Identification of a novel inflammatory mast cell subpopulation in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is associated with poor therapeutic outcomes, and increasing attention has been directed toward the tumor microenvironment (TME). Mast cells are an important component of the TME, yet their heterogeneity and functional roles in TNBC remain incompletely understood.

Single-cell RNA sequencing data of TNBC were analyzed using the Seurat pipeline. Mast cells were extracted and re-clustered, and the subpopulations were characterized by functional enrichment, transcription factor, pseudotime, drug sensitivity, and cell-cell communication analyses. The role of the inflammatory subpopulation was further examined using an in silico virtual knockout approach and validated in a 4T1 subcutaneous tumor model in BALB/c mice.

Four transcriptionally distinct mast cell subpopulations were identified. Among them, the C4 subpopulation exhibited a pronounced inflammatory signature and was highly enriched in the NF-κB pathway. C4 was predicted to have lower sensitivity to several first-line chemotherapeutic agents and was associated with an unfavorable treatment response. Cell-cell communication analysis suggested that C4 may suppress CD8⁺ T cells through multiple immune-inhibitory signals. In vivo, IL-33 administration increased the proportion of C4 and promoted tumor growth, and the proportion of C4 was positively correlated with tumor volume and weight.

We identified an inflammation-associated mast cell subpopulation (C4) that may contribute to TNBC progression, providing a potential target for future therapeutic exploration.