The induction of donor-specific transplantation tolerance, i.e. the indefinite acceptation of an allogeneic organ in the absence of immunosuppressive drugs and of allograft injury, remains the ultimate goal of research in the field of allograft tolerance. Among different possibilities is the use of so-called regulatory or suppressor T cells that have been extensively studied during the last decade. Naturally occurring CD4⁺CD25⁺ regulatory T cells (Tregs) play a major role in peripheral tolerance of T cells. The idea is to use donor antigen-presenting cells to expand large numbers of recipient regulatory T cells that could be re-injected to the recipient for preventing allograft rejection. However, the nature of the best antigen-presenting cells for promoting T regulatory cells expansion is not well defined. We have found that, among the different subsets of professional antigen-presenting cells or dendritic cells, only the subset of plasmacytoid dendritic cells (pDC) could induce strong proliferation of allogeneic regulatory CD4⁺ T cells endowed with potent suppressive activity. Interestingly, the adoptive transfer on the day of transplantation of low numbers of donor pDC-expanded regulatory T cells to non-irradiated syngeneic hosts induced significant prolongation of heart allograft survival. In this project, we will further study the role of pDC in regulatory T cell expansion and function by: 1) determining the optimal conditions for using donor pDC-expanded Tregs in vivo to control allograft rejection, 2) determining the molecular mechanisms involved in the capacity of pDC to stimulate Tregs in vitro and in vivo and to analyze in vivo the interaction between pDC as well as other DC subsets and Tregs, and 3) reproducing our in vitro data in primates and in human. These data will be important for designing in the future a preclinical model for using donor pDC expanded Tregs to promote allograft tolerance in primates.