| Regulatory T (Treg) cells that can prevent autoimmune disease and allograft rejection have been identified in a number of animal models. It has also been shown that a single class I mismatched donor lymphocyte infusion (DLI) prior to transplantation can prolong skin allograft survival. In this thesis, I studied the mechanisms involved in DLI-induced tolerance. It was observed that the infusion of allogeneic lymphocytes leads to the activation and expansion of αβ-T cell receptor (TCR)+CD3 +CD4−CD8− double-negative (DN) Treg cells in the periphery of recipient mice. Activated DN Treg cells can specifically suppress CD8+ Treg cells in vivo, attenuate graft versus host disease and prevent tumour outgrowth. Moreover, DN Treg cells preferentially migrate to donor-specific skin allografts and form the majority of graft-infiltrating T cells. Purified graft-infiltrating DN Treg cells were able to suppress and kill anti-donor CD8+ T cells in an antigen-specific manner, suggesting that these Treg cells are beneficial to graft survival. The nature and mechanism of DN Treg cell-mediated suppression were further characterized. DN T regulatory cells do not mediate suppression by competition for growth factors or antigen presenting cells (APC) or by modulation of APC, but required cell contact with the activated target CD8+ T cells. There are three ways that DN Treg cells can mediate suppression. Firstly, DN Treg cells can directly recognize allogeneic MHC on target cells. Secondly, allogeneic target cells can recognize constitutively expressed MHC on DN Treg cells. Thirdly, DN Treg cells are able to acquire specific allogeneic MHC molecules from APC, and present these molecules on their surface for more than 48 hours in vitro and 7 days in vivo. The acquisition is dependent on the TCR on DN Treg cells, and involves the transfer of membrane fragments from APC to DN T cells. The acquired MHC molecules can be recognized by target cells that carry the same TCR specificity as DN Treg cells. Together, these findings enhance our understanding of how DN Treg cells are activated, where and how they execute their function, and illustrate a variety of models in which DN Treg cells may have therapeutic potential. |
