD4+CD25+CTLA4+ T cells compared with healthy control blood. Increased percentages of Foxp3+ cells, which were negative for CD127, confirmed the enrichment of Tregs in perfusates. In MLR, CD4+CD25+ T cells from perfusates suppressed proliferation and IFN-γ production of donor and recipient T cells. In vivo within the first weeks after Tx, up to 5% of CD4+CD25+CTLA4+ T cells in recipient blood were derived from the donor liver. In conclusion, a substantial number of donor Tregs detach from the liver graft during perfusion and continue to migrate into the recipient after Tx. These donor Tregs suppress the direct pathway alloresponses and may in vivo contribute to chimerism-associated tolerance early after liver Tx.

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