Abstract 140: A novel signature of mesenchymal stromal cells in high-grade serous ovarian carcinoma

Abstract High grade serous ovarian cancer (HGSOC) is the most common and lethal subtype of ovarian cancer. Many cancers, including HGSOC, contain a “stromal” component comprised of cancer-associated fibroblasts (CAFs), immune cells, and blood vessels, as well as their secreted factors and extracellular matrix. This complex microenvironment plays a significant role in promoting tumor growth, therapy resistance, and invasion. The majority of studies of CAFs utilize cells cultured and passaged in vitro under crude conditions to maintain their viability. Such conditions alter the original phenotype of the cells and may select for specific subpopulations to grow out. To facilitate direct isolation of CAFs from primary tumor tissues, we screened a panel of antibodies to identify cell surface proteins that are uniquely expressed on CAFs. We discovered CD49e as a mesenchymal marker and then validated its specificity to CAFs through immunohistochemistry on tissue sections obtained from HGSOC patients. The discovery of CD49e as a CAF-specific cell surface marker facilitated fluorescence-activated cells sorting (FACS) to isolate CAFs directly from primary tumors, allowing us to avoid in vitro manipulation and to characterize their transcriptional and functional profiles in the primary setting. We next performed transcriptional profiling of primary CAFs isolated from 12 HGSOC patients and found that CAFs fall into two subgroups with unique gene expression signatures. A cell surface protein, fibroblast activation protein (FAP) is the defining marker for separating the two subgroups (FAP-Hi and FAP-Lo). The FAP-Hi subgroup possesses the classical gene signature of CAFs that is reported in the literature. When we isolated FAP-Hi and FAP-Lo cells and placed them into classical CAF growth conditions in vitro, both cell types had mesenchymal features, but FAP-Hi cells grow faster. Thus under the classical conditions for growing CAFs, FAP-Hi cells have a growth advantage, outcompeting FAP-Lo cells and becoming the dominant cells used for experimentation. Flow cytometry for FAP indicates that both CAF subtypes co-exist in every tumor, but their ratio varies from one patient to another. Patients whose tumors are dominated by FAP-Hi CAFs have worse clinical outcome than patients whose tumors are dominated by FAP-Lo CAFs. Thus, we have functionally characterized the role of FAP-Hi and FAP-Lo fibroblasts in the context of HGSOC. We have shown that FAP-Hi fibroblasts support tumor proliferation and invasion of ovarian cancer cells in vitro and in vivo. On the other hand, FAP-Lo cells suppress cancer cell proliferation and invasion in vitro and in vivo. Citation Format: Ali Hussain, Veronique Voisin, Stephanie Poon, Jalna Meens, Julia Dmytryshyn, Gary Bader, Benjamin Neel, Laurie Ailles. A novel signature of mesenchymal stromal cells in high-grade serous ovarian carcinoma [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 140.

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