Abstract 5173: BET inhibition reprograms pancreatic cancer associated fibroblasts to promote anti-tumor immunity
Authors: Minocha Trisha , Thomas Alison , Olson Brian M. 
Published: April 2023

Abstract
BACKGROUND: Pancreatic ductal adenocarcinoma (PDAC) is the most malignant neoplasm of the pancreas, and is refractory to most therapeutic strategies, including immunotherapy. We have previously demonstrated that loss of CDKN2A in PDAC tumor cells promotes an immunologically-cold tumor microenvironment, which can be counteracted through the use of bromodomain and extraterminal domain inhibitors (BETi). The immunosuppressive PDAC tumor microenvironment is also characterized by a dense desmoplastic stroma, rich in cancer associated fibroblasts (CAFs). These CAFs are a highly plastic population of cells, comprised of myofibroblastic, inflammatory, and antigen-presenting CAFs (myCAFs, iCAFs, and apCAFs, respectively). Previously research has suggested BETi can alter the function of CAFs, suggesting this therapeutic strategy may be effective at targeting both the tumor and stromal compartments of PDAC. Here we hypothesize that BETi enhances anti-tumor immunity in CDKN2A-deficient PDAC by modulating the phenotype and function of PDAC CAFs.
METHODS: Isogenic wild-type and CDKN2A-deficient KPCluc pancreatic cancer cell lines were used in conjunction with KPCluc-derived murine CAF cells. CAF spheroids were used to evaluate the effects of BETi on CAF phenotype (as measured by flow cytometry) and function (as measured by NanoString analysis, checkpoint ligand expression by flow cytometry, and T cell migration assays). Additionally, the impact of BETi-induced modulation of CAF phenotype and function on wild-type and CDKN2A-deficient tumor cell spheroids was measured by flow cytometry.
RESULTS: Treatment of pancreatic CAFs with BETi induced expression of genes associated with DNA damage responses and type I IFN expression. Additionally, while CAF supernatants suppressed T cell migration in vitro, treatment with BETi restored T cell migration across a transwell. This was due in part to DNA damage/STING/NF-κB signaling induced in PDAC CAF following BETi. Supernatants from BETi-treated PDAC CAFs were found to modulate PD-L1 expression on PDAC tumor cells. Direct treatment of CAFs with BETi induced a dramatic dose- and time-dependent polarization of CAF spheroids towards a myCAF phenotype. However, while BETi induced DNA damage/STING/NF-κB signaling in CAF cells, this signaling did not contribute towards this myCAF polarization, as treatment of CAF spheroids with BETi and STING or NF-κB inhibitors did not impact myCAF polarization. Additional mechanisms of polarization are currently being evaluated and will be updated at presentation.
CONCLUSIONS: BET inhibition modulates CAF cell phenotype and function to promote anti-tumor immune responses. Taken together with the impact of BET inhibition on CDKN2A-deficient PDAC tumor cells, these data suggest BETi is a rational treatment approach to promote anti-tumor immunity in this genetically defined immunotherapy resistant patient population.
Citation Format: Trisha Minocha, Alison Thomas, Brian M. Olson. BET inhibition reprograms pancreatic cancer associated fibroblasts to promote anti-tumor immunity. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5173.