SMPD3 enhances MHC-I surface presentation to drive CD8⁺ T-cell immunity in colorectal cancer

Colorectal cancer (CRC) is frequently accompanied by dysregulation of lipid metabolism. However, how sphingolipid metabolism–associated regulators influence key cellular processes in CRC remains unclear. By integrating TCGA and GEO datasets with differential expression analysis and machine learning, we identified SMPD3 as a candidate regulator of CRC progression and antitumor immunity. SMPD3 was downregulated in CRC tissues, whereas higher SMPD3 expression was associated with earlier-stage disease and improved survival. Single-cell transcriptomic analysis and immunohistochemistry further showed that high SMPD3 expression was associated with increased intratumoral CD8⁺ T-cell infiltration. In immunocompetent syngeneic mouse models, Smpd3 overexpression suppressed tumor growth and increased intratumoral CD8⁺ T-cell accumulation, and this antitumor effect depended on an immunocompetent host environment. SMPD3-high CRCs also exhibited higher PD-1-related immunophenoscores and a more favorable response pattern in a retrospective cohort receiving anti-PD-1-based therapy. Consistently, Smpd3 overexpression combined with anti-PD-1 treatment produced the lowest tumor burden in syngeneic CRC models. Mechanistically, SMPD3 enhanced cell-surface MHC-I presentation without substantially altering total MHC-I protein abundance. B2m knockdown reduced MHC-I surface expression and attenuated SMPD3-associated CD8⁺ T-cell-mediated cytotoxicity and tumor suppression, supporting the functional involvement of the B2m/MHC-I antigen-presentation axis. Brefeldin A blockade and washout assays further implicated intracellular trafficking in SMPD3-mediated MHC-I surface presentation, and SMPD3 may exert its antitumor immune effects through an intracellular mode of action. Together, these findings identify SMPD3 as a tumor-intrinsic regulator linking MHC-I surface presentation to CD8⁺ T-cell antitumor immunity and support its further evaluation as a candidate biomarker for PD-1 blockade responsiveness in CRC. SMPD3 enhances tumor cell-surface MHC-I availability through a B2m/MHC-I-dependent and trafficking-sensitive intracellular process, thereby promoting CD8⁺ T-cell-mediated antitumor immunity and supporting improved PD-1-related immune responsiveness in colorectal cancer.