Key Points• MDSC treatment prevents GVHD by skewing T cells toward type 2 T cells.• MDSCs proliferate in vivo, suppress independent of major histocompatibility complex class I expression, and do not impair allogeneic T-cell homing and the graft-versustumor effect.Myeloid-derived suppressor cells (MDSCs) inhibit T-cell expansion and functions by versatile mechanisms such as nutrient depletion, nitrosylation, or apoptosis. Since graftversus-host disease (GVHD) is characterized by the expansion of donor-derived T cells destroying recipient tissue, we analyzed whether MDSCs can be used for GVHD prevention in murine allogeneic bone marrow transplantation models. Transplantation of MDSCs, generated from bone marrow cells by granulocyte-macrophage colony-stimulating factor (GM-CSF)/G-CSF in vitro, inhibited GVHD-induced death and attenuated histologic GVHD, whereas antitumor cytotoxicity of alloantigen-specific T cells was maintained. MDSCs expanded in vivo and invaded lymphatic and GVHD target organs. Major histocompatibility complex class I expression on MDSCs was dispensable for their suppressive capacity. Inhibition of GVHD required the presence of MDSCs during T-cell priming, whereas allogeneic T-cell numbers and homing in lymphoid and GVHD target organs were not considerably affected in MDSC-treated mice. However, MDSCs skewed allogeneic T cells toward type 2 T cells upregulating T helper 2 (Th2)-specific cytokines. Type 2 T-cell induction was indispensable for GVHD prevention since MDSC treatment failed to prevent GVHD when allogeneic STAT6-deficient T cells, which are unable to differentiate into Th2 cells, were transplanted. MDSC-induced Th2 induction might be applicable for GVHD treatment in clinical settings. (Blood. 2015;126(9):1138-1148

The immunosuppressant rapamycin (RAPA) inhibits mammalian target of rapamycin (mTOR) functions and is applied after allogeneic bone marrow transplantation (BMT) to attenuate the development of graft-versus-host disease (GVHD), although the cellular targets of RAPA treatment are not well defined. Allogeneic T cells are the main drivers of GVHD, while immunoregulatory myeloid-derived suppressor cells (MDSCs) were recently identified as potent disease inhibitors. In this study, we analyzed whether RAPA prevents the deleterious effects of allogeneic T cells or supports the immunosuppressive functions of MDSCs in a BMT model with major histocompatibility complex (MHC) classes I and II disparities. RAPA treatment efficiently attenuated clinical and histological GVHD and strongly decreased disease-induced mortality. Although splenocyte numbers increased during RAPA treatment, the ratio of effector T cells to MDSCs was unaltered. However, RAPA treatment induced massive changes in the genomic landscape of MDSCs preferentially up-regulating genes responsible for uptake or signal transduction of lipopeptides and lipoproteins. Most importantly, MDSCs from RAPA-treated mice exhibited increased immunosuppressive potential, which was primarily inducible nitric oxide synthase (iNOS)-dependent. Surprisingly, RAPA treatment had no impact on the genomic landscape of T cells, which was reflected by unchanged expression of activation and exhaustion markers and cytokine profiles in T cells from RAPA-treated and untreated mice. Similarly, T cell cytotoxicity and the graft-versus-tumor effect were maintained as co-transplanted tumor cells were efficiently eradicated, indicating that the immunosuppressant RAPA might be an attractive approach to strengthen the immunosuppressive function of MDSCs without affecting T cell immunity.

Allogeneic bone marrow transplantation (BMT) is a curative therapy for various hematological malignancies, such as leukemias and lymphomas. Donor-derived alloantigenspecific T cells eradicate residual tumor cells, however, these T cells are also responsible for the induction of graftversus-host disease (GVHD). Therefore, one major challenge in BMT is the identification of T-cell subsets provoking antitumor cytotoxicity without causing GVHD. T helper (Th) subsets are programmed into various lineages dependent on ligand interactions and the cytokine milieu [1]. Th9 cells, which can be generated from naïve CD4 cells after activation in the presence of IL-4 and TGF-β are characterized by increased secretion of IL-9 and have been proven to exhibit efficient antitumor capacity especially toward melanomas [2][3][4]. Likewise, a few studies linked the presence of Th9/IL-9 with enhanced solid organ acceptance and decreased GVHD development [5][6][7]. However, the therapeutic effect of adoptive Th9 cell transfer on GVHD development and eradication of tumor cells is currently not well defined.To clarify whether Th9 cells are suitable for donor lymphocyte infusion in BMT settings, we differentiated naïve splenic CD4 + T cells derived from B6 mice on anti-CD3/CD28 coated plates in the presence of IL-4, TGF-β and the TNF-family cytokine TL1A. About 60% of the cells expressed IL-9 intracellulary after 5 days of differentiation and exhibited high IL-9 RNA expression. Expression of the Th1-specific cytokine IFN-γ was undetectable while IL-5, -13 and TNF-α were weakly expressed (Fig. 1a). Most importantly, Tregs were not differentiated, because Foxp3 expression was undetectable (Fig. S1). To clarify the impact of Th9 cells on GVHD induction, we used an allogeneic CD4 + -dependent parent → F1 BMT model with a 50% mismatch for MHC class I and II molecules (B6 → B6D2F1). The alloantigen H-2 d expressed on recipient tissue of lethally irradiated B6D2F1 (H-2 bxd ) mice activates transplanted B6-derived (H-2 b ) donor T cells. Acute, lethal GVHD in this model only develops if the transplant contains mature CD4 + T cells since the transfer of CD8 +