Background <p>Cancer stem cells (CSCs) play a crucial role in breast cancer (BRCA) progression and lymph node metastasis. This study aimed to elucidate how CSCs reshape the immune microenvironment during metastatic dissemination, with a particular focus on macrophage and T-cell regulation.</p> Methods <p>A mouse orthotopic BRCA model was established to obtain primary tumor (BRCA_PT) and lymph node metastatic (BRCA_LNMT) tissues. Single-cell RNA sequencing and spatial transcriptomics were used to characterize cellular heterogeneity, marker genes, and intercellular communication. TCGA-BRCA data were analyzed for differential expression, functional enrichment, and immune cell infiltration. In vitro, 4T1-S CSCs were used to assess self-renewal, migration/invasion, ISG15-mediated signaling, and interactions with macrophages and T cells. ELISA, western blotting, sphere formation, colony formation, CCK-8, Transwell, luciferase reporter assays, and ChIP were performed. In vivo, subcutaneous and orthotopic mouse models were used to evaluate the effect of ISG15 on tumor growth and lymph node metastasis.</p> Results <p>Bioinformatic analyses revealed an elevated proportion of CSCs in BRCA_LNMT, where CSCs likely induced M2 macrophage polarization through TAM-mediated communication. ISG15 was highly expressed in metastatic tumors and associated with M2 polarization and reduced T-cell activation. In vitro, ISG15 enhanced CSC self-renewal and invasiveness, promoted IL-10–mediated M2 polarization, and upregulated PD-L1 via JAK-STAT signaling to suppress T-cell activity. In vivo, ISG15 silencing significantly inhibited tumor growth and lymph node metastasis.</p> Conclusion <p>ISG15 in BRCA CSCs promotes lymph node metastasis by driving M2 macrophage polarization and suppressing T-cell activation, highlighting a critical role for ISG15-mediated immunomodulation and a potential therapeutic target.</p> Graphical Abstract <p></p>