**5. Phagocytosis alterations and their relationship with obesity comorbidities**

Lymphocyte subpopulations changes, both for those of the innate and the adaptive immune response, have been reported in obese individuals. These cells accumulate in the obese persons' VAT and could result from a survival increment and proliferation

#### *Phagocytosis: Inflammation-Obesity Relationship DOI: http://dx.doi.org/10.5772/intechopen.110510*

of resident immune cells, as well as greater cellular recruitment toward the VAT or a decrease in the cellular return to peripheral blood [36, 37]. There are also differences in the proportion of these cells among the different fat deposits. It has been observed that there are larger numbers of macrophages, T lymphocytes, and inflammatory molecules in the VAT compared to the subcutaneous tissue of obese individuals. Moreover, it was found that in the VAT from obese individuals with MetS, the number of Tregs is lower [11, 12, 38].

There are several innate immune system cells in the low-intensity chronic inflammation caused by obesity, but since this chapter deals with the phagocytic process and its relation to inflammation obesity, we will focus only on the phagocytic cells.

Alterations of the innate immune system in obesity include, among other aspects, a raised macrophage infiltration in AT, a place where these phagocytes interact with the adipocytes and endothelial cells, forming an inflammatory network. The interaction of these cells promotes the activation of the fat tissue macrophages, which are induced to produce diverse proinflammatory cytokines and chemokines such as TNF alpha and the monocyte chemoattractant protein-1 (MCP-1) [11].

Neutrophils are the first to migrate to the infection sites, and this happens in obesity, where neutrophils are the first cells to respond to inflammation, infiltrate the VAT approximately three days after a high-fat meal, and can stay there for up to 90 days [13, 39, 40].

Neutrophils depend mainly on glucose as the only energy source. In the diabetic patient, there is an excess of advanced glycation end products (AGEs), which are modified proteins that appear at the tissue and plasmatic level as a consequence of the reaction of blood monosaccharides with the protein's amino acids [41]. AGEs are formed in situations of sustained hyperglycemia or high oxidative stress [42]; this is a key part that explains why the neutrophil function is altered in diabetes [43–45].

Several clinical and epidemiological data report a higher incidence and severity of some specific types of infectious diseases, which are more frequent in obese persons than in lean ones. It has also been observed that the risk of developing cutaneous infections is increased, and the capacity to heal wounds is reduced in obese individuals. A decrease in the capacity of polymorphonuclear neutrophils to destroy bacteria was reported, which led to establishing the association of immune system alterations with obesity in children, adolescents, and adults [8, 46].

In obesity, circulating neutrophils are increased (associated with the BMI) as well as in individuals with MetS [47, 48]. These cells present an activated phenotype as indicated by an increase in the plasmatic concentrations of myeloperoxidase and elastase [48–50]. It is not well understood why the activated state of the neutrophils in obese individuals does not result in a more effective antimicrobial function. The following studies might partially explain this conundrum:

Four decades ago, it was described that diabetic patients have defects in their chemotactic response [51, 52]. Nevertheless, other studies showed controversial results, as no differences in the chemotactic response were observed between normal and diabetic patients [45, 53]. On the other hand, experimental studies in alloxan-induced diabetic mice showed that their neutrophils internalized the C-X-C motif chemokine receptor 2, which resulted in a reduced migration [54, 55]. It has also been shown that the administration of insulin to diabetic mice results in the reduction of alfa-1-acid glycoprotein (which is also increased in diabetic persons), restoring cellular migration [54].

Concerning adhesion, hyperglycemic stages increase the adhesion of phagocytic cells, especially for neutrophils, and due to the microenvironment, there is an increment in the protein C kinase (PKC) activator, which favors the expression on the

cell membrane of molecules such as P-selectin, E-selectin, and intercellular adhesion molecule-1. The adhesion mechanisms activated in phagocytic and endothelial cells have been associated with the increment in cytotoxic factors (free radicals and TNF alpha) and with transforming growth factor beta-1, fibroblast growth factor, and platelet-derived growth factor. This set of factors is related to the lesions at the vascular level, a bad reparation process, and they increment the appearance of atherosclerosis. Obese and hyperglycemic patients are characterized by presenting vascular and microvascular pathologies [45, 56, 57]. There are not many studies on the alterations of adhesion molecules that affect phagocytosis; it is only known since the 1970s that the presence of hyperglycemic states leads to phagocyte adherence abnormalities. Neutrophils from hyperglycemic patients showed a lower adherence, which is re-established by insulin [58]. Nevertheless, other studies show the opposite; in a diabetic mice and rat model, hyperglycemia (>500 mg/dL) increases the expression of adhesion molecules such as Fc gamma RII/III, ICAM-1, Mac-1, −2 [59, 60].

C3 is a central component of the complement system, and its activation into C3b is critical for bacterial opsonization and phagocytosis. Diabetic patients have elevated levels of C3 and C4 in addition to having a decreased ability to fix complement by IgG [61]. In hyperglycemic conditions, C3 suffers conformational changes that make it unable to initiate the complement pathway or act as an opsonin, despite the fact that it can adhere to bacteria such as *Staphylococcus aureus* [62, 63].

Phagocytic cells display in their cell membranes different types of Fc receptors (FcR), and depending on the activation of these receptors, the phagocyte will exert a different function through second messengers. Insulin can promote changes in the phosphorylation of second messengers, and therefore, it can modify the phagocytic cell response with respect to the glycemia levels based on the presence of the FcR activity, which uses cAMP for signal transduction. In hyperglycemic states, monovalent cations are altered through the FcR functions in the ionic channels so that phagocytosis would be affected by the modifications in the glycolysis pathway [64].

The production of intracellular ROI is often diminished in neutrophils from diabetic persons, which makes them more susceptible to infections. If the glycolysis pathway is modified, phagosome maturation is also altered, mainly with a reduction in the acidification and bactericidal capacity [65]. The molecule C5a has been found incremented in obesity and T2DM [66]. It has been observed that when neutrophils from critical patients are challenged with *S. aureus*, the molecule C5a impacts the phagosome maturation, preventing their acidification [67].

In diabetic rats, a decrease in the activity of the glyceraldehyde-6-phosphate dehydrogenase enzyme is observed, which indicates that the pentose pathway is diminished in the leukocytes from these animals. Leukocytes with reduced activity of this enzyme present damage in phagocytosis, bactericidal activity, and superoxide anion production. In addition, the decreased glucose flux through the pentose phosphate pathway reduces the NADPH and ribose 5-phosphate production, which might be related to a neutrophil malfunction in the diabetic state [65].

Another pathway that affects the bactericidal capacity is the polyol pathway. In hyperglycemic states and obesity, there is stress due to an increase in free radicals, which affects the endoplasmic reticulum of the phagocytic cells; enzymes such as the aldose reductase are activated, which reduces the glucose excess to sorbitol (polyol pathway). This pathway is characterized by an increase in NADPH consumption, leaving less and less substrate for the phagocytic function [68, 69] (**Figure 2**).

*Phagocytosis: Inflammation-Obesity Relationship DOI: http://dx.doi.org/10.5772/intechopen.110510*

#### **Figure 2.**

*Phagocytic alterations in obesity. Increased FFAs, ROS, NO, advanced glycation end products (AGEs), DAMPs, and glucose in the microenvironment impact phagocytic cells, especially Nt and Mo. (1) Chemotaxis is decreased due to the high expression of 1 alpha acidic glycoprotein (1-α AGP) and G-2 protein-coupled kinase (GRK-2) and the low expression of the chemotaxis molecule CXCR2. (2) High protein kinase C (PKC) levels increase the expression of adhesion molecules such as e-selectin, p-selectin, and intracellular adhesion molecule 1 (ICAM-1). (3) Endocytosis is affected due to the reduction of opsonins such as C3 and C4 and the decrease in the expression of the immunoglobulin Fc region receptor. (4) Lower function of the enzyme G6PDH and higher C5a prevents the proper maturation of the phagosome. (5) The production of intracellular ROS, which hold bactericidal activity, is also diminished due to decreased G6PDH activity. (6) Excess glucose is reduced to sorbitol, which increases the consumption of NADPH, making it less available for phagocytosis. Created with BioRender.com.*

### **6. Therapeutic strategies**

Even though obesity-related metabolic diseases are treated with drugs, some therapeutic alternatives that favor phagocytosis restoration are described here.

In search of improving the phagocytic capacity, which is deficient due to metabolic diseases such as obesity, several solutions have been proposed; among them, the use of probiotics stands out. Probiotics are defined as live microorganisms that have beneficial effects on the host's health when consumed [70]. These beneficial effects result from a wide range of actions that they exert, among which are the regulation of inflammation by increasing IL-10 expression [71] and the modulation of the expression of COX-2, and the activation of TLR4 [72]. In addition, probiotics can modulate insulin sensitivity [73] or decrease the individual's weight or dyslipidemia degree [73, 74], or act directly on the phagocyte, by increasing IFN gamma production, improving phagocytosis and increasing the expression of complement receptors [75].

Probiotics have an immunomodulatory function, and it has been found that their consumption can regulate the macrophage phagocytic activity against several pathogen agents, such as *Aggregatibacter actinomycetemcomitans*, a pathogen bacterium that affects the oral mucosa. When the *Lactobacillus johnsonii* NBRC 13952 probiotic is present, it increments the phagocytic activity and optimizes the bactericidal capacity of the macrophages, thus avoiding infection [76]. In the same way, the consumption of *Lactobacillus rhamnosus* HN001 and *Lactobacillus acidophilus* for four weeks by elderly subjects incremented their phagocytic capacity. With this immune stimulus, an improvement in the health of this population sector is sought [77].

Some of the mechanisms by which probiotics exert their action are still unknown, not to mention that these mechanisms also differ between the strains used for this purpose. Nevertheless, it has been demonstrated that probiotics secrete molecules that can regulate several functions, as is the case for *L. rhamnosus* strain GG (LGG). When macrophages were exposed to LGG-conditioned media, their phagocytic and bactericidal activity was increased up to sixfold. This activity was associated with an increment in free radicals production, with the activation of NADPH oxidase, and a slight increase in nitric oxide generation [78].

Another way that is being explored to counteract the metabolic changes and improve the phagocytic function is through organic compounds such as resolvins. These are a group of molecules derived from omega-3 fatty acids [79] that have a positive effect on decreasing obesity and increasing the phagocytic and bactericidal capacity. How this effect is attained is still under investigation, though a blockade of the Akt pathway and the mitogen activated protein kinase phosphorylation seems to be involved [80].

Macrophages from obese patients exhibit a deficiency in the expression of growth differentiation factor 15 (GDF-15), which is essential for the oxidative metabolism in M2 macrophages and suppresses M1 macrophages, increasing inflammation and

#### **Figure 3.**

*Alternatives for the recovery of an adequate phagocytosis. Statins increase opsonization improving phagocytosis. The presence of omega-3 fatty acids (ω3FAs) and their derivates, such as the resolvins, reduce obesity, help the M2 differentiation of macrophages, increase phagocytosis, and increase insulin sensitivity. Melatonin increments phagocytosis besides having antioxidant action and modulates obesity. Probiotics generate changes that immunomodulate the microenvironment leading to an improvement in the use of energetic resources, increase the production of anti-inflammatory cytokines (IL-10), diminish the presence of FFAs, and improve all the phagocytic process. Created with BioRender.com*

*Phagocytosis: Inflammation-Obesity Relationship DOI: http://dx.doi.org/10.5772/intechopen.110510*

IR. The administration of GDF-15 to obese mice reverts IR, mitochondrial oxidative alterations (improving bactericidal and phagocytosis capacity), and macrophage differentiation, making it a good prospect for obesity treatment [81].

Another condition that can modify the phagocytosis process is the presence of hormones such as melatonin. There is evidence that lactating obese women possess phagocytes with high melatonin concentrations compared to women with a normal BMI. Melatonin promotes the activity of the colostrum phagocytes through G protein-coupled receptors, improving dectin-1 expression, an important type C lectin receptor crucial in proinflammatory responses such as cytokine production, ROI production, and phagocytosis [82]. The melatonin in the colostrum macrophages increases superoxide release in phagocytosis, but it also has cytoprotective effects with an antioxidant function depending on the dose, cellular targets, and exposition time. Considering these functions, the high levels of melatonin present in the colostrum of high-BMI women could be a mechanism of protection against childhood obesity, as obese individuals have reduced melatonin levels. On the other hand, melatonin promotes colostrum phagocytes' activity, which could be important for the protection of the lactating newborn (**Figure 3**) [83, 84].

## **7. Conclusion**

Obesity is a chronic, multifactor illness. Data have been reported that relates obesity to alterations in the immune system in obese children, adolescents, and adults. Neutrophils from obese and diabetic individuals show a deteriorated phagocytic functionality that is manifested by a reduced chemotaxis, phagocytosis, and intracellular reactive oxygen species production.

Some therapeutic alternatives for the recovery of an adequate phagocytosis have been reported, such as probiotics, resolvins, statins, administration of GDF-15, and melatonine, but future research is needed to fully understand the aberrant neutrophil function in obesity and other obesity-related complications.
