The Role of IGF2BP3-Mediated Immune Response Activation in Enhancing Immunotherapy of Koumine for Gastric Cancer.

Resistance to immune checkpoint blockade in gastric cancer often arises from defective antigen presentation due to MHC-I downregulation, yet strategies to restore MHC-I expression remain limited. Here, using the natural alkaloid koumine as a chemical probe, we identified IGF2BP3 as a druggable regulator of MHC-I antigen presentation. Network pharmacology, proteomics, surface plasmon resonance, and cellular thermal shift assays were used to identify and validate koumine's target. Genetic rescue experiments, loss-of-function studies, in vitro T-cell co-culture assays, and in vivo gastric cancer mouse models were performed to confirm the causal relationship. m6A-seq and ChIP-seq analyses delineated the downstream epigenetic pathway. Koumine binds IGF2BP3 and promotes its proteasome-dependent degradation, which reduces m6A enrichment on EED mRNA, accelerates EED decay, lowers H3K27me3, and derepresses IRF transcription, thereby restoring MHC-I expression and enhancing CD8+ T-cell killing. Crucially, koumine enhanced the efficacy of anti-PD-1 therapy in gastric cancer mouse models, suppressing tumor growth and fostering an immunologically active tumor microenvironment characterized by CD8+ T-cell infiltration and activation. This study identifies IGF2BP3 as an actionable target for improving antigen presentation and establishes koumine as a valuable chemical probe to dissect this pathway, with implications for immunotherapy sensitization beyond gastric cancer.