Human amnion-derived stem cells (hADSCs) are referred as the cells of possessing the abilities of self-renew and differentiation, which are isolated from human amnion and include human amniotic mesenchymal stem cells (hAMSCs) and human amniotic epithelial stem cells (hAESCs).
Stem cells, defined by dual hallmark features of self-renewal and differentiation potential, can be derived from embryonic and adult tissues. Stem cells are classified to pluripotent stem cells (PSCs), multipotent stem cells, and unipotent stem cells based on their developmental potency. PSCs are able to form all tissues/cells with distinct functional properties which depend upon the derived and cultured conditions. Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) are the two most common types of PSCs. Multipotent stem cells, such as hematopoietic stem cells, are restricted to generating the mature cell types of their tissue of origin and they exist in the resting state under normal physiologic circumstances and are activated when these tissues receive nociceptive stimulation. Unipotent stem cells possess the capability of self-renewal and limited differentiation potential and only produce a single cell type. The most typical unipotent stem cells are spermatogonial stem cells, which can only differentiate into sperm. In the early embryo, PSCs represent progenitors for all tissues while later in the development, tissue-restricted adult stem cells (ASCs), including multipotent stem cells and unipotent stem cells, give rise to cells with highly specialized functions. Unlike ESCs and iPSCs, tissue-restricted ASCs are limited in their potency to the cell types of the tissue in which they reside. ASCs derived from different tissues showed an attractive application clinically due to their abilities to differentiate into a certain type or a designated type of specific cells and have little risk of tumorigenicity and immune rejection. When tissues and organs are damaged, sufficient tissue-ASCs are essential in maintaining tissue regeneration and functional integrity.
Although researchers have made endless efforts to improve the technologies of ESC and iPSCs, there still are two prominent hardship, tumorigenicity and low survival rate of transplanted cells/tissues, leading to enormous challenges in clinical application. In addition, the differentiation of ESCs and iPSCs to different cells is a stepwise process that is involved in a combination of transcription factors. During the in vitro inducing process, the cells generated from transdifferentiation of ESCs or iPSCs may not possess biological function. In addition, ASCs have also certain limitations, such as the limited pluripotency, the reduced numbers with aging and the ability of the restricted expansion in vitro. Some studies have showed that ASCs were not intrinsically immunoprivileged, and under appropriate conditions, allogeneic ASCs might also induce immune rejection of an allogeneic graft. In addition, studies also showed that the gradual accumulation of genetic mutations in human ASCs during life were able to be transmitted to daughter cells and initiate tumorigenesis.
Human amnion-derived stem cells (hADSCs) including human amniotic epithelial stem cells (hAESCs) and human amniotic mesenchymal stem cells (hAMSCs) have the great advantages over other stem cells such as renewal, multi-differentiation potential, no-tumorigenicity, low/no immunogenicity, no ethical or legal concerns and their potent paracrine effects, especially immunomodulatory effects, making them have a promising source of stem cells for cell therapy in various diseases.
Placenta is composed of the amnion member, chorionic plate, decidua basalis, chorionic villi, cotyledons/interuillous space, and placental septa (Figure 1A). Among these placental components, the amniotic membrane serves as a suitable raw materials for cell-based therapy due to the large number of cells . The amniotic membrane is a transparent, smooth, avascular and single-layered thin membrane (about 100 μm) composed of epithelium and mesenchyme. The membrane covers the fetus and holds the amniotic fluid. Generally, amnion membrane has five layers including epithelium, basement membrane, compact layer, fibroblast layer and spongy layer (Figure 1B). Epiblast-derived hAMSCs and hypoblast-derived hAESCs are two primary stem cell types in amniotic membrane which are responsible for the production of extracellular matrix (ECM), different cytokines and growth factors. hAESCs come from the innermost layer of amnion which directly contact with amniotic fluid and fetus, whereas hAMSCs are scattered in the membrane. Isolation protocols have been extensively described for both hAESCs and hAMSCs. Briefly, the amniotic membranes were treated with trypsin-EDTA for 45–60 min at 37 °C to release hAESCs. Then, the remaining amniotic membranes were digested with Collagenase IV on a rotator 40 min at 37 °C to isolate hAMSCs.
Figure 1. The anatomy of the human term placenta and amniotic membrane. (A) Schematic section of the human term placenta; (B) Schematic of amnion structure.
hADSCs are easily isolated and propagated ex vivo. hAMSCs show a fibroblast-like morphology in culture, while hAESCs exhibit a cobblestone-like morphology. Compared with stem cells from other sources, hADSCs have the following advantages: (1) Easy to obtain, abundant sources, and no ethical and moral disputes: as the remaining after fetal birth the amniotic membrane can be used for separation of hAMSCs and hAESCs, which will not harm the donors; (2) No tumorigenicity: numerous studies showed that hADSCs had no proliferation and growth on soft agar in vitro, no colonies formed, no teratoma formation after implanting NOD-SCID mice in vivo; (3) Low immunogenicity and high histocompatibility: hADSCs were considered as the immune-privileged cells and showed remarkable characteristics of low immunogenicity. hADSCs had a low expression of the major histocompatibility class I antigen (HLA-ABC), no expressions of the major histocompatibility class II antigen (HLA-DR) and β2 microglobulin, importantly, the cells did not express HLA-ABC costimulatory molecules such as CD80, CD86 and CD40. It has been reported that transplantation of hAMSCs into humans to treat lysosomal diseases showed no obvious rejection. A recent study also demonstrated that intravenous administration of hAESCs did not result in haemolysis, allergic reactions, toxicity or tumor formation. Akle et al. reported that an immunotype-mismatched human amniotic membrane did not elicit a host immune response when transplanted under the volunteers’ skin. However, a few studies have highlighted that human amniotic cells might not actually be considered immune privileged, but, on the contrary, could stimulate both an innate and adaptive immune response, indicating that the possible co-immunostimulatory effects of amniotic stem cells.
Both hAMSCs and hAESCs expressed the classical mesenchymal stem cells (MSCs) markers such as CD90, CD44, CD73, and CD105, and lack of cell surface markers such as CD45, CD34, CD45, HLA-CR, CD80, CD86. The molecular markers of hAMSCs and hAESCs are shown in Table 1. Expressions of MSCs markers in hAMSCs indicated that the cells possess the attractive clinical benefits of MSCs due to immune-privilege and the ability for immunomodulation.
|hADSCs||Epithelial Markers||Mesenchymal Stem Cell Markers||Pluripotent Markers||Hematopoietic Marker||The Major Histocompatibility
Complex and Their Co-Stimulatory Molecules
|CD29, CD166,||OCT4, NANOG, SSEA4, TRA-1-60||CD34, CD45||HLA-DR, HLA-DQ||Yang et al. |
|CK19||CD29, CD44, CD73, CD90, CD105,||SSEA-4, SOX2, OCT-4||CD31, CD34, CD45, CD49d||HLA-DR||Wu et al. |
|CK7, E-cadherin||CD29, CD73,
|OCT4, NANOG, SSEA4||CD34, CD45||HLA-ABC||HLA-DR, CD80, CD86, CD40||Liu et al. |
|E-cadherin||OCT4, SOX2, NANOG, TFE3, KLF4,
SSEA3, SSEA4, TRA-1-60, REX1
|Castro et al. |
|E-cadherin, CD49f, CK7, EpCAM||CD44, CD90, CD105, CD146, PDGFR-b, CD29||CD45||HLA-A-B-C||HLA-DP-DQ-DR, CD80, CD86, CD40||Pratama et al. |
|Cytokeratin||SSEA3, SSEA4, TRA-1-60, Oct-4||CD34||Evron et al. |
|CD29, CD73||CD34, CD45||HLA-ABC||HLA-A2, HLA-DQ, HLA-DR||Murphy et al. |
|CD9, CD10, CD29, CD104, CD49f, CD105, CD44||CD34, CD45||HLA-ABC||HLA-DR, CD80, CD86, CD40||Banas et al. |
|SSEA-3, SSEA-4, TRA 1-60, TRA 1-81||CD34||Miki et al. |
|Cytokeratin, E-cadherin||CD29, CD166, CD90,||OCT4, NANOG, TRA 1-60, SOX2||CD34, CD45||HLA-DR, HLA-DQ||Yang et al. |
|CK19, E-cadherin||CD29, CD44, CD90||OCT4, SOX2, SSEA4,||CD34||Wu et al. |
SSEA-4, TRA 1-60, TRA 1-81, Oct-4, Nanog
|CD34||Miki et al. |
|CD73, CD29,||Oct3/4, Sox2, Klf4, SSEA4, c-Myc.||CD34, CD45||Koike et al. |
|hAMSCs||CD29, CD44, CD49d, CD73,
|SSEA-4, SOX2, OCT-4||CD31, CD34, CD45||HLA-DR||Wu et al. |
|OCT4, NANOG, SSEA4||CD34, CD45, CD133||HLA-ABC||HLA-DR, CD80, CD86, CD40||Liu and Li et al.|
|CD44, CD90, CD105, CD146||Oct-3/4||CD45, CD34||HLA-ABC||HLA-DR||Bačenková et al. |
|CD90, CD44, CD73, CD166, CD105, CD29||SSEA-4, STRO-1||CD34, CD45||Prado and Sugiura et al. |
|CD29, CD44, CD73, CD90, CD105||SSEA-4, Oct4||CD34, CD45, CD133||HLA-ABC||HLA-DR||Coppi et al. |
|CD105, CD117,||CD34||HLA-ABC||HLA-DR||Borghesi et al. |
|CD29, CD105||Oct-3/4, SSEA-4, SOX2, NANOG, Rex-1||HLA-A, HLA-DQB1||Mihu et al. |
|CD44, CD90, CD105. CD90||CD31, CD34||Seo et al. |
|CD44, CD73, CD90, CD105, CD29, CD49f, CD271||Oct3/4, Sox2, Klf4,SSEA4, Nanog, TRA1-60||CD34, CD45||Koike et al. |
CD90, CD105, Vimentin
C-MYC, SOX2, NANOG, SSEA-3, SSEA-4
|CD34, CD45||HLA-DR||Nogami et al.|
hADSCs have the potential to differentiate into all three germ layers when exposed to exogenous growth factors or chemicals. Both hAMSCs and hAESCs expressed the typical surface markers of embryonic stem cells such as SSEA-3, SSEA-4, SOX-2, TRA1-60 and TRA1-81, especially the Oct-4 and Nanog, indicating the great potential of hAMSCs and hAESCs in regenerative medicine. So far, studies showed that hADSCs were able to differentiate into adipocytes, bone cells, nerve cells, cardiomyocytes, skeletal muscle cells , hepatocytes, hematopoietic cells, endothelial cells, kidney cells and retinal cells(Figure 2).
Figure 2. Multi-differentiation potential of hAMSCs and hAESCs.
Although various therapeutic approaches have been applied to promote angiogenesis, most of them were not able to fully mimic the process of natural vessel development. Amnion has both angiogenic and anti-angiogenic properties, which is surface dependent. The epithelial surface of amnion had inhibitory effects on vessel formation and hAESCs were able to incorporate into the arterial wall without immunosuppression, but failed to improve vascular function. However, the vessel length and sprout were increased in the amniotic mesenchymal side. hAMSCs shared similar capability with bone marrow MSCs in neovascularization and could initiate the cascade of signals by secreting factors needed for promoting formation of stable neo-vasculature and angiogenesis. Several studies have evaluated the proangiogenic potential of hAMSCs which expressed high levels of representative proangiogenic genes VEGF-A, angiopoietin-1, HGF, and FGF-2 and anti-apoptotic factor AKT-1. By directly transplantation of hAMSCs into the ischemic hindlimbs of mice, the augmented blood perfusion and capillary density were observed, indicating that hAMSCs might promote the formation of neovascularization. Moreover, hAMSCs exerted the beneficial paracrine effects on infarcted rat hearts through cardioprotection and angiogenesis.
hAMSCs and their conditional medium (hAMSC-CM) significantly improved the cutaneous blood flow after infusion into the ischemic leg whose femoral vessels were ligated. König et al. observed that hAMSCs were able to take up acetylated low-density lipoprotein and form endothelial-like networks. Studies showed that hAMSCs promoted angiogenesis by regulating the ERK1/2-MAPK signaling pathway and that knockdown of lnRNA H19 significantly inhibited the angiogenic function of hAMSCs, in which the mechanism might be related to EZH2 degradation and VASH1 activation. In addition, Tang et al. demonstrated that the angiogenesis of hAMSCs might be related to inhibit the functions of the Circ-ABCB10/miR-29b-3p/VEGFR, Circ-100290/miR-449a/eNOS, and Circ-100290/miR-449a/VEGFA axes in human umbilical vein endothelial cells. Taken together, although the underlying mechanism of hAMSCs in angiogenesis is not fully elucidated the cells may become a useful reagent in various vascular diseases.
hAMSCs-derived soluble factors including TGF-β, HGF, PGE2 and IDO suppress mitogen-induced peripheral blood mononuclear cell (PBMC) proliferation in a dose-dependent manner. Wolbank et al. demonstrated the contact- and dose-dependent inhibitions of PBMC immune responses of hAMSCs and hAESCs. Bulati et al. observed that INF-γ-induced immunomodulatory effects of hAMSC were dependent on the activated lymphomonocytes, cell-to-cell contact and soluble factors. hAMSCs significantly inhibited the proliferation of stimulated lymphocytes and T cells . Rossi et al. demonstrated that prostaglandins released by hAMSCs were responsible for the anti-proliferative effect on lymphocytes. Meesuk et al. found hAMSCs exhibited immunosuppressive effect when they were co-cultured with activated T-cells by secreting indoleamine 2,3-dioxygenase. Morandi et al. observed that HLA-G and -E molecules were involved in hAESCs-mediated suppression of T cell proliferation.
Furthermore, hADSCs also have immunomodulatory and immunosuppressive effects on inflammatory processes including reducing the activities of inflammatory cells and inhibiting migration of microglia and recruitment of immune cells to injury sites. hADSCs also exhibited angiogenic, cytoprotective, anti-scarring, and antibacterial properties. Therefore, it is reasonable to believe that hADSCs may be a potential cell source for cell-based therapy of diseases.