Abstract C010: Anti-FN14 CAR-T cells overcome TGF-β-mediated suppression, and their synergy with methylseleninic acid enhances cytotoxicity and effector function in kidney, prostate, and brain tumors

Abstract Background: Clear cell renal cell carcinoma (ccRCC), metastatic castration-resistant prostate cancer (mCRPC), and glioblastoma (GBM) are highly treatment-refractory tumors characterized by elevated TGF-β1, a key suppressor of antitumor immunity and antigen presentation. Because CAR-T cells infiltrating these tumors experience persistent physiologic TGF-β exposure, assessing CAR-T functional resilience across increasing TGF-β concentrations is critical for determining therapeutic feasibility. Methylseleninic acid (MSA), a pharmacologic selenium compound, potently inhibits TGF-β1, PD-L1, and VEGF with minimal off-target toxicity. FN14, the TWEAK receptor and a metastasis driver, is selectively overexpressed in these malignancies, making it a high-specificity CAR target. Methods: We developed second-generation anti-FN14 CARs containing CD28 or 4-1BB costimulatory domains and a CD3ζ signaling motif, then lentivirally transduced into primary human CD4+ and CD8+ T cells. Cytotoxicity against FN14+ ccRCC, mCRPC, GBM lines and patient-derived cells was assessed using LDH release and xCELLigence assays. Effector function was evaluated through intracellular cytokine profiling, multiplex analysis, and phenotypic characterization by flow cytometry. CAR-T persistence in TGF-β-rich conditions was measured using CCK-8 proliferation assays, and Ki-67 and activation/exhaustion markers (PD-1, CXCR3) in the presence of 0-10 ng/mL TGF-β. MSA’s immunomodulatory and synergistic effects were tested at physiologically concentrations to measure impacts on CAR-T viability, metabolic activity, cytokine secretion, and tumor cell susceptibility. Results: Anti-FN14 CAR-T cells mediated rapid, antigen-specific lysis across all tumor types, achieving >50% specific killing at a 1:1 E:T ratio. MSA (2.5-5 µM) reduced tumor PD-L1 and TGF-β expression by >40% and enhanced CAR-T metabolic activity, IL-2, and IFN-γ secretion. CAR-T cells maintained comparable proliferation and activation in 0-5 ng/mL TGF-β, indicating that antigen-driven signaling counteracted low-to-moderate TGF-β-mediated suppression. Ex vivo CAR-T manufacturing with MSA (2.5 µM) improved cytotoxicity and persistence versus untreated CAR-T cells (p < 0.01). MSA’s effects were dose- and schedule-dependent and remained within human-relevant exposure ranges without detectable toxicity. Conclusion: MSA enhances anti-FN14 CAR-T cytotoxicity, metabolic fitness, and sustained effector activity by suppressing tumor-derived TGF-β1 and PD-L1 and by directly improving CAR-T functional capacity. Consistent with this mechanism, physiological TGF-β levels (0-5 ng/mL) did not significantly impair CAR-T proliferation or activation, demonstrating intrinsic resilience of FN14 CAR-T cells in TGF-β-rich environments. As a clinically relevant TGF-β1 inhibitor and active metabolite of seleno-L-methionine, MSA represents a readily translatable strategy to reduce CAR-T exhaustion and enhance efficacy in TGF-β-rich solid tumors including ccRCC, mCRPC, and GBM. Citation Format: Obed B. Amissah, Casey Ager, Yousef Zakharia, Youcef M. Rustum, Gloria B. Kim. Anti-FN14 CAR-T cells overcome TGF-β-mediated suppression, and their synergy with methylseleninic acid enhances cytotoxicity and effector function in kidney, prostate, and brain tumors [abstract]. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr C010.