MacDonald first demonstrated MDSCs (CD11b+Gr-1+), previously described as the granulocyte-monocyte precursor population, could differentiate into class II+, CD80/CD86+, and CD40− APC during GVHD, which promoted transplant tolerance by MHC class II-restricted generation of IL-10-secreting, Ag-specific regulatory T cells, and more importantly, preserved GVL effects [24]. This evidence concerns the gene IL10 and graft versus host disease.