Abstract B067: Investigating the interplay between BRCA status and WT1-CD200 axis in stromal regulation of TLS formation in HGSOC

Abstract High-grade serous ovarian cancer (HGSOC) is the most lethal subtype of ovarian cancer, with more than 70% of patients presenting with metastatic disease at the time of diagnosis. While germline BRCA1/2 mutations carry a 30-40-fold higher risk of HGSOC, paradoxically, patients with BRCA mutations have improved responses to treatment and overall survival benefit compared to BRCA-wildtype patients. This study aimed to investigate how BRCA status alters the tumor microenvironment (TME), particularly the stromal compartment, to support anti-tumor immunity. Our lab has previously demonstrated that ovarian cancer cells epigenetically reprogram their resident tissue mesenchymal stromal/stem cells (MSCs) to develop a cancer-supportive phenotype. These cancer-associated mesenchymal stem cells (CA-MSCs) express high levels of WT1, a transcription factor associated with protumorigenic functions and immune evasion. Using our mRNA seq data, we identified a positive correlation between WT1 and CD200, an immune-modulatory protein, in CA-MSCs. With the help of multispectral flow analysis, we found that elevated CD200 expression in CA-MSCs impairs their differentiation into follicular dendritic cells (fDCs), a stromal subtype essential for the development and active function of tertiary lymphoid structures (TLS). These are ectopic lymphoid aggregates that develop in the TME and serve as a hub with multiple immune cells, including B cells and T cells, associated with enhanced anti-tumor immunity and a favorable response to immunotherapy. To investigate whether BRCA status influences this stromal differentiation axis, we isolated CA-MSCs from primary HGSOC tumors with known germline BRCA status and analyzed CD200 expression by qPCR and flow cytometry. We observed that CA-MSCs derived from germline BRCA-mutant tumors express significantly lower levels of CD200 compared to those from BRCA-wildtype tumors. Functionally, MSCs with reduced CD200 expression exhibit an enhanced capacity to differentiate into fDCs in vitro. Therefore, these results suggest that BRCA mutations may downregulate the WT1-CD200 axis in CA-MSCs, promoting stromal reprogramming that supports TLS formation and antitumor immune responses. Overall, our findings reveal a novel mechanism that highlights the WT1-CD200 axis in HGSOC stroma as a potential target for enhancing TLS formation and anti-tumor immune responses. Citation Format: Swathi Suresh, Erika Lampert, Grace Gorecki, Ian MacFawn, Huda Atiya, Tullia Bruno, Lan Coffman. Investigating the interplay between BRCA status and WT1-CD200 axis in stromal regulation of TLS formation in HGSOC abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85 (18Suppl): Abstract nr B067.