Among breast cancer subtypes, triple-negative breast cancer (TNBC) stands out for its aggressiveness and high frequency of brain metastases. However, the mechanisms driving BrM remain poorly understood.

We performed integrated single-cell RNA sequencing (scRNA-seq) analysis of TNBC among 15 patients (8 with metastases, 7 without) and combined these data with transcriptomic profiles of BrM from public datasets. B cell heterogeneity was characterized, and the prognostic value of cycling B cells (CD19⁺Ki67⁺) was validated in two independent RNA-seq cohorts (TCGA, GSE65194) and a Xiangya real-world cohort. Functional assays were performed using TNBC-derived organoids co-cultured with CD19⁺Ki67^+/− B cells, and multiplex immunofluorescence was used to evaluate activation of signaling pathways.

scRNA-seq revealed significant enrichment of cycling B cells in metastatic TNBC. High abundance of CD19⁺Ki67⁺ B cells correlated with poor overall survival across cohorts. Functional experiments demonstrated that CD19⁺Ki67⁺B cells enhanced TNBC organoid proliferation, invasion, and metastatic potential compared to CD19⁺Ki67⁻ B cells. Cell-cell communication analysis revealed that activation of the NAMPT/ITGA5/ITGB1 signaling pathway served as a critical mechanism by which B cells regulated crosstalk with cancer cells, which was further validated by multiplex immunofluorescence and a cohort of 74 patient samples.

CD19⁺Ki67⁺ B cells drive TNBC progression and brain metastasis by activating the NAMPT/ITGA5/ITGB1 pathway. These findings provide mechanistic insights into the immune regulation of TNBC BrM and identify potential therapeutic targets to improve clinical outcomes.