4.2.2. Macrophage Polarization

Two major macrophage phenotypes have been described, the classically activated M1 phenotype, characterized by pro-inflammatory properties, and alternatively the activated M2 phenotype, characterized by an anti-inflammatory and a tolerogenic activity [142]. One of the macrophages' fundamental features, besides the fact that they display an important phagocytic activity, is their plasticity. They are the most plastic cells of the entire hematopoietic system [143]. In specific terms, macrophages are able to modify their phenotype according to signals perceived in their environment (cytokines, microbial particles, apoptotic bodies, activated lymphocytes) [144]. One of the current challenges in cancer treatment is to find a way to switch TAMs from an M2-like pro-tumoral into an M1-like anti-tumoral phenotype [145].

Among others, the main *stimuli* of M1 polarization are those triggering a pro-inflamma tory response such as bacterial wall components (Lipopolysaccharide, LPS, and Lipoteichoic acid, LTA), viruses or cytokines (interferon gamma, IFN-γ, and granulocyte-macrophage colony-stimulating factor, GM-CSF). By contrast, the main *stimuli* promoting M2 polarization include interleukins IL-4, IL-13, IL-10, and the cytokine M-CSF (macrophage colonystimulating factor), which activate a tolerogenic or even anti-inflammatory phenotype [146].

This concept of M1 or M2 phenotype (derived from naïve macrophages or M0) is based on in vitro models (Figure 3) where many polarization markers (Table 2) have been identified [147]. Nevertheless, in vivo, given the complexity of the cellular and cytokine environment (specifically in the TME), M1-like or M2-like macrophage terms are preferentially used [148]. M1 and M2 polarization represent two extremes of the macrophage polarization spectrum [142] between which there are various degrees of polarization towards which macrophages are able to converge according to environmental signals and their concentration [149].

**Figure 3.** Spectrum of macrophage polarization. Depending on the signals perceived in its milieu (microbial products, damaged cells, cytokines), a naive M0 macrophage can be activated and polarize towards a plethora of different phenotypes. The two extremes of this continuous polarization spectrum are, on the one hand, M1 macrophages, known to have pro-inflammatory activity, and on the other hand, M2 macrophages, known to have anti-inflammatory function. These two extremes were obtained in in vitro models where their polarization markers (such as membrane receptors, transcription factors, cytokines) were identified. However, in vivo, macrophages present in an organism, or in a tumor, will tend towards an M1 or M2 phenotype. Due to the great complexity of the in vivo milieu, these macrophages will never reach the level of polarization that macrophages obtained in vitro. These macrophages in vivo will thus be called M1-like or M2-like [149–152]. Created with BioRender.com. Moreover, once a macrophage is polarized, this polarization is not definitive. Thus, depending on environmental signals variation, such as a treatment, an M1-like macrophage may switch to an M2-like phenotype or *vice versa*. This phenomenon, based on mac-**Figure 3.** Spectrum of macrophage polarization. Depending on the signals perceived in its milieu (microbial products, damaged cells, cytokines), a naive M0 macrophage can be activated and polarize towards a plethora of different phenotypes. The two extremes of this continuous polarization spectrum are, on the one hand, M1 macrophages, known to have pro-inflammatory activity, and on the other hand, M2 macrophages, known to have anti-inflammatory function. These two extremes were obtained in in vitro models where their polarization markers (such as membrane receptors, transcription factors, cytokines) were identified. However, in vivo, macrophages present in an organism, or in a tumor, will tend towards an M1 or M2 phenotype. Due to the great complexity of the in vivo milieu, these macrophages will never reach the level of polarization that macrophages obtained in vitro. These macrophages in vivo will thus be called M1-like or M2-like [149–152]. Created with BioRender.com.

repolarization of TAMs, such as SPIONs, might be a potential therapeutic lead to inhibit cancer development or even contribute to cancer regression in solid tumors. Moreover, once a macrophage is polarized, this polarization is not definitive. Thus, depending on environmental signals variation, such as a treatment, an M1-like macrophage may switch to an M2-like phenotype or *vice versa*. This phenomenon, based on macrophage plasticity, is known as repolarization [153]. Therefore, treatments promoting the repolarization of TAMs, such as SPIONs, might be a potential therapeutic lead to inhibit cancer development or even contribute to cancer regression in solid tumors.

rophage plasticity, is known as repolarization [153]. Therefore, treatments promoting the


**Table 2.** Markers of macrophage polarization.


**Table 2.** *Cont.*

Abbreviations: Arg1: Arginase 1; CCL2: Chemokine (C-C motif) Ligand 2; CD: Cluster of Differentiation; iNOS: inducible Nitric Oxyde Synthase; IL: Interleukins; TGF-β: Transforming Growth Factor beta; Tumor Necrosis Factor alpha; VEGF: Vascular Endothelial Growth Factor.
