Abstract
Sidedness influences colorectal cancer (CRC) prognosis and treatment response, yet the mechanism dictating differential EGFR inhibitor (EGFRI) sensitivity is unclear. This study investigated the tumor microenvironment (TME) in relation to EGFRI eligibility-clinically defined by factors such as tumor sidedness (e.g., left-sided), RAS/BRAF wild-type status, and microsatellite stability (MSS)-using integrated single-cell RNA sequencing (scRNA-seq), with bulk RNA-seq and spatial transcriptomics validation. We found cancer cell features reflected EGFRI eligibility more strongly than sidedness. EGFRI eligible tumors exhibited high Epiregulin (EREG) expression by cancer cells. Cell interaction analysis revealed a specific "EREG/EGFR/CSF axis" in EGFRI eligible CRC: EREG derived from cancer cell stimulates EGFR-expressing non-myCAF subtypes of cancer-associated fibroblasts (CAFs), which signal via CSF to M1/M2-like Tumor-Associated Macrophages/Monocytes (TAM/TAMo), potentially promoting M2 polarization. Spatial analysis confirmed the proximity of these interacting cell populations and localized EGFR pathway activation near cancer cells specifically in eligible tumors. This study provides a TME-centric view of EGFRI eligibility, identifying a key intercellular communication network driving differential responses. These findings suggest TME features could offer more precise patient stratification than sidedness alone, potentially improving CRC therapeutic strategies.