A Critical Role of DC-SIGN+ Tumor-Associated Macrophages in Colorectal Cancer Immune Evasion and Progression via BCL-3-Mediated PD-L1 Expression

Background: Tumor-associated macrophages (TAMs) play a pivotal role in facilitating tumor immune escape in colorectal cancer (CRC). C-type lectin Dendritic Cell-Specific ICAM-Grabbing Nonintegrin (DC-SIGN) is variably expressed in TAMs in tumor tissues. However, its role in CRC progression remains poorly defined. Methods: We analyzed The Cancer Genome Atlas (TCGA) data and an independent CRC cohort to evaluate the prognostic significance of DC-SIGN high TAMs. Immunofluorescence and flow cytometry were used to characterize DC-SIGN expression in CRC tissues. RNA sequencing and bioinformatics analyses were performed on sorted DC-SIGN high and DC-SIGN low TAMs. Functional assays using THP-1–derived macrophages and primary TAMs were conducted to examine how DC-SIGN regulates PD-L1 expression via the transcription factor BCL-3. Results: DC-SIGN was specifically expressed in TAMs within CRC tissues and was associated with increased stromal and immune cell infiltration. DC-SIGN expression correlated with worsened prognosis in CD8 high , but not CD8 low , patients with CRC across two independent cohorts, and served as an independent predictor of unfavorable survival in CD8 high CRC. Transcriptomic profiling revealed that DC-SIGN high TAMs exhibited distinct immune-related pathways, including marked upregulation of PD-L1 and PD-L1 immune checkpoint pathway. Mechanistically, Lewis x -ligated DC-SIGN upregulated PD-L1 expression at both mRNA and protein levels through BCL-3, which directly bound to the PD-L1 promoter. Conclusion: The DC-SIGN/BCL-3 axis in TAMs drives PD-L1 expression and contributes to CRC immune evasion. Targeting DC-SIGN + TAMs may represent a promising therapeutic strategy to reprogram the tumor microenvironment (TME) and improve the efficacy of immunotherapy in CRC. Plain Language Summary: Colorectal cancer is one of the most common cancers, and while the immune system is designed to recognise and destroy cancer cells, tumours can develop ways to avoid this attack. In this study, we investigated immune cells called macrophages, which normally protect the body but can be reprogrammed inside tumours to support cancer growth. We found that a molecule called DC-SIGN is present on many of these macrophages in colorectal cancer and that patients with higher levels of these DC-SIGN–positive macrophages had poorer outcomes, particularly when their tumours also contained many cancer-fighting T cells. By studying these cells in detail, we discovered that DC-SIGN increases the production of PD-L1, a protein that blocks T cells from killing cancer cells. We also showed that DC-SIGN uses another protein, BCL-3, as a switch to turn on PD-L1. These findings reveal a new way that colorectal cancer avoids the immune system and suggest that targeting DC-SIGN may help improve immunotherapy, a treatment that boosts the body’s natural defences against cancer. Keywords: colorectal cancer, tumor-associated macrophages, C-type lectin dendritic cell-specific ICAM-grabbing nonintegrin, programmed cell death ligand-1, BCL-3, immune evasion