New insights into M1/M2 macrophages: key modulators in cancer progression

In normal tissues, macrophages could be differentiated into microglial cells in the brain, Kupffer cells in the liver, and Langerhans cells in the skin [14, 15]. In the similar manner, the phenotypes of macrophages within tumor tissues (tumor associated macrophages, TAM) also represent of innate and adaptive immune responses. Tumor related inflammation represents one of the complex hallmarks of cancer especially due to an enrichment with monocyte-derived macrophages [16]. Theoretically, in response to various stimuli, TAM could be differentiated into distinct functional subsets, extremely into M1 phenotype macrophages by Th1 (IFN-γ, TNFα, and LPS et al.), and into M2 by Th2 (IL-4, IL-10, TGFβ1, and PGE2 et al.) cytokines and immunocomplexes [17] (Fig. 1). M1 phenotype macrophages are characterized by the expression of HLA-DR and CD197, whereas M2 is typical for the high expression of CD163, CD209, CD206, and CCL2 et al. [18, 19]. In addition, potential roles of IRF4 in M1-like polarization [20] and Trib1 in tissue-resident M2-like macrophages [21] have been demonstrated, which further confirms that macrophages consist of more than two subgroups. Underlying mechanisms of TAM polarization has been demonstrated to be correlated with several major signaling pathways, including C-Jun N-terminal kinase (JNK) signaling pathway, PI3K/Akt signaling pathway, Notch signaling pathway, JAK/STAT signaling pathway, et al. [22].

Herein, we would focus on the location-related distinct functional properties instead of these pathways. In normal tissues, macrophages contribute to maintaining tissue homeostasis through phases-depending mechanisms. In the initiation phase of host immune response, macrophages could generally be served as tissue sentinels via phagocytosis. Then the damaged cells and debris could be eliminated by macrophages in the phase of full inflammation. In the recovery phase, macrophages participate in the re-establishment of tissue integrity [23]. As known, tumor is partially a disease of homeostatic imbalance. Some mechanisms involved in keeping normal tissues homeostasis might also be utilized by TAM to acquire different functional phenotypes, thereafter influencing adaptive immunity towards different directions [24].

In this perspective, momentum TAM accumulated in and around tumor organs to monitor the abnormal tissue architecture, cellular compositions, and functional states [25], whereas TAM could finally be trained to perform key homeostatic functions in the maintenance of tumoral growth and progression [26]. Reasons could be offered by the issue of heterogeneity in the M1/M2. M1 phenotype macrophages are tumor-resistant due to intrinsic phagocytosis and enhanced antitumor inflammatory reactions. Contrastingly, M2 are endowed with a repertoire of tumor-promoting capabilities involving immuno-suppression, angiogenesis and neovascularization, as well as stromal activation and remodeling [27]. Functional properties of M1 and M2 would be discussed later in this review. Due to ignorance of distinct polarization of TAM, researches on exact prognostic values of TAM during cancer progression still failed to reach a consensus and remained contradictory [28]. In addition, histological distribution of TAM within cancer tissue has been analyzed and studied [29, 30], further proven to be related with corresponding functional potentials and overall influence on cancer outcomes [31, 32].

M1phenotype macrophages have intrinsic function to trap, phagocytose, and lyse tumor cells [33]. In addition, enhanced tumor antigen presenting ability of M1 would promote other leukocytes cytotoxic functions. For instance, CD8 + T cells and NK cells could be strengthened by immuno-stimulatory cytokines (IL-6, IL-12, TNF et al.) from M1 phenotype macrophages [34]. As a result, tumor cells apoptosis would be induced at this stage. Tumor stem cells have fewer immunogenic antigens, but more vigorous proliferative and differentiative abilities. Considering the heterogeneity and tumor-intrinsic mechanisms of immune escape [35], tumor stem cells might use M1 phenotype macrophages as a natural filter to avoid being destroyed and survived to next stage [36] (Fig. 2a).

For long it has been a consensus that angiogenesis and neovascularization are essential for tumor growth and metastasis [42]. Many studies have shown that M2 phenotype macrophages are crucial to support the evolution of tumor-related vasculature, a process in which neo-vessel sprout from existing blood vessels or by proliferation, motility and accumulation of vascular endothelial cells [43]. Recent studies have emphasized the significant correlation between M2 and tumor neo-vessels with focus on perivascular and peri-necrotic areas [44]. Additionally, morphological studies have demonstrated a disorganized and collapsed characteristic of tumor neo-vessels [45]. This reason, combined with over-rapid growth of tumor cells, coordinately contribute to hypoxia zones, especially in tumor center and peri-necrotic regions. Hypoxia in these areas induces the expression of inflammatory molecules (IL-4, IL-10 et al.), thus promoting the recruitment of macrophages followed by conversion to the M2 phenotype [46]. In this case, M2 participates in a “angiogenesis cascade”, which is capable to affect the onset and maintenance of the angiogenic process, including degradation of the extracellular matrix, endothelial cell proliferation and migration, and neovascularization [47]. Molecular mechanisms underlying each phase could resulted from a variety of pro-angiogenetic cytokines and growth factors, including VEGF, TNFα, IL-8, and bFGF, as well as angiogenesis-regulating enzymes like MMP and COX-2 [48] (Fig. 2c).

Emerging evidence has also illustrated the significance of tumor stromal transformation in tumor progression. Herein, M2 is discussed in terms of intimate correlations with other stromal components. Actually, stromal cells around incipient tumors have a futile attempt to “destroy and repair” tumor tissues [49]. However, M2 could assist tumor cells to gain a hallmark to activate and remodel stromal features to support tumors [50].

For instance, fibroblasts differentiated from mesenchymal cells are important to tissue-repair, whereas transformed to cancer-associated fibroblasts (CAFs). In turn, CAFs also promote tumor growth and metastasis by recruiting and reprogramming macrophages into M2 phenotype [51]. In addition, M2-related enzymes promote ECM digestion and deposition, as well as the proteolysis of basement membrane (BM) and collagens surrounding tumor nests [52]. In this case, intrinsic defensive abilities of collagens are dramatically reduced. The principal factors accounted for this transformation include cathepsin B, TNFα, MMPs et al. [53].

Subsequently, degraded BM and collagen fragments may serve as chemotactic stimuli to circulating monocytes. These recruited macrophages, especially M2, could further induce the degradation and re-arrangement, enhance the stiffness of collagens, and co-orchestrate angiogenesis [54]. Thus, all these have uncovered a positive feed-back transformation displayed among M2 and other representative stromal constitutions (Fig. 2d).

The specialization of macrophages in particular microenvironments explains their heterogeneity. In addition, the heterogeneous functional properties of macrophages would result from their location in tumor tissues [55]. There are three typical locations including tumor center, invasive front (the interface between tumor cells and stroma), and tumor stroma [56]. It has been documented that macrophages seem to be “hijacked” to perform distinct functions according to the location-related signals [22]. Briefly, M2 phenotype macrophages would be preferentially endowed with potentials to promote the motility of cancer cells in invasion areas [57], to promote metastasis in stromal and perivascular areas [31], and to stimulate angiogenesis in avascular and peri-necrotic hypoxic areas [54]. In other words, the distribution pattern of macrophages might be correlated with different cancer progression mechanisms.

Notably, we have recently found a timely novel result that the distribution pattern of macrophages could be an independent prognostic factor in gastric cancer [30]. According to relative macrophages densities in tumor nest or tumor stroma, gastric cases could be divided into nest-dominant pattern and stroma-dominant pattern. As a phenomenon, stroma-dominant pattern cases tended to have poorer survival and higher malignancy. In theory, accumulation of macrophages in tumor stroma might participate more actively in the process of stroma activation [58] and ECM remodeling, together with other stromal components including lysyl oxidase, MMP9, type IV collagen, which has been emphasized by Peng et al. [59]. In addition, Yang et al. [60] preliminarily elucidated the prognostic value of CD163+/CD68+ ratio in colorectal cancer invasive front. To make sense of macrophages polarization in different distribution patterns, studies could be conducted around macrophages phenotypes and ratios, tumor locations and biological functions in the future.