Mesenchymal stem cells (MSCs) are pluripotent stem cells which can attach to the wall where they grow and replicate (35). The sources of MSCs are relatively abundant; for example, they can be extracted from the bone marrow, umbilical cord, adipose tissue, peripheral blood, and so on (36). MSCs have become a research hotspot for their diverse functions, which include cell differentiation and proliferation, regulation of inflammatory processes, control of oxidative stress, and angiogenesis (37).

Currently, there are several classifications of MSCs; donor MSCs can be classified as either allogeneic or autologous. Allogeneic MSCs are derived from other donors for implantation into the recipient patient. Because they are extracted from a foreign body, the number of allogeneic MSCs can be quite large. However, since allogeneic MSCs are equivalent to foreign matter, they are not ethically recognized in humans (38). In contrast, autologous MSCs are cells that are extracted from the patient’s own tissues, induced and differentiated, and then re-implanted for treatment. Therefore, they adapt better to the patient’s body, and are less likely to suffer from immune rejection. In the past decade (39), genetic modification of autologous hematopoietic stem cells has been used for the treatment of single-gene diseases. Moreover, they are more in line with people’s ethical requirements and are recognized by more people. At present, autologous MSCs have been used to treat several diseases, including multiple sclerosis (40), post-burn scar treatment (41), refractory rectovaginal Crohn’s fistulas (42), and patients with advanced tumors (43), among others ( Table 2 ).

In addition, MSCs can be classified by source, as either dental pulp MSCs (44), human umbilical cord MSCs (45), bone marrow MSCs (46), and adipose tissue-derived MSCs (47). In addition, if they are classified by developmental potency, they can be divided into totipotent stem cells (TSCs), pluripotent stem cells, and unipotent stem cells.

MSCs have many notable characteristics. First, MSCs cannot differentiate and self-renew (52). Hsiao et al. (53) proposed that treatment with undifferentiated MSCs can improve insulin resistance in diabetic rats, rebalance inflammation, and improve blood sugar levels. Regarding the differentiation ability of MSCs, researchers have suggested that MSCs can differentiate into adipocytes, osteoblasts, and chondrocytes. In addition, detection of MSC surface antigens indicated that the expression of markers such as CD166, CD44, CD29, CD73, CD90, and CD105 increased significantly with the increase of transmission algebra (20). Moreover, MSCs also express CD11, CD14, CD45, and CD34. However, as the transmission algebra increases, these markers are reduced or lost. Therefore, MSCs can be detected using the above-mentioned markers.

Current research indicates that application of bone marrow MSCs can promote endometrial regeneration by suppressing the innate and adaptive immune systems (54). On the one hand, in the innate immune system, MSCs can inhibit the activation, proliferation and cytotoxicity of natural killer cells (NK cells) (55). Conversely, in adaptive immunity, MSCs (56) can inhibit the proliferation of B and T cells, and further prevent the proliferation and differentiation of T cells into pro-inflammatory TH1 and TH17 helper T cells. In addition, MSCs can promote the differentiation of T cells into tolerant Treg cells (57), and can regulate the action of monocytes, dendritic cells, and NK cells by secreting chemokines (58), such as IGF-1, TGF-β, bFGF, HGF, IL-6, SDF-1, M-CSF, VEGF, PIGF, and MCP-1 (55).

Toll-like receptors (TLR) are also involved in the immunotherapy of MSCs. When faced with a danger signal, TLRs recognize danger signals and are activated to trigger a cellular response, mobilizing innate and adaptive immune cells (59). The results showed that MSCs combined with TLR4 promoted the secretion of pro-inflammatory cytokines. In contrast, if combined with TLR3, MSCs promote the secretion of anti-inflammatory cytokines (60). When microorganisms are infected, exogenous danger signals such as endotoxins or lipopolysaccharides (LPS) disappear. Meanwhile, abnormal or damaged cells overflow into the circulatory system, resulting in the expression of heat shock proteins, an endogenous danger signal. Get rid of the “danger signal” to activate the sentinel innate immune cells (for example, dendritic cells) TLR. Various immune cells are recruited to the site of endometrial damage (61).

Moreover, researchers have found that the concentration of macrophages in the endometrium of patients with IUA decreased, which may be related to a decrease in CSF-1 (62). Otherwise, another study indicated (63) that transplantation of Human amniotic epithelial cells (HAECs) can be performed through autophagy-induced recovery of damaged endometrium. VEGF expression in the menstrual and proliferative phases can promote the reconstruction of endometrial tissue. HAECs can increase the expression of VEGF, promote angiogenesis, and promote the recovery of endometrial fibrosis.

Mesenchymal stem cells have the capability to differentiate After MSCs reach the tissue, they secrete stem cell cytokines such as SCF and M-CSF to reactivate the differentiation potential of the endogenous stem cells of the damaged tissue, thus promoting the generation of new tissue cells to replace damaged cells, resulting in tissue recovery; this is termed stem cell differentiation therapy (64). At present, MSC differentiation has been applied in various fields. A set of clinical treatment data of Ashman syndrome showed (65) that patients receiving autologous CD9 ⁺, CD44 ⁺, CD90 ⁺ bone marrow MSC transplantation experienced improved endometrial vascularization and an increase in endometrial thickness. One such patient subsequently underwent in vitro fertilization and embryo transfer, and successfully carried a fetus to term.

Differentiation therapy of MSCs can also be applied to other aspects, such as the treatment of Parkinson’s disease, in which EDSCs are promoted to differentiate into dopamine-producing cells (66). Studies have shown (67) that CD90, CD146, and PDGFR-β in HEDSCs can differentiate into neuron-like cells. Moreover, they can also differentiate into cholinergic neuron-like cells (68), oligodendrocyte-like cells (69), insulin-producing cells (70), cardiomyocyte-like cells (71), megakaryocyte-like cells (72), and urothelial cells (73).

MSCs coordinate tissue recovery by releasing soluble paracrine cytokines (74). Treatment of IUA through the paracrine pathway is currently the most popular research method as IUA is partially affected by inflammatory cytokines. One of the subgroups of inflammatory cytokines is IL-1ra in the acute inflammatory paracrine effect of MSCs. IL-1ra inhibits cytokine stimulation of the helper T lymphocyte system, as well as the production of the inflammatory cytokines TNF-α in macrophages in an IL-1ra-dependent manner (74).

Furthermore, because exosomes can make mesenchymal stem cells specialize in the treatment of certain diseases, they are now an active field. Exosomes are small, single-membrane secretory organelles with a diameter of 30–200 nm (75). Recent studies have shown (76) that the function, targeting, and mechanism-driven accumulation of specific cellular components in exosomes indicate that they play a role in regulating intercellular communication.

At present, exosomes are also used in the IUA. Yao et al. (77) demonstrated that exosomes from MSCs can reverse the epithelial to mesenchymal transition (EMT) through action of the TGF-β1/Smad pathway and promote the repair of damaged endometrium in 64 female rabbits. Liao et al. (78) also summarized the manner by which different exosomes could be used to treat female reproductive disorders, and concluded that MSCs can improve female fertility in in vitro and in vivo models. Therapeutic mechanisms include angiogenesis, immune regulation, anti-fibrosis, and anti-oxidative stress. In addition, after subcutaneous injection of a mixture of matrix gel and MSCs in mice, the number of hemoglobin or CD31⁺ cells increased, indicating that MSCs can promote the formation of functional capillaries (79). Furthermore, Zhao et al. (80) demonstrated that ADSC-ex promotes endometrial regeneration and fertility recovery. Moreover, Tao et al. (81) found through experiments that Mir-29a in bone marrow MSC exosomes can inhibit fibrosis in the process of endometrial adhesion repair.

The homing function of stem cells functions to recruit stem cells by disrupting the tissue-secreted chemokine system (82). It is well known that chemokine SDF1 and its special receptor CXCR4 play a key role in the homing of BMSCs (83). Moreover, studies have shown that ERα promotes the proliferation and migration of BMSCs through SDF1/CXCR4 (84). Furthermore, the CXCL12/CXCR4 protein ligand is a chemokine receptor complex that can transmit stem cells to the uterus (85), and thus promote endometrial repair. In addition, G-CSF has been shown to mobilize hematopoietic stem cells from the bone marrow into the blood, thereby reducing migration to the endometrium (86). G-CSF can also enhance the expression of cytokeratin, vimentin, integrin, and leukemia inhibitory cytokine (LIF) and regulate endometrial function (7).

Immunomodulatory macrophages can be categorized as either pro-inflammatory M₁ and anti-inflammatory M₂ macrophages (91). Some scholars have pointed out that MSCs can effectively promote the polarization of macrophages into the M₂ type. This process is mediated by a variety of soluble molecules, including PGE2, IDO, HGF, IL-1RA, TSG6, TGF-β, etc. (90). In addition, MSCs induce M2 polarization, the anti-inflammatory cytokines IL-10, arginase-1, and TGF-β increase, and the pro-inflammatory cytokines TNF-α, IL-12, and IL-1 decrease, inhibit T cell response, and induce the production of regulatory T cells. Finally, this leads to further immunosuppression, which is conducive to the treatment of allogeneic MSCs (92).

Furthermore, when human umbilical cord-derived MSCs (UC-MSCs) are co-cultured with monocytes and macrophages, the expression of HLA-DR/DP/DQ and CD86 is reduced, and the phagocytosis and antigen presentation ability of monocytes and macrophages is decreased (93). This indicates that, compared with ordinary stem cells, UC-MSCs can reduce the chance immune rejection caused by allogeneic transplantation. In addition, US-MSCs can induce CD14+, CD16+, and CD206+ in mice after being phagocytosed by monocytes, leading to an increase in the anti-inflammatory cytokines IL-10 and PD-L1 (94) ( Figure 3 ).

Dendritic cells (DCs) are antigen-presenting cells that can induce T cells to produce an immune response. MSCs can inhibit the maturation of dendritic cells by regulating the expression of HLA-DR, CD40, OX40L, CD80, and CD83 (95), and upregulating the expression of PD-L1 (96), thereby reducing its ability to activate T cells. In addition, when DCs are co-cultured with MSCs, the pro-inflammatory cytokines secreted by CD1c+ DCs are reduced, and the anti-inflammatory cytokine IL-10 secreted by plasmacytoid DCs is increased (97). Moreover, studies have shown that exosomes released by adipose-derived mesenchymal stem cells can inhibit IL-6 and increase the expression of the anti-inflammatory cytokines IL-10 and TGF-β (98). Furthermore, treatment of mouse dendritic cells with MSC-derived exosomes did not stimulate T cell proliferation after LPS activation (98)

In the T cell family, CD4 ⁺ helper T cells and CD8 ⁺ cytotoxic T cells play important roles in the immune regulation of MSCs. On the one hand, CD4 ⁺ T cells can bind to CD40 and CD40 ligands on their cell surface, thereby enhancing the ability of dendritic cells to induce CTL. On the other hand, CD40 can maintain CTL activity by triggering the secretion of IL-2 (99). However, MSCs inhibit the proliferation of these two tesla cells through paracrine and cell contact, thereby reducing the release of the pro-inflammatory cytokines TNF-α and IFN-γ, and reducing the immune rejection between allogeneic MSC receptors (100).

In addition, exosomes extracted from mouse MSCs can inhibit T cell proliferation by upregulating cyclin-dependent kinase inhibitor 1 B and downregulating cyclin-dependent kinase 2 (101). Bone marrow mesenchymal stem cells can transform Th1 to Th2 mediated by dendritic cells and induce Tregs (56), reduce pro-inflammatory cytokines IFN-α, IL-17, and IL-6, and increase the anti-inflammatory cytokines IL-4 and IL-10 (102). Moreover, exosomes extracted from umbilical cord-derived mesenchymal stem cells can inhibit CD8 + T cells and Th1 cells, reduce the secretion of pro-inflammatory cytokines IFN-γ and TNF-α, induce Tregs, and promote the secretion of the anti-inflammatory cytokines IL-10 (103) ( Figure 4 ).

In the treatment of osteoarthritis, Djouad et al. (110) initially proposed that although allogeneic mouse MSCs can be transplanted into mice with strong immunity to form bone, lymphocyte infiltration can be seen around newly formed tissues. Fischer et al. (111) also found more severe graft-versus-host disease (GVHD) in mice receiving allogeneic hematopoietic stem cell transplantation than in wild-type mice in clinical models. The main reason explaining this is that during allogeneic hematopoietic stem cell transplantation, damaged or dead epithelial cells release endogenous risk signals and are perceived by pattern recognition receptors on antigen-presenting cells, triggering the release of proinflammatory cytokines and T cells from major donor sources, which attack and destroy host cells and tissues, causing graft-versus-host disease (112).

In addition, studies have shown that the survival rate of allogeneic mesenchymal stem cells is limited when transplanted into the body. Liesz et al. proposed that even in uninjured adult brains, transplanted MSCs can be rejected within 14 days (113). This indicates that after allogeneic MSCs are transplanted into the body, the survival rate is still not guaranteed and some cannot coexist with the host. There are also reports that pigs (114), rats (115) and baboons (116) have dissolved allogeneic antibodies after inoculating allogeneic mesenchymal stem cells, which further confirmed the possibility of immune rejection between allogeneic and autologous MSCs.

Barnhoorn et al. (117) showed that the titer of anti-donor immunoglobulin G increased significantly 7 days after the subcutaneous injection of IFN-γ-activated allogeneic mesenchymal stem cells in pigs. Furthermore, in the use of MSCs to treat traumatic brain injury, it has been proposed (54) that allogeneic mesenchymal stem cells can be actively rejected by the host immune response, mainly due to the cytotoxic CD8+ T cell-mediated response, which limits the therapeutic effect. Melissa et al. (51) released Fas ligand (FasL) through agarose gel to induce the apoptosis of cytotoxic CD8⁺ T cells, thereby reducing immune rejection induced by allogeneic mesenchymal stem cells during transplantation.

In addition, CD4⁺ Tm cells have been shown to induce allograft rejection by activating cytotoxic CD8⁺ T cells and helping B cells produce allogeneic antibodies (118, 119). Moreover, some scholars have found (120) that activated CD4 + T cells secrete IL-2 and IFN-γ, thereby damaging the structure of the extracellular matrix, precipitating extracellular collagen, promoting the proliferation of fibroblasts, and ultimately leading to immune rejection. It has also been suggested (121) that CD4⁺ helper T cells and monocytes can be recruited into the neointima and secrete IL-1, IL-6, and TNF-α, which enables smooth muscle cells to migrate and proliferate in the elastic layer of the endometrium, ultimately leading to graft-versus-host reaction (GVHR).

Recent advances have shown that solid organ transplantation can induce mixed hematopoietic chimerism in allogeneic MSC transplantation through donor hematopoietic stem cell transplantation (122). Moreover, the immunization isolation membrane device is used to temporarily prevent rejection by wrapping differentiated allogeneic cells in collagen or gelatin gels. Zanotti et al. have shown that subcutaneous injection of bone marrow MSCs wrapped in alginate can significantly improve their survival in GVHD mice (123).

In addition, in order to overcome the immune rejection caused by transplantation of allogeneic mesenchymal stem cells into the body and prolong the survival time of allogeneic mesenchymal stem cells, it is also necessary to ensure their persistence through host and MSC modification. For example, mesenchymal stem cells can be pre-treated or loaded with biological agents to increase the expression of surface receptors or the production of immunosuppressive cytokines (124).

In addition to improving immune tolerance, we can improve the efficacy of allogeneic MSCs by extending their survival time in vivo. On the one hand, the immune tolerance of allogeneic MSCs can be improved by treatment with exosomes or secreting nutritional and immunoregulatory cytokines to mediate hit-and-run mechanisms (125). However, it has also been proposed that the immune system can be reprogrammed through apoptotic bodies, thereby prolonging the duration of allogeneic MSCs after injection.

Induction therapy can be divided into T cell consumption and T cell non-consumption strategies. The former involves anti-thymocyte globulin (ATG), anti-CD3 antibody (OKT3), and alemtuzumab, which reduce the release of the proinflammatory cytokines TNF-α and IFN-γ by consuming T cells. While anti-IL-2 antibodies such as daclizumab, brazillicoxib, and anti-CD20 antibodies such as rituximab and anti-cytotoxin are considered non-depleting agents for T cells (126). IL-2, IL-6, and IL-1 are autophagy-related cytokines (63), and anti-IL-2 antibodies can reduce immune rejection by reducing autophagy induction.