**2.1 Organized mucosal associated lymphoid tissue (O-MALT)**

O-MALT in the gastrointestinal tract includes Peyer's patches and isolated lymph follicles (ILF). The number and location of mucosal follicles vary greatly between species and in an individual also changes over time and exposure to antigens. Most of these centers are isolated and scattered throughout the airways and gastrointestinal tract, but their extent increases to the colon and rectum. Some of these lymphatic tissues together form large complexes such as the palatine, lingual, and pharyngeal tonsils called the Waldeyer's ring, mucosal follicles in the appendix, and Peyer's patches in the small intestine. Peyer's patches are more in the ileum (the last third of the small intestine) and less in the jejunum (not seen in the colon). Mucosal lymphoid follicles in both single form (ILF) and complex (Peyer's patches) are covered by a specific epithelium. The general structure of the lymph plaques of Peyer's patches is shown in **Figure 1**. Each Peyer's patches contain more than 100 lymphoid follicles, each with a dark border and a relatively lighter circular center. O-MALT in gastrointestinal lymphatic tissues includes Peyer's patches (in the ileum) and ILFs (in the colon).

**Figure 1.** *The general structure of Peyer's patche lymph follicles. FAE, Follicule associated epithelium.*

The follicles are separated from the mucosal epithelium by intercellular spaces and a dome-shaped area filled with lymphocytes called the corona. The mucosal surface above the corona of the follicle is free of villi on the surface of the epithelium of other areas and contains antigen-carrying cells or M cells (found only in this area). High endothelial venules (HEV) where lymphocytes leave the artery are located in the interfollicular section [1, 4, 5].

#### *2.1.1 Lymphocytes in O-MALT*

Lymphocytes in O-MALT and follicle associated Epithelium (FAE) has been studied in several species [6–8]. The follicles are the site of accumulation of B lymphocytes, dendritic cells, and macrophages. However, T lymphocytes are mainly predominant in the internal and parafollicular parts [6, 9]. Most of the parafollicular B lymphocytes and located in the corona are IgM+ and the B cells in the germinal centers are IgA+ . CD4 + T lymphocytes are mostly found in the corona, below the epithelium of the dome area, and the parafollicular regions, and CD8 + T cells are often found in the interfollicular regions.

#### *2.1.2 Antigen-presenting cells*

Antigen-presenting cells in O-MALT (such as Peyer's patches) include follicular dendritic cells within the germinal center, interdigitating cells near lymphocytes of parafollicular regions, macrophages, and B cells. Macrophages are mostly concentrated in the coronal and B lymphocytes are often found in the follicular regions [7, 10, 11]. Antigen-presenting cells trap antigens of extracellular origin in endosomes. In these phagosomes, antigens are digested and processed by specific proteolytic enzymes, and finally the peptides are presented to T lymphocytes by MHC II. Cells isolated from Peyer Patch mice can be stimulated with antigen in vitro, resulting in a primary and secondary immune response, leading to the production of IgM class antibodies and IgG and IgA class antibodies, respectively [12].

#### **2.2 Diffuse mucosal associated lymphoid tissue (D-MALT)**

Diffuse lymphoid tissue is scattered throughout the mucosal surface and includes lymphocytes, diffuse plasma cells in the lamina propria, mucosal connective tissue, and intraepithelial lymphocytes (IELs). Some of these cells are derived from O-MALT and contain effector and memory lymphocytes. These cells are caused by antigen stimulation in areas such as Peyer's patches. In a regular process, antigen-stimulated cells begin to migrate from the site of stimulation and settle in other mucosal tissues [13, 14].

#### *2.2.1 Intraepithelial lymphocytes (IELs)*

Intraepithelial lymphocytes are often T cells located in the epithelial layer. About 15 to 20% of the population make up epithelial cells. These cells are considered guarding cells in the immune system and react with antigens earlier than others, and therefore show memory phenotype (CD45RO+ ).

IELs are found in two types, Tαβ and Tγδ. The main function of these cells is to establish tolerance against symbiotic bacteria and to protect against pathogenesis. In humans, about 90% of IELs are Tαβ and only 10% are Tγδ. In mice, the percentage of Tγδ cells reaches 50%.

Most IEL cells are CD8<sup>+</sup> and are divided into two categories in terms of origin. Some of these are conventional Tαβ cells that have evolved in the thymus that

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

can express both the CD4<sup>+</sup> marker and the CD8+ marker. The other group is the unconventional or natural Tαβ cells and Tγδ, which have evolved in environments other than the thymus, such as the intestine. These lymphocytes have the power of self-renewal and are restricted to non-classical MHC molecules. These unconventional IELs usually show a specific CD8 consisting of α chain homodimer. Most intraepithelial Tγδ cells, as well as many Tαβ lymphocytes in the gut, express the CD8αα homodimer. For this reason, the expression of CD8αα has been considered an indicator of intraepithelial T cells in the intestine compared to peripheral blood T cells.

Few IELs are found with the CD4+ CD8+ or CD4− CD8− phenotype. Unlike conventional TCRs, which have a wide variety, TCRs in IELs have limited variability.

Most IELs are dormant under normal conditions but react as soon as they are exposed to the antigen due to a memory phenotype. TαβCD8<sup>+</sup> and Tγδ cells show cytotoxic activity against infection. Production and storage of perforins and granzymes can be done in IEL.

Conventional T cells, unlike unconventional T cells, must be activated to play their executive role. Both abnormal Tαβ and Tγδ cells in the intestinal epithelium detect antigens at the level of non-classical MHC molecules such as CD1, which allows factors expressed on the surface of damaged epithelial cells to respond to stress. Thus, Tγδ cells can also be activated in response to foreign antigen peptides and host cell-derived danger signals.

Tγδ cells have a more limited gene repertoire of TCR and in the gut often express the Vδ1 chain, which is different from blood Tγδ. Vδ1-expressing Tγδ cells can detect non-classical MHCs induced by MICA and MICB stress. MICA and MICB are known as the damage-associated molecular pattern (DAMP) and increase in response to cellular stress. Tγδ can respond to tissue damage in the shortest possible time. By secreting IFN-γ, these cells increase the cytotoxic response against virusinfected cells and enhance the neutrophilic response against bacteria.

Tγδ lymphocytes in the gut play an important role in protecting mucosal surfaces from damage caused by immune responses. Tγδ lymphocytes also regulate immune responses by increasing TGFβ and limiting the migration of inflammatory leukocytes to the intestinal tract. In addition, these cells produce Insulin-like growth factor-1 (IGF-1) and keratinocyte growth factor (KGF).

The proportion of Tγδ cells is higher among IEL cells in infancy. As you age, the proportion of Tαβ cells increases, so Tγδ cells in infancy are likely to play an effective role in defending against pathogens [2, 5].

#### *2.2.2 Lamina properia lymphocytes*

Lamina Properia Lymphocytes include B cells (often transformed into plasma cells) and T lymphocytes. In mice, 40% of lamina properia lymphocytes are B cells that produce mainly IgA. 25% of the cell population are T lymphocytes (mainly with the CD4 + TH2 phenotype) [15–19]. Human lamina propria CD4 + T cells provide memory cell markers and do not proliferate in response to antigenic stimuli. Rather, they produce cytokines such as IFN-γ [20]. The predominant population of T lymphocytes in the lamina propria is CD4 + T (60–70%), the majority of which exhibit the TCRα/β phenotype. Most of these cells have the CD45RO (specific for memory cells) and are different from peripheral blood T lymphocytes in this respect. Lamina propria is an important center for IgA production. In these areas, O-MALT derived B lymphoblasts (such as Peyer's patches) are affected by cytokines such as IL-6 and undergo differentiation [21]. Lamina properia TH1 cells proliferate TH2 cells by secreting IL-2 and IFN-γ. On the other hand, TH2 cells, by producing IL-5 and IL-6, prepare for the differentiation of B cells into IgA-producing plasma cells [22].

The lymphocytes in the lamina propria are mainly in the late stages of differentiation and often turn into plasma cells. Furthermore, In the intestinal lamina propria cells such as macrophages, neutrophils, eosinophils. There are dendritic cells and mast cells. Lamina propria CD4 + T cells can react with these cells, enhancing their phagocytic and antimicrobial capacity. Macrophages may also be involved in the processing and delivery of antigens to T cells.

#### **3. Innate lymphoid cells in intestinal mucosa**

Innate Lymphoid Cells (ILCs) in the intestinal mucosa are involved in defense against pathogens, enhancing the function of the physical barrier, and tolerance to the microbial flora. There are two types of ILC2 and ILC3 in the mucosa, and ILC2 is involved in the defense against worms in the gut. Besides, in response to cytokines IL-33 and IL-25, they can secrete cytokines IL-5 and IL-13, the former of which is effective in activating eosinophils and the latter in increased mucus production and thus repelling worm parasites. ILC3 is also present in the gut and can produce the cytokines IL-17 and IL-22 in response to stimulation with IL-18 and IL-23 cytokines. The cytokines produced by these cells are involved in enhancing the physical function of the mucosa by stimulating the production of defensins and strengthening strong epithelial connections.

Other cells in the mucosa are Mucosal associated invariant T (MAIT), which are a subset of CD8 + T cells with invariant TCR Va7.2-Ja33. The main role of these cells is to defend against bacteria and fungi that cross the intestinal epithelial barrier and enter the bloodstream. Intestinal bacteria (normal flora or other bacteria) enter the liver through the portal vein and encounter the MAIT cells if they pass through the intestinal epithelium and enter the bloodstream. These cells detect fungal and bacterial metabolites through an MHC-like protein class 1 called MRI and, once activated, produce a cytotoxic role by producing inflammation-promoting cytokines. 50% of the population of T cells located in the liver belongs to this group [1, 4, 5].

#### **3.1 Enterocytes and antigen-presenting**

Mucosal epithelial cells (especially small intestinal enterocytes) act as antigenpresenting cells and present MHC II molecules [23–26]. Besides, CD1d (MHC I-like) molecules are present on the surface of these cells. Mature enterocytes from intestinal villi express class II molecules whereas crypt cell production may be affected by cytokines such as IFN-γ [27]. Enterocytes are able to present antigens to T cells in vitro. However, the T cell response is suppressive [28, 29] and this mechanism seems to be involved in mucosal tolerance.

### **4. Antigen penetration into O-MALT**

#### **4.1 Follicule associated epithelium (FAE)**

The intestinal epithelium can be thought of as a complex of crypt-centered cells. In the small intestine, each crypt contains a large number of undifferentiated germ cells from which other cells are formed. Differentiated crypt cells then cover adjacent villi [30].

Goblet cells, enterochromaffin, and pIgR-containing enterocytes are located in the lateral wall of the villi. Cells that move from the crypt to the dome of the lymphoid follicles become pIgR-containing enterocytes and M cells [31].

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

The location of Peyer's patches and other parts of O-MALT in members of an animal species is known. Immature M cells remain even after lymphocyte depletion with radiotherapy [32, 33]. The formation of mucosal tissues is organized before birth [34]. However, the antigen transfer process causes the mucosal follicles to expand. In general, it can be said that the superficial components of epithelial cells together with local secretory products are involved in the formation of O-MALT.
