VISTA×MSLN Bispecific Antibody Therapies for Epithelioid Malignant Pleural Mesothelioma

Background Malignant pleural mesothelioma (MPM) is a rare, lethal cancer largely linked to asbestos exposure, with a 5-year overall survival <20%. Until the approval of nivolumab plus ipilimumab (CheckMate 743), platinum-based chemotherapy was the only systemic standard of care. The epithelioid subtype (∼70% of cases) of MPM derives limited benefit from current ICIs, underscoring the need for biomarker-guided therapies. We identified co-expression of the immune checkpoint V-domain Ig suppressor of T-cell activation (VISTA) and the tumor-associated antigen mesothelin (MSLN) in epithelioid MPM and validated this axis by scRNA-seq, which demonstrated concordant VISTA and MSLN transcription within malignant epithelioid clusters. Based on this biomarker pair, we engineered an anti-VISTA×MSLN bispecific antibody (BsAb) and explored multiple modalities, including an IFN-β mutein immunocytokine and bispecific ADC concepts. Method Anti-VISTA×MSLN BsAb formats were designed and screened for optimal binding/cell-surface coverage on VISTA/MSLN co-expressing cells. A stability/production/PK-enhanced IFN-β mutein relative to recombinant IFN-β was fused to the BsAb to generate a trispecific immunocytokine. In vitro binding, internalization, and effector assays were performed in human MPM cell lines. In vivo antitumor activity and TME modulation were tested in xenografts and a syngeneic mouse model; a subset used human IFNAR1/2 knock-in (hIFNAR1/2-KI) mice. To assess suitability for payload delivery, we analyzed antigen-dependent internalization and early bispecific ADC feasibility in vitro. Results scRNA-seq confirmed VISTA/MSLN co-expression in malignant epithelioid populations, supporting VISTA×MSLN as a therapeutic axis. The anti-VISTAxMSLN BsAb showed enhanced binding to dual-positive cells relative to the parental monospecific antibodies and efficient, antigen-driven internalization. The trispecific immunocytokine retained BsAb targeting and induced type I IFN-dependent tumor-cell killing and immune activation in vitro. In vivo, it produced significant antitumor activity in MPM models, including in hIFNAR1/2-KI mice, and remodeled the TME consistent with enhanced antitumor immunity. Internalization/trafficking supported feasibility of developing anti-VISTA×MSLN BsAb ADCs as an additional therapeutic modality. Conclusion VISTA/MSLN co-expression defines a biomarker-guided therapeutic axis in epithelioid MPM. The engineered anti-VISTAxMSLN BsAb is a versatile scaffold enabling multiple modalities— including an IFN-β mutein immunocytokine with in vivo efficacy and TME reprogramming—and exhibits internalization compatible with bispecific ADC development. Collectively, these findings support anti-VISTA×MSLN-based therapeutics as promising preclinical candidates for epithelioid MPM. Citation Format: SangMoo Lee, Heegeon Park, Gyo-Jin Choi, Myeung Ryun Seo, Ji Eun Park, Seokgyeong Yun, Naeun Jeong, Soo Jin Ryu, Sungyoul Hong, Yeong Jeong Jeon, YOON LA Choi, Young Kee Shin. VISTA×MSLN bispecific antibody-directed therapies as a biomarker-guided strategy for epithelioid malignant pleural mesothelioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5533.