Induction and maintenance of T-dependent or T-independent xenotolerance by nonprimarily-vascularized skin or thymus grafts.

BACKGROUND: The success of clinical xenotransplantation will depend on induction of xenotolerance. We have previously shown that combined xenothymus and vascularized xenoheart transplantation under the coverage of a tolerizing regimen (TR) can induce and maintain full xenotolerance. Here, induction/maintenance of xenotolerance using nonprimarily-vascularized thymus and/or skin grafts was investigated. MATERIALS AND METHODS: Hamster skin or thymus or combined skin and thymus transplantation was performed in nude rat recipients with or without administering a TR (NK cell depletion, day -14; xenoantigen infusion, day -14; Leflunomide, day -14 through +14). Xenotolerance was confirmed by subsequent transplantation of a vascularized hamster heart, measurement of xenoantibody formation, or mixed lymphocyte reaction (MLR). RESULTS: Skin grafts were as effective as vascularized heart grafts to induce/maintain T-independent xenotolerance. Even without TR and despite being rejected themselves, xenoskin grafts lead to progressively developing xenononreactivity. Xenothymus transplantation induced xenotolerance in the T-dependent but not in the T-independent immune compartment, leading to rejection of subsequently transplanted hamster hearts by T-independent mechanisms (production of IgM but not IgG xenoantibodies (Xabs), presence of antihamster MLR nonresponsiveness). Combined skin and thymus xenotransplantation sensitized the T-cell compartment, leading to hyperacute rejection of subsequently transplanted hamster hearts. This was not the case when the skin grafts were transplanted late (2 months) after the thymus grafts. CONCLUSIONS: Xenogeneic skin and xenogeneic thymus grafts have opposite xenotolerance inducing capacities in the T-independent as compared to the T-dependent immune compartment. Thymus grafts induce and maintain T-dependent but not T-independent xenotolerance. Skin grafts alone induce T-independent xenotolerance but sensitize the T-cell compartment when transplanted concomitantly with thymus grafts.

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(+)CD45RC(low) 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.