Abstract 3519: PD-L2-RGMb interaction modulates anti-cancer immunity through swinging the shift of Th1/Th2 cell balance

Abstract Immune checkpoint blockade has revolutionized gastric cancer (GC) treatment, yet primary and acquired resistance remain major clinical challenges. Here, we identified extracellular vesicles (EVs) expressing programmed death-ligand 2 (PD-L2-EVs) as a key determinant of anti-PD1 response in GC. Through profiling the extracellular vesicle protein expression profiles in plasma samples from a retrospective cohort of 76 patients, we demonstrated that high baseline PD-L2-EV levels correlate with improved immunotherapy response and survival. Using multiple immunocompetent murine models, we showed that PD-L2-EVs synergize with anti-PD1 to suppress tumor growth by remodeling the tumor immune microenvironment, enhancing CD8+ T cell and NK cell infiltration while reducing immunosuppressive cell populations. Mechanistically, PD-L2 possesses differential binding dynamics between PD1 and repulsive guidance molecule b (RGMb), its binding to the latter mediates Th1 differentiation through promoting STAT4-signaling and augments CD8+ T cell’s anti-tumor immunity. On the other hand, AFP reduces cancer cell’s expression of PD-L2 and impaired Th1 differentiation, which explained the resistance to immunotherapy for the subtype of AFP-GC. Importantly, through shifting T helper 1/2 cells’ balance towards Th1 under PD1 blockade, PD-L2-EV enhances the efficacy of anti-PD1 but not for anti-PD-L1 therapy. Our work nominated PD-L2-EVs as a predictive biomarker in guiding the selection of different types of immunotherapy and potentiates engineered PD-L2-EV as a candidate for therapeutic combination, particularly in AFP-positive patients. Citation Format: Siyi He, Xiaoyi Chong, Fangli Jiang, Xinru Hua, Chenlin Cao, Cheng Zhang, Xiaotian Zhang, Lin Shen. PD-L2-RGMb interaction modulates anti-cancer immunity through swinging the shift of Th1/Th2 cell balance 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 3519.