**2. Immunoregulatory function of MSCs in the inflammatory microenvironment**

A key factor of survival in multicellular organisms is the maintenance and balance of homeostatic state. In the absence of inflammation, phagocytic cell is recruited to remove the apoptotic cells, whereas during acute injury, it is accompanied by inflammation, and the cell components that are released from necrotic cells result in microvascular damage due to increased vasopermeability and infiltration of macrophages and neutrophils [13]. During the process of phagocytosis of necrotic cells, there is secretion of pro-inflammatory mediators such as interleukin-1 (IL-1), interferon-γ (IFN-γ), and tumor necrosis factor-α (TNF-α), various chemokines which further initiate downstream signaling pathways [14]. Adaptive immune response actively participates in the repair of damaged tissues in close association with CD4+, CD8+, T, and B cells [13]. Recently, MSCs have been recognized to be actively involved in damaged tissue repair processes. As a functional unit for

*Immune Response Activation and Immunomodulation*

transdifferentiate into different tissue types.

concerns. Fetal stem cells can be isolated from extraembryonic tissues like cord blood, amniotic fluid, Wharton's jelly, the placenta, and amniotic membrane [1–3]. However, adult stem cells (ASCs) are multipotent cells and are usually harvested from the bone marrow, adipose tissue, dental pulp, etc. All together, these cells possess clonogenic and self-renewing potential and plasticity to differentiate and often

Isolated stem cells from both adult and fetal tissues are multipotent and are recognized as MSCs. Notably, despite similar morphology and phenotypic properties, these tissue-specific MSCs have subtle differences in their regenerative potential due to the impact of stem cell niche on cell fate, known as stem cell niche theory, genetic variability, and/or epigenetic alterations [4]. Several studies have been carried out to show that there are differences in regenerative capacity of MSCs populations of

*A diagrammatic representation of cellular characteristics, mode of action, and their therapeutic potential of* 

*mesenchymal stem cells with current status of clinical trial.*

**104**

**Figure 1.**

development and regeneration in various tissues, they hold utmost importance in maintaining proper functioning of tissues [15, 16]. In their undifferentiated and self-renewable state, the balance among interaction and protection of MSCs appears to be achieved by maintaining the stem cells in a specialized microenvironment called "niche." This niche provides accommodation to different molecules to brace and coordinate stem cell activities pertaining to growth, proliferation, differentiation, and functionality. In particular, cell-cell interaction in the niche provides structural support, regulates adhesive interaction, and activates signals by secretion of certain molecules that can control stem cell functions. Stem cell immunoregulatory responses occur due to its close association with vasculature which provides metabolic cues and a conduit due to which inflammatory cytokines and immune cells, as well as humoral factors, can be delivered to the niche. In addition, the niche also provides biochemical and biomechanical parameters such as temperature, shear force, and chemical signals, which also influence stem cell behavior and fate in response to the external environment. In the process of tissue repair, MSCs are able to affect the inflamed microenvironment by secreting a cascade of various adhesion molecules, growth factors, and pro- and anti-inflammatory cytokines [17]. Since MSCs display notable immunomodulatory properties and they are able to dodge the immune system recognition mechanisms, they can potentially modulate the defense mechanisms of the host. In inflammatory condition, MSCs located in immediate location or originating from the bone marrow region start migrating to the site of injury. At the site, these MSCs associate themselves closely with numerous types of immune cells in order to initiate regeneration process of damaged tissue, which is typically accompanied with cytokine storm. The combinational sensitization of MSCs by IFN-γ and TNF-α induces the release of chemokines where they participate in chemotaxis and are able to inhibit proliferation of inflammatory effector cells. Several molecules participate in the activation, homing, and functionality of MSCs [18] (**Figure 2**). The commencement of homing process is led by selectins present on the endothelium. Specifically, for bone marrow homing, the expression of hematopoietic cell E-/L-selectin ligand (HCELL) is very important which is a functional glycoform of CD44 present on the migrating cells, while MSCs do not express HCELL but express CD44. The subsequent molecules participated in the activation of MSCs are mainly chemokine receptors [19, 20].

Chemokines are defined as positively charged short peptides (7–13 kDa). Broadly, four families of chemokines have been recognized: CCL family with adjacent cysteine residues, CXCL family with cysteine residues separated by a single amino acid, CXCL family with two instead of four cysteines, and CX3CL family with cysteines separated by three amino acids. For MSCs to home to injury site, these cells bear chemokine receptors and are identified at site of injury due to production of the chemokines. Hence a thorough understanding of the functioning of chemokine receptor profile is important for optimizing the process of both internal and external homing processes of MSCs to wound site. The site of injury produces abundant chemokines which may provide signal to MSCs or may function as chemoattractants [21]. During the process of regeneration or repair, the injury site tightly regulates the process of chemokine expression profile or expression pattern of each chemokine, which plays a unique role in directed migration of cells toward the site of injury. The subsequent expression of chemokines to attract specific immune cell types is conducted by immune response of injury.

Integrins are key players associated with the balance activation-dependent arrest of MSCs in the second last step of homing. They are known for cell-cell mediated matrix and adhesion, and they belong to the largest family of receptors. Mammals contain 18α and 18β subunits of integrin that combine to form at least 24 different heterodimers, each of which corresponds to a specific set of cell surface,

**107**

**Figure 2.**

*Mesenchymal Stem Cells Modulate the Immune System in Developing Therapeutic Interventions*

extracellular matrix (ECM) or soluble protein ligands. They are multifaceted receptors, transmitting bidirectional signals across the cell membrane, which is crucial for building a suitable interaction between the exterior and interior of the cell. Numerous cell processes like morphology, migration, proliferation, differentiation, and apoptosis are unexpected on this recurrent discussion. According to few reports, it was suggested that MSC homing will be affected if integrin-β1 is inhibited. Also, interaction of vascular cell adhesion molecule-1-very late antigen-4

*Immunoregulatory action of MSCs at chronic inflammation (left panel) versus acute inflammation (right panel). MSCs home to the injury site due to local cytokine storm secreted by activated immune cells. Activation and migration of MSCs lead to secretion of multiple immunomodulatory and growth factors. Depending on the cytokine signal (acute versus chronic inflammation), MSCs initiate the immunoregulatory response and repair the injury site or are unable to inhibit the persisting chronic inflammatory signals resulting in cellular fibrosis.*

During the last stage of transmigration across the endothelial cell layer and below the basement membrane, various lytic enzymes are essential to cleave components of the basement membrane, such as the matrix metalloproteinases (MMPs) [23]. Specifically, gelatinases, MMP-2 and MMP-9, preferentially degrade collagen and gelatin, two of the major components of the basement membrane which facilitate MSC migration. The MSCs reportedly help migrate MMP-2 and tissue inhibitor

The initial reports pointing to MSC homing were toward investigating the origin

of BM-MSCs after allogeneic bone marrow transplantation. Those studies also concluded that the hematopoietic cell population was provided by the donor, but

(VCAM1-VL4) is involved functionally in MSC homing [22].

of metalloproteinases 3 (TIMP-3) [23].

*DOI: http://dx.doi.org/10.5772/intechopen.80772*

*Mesenchymal Stem Cells Modulate the Immune System in Developing Therapeutic Interventions DOI: http://dx.doi.org/10.5772/intechopen.80772*

#### **Figure 2.**

*Immune Response Activation and Immunomodulation*

development and regeneration in various tissues, they hold utmost importance in maintaining proper functioning of tissues [15, 16]. In their undifferentiated and self-renewable state, the balance among interaction and protection of MSCs appears to be achieved by maintaining the stem cells in a specialized microenvironment called "niche." This niche provides accommodation to different molecules to brace and coordinate stem cell activities pertaining to growth, proliferation, differentiation, and functionality. In particular, cell-cell interaction in the niche provides structural support, regulates adhesive interaction, and activates signals by secretion of certain molecules that can control stem cell functions. Stem cell immunoregulatory responses occur due to its close association with vasculature which provides metabolic cues and a conduit due to which inflammatory cytokines and immune cells, as well as humoral factors, can be delivered to the niche. In addition, the niche also provides biochemical and biomechanical parameters such as temperature, shear force, and chemical signals, which also influence stem cell behavior and fate in response to the external environment. In the process of tissue repair, MSCs are able to affect the inflamed microenvironment by secreting a cascade of various adhesion molecules, growth factors, and pro- and anti-inflammatory cytokines [17]. Since MSCs display notable immunomodulatory properties and they are able to dodge the immune system recognition mechanisms, they can potentially modulate the defense mechanisms of the host. In inflammatory condition, MSCs located in immediate location or originating from the bone marrow region start migrating to the site of injury. At the site, these MSCs associate themselves closely with numerous types of immune cells in order to initiate regeneration process of damaged tissue, which is typically accompanied with cytokine storm. The combinational sensitization of MSCs by IFN-γ and TNF-α induces the release of chemokines where they participate in chemotaxis and are able to inhibit proliferation of inflammatory effector cells. Several molecules participate in the activation, homing, and functionality of MSCs [18] (**Figure 2**). The commencement of homing process is led by selectins present on the endothelium. Specifically, for bone marrow homing, the expression of hematopoietic cell E-/L-selectin ligand (HCELL) is very important which is a functional glycoform of CD44 present on the migrating cells, while MSCs do not express HCELL but express CD44. The subsequent molecules participated in the activation of MSCs are mainly chemokine receptors [19, 20]. Chemokines are defined as positively charged short peptides (7–13 kDa). Broadly, four families of chemokines have been recognized: CCL family with adjacent cysteine residues, CXCL family with cysteine residues separated by a single amino acid, CXCL family with two instead of four cysteines, and CX3CL family with cysteines separated by three amino acids. For MSCs to home to injury site, these cells bear chemokine receptors and are identified at site of injury due to production of the chemokines. Hence a thorough understanding of the functioning of chemokine receptor profile is important for optimizing the process of both internal and external homing processes of MSCs to wound site. The site of injury produces abundant chemokines which may provide signal to MSCs or may function as chemoattractants [21]. During the process of regeneration or repair, the injury site tightly regulates the process of chemokine expression profile or expression pattern of each chemokine, which plays a unique role in directed migration of cells toward the site of injury. The subsequent expression of chemokines to attract

specific immune cell types is conducted by immune response of injury.

Integrins are key players associated with the balance activation-dependent arrest of MSCs in the second last step of homing. They are known for cell-cell mediated matrix and adhesion, and they belong to the largest family of receptors. Mammals contain 18α and 18β subunits of integrin that combine to form at least 24 different heterodimers, each of which corresponds to a specific set of cell surface,

**106**

*Immunoregulatory action of MSCs at chronic inflammation (left panel) versus acute inflammation (right panel). MSCs home to the injury site due to local cytokine storm secreted by activated immune cells. Activation and migration of MSCs lead to secretion of multiple immunomodulatory and growth factors. Depending on the cytokine signal (acute versus chronic inflammation), MSCs initiate the immunoregulatory response and repair the injury site or are unable to inhibit the persisting chronic inflammatory signals resulting in cellular fibrosis.*

extracellular matrix (ECM) or soluble protein ligands. They are multifaceted receptors, transmitting bidirectional signals across the cell membrane, which is crucial for building a suitable interaction between the exterior and interior of the cell. Numerous cell processes like morphology, migration, proliferation, differentiation, and apoptosis are unexpected on this recurrent discussion. According to few reports, it was suggested that MSC homing will be affected if integrin-β1 is inhibited. Also, interaction of vascular cell adhesion molecule-1-very late antigen-4 (VCAM1-VL4) is involved functionally in MSC homing [22].

During the last stage of transmigration across the endothelial cell layer and below the basement membrane, various lytic enzymes are essential to cleave components of the basement membrane, such as the matrix metalloproteinases (MMPs) [23]. Specifically, gelatinases, MMP-2 and MMP-9, preferentially degrade collagen and gelatin, two of the major components of the basement membrane which facilitate MSC migration. The MSCs reportedly help migrate MMP-2 and tissue inhibitor of metalloproteinases 3 (TIMP-3) [23].

The initial reports pointing to MSC homing were toward investigating the origin of BM-MSCs after allogeneic bone marrow transplantation. Those studies also concluded that the hematopoietic cell population was provided by the donor, but

the stem cells were provided by the recipient [24]. Therefore, a number of trials at both the preclinical and clinical levels have been carried out, and MSCs are seen to help migrate in a variety of tissues. Initial studies in animal models also confirmed that the presence of MSCs transplanted to donor was present in the bone marrow, thymus, spleen, and liver [12, 24, 25].

To elucidate the dynamics of MSCs migration, a systemic infusion of MSCs was studied by using varied techniques, i.e., after infusion of MSCs, they were first trapped in the lungs, and eventually, the cells disappear from the lungs and are distributed to other organs. Other aspects of MSCs homing was also studied by few groups under which they studied the factors such as early cell passage, irradiation and younger animals and observed that they influence the short-term bone marrow homing and condition which in result increases the homing [26–30].

Once MSCs are activated and recruited to the site of injury, there is onset of T-cell activation because of the presence of various pro-inflammatory cytokines such as IFN-γ, TNF-α, IL-1β, etc. IFN-γ is a critical player in providing stimulatory signals for activation and expansion of T cells and its subsets, such as it begins to suppress the T cell proliferation, differentiation, and inhibition of various biological functions. Other than IFN-γ, TNF-α and IL-1β also activate MSCs, either in synergy or alone. After stimulation with pro-inflammatory cytokines, MSCs also release other significant immunomodulatory factors. These stimulated cells modulate many immune effecters in vitro as well as in an animal model [31, 32]*.*
