Vascularized thymic lobe transplantation in miniature swine: thymopoiesis and tolerance induction across fully MHC-mismatched barriers

Abstract

As the major site of self-nonself discrimination in the immune system, the thymus, if successfully transplanted, could potentially carry with it the induction of central tolerance to any other organ or tissue from the same donor. We have recently developed a technique for transplantation of an intact, vascularized thymic lobe (VTL) in miniature swine. In the present study, we have examined the ability of such VTL allografts to support thymopoiesis and induce transplantation tolerance across fully MHC-mismatched barriers. Six miniature swine recipients received fully MHC-mismatched VTL grafts with a 12-day course of tacrolimus. Three of these recipients were thymectomized before transplantation and accepted their VTL allografts long-term, with evidence of normal thymopoiesis. In contrast, three euthymic recipients rejected their VTL allografts. Donor renal allografts, matched to the donor VTL grafts, were transplanted without immunosuppression into two of the three thymectomized recipients, and one of the three euthymic recipients. These renal allografts were accepted by thymectomized recipients, but rejected by the euthymic recipient in an accelerated fashion. This study thus demonstrates that successful transplantation of a vascularized thymus across a fully MHC-mismatched barrier induces tolerance in this preclinical, large-animal model. This procedure should enable studies on the role of the thymus in transplantation immunology as well as offer a potential strategy for tolerance induction in clinical transplantation.

Fig. 1.

Representative histology of VTL grafts stained with hematoxylin/eosin. Successive biopsies from thymectomized (A) and euthymic (B) hosts on days 14 (Aa and Ba), 30 (Ab and Bb), and 60 (Ac and Bc). Immunohistochemical staining of a thymectomized recipient with anti-donor MHC class I Abs on day 60 (Ad).

Fig. 2.

FACS analysis of thymocytes in VTL grafts. Analysis of successive biopsies from a representative PAA(-) VTL graft transplanted into a thymectomized PAA(+) pig. On the day of transplant (day 0), all cells were negative for PAA staining (a and b). The pattern of staining became progressively more similar to that of the naiïve PAA(+) control thymus (i and j) on days 30 (c and d), 60 (e and f), and 255 (g and h).

Fig. 3.

In vitro responses from representative CML and MLR assays. (A) Thymectomized animals developed anti-donor-specific unresponsiveness after transplantation in both MLR and CML assays. (B) Euthymic recipients failed to develop donor-specific unresponsiveness.