Abstract. Hematologic malignancies are often treated with allogeneic hematopoietic stem cell transplantation (HSCT). However, HSCT is complicated by graft-vs-host disease (GvHD), a severe, potentially lethal complication initiated when donor alloreactive T cells attack host cells and organs. Despite the administration of prophylactic regimens for aGVHD as standard pre-transplantation therapy, up to 60% of these patients develop aGVHD of grade II or higher, and require additional immunosuppressive intervention. Thus, there is an urgent need to improve pre-transplantation therapies to prevent aGVHD. We identified a probiotic exopolysaccharide (EPS) that induces tolerogenic dendritic cells (DCs) that inhibit T cell proliferation, and we found that EPS-treated human DCs (EPS-DCs) both inhibited T cell proliferation in mixed lymphocyte reactions, and significantly increased survival of humanized NSG-HLA-A2 mice after transplantation with human peripheral blood mononuclear cells (hPBMC). The data indicate that a cell-based therapy using EPS-DCs can mitigate aGvHD, and the goal of this grant is to optimize the EPS-DC dosage for maximum protection from GvHD, and to determine the molecular mechanism by which these EPS-DCs inhibit proliferation of alloreactive T cells both in vitro and in vivo. These studies will provide a novel cell-based therapy for preventing GvHD in patients receiving HSCT. This cell therapy will be translatable to humans because large numbers of EPS-DCs can be generated and frozen in bulk and used as “off the shelf” treatment, for all patients, independent of MHC types.