In this study, we identified 4-1BBL/TNFSF9 as a transcriptionaly activated target of three distinct HDACi in leukemia cell lines and most importantly that MS-275-induced 4-1BB/4-1BBL interactions potentiated anti-leukemia allogeneic leukocyte response, hence providing novel molecular insights into the antileukemia mechanisms of action of HDACi. This evidence concerns the gene TNFSF9 and leukemia.