Tissue regeneration approach using stem cells in different clinical settings is a medical sensation now-a-days. Especially Mesenchymal Stem cells have been focused in a plethora of preclinical and clinical experiments over the five decades due to extraordinary capacity in differentiation to multiple lineages, immunomodulation and angigenic properties.
The presence of stem cells in the endometrium was first speculated by Prianishnikov in 1978. In 2004 Chan et al. confirmed the presence of clonogenic stem cell in the endometrium. This regenerative capacity is due to presence of EnSCs that were demonstrated to be immuno-privileged in comparison with other cell types, shedding a new light for cell-based therapies and rendering these cells as a promising resource in regenerative medicine. Human endometrium is highly regenerative tissue since its complete regeneration of the uterus was achieved after an almost total resection of endometrium and the renovated endometrium could support the gestation.
Monthly cyclical endometrial growth, differentiation, and regression are governed by a fine tuned inter play between ovarian sex steroid hormones and numerous cell types. Study shown that oestrogen replacement therapy could regenerate atrophied endometrium. Human endometrial tissue composed of epithelial stem cells (0.22%), mesenchymal stem cells (1.25%) and side population stem cells. During menstruation, all these cells could be found as menstrual stem cells or endometrial regenerative cells (ERC).(13) Like other sources, endometrial MSCs have recently been emerged as a promising alternative source of therapeutic MSCs due to its unique ability to undergo smooth muscle differentiation, migratory, angiogenic. and robust immunomodulation properties. In vitro analysis reported that EMSCs express SSEA-4 and OCT4 and decreases OCT4 expression with increasing passages that confirms its stemness.
EMSCs also expressed high levels of CD73, CD90, CD105, and CD166, but they lacked CD14, CD34, and CD45 expression that is very similar with the minimum characteristics of MSCs defined by the Committee of the International Society for Cellular Therapy. EMSCs also have the high capacity proliferation, multidirectional differentiation that is indicative for the potentiality for tissue regeneration. Large body of data of experiment concludes that EMSCs have the ability to differentiation in multiple lineages such as osteoblast, cartilage, adipose, smooth muscle cell(1/18), hepatocytes, cardiomyocyte, neural cells. Moreover, EMSCs expressed MHC I HLA-ABC but not MHC II HLA-DR. They have the capability to reduce IFN-γ and TNF-α and immunomodulation capacity and shown to inhibit mixed lymphocyte reaction (MLR) in different experiments.(32) So EMSCs showed low immunogenicity. In vivo experiment performed without occurrence of immune rejection.
EnSCs synthesize and discharge several growth factors including transforming growth factor-α (TGFα), epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I). IGF-I, TGFα, epidermal growth factor receptor (EGFR), basic fibroblast growth factor (bFGF), and platelet derived growth factor-β (PDGFβ) receptor expression in epithelial cells are upregulated during expeditious growth in the proliferative stage. Leukemia inhibitory factor (LIF), Hepatocyte growth factor (HGF) and stem-cell factor (SCF) have the essential contribution in endometrial regeneration.
Vascular systems are responsible to supply nutrients, oxygen that provides various signaling molecules. During embryonic development mesodermal differentiate into endothelial cells, hemangioblast and hematopoietic progenitors give rise to blood vessel, the vasculogenesis.(39) The formation of new vessels from already existing ones is called angiogenesis. Angiogenesis starts from local destruction of the wall of preexisting blood vessel, activation of endothelial cell proliferation, and migration. Some known molecules, notably FGF, TGFα and TGFβ, HGF, TNFα, angiogenin, angiopoietins and interleukin-8 etc, serve as positive regulators of angiogenesis. Several studies have shown that the critical event in angiogenesis through the regulation of signaling cascade of endothelium is attributed to VEGF.
VEGF is also involved in regulation of vascular permeability. MSCs exert its angiogenic properties by not only paracrine signaling but also direct cellular involvement. Some angiogenic factors secreted by MSCs, for example, VEGF, SDF1, FGF2, HGF, ANG1, MCP-1 etc. acts as key growth factor for primary vessel formation and its stabilization Moreover in vitro and in vivo studies showed that the microvesicles (>200 μm) and exosomes (~50–200 μm) secreted from MSCs transport proangiogenic growth factors and miRNA. MSC derived exosomal proteins (PDGF, FGF, EGF and NF-κB pathway-affiliated proteins etc) has been found upregulated when MSCs exposed to ischemic, shown to enhance angiogenesis.(51) X. Liang, et al (2016) demonstrated that exosomes from adipose derived MSCs transfer miR-125a to endothelial cells and promote angiogenesis. Recently M. Gong (qPCR study reported ) that known proangiogenic exosomal miRNA (miR-424, miR-30c, miR-30b, and let-7f etc) derived from MSCs are transferred to endothelial cells. In vitro study reported that endothelial cells form capillary like structures when cocultured with MSCs embedded on hydrogels and assuming perivascular position.(54) and phenotype. Thus MSCs play critical role in vessel formation and stabilization.
Since it has been reported that bone marrow derived MSCs have the strong capability for angiogenesis.(56) EMSCs and BMMSCs share some common cell surface markers like CD90, CD105 and lack of CD45 and CD34. Michael P Murphy et al (2008) performed a series of experiments in vitro and in vivo to assess the proof of angiogenic concept of endometrial derived stem cells and reported that EMSCs could stimulate the angiogenic Esfandiari et al. (2007, 2008) showed human ensc could proliferate and sprout new blood vessel in 3D fibrin matrix supported culture.(58-60). Jafar Ai et al (2012) reported EnSCs were differentiated to endothelial cells (CD31+).
Data found from an in vitro experiment that human ensc could differentiated into endothelial like cells in the presence of FGF-2 and VEGF on nanfibrous scaffold. M Khazaei – 2004 reported first time in an in vitro study of endothelial network formation and angiogenesis from ensc in vitro. (63) Enmsc are found to be clonogenic mesenchymal like cells expresses pericyte markers, and localized in the perivascular space of endometrial small vessels play role in the formation of the endometrial stromal vascular tissue and the vascularization through secretion of pro-angiogenic and growth supporting factors. S. Canosa (2016) concluded enmsc can support the endothelial cell to differentiate taking part in formation of endothelium and new blood vessels.
Mesenchymal Stem Cell-Based Immunomodulation: Properties and Clinical Application] Bartholomew et al. (2002) first shown MSC have the ability to modulate immunosuppression by suppressing a mixed lymphocyte response in vitro and prevention of allograft rejection in baboon model in vivo.(69) MSCs interact with different kinds of immune cells, including B cells, T cells, natural killer (NK) cells, dendritic cells (DCs) and macrophages. This immunomodulation phenomena is performed by the cell to cell interaction in collaboration with specific immune modulatory factors, including prostaglandin E2 (PGE-2), nitric oxide (NO), indoleamine 2, 3-dioxygenase (IDO) and chemokines secreted by MSCs. Extracellular vesicles provided by MSCs have the ability to generate M2 macrophages, regulatory T cells and suppress the maturation of monocytes and proliferation of T cells and B cells. MSCs have the ability to adhere to the inflammation site to regulate the inflammation.
In some experiments MSCs secretoms including HGF, tumor-specific glycoprotein (TSG6) found successful in treating immune disorders such as Crohn’s disease and SLE(77, 78). MSCs show significant effects on immunosuppression by refraining immune cells in both the innate and adaptive immune systems. The components of innate immune system including DCs, NK cells, and macrophages interact with MSCs that inhibit inflammatory responses. MSCS could downregulate interferon-γ (IFN-γ) and TNF-α expression of DCs and accelerates production of IL-10. Moreover proinflammatory capacity of DCs is suppressed by inhibiting TNF production. NK cells is known to produce proinflammatory cytokines but MSCs inhibit the effect of NK cells by immunosuppressive secretion such as PGE2, TGF-β, and sHLA-G and reduction of IFN-γ secretion. Depending on microenvironment MSCs can activate M1 macrophages or activated M2 macrophages(84). M2 macrophages secretes IL-10 and downregulates inflammatory cytokines such as IFNγ, TNF-α, IL-1β, and IL-12. IL-10 is a key role player in MSC regulated immunosuppression. Antigen-presenting cells, including monocytes and DCs interact with MSCs to induce secretion of IL-10.
MSCs have the ability to inhibit B cells proliferation, activation and suppress B cells differentiation, eventually depress the antibody production. However, it depends on the strength of the inflammatory stimulation. Immunosuppressive factors, chemokines and adhesion molecules secreted from MSCs can effectively suppress T-cells proliferation through cellular or nonspecific mitogenic stimuli or coinhibitory molecule B7-H4 and HLA-G. Experiments shown that MSCs play important role by directly inhibiting the proliferation of αβ T cells and γδ T cells as well. However the immunosuppressive activities of MSCs also depends on microenvironment, for example, in the presence of Pathogen associated molecule and TLR (TLR3,TLR4), MSCs can no more suppress T cells proliferation. MSCs can protect neutrophils from apoptosis by releasing IL-10.
Cassatella et al. (2011) showed that TLR3-activated MSCs are potent in preserving neutrophils viability and function by endogenous IL-6, GM-CSF and IFN-b. In another study, Maqbool et al. (2011) also reported that MSCs could rescue neutrophils from nutrient or serum deprived cell death. Thus Neutrophil, one of the most important immune cells, exert immunomodulation performance is supported by MSCs. Nevertheless we should consider that the effect of MSCs is not always strictly suppressive. It depends on strength of factors. For example, low number of MSCs can promote T cell activation and high number of MSCs cause opposite effect. Likewise, low dose of IFN-γ allow MSCs to express MHC class II antigen, on the other hand, high dose of IFN-γ reduces the expression class II antigen. Currently, MSCs has been used as cell based therapy in numerous diseases including steroid refractory GVHD, Multiple Sclerosis , Rheumatoid Arthritis, Aplastic Anemia Crohns Disease and SLE (106) and showed satisfactory safety and efficacy.