Single-cell analysis reveals crosstalk between TREM1-positive myeloid cells and cancer-associated fibroblasts in colorectal cancer progression.

[BACKGROUND] The limited efficacy of immunotherapy in colorectal cancer (CRC) underscores the need to better define immunosuppressive mechanisms within the tumor microenvironment (TME). We applied single-cell RNA sequencing (scRNA-seq) to characterize stromal-immune interactions shaping the CRC TME.

[METHODS] Paired tumor and adjacent normal mucosa samples from eight patients with CRC were analyzed by scRNA-seq. Integrated bioinformatic approaches, including trajectory inference, transcriptional regulon analysis, and cell-cell communication modeling, were used to define cellular differentiation and intercellular signaling. Key findings were validated using The Cancer Genome Atlas datasets, multiplex immunofluorescence staining, and in vitro functional assays.

[RESULTS] Trajectory analysis demonstrated a progressive increase in triggering receptor expressed on myeloid cells 1 (TREM1) expression during tumor-associated myeloid differentiation. TREM1-positive myeloid cells exhibited enriched M2-like signatures and were preferentially associated with consensus molecular subtype 4 tumors, characterized by stromal activation and poor clinical outcomes. Stromal profiling identified an α-smooth muscle actin (ACTA2)-positive population associated with CRC progression. Spatial analyses revealed close localization of TREM1-positive myeloid cells and ACTA2-positive cancer-associated fibroblasts (CAFs) in tumor tissues. Mechanistically, integrated bioinformatic analyses indicated that TREM1-positive myeloid cells engage CAFs primarily through secreted phosphoprotein 1 (SPP1) signaling, while CAFs reinforce immunosuppressive myeloid phenotypes via TGF-β-associated extracellular matrix pathways. Functional assays showed that TREM1 inhibition reduced SPP1 expression and attenuated M2 macrophage polarization.

[CONCLUSIONS] These findings identify a bidirectional interaction between TREM1-positive myeloid cells and CAFs that contributes to a profibrotic and immunosuppressive CRC microenvironment, highlighting the TREM1-SPP1 axis as a pathway of potential translational relevance.