Understanding the Immunological Mechanisms of Mesenchymal Stem Cells in Allogeneic Transplantation: From the Aspect of Major Histocompatibility Complex Class I

Abstract

Mesenchymal stem cell (MSC) transplantation therapy appears to be an ideal strategy for repairing structural defects and restoring the functions of diseased tissues and organs. Additionally, MSCs are also used as immunosuppressants in allogeneic organ transplantation. However, owing to their inherent immunogenicity, MSC transplantation can induce the activation of an immune response, which can lead to the death and clearance of the transplanted MSCs. Major histocompatibility complex (MHC) molecules are responsible for antigen presentation, help T lymphocytes to recognize endogenous/extrinsic antigens, and trigger immune activation. Many studies have shown that MHC molecules (particularly class I) play key roles in the immunogenicity of MSCs. This review, therefore, focuses on the relationship between MHC-I surface expression on MSCs and its immunogenicity, as well as potential strategies to overcome the hurdle of MHC incompatibility.

Introduction

Mesenchymal stem cells (MSCs) have long been a preferred cell type for clinical transplantation, due to their lack of ethical controversy and various favorable biological characteristics, such as extensive proliferative capacity, multilineage differentiation potential, and immunomodulatory properties. As Mehdi et al. reviewed recently [1], MSCs are tissue precursor cells with immunoregulatory capacity rather than bona fide immune cells. Their immune properties are environment sensitive and variable among different species, and their immunoregulatory effect has to be activated by inflammatory cytokines. These special immune properties profoundly affect their therapeutic effectiveness. Therefore, it is imperative that MSCs should be a well-defined cell population in terms of molecular marker expression, biological functions, and properties, including powerful immunomodulatory effects; which are future challenges to be overcome, to ensure the efficacy and safety of MSC-based therapy.

In the field of MSC transplantation, there are autogenic MSC transplantation, allogeneic MSC transplantation, and xenogenic MSC transplantation. Autogenic MSC transplantation has several obvious limitations, including donor-site morbidity resulting in difficulty in harvesting autologous MSCs, and also affecting the selected cells' optimal stem cell-like qualities. Meanwhile, expanding these scarce cells to a sufficient number for therapy is time consuming. Xenogenic MSC transplantation is often unsuccessful due to irreconcilable interspecies differences. Consequently, allogeneic MSCs (allo-MSCs) appear to be one of the most promising candidates for regenerative medicine applications.

Immunological rejection, however, still poses a formidable barrier to allogeneic MSC transplantation [1,2]. Transplant rejection is mainly caused by the mismatch of major histocompatibility complex molecules (MHC, which have two classes: class I and class II). In addition, MHC-I has been considered as one of the most essential molecules for recognition of self-cells, abnormal self-cells, and non-self-cells and activation of the acquired immune system to eliminate the abnormal self-cells, and non-self-cells. MSCs from allogeneic donors bearing allo-MHC-I molecules that differ from those of the recipients, even by as little as one amino acid, will be rejected through either cellular or humoral immune mechanisms [3,4]. Therefore, it is important to review the immunogenicity of MSCs and the crosstalk between MSCs and their MHC-I expression, which may provide some cues for minimizing the potential of immune rejection in allogeneic MSC transplantation.

Current Understanding About the Immunogenicity of MSCs

Most studies show that MSCs expressed low levels of MHC-I, and did not express MHC-II and costimulatory molecules, like CD80 or CD86 in vitro [5]. Kotobuki et al. found that when they cocultured MSCs of ACI or LEW rats with lymphocytes of F344 rat separately, the number of F344 lymphocytes did not increase substantially, unlike stimulation with ACI or LEW lymphocytes. Additionally, the implanted allogeneic MSCs also had the capacity for biocompatibility and differentiation. Le Blanc et al. reported that there was no proliferative response being observed when allogeneic human MSCs were cocultured with peripheral blood lymphocytes [6]. Niemeyer et al. show that undifferentiated human bone marrow derived mesenchymal stem cells (BMSCs) and adipose-derived stem cells (ADSCs) can survive in immunocompetent mice [7]. Other studies have shown that MSC transplantation does not evoke strong immune responses and that MSCs play a positive role in the treatment of many diseases such as GvHD [1], ischemic stroke [8], tuberculosis [9], osteoarthritis [10], kidney disease [11], and hyperglycemia [12]. Therefore, these data suggest that MSCs are immunoprivileged and that MSCs can be transplanted between HLA-incompatible individuals.

Nevertheless, there are also contradictory results. Ishikane et al. reported that allogeneic rat MSCs can exert a therapeutic effect on angiogenesis, but the transplanted MSCs were observed to decrease within 3 weeks after injection, based on the histological analysis [13]. Toma et al. tracked intra-arterially delivered rat MSCs through intravital microscopy, and found that the surviving MSCs decreased to 14% of the initial number of transplanted cell at 3 days postdelivery [14]. Furthermore, the autopsy of patients who had received allogeneic MSC infusion within a year revealed that allogeneic MSCs trended toward cell death after transplantation [15]. Isakova et al. carried out a study on rhesus macaques, which proved that allogeneic MSCs could induce stronger allorecognition responses, including the expansion of NK, B, and T cell subpopulations in peripheral blood and detectable levels of allospecific antibodies, with a higher degree of MHC-I and MHC-II mismatch between the donor MSCs and recipients [16].

Recently, both Ankrum et al. and Berglund et al. reviewed the immunogenic characteristics of MSCs and raised the point that MSCs are not immunoprivileged, but immune evasive [17,18]. It has also been suggested that the immune properties of MSCs are binary. They could shift to either a proinflammatory phenotype in a mild inflammatory microenvironment or an anti-inflammtory phenotype in a strong inflammatory milieu [19]. This may depend on the sensitivity of toll-like receptors or some other yet undefined signaling molecules. This milieu sensitivity may cause unstable immunogenicity of MSCs. As the MHC, particularly MHC-I, plays key roles of modulating the immunogenicity of MSCs, we therefore focus on this molecule in our review.

The Synthesis and Expression Pathways of MHC-I

MHC-I is an endogenous antigen molecule that is either expressed on the nucleated cell surface or secreted into the serum. In this study, we illustrated the pathway of classical MHC-I expression and localization in homo sapiens. The MHC-I molecule is also called HLA-I, which is a heterodimeric transmembrane protein that contains the polymorphic α1 and α2 domains (both domains contribute to combining the antigen peptide), the monomorphic transmembrane α3 domain, and beta-2 microglobulin. The classical class I transmembrane proteins (HLA-I heavy chain) are encoded by three locis: HLA-A, HLA-B, and HLA-C. After the transcription and translation of HLA class I heavy chain, the polypeptide will be recognized, sheared, and modified by the function of the translocon-associated proteins, including Calnexin and Bip on the endoplasmic reticulum (ER) [20]. Subsequently, chaperones assist in the folding and assembly of the HLA-I heavy chain and light chain (β2m) into heterodimers. Then the heterodimers, along with tapasin/TAPBPR, ERp57, TAP, etc. form a transient multisubunit membrane complex called peptide-loading complex, which coordinates the loading of peptides to MHC-I molecules [21].

Except for the formation of MHC-I molecules per se, the generation of antigen peptides and the combination of peptides and MHC-I are multistep processes. They involve several molecules that constitute the antigen processing and presenting machinery. The antigen peptides are originally degraded from cytosolic proteins through the ubiquitin/proteasome pathway, which is highly dependent on the degradation pathway in the cytoplasma [21,22]. Hence, the peptides can be transported into ER by transporters associated with antigen processing (TAP: which is a heterodimer of two subunits—TAP1 and TAP2), the proper length and sequence of antigen peptides modified by peptidase can be bound by a binding pocket constituted of α1and α2 subunits of class I molecules [23]. The MHC-I molecules are then presented onto the cell surface through the ER and Golgi bodies, either as fully conformed trimolecular complexes or as various open forms (being devoid of the peptide and/or β2m). Mahmutefendić et al. studied the role of open conformers, showing that most of the internalized open-form MHC-I go directly into the acidic late endosomal compartments, although only a small part of these are degraded [24].

In addition, the MHC-I can appear as soluble forms (sMHC-I), which are derived by proteolysis of full-length MHC-I. Devito-Haynes et al. utilized a specific membrane-bound metalloprotease (MPase) inhibitor BB-94 to not only block the release of soluble β2m -free MHC, but also the appearance of sMHC/β2m within the cell supernatants, thus confirming that MPase participates in the process of sMHC-I generation [25]. It is also reported that the β2m-free class I heavy chains may be partially responsible for controlling the expression level of surface MHC-I on activated cells. The surface and soluble MHC-I play different roles in immune pathways, and we will discuss their functions separately.

The Activating Role of Surface-Bound MHC-I in Immune System

MHC-I is one of the major subgroups of MHC encoded by the MHC gene family, which is not only polygenic (containing several different MHC-I loci), but also polymorphic (each gene having multiple variants). MHC-I molecules are expressed by all nucleated cells and present cytosolic and nuclear protein antigens to the cell surface, presenting peptides to the immune system. The virus-infected host cells are hijacked to translate viral proteins and replicate the viral genome, so the MHC-I will present viral peptide antigens on the cell surface to alert the immune system to detect a virally infected cell. However, many viruses have evolved proteins that interfere with antigen presentation by MHC-I, either through impairing MHC-I presentation, such as in the case of cytomegalovirus [26], herpes simplex virus [27], and porcine respiratory reproductive syndrome virus [28]; or inducing degradation or mislocalization of MHC-I molecules, such as in the case of Kaposi's sarcoma-associated herpesvirus [29] and HIV-1 [30]. If the cells were infected with the virus, they would express abnormal MHC-I molecules, but the surface expression level may be hijacked to a low level. Meanwhile, the donor cells which express non-self MHC-I molecules may also be detected by the host immune system and subsequently be elimitated [31]. Garza-Rodea and colleagues modified the human MSCs with Herpesvirus to downregulate MHC-I surface expression, which successfully evaded the immune response [32]. However, modification of MSCs with viral infection may also raise safety and efficacy concerns.

On the other hand, although MSCs are known to express a low level of surface MHC-I in vitro, it is difficult to ensure they maintain a similar expression level after transplantation. It is likely that the inflammatory environment will increase the MHC-I expression [33,34]. More specifically, previous studies showed that the inflammatory cytokine IFN-γ can increase the MHC-I surface expression on MSCs. MHC-I functions mainly through the presentation of abnormal or non-self peptide antigens to CTLs [35], and to modulate the immunoregulatory activity exerted by NK cells [36]. In this review, we discussed the functions of allo-MHC-I on both CD8⁺ and NK cells.

CD8⁺ T cells, which are commonly known as cytotoxic T cells, are critical for recognizing the MHC-I/peptide complex. The complex can strengthen MHC-I-restricted T cell receptor (TCR) signaling by binding with the MHC-I molecules transferred on antigen-presenting cells (APCs). In a previous study, Hausmann et al. measured the percentages of allo-specific CD8⁺ T cells with antispecific MHC reactivity and showed that even a single amino acid substitution between MHC-I alleles can induce significant changes in CD8⁺T cell allo-responses [37]. These recognition responses convert the CD8⁺T cells into cytotoxic T lymphocytes (CTLs) that secrete cytokines, such as IFN-γ, Interleukin-2, and tumor necrosis factor (TNF), as well as some T helper cells (such as Th1,Th2,Th17) cytokines, to destroy their target cells through either the granule exocytosis pathway (being dominant in CD8⁺ CTL) or the Fas pathway (associated with the TNF family) [38,39].

Natural killer (NK) cells are frequently found in rejected allografts, which directly lyse target cells that have inappropriate self-MHC-I molecule expression. NK cells undergo a maturation step by recognizing the self-MHC molecules. This education process involves identifying self-MHC-I by cognate inhibitory receptors. In the context of allotransplantation, NK cells can respond to the allografts by recognizing “missing self” MHC-I molecules [40]. The killer cell activation receptor (KAR) family is a set of receptors that are responsible for activating the NK cells' reactive response. Meanwhile, the killer cell inhibitory receptor (KIR) family are responsible for inhibiting NK cells' KAR, causing an active response to target cells through interaction with compromised MHC-I. So, they confer upon NK cells the ability to discriminate between MHC-I-positive and -negative target cells.

The functions of NK cells are regulated by the balanced signals of both inhibitory and activating receptors on the cell surface. Activated NK cells can induce two completely different immune response after transplantation, either promoting allograft rejection or contribute to the tolerance of the allograft. There is a two-way crosstalk between the transplanted graft and NK cells. NK cells are potent effectors that mediate allograft rejection and provide the necessary cues to aid T cell-dependent immune responses [41]. On the other hand, accumulating evidence demonstrates that NK cells contribute to the establishment of allograft tolerance, by altering the proliferation of CD8⁺T cells [42]. Hence, NK cells influence and may regulate the adaptive immune response both in graft rejection and the induction of tolerance. An appropriate level of MHC-I surface expression on allogeneic MSCs is crucial for successful cell transplantation. The effects of surface MHC-I function on T cells and NK cells are summarized in Fig. 1.

FIG. 1.

The effects of membranal MHC-I function on T cells and NK cells. The MHC-I expressed on allogeneic donor's MSCs can be recognized by CD8⁺T cells both in direct way or in APC-dependent way. In addition, CD8⁺T cells will transform into CTL to lysis the allo-MSCs with perforin and granule exocytosis or induced the apotosis of allo-MSC in Fas pathway. Meanwhile, the CD8⁺T cells will differentiate to memory T cells, which will be more sensitive for the following allo-MSCs stimulation; the NK cell-expressed KIR can inhibit the NK cell lysis function through interaction with MHC-I molecules. MHC, major histocompatibility complex; MSC, mesenchymal stem cells; APC, antigen-presenting cells; CTL, cytotoxic T lymphocytes; KIR-L, killer inhibitory receptor legand; TCR, MHC-I-restricted T cell receptor; Fas-L, Fas ligand.

The Inhibitory Role of Soluble MHC-I in the Immune System

The level of sMHC-I in the serum of healthy individuals is naturally low, but a marked increase in the patients' sMHC-I level is associated with some pathological conditions. Furthermore, it is well known that the serum levels of sMHC-I will dramatically increase during the acute rejection phase of transplantation [43]. As summarized above, the surface-bound MHC-I stimulates immune responses, while the secreted sMHC-I may play an immunoregulatory role by functioning as an immune suppressor. Several studies have suggested that the sMHC-I exert an immunomodulatory effect through directly inhibiting T cell functions through either receptor blocking or inducing apoptosis of activated CD8⁺T cell [44,45]. Additionally, sMHC-I can induce the secretion of soluble Fas ligand (Fas-L) and trigger Fas⁺ T lymphocytes to undergo apoptosis [46]. sMHC-I initiates an apoptotic signal to these cells through sMHC-I/CD8 interactions and FasL/Fas interaction. In the aspect of NK cells, sMHC-I can downregulate the killing effect of NK cells by blocking its recognition of target cells, and different NK clones are discriminated according to different sMHC-I specificities [47]. Additionally, sMHC-I can decrease the NK cell-mediated innate response by increasing intracellular transforming growth factor (TGF)-β1 expression in CD8⁺T lymphocytes and neutrophils, and then stimulating TGF-β1 release (Fig. 2). So the presence of sMHC-I seems beneficial to the immune tolerance process.

FIG. 2.

Soluble MHC-I modulates the transplanted rejection. sMHC-I triggers apoptosis of CD8⁺T cells either through interaction with their TCR directly or triggering Fas/Fas-L pathway. sMHC-I can downregulate the killing effect of NK cells by blocking its KIR and increasing TGF-β1 expression in both CD8⁺ T lymphocytes and neutrophils to decrease the NK cell-mediated innate response. TGF, transforming growth factor.

The MHC-I Expression on MSCs

As early as 2013, it has already been reported that there was poor correlation between in vitro and in vivo studies of porcine bone marrow MSCs [48], whereas the umbilical cord tissue-derived MSCs from pigs and horses would not elicit any immune responses after being administered in vivo multiple times, which paralleled the in vitro findings. These suggest that the in vivo immune activities of MSCs may vary because of different tissue and species sources. However, except for porcine bone marrow MSCs, consistent immunomodulatory effect in vitro could be observed in vivo among different sources of horse MSCs, including umbilical cord blood, umbilical cord tissue, bone marrow, and adipose tissue. These would imply that the tissue sources of domesticated species may not be a key factor in causing the differences of immune properties in vitro and in vivo. As for humans, the MHC expression on human MSCs can be influenced by many factors, including different donors, tissue origins, medical treatments, and passage numbers. These information have been summarized in Table 1.

Table 1.

Comparison of the Major Histocompatibility Complex Expression on Different Human Mesenchymal Stem Cells

Comparison of the MHC expression on different human MSCs		Reference
Comparison	Results	Reference
Origins	Lee et al. found that higher expression levels of both HLA-ABC and HLA-G were detected in Chorionic Plate-derived MSCs (96.7%) compared with Bone Marrow-derived MSCs (87.2%) and adipose-derived MSCs (86.4%).	[49]
Origins	Sieps et al. reported that human neonatal thymus-derived MSCs lack the expression of MHC-I and MHC-II.	[50]
Passages	Mclntosh et al. reported that after a progressive passage and expansion of adherent adipose MSCs, the MHC-I expression was upregulated, whereas the MHC-II is downregulated.	[51]
Cell niche	Lange et al. compared MSCs expanded in platelet lysate-supplemented medium and normal serum-containing medium. The results indicated that the MSCs expanded in serum-free medium would have lower MHC-I as well as MHC-II expression.	[52]
Cell niche	Chan et al. demonstrated that the surface expression profile of MHC-I and MHC-II on MSCs was markedly upregulated by IFN-γ.	[53]
	Lim et al. isolated MSCs from three anatomical segments of the umbilical cord (maternal, middle, and fetal). The flow cytometry results indicated that the maternal and fetal segment expresses significantly lower levels of MHC-I.	[54]
Differentiation States	Liu et al. found that the expression of MHC-I and MHC-II on MSCs were increased from 34% to 65% and 0% to 12%, respectively, after neuronal differentiation for 2 weeks.	[55]
Differentiation States	Technau et al. reported that undifferentiated ADSC was negative for MHC-I and MHC-II expression by immunofluorescence, however, both MHC-I and MHC-II expressions appeared after chondrogenic differentiation.	[56]
Donor age	Stolzing et al. reported that the percentage of MHC-II expression would decrease on MSCs from Young donors (11.7%) versus Aged donors (2.4%)	[57]

MHC, major histocompatibility complex; MSC, mesenchymal stem cell.

Our study showed that human embryonic stem cell-derived MSCs treated with low doses of IFN-γ would markedly upregulate MHC-I expression on their surface after 1 day, while the MSCs can autodownregulate their surface MHC-I expression to maintain low immunogenicity on subsequent following days. The higher the MHC-I surface expression on MSCs, the stronger were the allogenic immune responses being induced both in vitro and in vivo [58,59]. Therefore, MHC-I expression level is an important parameter in determining the compatibility of donors for organ or cell transplant, where any mismatch will significantly increase responder T cell proliferation. Meanwhile, Kayleign et al. also found that human bone marrow-derived MSCs treated with high doses of IFN-γ could upregulate MHC-II expression and antigenic peptide presentation. However, these MSCs did not induce cellular alloreactivity in vitro [60]. These two studies indicated that the inflammatory environment, particularly the presence of IFN-γ, will affect MSCs' immunogenicity, while the converse may have a different effect. Based on our research [58], one mechanism of adapting MSCs to the niche is through accelerating the endocytosis of MHC-I. Whereas the mechanism involved in the study of Kayleign is the deficiency of CD80 expression on IFN-γ-activated MSCs. The efficacy of MSCs' transplantation may be enhanced through a better understanding of their immune molecular profile and immunoregulatory mechanisms.

It can be concluded that MSCs have been recognized to be immunoprivileged, but this characteristic, especially the expression level of MHC-I, may differ according to the MSC differentiation status. It is better to set formal criteria to evaluate the MSCs' immunogenicity before allotransplantation.

The Relationship Between the Failure of Allogeneic Transplantation and the Donor's MHC-I Expression

As discussed above, the immunogenicity of MSCs may vary according to the cell status and the cell niche. So the failure of allo-MSCs' transplantation may be caused by a phenotypic change on MSCs after injection, particularly the changed surface MHC-I expression level.

Nauta et al. have shown that infusion of syngeneic MSCs can promote long-term chimerism, whereas allo-MSC hampers the engraftment of bone marrow [61]. Furthermore, allo-MSCs may trigger an allo-response particularly through the T cell response, including a memory T cell response [62]. Similar results were discovered in swine- [63] and nonhuman primate [16]-derived MSCs. These results thus suggest that in vivo characteristics of allogeneic MSCs might differ from that in vitro. Many studies reported that allo-MSCs are immunogenic due to their inherent expression of MHC molecules [64]. Our previous study demonstrated that after at 48 h after murine MSC implantation, the MHC-I displayed positive staining for the MSC graft [59]. Additionally, the MSCs decreased TGF-β secretion and increased MHC-I expression after IFN-γ treatment. The immunoprivileged and immunomodulatory properties of MSCs were lost following transplantation [65].

It has been reported that using adenoviral gene transfer to decrease the surface MHC-I expression of human endothelial cells can protect these cells from CTL-mediated lysis [66]. Pollak and Blanchard discovered that previous methods of pretreating donor or graft with monoclonal antibodies that directed against graft APCs, adhesion, or costimulation molecules have little efficacy in promoting acceptance of cardiac allograft. However, the hearts from MHC-I and MHC-II-deficient donors showed significantly longer survival than the control, which suggested a novel method to modify the immunogenicity of the graft [67]. Because CD8⁺T cells are key effective components in executing the alloimmune response, NK cells contribute to both graft rejection and tolerance to allografts [68]. MHC-I can induce not only CD8⁺T cells but also NK cell reaction, so MHC-I is one of the most important factors in allorejection. Hence, it would be meaningful to focus on controlling the MHC-I expression to avoid transplant rejection [69].

Current Contributions to Avoid Transplant Rejection

Transplantation is the last hope for patients with severe organ degeneration or tissue injury. However, because of antigenic disparities between donors and recipients, it is still a challenge to overcome allograft rejection. In clinical practice and experimental models, immunosuppressants such as Cyclosporine A (CsA), Rapamune (RAPA), and Mycophenolate Mofetil are frequently used to reduce the host's antiallogeneic grafts' immune response by regulating alloimmunity and early adverse outcomes. The continuing search for more selective and specific agents has become one of the priorities for transplant medicine in the past decade. Some of these compounds are now entering routine clinical practice, and other new pharmacological strategies and innovative approaches to transplantation are also under development. Huang et al. discovered a T cell-specific inhibitor, SP100030, which can suppress T cell division and cytokine synthesis, and this compound has been shown to prolong cardiac allograft survival from 10 to 25 days [70]. Kawabe and Ochi found a reduction of the Vβ8⁺CD4⁺ T cells following the injection of superantigen staphylococcal enterotoxin B into BaiB/c mice due to in vivo-triggered apoptosis [71]. This finding suggested the possibility of preventing immune rejection through impairing the function of T cells.

Meanwhile, as one of the main inducers of T cell activation, it is also meaningful to consider MHC-I expression during transplantation. Li et al. established an allogeneic MSC line with B2M downregulation through CRISPR technology, which could inhibit immune rejection [72]. Similarly, Huang et al. suggested that CIITA-MSCs, which is deficient in MHC-II expression, could diminish immune rejection and improve survival of allo-MSCs [73]. In addition, Cooper and colleagues developed designer zinc finger nucleases to eliminate HLA-A expression in ESCs, and the established cell line lacked MHC-I expression even under IFN-γ and TNF-α treatment [74]. These findings thus established the therapeutic utility of genetically edited allogeneic stem cells that may evade immune rejection, although they were previously derived from donors with disparate HLA expression. Taken together, application of these methods will offer potential therapeutic intervention but still needs further study and validation.

Future Perspectives

As we have concluded before, although MSCs have been reported to exhibit low immunogenicity and immunomodulatory properties both in vitro and in vivo [75], many studies have demonstrated that allogeneic MSCs may also induce immunological rejection, owing to their MHC barrier [61,63,68]. To avoid immune rejection, the MHC-I expression on MSCs is an important factor to be carefully considered.

It is known that the TGF-β2 can downregulate the expression of MHC-I and MHC-II by MSCs without changing other phenotype surface markers [76]. This gave the idea that implanting the MSCs together with TGF-β2 or similar molecules may be helpful in preventing immune rejection of allogeneic MSCs. Another possibility is that IFN-γ will upregulate the MHC expression on MSCs [53], while increasing the secretion of immunomodulatory soluble molecules; so it may be helpful to separate allogeneic MSC grafts from contact with immune cells to maximize and extend their immunomodulatory effects. Hence, creating an immunosuppressive environment for transplantation can reduce the potential of immune rejection. It is important to evaluate the cell niche during allogeneic MSC transplantation.

Previous studies that applied the gene-editing technique to modify the MHC expression of MSCs showed that it can reduce immune rejection. However, further validation studies on both the safety and efficacy of transplantation of these genetically modified cells need to be rigorously performed, to obtain a highly immunocompatible “off-the-shelf” allogeneic MSCs. Previous studies on MHC-I expression by MSCs were mainly based on in vitro evaluation. It is known that the MHC-I expression by human MSCs will be affected by many factors, and is expected to vary between MSCs of different passages. However, the in vivo niche is more complex. So we should utilize new techniques and put in more effort to evaluate the MHC-I expression of MSCs after implantation, particularly in vivo. In conclusion, owing to the importance of MHC-I in allogeneic MSCs' rejection, we need to evaluate and also monitor the MHC-I expression of allogeneic MSCs to ensure the safety and efficacy of allogeneic MSC treatment.

Footnotes

Acknowledgments

The authors would like to thank the staff in Core Facilities, School of Medicine, Zhejiang University. This work was supported by the National Key Research and Development Program of China (2017YFA0104902,2018YFC1105104), National Natural Science Foundation of China grants (31200739, 31470948), as well as sponsored by the China Scholarship Council and Fundamental Research Funds for the Central Universities.

Author Disclosure Statement

The authors declare that they have no competing financial interests.

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