Single-Cell Profiling Identifies a CCR2+ Neutrophil-like Population Associated with Colorectal Cancer Liver Metastasis in a Murine Model

Background/Objectives: Colorectal liver metastases (CRLMs) are a major contributor to recurrence and mortality in colorectal cancer (CRC), with approximately a quarter of patients developing liver metastases over the course of the disease. Bone-marrow-derived myeloid lineages are sent into the circulatory system and colonize pre-metastatic niches, yet the transcriptional programs by which they establish a pro-metastatic microenvironment remain incompletely defined. Methods: Using an MC38 splenic-injection CRLM mouse model, we generated single-cell RNA sequencing (scRNA-seq) profiles of FACS-sorted CD11b+Gr1+ bone marrow myeloid cells, together with bulk RNA sequencing profiles of bone marrow and peripheral blood. Downstream analyses were performed in silico, including clustering and annotation, trajectory inference, cell–cell communication analysis, weighted gene co-expression network analysis (WGCNA), and pathway enrichment, with subset specificity examined against a public dataset of E. coli (Escherichia coli)-infected mice. Results: Within the CD11b+Gr1+ compartment, a CCR2+ neutrophil-like population (Ly6g+S100a8/9+) emerging during terminal differentiation was identified, which was enriched in CRLM mice but nearly absent in controls. Communication inference revealed an FN1-CD44 interaction involving mature neutrophils, which was associated with an epithelial–mesenchymal transition signature and upregulation of Tgfb1 and Il1b. This subpopulation was not recovered in the infection dataset, suggesting relative specificity to CRLMs. Conclusions: Within the constraints of a splenectomized hepatic colonization model, integrated transcriptomic analysis highlighted a CCR2+ bone marrow neutrophil-like population as a candidate contributor to CRLM, challenging the view that CCR2+ pro-metastatic myeloid cells are exclusively monocytic and suggesting candidate biomarkers and therapeutic targets for further study.