Macrophage-associated immune-stromal crosstalks correlate with the tumor microenvironment in microsatellite-stable mucinous colorectal cancer.

Mucinous adenocarcinoma (MAC) of the colorectum is increasingly recognized as a subtype characterized by an immunosuppressive tumor microenvironment and poor response to immunotherapy, yet its immune and stromal landscape remains incompletely defined. In this study, we integrated three publicly available single-cell RNA sequencing datasets to comprehensively profile the cellular ecosystem of microsatellite-stable (MSS) MAC. Compared to classical adenocarcinoma (AC), MAC exhibited a distinct tumor microenvironment marked by elevated infiltration of myeloid and fibroblast populations, along with reduced lymphocyte and epithelial cell proportions. Notably, MAC harbored abundant immunosuppressive neutrophils that interacted intensively with interstitial resident tissue macrophage-like tumor-associated macrophages (RTM-TAMs) through IL1B-IL1R2 and CXCL8-CXCR2 signaling pathways, related to a pro-tumor myeloid network. Fibroblast analysis revealed a significant enrichment of VEGFA⁺ myofibroblastic cancer-associated fibroblasts (myCAFs), particularly in hypoxic, mucin-rich tumor regions, which were spatially associated with RTM-TAMs via SPP1-CD44/ITGA5/ITGB1 interactions. Leveraging cell-type-specific genes and key ligand-receptor pairs, we developed a Mucinous Colorectal cancer Immune Module (MCIM) comprising 18 genes, which stratified patient prognosis and was associated with overall survival in colorectal cancer cohorts. Together, these findings provide a detailed map of the immune-stromal architecture in MAC with MSS status, reveal macrophage-associated immunosuppressive features, and propose MCIM as a potential biomarker for prognostication in mucinous colorectal cancer.