CTSG-expressing mast cells confer resistance to immunotherapy in colorectal cancer

Mast cells are increasingly recognized as an important regulator of the tumor microenvironment. However, their role in colorectal cancer (CRC) is controversial, and there is a lack of targetable mast cell-specific molecules to improve the tumor microenvironment and overcome resistance to immunotherapy.

We used single-cell RNA sequencing, bulk RNA sequencing, and bioinformatics analyses to characterize mast cell subsets in the CRC tumor microenvironment and assess their association with immunotherapy response. We further employed a murine orthotopic CRC model and multi-omics approaches to explore how mast cells regulate anti-tumor immunity.

Tumor-infiltrating mast cells displayed altered transcriptional state in CRC and closely associated with tumor molecular features. Mast cell deficiency significantly attenuated tumor growth and increased immune cell infiltration in an orthotopic CRC model. Strikingly, a subset of mast cells specifically expressing cathepsin G (CTSG) correlated with immunotherapy resistance and poor prognosis in patient with CRC. Mast cell-derived CTSG contributed to tumor metabolic adaptation and limited chemokine-mediated immune recruitment. The interaction between mast cells and tumor cells was mainly mediated via protease-activated receptor 2 (PAR2) signaling. Furthermore, pharmacological inhibition of CTSG improved the recruitment and function of CD8⁺ T cell and enhanced the efficacy of anti-PD-1 in vivo.

Our findings uncover a pro-tumoral CTSG⁺ mast cell subset associated with a metabolically active and immune-restricted microenvironment in CRC, and highlight CTSG as a promising target for improving immunotherapy response.