<i>CEBPB</i> Expression in Tumor Cells Drives Immune Evasion in Colorectal Cancer via <i>CTLA4</i> Up-regulation in T Cells

<b>Background:</b> Immune checkpoint inhibitors are ineffective in the majority of colorectal cancers (CRCs) that are microsatellite stable. However, the underlying reasons for their unresponsiveness and mechanisms of immune evasion are poorly understood. In the present study, we aimed to elucidate the mechanisms underlying the immune evasion driven by CRC cells. <b>Methods:</b> We performed single-cell RNA sequencing of tumor tissues from 30 CRC patients and syngeneic mice implanted with transformation-related protein 53 (<i>Trp53</i>) knockout CT26 cells. Gene expression and correlations of individual tumor microenvironment (TME) components were analyzed, and their functional significance was investigated using syngeneic mouse models and cell line co-culture experiments. <b>Results:</b> CCAAT enhancer-binding protein beta (<i>CEBPB</i>) expression was increased in tumor protein 53 (<i>TP53</i>)-mutated CRCs. We confirmed that wild-type <i>TP53</i> negatively regulated <i>CEBPB</i> expression in CRC cell lines. <i>CEBPB</i> expression was associated with decreased intratumoral T cell infiltration and negatively impacted survival in CRC patients. In the intercellular correlation analysis of gene expression, tumor epithelial cell <i>CEBPB</i> expression was significantly correlated with cytotoxic T-lymphocyte associated protein 4 (<i>CTLA4</i>) expression in T cells, especially in regulatory and exhausted T cells. <i>Cebpb</i> overexpression promoted tumor growth in the immunocompetent syngeneic mouse models, which was accompanied by increased CTLA-4 expression in tumor-infiltrating CD4⁺ T cells. In vitro co-culture experiments also showed that tumor cell <i>CEBPB</i> overexpression increased <i>CTLA4</i> in T cells. <b>Conclusions:</b> Tumor cell <i>CEBPB</i> expression, up-regulated by <i>TP53</i> mutation, can increase <i>CTLA4</i> expression in T cells and negatively affect patient outcomes. These findings suggested a central role of tumor cell <i>CEBPB</i> in shaping an immunosuppressive TME.