<p>Human epidermal growth factor receptor 2 (HER2) -positive breast cancer exhibits aggressive clinical progression and unfavorable outcomes. While HER2-directed agents and immune checkpoint inhibitors have transformed clinical management, persistent challenges include suboptimal response rates and acquired resistance. These limitations underscore the critical need for alternative therapeutic approaches. Our investigation, leveraging single-cell transcriptomic data derived from both healthy and neoplastic breast tissues, sought to elucidate the specific cellular subsets and regulatory pathways that contribute to the aggressive characteristics of HER2 + breast cancer. Subsequent validation of these findings was achieved through the incorporation of bulk transcriptomic datasets from various breast cancer cohorts, alongside spatial transcriptomics data pertaining to HER2 + breast malignancies, and comprehensive multiplex immunohistochemistry examinations. We identified a marked elevation of SPP1 + macrophages, displaying an M2-like immune-suppressive phenotype, within the tumor microenvironment. These particular macrophage populations were deeply implicated in modulating T cell activities and consistently correlated with adverse patient outcomes. At a mechanistic level, we ascertained that SPP1 + macrophages bolster the immune-suppressive capacity of Tregs via engagement of the CD86-CTLA4 axis, thereby facilitating the advancement of the tumor. Furthermore, the strong association between SPP1 + macrophages and both unfavorable prognosis and advanced disease stages was corroborated in an independent, in-house HER2 + breast cancer patient group. Our work uncovered the pro-tumorigenic capabilities and underlying molecular mechanisms of SPP1 + macrophages in HER2 + breast cancer, suggesting these cells represent a promising new therapeutic target.</p>