Immune evasion in locally advanced mismatch repair-deficient microsatellite instability-high colorectal cancer: Reduced T-cell infiltration and upregulation of epithelial IDO1 expression.

Early-stage colorectal cancer (CRC) with confirmed microsatellite instability generally has a favorable prognosis associated with pronounced immune infiltration. However, some patients still develop metastases. The underlying mechanisms, particularly those related to the tumor immune microenvironment, remain incompletely understood. The study included tissue samples from 217 patients with dMMR/MSI-H CRC, comprising 89 stage III/IV and 128 stage I/II cases. Tissue microarrays and immunohistochemical analyzes were performed for all cases. We evaluated immune markers identifying T cells, B cells, dendritic cells, natural killer cells, macrophages, immunosuppressive markers, and immune checkpoint targets in epithelial and stromal compartments, and additionally performed a cohort-derived immune infiltration score (IIS). The survival analysis assessed the prognostic impact of immune markers stratified by tumor stage. Stage I/II dMMR/MSI-H CRCs showed significantly higher CD3⁺ T-cell and natural killer cell levels, higher IIS metrics across all regions, and higher CD4⁺ T helper cell levels in the stroma. Stage III/IV cases exhibited increased epithelial expression of indoleamine 2,3-dioxygenase 1. Given the limited number of stage IV patients, an additional stage III vs. stage I/II comparison was performed, revealing that these immune differences were already evident at the level of nodal progression. In addition, CD3⁺, CD4⁺, and CD8⁺ T cells and the IIS showed varying prognostic associations across tumor stages. These findings suggest that the overall immunogenicity and prognostic relevance of immune markers in dMMR/MSI-H CRC depend on tumor stage. Immune profiling could be used for early-stage patient stratification and also suggests the potential benefit of early immunotherapeutic intervention.