**3.3 Prostaglandin E2 (PGE-2)**

*Immune Response Activation and Immunomodulation*

**3.2 Human leukocyte antigen-G (HLA-G)**

*of the biological active content for immunomodulatory effect.*

The studies carried out with placental cells showed that they are capable of preventing maternal T-cell destruction of the fetus during pregnancy, which happens due to the expression of IDO in placental cells. During pregnancy, the fetus expresses paternal antigens that do not provoke rejection by the mother like other semi-allogeneic grafts [36, 37]*.* Dendritic cells can also express IDO and thus induce a tolerogenic response. Su et al. suggest that MSCs do not have the innate ability to express IDO but gain this ability following stimulation by the pro-inflammatory cytokines IFN-γ and TNF-α in combination with IL-1β [38]. Recently, the role of IDO in MSC-mediated immunoregulation has been demonstrated in the suppression of various immune cell populations, including T cells and NK cells [39, 40].

*Immunoregulatory function of MSCs on different cell types of the innate and adaptive immune cells. (Right panel) paracrine effect of MSCs through secretion of exosomes and their fusion with the target cell membrane and release* 

*Representative image shows the immune response of MSCs by secretion of IM factors (left panel).* 

HLA-G is a major histocompatibility complex class I antigen encoded by a gene on chromosome 6p21. It differs from classical HLA class I molecules by its restricted tissue distribution and limited polymorphism in the coding region. HLA-G can be expressed as seven distinct protein isoforms, each encoded by a specific, alternatively spliced transcript. Four isoforms are membrane-bound proteins (HLA-G1, HLA-G2, HLA-G3, and HLA-G4), and the other three isoforms are soluble proteins (HLA-G5, HLA-G6, and HLA-G7) [41]. It exerts its immunomodulatory functions by interacting with multiple receptors such as LILRB1(ILT2/ CD85j), LILRB2 (ILT4/CD85d), and KIR2DL4 (CD158d) which are differentially expressed by immune cells. Besides these receptors, HLA-G can also bind to CD8 without T-cell receptor (TCR) interaction, provoking NK cells and activated CD8 + T cell-induced apoptosis as well as FASL upregulation and secretion [42, 43]. HLA-G plays a fundamental role in maternal tolerance and transplantation. HLA-G expression by MSCs can be positively modulated by IL-10 and leukemia inhibitory factor (LIF). Other molecules such as glucocorticoid and interferon-β (IFN-β) are found to regulate HLA-G expression in immune cells. HLA-G has been

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**Figure 3.**

Prostaglandins are small molecule derivatives of arachidonic acid (AA), produced by cyclooxygenase (COX, constitutively active cyclooxygenase COX1 and inducible COX2) and PG synthases. It can be produced by all cell types of the body, with epithelia, fibroblast, and infiltrating inflammatory cells representing the major sources of PGE-2 in the course of an immune response. The receptors of PGE2 (EP1–EP4) are present on multiple cell types, reflecting the ubiquitous function of PGE-2. It is relatively stable in vitro although its decay is accelerated by albumin [46]. In contrast, PGE-2 has a very rapid throughput rate in in vivo conditions and is quickly eliminated from tissues and circulation. This property of PGE-2 is most likely to contribute toward immune pathology and constitutes a potential target for immunomodulation. It is worth noting that the effect of PGE-2 in MSCmediated immunoregulation in most cases is exerted in combination with other immunosuppressive molecules. With human MSCs, PGE-2 has been found to act with IDO to alter T-cell proliferation, during proliferation, cytotoxicity, and cytokine production by NK cells [47].

## **3.4 Inducible nitric oxide synthase (iNOs)**

Nitric oxide synthases are family of enzymes catalyzing the production of nitric oxide from L-arginine. The enzymes convert arginine into citrulline and produce NO in the process. NO activity is independent of the level of calcium in the cell. However, its activity as other NO isoforms is dependent upon the binding of calmodulin (CaM). NO in high concentration is known to inhibit immune responses through mechanisms that remain largely unidentified. In addition, upon induction cytokines such as TNF-α and IFN-γ, alone or in combination, stimulate NO. This has a significant impact on both primary and secondary immune responses. For example, NO targets dendritic cells (DCs) that have a crucial role in making powerful immune response. It was found to prevent maturation of rat lung DCs by inhibiting granulocyte-macrophage colony-stimulating factors. Similarly, NO inhibits TNF-α and prevents DC maturation in humans [48]. MSCs produce large amounts of chemokines and adhesion molecules; immune cells accumulate in close proximity to the MSCs, where the high concentration of secreted NO can suppress the immune cells [49, 50].

### **3.5 Interleukin 10 (IL-10)**

IL-10 is produced by both myeloid and lymphoid cells. While it is good immune suppressor, it has some immune stimulatory effects. IL-10 is recognized by its effect on T cells, macrophages, and monocytes which ultimately prohibit inflammatory responses. Thus, it regulates growth and differentiation of B cells, T cells, NK cells, and other cells of the immune system hence influencing inflammatory responses. IL-10 has the capability to inhibit the production of IL-2, TNF-α, IL-12, and IFN-γ. Furthermore, it will downregulate HLA class I. Although IL-10 has been implicated in MSC-mediated immunosuppression, direct IL-10 production by MSCs has not been demonstrated so far. Instead, contact of antigen-presenting cells such as dendritic cells or monocytes with MSCs has been found to induce IL-10 production [51–53].
