Tumor-Associated Macrophages (TAMs) in Cancer: Functional Programs, Metastatic Mechanisms, and Therapeutic Targeting

Tumor-associated macrophages (TAMs) are among the most abundant immune cell populations in breast cancer and have emerged as central regulators of tumor progression, metastatic dissemination, immune evasion, and therapeutic resistance. While TAMs were historically described using a simplified M1/M2 polarization framework, accumulating evidence indicates that TAMs in breast cancer comprise a continuum of phenotypic and functional states shaped by ontogeny (tissue-resident vs monocyte-derived), spatial localization (including hypoxic, perivascular, and invasive niches), tumor-intrinsic programs, and therapy-induced selective pressures. In breast cancer, mechanistic studies integrating lineage tracing, intravital imaging, single-cell and spatial profiling, and clinical analyses have established that TAMs actively coordinate rate-limiting steps of the metastatic cascade. These include promotion of angiogenesis and vascular permeability, orchestration of tumor cell invasion and TMEM-mediated intravasation, facilitation of metastatic seeding and niche formation, and suppression of anti-tumor immunity. TAMs also critically influence therapeutic response by modulating chemotherapy efficacy and limiting the activity of immune checkpoint blockade. Therapeutic strategies targeting TAMs in breast cancer have evolved from depletion approaches (CSF1/CSF1R blockade) to inhibition of monocyte recruitment (CCL2/CCR2 axis), functional reprogramming (CD40 agonism, PI3Kγ inhibition), and macrophage-directed checkpoint modulation (CD47–SIRPα axis). Early clinical studies demonstrate clear pharmacodynamic activity but highlight the need for context-specific and combination-based approaches. This review focuses on TAM biology in breast cancer progression and metastasis, synthesizing key mechanistic and translational evidence and proposing a framework in which spatially and functionally defined macrophage states act as rate-limiting regulators of dissemination and therapy response. We further outline principles for rational TAM-targeting strategies that integrate tumor stage, metastatic niche, and treatment context.