Syndecan-4 drives immune escape in microsatellite-stable colorectal cancer by suppressing the formation of cytotoxic effector T cells via the PRKCA–TNFα–NF-κB axis

Abstract Approximately 85% of colorectal cancers (CRCs) are mismatch repair proficient/microsatellite stable (pMMR/MSS), a subtype that is resistant to immunotherapy because of its immunosuppressive tumor microenvironment (TME). However, the molecular determinants underlying immune evasion in MSS-type CRC remain incompletely understood. We therefore investigated the role of syndecan-4 (SDC4) in mediating immune evasion in MSS-type CRC. Using an in vivo genome-wide CRISPR-Cas9 screen, we identified SDC4 as a key candidate. We then analyzed its histological and clinical relevance in MSS-type CRC tissues from the TCGA, GEO, and spatial transcriptomics databases. For functional validation, we used SDC4-knockout (KO) CT26 cells and Apc / p53 -deficient, Kras -mutant (AKP) organoids in mouse models, whereas for mechanistic studies, we utilized bulk and single-cell RNA sequencing, coimmunoprecipitation, and molecular docking. Our results indicate that high SDC4 expression in tumors from patients with CRC is negatively correlated with CD8 + T-cell accumulation in the TME. In tumor models, SDC4 KO activated TNFα/NF-κB signaling, induced chemokine production, and promoted CD8 + T-cell formation and infiltration, which collectively restrain tumor growth. We further found that SDC4 can directly interact with protein kinase C alpha (PRKCA) and limits the phosphorylation of IKKα/β, IKBα, and p65, leading to the inactivation of the NF-κB signaling pathway. Targeting SDC4/PRKCA may thus reverse immunosuppression and enhance the efficacy of immunotherapy in MSS-type CRC.