Abstract 797: Targeting HDAC3 reactivates immunosurveillance and enhances immunocheckpoint activity in B-cell lymphoma

Somatic mutation of the histone acetyltransferase CREBBP occurs in ~40% of diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma (FL) patients. CREBBP mutation leads to transcriptional repression of genes involved in B-cell differentiation, antigen presentation and cell growth, and this repressive effect is mediated specifically through HDAC3. Hence CREBBP mutant DLBCL cells are biologically dependent on HDAC3 for both cell autonomous effects on cell proliferation and evasion of immune surveillance. Because of this we hypothesize that CREBBP mutant DLBCL would be highly responsive to HDAC3 specific inhibition.Therefore we treated a panel of DLBCL cell lines with increasing dose of the clinically relevant HDAC3 selective inhibitor (HDAC3i) KDAC0001 (from 0.1 to 20 μM), which resulted in a dose- and time-dependent growth inhibition and cell death. HDAC3i treatment demonstrated a stronger efficacy in cells carrying CREBBP mutation compared to the wild type with an IC50 20-30 fold lower at 72 Hrs. Knockdown by shRNA or loss of function mutations introduced via CRISPR in CREBBP wt cell lines (MD901, OCI-LY18 and RL) led to increased sensitivity to KDAC0001 compared to the parental lines. Along these lines we placed six primary DLBCL specimens (3 GCB and 3 ABC) in our 3D organoid culture system and observed potent growth suppression by KDAC001 vs vehicle after 6 days. To identify the genes through which HDAC3 drives the lymphoma phenotype we performed RNA Seq analysis using 3 DLBCL cell lines (OCI-LY1 CREBBP wt, OCI-LY19 and OZ CREBBP m) treated with either DMSO or KDAC0001 at 10 μM for 24 Hrs (a timepoint prior to cells manifesting any response to drug). We observed induction of over 1100 genes whereas 74 genes were downregulated. Gene pathway analysis showed that HDAC3 induced genes corresponded to those that are repressed in CREBBP mutant lymphomas, and regulated by BCL6-HDAC3 complexes linked to gene enhancers, including their known downstream p53 and NFKB target genes. On the other hand KDAC0001 treatment also broadly induced MHC class II and antigen presentation genes. We confirmed the increase in MHC class II genes, CIITA, PD-1 and PD-L1 not only at the transcriptional level, as determined by qPCR, but also at the protein level, as determined by flow cytometry. Co-culturing OCI-LY18 DLBCL cells and human lymphocytes with increasing amounts of HDAC3i was associated with increased cytotoxicity in a time-dependent manner. We engrafted three CREBBP wild type primary human PDX in NSG mice and treated with either vehicle or KDAC0001 at 25 or 50 mh/kg daily for 21 days. Even thought these PDX were not CREBBP mutant, there was still cell autonomous activity against all these tumors. In conclusion, HDAC3 inhibitor KDAC0001 has both cell autonomous and immune related activities.Therefore, it can be used as a novel form of “epigenetic immunotherapy” to restore immune clearance of resistant DLBCL and FL. Citation Format: Patrizia Mondello, Matt Teater, Lorena Fontan, Matthew Durant, Elisa de Stanchina, Giorgio Inghirami, Michael Green, Ari Melnick. Targeting HDAC3 reactivates immunosurveillance and enhances immunocheckpoint activity in B-cell lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 797.