So far, infusions of regulatory T-cells (Tregs), mesenchymal stromal cells (MSCs), and the Myeloid-Derived Suppressor Cells (MDSCs) have been the most studied, with promising results (9, 85), while the exact mechanism by which these cells improve GvHD outcome remains incompletely elucidated; multiple immunosuppressive properties such as IL-10, indoleamine 2-3 dioxygenase, ectonucleotidase, and arginase-1 production, and inhibitory T cell co-receptor expression have been suggested as mechanisms to suppress T cell function (86–88). This evidence concerns the gene IDO2 and graft versus host disease.