Breast Cancer Milieu Maneuvers Cancer-Associated Macrophages to Synergize Neoplastic Repertoires

Breast cancer is one of the most devastating malignancies in women worldwide. A growing body of evidence has linked neoplastic growth, invasion, metastasis, immune escape, and therapeutic resistance to infiltrating tumor-associated macrophages. In a breast cancer mass, macrophages are largely polarized to two main subtypes, M1 and M2, albeit with continuum intermediates, based on their immunological behaviors, gene signatures, and functional roles. While the former portrays proinflammatory and anti-cancer effects, the latter elicits the opposite impacts. M2 macrophages have gained rising attention as they are largely involved in fostering an immune-suppressive, cancer-promoting landscape and are imperative for malignant features across breast cancer subtypes. Through a positive feedback paracrine loop, M2 macrophages can be enriched by a plethora of dysregulated oncogenic signaling mediators, exemplified by CSF1/CSF1R, STAT3, IL-6, YAP, PI3K, PDK1, and AKT. These modulators could be released from or activated by surrounding malignant cells, fibroblasts, secreted extracellular vesicles, cell fragments generated after chemotherapies, hypoxia, dysregulated immune checkpoint pathways or oncometabolites. This review aims to discern the molecular cues fortifying M2 subpopulations. Moreover, recent advances in single-cell sequencing, spatial, and computational approaches have refined the understanding of TAM heterogeneity, while clinical translation remains limited by low therapeutic specificity, compensatory signaling, and differences between murine and human macrophage biology. Future therapeutic regimens should include strategies aimed at correcting aberrations that favor M2 polarization and are justified with divergences between humans and mice.