HDAC9 and graft versus host disease: Histone deacetylase (HDAC) inhibitors were found to decrease the allo-stimulatory function of dendritic cells, a group of potent antigen-presenting cells known to be instrumental in the induction of acute GVHD, as well as enhance natural regulatory T-cell function, and hence reduce GVHD while preserving GVL effects (89–92).