**5. The production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in the function of the macrophage**

In an inflammatory process, the function of the macrophage is crucial since it is responsible for limiting that inflammation. After phagocytosis, in late phagosome, the phagosome binds with the lysosome presenting an acid pH due to the action of several V-ATPases and proteases with the stimulation of the foreign agent, the

macrophage produces reactive oxygen species (ROS) and nitrogen (ON) (superoxide ion and hydrogen peroxide) secondary the catalytic activity of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase enzyme complex. This reaction is preferably intracellular through electron transfer reactions within the phagolysosome, especially in the mitochondrial respiratory chain. The increase in NADPH causes oxygen consumption (respiratory burst) and the creation of toxic products such as ROS, NO\*. NO is produced by inducible nitric oxide synthase (iNOS), which, in turn, stimulates further NO production.

This event is associated with various pathophysiological processes, such as the oxidation of low-density lipoproteins (LDL) that are phagocytosed by the macrophage, becoming a foamy macrophage itself, which is associated with an increased risk of atherosclerosis [10].

Despite being a mechanical-biological process studied for several years, phagocytosis still has unknown events. It is a process that is not the same for any particle that will be engulfed since there are variations according to the characteristics of the particle and the type of receptor that binds to the ligand. The examples mentioned below depend on the target particle to be phagocytosed.

### **6. Phagocytosis in infectious diseases**

Bacteria, viruses, fungi, and parasites present various PAMPs not detected by cellular receptors called pattern recognition receptors (PRRs). There is an extensive variety of PRRs that, according to the characteristics of the receptor, will identify and bind to a specific ligand. The phagocytosis of a PAMP occurs by binding to one or various receptors and one or different PAMPs of the same pathogen in a single event. There are several examples in this regard; the polysaccharides present on the surface of some yeasts bind to the mannose receptor or the dectin-1 receptor, while the lipopolysaccharide (LPS) of gram-negative bacteria is detected by the scavenger-A receptor (SR-A). The phagocytosis of mycobacteria occurs through complement receptors (opsonization of mycobacteria by complement) or by the mannose receptor that recognizes lipoarabinomannan (LAM), a structure that is part of the wall of mycobacteria; the coating of mycobacteria by surfactant protein A (Sp-A) has also been described [40]. Fungal phagocytosis is less studied; beta-glucans of the fungal cell wall bind to Dectin-1 receptors to initiate phagocytosis [41]. While the human serum amyloid protein (SAP) is considered a Trojan horse since some fungi and bacteria have a functional SAP on their wall that allows the fungus to bind to cells and be more invasive [42].

Interestingly, it has been reported how bacteria of the genus *Treponema pallidum* are phagocytosed when they are covered by opsonins or without opsonins [43]. Regarding the phagocytosis of the virus by the macrophage, we have the example of the person responsible for the current pandemic, the Coronavirus type 2 (severe acute respiratory syndrome coronavirus 2 [SARS-Cov-2]); the critical entry of the virus into the cell is the angiotensin 2 receptor (ACE-2). Different lectin-like receptors (CLRs) act as endocytic receptors for macrophages and are compromised when ingesting viruses or other pathogens [44].

The importance of removing apoptotic bodies through phagocytosis is known. Many cells die every day in healthy subjects, and phagocytes must remove their apoptotic bodies. Apoptotic cells display on their surface several molecules that distinguish them from healthy cells, such as phosphatidylserine (PS), a molecule restricted to the inner layer of the plasma membrane in healthy cells, which appears on the surface during the apoptosis process. In a sterile inflammation event produced by cells such as neutrophils

#### *Macrophage: From Recognition of Foreign Agents to Late Phagocytosis DOI: http://dx.doi.org/10.5772/intechopen.110508*

that have been recruited to the site of inflammation and undergo cell death by apoptosis, they are phagocyted to decrease or eliminate tissue-damaging proinflammatory factors and ROS. This process called spherocytosis is the part of the interaction of a complex network involving binding molecules, molecules that signal the cell through PS that helps tissue homeostasis [45]. It is a complex mechanism by which various interactions are related as ligand-receptor and signals. Cells undergoing apoptosis release multiple molecules such as ATP, lysophosphatidylcholine, fractalkine, and sphingosine 1-phosphate. These molecules act as chemotactic factors that recruit phagocytes to the site of cell death. Multiple phagocytic receptors bind PS. Direct binding to PS is mediated by receptors such as TIM1, brain-specific angiogenesis inhibitor 1 (BAI1), and stabilin-2 [30]. In other cases, the molecules can bind to PS and to surface receptors forming a bridge; an example of this is MFG-E8 that links PS to αVβ3 integrins, which are effective phagocytic receptors.

Another example is Gas6 and protein S molecules that are between PS and phagocytic receptors, such as TAMs (Tyro3, Axl, Mer) [46]. Derivatives of PS metabolism may also contribute to the recognition of apoptotic bodies. PS appears to undergo oxidation, and some phagocytic receptors, such as CD36 and CD68, bind modified lipids, including oxidized PS [30].

### **7. Macrophage response in phagocytosis**

The macrophage presents high metabolic plasticity, which is associated with the polarization of the macrophage and the molecules and factors they produce, so their response will be unique in each case. The macrophage response can be controlled by the target particle inducing specific signaling pathways directed by receptors that recognize the target particle and by overlapping signaling pathways.

An example is phagocytosis secondary to antibodies recognition that is controlled by protein kinase C (KPC) without stimulating phosphatidylinositol 3-kinase or extracellular signal regulated kinases (ERK). However, antibody phagocytosis stimulates these last two molecules through cytokines and depending on these multiple factors, we have the macrophages 0 (M0), which are naïve macrophages, M1 characterized by proinflammatory and accompanied by IL-6, IL-12, and TNF alpha, M2, which are anti-inflammatory and produce IL-10, TGF-beta, and Arginase; Mreg are regulatory macrophages with anti-inflammatory characteristics and IL-10 producers. Other recently reported macrophages, such as M-mox and M4, are mentioned, but less is known about them. The M2 group is classified into M2a, M2b, M2c, and M2d.

This classification, carried out practically for a better understanding, is based on the expressed transcription factors and the signaling pathways used by macrophages. However, these macrophages display high plasticity and change their status depending on the medium and environmental signals. Despite the plasticity of macrophages, three responses are recognized, two of which are well characterized. The description of the macrophage's immune response is diverse and changing since it depends on the characteristics of the target particle, especially if it is a pathogen, the receptors responsible for binding to that particle, and whether or not it is opsonized and the capacity of the macrophage to remove the foreign agent.

One of many examples is the binding of polysaccharide from fungi to mannose or Dectin-1 receptors, the binding of lipopolysaccharide from a gram negative bacterium to TLRs, or the binding of bacteria to SR-A or framework. Each of these events

will stimulate transcription factors and stimulus-dependent signaling pathways. Even with this diversity, we can state in general that there is an immune response that is characterized by the production of various molecules such as lipases, nucleases, proteases, glycosidases, and phosphatases responsible for degrading the target particle, the expression of NADPH oxidase (Nox2), and oxide synthase 2 (Nos2) responsible to produce reactive oxygen and nitrogen species. In infections, the macrophage activates proteins that sequester iron (Fe) and Mn, essential elements for microorganisms.
