Unmatched hematopoietic stem cell transplantation (U-HSCT) for the treatment of aplasia in victims of radiation exposure secondary to terrorist attack is largely limited by severe graft-versus-host disease (GVHD). The use of regulatory T cells (Treg) has recently emerged as a therapeutic approach to control alloimmune inflammation in several mouse models. However, prior to use of Treg in human recipients of U-HSCT, testing this strategy first in a relevant large animal model is critical. The hypothesis tested in this proposal is that Treg can serve as effective adoptive immunotherapy for the prevention of severe GVHD in a canine model of U-HSCT. To overcome the difficulty encountered with expansion of polyclonal allospecific Treg without loss of phenotype and function, the use of novel modular immunostimulatory lipid rafts (ISLR) is proposed for the production of therapeutic quantities of Treg. From recent preliminary data using canine cells, we have i] isolated Treg from primary allogeneic mixed lymphocyte reaction, and ii] expanded these cells approximately 22-fold in 5 days of culture. Thus, the goals of this proposal are to demonstrate the functionality of ISLR-expanded Treg , and subsequently to prevent development of severe GVHD by adoptive transfer of “off-the-shelf” allospecific Treg in canine recipients of U-HSCT.