HDAC6 Inhibition by ITF3756 Modulates PD-L1 Expression and Monocyte Phenotype: Insights for a Promising Immune Checkpoint Blockade co-treatment therapy

Valeria Spadotto¹Chiara Ripamonti¹Andrea Ghiroldi¹Roberta Noberini²Tiziana Bonaldi²Christian Steinkühler¹Gianluca Fossati^1*

• ¹Italfarmaco SpA, Cinisello Balsamo, Italy
• ²Department of Experimental Oncology, European Institute of Oncology (IEO), Milano, Lombardy, Italy

Tumor immunotherapy has revolutionized cancer treatment, particularly through the use of immune checkpoint inhibitors targeting the PD-L1/PD-1 axis. While PD-L1 expression on tumor cells has been established as a predictive biomarker for therapeutic response, emerging evidence highlights the significance of PD-L1 expression on myeloid cells in the periphery and in the tumor microenvironment (TME). This study investigates the effects of the selective HDAC6 inhibitor ITF3756 on PD-L1 expression and on the immune functionality of monocytes stimulated with the pro-inflammatory cytokine TNF-α. Our findings demonstrate that ITF3756 effectively downregulates PD-L1 expression in TNF-α-activated monocytes, simultaneously enhancing their costimulatory capacity through increased CD40 expression. Transcriptomic and proteomic analyses reveal that ITF3756 counteracts TNF-α pathway activation and downregulates several inhibitory immune checkpoint (ICP) molecules, leading to a less immunosuppressive phenotype while promoting T cell proliferation in co-culture assays. Additionally, ITF3756 enhances the activation status of dendritic cells (DCs), further supporting T cell activation. In an in vivo model of colon cancer, ITF3756 reduces tumor growth, thereby supporting its potential application as an anti-tumor therapy. These results suggest that selective HDAC6 inhibition could serve as a promising immunomodulatory strategy to augment the efficacy of ICP blockade, positioning ITF3756 as a valuable agent in cancer immunotherapy.