In vivo induction of myeloid suppressor cells and CD4+Foxp3+ T regulatory cells prolongs skin allograft survival in mice.

Natural CD4+Foxp3+ T regulatory (Treg) cells can promote transplantation acceptance across MHC barriers, while Myeloid-derived suppressor cells (MDSCs) inhibit effector T cell responses in tumor-bearing mice. One outstanding issue is whether combining the potent suppressive function of MDSCs with that of Treg cells might synergistically favor graft tolerance. In the present study, we evaluated the therapeutic potential of MDSCs and natural Treg cells in promoting allograft tolerance in mice by utilizing immunomodulatory agents to expand these cells in vivo. Upon administration of recombinant human G-CSF (Granulocyte-Colony Stimulating Factor; Neupogen), or interleukin-2 complex (IL-2C), Gr-1+CD11b+ MDSCs or CD4+Foxp3+ Treg cells were respectively induced at a high frequency in the peripheral lymphoid compartments of treated mice. Interestingly, induced MDSCs exhibited a more potent suppressive function in vitro when compared to MDSCs from naïve mice. Furthermore, in vivo co-administration of Neupogen and IL-2C induced MDSCs at percentages that were higher than those seen when either agent was administered alone, suggesting an additive effect of the two drugs. Although treatment with either IL-2C or Neupogen led to a significant delay of major histocompatibility complex (MHC) class II disparate allogeneic donor skin rejection, the combinatorial treatment was superior to either alone. Importantly, histological assessment of surviving grafts revealed intact morphology and minimal infiltrates at 60 days post transplant. Collectively, our findings demonstrate that concurrent induction of MDSCs and Tregs is efficacious in downmodulating alloreactive T cell responses in a synergistic manner and highlight the therapeutic potential of these naturally occurring suppressive leukocytes to promote transplantation tolerance.