**4.1 CD103**

*Cells of the Immune System*

phoid tissues [1, 58].

**3. Migration**

as a characteristic of TRM phenotype [59–61].

tion for migration of T cell subsets is shown in **Figure 2**.

and non-lymphoid tissues, same as naive T cells.

*Schematic illustration of circulation and migration of T cell subsets.*

lymphoid tissues [62, 63].

lymph nodes [64, 65].

settled [8, 11, 66, 67].

Secondary lymphoid organs and lymph nodes are the tissues where TCM and TEM cells are more common and pass through. However, recent studies have shown that a small number of non-circulating memory T cells are present in these tissues. TRM cells in SLO show phenotypic characterization similar to those in non-lym-

Primary lymphoid organs (PLO) are bone marrow and thymus. Antigen-specific TRM cells have also been found in these tissues and have been shown to facilitate long-term maintenance in the PLO. TRM cells in the PLO express CD69 and CD103

Circulation of T cell in the blood, secondary lymphoid organs, and nonlymphoid tissues is a complex system. Numerous receptors, ligands, chemokines, cytokines, and transcription factors has a role on this [31, 32]. T cells can be classified according to the organs or tissues in which they recirculate. Schematic illustra-

• Naive T cells: recirculate in the blood, secondary lymphoid organs, and non-

• Effector memory T cells: recirculate in the blood, secondary lymphoid organs,

• Central memory T cells: recirculate between nonlymphoid tissues, lymph, and

• Tissue-resident memory T cells: do not recirculate between blood, secondary lymphoid organs, nonlymphoid tissues, but may migrate within the tissue it

CC-chemokine receptor 7 (CCR7), CD69, CD49, S1PR1, KLF2, and integrins are the main factors responsible for the migration of T cell subsets. The role of these factors may differ depending on the location of the host tissue [68, 69]. These will

be further explained in more detail in phenotype and localization parts.

**52**

**Figure 2.**

αEβ7 integrin (CD103) was first discovered in the late 1980s. After that several new monoclonal antibodies were produced as a specific marker for intestinal intraepithelial T cells in humans, mice, and rats, presumably contributing to their tissue-specific localization [70]. Integrins are transmembrane αβ heterodimers that bind to extracellular matrix components and to cellular counter receptors. They have important roles on cell localization, migration, and signaling and are important for T lymphocyte adhesion and stimulation [71].

Following the discovery of the ligand called E-cadherin, interest in CD103 has been increased considerably. E-cadherin is a transmembrane protein with an extracellular region containing extracellular cadherin domain repeats, which mediates cell-cell adhesion by homodimerizing in trans with E-cadherin domains of neighboring cells [72].

CD103 is important in adhesion as well as T cell activation and TGF-B induced defense in tumor microenvironment. In TGF-B environment, CD103 TRM cells have been shown to release more efficient granzyme. Although CD103 is an important marker, CD103 alone is not sufficient to detect TRM cells. CD103 negative TRM cells were found in several tissues. Furthermore, there are different types of CD103 T cells such as CD4 CD103 T cells and CD8 CD103 Treg cells.

#### **4.2 CD69**

CD8+ TRM cells can be characterized by their expression of the surface molecules CD69 and CD103. These markers are usually not expressed on circulation T cells [73]. CD69 is a type II C-lectin membrane receptor with a scarce expression in resting lymphocytes that is rapidly induced upon cell activation [74]. Because of these features, CD69 was considered as early activation antigen of immune cells. However, recent studies have shown that this molecule is an important indicator of TRM differentiation as well as activation of the immune response.

CD69 has been found to suppress the activity of sphingosine-1 phosphate receptor 1 (S1P1), helping the TRM cells that remain in peripheral tissues [75]. The S1P1 receptor/gene, originally known as endothelial differentiation gene 1, acts by binding with a bioactive signaling molecule S1P1 [76]. It was suggested that CD69 expression might help retaining TRM cells in peripheral tissues by suppressing the activity of S1P1. Decreased expression of transcription factor of KLF2 is another factor affecting S1P1 expression to remain down-regulated in TRM cells [77, 78].

Moreover, CD69 expression is not limited to TRM cells and is not essential for TRM formation. CD69 has also been shown to be expressed in cells such as natural killer cells, dendritic cells, and in the absence of CD69, TRM formation decreased but is not completely eliminated [32, 79]. Therefore, CD69 is a good TRM marker, but it is not sufficient to be the sole determinant.

#### **4.3 CD49a**

CD49a or integrin α1 paired with CD29 (integrin-β1) to form very late antigen (VLA-1). VLA-1 is a collagen-binding integrin and receptor for collagen and laminin such as ColIV and ColI [9, 80, 81].

Collagen IV enriched in the basement membrane separating epidermis and dermis. CD49a is therefore a good marker for skin TRM cells. In human skin epithelia, CD8+ CD49a+ TRM cells produced interferon-γ, whereas CD8+ CD49a TRM cells produced interleukin-17 (IL-17). It has been reported that CD8+ T cells with a TRM phenotypes (CD103+ and CD49a+) are present in solid tumors as well as lung interstitium [9, 35, 82].

VLA1 is a receptor not only involved in adhesion but also to migration and survival. In the formation and proliferation of TRM cells, CD49a together with CD103 and CD69 are the most determinative markers of TRM presence.

#### **4.4 CD44**

The CD44 antigen is a cell-surface glycoprotein involved in cell-cell interactions, cell adhesion, and migration [83]. The most well-studied function of CD44 is as a receptor for hyaluronic acid, a component of the extracellular matrix. In regard to accessing peripheral tissues during an immune challenge, CD44 can bind hyaluronic acid expressed on vascular endothelial cells and facilitate transmigration. CD44 is a classical marker of previous activation, expressed on newly generated effector cells as well as resting memory cells [23, 84, 85].

It is important to specify that TRM cells express different markers depending on the host tissues. It should not be ignored that there may be some differences between TRM subsets in various tissue types. The results obtained by using in vivo techniques such as parabiosis, organ transplantation, using transgenic mice, and bone marrow chimera techniques were more effective in the identification TRM cell proliferation. The main factors that enable scientists to identify TRM cells as a subset of T cells have been obtained by these methods.

#### *4.4.1 Parabiosis*

Parabiosis is a surgical process that allows the sharing of blood circulation in two organisms. Bringing the skin of the two animals, in particular mice, together stimulate the capillary blood vessel formation in this region. Blood and immune cells circulate between parabiotic partners [86]. Therefore, migration or residence can be examined by investigating whether the immune system cell in one organism is in equilibrium with the other.

#### *4.4.2 Bone marrow chimera (BMC)*

BMC is another widely used technique to study donor organism, which has congenitally distinctive or labeled bone marrow, and a recipient organism, which have been irradiated, thus losing its all bone marrow-derived cells (lymphocytes) are two component of this method. Then, bone marrow cells of donor organism are transferred to the recipient organism [87, 88].

#### *4.4.3 Organ transplantation*

Transplantation is a similar approach to BMC in TRM cell studies. In this method, organ or skin graft of the donor organism is transplanted into the recipient. The equilibrium between the established T cell populations of donor and recipient organisms are examined to investigate the TRM cells. Moreover, TRM cells have important roles in organ transplantation and tissue rejection [89, 90].

**55**

**Figure 3.**

*Resident Memory T Cells*

**5. Phenotype**

andCCL21 [2, 93, 94].

*4.4.4 Transgenic organism*

*DOI: http://dx.doi.org/10.5772/intechopen.90334*

or tissue exit cannot be expressed [32, 57, 91, 92].

the downregulation of the transcription factor KLF2 [93].

trated in **Figure 3** and each is described in detail in **Table 1**.

Transgenic organisms are widely used in TRM studies. Numerous studies have been performed using knockout mice where proteins involved in tissue localization

There is not a single phenotypic character to be used to identify TRM cells. Many researchers have examined the TRM cell phenotype in different tissues including lungs, liver, lymphoid sites, skin, and intestines both in mice and humans.

Characteristically, TRM cells express CD103 and CD69. CD49a, which binds to the extracellular collagen and laminin, can be added to these two for the skin tissue [21, 23, 93]. TRM cells do not express or express very low levels of lymph node homing molecules which are required for tissue exit such as CD62L, CCR7, and S1PR1 and it is critical for TRM cell tissue residency [1, 15, 53, 67, 69]. S1P1 is mediated by

TRM cells also express cluster of chemokines and chemokine receptors including CXCR3 and CCR6, and was able to produce chemokine ligands such as CCL19

Tissue microenvironment also promotes TRM differentiation. TRM precursors that are KLRG1 negative, are more likely to differentiate into TRM cells [53, 55]. Broad range of transcription factors is associated with TRM formation. Most common transcription factors are AHR (aryl hydrocarbon receptor), Notch, Blimb1, Hobit, Eomes, and T-bet [30, 95]. These phenotypic structures are illus-

*Schematic illustration of some of the most common, receptors, transcription factors, ligands, and molecules involved in differentiation and maintenance of TRM cells and their regulation for TRM formation.*

#### *Resident Memory T Cells DOI: http://dx.doi.org/10.5772/intechopen.90334*
