Induced Tolerance to Liver Allografts Involves Apoptosis and the Generation of Regulatory T Cells (141.40)

Abstract

Abstract Post-transplant total lymphoid irradiation (TLI) is a non-myeloablative regimen that has been studied in rodent models for induction of tolerance to allografts. We show in a rat model of orthotopic liver transplantation that 88% of Lewis recipients of DA livers (n=16) survive more than 100 days (and accept donor skin grafts) after receiving post-transplant TLI alone whereas the control group (liver transplantation without host conditioning) rejected their liver allografts within 14 days. To establish the mechanism of this induced tolerance, lymphocytes were isolated from the livers of groups (n=3-5) of Lewis recipients of Lewis or DA liver grafts, on day 7 post-transplant, and analyzed for apoptosis. Apoptosis of T cells in allografts was increased by TLI treatment (Allo+TLI vs. Allo, CD4+ 41.5 % vs 13.4%, p=0.002, CD8+, 22.5% vs. 9.7% p=0.034). Syngeneic grafts from recipients treated with TLI had minimal apoptosis similar to the levels detected in normal livers. Further, intragraft CD4+CD25+FoxP3+ cells were increased in the TLI group in the first week post-transplant and remained elevated in the graft and in the spleen. Importantly, adoptive transfer of splenocytes from day >100 liver allograft recipients that received TLI prolonged the survival of donor, but not third-party, heart grafts in naïve recipients. Depletion of CD4+CD25+ cells from transferred splenocytes abrogated this prolongation. We conclude that post-transplant TLI significantly increases apoptosis of infiltrating T cells in the allograft during the first week post-transplant. We suggest that apoptosis of host lymphocytes helps to promote an environment whereby CD4+CD25+FoxP3+ T regulatory cells can develop and accumulate in the graft and periphery, and maintain graft non-responsiveness. Supported in part by NIH AI044095, AI037683