Bone marrow-derived mesenchymal stem cells inhibit acute rejection of rat liver allografts in association with regulatory T-cell expansion

Cellular strategies to induce immune tolerance after liver transplantation: Clinical perspectives

Liver transplantation (LT) has become the most efficient treatment for pediatric and adult end-stage liver disease and the survival time after transplantation is becoming longer due to the development of surgical techniques and perioperative management. However, long-term side-effects of immunosuppressants, like infection, metabolic disorders and malignant tumor are gaining more attention. Immune tolerance is the status in which LT recipients no longer need to take any immunosuppressants, but the liver function and intrahepatic histology maintain normal. The approaches to achieve immune tolerance after transplantation include spontaneous, operational and induced tolerance. The first two means require no specific intervention but withdrawing immunosuppressant gradually during follow-up. No clinical factors or biomarkers so far could accurately predict who are suitable for immunosuppressant withdraw after transplantation. With the understanding to the underlying mechanisms of immune tolerance, many strategies have been developed to induce tolerance in LT recipients. Cellular strategy is one of the most promising methods for immune tolerance induction, including chimerism induced by hematopoietic stem cells and adoptive transfer of regulatory immune cells. The safety and efficacy of various cell products have been evaluated by prospective preclinical and clinical trials, while obstacles still exist before translating into clinical practice. Here, we will summarize the latest perspectives and concerns on the clinical application of cellular strategies in LT recipients.

Stem Cell-Derived Extracellular Vesicles for Cancer Therapy and Tissue Engineering Applications

Recently, stem cells and their secretomes have attracted great attention in biomedical applications, particularly extracellular vesicles (EVs). EVs are secretomes of cells for cell-to-cell communication. They play a role as intercellular messengers as they carry proteins, nucleic acids, lipids, and therapeutic agents. They have also been utilized as drug-delivery vehicles due to their biocompatibility, low immunogenicity, stability, targetability, and engineerable properties. The therapeutic potential of EVs can be further enhanced by surface engineering and modification using functional molecules such as aptamers, peptides, and antibodies. As a consequence, EVs hold great promise as effective delivery vehicles for enhancing treatment efficacy while avoiding side effects. Among various cell types that secrete EVs, stem cells are ideal sources of EVs because stem cells have unique properties such as self-renewal and regenerative potential for transplantation into damaged tissues that can facilitate their regeneration. However, challenges such as immune rejection and ethical considerations remain significant hurdles. Stem cell-derived EVs have been extensively explored as a cell-free approach that bypasses many challenges associated with cell-based therapy in cancer therapy and tissue regeneration. In this review, we summarize and discuss the current knowledge of various types of stem cells as a source of EVs, their engineering, and applications of EVs, focusing on cancer therapy and tissue engineering.

Immunomodulatory Role of Mesenchymal Stem Cells in Liver Transplantation: Status and Prospects

BACKGROUND

Liver transplantation (LT) is the only effective therapy for end-stage liver diseases, but some patients usually present with serious infection and immune rejection. Those with immune rejection require long-term administration of immunosuppressants, leading to serious adverse effects. Mesenchymal stem cells (MSCs) have various advantages in immune regulation and are promising drugs most likely to replace immunosuppressants.

SUMMARY

This study summarized the application of MSCs monotherapy, its combination with immunosuppressants, MSCs genetic modification, and MSCs derivative therapy (cell-free therapy) in LT. This may deepen the understanding of immunomodulatory role of MSCs and promote the application of MSCs in immune rejection treatment after LT.

KEY MESSAGES

MSCs could attenuate ischemia-reperfusion injury and immune rejection. There is no consensus on the effects of types and concentrations of immunosuppressants on MSCs. Although genetically modified MSCs have contributed to better outcomes to some extent, the best modification is still unclear. Besides, multiple clinical complications developed frequently after LT. Unfortunately, there are still few studies on the polygenic modification of MSCs for the simultaneous treatment of these complications. Therefore, more studies should be performed to investigate the potency of multi-gene modified MSCs in treating complications after LT. Additionally, MSC derivatives mainly include exosomes, extracellular vesicles, and conditioned medium. Despite therapeutic effects, these three therapies still have some limitations such as heterogeneity between generations and that they cannot be quantified accurately.

Timing of Mesenchymal Stromal Cell Therapy Defines its Immunosuppressive Effects in a Rat Lung Transplantation Model

Cell therapy using mesenchymal stromal cells (MSCs) is being studied for its immunosuppressive effects. In organ transplantation, the amount of MSCs that accumulate in transplanted organs and other organs may differ depending on administration timing, which may impact their immunosuppressive effects. In vitro, adipose-derived mesenchymal stem cells (ADMSCs) suppress lymphocyte activation under cell-to-cell contact conditions. However, in vivo, it is controversial whether ADMSCs are more effective in accumulating in transplanted organs or in secondary lymphoid organs. Herein, we aimed to investigate whether the timing of ADMSC administration affects its immunosuppression ability in a rat lung transplantation model. In the transplantation study, rats were intramuscularly administered half the usual dose of tacrolimus (0.5 mg/kg) every 24 h after lung transplantation. ADMSCs (1 × 106) were administered via the jugular vein before (PreTx) or after (PostTx) transplantation. Cell tracking using quantum dots was performed. ADMSCs accumulated predominantly in the lung and liver; fewer ADMSCs were distributed in the grafted lung in the PreTx group than in the PostTx group. The rejection rate was remarkably low in the ADMSC-administered groups, particularly in the PostTx group. Serum tumor necrosis factor-α (TNF-α), interferon-γ, and interleukin (IL)-6 levels showed a greater tendency to decrease in the PreTx group than in the PostTx group. The proportion of regulatory T cells in the grafted lung 10 days after transplantation was higher in the PostTx group than in the PreTx group. PostTx administration suppresses rejection better than PreTx administration, possibly due to regulatory T cell induction by ADMSCs accumulated in the transplanted lungs, suggesting a mechanism different from that in heart or kidney transplantation that PreTx administration is more effective than PostTx administration. These results could help establish cell therapy using MSCs in lung transplantation.

MSCs can be a double-edged sword in tumorigenesis

Mesenchymal stem cells (MSCs) have been used to treat various diseases including Alzheimer's disease and cancer. In particular, the immunomodulatory function of MSCs plays a major role in cancer therapy using stem cells. However, MSCs exert promotive and inhibitory effects on cancer. The immunomodulatory effects of MSCs in the tumor microenvironment (TME) are ambiguous, which is the primary reason for the different outcomes of MSCs therapies for tumors. This review discusses the use of MSCs in cancer immunotherapy and their immunomodulatory mechanisms in cancers.

Normothermic liver machine perfusion as a dynamic platform for regenerative purposes: What does the future have in store for us?

Liver transplantation has become an immense success; nevertheless, far more recipients are registered on waiting lists than there are available donor livers for transplantation. High-risk, extended criteria donor livers are increasingly used to reduce the discrepancy between organ demand and supply. Especially for high-risk livers, dynamic preservation using machine perfusion can decrease post-transplantation complications and may increase donor liver utilisation by improving graft quality and enabling viability testing before transplantation. To further increase the availability of donor livers suitable for transplantation, new strategies are required that make it possible to use organs that are initially too damaged to be transplanted. With the current progress in experimental liver transplantation research, (long-term) normothermic machine perfusion may be used in the future as a dynamic platform for regenerative medicine approaches, enabling repair and regeneration of injured donor livers. Currently explored therapeutics such as defatting cocktails, RNA interference, senolytics, and stem cell therapy may assist in the repair and/or regeneration of injured livers before transplantation. This review will provide a forecast of the future utility of normothermic machine perfusion in decreasing the imbalance between donor liver demand and supply by enabling the repair and regeneration of damaged donor livers.

Myeloid and Mesenchymal Stem Cell Therapies for Solid Organ Transplant Tolerance

Transplantation is now performed globally as a routine procedure. However, the increased demand for donor organs and consequent expansion of donor criteria has created an imperative to maximize the quality of these gains. The goal is to balance preservation of allograft function against patient quality-of-life, despite exposure to long-term immunosuppression. Elimination of immunosuppressive therapy to avoid drug toxicity, with concurrent acceptance of the allograft-so-called operational tolerance-has proven elusive. The lack of recent advances in immunomodulatory drug development, together with advances in immunotherapy in oncology, has prompted interest in cell-based therapies to control the alloimmune response. Extensive experimental work in animals has characterized regulatory immune cell populations that can induce and maintain tolerance, demonstrating that their adoptive transfer can promote donor-specific tolerance. An extension of this large body of work has resulted in protocols for manufacture, as well as early-phase safety and feasibility trials for many regulatory cell types. Despite the excitement generated by early clinical trials in autoimmune diseases and organ transplantation, there is as yet no clinically validated, approved regulatory cell therapy for transplantation. In this review, we summarize recent advances in this field, with a focus on myeloid and mesenchymal cell therapies, including current understanding of the mechanisms of action of regulatory immune cells, and clinical trials in organ transplantation using these cells as therapeutics.

Immune response to allogeneic equine mesenchymal stromal cells

Background

Mesenchymal stromal cells (MSCs) are believed to be hypoimmunogeneic with potential use for allogeneic administration.

Methods

Bone marrow was harvested from Connemara (n = 1), Standardbred (n = 6), and Thoroughbred (n = 3) horses. MSCs were grouped by their level of expression of major histocompatibility factor II (MHC II). MSCs were then sub-grouped by those MSCs derived from universal blood donor horses. MSCs were isolated and cultured using media containing fetal bovine serum until adequate numbers were acquired. The MSCs were cultured in xenogen-free media for 48 h prior to use and during all assays. Autologous and allogeneic MSCs were then directly co-cultured with responder leukocytes from the Connemara horse in varying concentrations of MSCs to leukocytes (1:1, 1:10, and 1:100). MSCs were also cultured with complement present and heat-inactivated complement to determine whether complement alone would decrease MSC viability. MSCs underwent haplotyping of their equine leukocyte antigen (ELA) to determine whether the MHC factors were matched or mismatched between the donor MSCs and the responder leukocytes.

Results

All allogeneic MSCs were found to be ELA mismatched with the responder leukocytes. MHC II-low and universal blood donor MSCs caused no peripheral blood mononuclear cell (PBMC) proliferation, no increase in B cells, and no activation of CD8 lymphocytes. Universal blood donor MSCs stimulated a significant increase in the number of T regulatory cells. Neutrophil interaction with MSCs showed that universal blood donor and MHC II-high allogeneic MSCs at the 6 h time point in co-culture caused greater neutrophil activation than the other co-culture groups. Complement-mediated cytotoxicity did not consistently cause MSC death in cultures with active complement as compared to those with inactivated complement. Gene expression assays revealed that the universal blood donor group and the MHC II-low MSCs were more metabolically active both in the anabolic and catabolic gene categories when cultured with allogeneic lymphocytes as compared to the other co-cultures. These upregulated genes included CD59, FGF-2, HGF, IDO, IL-10, IL-RA, IL-2, SOX2, TGF-β1, ADAMSTS-4, ADAMSTS-5, CCL2, CXCLB/IL-8, IFNγ, IL-1β, and TNFα.

Conclusions

MHC II-low MSCs are the most appropriate type of allogeneic MSC to prevent activation of the innate and cell-mediated component of the adaptive immune systems and have increased gene expression as compared to other allogeneic MSCs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02624-y.

Cellular Therapies in Solid Organ Allotransplantation: Promise and Pitfalls

Donor specific transfusions have been the basis of tolerance inducing protocols since Peter Medawar showed that it was experimentally feasible in the 1950s. Though trials of cellular therapies have become increasingly common in solid organ transplantation, they have not become standard practice. Additionally, whereas some protocols have focused on cellular therapies as a method for donor antigen delivery—thought to promote tolerance in and of itself in the correct immunologic context—other approaches have alternatively focused on the intrinsic immunosuppressive properties of the certain cell types with less emphasis on their origin, including mesenchymal stem cells, regulatory T cells, and regulatory dendritic cells. Regardless of intent, all cellular therapies must contend with the potential that introducing donor antigen in a new context will lead to sensitization. In this review, we focus on the variety of cellular therapies that have been applied in human trials and non-human primate models, describe their efficacy, highlight data regarding their potential for sensitization, and discuss opportunities for cellular therapies within our current understanding of the immune landscape.

Third-party bone marrow-derived mesenchymal stromal cell infusion before liver transplantation: A randomized controlled trial

Mesenchymal stromal cells (MSC) have emerged as a promising therapy to minimize the immunosuppressive regimen or induce tolerance in solid organ transplantation. In this randomized open-label phase Ib/IIa clinical trial, 20 liver transplant patients were randomly allocated (1:1) to receive a single pretransplant intravenous infusion of third-party bone marrow-derived MSC or standard of care alone. The primary endpoint was the safety profile of MSC administration during the 1-year follow-up. In all, 19 patients completed the study, and none of those who received MSC experienced infusion-related complications. The incidence of serious and non-serious adverse events was similar in the two groups. Circulating Treg/memory Treg and tolerant NK subset of CD56^bright NK cells increased slightly over baseline, albeit not to a statistically significant extent, in MSC-treated patients but not in the control group. Graft function and survival, as well as histologic parameters and intragraft expression of tolerance-associated transcripts in 1-year protocol biopsies were similar in the two groups. In conclusion, pretransplant MSC infusion in liver transplant recipients was safe and induced mild positive changes in immunoregulatory T and NK cells in the peripheral blood. This study opens the way for a trial on possible tolerogenic efficacy of MSC in liver transplantation. ClinicalTrials.gov identifier: NCT02260375.

Cell therapy in transplantation: A comprehensive review of the current applications of cell therapy in transplant patients with the focus on Tregs, CAR Tregs, and Mesenchymal stem cells

Organ transplantation is a practical treatment for patients with end-stage organ failure. Despite the advances in short-term graft survival, long-term graft survival remains the main challenge considering the increased mortality and morbidity associated with chronic rejection and the toxicity of immunosuppressive drugs. Since a novel therapeutic strategy to induce allograft tolerance seems urgent, focusing on developing novel and safe approaches to prolong graft survival is one of the main goals of transplant investigators. Researchers in the field of organ transplantation are interested in suppressing or optimizing the immune responses by focusing on immune cells including mesenchymal stem cells (MSCs), polyclonal regulatory Tcells (Tregs), and antigen-specific Tregs engineered with chimeric antigen receptors (CAR Tregs). We review the mechanistic pathways, phenotypic and functional characteristics of these cells, and their promising application in organ transplantation.

A Large-Scale Bank of Organ Donor Bone Marrow and Matched Mesenchymal Stem Cells for Promoting Immunomodulation and Transplant Tolerance

Induction of immune tolerance for solid organ and vascular composite allografts is the Holy Grail for transplantation medicine. This would obviate the need for life-long immunosuppression which is associated with serious adverse outcomes, such as infections, cancers, and renal failure. Currently the most promising means of tolerance induction is through establishing a mixed chimeric state by transplantation of donor hematopoietic stem cells; however, with the exception of living donor renal transplantation, the mixed chimerism approach has not achieved durable immune tolerance on a large scale in preclinical or clinical trials with other solid organs or vascular composite allotransplants (VCA). Ossium Health has established a bank of cryopreserved bone marrow (BM), termed "hematopoietic progenitor cell (HPC), Marrow," recovered from deceased organ donor vertebral bodies. This new source for hematopoietic cell transplant will be a valuable resource for treating hematological malignancies as well as for inducing transplant tolerance. In addition, we have discovered and developed a large source of mesenchymal stem (stromal) cells (MSC) tightly associated with the vertebral body bone fragment byproduct of the HPC, Marrow recovery process. Thus, these vertebral bone adherent MSC (vBA-MSC) are matched to the banked BM obtained from each donor, as opposed to third-party MSC, which enhances safety and potentially efficacy. Isolation and characterization of vBA-MSC from over 30 donors has demonstrated that the cells are no different than traditional BM-MSC; however, their abundance is >1,000-fold higher than obtainable from living donor BM aspirates. Based on our own unpublished data as well as reports published by others, MSC facilitate chimerism, especially at limiting hematopoietic stem and progenitor cell (HSPC) numbers and increase safety by controlling and/or preventing graft-vs.-host-disease (GvHD). Thus, vBA-MSC have the potential to facilitate mixed chimerism, promote complementary peripheral immunomodulatory functions and increase safety of BM infusions. Both HPC, Marrow and vBA-MSC have potential use in current VCA and solid organ transplant (SOT) tolerance clinical protocols that are amenable to "delayed tolerance." Current trials with HPC, Marrow are planned with subsequent phases to include vBA-MSC for tolerance of both VCA and SOT.

Mesenchymal Stromal Cell Therapy in Solid Organ Transplantation

Transplantation is the gold-standard treatment for the failure of several solid organs, including the kidneys, liver, heart, lung and small bowel. The use of tailored immunosuppressive agents has improved graft and patient survival remarkably in early post-transplant stages, but long-term outcomes are frequently unsatisfactory due to the development of chronic graft rejection, which ultimately leads to transplant failure. Moreover, prolonged immunosuppression entails severe side effects that severely impact patient survival and quality of life. The achievement of tolerance, i.e., stable graft function without the need for immunosuppression, is considered the Holy Grail of the field of solid organ transplantation. However, spontaneous tolerance in solid allograft recipients is a rare and unpredictable event. Several strategies that include peri-transplant administration of non-hematopoietic immunomodulatory cells can safely and effectively induce tolerance in pre-clinical models of solid organ transplantation. Mesenchymal stromal cells (MSC), non-hematopoietic cells that can be obtained from several adult and fetal tissues, are among the most promising candidates. In this review, we will focus on current pre-clinical evidence of the immunomodulatory effect of MSC in solid organ transplantation, and discuss the available evidence of their safety and efficacy in clinical trials.

Immunomodulatory and Regenerative Effects of Mesenchymal Stem Cells and Extracellular Vesicles: Therapeutic Outlook for Inflammatory and Degenerative Diseases

Mesenchymal stem cells (MSCs) are non-hematopoietic, multipotent stem cells derived from mesoderm, which can be easily isolated from many sources such as bone marrow, umbilical cord or adipose tissue. MSCs provide support for hematopoietic stem cells and have an ability to differentiate into multiple cell lines. Moreover, they have proangiogenic, protective and immunomodulatory properties. MSCs have the capacity to modulate both innate and adaptive immune responses, which accompany many diseases, by inhibiting pro-inflammatory reactions and stimulating anti-inflammatory activity. Recent findings revealed that the positive effect of MSCs is at least partly associated with the production of extracellular vesicles (EVs). EVs are small membrane structures, containing proteins, lipids and nuclei acids, which take part in intra-cellular communication. Many studies indicate that EVs contain protective and pro-regenerative properties and can modulate an immune response that is activated in various diseases such as CNS diseases, myocardial infarction, liver injury, lung diseases, ulcerative colitis or kidney injury. Thus, EVs have similar functions as their cells of origin and since they do not carry the risk of cell transplantation, such as tumor formation or small vessel blockage, they can be considered a potential therapeutic tool for cell-free therapy.

Graft infusion of adipose-derived mesenchymal stromal cells to prevent rejection in experimental intestinal transplantation: A feasibility study

BACKGROUND

Mesenchymal stromal cells (MSC) have been proposed as a promising complement to standard immunosuppression in solid organ transplantation because of their immunomodulatory properties. The present work addresses the role of adipose-derived MSC (Ad-MSC) in an experimental model of acute rejection in small bowel transplantation (SBT).

MATERIAL/METHODS

Heterotopic allogeneic SBT was performed. A single dose of 1.5x106 Ad-MSC was intra-arterially delivered just before graft reperfusion. Animals were divided into CONTROL (CTRL), CONTROL+Ad-MSC (CTRL_MSC), tacrolimus (TAC), and TAC+Ad-MSC (TAC_MSC) groups. Each Ad-MSC groups was subdivided in autologous and allogeneic third-party groups.

RESULTS

Rejection rate and severity were similar in MSC-treated and untreated animals. CTRL_MSC animals showed a decrease in macrophages, T-cell (CD4, CD8, and Foxp3 subsets) and B-cell counts in the graft compared with CTRL, this decrease was attenuated in TAC_MSC animals. Pro- and anti-inflammatory cytokines and some chemokines and growth factors increased in CTRL_MSC animals, especially in the allogeneic group, whereas milder changes were seen in the TAC groups.

CONCLUSION

Ad-MSC did not prevent rejection when administered just before reperfusion. However, they showed immunomodulatory effects that could be relevant for a longer-term outcome. Interference between tacrolimus and the MSC effects should be addressed in further studies.

Interactions Between Allogeneic Mesenchymal Stromal Cells and the Recipient Immune System: A Comparative Review With Relevance to Equine Outcomes

Despite significant immunosuppressive activity, allogeneic mesenchymal stromal cells (MSCs) carry an inherent risk of immune rejection when transferred into a recipient. In naïve recipients, this immune response is initially driven by the innate immune system, an immediate reaction to the foreign cells, and later, the adaptive immune system, a delayed response that causes cell death due to recognition of specific alloantigens by host cells and antibodies. This review describes the actions of MSCs to both suppress and activate the different arms of the immune system. We then review the survival and effectiveness of the currently used allogeneic MSC treatments.

Mesenchymal Stromal Cells in Solid Organ Transplantation

Over the past decade, the clinical application of mesenchymal stromal cells (MSCs) has generated growing enthusiasm as an innovative cell-based approach in solid organ transplantation (SOT). These expectations arise from a significant number of both transplant- and non-transplant-related experimental studies investigating the complex anti-inflammatory, immunomodulatory, and tissue-repair properties of MSCs. Promising preclinical results have prompted clinical trials using MSC-based therapy in SOT. In the present review, the general properties of MSCs are summarized, with a particular emphasis on MSC-mediated impact on the immune system and in the ischemic conditioning strategy. Next, we chronologically detail all clinical trials using MSCs in the field of SOT. Finally, we envision the challenges and perspectives of MSC-based cell therapy in SOT.

The Immunological Basis of Liver Allograft Rejection

Liver allograft rejection remains a significant cause of morbidity and graft failure in liver transplant recipients. Rejection is caused by the recognition of non-self donor alloantigens by recipient T-cells. Antigen recognition results in proliferation and activation of T-cells in lymphoid tissue before migration to the allograft. Activated T-cells have a variety of effector mechanisms including direct T-cell mediated damage to bile ducts, endothelium and hepatocytes and indirect effects through cytokine production and recruitment of tissue-destructive inflammatory cells. These effects explain the histological appearances of typical acute T-cell mediated rejection. In addition, donor specific antibodies, most typically against HLA antigens, may give rise to antibody-mediated rejection causing damage to the allograft primarily through endothelial injury. However, as an immune-privileged site there are several mechanisms in the liver capable of overcoming rejection and promoting tolerance to the graft, particularly in the context of recruitment of regulatory T-cells and promotors of an immunosuppressive environment. Indeed, around 20% of transplant recipients can be successfully weaned from immunosuppression. Hence, the host immunological response to the liver allograft is best regarded as a balance between rejection-promoting and tolerance-promoting factors. Understanding this balance provides insight into potential mechanisms for novel anti-rejection therapies.

Effect of Pomegranate Extract in Mesenchymal Stem Cells by Modulation of microRNA-155, microRNA-21, microRNA-23b, microRNA-126a, and PI3K\AKT1\NF-[Formula: see text]B Expression

Today, mesenchymal stem cells (MSCs) are candidates for various autoimmune disease treatments due to immunomodulatory activity in these cells. Much research has recently been done to improve the immunomodulatory activity of MSCs. Genetic variation is one of these methods. microRNAs (miRNAs) are small noncoding RNAs that control most of the cell's biological activities. Recent studies have shown that miRNAs play a significant role in the regulation of MSC immunomodulatory activity. Pomegranate is a fruit that has antioxidant, anti-inflammatory, and anticancer properties and has been used for many years for therapeutic purposes. The objective of this research is to evaluate the immunoregulatory-related miRNAs level of adipose-derived MSCs (Ad-MSCs) obtained from adipose tissue in the presence or lack of pomegranate (Punica granatum) extract (PGE). Our results showed that miRNA-23 and miRNA-126 were upregulated by PGE treatment in MSCs, and in contrast, miRNA-21 and miRNA-155 were downregulated by PGE treatment in MSCs. In addition this research shows that PGE can downregulate the expression of PI3K\AKT1\NF-[Formula: see text]B in Ad-MSCs. Our bioinformatics data have shown that the target of these four miRNAs and the signaling pathways, in which these targets are involved, can play an important role in regulating the immunomodulation function of stem cells. In conclusion, PGE can inhibit the expression of PI3K\AKT1\NF-[Formula: see text]B genes involved in inflammatory pathways via miRNA-23 and miRNA-126 overexpression or miRNA-21 and miRNA-155 downregulation that plays a role in the pathways of immune modulation in Ad-MSCs. These results may provide insight into the mechanism underlying the regulation of the immunomodulatory activity of Ad-MSCs by PGE.

Co-transplantation of bone marrow mesenchymal stem cells and monocytes in the brain stem to repair the facial nerve axotomy

After the facial nerve axotomy (FNA), the distal end of the axon would gradually decay and disappear. Accumulated evidence shows that transplantation of bone marrow mesenchymal stem cells (BMSCs) reveals potential in the treatment of nervous system diseases or injuries. This study is aimed at investigating the therapeutic effects of co-transplantation of BMSCs and monocytes in FNA. We found that co-culture significantly elevated the CD4+/CD8+ ratio and CD4+ CD25+ T cell proportion compared with monocytes transplantation, and enhanced the differentiation of BMSCs into neurons. After the cell transplantation, the lowest apoptosis in the facial nerve nucleus was found in the co-transplantation group 2 (BMSCs:monocytes= 1:30). Moreover, the lowest expression levels of pro-inflammatory cytokines and the highest expression levels of anti-inflammatory cytokines were observed in the co-transplantation group 2 (BMSCs: monocytes= 1:30). The highest expression levels of protein in the JAK/STAT6 pathway and the SDF-1/CXCR4 axis were found in the co-transplantation group 2. BMSC/monocyte co-transplantation significantly improves the microenvironment in the facial nerve nucleus in FNA rats; therefore these findings suggest that it could promote the anti-/pro-inflammatory balance shift towards the anti-inflammatory microenvironment, alleviating survival conditions for BMSCs, regulating BMSC the chemotaxis homing, differentiation, and the section of BMSCs, and finally reducing the neuronal apoptosis. These findings might provide essential evidence for the in-hospital treatment of FNA with co-transplantation of BMSCs and monocytes.

Mesenchymal Stromal Cells, a New Player in Reducing Complications From Liver Transplantation?

In response to the global burden of liver disease there has been a commensurate increase in the demand for liver transplantation. However, due to a paucity of donor organs many centers have moved toward the routine use of marginal allografts, which can be associated with a greater risk of complications and poorer clinical outcomes. Mesenchymal stromal cells (MSC) are a multi-potent progenitor cell population that have been utilized to modulate aberrant immune responses in acute and chronic inflammatory conditions. MSC exert an immunomodulatory effect on innate and adaptive immune systems through the release of both paracrine soluble factors and extracellular vesicles. Through these routes MSC can switch the regulatory function of the immune system through effects on macrophages and T regulatory cells enabling a switch of phenotype from injury to restoration. A key benefit seems to be their ability to tailor their response to the inflammatory environment without compromising the host ability to fight infection. With over 200 clinical trials registered to examine MSC therapy in liver disease and an increasing number of trials of MSC therapy in solid organ transplant recipients, there is increasing consideration for their use in liver transplantation. In this review we critically appraise the potential role of MSC therapy in the context of liver transplantation, including their ability to modulate reperfusion injury, their role in the reduction of medium term complications in the biliary tree and their potential to enhance tolerance in transplanted organs.

Effect of Timing and Complement Receptor Antagonism on Intragraft Recruitment and Protolerogenic Effects of Mesenchymal Stromal Cells in Murine Kidney Transplantation

BACKGROUND

Mesenchymal stromal cells (MSCs) have protolerogenic effects in renal transplantation, but they induce long-term regulatory T cells (Treg)-dependent graft acceptance only when infused before transplantation. When given posttransplant, MSCs home to the graft where they promote engraftment syndrome and do not induce Treg. Unfortunately, pretransplant MSC administration is unfeasible in deceased-donor kidney transplantation.

METHODS

To make MSCs a therapeutic option also for deceased organ recipients, we tested whether MSC infusion at the time of transplant (day 0) or posttransplant (day 2) together with inhibition of complement receptors prevents engraftment syndrome and allows their homing to secondary lymphoid organs for promoting tolerance. We analyzed intragraft and splenic MSC localization, graft survival, and alloimmune response in mice recipients of kidney allografts and syngeneic MSCs given on day 0 or on posttransplant day 2. C3a receptor (C3aR) or C5a receptor (C5aR) antagonists were administered to mice in combination with the cells or were used together to treat MSCs before infusion.

RESULTS

Syngeneic MSCs given at day 0 homed to the spleen increased Treg numbers and induced long-term graft acceptance. Posttransplant MSC infusion, combined with a short course of C3aR or C5aR antagonist or administration of MSCs pretreated with C3aR and C5aR antagonists, prevented intragraft recruitment of MSCs and graft inflammation, inhibited antidonor T-cell reactivity, but failed to induce Treg, resulting in mild prolongation of graft survival.

CONCLUSIONS

These data support testing the safety/efficacy profile of administering MSCs on the day of transplant in deceased-donor transplant recipients and indicate that complement is crucial for MSC recruitment into the kidney allograft.

Therapeutic Mesenchymal Stromal Cells for Immunotherapy and for Gene and Drug Delivery

Mesenchymal stromal cells (MSCs) possess several fairly unique properties that, when combined, make them ideally suited for cellular-based immunotherapy and as vehicles for gene and drug delivery for a wide range of diseases and disorders. Key among these are: (1) their relative ease of isolation from a variety of tissues; (2) the ability to be expanded in culture without a loss of functionality, a property that varies to some degree with tissue source; (3) they are relatively immune-inert, perhaps obviating the need for precise donor/recipient matching; (4) they possess potent immunomodulatory functions that can be tailored by so-called licensing in vitro and in vivo; (5) the efficiency with which they can be modified with viral-based vectors; and (6) their almost uncanny ability to selectively home to damaged tissues, tumors, and metastases following systemic administration. In this review, we summarize the latest research in the immunological properties of MSCs, their use as immunomodulatory/anti-inflammatory agents, methods for licensing MSCs to customize their immunological profile, and their use as vehicles for transferring both therapeutic genes in genetic disease and drugs and genes designed to destroy tumor cells.

The immunoregulation of mesenchymal stem cells plays a critical role in improving the prognosis of liver transplantation

The liver is supplied by a dual blood supply, including the portal venous system and the hepatic arterial system; thus, the liver organ is exposed to multiple gut microbial products, metabolic products, and toxins; is sensitive to extraneous pathogens; and can develop liver failure, liver cirrhosis and hepatocellular carcinoma (HCC) after short-term or long-term injury. Although liver transplantation (LT) serves as the only effective treatment for patients with end-stage liver diseases, it is not very popular because of the complications and low survival rates. Although the liver is generally termed an immune and tolerogenic organ with adaptive systems consisting of humoral immunity and cell-mediated immunity, a high rejection rate is still the main complication in patients with LT. Growing evidence has shown that mesenchymal stromal cell (MSC) transplantation could serve as an effective immunomodulatory strategy to induce tolerance in various immune-related disorders. MSCs are reported to inhibit the immune response from innate immune cells, including macrophages, dendritic cells (DCs), natural killer cells (NK cells), and natural killer T (NKT) cells, and that from adaptive immune cells, including T cells, B cells and other liver-specific immune cells, for the generation of a tolerogenic microenvironment. In this review, we summarized the relationship between LT and immunoregulation, and we focused on how to improve the effects of MSC transplantation to improve the prognosis of LT. Only after exhaustive clarification of the potential immunoregulatory mechanisms of MSCs in vitro and in vivo can we implement MSC protocols in routine clinical practice to improve LT outcome.

Investigating the route of administration and efficacy of adipose tissue-derived mesenchymal stem cells and conditioned medium in type 1 diabetic mice

Type 1 diabetes (T1D) is a chronic autoimmune disease destroying the insulin-producing beta cells. Recently, stem cell therapy has been tested to treat T1D. In the present study, we aim to investigate the effects of intraperitoneal and intravenous infusion of multipotent mesenchymal stem/stromal cells (MSCs) and MSC-conditioned medium (MSC-CM) in an experimental model of diabetes, induced by multiple injections of Streptozotocin (STZ). The adipose tissue-derived MSC and MSC-CM were isolated from C57Bl/6 male mice and characterized. Later, MSC and MSC-CM were injected intraperitoneally or intravenously into mice. The blood glucose, urinary glucose, and body weight were measured, and the percentages of CD4+ CD25+ FOXP3+ T cells as well as the levels of IFN-γ, TGF-β, IL-4, IL-17, and IL-10 were evaluated. Our results showed that both intraperitoneal and intravenous infusions of MSC and MSC-CM could decrease the blood glucose, recover pancreatic islets, and increase the levels of insulin-producing cells. Furthermore, the percentage of CD4+ CD25+ FOXP3+ T cells was increased after intraperitoneal injection of MSC or MSC-CM and intravenous injection of MSCs. After intraperitoneal injection of the MSC and MSC-CM, the levels of inflammatory cytokines reduced, while the levels of anti-inflammatory cytokines increased. Together current data showed that although both intraperitoneal and intravenous administration had beneficial effects on T1D animal model, but intraperitoneal injection of AD-MSC and AD-MSC-CM was more effective than systemic administration.

Pre-treatments enhance the therapeutic effects of mesenchymal stem cells in liver diseases

Liver diseases caused by viral infection, alcohol abuse and metabolic disorders can progress to end‐stage liver failure, liver cirrhosis and liver cancer, which are a growing cause of death worldwide. Although liver transplantation and hepatocyte transplantation are useful strategies to promote liver regeneration, they are limited by scarce sources of organs and hepatocytes. Mesenchymal stem cells (MSCs) restore liver injury after hepatogenic differentiation and exert immunomodulatory, anti‐inflammatory, antifibrotic, antioxidative stress and antiapoptotic effects on liver cells in vivo. After isolation and culture in vitro, MSCs are faced with nutrient and oxygen deprivation, and external growth factors maintain MSC capacities for further applications. In addition, MSCs are placed in a harsh microenvironment, and anoikis and inflammation after transplantation in vivo significantly decrease their regenerative capacity. Pre‐treatment with chemical agents, hypoxia, an inflammatory microenvironment and gene modification can protect MSCs against injury, and pre‐treated MSCs show improved hepatogenic differentiation, homing capacity, survival and paracrine effects in vitro and in vivo in regard to attenuating liver injury. In this review, we mainly focus on pre‐treatments and the underlying mechanisms for improving the therapeutic effects of MSCs in various liver diseases. Thus, we provide evidence for the development of MSC‐based cell therapy to prevent acute or chronic liver injury. Mesenchymal stem cells have potential as a therapeutic to prolong the survival of patients with end‐stage liver diseases in the near future.

Towards Cell free Therapy of Premature Ovarian Insufficiency: Human Bone Marrow Mesenchymal Stem Cells Secretome Enhances Angiogenesis in Human Ovarian Microvascular Endothelial Cells

Primary Ovarian Insufficiency (POI) refers to an ovarian loss of function in women under the age of 40. Unfortunately, currently, there is no effective treatment available for POI-related infertility. Alternatives such as the use of egg donations are culturally and ethically unacceptable to many couples. Human Bone marrow-derived Mesenchymal Stem Cells (MSCs) are known for their ability to differentiate into other cell types, once primed by the organ microenvironment. Importantly MSCs produce a vast array of bioactive factors many of them have been shown to enhance neovascularization in various tissues. Recently, preliminary data from our ongoing clinical trial revealed encouraging preliminary data after autologous MSC engraftment into the ovaries of 2 POI patients with durable elevation in serum estrogen levels and increase in size of treated ovaries sustained up to one-year post cell therapy.

In this study, we investigated the action of the mechanisms of MSCs treatment on a POI ovary. We designed an in vitro study using MSC secretome and Human Ovarian Endothelial Cells (HOVECs) to understand the molecular mechanisms by which MSC mediates their angiogenic properties and regenerative effects. Human primary HOVECs were treatment with MSC secretome and examined by FACS for the expression of angiogenesis markers such as Endoglin, Tie-2, and VEGF. The formation of vessels was evaluated by using a 3D Matrigel tubulogenesis assay.

We observed that the expression of proliferation marker Ki67 was significantly increased under treatment with MSC secretome in HOVEC cells (P4). MSCs secretome treatment also induced significantly higher expression of several angiogenic markers such as VEGFR2, Tie2/Tek, VE-Cadherin, Endoglin, and VEGF compared to matched control (P4). Furthermore, MSC secretome significantly increased the number of branching points in tubulogenesis assay (P4). Our study suggests that MSC secretome likely contains bioactive factors that can enhance ovarian angiogenesis. Further characterization of these factors can lead to novel therapeutic options for women with premature ovarian insufficiency and other related causes of female infertility.

Research Status of Mesenchymal Stem Cells in Liver Transplantation

Liver transplantation has been deemed the best choice for end-stage liver disease patients but immune rejection after surgery is still a serious problem. Patients have to take immunosuppressive drugs for a long time after liver transplantation, and this often leads to many side effects. Mesenchymal stem cells (MSCs) gradually became of interest to researchers because of their powerful immunomodulatory effects. In the past, a large number of in vitro and in vivo studies have demonstrated the great potential of MSCs for participation in posttransplant immunomodulation. In addition, MSCs also have properties that may potentially benefit patients undergoing liver transplantation. This article aims to provide an overview of the current understanding of the immunomodulation achieved by the application of MSCs in liver transplantation, to discuss the problems that may be encountered when using MSCs in clinical practice, and to describe some of the underlying capabilities of MSCs in liver transplantation. Cell–cell contact, soluble molecules, and exosomes have been suggested to be critical approaches to MSCs’ immunoregulation in vitro; however, the exact mechanism, especially in vivo, is still unclear. In recent years, the clinical safety of MSCs has been proven by a series of clinical trials. The obstacles to the clinical application of MSCs are decreasing, but large sample clinical trials involving MSCs are still needed to further study their clinical effects.

Mesenchymal stromal cells in kidney transplantation

PURPOSE OF REVIEW

Mesenchymal stromal cells (MSC) have emerged as one of the most promising candidates for immunomodulatory cell therapy in kidney transplantation. Here we describe novel insights into the MSC mechanism of action and provide an overview of initial safety and feasibility studies with MSC in kidney transplantation.

RECENT FINDINGS

Clinical studies of MSC-based cell therapy in kidney transplant recipients demonstrated the safety and feasibility of cell therapy and provide the first encouraging evidence of the efficacy of MSC in enabling the minimization of immunosuppressive drugs. In our initial experience with MSC-based therapy in kidney transplant recipients we carried out extensive clinical and immunological monitoring of MSC-treated patients and found possible biomarkers of MSC immunomodulation in some of them. Based on these biomarkers we identified a patient in whom complete discontinuation of immunosuppression has been achieved safely and successfully.

SUMMARY

Many issues should be addressed before MSC-based therapy becomes a standard treatment protocol for kidney transplantation. A better understanding of the MSC mechanism of action and the identification of biomarkers of response to therapy will inform the rational design of the most effective clinical protocol and the selection of patients amenable to safe immunosuppressive drug withdrawal.

The Immunomodulatory Functions of Mesenchymal Stromal/Stem Cells Mediated via Paracrine Activity

Mesenchymal stromal/stem cells (MSCs) exist in almost all tissues, possessing the potential to differentiate into specialized cell types and exert immunomodulatory functions. Thus, they have attracted much attention as a promising therapeutic candidate. Recent studies have demonstrated that paracrine signaling is mainly responsible for the involvement of MSCs in the modulation of immune responses and the progression of diseases. Through release of secretome consisting of a diverse range of cytokines, chemokines, and extracellular vesicles (EVs), MSCs convey regulatory messages to recipient immune cells in the microenvironment. In this review, we focus on the recent advances in how MSCs contribute to immunomodulation through the secretion of paracrine factors. The further improved understanding of the molecular mechanism underlying the interactions between MSCs and immune cells highlights the paracrine biology of MSCs in the modulation of the immune microenvironment and promotes the clinical application of MSCs in regenerative medicine and immune diseases.

A Pilot Study of Mesenchymal Stem Cell Therapy for Acute Liver Allograft Rejection

Acute allograft rejection remains common after liver transplantation despite modern immunosuppressive agents. In addition, the long-term side effects of these regimens, including opportunistic infections, are challenging. This study evaluated the safety and clinical feasibility of umbilical cord-derived mesenchymal stem cell (UC-MSC) therapy in liver transplant patients with acute graft rejection. Twenty-seven liver allograft recipients with acute rejection were randomly assigned into the UC-MSC infusion group or the control group. Thirteen patients received one infusion of UC-MSCs (1 × 10⁶ /kg body weight); one patient received multiple UC-MSC infusions; 13 patients were used as controls. All enrolled patients received conventional immunosuppressive agents with follow-up for 12 weeks after UC-MSC infusions. No side effects occurred in treated patients. Four weeks after UC-MSC infusions, alanine aminotransferase levels had decreased markedly and remained lower throughout the 12-week follow-up period. Importantly, allograft histology was improved after administration of UC-MSCs. The percentage of regulatory T cells (Tregs) and the Treg/T helper 17 (Th17) cell ratio were significantly increased 4 weeks after infusions; in contrast, the percentage of Th17 cells showed a decreasing trend. In controls, the percentages of Tregs and Th17 cells and the Treg/Th17 ratio were statistically unchanged from the baseline measurements. Transforming growth factor beta 1 and prostaglandin E2 were increased significantly after UC-MSC infusions; by contrast, there were no significant changes in controls. Our data suggest that UC-MSC infusion for acute graft rejection following liver transplantation is feasible and may mediate a therapeutic immunosuppressive effect. Stem Cells Translational Medicine 2017;6:2053-2061.

Mesenchymal Stromal Cells for Transplant Tolerance

In solid organ transplantation lifelong immunosuppression exposes transplant recipients to life-threatening complications, such as infections and malignancies, and to severe side effects. Cellular therapy with mesenchymal stromal cells (MSC) has recently emerged as a promising strategy to regulate anti-donor immune responses, allowing immunosuppressive drug minimization and tolerance induction. In this review we summarize preclinical data on MSC in solid organ transplant models, focusing on potential mechanisms of action of MSC, including down-regulation of effector T-cell response and activation of regulatory pathways. We will also provide an overview of available data on safety and feasibility of MSC therapy in solid organ transplant patients, highlighting the issues that still need to be addressed before establishing MSC as a safe and effective tolerogenic cell therapy in transplantation.

Induction of CD4+CD25+Foxp3+ regulatory T cells by mesenchymal stem cells is associated with RUNX complex factors

Among the particular immunomodulation properties of mesenchymal stem cells (MSCs), one relies on their capacity to regulatory T cell (Treg) induction from effector T cells. Stable expression of Foxp3 has a dominant role in suppressive phenotype and stability of induced regulatory T cells (iTregs). How MSCs induce stable Foxp3 expression in iTregs remains unknown. We previously showed MSCs could enhance demethylation of Treg-specific demethylated region (TSDR) in iTregs in cell-cell contact manner (unpublished data). Here, we evaluated the possible effect of MSCs on the mRNA expression of Runx complex genes (Runx1, Runx3, and CBFB) that perch on TSDR in iTregs and play the main role in suppressive properties of Tregs, a regulatory pathway that has not yet been explored by MSCs. Also, we investigated the mRNA expression of MBD2 that promotes TSDR demethylation in Tregs. We first showed that in vitro MSC-iTreg induction was associated with strong mRNA modifications of genes involved in Runx complex. We next injected high doses of MSCs in a murine model of C57BL/6 into Balb/C allogeneic skin transplantation to prolong allograft survival. When splenocytes of grafted mice were analyzed, we realized that the Foxp3 expression was increased at day 5 and 10 post-graft merely in MSC-treated mice. Furthermore, Foxp3 mRNA expression was associated with modified Runx complex mRNA expression comparable to what was shown in in vitro studies. Hence, our data identify a possible mechanism in which MSCs convert conventional T cells to iTreg through strong modifications of mRNA of genes that are involved in Runx complex of Foxp3.

Liver regeneration therapy through the hepatic artery-infusion of cultured bone marrow cells in a canine liver fibrosis model

Background

We previously reported regenerative therapies for decompensated cirrhosis based on peripheral venous drip infusion using non-cultured whole bone marrow (BM) cells, or the less invasive cultured BM-derived mesenchymal stem cells (BMSCs). Here, we assessed the efficacy and safety of hepatic arterial infusion using cultured autologous BMSCs, comparing it with peripheral infusion, using our established canine liver fibrosis model.

Methods

Canine BM cells were harvested and cultured, and the resultant BMSCs were returned to carbon tetrachloride (CCl₄)-induced liver cirrhosis model canines via either a peripheral vein (Vein group) or hepatic artery (Artery group). A variety of assays were performed before and 4, 8, and 12 weeks after BMSC infusion, and liver fibrosis and indocyanine green (ICG) half-life (t_1/2) were compared to those in a control group that received CCl₄ but not BMSCs. The safety of this approach was evaluated by contrast-enhanced computed tomography (CT) and serial blood examinations after infusion.

Results

Four weeks after infusing BMSCs, a significant improvement was observed in the Vein group (n = 8) compared to outcome in the Control group (n = 10), along with a decrease in ICG t_1/2. In the Artery group (n = 4), ICG t_1/2 was significantly shorter than that in the Vein group at 8 weeks (Δt_1/2: −3.8 ± 1.7 min vs. +0.4 ± 2.4 min; p < 0.01) and 12 weeks (Δt_1/2: −4.2 ± 1.7 min vs. +0.4 ± 2.7 min; p < 0.01) after BMSC administration. Post-infusion contrast-enhanced CT showed no liver infarction, and blood tests showed no elevations in either serum lactate dehydrogenase concentrations or hypercoagulability.

Conclusions

We confirmed the efficacy and safety of the hepatic arterial infusion of cultured autologous BMSCs using a canine model, thereby providing non-clinical proof-of-concept.

Bone marrow mesenchymal stromal cells attenuate liver allograft rejection may via upregulation PD-L1 expression through downregulation of miR-17-5p

Studies have reported that bone marrow mesenchymal stem cells (BMSCs) play an important role in immune regulation after organ transplantation. Some of the BMSC-mediated regulatory mechanisms are related to PD-L1 expression. However, the regulatory mechanism of PD-L1 expression is not fully understood. In this experiment, we confirmed the regulatory role of BMSCs in the rejection of liver transplantation and explored the possible mechanism by which PD-L1 expression is regulated in BMSCs. An orthotopic liver transplantation model in rats was established based on the "double-cuff technique". Third-generation BMSCs were injected into the rejection model rats via portal vein. At the same time, sera were obtained from rats in the allograft, isograft and sham-operated groups. The sera from these groups were separately added to BMSCs complete medium to partially mimic the in vivo environment in which BMSCs are exposed. After predicting and analysing microRNAs that regulate PD-L1 expression, we examined the microRNA and PD-L1 expression in BMSCs of the three groups cultured in the conditioned media. Moreover, a luciferase reporter assay was used to confirm the interaction between PD-L1 and microRNA. The study found that the liver function and liver graft histopathology of the BMSC-treated group were better than those of the allograft group. The Kaplan-Meier survival curve analysis showed that the median survival time (MST) of the BMSC-treated group was longer than that of the allograft group. Bioinformatics prediction and analysis showed that miR-17-5p is highly likely to regulate PD-L1. Compared with the other two groups of cells, BMSCs cultured with serum from allograft models showed higher PD-L1 expression, but lower miR-17-5p expression. Pearson correlation analysis showed that miR-17-5p and PD-L1 expression was negatively correlated, and further luciferase reporter assays confirmed that miR-17-5p interacted directly with the 3´-untranslated region (UTR) of PD-L1 mRNA. The results of this study demonstrate that BMSCs can attenuate liver allograft rejection and provide us with the idea that BMSCs may further upregulate PD-L1 expression by downregulating miR-17-5p expression, thereby attenuating liver allograft rejection.

Historical Perspectives and Advances in Mesenchymal Stem Cell Research for the Treatment of Liver Diseases

Liver transplantation is the only effective therapy for patients with decompensated cirrhosis and fulminant liver failure. However, due to a shortage of donor livers and complications associated with immune suppression, there is an urgent need for new therapeutic strategies for patients with end-stage liver diseases. Given their unique function in self-renewal and differentiation potential, stem cells might be used to regenerate damaged liver tissue. Recent studies have shown that stem cell-based therapies can improve liver function in a mouse model of hepatic failure. Moreover, acellular liver scaffolds seeded with hepatocytes produced functional bioengineered livers for organ transplantation in preclinical studies. The therapeutic potential of stem cells or their differentiated progenies will depend on their capacity to differentiate into mature and functional cell types after transplantation. It will also be important to devise methods to overcome their genomic instability, immune reactivity, and tumorigenic potential. We review directions and advances in the use of mesenchymal stem cells and their derived hepatocytes for liver regeneration. We also discuss the potential applications of hepatocytes derived from human pluripotent stem cells and challenges to using these cells in treating end-stage liver disease.

Mesenchymal stromal cells for tolerance induction in organ transplantation

The primary challenge in organ transplantation continues to be the need to suppress the host immune system long-term to ensure prolonged allograft survival. Long-term non-specific immunosuppression can, however, result in life-threatening complications. Thus, efforts have been pursued to explore novel strategies that would allow minimization of maintenance immunosuppression, eventually leading to transplant tolerance. In this scenario, bone marrow-derived mesenchymal stromal cells (MSC), given their unique immunomodulatory properties to skew the balance between regulatory and memory T cells, have emerged as potential candidates for cell-based therapy to promote immune tolerance. Here, we review our initial clinical experience with bone marrow-derived MSC in living-donor kidney transplant recipients and provide an overview of the available results of other clinical programs with MSC in kidney and liver transplantation, highlighting hurdles and success of this innovative cell-based therapy.

Mesenchymal stem cells and their immunosuppressive role in transplantation tolerance

Since they were first described, mesenchymal stem cells (MSCs) have been shown to have important effector mechanisms and the potential for use in cell therapy. A great deal of research has been focused on unveiling how MSCs contribute to anti-inflammatory responses, including describing several cell populations involved and identifying soluble and other effector molecules. In this review, we discuss some of the contemporary evidence for use of MSCs in the field of immune tolerance, with a special emphasis on transplantation. Although considerable effort has been devoted to understanding the biological function of MSCs, additional resources are required to clarify the mechanisms of their induction of immune tolerance, which will undoubtedly lead to improved clinical outcomes for MSC-based therapies.

Mesenchymal stem cells can induce regulatory T cells via modulating miR-126a but not miR-10a

Among the different immunosuppressive properties attributed to mesenchymal stem cells (MSCs), one relies on their ability to induce regulatory T cells (iTregs) from conventional T cells under particular inflammatory context. Stable Foxp3 expression plays a major role in the phenotypic and functional stability of iTregs. However, the mechanism behind Foxp3 induction in iTregs by MSCs remains unknown. Here, we assessed the possible effect of MSCs on miR-126a and miR-10a expression in iTregs and, consequently on Foxp3 stability, a regulatory pathway that has not yet been explored. We first demonstrated that in vitro MSC-iTreg generation was directly associated with strong modifications of miR-126a. We next infused high doses of MSCs in a murine model of allogeneic skin transplantation (C57BL/6 into Balb/c). This treatment significantly prolonged skin allograft survival compared to PBS treated mice. When splenocytes from grafted mice were collected, we observed that the expression of Foxp3 gene was elevated at day 5 and 10 post-graft merely in MSCs treated mice. Moreover, Foxp3 expression was not associated with modified miR-10a expression comparable to in vitro experiments. Thus, our data identify a solid mechanism where MSCs induce conversion of conventional T cells to iTregs through strong modifications of miR-126a. Although miR-10a expression level remains unchanged in vitro and in vivo, we observed expression of this miR in MSC-DC condition.

Mesenchymal stem cells overexpressing IL-35 effectively inhibit CD4+ T cell function

Mesenchymal stem cells (MSCs) have recently emerged as promising candidates for cell-based immune tolerance therapy. Interleukin 35 (IL-35) is a relatively newly identified cytokine required for the regulatory and suppressive functions of regulatory T cells (Treg), playing an important role in the prevention of autoimmune diseases. In this study, we isolated adipose tissue-derived MSCs, a good vehicle for cell therapy, which were transfected with a lentivirus vector for the overexpression of the therapeutic murine IL-35 gene. IL-35 levels in transfected MSCs (IL-35-MSCs) were quantified by ELISA. Co-culture of CD4⁺ T cells and IL-35-MSCs resulted in the inhibition of CD4⁺ T cell proliferation and IL-17A secretion. In addition, IL-35-MSCs induced IL-10 production by CD4⁺ T cells, but did not affect IFN-γ. These findings suggested that MSCs over-expressing IL-35 had higher immunosuppressive capacity compared with non-transfected MSCs, and may provide a useful approach for basic research on gene therapy for autoimmune disorders.

TLR3 or TLR4 Activation Enhances Mesenchymal Stromal Cell-Mediated Treg Induction via Notch Signaling

Mesenchymal stromal cells (MSCs) are the subject of numerous clinical trials, largely due to their immunomodulatory and tissue regenerative properties. Toll-like receptors (TLRs), especially TLR3 and TLR4, are highly expressed on MSCs and their activation can significantly modulate the immunosuppressive and anti-inflammatory functions of MSCs. While MSCs can recruit and promote the generation of regulatory T cells (Tregs), the effect of TLR activation on MSC-mediated Treg induction is unknown. In this study, we investigated the effect of ligand-mediated activation of TLR3 and TLR4 on Treg induction by human MSCs. We found that generation of Tregs in human CD4(+) lymphocyte and MSC cocultures was enhanced by either TLR3 or TLR4 activation of MSCs and that the increase was abolished by TLR3 and TLR4 gene-silencing. Augmented Treg induction by TLR-activated MSCs was cell contact-dependent and associated with increased gene expression of the Notch ligand, Delta-like 1. Moreover, inhibition of Notch signaling abrogated the augmented Treg levels in the MSC cocultures. Our data show that TLR3 or TLR4 activation of MSCs increases Treg induction via the Notch pathway and suggest new means to enhance the potency of MSCs for treating disorders with an underlying immune dysfunction, including steroid resistant acute graft-versus-host disease. Stem Cells 2017;35:265-275.

Mesenchymal stem cells improve the outcomes of liver recipients via regulating CD4+ T helper cytokines in rats

BACKGROUND

Bone marrow mesenchymal stem cells (BMMSCs) exert immunosuppressive activities in transplantation. This study aimed to determine whether BMMSCs reduce acute rejection and improve outcomes of liver transplantation in rats.

METHODS

Orthotopic liver transplantation from Lewis to Brown Norway rats was performed, which was followed by the infusion of BMMSCs through the penile superficial dorsal vein. Normal saline infusion was used as a control. Animals were sacrificed at 0, 24, 72, or 168 hours after BMMSCs infusion. Liver grafts, and recipient serum and spleen tissues were obtained. Histopathology, apoptosis, serum liver enzymes, serum cytokines, and circulating regulatory T (Treg), Th1, Th2 and Th17 cells were assessed at each time point.

RESULTS

BMMSCs significantly attenuated acute rejection and improved the survival rate of allogeneic liver transplantation recipients. Liver enzymes and liver apoptosis were significantly alleviated. The levels of the Th1/Th2 ratio-associated cytokines such as IL-2 and IFN-gamma were significantly reduced and IL-10 was significantly increased. The levels of the Th17/Tregs axis-associated cytokines such as IL-6, IL-17, IL-23, and TNF-alpha were significantly reduced, whereas TGF-beta concentration was significantly increased. Moreover, flow cytometry analysis showed that the infusion of BMMSCs significantly increased Th2 and Treg cells and decreased Th1 and Th17 cells.

CONCLUSION

BMMSCs had immunomodulatory effects, attenuated acute rejection and improved outcomes of allogeneic liver transplantation in rats by regulating the levels of cytokines associated with Th1/Th2 and Th17/Treg ratios.

Transplantation Tolerance Induction: Cell Therapies and Their Mechanisms

Cell-based therapies have been studied extensively in the context of transplantation tolerance induction. The most successful protocols have relied on transfusion of bone marrow prior to the transplantation of a renal allograft. However, it is not clear that stem cells found in bone marrow are required in order to render a transplant candidate immunologically tolerant. Accordingly, mesenchymal stem cells, regulatory myeloid cells, T regulatory cells, and other cell types are being tested as possible routes to tolerance induction, in the absence of donor-derived stem cells. Early data with each of these cell types have been encouraging. However, the induction regimen capable of achieving consistent tolerance, while avoiding unwanted sided effects, and which is scalable to the human patient, has yet to be identified. Here, we present the status of investigations of various tolerogenic cell types and the mechanistic rationale for their use in tolerance induction protocols.

Immunogenicity of allogeneic mesenchymal stromal cells: what has been seen in vitro and in vivo?

Mesenchymal stromal cells (MSC) are promising candidates for supporting regeneration and suppressing undesired immune reactivity. Although autologous MSC have been most commonly used for clinical trials, data on application of allogeneic MHC-unmatched MSC were reported. The usage of MSC as an 'off-the-shelf' product would have several advantages; however, it is an immunological challenge. The preclinical studies on the (non)immunogenicity of MSC are contradictory and, unfortunately, solid data from clinical trials are missing. Induction of an alloresponse would be a major limitation for the application of allogeneic MSC. Here we discuss the key elements for the induction of an alloresponse and targets of immunomodulation by MSC as well as preclinical and clinical hints on allo(non)response to MSC.

Mesenchymal stromal cell therapy in liver disease: opportunities and lessons to be learnt?

End-stage liver disease is responsible for 30,000 deaths per year in the United States alone, and it is continuing to increase every year. With liver transplantation the only curative treatment currently available, new therapies are in great demand. Mesenchymal stem cells (MSC) offer an opportunity to both treat liver inflammatory damage, as well as reverse some of the changes that occur following chronic liver injury. With the ability to regulate both the innate and adaptive immune system, as well as both inhibit and promote apoptosis of effector inflammatory cells, there are numerous therapeutic opportunities for MSC in acute and chronic liver disease. This article critically appraises the potential therapeutic roles of MSC in liver disease, as well as the barriers to their adoption into clinical practice.

Foxp3-modified bone marrow mesenchymal stem cells promotes liver allograft tolerance through the generation of regulatory T cells in rats

Background

The transcription factor forkhead box P3 (Foxp3) is a master regulatory gene necessary for the development and function of CD4⁺CD25⁺ regulatory T cells (Tregs). Mesenchymal stem cells (MSC) have recently emerged as promising candidates for cell-based immunosuppression/tolerance induction protocols. Thus, we hypothesized that MSC-based Foxp3 gene therapy would improve immunosuppressive capacity of MSC and induce donor-specific allograft tolerance in rat’s liver allograft model.

Methods

The present study utilized a lentivirus vector to overexpress the therapeutic gene Foxp3 on MSC. In vivo, Injections of 2 × 10⁶ MSC, FUGW-MSC or Foxp3-MSC into the portal vein were carried out immediately after liver transplantation.

Results

Successful gene transfer of Foxp3 in MSC was achieved by lentivirus carrying Foxp3 and Foxp3-MSC engraftment in liver allograft was confirmed by fluorescence microscopy. Foxp3-MSC treatment significantly inhibited the proliferation of allogeneic ACI CD4⁺ T cells to splenocytes (SC) from the same donor strain or third-party BN rat compared with MSC. Foxp3-MSC suppressive effect on the proliferation of CD4⁺ T cells is contact dependent and associated with Programmed death ligand 1(PD-L1) upregulation in MSC. Co-culture of CD4⁺ T cells with Foxp3-MSC results in a shift towards a Tregs phenotype. More importantly, Foxp3-MSC monotherapy achieved donor-specific liver allograft tolerance and generated a state of CD4⁺CD25⁺Foxp3⁺ Tregs-dependent tolerance.

Conclusion

Foxp3-engineered MSC therapy seems to be a promising and attractive cell therapy approach for inducing immunosuppression or transplant tolerance.