Enrichment of IL-12-producing plasmacytoid dendritic cells in donor bone marrow grafts enhances graft-versus-leukemia activity in allogeneic hematopoietic stem cell transplantation.

A critical question in the field of allogeneic hematopoietic stem cell transplantation (HSCT) is how to enhance graft-versus-leukemia (GVL) activity while limiting graft-versus-host-disease (GVHD). We have previously reported that donor bone marrow (BM) precursors of plasmacytoid dendritic cells (pre-pDCs) can polarize donor T cells toward Th1 immunity and augment the GVL activity of donor T cells while attenuating their GVHD activity in a murine model of allogeneic HSCT. Clinical data on the role of donor pre-pDCs and conventional DCs (cDCs) on transplantation outcomes has been conflicting. To test the effect of increasing the proportion of pre-pDCs versus cDCs in a BM graft, we enriched CD11b(-) pDCs by selectively depleting the CD11b(+) myeloid DC (mDC) population from BM using FACS sorting in a murine model of allogeneic BM transplantation. Donor T cell expansion and GVL activity were greater in mice that received BM depleted of mDCs compared with mice that received undepleted BM. GVHD was not increased by depleting mDCs. To examine the mechanism through which mDC depletion enhances the GVL activity of donor T cells, we used BM and pDCs from IL-12p40KO mice, and found that the increased GVL activity of mDC-depleted BM was IL-12-dependent. This study indicates that a clinically translatable strategy of engineering the DC content of grafts can improve clinical outcomes in allogeneic HSCT through the regulation of donor T cell activation and GVL activity.

IFN-γ and indoleamine 2,3-dioxygenase signaling between donor dendritic cells and T cells regulates graft versus host and graft versus leukemia activity.

Allogeneic hematopoietic stem cell transplantation (HSCT) can eradicate chemorefractory leukemia through the graft-versus-leukemia (GVL) activity of donor T cells. However, the clinical success of allo-HSCT is limited by the graft-versus-host disease (GVHD) activity of donor T cells. We have reported previously that donor bone marrow precursors of plasmacytoid dendritic cells (pre-pDCs) can activate donor T cells toward T-helper 1 immune polarization in murine allogeneic HSCT. To optimize the GVL activity of these activated donor T cells and limit their graft versus host activity, we engineered the cellular constituents of an allogeneic hematopoietic stem cell graft with highly purified hematopoietic stem cells, T cells, and pre-pDCs and studied their GVL and GVHD activities in a murine model of allogeneic HSCT. Transplanted donor pre-pDCs expanded in vivo for 2 weeks after transplant, and they markedly augmented the activation and GVL activity of donor T cells while attenuating their GVHD activity, leading to an improved therapeutic index. Bidirectional signaling between donor T cells and donor pDCs with IFN-γ synthesis by donor T cells inducing indoleamine 2,3-dioxygenase synthesis by donor pDCs limited GVHD by altering the balance between donor T-reg and inflammatory T cells. Manipulating the content of donor DC precursors in allogeneic HSCT is a novel method to optimize the balance between GVL and GVHD.