Immunological Microenvironment Differences Between Left and Right Colon Cancer: Dynamic Interactions of CASC15+KLK6+ Epithelial Subpopulation with T Cells and Mast Cells

BACKGROUND: Right-sided (RCC) and left-sided colorectal cancers (LCC) exhibit distinct clinical behaviors and molecular characteristics, but immune microenvironmental differences remain unclear. METHODS: Single-cell RNA sequencing was performed on 3 LCC and 3 RCC tumors. Validation studies included immunofluorescence on 70 patients (30 LCC and 40 RCC). Analytical methods comprised cellular heterogeneity assessment (Seurat, inferCNV), pseudotime trajectory reconstruction (Monocle2), pathway enrichment (GSVA), and cell-cell communication inference (CellChat). RESULTS: RCC displayed significantly increased exhausted CD8+ T cells and decreased activated CD8+ T cells compared to LCC. Concurrently, RCC showed reduced proportions of TNF+ mast cells and increased VEGFA+ mast cells. We identified a CASC15+KLK6+ epithelial subpopulation specifically enriched in RCC tumors, exhibiting high copy number variation scores, elevated malignant pathway activity, and significant negative correlation with patient survival (r = -0.2919, p = 0.0142). Enhanced intercellular communication involving MIF signaling (CD74/CXCR4/CD44) and CD99 pathways was observed between epithelial cells, T cells, and mast cells in RCC. CONCLUSION: RCC harbors an immunosuppressive niche driven by T-cell exhaustion, mast cell reprogramming, and expansion of a malignant CASC15+KLK6+ epithelial subpopulation. Targeting MIF and CD99 signaling networks represents a potential therapeutic strategy for right-sided colon cancer.