Background Ovarian cancer is a leading cause of mortality in women due to late detection and lack of durable therapy responses. The most aggressive subtype is high grade serous ovarian cancer (HGSC). Within the ascites, high levels of the immunosuppressive cytokine Transforming Growth Factor-β (TGF-β) is linked to poor prognosis. We hypothesize that aberrant TGF-β signaling in the tumor microenvironment promotes resistance to immunotherapies, including immune checkpoint inhibitors targeting PD-1/PD-L1. Here, we test whether dual blockade of TGF-β and PD-L1 with bintrafusp alfa provokes durable anti-tumor immune responses in preclinical HGSC models.

Methods Two murine HGSC syngeneic engraftment models were used to model advanced metastatic HGSC. Mouse ID8-Trp53^-/-/Brca2^-/--Luc HGSC cells were injected intraperitoneally into female B cell-deficient μMT^- mice (C57BL/6 background). Control IgG or bintrafusp alfa treatments were administered twice weekly, and HGSC was monitored using IVIS. Treatment effects on survival, tumor burden, ascites volume, cytokines, and immunophenotypes were compared. Mouse BR5-Luc (Trp53^-/-/Brca1^-/-;Myc^O/E/Akt^O/E) HGSC cells were engrafted into female FVB mice. During control IgG or bintrafusp alfa treatments, B cells were depleted using anti-CD20 injections every 3 weeks to avoid neutralization of the humanized proteins. Survivor mice were rechallenged to test memory responses and immunophenotypes compared to naïve FVB control mice. A simplified co-culture model of HGSC killing by cytotoxic T cells was established using T cells from OT-1 transgenic mice and BR5-Luc cells transduced with chicken ovalbumin antigen and a Granzyme B-cleavable FRET reporter.

Results In the ID8 syngeneic HGSC model, bintrafusp alfa treatments reduced ascites development and HGSC tumor burden. Analysis of the ascites revealed depletion of TGF-β and VEGF, with increased CD8 T cell activation and M1 tumor-associated macrophages. In the BR5 syngeneic model, bintrafusp alfa treatments led to HGSC rejection and ∼ 50% survivors. Upon rechallenge, 75% of survivor mice were protected from HGSC without further treatments. These mice had increased peritoneal CD4 and CD8 T effector memory cells and NK cells compared to naïve mice. Testing bintrafusp alfa in an HGSC/T cell co-culture model, we found increased expression of Granzyme B and improved Granzyme B-mediated killing of BR5-Luc cells compared to controls. Bintrafusp alfa also reduced the HGSC-targeting integrin CD103 compared to controls.

Conclusions These results show that coordinated blockade of TGF-β and PD-L1 by bintrafusp alfa leads to acquired anti-tumor immunity in HGSC models. Since bintrafusp alfa has advanced to clinical trials in other cancers, this project has potential to inform new immunotherapy regimes for ovarian cancer patients.

Acknowledgements We acknowledge funding and in-kind contributions from CIHR and EMD Serono.