BACKGROUND: The production of extracellular mucus and expression of mucins are commonly aberrant in colorectal cancer, yet their roles in tumour progression remain unclear. METHODS: To investigate the potential influence of mucus on immune response and prognosis, we analysed mucinous differentiation (non-mucinous, 0%
mucinous component, 1-50%
mucinous, >
50%) and its associations with immune cell densities (determined with three multiplex immunohistochemistry assays or conventional immunohistochemistry) and survival in 1049 colorectal cancer patients and a validation cohort of 771 patients. We also assessed expression patterns of transmembrane (MUC1, MUC4) and secreted (MUC2, MUC5AC and MUC6) mucins using immunohistochemistry. RESULTS: Mucinous differentiation was associated with higher densities of CD14 CONCLUSIONS: Although mucinous differentiation did not independently predict survival, extracellular mucus and MUC1 expression could promote tumour progression through immunosuppression.