Importance of M1/M2 state macrophage switching during inflammation (scientific review)

In recent studies, it has been demonstrated that macrophages not only play an essential role in adaptive processes in many human diseases but are also capable of over-increasing inflammatory effects. That is, they have both positive and negative effects. Their specific functions depend on the tissue microenvironment and, consequently, on the phenotype acquired under these conditions. Accordingly, it is of primary importance for the organism what kind of phenotype macrophages have in certain situations and whether it is possible to influence the process of their phenotyping.

What is inflammation?

Inflammation is a biological immune system’s response to harmful stimuli, such as pathogens, damaged cells, toxic compounds, or irradiation (1) and acts by removing injurious stimuli and initiating the healing process (2). Usually, during acute inflammatory responses, cellular and molecular events and interactions efficiently minimize impending injury or infection. This mitigation process contributes to the restoration of tissue homeostasis and the resolution of acute inflammation. Although inflammatory response processes depend on the precise nature of the initial stimulus and its location in the body, they all share a common mechanism, which can be summarized as follows: 1) cell surface pattern receptors recognize detrimental stimuli; 2) inflammatory pathways are activated; 3) inflammatory markers are released; 4) inflammatory cells are recruited.

However, uncontrolled acute inflammation may become chronic, contributing to a variety of chronic inflammatory diseases (3) or be inadequately strong, which leads to the destruction of their own cells and autoimmune diseases (4). The inflammatory response involves a highly coordinated network of many cell types. Activated macrophages, monocytes, and other cells mediate local responses to tissue damage and infection. At sites of tissue injury, damaged epithelial and endothelial cells release factors that trigger the inflammatory cascade, along with chemokines and growth factors, which attract neutrophils and monocytes. The first cells attracted to a site of injury are neutrophils, followed by monocytes, lymphocytes (natural killer cells [NK cells], T cells, and B cells), and mast cells (5; 6; 7). Inflammation-mediated immune cell alterations are associated with many diseases, including asthma (8), cancer (9), chronic inflammatory diseases (10), coronary heart disease (11), chronic kidney disease (12), atherosclerosis (13), diabetes (14), autoimmune and degenerative diseases (15).

Macrophage’s role in inflammation

Different investigations proved that macrophages, differentiated from monocytes, are important components of the mononuclear phagocyte system, and are critical in inflammation initiation, maintenance, and resolution (16; 17; 18). During inflammation, macrophages present antigens, undergo phagocytosis, and modulate the immune response by producing cytokines and growth factors. Mast cells, which reside in connective tissue matrices and on epithelial surfaces, are effector cells that initiate inflammatory responses. Activated mast cells release a variety of inflammatory mediators, including cytokines, chemokines, histamine, proteases, prostaglandins, leukotrienes, and serglycin proteoglycans (19). 

Macrophages have at least three major functions: antigen presentation, phagocytosis, and immunomodulation (20). The functional responses of macrophages in host defence consist of sequential steps: active recruitment of the cells to the site of infection, recognition of microbes, phagocytosis, and destruction of ingested microbes. In addition, macrophages produce biologically active molecules that serve many important roles in innate and adaptive immune responses. Thus, macrophages play a key role in the up- and down-regulation of the inflammation process. 

These cells have a plastic phenotype that is highly dependent on the prevalent cytokines and growth factors found in the microenvironment, and the phenotype can change in response to changes in the microenvironment. The scientific community has attempted to define the types of macrophages by classifying them on a spectrum ranging from M1 to M2 phenotype. Classically activated macrophages (M1) are characterized as pro-inflammatory, secrete cytokines like IL-6,12 and TNF-α, and have tumoricidal activity (21). 

Alternatively activated macrophages (M2) are characterized as immunosuppressive and express cytokines such as IL-10 and TGF-β, MMP12, arginase (22), promote tissue remodelling and repairing through collagen formation, and clear dying cells and debris by efferocytosis (23). Thus, the normal course of the inflammatory process directly depends on the balance of M1 and M2 macrophages. 

Inflammation and phenotype reprogramming macrophages

To date, scientists have discovered different approaches to the impact on the activation of the formation of macrophages and their polarization. The phenotype and functional polarization of macrophages are regulated by many factors. Signal transducers and transcriptional activators that affect macrophage polarization are as follows: STATs, interfering regulatory factors (IRFs), nuclear factors (NF-kappa b), activating proteins (AP1), peroxisome proliferators activating receptors (PPAR-gamma), and cAMP response element-binding protein (CREB). They interact with each other to regulate the phenotype of macrophages. The JAK-STAT signalling pathway is also closely related to the phenotypic activity of macrophages (24). IFN (interferon) works through this signalling pathway. IFN-γ can induce polarization of M1 macrophages. The role of IFN-α/IFN-β-mediated signalling pathways in macrophage polarization is not well understood, but it is known to enhance anti-inflammatory effects under certain conditions.PI3K pathway plays an important role in the survival of macrophages (25). Different AKt kinases have different effects on macrophage polarization. Among them, AKt1 can be activated by PI3K, and the ablation of AKt1 leads to polarization of M1-type macrophages, while the ablation of AKt2 leads to polarization of M2-type macrophages (26). In addition to the above signalling pathways, mitochondrial biosynthesis also plays an important role in macrophage polarization (27).

Macrophage’s role in cancer

One of the functions of macrophages is to provide a defence mechanism against tumour cells. In the last decades, the mechanism of tumour cell killing by macrophages has been studied extensively (28). The tumour cytotoxic function of macrophages requires stimulation either with bacterial cell wall products such as lipopolysaccharide (LPS) or muramyl dipeptide (MDP) or with cytokines such as interferon-gamma (IFN-gamma) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Activated macrophages secrete several substances that are directly involved in tumour cell killing i.e., tumour necrosis factor (TNF) and nitric oxide (NO) (29). On the other hand, substances are secreted that can stimulate tumour cell growth, depending on the stage and the nature of the tumour. Several clinical trials have been performed aiming at the activation of macrophages or dendritic cells, a subpopulation of the macrophages.

But the same time, macrophages are a major component of the tumour microenvironment and orchestrate various aspects of immunity reaction in tumorigenesis. Depending on their activation status, macrophages can exert dual influences on tumorigenesis by either antagonizing the cytotoxic activity immune cells or by enhancing antitumor responses. In most solid cancers, increased infiltration with tumour-associated macrophages (TAMs) has long been associated with poor patient prognosis, highlighting their value as potential diagnostic and prognostic biomarkers in cancer (30). Many researchers are now taking advantage of macrophages’ plasticity to re-educate the cells to work for the patient. One way to switch TAMs from the M2 phenotype, which promotes cancer growth, to the immune-boosting M1 phenotype is to provide the cells with proinflammatory stimuli, such as interferons or ligands for Toll-like receptors. Alternatively, researchers can directly target molecular switch proteins responsible for driving M2 characteristics, such as PI3-kinase and the transcription factor STAT3. In animal models, drugs that inhibit these molecules have successfully skewed TAMs toward M1 phenotypes and shrunk tumours (31; 32).

Can we change the macrophage polarization?

The possibilities to influence the polarization of macrophages are widely studied. The use of immune modulators now has the task of shifting the differently pronounced stages of this disturbed balance back towards the biological ‘normal’. This means that on the one hand excessive inflammations must be suppressed, on the other hand, exhausted immune processes must be reactivated. To suppress inflammations is necessary with illnesses of the rheumatic type, yet to activate immune performance again with illnesses with reduced immune function and cancer (Dr. med. Henry Krah, Merbelsrod, 23.07.2019). Especially important due to a large number of diseases of chronic inflammatory genesis are type M2 macrophages that are critical in anti-inflammatory, clearance of parasites, Th2 response, immunoregulation, tissue remodelling, angiogenesis, matrix deposition, etc. (23). According to numerous studies of common diseases (cancer, autoimmune, chronic inflammation, etc.), the use of polyphenols, polypeptides, and bacteria are among the most promising ways in influencing the inflammation process, its resolution, balance of M1/M2 macrophage, and switching their phenotypes (33; 34; 35). Recently, scientists created a new innovative product, that can influence macrophage polarization in different ways – via macrophage direct and indirect activation, regulation of the TNF-α, increasing the amount of NO and NK killer cells (36; 37). The analysis of the immunomodulatory properties of Immuno-m^® shows an impact on important cells and proteins of the immune system, which play a special role in the regulation of the inflammation, for tumour control and directly affect tumour cells – lymphocytes, macrophages, interleukins, interferon, and others, resulting into switching the macrophage polarization for effective immune response (38; 39). Thus, Immuno-m^® is a promising new key to the regulation of the inflammatory process in patients with a variety of diseases, and further studies of the possibilities of its use are required.

Dr. Oksana Klymenko M.D., PhD, 
SNHS Dip. (Holistic Nutrition), Medical Doctor, Researcher in the fields 
of molecular physiology and pathophysiology, 
molecular biology, genetics, cell biology

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