Macrophage activation has been historically categorized into two polarized states: M1 and M2 (20–22). Classically activated (M1) macrophages are induced by pro-inflammatory stimuli such as interferon-γ (IFN-γ), lipopolysaccharide (LPS) and other Toll-like receptor (TLR) ligands (21, 23, 24). They are characterized by elevated antigen-presenting capacity, potent microbicidal and tumoricidal activity, a strong pro-inflammatory profile through production of cytokines including TNF-α, interleukin-1 beta (IL-1β) and IL-6, reactive oxygen and nitrogen species, and typically express markers such as iNOS and MHC II. Functionally, M1 macrophages contribute to host defense, pathogen clearance, inflammatory responses and adaptive immune response by activating T helper 1 (Th1) cells. Alternatively activated (M2) macrophages arise in response to anti-inflammatory signals such as IL-4, IL-13, IL-10, glucocorticoids, and TGF-β (21, 23, 24). They support tissue remodeling, wound healing, angiogenesis, and inflammation resolution, express markers such as arginase 1 (Arg1), CD163, and CD206, and produce anti-inflammatory cytokines including IL-10. Functionally, M2 macrophages are involved in immunoregulation, suppression of inflammation and tissue repair. Also, they reinforce Th2-type immunity limiting Th1-mediated activity. Beyond the classical M1/M2 dichotomy, the M2 compartment comprises heterogeneous subpopulations (M2a, b, c and d) defined by distinct activating cues and functional programs, such as tissue repair, immune regulation and pro-angiogenic activity, indicative of macrophage functional heterogeneity (25, 26). Recently, single-cell RNA-seq analysis of human BC TAMs identified seven distinct TAM subtypes, ranging from pro-inflammatory/anti-tumor to pro-tumoral functional state (12). These findings corroborate that the classic M1/M2 dichotomy is an oversimplification, as TAM phenotypes form a dynamic continuum with overlapping activities affecting inflammation, immune modulation, angiogenesis and tumor progression. Interestingly, the presence of pro-inflammatory TAM subsets highlights the controversial role of inflammation in cancer, which can both constrain and fuel tumor progression depending on context. This further supports the idea that macrophage activation does not occur in discrete polarization states but rather encompasses a dynamic and context-dependent spectrum of phenotypes that can shift in response to microenvironmental stimuli (27). However, the term “M2 phenotype” is commonly used to describe TAM populations with immunosuppressive and pro-tumoral functions.

In cancer, circulating monocytes are recruited into TME by soluble factors or extracellular vesicle (EVs)-loaded molecules released from tumor cells (28). Key chemoattractants include chemokines such as CCL2, CCL5 and CXCL12, growth factors like CSF−1 and CSF−2, and VEGFA (29, 30). Tumor−derived EVs further contribute to this process by delivering proteins and RNAs that enhance monocyte chemotaxis and survival (31). Major drivers of M2 polarization are anti−inflammatory cytokines and growth factors, including IL−4, IL−10, TGF−β, and CSF−1, secreted by tumor, stromal and immune cells within the TME (32). In parallel, hypoxic conditions in poorly vascularized tumor regions stabilize HIF−1α and HIF−2α in infiltrating macrophages, driving metabolic adaptation (33). In addition, highly glycolytic cancer cells produce metabolites such as lactate, which act as environmental cues (34). Moreover, tumor−derived EVs play a critical role in macrophage reprogramming by transferring miRNAs, lipids, and immunomodulatory proteins that alter their transcriptional and epigenetic landscapes, dampen inflammatory pathways, and consolidate immunosuppressive functions (35, 36). Functionally, TAMs promote tumor growth and progression through different mechanisms. They secrete cytokines and growth factors, including VEGF, PDGF, and TGF−β that directly stimulate angiogenesis, cancer cell proliferation and survival, contributing to tumor expansion and poor clinical outcomes (37). In addition, TAMs facilitate metastasis and invasion by secreting matrix metalloproteinases (MMPs) able to remodel the ECM and promoting epithelial−mesenchymal transition (EMT), migration and the formation of pro-metastatic niches (38, 39). They also contribute to therapy resistance by activating survival pathways, impairing drug penetration through ECM remodeling, and supporting cancer stem cell niches (40). A central role of TAMs is the establishment of an immunosuppressive milieu that weakens effective anti−tumor immunity. TAMs release indeed anti−inflammatory cytokines such as IL−10, TGF−β, and IL−6, inhibit cytotoxic CD8⁺ T cell function, and recruit regulatory T cells, thereby suppressing adaptive immune responses (41). Importantly, TAMs also modulate immune checkpoint pathways: they can induce and express PD−L1 and other inhibitory ligands, further dampening cytotoxic T cell activity and contributing to immune evasion (42). Overall, these observations highlight the crucial role of the cross-talk between cancer cells and TME in orchestrating TAM reprogramming and function, thereby influencing the biological and clinical course of the disease. Among the key signals involved in this fundamental bidirectional dialogue, miRNAs have emerged as critical mediators.

In BC, miRNA expression patterns correlate with key clinicopathological features such as hormone receptor status, lymph node involvement, proliferation index, and p53 status, highlighting their biological and clinical relevance (52). For instance, miR-29a promoted EMT, invasion, and lung metastasis by targeting PTEN and activating AKT signaling in ERα-positive luminal BC models both in vitro and in vivo (53). Response to therapy is also affected by miRNA dysregulation. Normann et al. detected improved response to the anti-HER2 treatment trastuzumab upon miR-101-5p overexpression and consequent alterations in key HER2 signaling-related pathways in HER2-positive BC cell lines, findings validated in TCGA and METABRIC datasets (54). MiRNAs can also play a tumor-suppressive role. Indeed miR-33b ectopic expression in HER2-positive BC resulted in a notable reduction in EMT, proliferation, invasion, and migration, while simultaneously promoting apoptosis; lower miR-33b levels in patient-derived tumor samples were associated with poorer survival (55). In addition, tumor cells exploit miRNAs as paracrine signals. Tumor-derived EVs carrying miRNAs contribute to mold a protumoral TME, including the reprogramming of macrophages toward an immunosuppressive M2 phenotype. EVs enriched with miR-92a-3p from BC in vitro models have been shown to be taken up by macrophages, where miR-92a-3p targets TLR4 thus increasing M2 markers and enhancing immune evasion and tumor invasiveness (Figures 1A, B) (56). Also, paclitaxel-resistant BC cells secrete exosomes enriched in miR-99b-3p, which enhances neighboring tumor cell migration and drug resistance by activating the AKT/mTOR pathway (53). Exosomal miR-99b-3p is also transferred to macrophages, where it promotes M2 polarization, indirectly reinforcing chemoresistance and invasiveness in drug-sensitive pre-clinical BC models both in vitro and in vivo. Moreover, tumor cells exhibit reduced release of tumor-suppressive miRNAs. For instance, hypoxic in vitro BC cells secrete exosomes with reduced miR-143-3p, leading to increased expression of a subunit of the mTORC2 complex in macrophages, which promotes an M2-like status and enhances BC cell invasion and migration (57). This is however a rare example because the current literature mainly describes miRNAs directly downregulated in TAMs and subsequently restored using engineered exosomes (58, 59). Moreover, tumor-suppressive miRNAs associated with favorable prognosis may indirectly influence immune–tumor cross-talk, but their specific roles in macrophages have not yet been fully elucidated (60). Overall, even though further investigation is certainly needed to elucidate how miRNAs influence the more complex and nuanced macrophage plasticity, all these studies illustrate a complex network in which BC cells actively modulate macrophage phenotype through miRNA-mediated signaling pathways, revealing exploitable targets for immunomodulatory therapies.

The modulation of miRNA expression within macrophages plays a central role in driving their phenotypic switch toward an M2-like state, by regulating signaling pathways and transcriptional programs (61, 62). For instance, miR-182 is induced in macrophages via tumor-secreted TGFβ signaling, where it downregulates TLR4 and promotes an M2-like functional program (Figures 1A, B); constitutive deletion of miR-182, as well as conditional knockout of miR-182 in macrophages, compromises M2-like TAM differentiation and limits BC progression in in vivo murine models (63). Conversely, miR-382 acts within macrophages to reduce M2 skewing by targeting metabolic regulators such as peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), limiting the tumor-promoting functions of TAMs and consequently cancer progression and metastasis in in vitro and in vivo TNBC models (64). MiRNAs also serve as key messengers in TAM-tumor cell crosstalk (65). Indeed, TAMs actively promote BC progression through the secretion of EVs enriched with miRNAs that reprogram malignant cells. For instance, TAM-derived exosomal miR-660 promotes TNBC cell growth and metastasis through modulation of oncogenic signaling pathways such as the IκB kinase beta (IKKβ)/NF-κB p65 axis by direct inhibition of kelch like family member 21 (KLHL21), whereas miR-223-3p facilitates pulmonary metastasis by enhancing invasive and migratory capacities of TNBC cells in both in vitro and in vivo models (Figures 1C, D) (66–68). TAM signaling also contributes to metabolic plasticity, for instance macrophage-derived miR-503-3p suppresses glycolysis and promotes oxidative phosphorylation in breast cancer cells (Figures 1C, D) (69). Interestingly, Skourti E. et, by using dual radionuclide–fluorescence reporter system for spatiotemporal in vivo miRNA quantification, showed that miR-155 upregulation is dependent on macrophage infiltration, revealing immune-driven miRNA dynamics within the TME (70). Considering that miR-155 has also been reported as a LPS-induced pro-inflammatory miRNA in macrophages, it is clear that the effects mediated by miRNAs are context-dependent (71). Further highlighting the importance of miRNAs as key mediators of TAM–tumor cell crosstalk, exosomes from TAMs lacking protumoral progranulin are characterized by enhanced miR-5100 levels, a tumor-suppressor miRNA which directly suppresses the chemokine CXCL12 thus inhibiting BC cells invasion, migration, and EMT and reduce lung metastasis in vivo (72). Collectively, these studies underscore TAM-derived miRNAs as critical effectors of tumor-TME communication in BC, reinforcing an immunosuppressive and pro-metastatic microenvironment and identifying macrophage-derived miRNAs as potential therapeutic targets. However, the temporal dynamics and organization of these bidirectionality are still unclear and warrant further investigation.