Thymic rejuvenation and the induction of tolerance by adult thymic grafts

Abstract

The thymus, the site of origin of T cell immunity, shapes the repertoire of T cell reactivity through positive selection of developing T cells and prevents autoimmunity through negative selection of autoreactive T cells. Previous studies have demonstrated an important role for the thymus not only in central deletional tolerance, but also in the induction of peripheral tolerance by vascularized renal allografts in juvenile miniature swine recipients. The same protocol did not induce tolerance in thymectomized recipients nor in recipients beyond the age of thymic involution. We subsequently reported that vascularized thymic lobe grafts from juvenile donors were capable of inducing tolerance in thymectomized juvenile hosts. However, the important question remained whether aged, involuted thymus could also induce tolerance if transplanted into thymectomized hosts, which, if true, would imply that thymic involution is not an intrinsic property of thymic tissue but is rather determined by host factors extrinsic to the thymus. We report here that aged, involuted thymus transplanted as a vascularized graft into juvenile recipients leads to rejuvenation of both thymic structure and function, suggesting that factors extrinsic to the thymus are capable of restoring juvenile thymic function to aged recipients. We show furthermore that rejuvenated aged thymus has the ability to induce transplant tolerance across class I MHC barriers. These findings indicate that it may be possible to manipulate thymic function in adults to induce transplantation tolerance after the age of thymic involution.

Fig. 1.

Morphometric analysis and histology of naive thymus at various stages. (A) Morphometric analysis showing c/m areas of porcine thymus at various times during the aging process. Histologic findings of porcine thymus at 4 months (H&E, ×100) (B) and 20 months (H&E, ×100) (C) of age. Cortical regions contain small, dense thymocytes showing strong hematophilic staining (c) compared with medullary regions (m).

Fig. 2.

Morphometric analysis showing comparison of c/m ratios of aged porcine thymus grafts after transplantation to juvenile recipients versus recipient age-matched naive thymuses. Shown are results for recipient-age-matched naive thymus (average) (♦) and for recipients 1 (▵), 2 (◇), and 3 (□).

Fig. 3.

Representative histology of VTL grafts. H&E staining (×40) of biopsy samples from an aged VTL graft (from a 20-month-old donor) in a juvenile 4-month-old recipient pig on day 0 (c/m ratio 0.79) (A), day 60 (c/m ratio 2.68) (B), and day 100 (c/m ratio 3.62) (C).

Fig. 4.

Histology of aged VTL grafts transplanted across a class I-mismatched barrier with a 28-day course of FK506. (A) Results for day 60 (H&E, ×100) (Aa), day 60 (cytokeratin, ×100) (Ab), day 100 (H&E, ×100) (Ac), day 180 (H&E, ×100) (Ad), and day 260 (H&E, ×100) (Ae). (B) Immunohistochemistry of VTL grafts stained with anti-recipient MHC class I at day 100 (×400) (Ba) and day 180 (×400) (Bb) and with anti-donor MHC class I antibody at day 180 (×400) (Bc). Arrows in Bc indicate donor-type cells with dendritic cell morphology at the corticomedullary junction. (Bd and Be) Double-staining with donor-type class I phycoerythrin-conjugated antibody and class II pig-specific FITC-conjugated antibody (×800) at day 100 (Bd) and day 180 (Be). Double-stained cells (donor class I-positive/pig class II-positive) appear yellow. Morphologically dendritic-like cells double-positive for donor class I and pig class II were detected at days 100 and 180 (white arrows in Bd and Be). Class II-positive recipient-type cells with dendritic cell-like morphology (green cells in Bd and Be) increased between days 100 and 180.

Fig. 5.

Plasma creatinine levels after donor-matched kidney transplant in recipients of aged VTLs with 28 days of FK506 across a class I-mismatched barrier (A), donor-matched kidney biopsy at day 153 after kidney transplant (H&E, ×100) (B), and third-party kidney biopsy rejected on day 7 (H&E, ×100) (C).