Ovarian cancer frequently shows limited clinical benefit from immune checkpoint blockade, suggesting that malignant cells possess intrinsic programs that suppress effective T cell–mediated antitumor immunity. To identify such tumor-intrinsic mechanisms independent of variable antigen recognition, the authors used a controlled redirected cytotoxicity approach to expose pathways that modulate sensitivity to T cell killing.
The investigators established and optimized an MHC‑independent redirected cytotoxicity platform based on a B7H3×CD3 engager that enforces a fixed recognition signal between T cells and target tumor cells. This platform produces a reproducible partial‑killing window, enabling discovery of perturbations that either sensitize tumors to T cell cytotoxicity or act as direct cytotoxics. The fixed recognition conditions allow interrogation of tumor-intrinsic resistance programs while holding effector-target recognition constant.
Using the redirected cytotoxicity platform, the team screened a library of 1,796 bioactive compounds. Screening was performed in paired assays comparing SKOV3 tumor cell monocultures with SKOV3 cells co-cultured with peripheral blood mononuclear cells (PBMCs). This paired design enabled discrimination between compounds that cause direct tumor cell killing and those that specifically enhance T cell–mediated cytotoxicity.
The screen identified several immune‑sensitizing perturbations; among these, a selective bromodomain inhibitor emerged as a leading candidate for further study.
The selective BRD9 bromodomain inhibitor I‑BRD9 was highlighted as a top hit from the compound screen. Functionally, I‑BRD9 enhanced T cell–mediated killing across multiple ovarian cancer models, including B7‑H3–positive benchmark cell lines and patient‑derived ovarian tumor suspensions. Importantly, I‑BRD9 did not reduce viability of tumor cells in monoculture nor of PBMCs, indicating that its effect was to increase susceptibility to T cell cytotoxicity rather than acting as a general cytotoxic agent.
To investigate mechanisms, the authors performed cross‑cell‑line RNA‑sequencing following BRD9 inhibition. Analysis revealed that BRD9 inhibition reshapes a coordinated tumor-intrinsic immune resistance program. Components of this program included genes involved in PGE2 biosynthesis (notably the PTGES node), expression of inhibitory ligands, transcripts related to antigen presentation, chemokines associated with T cell recruitment, and features of the extracellular matrix. Together, these transcriptional changes suggest BRD9 controls a multifaceted program that limits effective T cell responses.
To test whether modulation of the PGE2 axis could account for key effects of BRD9 inhibition, the investigators used siRNA to knock down PTGES, a prostaglandin E synthase implicated in PGE2 biosynthesis. PTGES knockdown reproduced several important outcomes seen with I‑BRD9: restoration of chemokine and PGE2‑axis transcripts, promotion of CD8+ T‑cell proliferation and interferon‑gamma (IFN‑γ) production, and upregulation of T‑cell effector–associated gene expression. These functional data support a model in which BRD9 controls tumor immune resistance in part through regulation of the PTGES/PGE2 axis.
The combined screening, transcriptional profiling, and functional knockdown experiments support the existence of a tumor‑intrinsic BRD9–PTGES/PGE2 axis that limits ovarian cancer sensitivity to T cell–mediated cytotoxicity. Because BRD9 inhibition enhanced redirected T cell killing without directly killing tumor cells or PBMCs, targeting BRD9 or downstream components of the PGE2 pathway could be an approach to sensitize ovarian tumors to T cell‑based immunotherapies, including redirected T‑cell strategies.
These results provide a rationale for further preclinical evaluation of BRD9 inhibitors or PTGES/PGE2‑directed interventions in ovarian cancer models, and for exploration of whether modulation of this axis can increase the efficacy of existing immune therapies.
The authors declared that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in the paper.
BRD9; Immune sensitization; Ovarian cancer; PTGES/PGE2; Redirected T‑cell cytotoxicity
Note: This summary is based on the PubMed abstract and related metadata for the cited article. Detailed experimental methods, numerical results, and additional data were not reported in the abstract and therefore are not described here.