Yehong Yan

Effects of a short course of leflunomide on T-independent B-lymphocyte xenoreactivity and on susceptibility of xenografts to acute or chronic rejection.

Background. Leflunomide is a novel immunosuppressive agent with promising activity for xenotransplantation. It is not clear yet which mechanisms of action of leflunomide are responsible for that. Methods. In a hamster-to-C57BL/6 nude mouse heart transplantation model, a 2-week course of leflunomide was used after transplantation or for pretreating donors. Nontolerant B lymphocytes were transferred to recipients after transplantation of first or second xenogeneic heart grafts that were transplanted with or without leflunomide treatment. Results. Hamster xenogeneic hearts transplanted into athymic C57BL/6 nude mice receiving leflunomide did not induce immunoglobulin (Ig) M xenoantibodies (XAb) and survived without signs of chronic rejection. Second xenogeneic hearts transplanted 4 weeks after withdrawal of leflunomide survived without induction of XAb but developed chronic vascular lesions. After injection of naive B lymphocytes at 6 weeks after grafting a first or second hamster heart, only in the latter case were XAb induced. These were deposited in, and provoked acute rejection of, only the second grafts. Pretreatment of donors with leflunomide decreased the ex vivo xenoantibody deposition on the xenogeneic heart endothelia. Conclusions. A short posttransplant course of leflunomide induces T-independent B-lymphocyte xenotolerance. Leflunomide treatment also influences xenoantigen expression, as nontolerant B lymphocytes provoke IgM XAb formation and rejection of only second xenografts (transplanted without leflunomide) and not of first xenografts (transplanted with leflunomide treatment). The ex vivo experiments that show that XAb deposition is decreased in leflunomide-pretreated xenografts further confirm this. The latter may also explain the resistance of first and not second xenografts against chronic rejection.

Pathogenesis of Autoimmunity After Xenogeneic Thymus Transplantation

Thymus transplantation is a promising strategy to induce xenotolerance, but may also induce an autoimmune syndrome (AIS). The pathogenesis of this AIS was explored using nude rats as recipients. Thymus grafts consisted of fetal hamster thymic tissue with or without mixing with fetal rat tissue such as thymus, thyroid, salivary gland, and heart. All hamster thymus recipients died of AIS within 2–3 mo. In most recipients of xenothymus mixed with rat tissues such as thymus, thyroid, and salivary gland, but not heart, AIS was prevented, indicating an insufficient presence of rat epithelial cell Ags within the xenothymus. AIS could be transferred to control nude rats by whole splenocytes or by splenocyte subpopulations such as CD3+, CD3−, and B lymphocytes, but not by non-T, non-B cells from AIS animals. This transfer could be suppressed by cotransferring either CD4+ or CD8+ lymphocytes from euthymic rats, but not by splenocytes from recipients of syngeneic or xenogeneic thymus mixed with rat tissue, indicating a defective generation of regulatory lymphocytes. As for CD4+ regulatory cells this defect was probably qualitative, because the percentages of CD4+CD25+ or CD4+CD45RClow populations were normal after xenothymus transplantation. As for the CD8+ regulatory cells, the defect was quantitative, as CD8+ cell levels always remained low. The latter was related to the nonvascularized nature of thymus grafts. In conclusion, AIS after xenothymus transplantation in nude rats is due to a combination of insufficient intrathymic presence of host-type epithelial cell Ags and a defective generation of regulatory T lymphocytes.

Late CD8+ T cell-dependent xenoantibody production in innate tolerant nude rats after hamster islet grafting but not after hamster heart grafting.

Background. Xenograft rejection can be provoked by both the innate and adaptive immune compartments and close reciprocal interactions exist between these two systems. We investigated the interdependent roles of T and B lymphocytes in vascularized (heart) and cellular (islet) xenograft rejection in a model with established xeno-nonreactivity of the innate immune system. Methods. Specific innate xenotolerance was induced in nude rats bearing either a hamster heart or a hamster pancreatic islet graft by a tolerizing regimen consisting of donor antigen infusion, temporary natural killer cell depletion and a 4-week administration of leflunomide. One month after transplantation, syngeneic CD4+ and CD8+ T cells were adoptively transferred. Results. Both vascular and cellular xenografts were rejected after CD4+ T cell reconstitution, corresponding with production of high IgM and IgG xenoantibody titers. Deposition of xenoantibodies and complement was seen in the heart but not in the islet xenografts. After infusion of CD8+ T cells, xenohearts underwent a delayed type of rejection without xenoantibody production and xenoislets were not rejected. In xenoislet recipients, CD8+ dependent B cells were not tolerized, resulting in the production of IgG xenoantibodies belonging to Th2-dependent isotypes, known not to cause graft rejection, and deposited at the graft implantation site. Conclusions. We conclude that distinct mechanisms of immune activation underlie xenogeneic reactions against vascular and cellular grafts.

Combined use of FTY720 and cyclosporine a prevents chronic allograft vasculopathy

FTY720, A PROMISING NEW type of immunosuppressive agent, is a synthetic structural analog of myriocin, a metabolite of the ascomycete Isaria Sinclairii. Its chemical structure and mechanism of action are different from those of cyclosporine A (CsA), FK506, and other current immunosuppressants. FTY720 modifies lymphocyte trafficking through alteration of the expression or function of certain adhesion molecules. FTY 720 provokes migration of lymphocytes from the peripheral blood to the secondary lymphoid tissues and, as a consequence, a peripheral lymphocytopenia is observed. We already reported that FTY720 has marked anti-acute rejection properties. We also showed that FTY720 does not block tolerance in a model where an heart alloTx is spontaneously accepted after donor specific blood transfusion. Chronic allograft vasculopathy (CAV) remains a leading cause of graft failure after organ Tx. There are no data available on the effect of FTY720 on CAV. Here we investigated the effect of combined use of FTY720 and CsA on CAV in a rat heart transplantation model.