The PD-1:PDL pathway plays an important role in regulating alloimmune responses but its role in transplantation tolerance is unknown. We investigated the role of PD-1:PDL costimulatory pathway in peripheral and a well established model of central transplantation tolerance. Early as well as delayed blockade of PDL1 but not PDL2 abrogated tolerance induced by CTLA4Ig in a fully MHC-mismatched cardiac allograft model. Accelerated rejection was associated with a significant increase in the frequency of IFN-γ-producing alloreactive T cells and expansion of effector CD8+ T cells in the periphery, and a decline in the percentage of Foxp3+ graft infiltrating cells. Similarly, studies using PDL1/L2-deficient recipients confirmed the results with Ab blockade. Interestingly, while PDL1-deficient donor allografts were accepted by wild-type recipients treated with CTLA4Ig, the grafts developed severe chronic rejection and vasculopathy when compared with wild-type grafts. Finally, in a model of central tolerance induced by mixed allogeneic chimerism, engraftment was not abrogated by PDL1/L2 blockade. These novel data demonstrate the critical role of PDL1 for induction and maintenance of peripheral transplantation tolerance by its ability to alter the balance between pathogenic and regulatory T cells. Expression of PDL1 in donor tissue is critical for prevention of in situ graft pathology and chronic rejection.

No abstract

We studied T-cell clones generated from grafts of rejecting and tolerant animals and investigated the regulatory function of Th2 clones in vitro and in vivo. To prevent allograft rejection, we treated LEW strain recipient rats of WF strain kidney grafts with CTLA4Ig to block CD28-B7 costimulation. We then isolated epitope-specific T-cell clones from the engrafted tissue, using a donor-derived immunodominant class II MHC allopeptide presented by recipient antigen-presenting cells. Acutely rejected tissue from untreated animals yielded self-restricted, allopeptide-specific T-cell clones that produced IFN-γ, whereas clones from tolerant animals produced IL-4 and IL-10. Adoptive transfer into naive recipients of Th1 clones, but not Th2 clones, induced alloantigen-specific delayed-type hypersensitivity (DTH) responses. In addition, Th2 clones suppressed DTH responses mediated by Th1 clones in vivo and blocked Th1 cell proliferation and IFN-γ production in vitro. A pilot human study showed that HLA-DR allopeptide-specific T-cell clones generated from patients with chronic rejection secrete Th1 cytokines, whereas those from patients with stable graft function produce Th2 cytokines in response to donor-specific HLA-DR allopeptides. We suggest that self-restricted alloantigen-specific Th2 clones may regulate the alloimmune responses and promote long-term allograft survival and tolerance. See related Commentary on pages 797-798rats undergoing acute vascularized allograft rejection express a restricted TCR Vβ repertoire and transfer DTH responses in vivo (16). In this study, we compared for the first time, to our knowledge, the functions of self-restricted alloreactive T-cell clones generated from grafts of rejecting and tolerant animals and analyzed the putative regulatory functions of Th2 clones in vitro and in vivo. Results of a pilot study in kidney transplant recipients with chronic rejection or stable graft function establish the biological relevance of our animal studies in humans. Our results confirm the regulatory functions of alloreactive Th2 clones and provide an invaluable tool for the study of the functions of Th1 and Th2 clones in acute/chronic allograft rejection and tolerance in vivo. MethodsAnimals. Inbred 200-250 g male Lewis (LEW; RT1 l ) rats were used as recipients and Wistar Furth (WF; RT1 u ) rats served as donors. They were purchased from Harlan Sprague-Dawley Inc. (Indianapolis, Indiana, USA).Rat kidney transplantation. LEW rats underwent bilateral nephrectomies and received heterotopic MHCincompatible WF renal allografts. For the purpose of this study, we used two groups of animals. The first group was unmodified and the rejecting graft was harvested on day 7. The second group was treated with a single injection of human cytotoxic lymphocyte activation factor 4 (CTLA4Ig; Bristol Myers Squibb Co., Princeton, New Jersey, USA) on day 2 after transplant and the graft was harvested after 100 days. This protocol of CTLA4Ig administration has previously been shown by Sayegh's group to induce long-term a...

No abstract