Single agent treatment with T-cell checkpoint inhibitors has not been effective in pancreatic ductal adenocarcinoma (PDAC) patients. The PDAC stroma, which can account for as much as 80-90% of the tumor mass, can act as a physical and an immunologic barrier to T-cell-mediated therapies. Transgenic mouse models have shown that ablation of pancreatic stellate cells (PSCs), key regulators of fibrosis in vivo, can sensitize PDAC tumors to immune checkpoint therapies. Recently, inhibitors targeting bromodomains and extra-terminal (BET) proteins, a number of which are currently being evaluated in clinical trials for solid tumors, were shown to induce stellate cells to become quiescent and decrease collagen production. The BET family of proteins binds to acetylation motifs present in histones and enables recruitment of transcription factors and other chromatin regulators during RNA transcription. We have found that PSCs express significantly increased PD-L1 levels compared to pancreatic cancer cell lines. We show that BET inhibitors, and in particular specific knockdown of BRD4 protein, decrease basal and IFN-γ-mediated PD-L1 expression in primary stellate cells. We also show that, in contrast to a recent report using cancer cells, c-MYC does not mediate basal or IFN-γ-mediated PD-L1 expression in stellate cells. Instead, we show that the IFN-γ-mediated PD-L1 expression is regulated by IRF1, suggesting cross talk between BRD4 and IRF1 in the regulation of PD-L1 expression. Ongoing in vivo experiments will evaluate the role of BET inhibitors in the regulation of PD-L1 in mouse models of pancreatic cancer.

Citation Format: Kazumi Ebine, Brian T. DeCant, Katharine A. Collier, Thao N. Pham, Krishan Kumar, Hidayatullah G. Munshi. BET inhibitors suppress PD-L1 expression in pancreatic stellate cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1679. doi:10.1158/1538-7445.AM2017-1679