**2. Plasticity of MDSCs in cancer and wound healing**

MDSCs represent a group of heterogeneous monocytes during myeloid cell development with a major attribute of immunosuppressive activities. The population of these cells increases in a number of conditions associated with chronic inflammation, autoimmune diseases, and cancer. These heterogeneous cells are now further divided into two major subgroups including polymorphonuclear (PMN) and monocytic (M)-MDSCs [23]. Although non-immunosuppressive MDSCs exist in tumor-bearing hosts or in conditions of chronic inflammation [24], in which MDSCs can be classified as MDSC-like cells (MDSC-LC), demonstration of immunosuppressive activities is required to accurately define MDSCs after the initial phenotypical characterization by cell surface markers. In term of immunosuppressive activities of MDSCs, different mediators were reported, such as arginases, nitric oxide (NO), reactive oxygen species (ROS), indoleamine 2,3-dioxygenase (IDO), transforming growth factor-β1 (TGF-β1), and prostaglandin E2 (PGE2) among others, depending on specific conditions. As MDSCs are heterogeneous and suppress immune functions with different mechanisms, it is not surprising that they possess phenotypical and functional plasticity [25], reflecting their adaptation to varied environmental conditions. Note that immune cell plasticity could be understood from two different and important senses [16]. The first one is *intra-lineage cell plasticity*, that is, changes in cell function within a given cell lineage. This is also known as functional plasticity. The second sense is *trans-lineage cell plasticity*, that is, the switch from one lineage to another. Alternatively, this can be called "transdifferentiation" or "phenotypical plasticity." We will mainly use "phenotypical plasticity" and "functional plasticity" to discuss MDSC functions in this chapter.

## **2.1 MDSC plasticity in cancer**

Immunotherapies against cancer rely on activated T cells or NK cells to recognize and eliminate tumor cells. However, the effector cells in the tumor microenvironment encounter a wide array of factors that limit their activities. MDSC-mediated immune suppression represents one of the major mechanisms by which the functions of immune effector cells are blocked in cancer. In addition, MDSCs are implicated not only in regulating tumor immune response, but also in tumor angiogenesis, tumor cell invasion, and formation of pre-metastatic niches [26].

**111**

*KLF4-Mediated Plasticity of Myeloid-Derived Suppressor Cells (MDSCs)*

in elimination of MDSC-mediated immune suppression [13].

Phenotypical plasticity of MDSCs in cancer could be first understood from the capacity of myeloid regulatory cells to convert from each other under certain conditions. Such plasticity could explain confusing observations on the role of MDSCs in tumor growth or tumor inhibition [13]. For example, while MDSCs are well known for their tumor promoting function because of their immunosuppressive activities against T cells, they can be converted to dendritic cells (DCs) in the presence of nature killer T (NKT) cells and α-galactosylceramide, leading to an anti-tumor immune response against HER2/CT26 tumor [27]. Mechanistically, it was proposed that NKT cells interact with MDSCs. This interaction leads to the conversion of MDSCs to DCs by increasing gene expression of CD80, CD86 and CD70. Consequently, interactions of CD80 and CD70 on newly converted DCs with CD28 and CD27 on T cells support these T cell responses to the tumor cells resulting

Phenotypical plasticity of MDSCs could also be understood from the existence of MDSC subtypes and their differentiation into macrophages under normal and abnormal conditions. Because PMN-MDSCs are short lived, M-MDSCs have been studied in a more detail. In addition, most studies did not correlate M-MDSCs with monocytes expressing high levels of Ly-6C (Ly-6Chi cells). These Ly-6Chi cells are frequently referred to inflammatory monocytes. Given their elevated function at the tumor site and their potent immunosuppressive activities, Ly-6Chi monocytes in the tumor microenvironment most likely represent *bona fide* M-MDSCs [14]. M-MDSCs have been shown to differentiate into tumor-associated macrophages (TAMS) after they are recruited to the tumor site [28]. It was shown that the CD45-mediated inhibition of STAT3 in MDSCs promotes TAM differentiation [29]. Besides TAMs and DCs as we discussed earlier, MDSCs differentiate into fibrocytes, an emerging group of cells with multiple functions in inflammation and cancer [19, 20, 30, 31]. Functional plasticity of MDSCs could be understood by their intrinsic features especially their immunosuppressive activities. It is known that immunosuppressive activities of MDSCs are mainly detected in tumors, but rarely in other tissues or organs including bone marrow or spleen. However, MDSCs in tumor and other chronic inflammatory conditions may not always be immunosuppressive. For example, in the initiation stage of chronic inflammation or early stage tumors, there are cells with MDSC phenotypical markers but without potent immunosuppressive activities. Moreover, even in advanced stage tumors, not all cells with a MDSC phenotype possess immune suppressive activity. For example, recent studies showed that in chronic inflammation, cells with an MDSC phenotype lacking suppressive activity actually contribute to the early stages of tumor inflammation [32]. However, the exact nature and the mechanism of how MDSCs acquire their immune

*DOI: http://dx.doi.org/10.5772/intechopen.89151*

suppressive activities are not entirely clear.

**2.2 Potential role of MDSC plasticity in tissue repair**

Though immunologists generally consider the immune system as a system of defense, recent studies suggest a key role of the system in tissue robustness, the capability of an organism to maintain its function and performance despite perturbations [33, 34]. One of the major ways by which the immune system contributes to robustness is through immune cell plasticity. Most studies of tissue repair have focused on the innate immune system, which may reflect the evolutional conservation of the repair-mediated robustness. Although plasticity of γδT cells [35, 36], innate lymphoid cells [37], and regulatory T cells [38] is also involved in tissue repair, we will mainly discuss the role of neutrophils, macrophages, and MDSCs in the process. Neutrophils are the major innate cells recruited to the damage site and are considered as the first line of defense against infection [39]. However, these cells

#### *KLF4-Mediated Plasticity of Myeloid-Derived Suppressor Cells (MDSCs) DOI: http://dx.doi.org/10.5772/intechopen.89151*

*Cells of the Immune System*

role of their plasticity in wound healing has not been fully examined. On the other hand, two immune cell lineages closely related to MDSCs, namely neutrophils and macrophages, demonstrated their phenotypical and functional plasticity in wound repair [16]. In addition, we showed that in wound healing MDSCs not only execute their immunosuppressive function to inhibit inflammation, but also stimulate cell proliferation once they adopt a fibrocyte fate [11]. Collectively, these observations support a key role of MDSC plasticity in wound healing leading to tissue robustness,

We recently reported that KLF4 promotes cancer development by regulating the recruitment and function of MDSCs [8, 17, 18]. In addition, we found that KLF4 regulates generation of fibrocytes, emerging effector cells in chronic inflammation [19, 20], from MDSCs in cancer [8], wound healing [11], allergic asthma [21]. Given the importance of plasticity of macrophages, a highly relevant cell type to MDSCs, in tissue repair and regeneration [22], we postulate that KLF4 also regulates myeloid plasticity in wound healing. In this review, the role of KLF4 in regulating plasticity of MDSCs in wound healing and the underlying molecular mechanisms will be discussed.

MDSCs represent a group of heterogeneous monocytes during myeloid cell development with a major attribute of immunosuppressive activities. The population of these cells increases in a number of conditions associated with chronic inflammation, autoimmune diseases, and cancer. These heterogeneous cells are now further divided into two major subgroups including polymorphonuclear (PMN) and monocytic (M)-MDSCs [23]. Although non-immunosuppressive MDSCs exist in tumor-bearing hosts or in conditions of chronic inflammation [24], in which MDSCs can be classified as MDSC-like cells (MDSC-LC), demonstration of immunosuppressive activities is required to accurately define MDSCs after the initial phenotypical characterization by cell surface markers. In term of immunosuppressive activities of MDSCs, different mediators were reported, such as arginases, nitric oxide (NO), reactive oxygen species (ROS), indoleamine 2,3-dioxygenase (IDO), transforming growth factor-β1 (TGF-β1), and prostaglandin E2 (PGE2) among others, depending on specific conditions. As MDSCs are heterogeneous and suppress immune functions with different mechanisms, it is not surprising that they possess phenotypical and functional plasticity [25], reflecting their adaptation to varied environmental conditions. Note that immune cell plasticity could be understood from two different and important senses [16]. The first one is *intra-lineage cell plasticity*, that is, changes in cell function within a given cell lineage. This is also known as functional plasticity. The second sense is *trans-lineage cell plasticity*, that is, the switch from one lineage to another. Alternatively, this can be called "transdifferentiation" or "phenotypical plasticity." We will mainly use "phenotypical plastic-

ity" and "functional plasticity" to discuss MDSC functions in this chapter.

genesis, tumor cell invasion, and formation of pre-metastatic niches [26].

Immunotherapies against cancer rely on activated T cells or NK cells to recognize and eliminate tumor cells. However, the effector cells in the tumor microenvironment encounter a wide array of factors that limit their activities. MDSC-mediated immune suppression represents one of the major mechanisms by which the functions of immune effector cells are blocked in cancer. In addition, MDSCs are implicated not only in regulating tumor immune response, but also in tumor angio-

though the underlying cellular and molecular mechanisms are not clear.

**2. Plasticity of MDSCs in cancer and wound healing**

**110**

**2.1 MDSC plasticity in cancer**

Phenotypical plasticity of MDSCs in cancer could be first understood from the capacity of myeloid regulatory cells to convert from each other under certain conditions. Such plasticity could explain confusing observations on the role of MDSCs in tumor growth or tumor inhibition [13]. For example, while MDSCs are well known for their tumor promoting function because of their immunosuppressive activities against T cells, they can be converted to dendritic cells (DCs) in the presence of nature killer T (NKT) cells and α-galactosylceramide, leading to an anti-tumor immune response against HER2/CT26 tumor [27]. Mechanistically, it was proposed that NKT cells interact with MDSCs. This interaction leads to the conversion of MDSCs to DCs by increasing gene expression of CD80, CD86 and CD70. Consequently, interactions of CD80 and CD70 on newly converted DCs with CD28 and CD27 on T cells support these T cell responses to the tumor cells resulting in elimination of MDSC-mediated immune suppression [13].

Phenotypical plasticity of MDSCs could also be understood from the existence of MDSC subtypes and their differentiation into macrophages under normal and abnormal conditions. Because PMN-MDSCs are short lived, M-MDSCs have been studied in a more detail. In addition, most studies did not correlate M-MDSCs with monocytes expressing high levels of Ly-6C (Ly-6Chi cells). These Ly-6Chi cells are frequently referred to inflammatory monocytes. Given their elevated function at the tumor site and their potent immunosuppressive activities, Ly-6Chi monocytes in the tumor microenvironment most likely represent *bona fide* M-MDSCs [14]. M-MDSCs have been shown to differentiate into tumor-associated macrophages (TAMS) after they are recruited to the tumor site [28]. It was shown that the CD45-mediated inhibition of STAT3 in MDSCs promotes TAM differentiation [29]. Besides TAMs and DCs as we discussed earlier, MDSCs differentiate into fibrocytes, an emerging group of cells with multiple functions in inflammation and cancer [19, 20, 30, 31].

Functional plasticity of MDSCs could be understood by their intrinsic features especially their immunosuppressive activities. It is known that immunosuppressive activities of MDSCs are mainly detected in tumors, but rarely in other tissues or organs including bone marrow or spleen. However, MDSCs in tumor and other chronic inflammatory conditions may not always be immunosuppressive. For example, in the initiation stage of chronic inflammation or early stage tumors, there are cells with MDSC phenotypical markers but without potent immunosuppressive activities. Moreover, even in advanced stage tumors, not all cells with a MDSC phenotype possess immune suppressive activity. For example, recent studies showed that in chronic inflammation, cells with an MDSC phenotype lacking suppressive activity actually contribute to the early stages of tumor inflammation [32]. However, the exact nature and the mechanism of how MDSCs acquire their immune suppressive activities are not entirely clear.

#### **2.2 Potential role of MDSC plasticity in tissue repair**

Though immunologists generally consider the immune system as a system of defense, recent studies suggest a key role of the system in tissue robustness, the capability of an organism to maintain its function and performance despite perturbations [33, 34]. One of the major ways by which the immune system contributes to robustness is through immune cell plasticity. Most studies of tissue repair have focused on the innate immune system, which may reflect the evolutional conservation of the repair-mediated robustness. Although plasticity of γδT cells [35, 36], innate lymphoid cells [37], and regulatory T cells [38] is also involved in tissue repair, we will mainly discuss the role of neutrophils, macrophages, and MDSCs in the process.

Neutrophils are the major innate cells recruited to the damage site and are considered as the first line of defense against infection [39]. However, these cells can switch phenotypes, display distinct subpopulations, and produce a large variety of cytokines and chemokines [40]. In tissue repair, neutrophils can show their intra-lineage or functional plasticity by pro- or anti-inflammation, during the early stage of a typical wound repair. In addition, in an inflammatory and pro-type 2 microenvironment of a lesion, neutrophils transdifferentiate into antigen presenting cells (APCs) [41]. Such transdifferentiation into APCs has also been studied in rheumatism, where it could drive sustained inflammation, thereby preventing normal repair [42]. Besides neutrophils, macrophages fulfill roles that change over the duration of wound healing [43]. Initially they are bactericidal, and voraciously phagocytose cell and matrix debris, particularly red blood cells and any spent neutrophils at the wound site. These early stage macrophages are called M1 macrophages, and they are pro-inflammatory. Later in the repair process, macrophages develop the pro-repair capacity. These macrophages are called M2 macrophages, and they are anti-inflammatory and pro-reparative. The resting macrophages are called M0 macrophages. Not surprisingly, the plasticity of macrophages, namely the changeable cellular phenotypes and the range of differentiation and activation states, helps to explain the pleiotropic nature of these cells and their complex functions in wound repair [22, 44]. Beside their role in the early inflammatory stage of wound healing, macrophages contribute to tissue remodeling in wound healing by transdifferentiation, notably into endothelial cells [45, 46], a phenotypical plasticity.

When compared to those of neutrophils and macrophages, the role of MDSCs and their plasticity in wound healing are less studied [47]. However, there is ample evidence supporting a critical role of MDSC plasticity in repair. For example, as a heterogeneous and immature population of myeloid cells, recruited MDSCs at wound sites can differentiate into macrophages, DCs, and neutrophils [25]. In addition, because of their immunosuppressive function, MDSCs appear to dampen inflammation at the early stage but then promote healing after inflammation wanes by adopting a fate of fibrocytes [11], a cell type that can further differentiate into myofibroblasts that produce extracellular matrix in wound closure [48, 49]. In cancer, a pathological condition considered as "wounds that do not heal," fibrocytes are viewed as a subpopulation of MDSCs [50, 51], further highlighting a dynamic and plastic nature of MDSCs in wound healing.
