A CD8 T Cell Infiltration-Driven Prognostic Signature for Gastric Cancer: Bridging Tumor Immunity and Clinical Outcomes.

CD8 T cells play pivotal roles in antitumor immunity, where infiltration levels often correlate with favorable prognosis. However, the functional heterogeneity of CD8 T cell subsets within the gastric cancer (GC) tumor microenvironment (TME)-particularly their divergent impacts on tumor progression, immunotherapy response, and clinical outcomes-remains poorly characterized. We integrated single-cell RNA sequencing (scRNA-seq) data from 23 GC tissues (GEO: GSE150290) with bulk transcriptomic profiles from TCGA-STAD to dissect CD8 T cell heterogeneity. Analytical pipelines included unsupervised clustering, pseudotime trajectory analysis, and protein-protein interaction (PPI) network construction to identify survival-associated hub genes. Differential gene expression, functional enrichment, and experimental validation were performed to confirm clinical relevance. scRNA-seq resolved CD8 T cells into five functionally distinct subsets: naïve/memory, exhausted, and three cytotoxic subpopulations. Among these, cytotoxic CD8 T1 cells exhibited the strongest prognostic relevance, with high infiltration correlating to improved survival and enrichment in G2-grade tumors. Pseudotime analysis revealed differentiation trajectories from naïve to exhausted subsets, accompanied by metabolic and immune checkpoint pathway alterations. PPI network analysis identified SELL, CD79B, and RAMP2 as hub genes, all significantly linked to survival and differentially expressed across tumor grades/stages. Experimental validation confirmed that SELL, CD79B, and RAMP2 knockdown suppressed GC cell proliferation, underscoring their functional roles. Our study unveils the landscape of CD8 T cell heterogeneity in GC and proposes a three-gene signature (SELL/CD79B/RAMP2) with dual prognostic and therapeutic potential. These findings provide actionable insights for stratifying patients, tailoring immunotherapy regimens, and developing novel targets to enhance antitumor immunity in GC.