**5. Dendritic cells in the gastrointestinal tract**

In mucosal tissues, dendritic cells are known to be the main controllers of immune responses These cells act as a protective system and, by identifying pathogens, can stimulate naive T and B cells. Both O-MALT and D-MALT tissues contain dendritic cells. There are several subgroups of DCs in the mucosa, each with unique properties. DCs in the Peyer's patches are often located in the M cell envelope and the subepithelial dome (SED) and are CD11b+ , CD8α<sup>−</sup> , CCR1+ , and CCR6+ . CCR1 and CCR6 receptors bind to CCL9 (MIP-1γ) and CCL20 (MIP-3α) chemokines, respectively.

CCL9 and CCL20 are continuously secreted from FAE cells and are located by the CCR1 and CCR6 receptors, causing these DCs to be located in the Peyer's patches epithelium.

DCs of Peyer's patches secrete 10-IL in the absence of infection in response to the uptake of dietary antigens or microbiome, which inhibits the inflammatory response to these antigens. When exposed to pathogens, these DCs are rapidly recalled below the FAE by increasing CCL20 secretion from the epithelium. Microbial products cause the expression of co-stimulatory molecules on the surface of DCs, and excited DCs lead to the activation and differentiation of naive T cells into effector cells. In Peyer's patches, in addition to the above-mentioned DCs, there is another DC subclass, which, unlike the first type, is CD11b− , CD8a+ and CCR6− . These cells are found in T cell-rich areas in Peyer's patches and produce IL-12 inflammatory cytokines.

A major route of antigen transport to Peyer's patches (O-MALT) is M cells. Other ways to transport antigens to the O-MALT region include the entry of food and soluble antigens through the epithelium. Moreover, the presence of FcRn on the surface of enterocytes enables these cells to detect IgA-coated antigens. The binding of FcRn to the antigen and antibody complex can trigger the entry of immune complexes from the luminal surface to the basal surface of enterocytes by transcytosis. When apoptosis kills pathogen-infected enterocytes, antigens can penetrate the subepithelial layer. More specifically, DCs uptake apoptotic cell debris and associated antigens (**Figure 3**).

**Figure 3.**

*Different ways antigen enters mucosal tissues.*

#### *Mucosal Immunology DOI: http://dx.doi.org/10.5772/intechopen.98863*

Another way to pick up antigens in the gastrointestinal tract is through DCs and macrophages, can send their appendages into the intestinal lumen without disrupting the integrity of the epithelial cells and actively sampling the antigens in the lumen, thereby transporting the antigen to the Transmit lamina propria.

Lamina propria dendritic cells (LPDCs) that pick up antigens in ways other than M cells play an important role in maintaining tolerance to non-pathogenic intestinal antigens.

LPDCs express the CD103 index (integrin αE: B7) at their surface and can migrate to T-cell-rich regions of the mesenteric lymph nodes through afferent lymphatics. In the mesenteric lymph nodes, LPDCs can react with naive T cells and activate them, inducing intestinal homing characteristics in these cells. As a result, active T cells can return to the gut and differentiate into effector cells. The migration of CD103<sup>+</sup> DCs to the lymph nodes is dependent on CCR7 expression. CCR7 is constantly expressed on the surface of these DCs, but its expression increases during infection. When there is no infectious agent, about 5 to 10 percent of mucosal DCs migrate to the mesenteric lymph nodes.

CD103<sup>+</sup> dendritic cells produce the non-protein retinoic acid (RA) molecule that is involved in cell signaling. RA is the product of the effect of retinal dehydrogenase enzyme on vitamin A. RA production from these DCs induces CCR9 and integrin α4: β7 markers on the surface of B and T cells, which is effective in implanting these cells in the intestine. LPDCs respond poorly to inflammatory stimuli such as TLR ligands and produce more IL-10. For this reason, the migration of CD103<sup>+</sup> DCs into the mesenteric lymph nodes in the absence of an infectious agent causes differentiation into Treg FoxP3<sup>+</sup> (iTreg) cells. RA secreted from DCs and TGFβ plays an important role in differentiating these Treg. TGFβ is abundantly produced by intestinal cells. In addition, intestinal DCs produce a substance called Indoleamine 2, 3-Dioxygenase (IDO). This enzyme catalyzes tryptophan and leads to the differentiation and induction of Treg cells in the intestine.

CD103+ DCs in the small intestinal mucosa are effective in combating inflammation. Factors such as RA, TGFβ, PGE2, and TSLP4 are effective in perpetuating this antiinflammatory response. TSLP, RA, and TGFβ are made by intestinal epithelial cells.

Macrophages located in mucosal tissue naturally produce IL-10. This cytokine deactivates DCs and preserves mucosal Tregs.

Studies have indicated that DC103+ DCs, located in the large intestine, play a role in maintaining tolerance and the immune response to symbiotic bacteria and are rarely seen in Peyer's patches. In addition to CD103+ DCs, other myeloid cells are found in the lamina propria, which stimulate inflammatory responses. These cells produce cytokines such as IL 6, IL 23, TNF a, and nitric oxide (NO), which are involved in differentiation into executive TH17 cells and class switching to IgA in B lymphocytes. These CD103− DCs are stimulated by TLR5 and express the CX3CR1 index, which is the receptor, and chemokine fractalkine. The aforementioned cells cannot migrate to the lymph nodes and are not able to present antigen to the naive T cell and produce RA. Furthermore, in addition, they are not classified as classical DCs, are more like macrophages, and are involved in the production of inflammatory cytokines.

#### **6. Adaptive immunity in the gastrointestinal tract**

Humoral immunity and mucosal IgA production are the main forms of acquired immunity in the gastrointestinal tract. Secretory IgA dimer into the lumen, IgG and

<sup>4</sup> Thymic Stromal Lymphopoietin.

IgM participate in the defense against pathogens. The role of cellular immunity in the gastrointestinal tract is to control responses in the gut with the help of Treg and TH17 cells.

After capturing the antigen, dendritic cells migrate to the mesenteric lymph nodes and Peyer's patches, and acquired intestinal immune responses are formed5 . Active T and B lymphocytes enter the bloodstream through the lymph flow at the site of the thoracic duct. They then settle in the mucosal tissues through the appearance of implanted surface molecules in the intestinal mucosa [1, 4].

#### **6.1 Mucosal B lymphocytes and IgA production**

In Peyer's patches, most B cells in the corona and dark zone of the follicular germinal centers are IgM<sup>+</sup> / IgD<sup>+</sup> , while in the light zone the germinal centers cells are more than 90% IgA<sup>+</sup> cells. IgA cells in the germinal centers leave the O-MALT, enter the mesenteric lymphatic ducts, and then the blood flows from there to the mucosal and glandular areas of different parts of D-MALT and become IgAproducing plasma cells. IgA cells in the germinal centers are called immune cells. Unlike villi capillaries, which allow the release of serum proteins into lamina propria, capillaries in Peyer's patches have no pores and are impermeable to serum proteins. Therefore, it can be said that immune response interactions such as antibody response, cell accumulation, and secretion of cytokines against intestinal O-MALT antigens are not affected by systemic processes. Based on this, it can be acknowledged that circulating IgA is unable to prevent viral invasion of Peyer's patches and the proliferation of infectious agents in the mucosa. Class switching to IgA occurs in O-MALT. The predominant class of antibodies in the gastrointestinal mucosa is the IgA dimer. In humans, two IgA subclasses are encoded in the genome by two separate and distant sequences. Class switching is associated with the removal of genes upstream of the CH fragment.

In the intestinal mucosa, by two mechanisms dependent or independent of T cells, the class is selectively switched to lgA. Cytokines are extremely important in any phenomenon of class change. In the gut, TGFβ also plays an important role in switching classes to IgA. If class switching is T-dependent, IgA is produced with a higher affinity for the antigen. The DCs capture the antigen, move it to the interfollicular zone (in Peyer's patches) or the mesenteric lymph nodes, and deliver it to the naive CD4 + T. CD4 + T cells are then activated and differentiated into TFH (follicular helper T cells). Then, they react with B IgM<sup>+</sup> / IgD<sup>+</sup> cells and induce class switching to IgA. The prerequisite for this is TGFβ and CD40L binding of T cell surface to CD40 expressed in B cell. NO production from dendritic cells can increase the expression of TGFβ receptor on B cells. In T-cell-independent switching, active dendritic cells produce cytokines such as APRIL6 , BAFF<sup>7</sup> , and TGFβ, leading to the induction of class switching in B IgM<sup>+</sup> / IgD<sup>+</sup> cells (especially B1 cells). In this case, IgA is produced with less binding affinity than in the T celldependent state.

In the process of differentiating BIgA<sup>+</sup> cells into IgA-producing plasma cells, the cell secretory system is fully developed, α-CH fusion occurs at the mRNA level, and a J chain is produced. IL-2 is involved in regulating J chain production in B lymphocytes and plasma cells. In vitro, B cells committed to producing IgA of O-MALT origin undergo 6-IL differentiation in the final stages of differentiation. But in vivo studies do not confirm this finding. Therefore, it can be concluded that there are no

<sup>5</sup> Inductive Sites.

<sup>6</sup> A proliferation-inducing ligand.

<sup>7</sup> B-cell activating factor of the TNF family.

factors required for IgA differentiation and secretion. By migrating these lymphocytes to D-MALT regions and effector sites, the conditions for differentiation into end-cell cells are provided [1, 4, 5].

#### **6.2 The role of secretory IgA in the regulation of immune responses**

IgA B cells do not differentiate in O-MALT and therefore IgA concentration is low in these areas. Serum immunoglobulin concentrations are also very low in these areas [61]. However, sIgA located in the lamina propria and glandular secretions enter the O-MALT by binding to the apical membrane of M cells in the FAE [62].

T cells containing the Fc receptor in Peyer's patches act as helper cells and increase BIgA + cells. Fcα receptor T and B cells are involved in the specific regulation of the isotype of the mucosal immune system [63].

Antigen-IgA complexes are also transported to O-MALT by M cells [62], so it can be said that the Fcα receptor of B cells or macrophages enhances the immune response by increasing antigen uptake and processing. In conclusion, IgA reabsorption by M cells and reaction with Fcα receptors are involved in modulating the immune response [64].

Also, in mucous secretions and glands, anti-idiotypes can enhance the immune response by such a mechanism. This clarifies the reason for the reaction of breastfed infants (sIgA absorption) to oral and injectable vaccines [65].

### **6.3 Lymphocyte migration and homing**

Lymphocyte and monocyte migration and implantation play an important role in the mucosal immune response. This process causes a set of specific cells to migrate to areas such as the Peyer's patches where antigens are present, and the widespread effector and memory cells to different parts of the mucosal surface provide comprehensive protection for the body.

Numerous molecules and receptors are involved in the lymphocytes homing into the intestinal mucosa, including homing receptors, cell adhesion molecules (integrins) of chemokines, and chemokine receptors.

Naive lymphocytes enter the mesenteric lymph node and O-MALT (Peyer's patches) through HEV. Lymphatic tissues facilitate the entry of naive lymphocytes expressing CCR7 and L-selectin by secreting CCL19 and CCL21. If in O-MALT and lymph nodes, these lymphocytes are exposed to specific antigens presented at the APC, the incidence of CCR7 and L-selectin is reduced. Once the cells are activated, they leave the mesenteric lymph nodes through the lymph and Peyer's patches and enter the bloodstream through the thoracic duct. Dendritic cells in the mucosa can induce specific molecules to localize activated lymphocytes in the gastrointestinal tract. Activated lymphocytes increase the expression of α4: β7 integrins that bind to MadCAM1 on their surface. MadCAM1 is expressed on the endothelial surface lining the blood vessels of the intestine and its associated lymphatic tissues. Due to this interaction, it provides the conditions for the adhesion of active lymphocytes to the endothelial vessels of the gastrointestinal tract. Activated T and B cells express the CCR9 chemokine receptor on their surface after initial exposure to antigen in the small intestine. This receptor binds to TECK (CCL25) at the epithelial surface of the small intestine, leading to the re-implantation of these cells in this area. Primary activation of lymphocytes in the colon leads to the development of the chemokine receptor CCR10, which binds to the MEG (CCL28) surface of the colon epithelial cells. Furthermore, CCL28 can be secreted by the mammary and salivary glands [1, 2].

Lymphocytes that have first been exposed to the antigen and have detected it on the surface of intestinal mucosal DCs have identified implantation molecules and can implant in the gastrointestinal mucosa. For this reason, it seems that vaccination against intestinal infections requires the administration of the vaccine in the mucosa because DCs in the mucosa will have the power to induce specific implantation molecules [4].

With the passage of active lymphocytes through the vascular endothelium, the expression of α4: β7 integrins stops on their surface, and instead another integrin called αE: β7 appears on their surface. αE: β7 can attach to the cadmium E molecule on the surface of intestinal mucosal epithelial cells. In this way, the lymphocytes are kept in the vicinity of the epithelial cells after entering the lamina propria (**Figure 4**).

#### **6.4 Secretory IgA**

In an adult human, more than 3 grams of IgA is secreted daily in the mucosa and glands. Secretory IgA is made up of two interconnected molecules (each containing four immunoglobulin chains).

In mice, rats, and rabbits there is only one IgA isotype, but in humans, there are two isotopes IgA1 and IgA2 encoded by two separate genes [66].

IgA2 is often made by mucosal plasma cells, and the lack of 13 specific amino acids in the α2 chain makes IgA2 resistant to specific anti-IgA1 proteases produced by purulent bacteria.

dimeric IgA also contains the J chain and the secretory component (SC). The carboxylic part of Fc is the two IgA molecules next to each other and their Fab is outward. In humans, mice, and rabbits, the penultimate cysteine of the two α chains binds to the cysteine J chain through disulfide bonding.

The J chain has an Ig-like domain and the SC has five Ig-like domains. A complete slgA molecule consists of two IgA monomeric molecules of a J chain and a secretory component. The secretory component covers areas sensitive to proteolytic digestion and the IgA hinge, and the secretory variants of this immunoglobulin are highly resistant to proteases [1, 4].

#### **Figure 4.**

*Homing in gastrointestinal mucosa. Effector T lymphocytes attach to MadCAM-1 surface endothelial cells for homing in the gut (A). Intestinal epithelial cells express specific chemokines for T cells that intend to home in the gut (B).*
