Objective:

To characterize mast cell subsets in the colorectal cancer (CRC) tumor microenvironment and assess their association with immunotherapy response.

Approach:

• Single-cell RNA sequencing: Used to characterize mast cell subsets in the CRC tumor microenvironment.
• Bulk RNA sequencing: Applied to assess the association of mast cells with immunotherapy response.
• Murine orthotopic CRC model: Employed to explore how mast cells regulate anti-tumor immunity.
• Multi-omics approaches: Utilized to investigate the role of mast cells in the tumor microenvironment.

Key Findings:

• Tumor-infiltrating mast cells displayed altered transcriptional states in CRC.
• Mast cell deficiency significantly reduced tumor growth and increased immune cell infiltration.
• A subset of mast cells expressing cathepsin G (CTSG) was associated with immunotherapy resistance and poor prognosis.
• CTSG contributed to tumor metabolic adaptation and limited immune recruitment.
• Interactions between mast cells and tumor cells were mediated via protease-activated receptor 2 (PAR2) signaling.
• Inhibition of CTSG improved CD8+ T cell recruitment and enhanced the efficacy of anti-PD-1.

Interpretation:

CTSG+ mast cells are linked to a metabolically active and immune-restricted microenvironment in CRC.

Limitations:

• The study primarily focused on murine models, which may not fully replicate human CRC.
• Further research is necessary to validate findings in larger human cohorts.

Conclusion:

CTSG+ mast cells represent a previously underappreciated component of the CRC tumor microenvironment that contributes to immunotherapy resistance.