Abstract B111: HDAC inhibitors modulate immune checkpoint blockade in breast cancer
Authors: Barberio Manuela Terranova , Thomas Scott , Ali Niwa H. , Park Jeenah , Rosenblum Michael D. , Budillon Alfredo , Munster Pamela N. 
Published: November 2016

Abstract
Background: Breast cancer is one of the most common diseases, second only to lung cancer as the leading cause of cancer death in women. In particular hormone sensitive metastatic breast cancer (MBC) and triple-negative breast cancer (TNBC) remains a therapeutic challenge as resistance develops in almost all patients. Despite aggressive treatment strategies, survival is poor due to the development of resistance, prompting the need for novel approaches. Here, we characterize the molecular mechanisms underlying epigenetic modifiers and in particular histone deacetylase inhibitors (HDACi) as priming modulators of immunotherapy, with a specific focus on TNBC and hormone resistant breast cancers. HDACi represent a new class of anticancer agents that can reverse hormone therapy resistance, resulting in prolonged anti-tumor responses in patients. In addition to their effects on estrogen receptor (ER) signaling, HDACi can also influence immune cell function, including but not limited to modulation of Foxp3+ regulatory T-cells (Tregs), tumor-infiltrating lymphocyte (TILs) composition, as well as induction of the co-inhibitory receptors PD-L1 and PD-1. Under normal conditions the PD-1/PD-L1 pathway down-regulates cytotoxic T-cell activity to maintain immune homeostasis. Cancer cells exploit this pathway in the tumor microenvironment to suppress cytotoxic T-cell activation, significantly diminishing the anti-tumor immune response. In breast cancer, PD-L1 expression is less frequent and mainly found in TNBC, HER2+, ER- and PR- tumors. Increased PD-L1 expression correlates with increased TILs and these criteria together are indicative of improved response rates in breast cancer patients. The PD-1/PD-L1 pathway represents one of the primary immunosuppressive drivers in multiple types of cancer. Thus, inhibiting PD-1/PD-L1 interactions may prevent T-cell suppression and reactivate immunosurveillance mechanisms necessary for tumor cell eradication.
Methods and Results: Evaluation of basal PD-L1 expression in a range of human and mouse breast cancer cell lines by western blotting and real-time PCR identified TNBC and HER2+ cells as the highest expressing cells. Testing different epigenetic modifiers, we found that HDACi were able to up-regulate PD-L1 mRNA and protein in a time-dependent manner up to 72 hours. This was a direct transcriptional effect induced by HDACi and was confirmed even in tamoxifen resistant breast cancer cells, characterized by increased basal expression of PD-L1 as compared to the parental cells. To define the role of epigenetic priming in promoting immune cell activation, we co-cultured tumor cells and human peripheral blood mononuclear cell (PBMCs) and performed comprehensive immunophenotyping by flow cytometry. HDACi were able to up-regulate PD-L1 on tumor cells independent of PBMCs, while exhibiting a selective decrease in the frequency of immunosuppressive Tregs.
Conclusion: Our data demonstrate that the combination of HDACi with immune checkpoint inhibitors represents a novel therapeutic anti-tumor strategy and warrants further clinical evaluation for the treatment of TNBC and hormone resistant breast cancer.
Citation Format: Manuela Terranova Barberio, Scott Thomas, Niwa H. Ali, Jeenah Park, Michael D. Rosenblum, Alfredo Budillon, Pamela N. Munster. HDAC inhibitors modulate immune checkpoint blockade in breast cancer [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B111.