Causal Role of Metabolites in HLA-DR⁺CD4⁺T Cell-Mediated Gastric Cancer Risk

Background & aims Gastric cancer (GC) ranks as the fifth most prevalent malignancy and the fourth leading cause of cancer-related mortality globally. Despite declining incidence and mortality rates, GC continues to impose a substantial public health burden, particularly in high-risk regions. Emerging evidence suggests that the immune microenvironment and metabolic reprogramming play pivotal roles in gastric carcinogenesis; however, their intricate interplay remains poorly understood. Leveraging Mendelian randomization (MR) combined with single-cell transcriptomic profiling, this study sought to elucidate the causal relationships between immune-metabolic factors and GC risk. Methods Univariate and multivariate MR analyses were conducted using 731 immunophenotypes, 1,400 plasma metabolite levels, and genome-wide association study (GWAS) summary statistics for GC. The inverse variance weighted (IVW) method served as the primary analytical approach, supplemented by complementary MR methods to ensure robustness. Additionally, single-cell RNA sequencing (scRNA-seq) and bulk transcriptomic data from GC tissues were integrated to characterize immune cell heterogeneity, assess functional states, and decipher cell-cell communication networks. Results IVW analysis revealed that 26 immune traits and 40 metabolite levels were causally associated with GC risk. Mediated MR analysis showed that the bilirubin degradation product C ₁₇ H ₁₈ N ₂ O ₄ (2) mediates the protective effect of HLA-DR on HLA-DR⁺ CD4⁺ T cells in GC. Single-cell RNA sequencing identified five subtypes of HLA-DR⁺ CD4⁺ T cells within the GC microenvironment, with HLA_DR_CD4T1 and HLA_DR_CD4T5 cell subsets significantly associated with GC risk. Conclusions This study identified HLA-DR⁺ CD4⁺ T cells as being associated with a reduced risk of GC, with bilirubin degradation product C ₁₇ H ₁₈ N ₂ O ₄ (2) mediating their protective effect. These findings may provide new immune and metabolic targets for the prevention and treatment of GC.