Undifferentiated Murine Embryonic Stem Cells Cannot Induce Portal Tolerance but May Possess Immune Privilege Secondary to Reduced Major Histocompatibility Complex Antigen Expression

Neural induction drives body axis formation during embryogenesis, but a neural induction-like process drives tumorigenesis in postnatal animals

Characterization of cancer cells and neural stem cells indicates that tumorigenicity and pluripotency are coupled cell properties determined by neural stemness, and tumorigenesis represents a process of progressive loss of original cell identity and gain of neural stemness. This reminds of a most fundamental process required for the development of the nervous system and body axis during embryogenesis, i.e., embryonic neural induction. Neural induction is that, in response to extracellular signals that are secreted by the Spemann-Mangold organizer in amphibians or the node in mammals and inhibit epidermal fate in ectoderm, the ectodermal cells lose their epidermal fate and assume the neural default fate and consequently, turn into neuroectodermal cells. They further differentiate into the nervous system and also some non-neural cells via interaction with adjacent tissues. Failure in neural induction leads to failure of embryogenesis, and ectopic neural induction due to ectopic organizer or node activity or activation of embryonic neural genes causes a formation of secondary body axis or a conjoined twin. During tumorigenesis, cells progressively lose their original cell identity and gain of neural stemness, and consequently, gain of tumorigenicity and pluripotency, due to various intra-/extracellular insults in cells of a postnatal animal. Tumorigenic cells can be induced to differentiation into normal cells and integrate into normal embryonic development within an embryo. However, they form tumors and cannot integrate into animal tissues/organs in a postnatal animal because of lack of embryonic inducing signals. Combination of studies of developmental and cancer biology indicates that neural induction drives embryogenesis in gastrulating embryos but a similar process drives tumorigenesis in a postnatal animal. Tumorigenicity is by nature the manifestation of aberrant occurrence of pluripotent state in a postnatal animal. Pluripotency and tumorigenicity are both but different manifestations of neural stemness in pre- and postnatal stages of animal life, respectively. Based on these findings, I discuss about some confusion in cancer research, propose to distinguish the causality and associations and discriminate causal and supporting factors involved in tumorigenesis, and suggest revisiting the focus of cancer research.

Hypoproliferative human neural progenitor cell xenografts survived extendedly in the brain of immunocompetent rats

Background

There is a huge controversy about whether xenograft or allograft in the “immune-privileged” brain needs immunosuppression. In animal studies, the prevailing sophisticated use of immunosuppression or immunodeficient animal is detrimental for the recipients, which results in a short lifespan of animals, confounds functional behavioral readout of the graft benefits, and discourages long-term follow-up.

Methods

Neuron-restricted neural progenitor cells (NPCs) were derived from human embryonic stem cells (ESCs, including H1, its gene-modified cell lines for better visualization, and HN4), propagated for different passages, and then transplanted into the brain of immunocompetent rats without immunosuppressants. The graft survivals, their cell fates, and HLA expression levels were examined over time (up to 4 months after transplantation). We compared the survival capability of NPCs from different passages and in different transplantation sites (intra-parenchyma vs. para- and intra-cerebroventricle). The host responses to the grafts were also investigated.

Results

Our results show that human ESC-derived neuron-restricted NPCs survive extendedly in adult rat brain parenchyma with no need of immunosuppression whereas a late-onset graft rejection seems inevitable. Both donor HLA antigens and host MHC-II expression level remain relatively low with little change over time and cannot predict the late-onset rejection. The intra-/para-cerebroventricular human grafts are more vulnerable to the immune attack than the intrastriatal counterparts. Prevention of graft hyperplasia by using hypoproliferative late passaged human NPCs further significantly extends the graft survival time. Our new data also shows that a subpopulation of host microglia upregulate MHC-II expression in response to the human graft, but fail to present the human antigen to the host immune system, suggestive of the immune-isolation role of the blood–brain barrier (BBB).

Conclusions

The present study confirms the “immune privilege” of the brain parenchyma and, more importantly, unveils that choosing hypoproliferative NPCs for transplantation can benefit graft outcome in terms of both lower tumor-genic risk and the prolonged survival time without immunosuppression.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02427-1.

Strategies for Cancer Immunotherapy Using Induced Pluripotency Stem Cells-Based Vaccines

Despite improvements in cancer therapy, metastatic solid tumors remain largely incurable. Immunotherapy has emerged as a pioneering and promising approach for cancer therapy and management, and in particular intended for advanced tumors unresponsive to current therapeutics. In cancer immunotherapy, components of the immune system are exploited to eliminate cancer cells and treat patients. The recent clinical successes of immune checkpoint blockade and chimeric antigen receptor T cell therapies represent a turning point in cancer treatment. Despite their potential success, current approaches depend on efficient tumor antigen presentation which are often inaccessible, and most tumors turn refractory to current immunotherapy. Patient-derived induced pluripotent stem cells (iPSCs) have been shown to share several characteristics with cancer (stem) cells (CSCs), eliciting a specific anti-tumoral response when injected in rodent cancer models. Indeed, artificial cellular reprogramming has been widely compared to the biogenesis of CSCs. Here, we will discuss the state-of-the-art on the potential implication of cellular reprogramming and iPSCs for the design of patient-specific immunotherapeutic strategies, debating the similarities between iPSCs and cancer cells and introducing potential strategies that could enhance the efficiency and therapeutic potential of iPSCs-based cancer vaccines.

Characterization of embryonic stem cell-differentiated fibroblasts as mesenchymal stem cells with robust expansion capacity and attenuated innate immunity

BACKGROUND

Mesenchymal stem cells (MSCs) isolated from adult tissues (Ad-MSCs) have shown great promise for use in regenerative medicine. However, their poor in vitro expansion capacity and tissue scarcity have been major limitations. In this study, we demonstrate that mouse embryonic stem cells (mESCs) can differentiate into cells with MSC properties.

METHODS

Using previously established methods that characterize Ad-MSCs, we analyzed mESC-differentiated fibroblasts (mESC-FBs), including plastic adherence, clonogenic growth, MSC marker expression, tri-lineage differentiation potential, and the capacity to express immunomodulators.

RESULTS

Although previously characterized as mESC-differentiated fibroblasts (mESC-FBs), these cells exhibit major properties of Ad-MSCs. However, mESC-FBs also display unique features inherited from ESCs, including robust expansion capacity, senescence resistance, and attenuated innate immunity. In particular, mESC-FBs are insensitive to bacterial endotoxin (lipopolysaccharide, LPS) and do not express LPS-induced inflammatory molecules, in contrast to bone marrow (BM)-MSCs. We further demonstrate that mESC-FBs are resistant to the cytotoxicity associated with inflammatory cytokines, bacterial endotoxins (LPS and heat-killed bacteria), and macrophage-mediated inflammation.

CONCLUSIONS

While it remains to be determined how the unique properties of mESC-FBs will affect their immunoregulatory activity under an in vivo condition, our findings demonstrate that ESCs could be used as an alternative source to generate a new class of ESC-MSCs with unique features potentially useful in regenerative medicine.

Recent progress of national banking project on homozygous HLA-typed induced pluripotent stem cells in South Korea

Induced pluripotent stem cells (iPSCs) can be generated by introducing several factors into mature somatic cells. Banking of iPSCs can lead to wider application for treatment and research. In an economical view, it is important to store cells that can cover a high percentage of the population. Therefore, the use of homozygous human leukocyte antigen-iPSCs (HLA-iPSCs) is thought as a potential candidate for effective iPSC banking system for further clinical use. We screened the database stored in the Catholic Hematopoietic Stem Cell Bank of Korea and sorted the most frequent homozygous HLA types of the South Korean population. Blood cells with the selected homozygous HLA types were obtained and transferred to the GMP facility in the Catholic Institute of Cell Therapy. Cells were reprogrammed to iPSCs inside the facility and went through several quality controls. As a result, a total of 13 homozygous GMP-grade iPSC lines were obtained in the facility. The generated iPSCs showed high pluripotency and normal karyotype after reprogramming. Five HLA-homozygous iPSCs had the type that was included in the top five most frequent HLA types. Homozygous HLA-iPSCs can open a new opportunity for further application of iPSCs in clinical research and therapy.

Comparative global immune-related gene profiling of somatic cells, human pluripotent stem cells and their derivatives: implication for human lymphocyte proliferation

Human pluripotent stem cells (hPSCs), including embryonic stem cells (ESCs) and induced PSCs (iPSCs), represent potentially unlimited cell sources for clinical applications. Previous studies have suggested that hPSCs may benefit from immune privilege and limited immunogenicity, as reflected by the reduced expression of major histocompatibility complex class-related molecules. Here we investigated the global immune-related gene expression profiles of human ESCs, hiPSCs and somatic cells and identified candidate immune-related genes that may alter their immunogenicity. The expression levels of global immune-related genes were determined by comparing undifferentiated and differentiated stem cells and three types of human somatic cells: dermal papilla cells, ovarian granulosa cells and foreskin fibroblast cells. We identified the differentially expressed genes CD24, GATA3, PROM1, THBS2, LY96, IFIT3, CXCR4, IL1R1, FGFR3, IDO1 and KDR, which overlapped with selected immune-related gene lists. In further analyses, mammalian target of rapamycin complex (mTOR) signaling was investigated in the differentiated stem cells following treatment with rapamycin and lentiviral transduction with specific short-hairpin RNAs. We found that the inhibition of mTOR signal pathways significantly downregulated the immunogenicity of differentiated stem cells. We also tested the immune responses induced in differentiated stem cells by mixed lymphocyte reactions. We found that CD24- and GATA3-deficient differentiated stem cells including neural lineage cells had limited abilities to activate human lymphocytes. By analyzing the transcriptome signature of immune-related genes, we observed a tendency of the hPSCs to differentiate toward an immune cell phenotype. Taken together, these data identify candidate immune-related genes that might constitute valuable targets for clinical applications.

Precise immune tolerance for hPSC derivatives in clinical application

Human pluripotent stem cells (hPSCs) promise a foreseeing future for regeneration medicine and cell replacement therapy with their abilities to produce almost any types of somatic cells of the body. The complicated immunogenicity of hPSC derivatives and context dependent responses in variable transplantations greatly hurdle the practical application of hPSCs in clinic. Especially for applications of hPSCs, induction of immune tolerance at the same time increases the risks of tumorigenesis. Over the past few years, thanks to the progress in immunology and practices in organ transplantation, endeavors on exploring strategies to induce long term protection of allogeneic transplants have shed light on overcoming this barrier. Novel genetic engineering techniques also allow to precisely cradle the immune response of transplantation. Here we reviewed the current understanding on immunogenicity, and efforts have been attempted on inducing immune tolerance for hPSC derivatives, with extra focus on modifying the graft cells. We also glimpse on employing cutting-edge genome editing technologies for this purpose, which will potentially endow hPSC derivatives with the nature of wide spectrum drugs for therapy.

Inherent Immunogenicity or Lack Thereof of Pluripotent Stem Cells: Implications for Cell Replacement Therapy

Donor-specific induced pluripotent stem cells (iPSCs) offer opportunities for personalized cell replacement therapeutic approaches due to their unlimited self-renewal potential and ability to differentiate into different somatic cells. A significant progress has been made toward generating iPSC lines that are free of integrating viral vectors, development of xeno-free culture conditions, and differentiation of pluripotent stem cells (PSCs) into functional somatic cell lineages. Since donor-specific iPSC lines are genetically identical to the individual, they are expected to be immunologically matched and these iPSC lines and their cellular derivatives are not expected to be immunologically rejected. However, studies in mouse models, utilizing rejection of teratomas as a model, have claimed that syngenic iPSC lines, especially the iPSC lines derived with integrating viral vectors, could be inherently immunogenic. This manuscript reviews current understanding of inherent immunogenicity of PSC lines, especially that of the human iPSC lines and their cellular derivatives, and strategies to overcome it.

Attenuated Innate Immunity in Embryonic Stem Cells and Its Implications in Developmental Biology and Regenerative Medicine

Embryonic stem cells (ESCs) represent a promising cell source for regenerative medicine. Intensive research over the past 2 decades has led to the feasibility of using ESC-differentiated cells (ESC-DCs) in regenerative medicine. However, increasing evidence indicates that ESC-DCs generated by current differentiation methods may not have equivalent cellular functions to their in vivo counterparts. Recent studies have revealed that both human and mouse ESCs as well as some types of ESC-DCs lack or have attenuated innate immune responses to a wide range of infectious agents. These findings raise important concerns for their therapeutic applications since ESC-DCs, when implanted to a wound site of a patient, where they would likely be exposed to pathogens and inflammatory cytokines. Understanding whether an attenuated immune response is beneficial or harmful to the interaction between host and grafted cells becomes an important issue for ESC-based therapy. A substantial amount of recent evidence has demonstrated that the lack of innate antiviral responses is a common feature to ESCs and other types of pluripotent cells. This has led to the hypothesis that mammals may have adapted different antiviral mechanisms at different stages of organismal development. The underdeveloped innate immunity represents a unique and uncharacterized property of ESCs that may have important implications in developmental biology, immunology, and in regenerative medicine.

Pluripotent stem cells and tolerance induction in organ transplantation

PURPOSE OF REVIEW

Ongoing research is constantly looking for means to modulate the immune system for long-lasting engraftment of pluripotent stem cells (PSC) during stem cell-based therapies. This study reviews data on in-vitro and in-vivo immunogenicity of embryonic and induced-PSC and describes how their immunological properties can be harnessed for tolerance induction in organ transplantation.

RECENT FINDINGS

Although PSC display immunomodulatory properties in vitro, they are capable of eliciting an immune response that leads to cell rejection when transplanted into immune-competent recipients. Nevertheless, long-term acceptance of PSC-derived cells/tissues in an allogeneic environment can be achieved using minimal host conditioning. Protocols for differentiating PSC towards haematopoietic stem cells, thymic epithelial precursors, dendritic cells, regulatory T cells and myeloid-derived suppressor cells are being developed, suggesting the possibility to use PSC-derived immunomodulatory cells to induce tolerance to a solid organ transplant.

SUMMARY

PSC and/or their derivatives possess unique immunological properties that allow for acceptance of PSC-derived tissue with minimal host conditioning. Investigators involved either in regenerative or in transplant medicine must join their efforts with the ultimate aim of using PSC as a source of donor-specific cells that would create a protolerogenic environment to achieve tolerance in solid organ transplantation.

Human Induced Pluripotent Stem Cells Are Targets for Allogeneic and Autologous Natural Killer (NK) Cells and Killing Is Partly Mediated by the Activating NK Receptor DNAM-1

Human induced pluripotent stem cells (hiPSCs) could be used to generate autologous cells for therapeutic purposes, which are expected to be tolerated by the recipient. However, iPSC-derived grafts are at risk of giving rise to teratomas in the host, if residuals of tumorigenic cells are not rejected by the recipient. We have analyzed the susceptibility of hiPSC lines to allogeneic and autologous natural killer (NK) cells. IL-2-activated, in contrast to resting NK cells killed hiPSC lines efficiently (P = 1.69 x 10(-39)). Notably, the specific lysis of the individual hiPSC lines by IL-2-activated NK cells was significantly different (P = 1.72 x 10(-6)) and ranged between 46 % and 64 % in 51Cr-release assays when compared to K562 cells. The hiPSC lines were killed by both allogeneic and autologous NK cells although autologous NK cells were less efficient (P=8.63 x 10(-6)). Killing was partly dependent on the activating NK receptor DNAM-1 (P = 8.22 x 10(-7)). The DNAM-1 ligands CD112 and CD155 as well as the NKG2D ligands MICA and MICB were expressed on the hiPSC lines. Low amounts of human leukocyte antigen (HLA) class I proteins, which serve as ligands for inhibitory and activating NK receptors were also detected. Thus, the susceptibility to NK cell killing appears to constitute a common feature of hiPSCs. Therefore, NK cells might reduce the risk of teratoma formation even after autologous transplantations of pluripotent stem cell-derived grafts that contain traces of pluripotent cells.

Current proceedings of cerebral palsy

Cerebral palsy (CP) is a complicated disease with varying causes and outcomes. It has created significant burden to both affected families and societies, not to mention the quality of life of the patients themselves. There is no cure for the disease; therefore, development of effective therapeutic strategies is in great demand. Recent advances in regenerative medicine suggest that the transplantation of stem cells, including embryonic stem cells, neural stem cells, bone marrow mesenchymal stem cells, induced pluripotent stem cells, umbilical cord blood cells, and human embryonic germ cells, focusing on the root of the problem, may provide the possibility of developing a complete cure in treating CP. However, safety is the first factor to be considered because some stem cells may cause tumorigenesis. Additionally, more preclinical and clinical studies are needed to determine the type of cells, route of delivery, cell dose, timing of transplantation, and combinatorial strategies to achieve an optimal outcome.

Teratoma formation in immunocompetent mice after syngeneic and allogeneic implantation of germline capable mouse embryonic stem cells

BACKGROUND

Embryonic stem cells (ESCs) have the potential to form teratomas when implanted into immunodeficient mice, but data in immunocompetent mice are limited. We therefore investigated teratoma formation after implantation of three different mouse ESC (mESC) lines into immunocompetent mice.

MATERIALS AND METHODS

BALB/c mice were injected with three highly germline competent mESCs (129Sv, BALB/c and C57BL/6) subcutaneously or under the kidney capsule. After 4 weeks, mice were euthanized and examined histologically for teratoma development. The incidence, size and composition of teratomas were compared using Pearson Chi-square, t-test for dependent variables, one-way analysis of variance and the nonparametric Kruskal- Wallis analysis of variance and median test.

RESULTS

Teratomas developed from all three cell lines. The incidence of formation was significantly higher under the kidney capsule compared to subcutaneous site and occurred in both allogeneic and syngeneic mice. Overall, the size of teratoma was largest with the 129Sv cell line and under the kidney capsule. Diverse embryonic stem cell-derived tissues, belonging to the three embryonic germ layers, were encountered, reflecting the pluripotency of embryonic stem cells. Most commonly represented tissues were nervous tissue, keratinizing stratified squamous epithelium (ectoderm), smooth muscle, striated muscle, cartilage, bone (mesoderm), and glandular tissue in the form of gut- and respiratory-like epithelia (endoderm).

CONCLUSIONS

ESCs can form teratomas in immunocompetent mice and, therefore, removal of undifferentiated ESC is a pre-requisite for a safe use of ESC in cell-based therapies. In addition the genetic relationship of the origin of the cell lines to the ability to transplant plays a major role.

Stem cell imaging: from bench to bedside

Although cellular therapies hold great promise for the treatment of human disease, results from several initial clinical trials have not shown a level of efficacy required for their use as a first line therapy. Here we discuss how in vivo molecular imaging has helped identify barriers to clinical translation and potential strategies that may contribute to successful transplantation and improved outcomes, with a focus on cardiovascular and neurological diseases. We conclude with a perspective on the future role of molecular imaging in defining safety and efficacy for clinical implementation of stem cell therapies.

Equine mesenchymal stromal cells and embryo-derived stem cells are immune privileged in vitro

Introduction

Autologous mesenchymal stem cells (MSCs) are an attractive concept in regenerative medicine, but their mechanism of action remains poorly defined. No immune response is reported after in vivo injection of allogeneic equine MSCs or embryo-derived stem cells (ESCs) into the equine tendon, which may be due to the cells’ immune-privileged properties. This study further investigates these properties to determine their potential for clinical application in other tissues.

Methods

Mitomycin C-treated MSCs, ESCs, or differentiated ESCs (dESCs) were cultured with allogeneic equine peripheral blood mononuclear cells (PBMCs), and their effect on PBMC proliferation, in the presence or absence of interferon-gamma (IFN-γ) was determined. MSCs and super-antigen (sAg)-stimulated PBMCs were co-cultured directly or indirectly in transwells, and PBMC proliferation examined. Media from MSC culture were harvested and used for PBMC culture; subsequent PBMC proliferation and gene expression were evaluated and media assayed for IFN-γ, tumor necrosis factor alpha (TNF-α), and interleukin (IL)-10 and IL-6 proteins with enzyme-linked immunosorbent assay (ELISA).

Results

Co-culture of PBMCs with ESCs or dESCs did not affect baseline proliferation, whereas co-culture with MSCs significantly suppressed baseline proliferation. Stimulation of PBMC proliferation by using super-antigens (sAgs) was also suppressed by co-culture with MSCs. Inhibition was greatest with direct contact, but significant inhibition was produced in transwell culture and by using MSC-conditioned media, suggesting that soluble factors play a role in MSC-mediated immune suppression. The MSCs constitutively secrete IL-6, even in the absence of co-culture with PBMCs. MSC-conditioned media also brought about a change in the cytokine-expression profile of sAg-stimulated PBMCs, significantly reducing PBMC expression of IL-6, IFN-γ, and TNF-α.

Conclusions

Equine MSCs and ESCs possess a degree of innate immune privilege, and MSCs secrete soluble factors that suppress PBMC proliferation and alter cytokine expression. These properties may make possible the future clinical use of allogeneic stem cells to help standardize and broaden the scope of treatment of tissue injuries.

Development of Experimental Tumors Formed by Mouse and Human Embryonic Stem and Teratocarcinoma Cells after Subcutaneous and Intraperitoneal Transplantations into Immunodeficient and Immunocompetent Mice

Pluripotent stem cells represent an attractive cell source for regenerative medicine. However, the risk of teratoma formation after transplantation restricts their clinical application. Therefore, to adequately evaluate the potential risk of tumorigenicity after cell transplantation into human tissues, effective animal transplantation assays need to be developed. We performed a multiparameter (cell number, transplantation site, cell type, host) comparative analysis of the efficiency of tumor development after transplantation of mouse and human embryonic stem (ES) cells and their malignant counterparts, teratocarcinoma (EC) cells, into animal recipients and revealed several key correlations. We found that the efficiency of tumor growth was higher after intra-peritoneal than after subcutaneous transplantations of all cell lines studied. The minimal cell numbers sufficient for tumor growth in immunodeficient nude mice were 100-fold lower for intraperitoneal than for subcutaneous transplantations of mouse and human ES cells (103 vs. 105 and 104 vs. 106, respectively). Moreover, mouse ES and EC cells formed tumors in immunodeficient and immunocompetent mice more effectively than human ES and EC cells. After intraperitoneal transplantation of 103, 104, and 105 mouse ES cells, teratomas developed in 83%, 100%, and 100% of nude mice, whereas after human ES cell transplantation, teratomas developed in 0%, 17%, and 60%, respectively. In addition, malignant mouse and human EC cells initiated tumor growth after intraperitoneal transplantation significantly faster and more effectively than ES cells. Mouse and human ES cells formed different types of teratomas containing derivatives of three germ layers but different numbers of undifferentiated cells. ES cell-like sublines with differentiation potential similar to the parental cell line were recloned only from mouse, but not from human, ES cell teratomas. These findings provide new information about the possibility and efficiency of tumor growth after transplantation of pluripotent stem cells. This information allows one to predict and possibly prevent the possible risks of tumorigenicity that could arise from stem cell therapeutics.

Immunogenicity of embryonic stem cell-derived progenitors after transplantation

PURPOSE OF REVIEW

This review focuses on the immunogenicity of embryonic stem cell (ESC)-derived progenitors and the impact of the immune response on applications of cell replacement therapy (CRT). Possible strategies to induce immunological tolerance to ESC-derived progenitor cells will also be discussed.

RECENT FINDINGS

Evidence for the differential epigenetic control of major histocompatibility (MHC) and antigen processing molecules in ESCs and differentiated ESCs has recently been described. The presence of T cells recognizing the pluripotency-associated transcription factor octamer-binding transcription factor 4 (OCT4) in healthy patient-derived peripheral blood mononuclear cells adds further complexity to the immune response against ESCs and ESC-derived progenitors.

SUMMARY

Although ESCs and ESC-derived progenitors appear to exert some level of immune privilege in specific circumstances, these allogeneic cells are indeed recognized by the immune system and can be subject to mechanisms of rejection. Herein, we discuss the importance of the recent reports describing an immunosuppressive capacity of ESCs, and the epigenetic control of MHC in ESCs and how these characteristics may be harnessed in the development of strategies to induce immunological tolerance.

Immunogenicity of pluripotent stem cells and their derivatives

The ability of pluripotent stem cells to self-renew and differentiate into all somatic cell types brings great prospects to regenerative medicine and human health. However, before clinical applications, much translational research is necessary to ensure that their therapeutic progenies are functional and nontumorigenic, that they are stable and do not dedifferentiate, and that they do not elicit immune responses that could threaten their survival in vivo. For this, an in-depth understanding of their biology, genetic, and epigenetic make-up and of their antigenic repertoire is critical for predicting their immunogenicity and for developing strategies needed to assure successful long-term engraftment. Recently, the expectation that reprogrammed somatic cells would provide an autologous cell therapy for personalized medicine has been questioned. Induced pluripotent stem cells display several genetic and epigenetic abnormalities that could promote tumorigenicity and immunogenicity in vivo. Understanding the persistence and effects of these abnormalities in induced pluripotent stem cell derivatives is critical to allow clinicians to predict graft fate after transplantation, and to take requisite measures to prevent immune rejection. With clinical trials of pluripotent stem cell therapy on the horizon, the importance of understanding immunologic barriers and devising safe, effective strategies to bypass them is further underscored. This approach to overcome immunologic barriers to stem cell therapy can take advantage of the validated knowledge acquired from decades of hematopoietic stem cell transplantation.

Immunogenicity of in vitro maintained and matured populations: potential barriers to engraftment of human pluripotent stem cell derivatives

The potential to develop into any cell type makes human pluripotent stem cells (hPSCs) one of the most promising sources for regenerative treatments. Hurdles to their clinical applications include (1) formation of heterogeneously differentiated cultures, (2) the risk of teratoma formation from residual undifferentiated cells, and (3) immune rejection of engrafted cells. The recent production of human isogenic (genetically identical) induced PSCs (hiPSCs) has been proposed as a "solution" to the histocompatibility barrier. In theory, differentiated cells derived from patient-specific hiPSC lines should be histocompatible to their donor/recipient. However, propagation, maintenance, and non-physiologic differentiation of hPSCs in vitro may produce other, likely less powerful, immune responses. In light of recent progress towards the clinical application of hPSCs, this review focuses on two antigen presentation phenomena that may lead to rejection of isogenic hPSC derivates: namely, the expression of aberrant antigens as a result of long-term in vitro maintenance conditions or incomplete somatic cell reprogramming, and the unbalanced presentation of receptors and ligands involved in immune recognition due to accelerated differentiation. Finally, we discuss immunosuppressive approaches that could potentially address these immunological concerns.

Immune privilege of stem cells

Immune privilege provides protection to vital tissues or cells of the body when foreign antigens are introduced into these sites. The modern concept of relative immune privilege applies to a variety of tissues and anatomical structures, including the hair follicles and mucosal surfaces. Even sites of chronic inflammation and developing tumors may acquire immune privilege by recruiting immunoregulatory effector cells. Adult stem cells are no exception. For their importance and vitality, many adult stem cell populations are believed to be immune privileged. A preimplantation-stage embryo that derives from a totipotent stem cell (i.e., a fertilized oocyte) must be protected from maternal allo-rejection for successful implantation and development to occur. Embryonic stem cells, laboratory-derived cell lines of preimplantation blastocyst-origin, may, therefore, retain some of the immunological properties of the developing embryo. However, embryonic stem cells and their differentiated tissue derivatives transplanted into a recipient do not necessarily have an ability to subvert immune responses to the extent required to exploit their pluripotency for regenerative medicine. In this review, an extended definition of immune privilege is developed and the capacity of adult and embryonic stem cells to display both relative and acquired immune privilege is discussed. Furthermore, we explore how these intrinsic properties of stem cells may one day be harnessed for therapeutic gain.

Embryonic stem cell-derived factors inhibit T effector activation and induce T regulatory cells by suppressing PKC-θ activation

Embryonic stem cells (ESCs) possess immune privileged properties and have the capacity to modulate immune activation. However, the mechanisms by which ESCs inhibit immune activation remain mostly unknown. We have previously shown that ESC-derived factors block dendritic cell maturation, thereby indirectly affecting T cell activation. Here, we show that ESC-derived factors also directly affect T cell activation. We provide the first demonstration that ESC-derived factors significantly down-regulated the expressions of IL-2 and IFN-γ, while markedly up-regulating the expression of IL-10, TGF-β, and Treg transcription factor Foxp3 in CD4+ CD25+ T cells. Furthermore, ESC-derived factors robustly suppressed T cell proliferation in response to the protein kinase C-θ (PKC-θ) activator phorbol 12-myristate 13-acetate (PMA). Western blot analysis indicated that ESC-derived factors prevented PKC-θ phosphorylation without influencing total PKC-θ levels. Moreover, IκB-α degradation was abrogated, confirming absence of PKC-θ activity. The impact of ESC-derived factors on PKC-θ activation appeared to be specific since other upstream T cell signaling components were not affected. In conclusion, ESCs appear to directly impact T cell activation and polarization by negatively regulating the PKC-θ pathway.

In vitro and in vivo cardiomyogenic differentiation of amniotic fluid stem cells

Cell therapy has developed as a complementary treatment for myocardial regeneration. While both autologous and allogeneic uses have been advocated, the ideal candidate has not been identified yet. Amniotic fluid-derived stem (AFS) cells are potentially a promising resource for cell therapy and tissue engineering of myocardial injuries. However, no information is available regarding their use in an allogeneic context. c-kit-sorted, GFP-positive rat AFS (GFP-rAFS) cells and neonatal rat cardiomyocytes (rCMs) were characterized by cytocentrifugation and flow cytometry for the expression of mesenchymal, embryonic and cell lineage-specific antigens. The activation of the myocardial gene program in GFP-rAFS cells was induced by co-culture with rCMs. The stem cell differentiation was evaluated using immunofluorescence, RT-PCR and single cell electrophysiology. The in vivo potential of Endorem-labeled GFP-rAFS cells for myocardial repair was studied by transplantation in the heart of animals with ischemia/reperfusion injury (I/R), monitored by magnetic resonance imaging (MRI). Three weeks after injection a small number of GFP-rAFS cells acquired an endothelial or smooth muscle phenotype and to a lesser extent CMs. Despite the low GFP-rAFS cells count in the heart, there was still an improvement of ejection fraction as measured by MRI. rAFS cells have the in vitro propensity to acquire a cardiomyogenic phenotype and to preserve cardiac function, even if their potential may be limited by poor survival in an allogeneic setting.

Immunosuppressive mechanisms of embryonic stem cells and mesenchymal stem cells in alloimmune response

Although both embryonic stem cells (ESCs) and mesenchymal stem cells (MSCs) are known to have immunosuppressive effects, the mechanisms of immunosuppression are still controversial. Both types of stem cells suppressed not only the proliferation but also survival of CD4(+) T cells in vitro. They suppressed secretion of various cytokines (IL-2, IL-12, IFN-γ, TNF-α, IL-4, IL-5, IL-1β, and IL-10), whereas there was no change in the levels of TGF-β or IDO. Classic and modified transwell experiments demonstrated that immunosuppressive activities were mainly mediated by cell-to-cell contact. Granzyme B in the ESCs played a significant role in their immunosuppression, whereas PDL-1, Fas ligand, CD30 or perforin was not involved in the contact-dependent immunosuppression. However, none of the above molecules played a significant role in the immunosuppression by the MSCs. Interestingly, both stem cells increased the proportion of Foxp3(+) regulatory T cells. Our results showed that both ESCs and MSCs suppressed the survival as well as the proliferation of T cells by mainly contact-dependent mechanisms and increased the proportion of regulatory T cells. Granzyme B was involved in immunosuppression by the ESCs in a perforin-independent manner.

Potential barriers to therapeutics utilizing pluripotent cell derivatives: intrinsic immunogenicity of in vitro maintained and matured populations

The potential to develop into any tissue makes pluripotent stem cells (PSCs) one of the most promising sources for cellular therapeutics. However, numerous hurdles exist to their clinical applications, three of the most concerning include the inability to separate therapeutic population from heterogeneously differentiated cultures, the risk of teratoma formation from residual pluripotent cells, and immunologic rejection of engrafted cells. The recent development of induced PSCs has been proposed as a solution to the histocompatibility barrier. Theoretically, creation of patient-specific induced PSC lines would exhibit a complete histocompatibility antigen match. However, regardless of the PSC source, in vitro propagation and nonphysiologic differentiation may result in other, likely less powerful, mechanisms of immune rejection. In light of recent progress towards clinical application, this review focuses on two such potential immunologic mechanisms applicable to isogenic PSC derivates: namely, the immunogenicity of aberrant antigens resulting from long-term in vitro maintenance and alterations in immunologic properties due to rapid in vitro differentiation. These issues will be considered with attention to their relation to effector cells in the adult immune system. In addition, we highlight immunosuppressive approaches that could potentially address the immunogenicity of these proposed mechanisms.

Effects of histocompatibility and host immune responses on the tumorigenicity of pluripotent stem cells

Pluripotent stem cells hold great promises for regenerative medicine. They might become useful as a universal source for a battery of new cell replacement therapies. Among the major concerns for the clinical application of stem cell-derived grafts are the risks of immune rejection and tumor formation. Pluripotency and tumorigenicity are closely linked features of pluripotent stem cells. However, the capacity to form teratomas or other tumors is not sufficiently described by inherited features of a stem cell line or a stem cell-derived graft. The tumorigenicity always depends on the inability of the recipient to reject the tumorigenic cells. This review summarizes recent data on the tumorigenicity of pluripotent stem cells in immunodeficient, syngeneic, allogeneic, and xenogeneic hosts. The effects of immunosuppressive treatment and cell differentiation are discussed. Different immune effector mechanisms appear to be involved in the rejection of undifferentiated and differentiated cell populations. Elements of the innate immune system, such as natural killer cells and the complement system, which are active also in syngeneic recipients, appear to preferentially reject undifferentiated cells. This effect could reduce the risk of tumor formation in immunocompetent recipients. Cell differentiation apparently increases susceptibility to rejection by the adaptive immune system in allogeneic hosts. The current data suggest that the immune system of the recipient has a major impact on the outcome of pluripotent stem cell transplantation, whether it is rejection, engraftment, or tumor development. This has to be considered when the results of experimental transplantation models are interpreted and even more when translation into clinics is planned.

Embryonic stem cells, derived either after in vitro fertilization or nuclear transfer, prolong survival of semiallogeneic heart transplants

Tolerance induction toward allogeneic organ grafts represents one of the major aims of transplantation medicine. Stem cells are promising candidates for promoting donor-specific tolerance. In this study, we investigated the immunomodulatory properties of murine embryonic stem cells (ESCs), obtained either by in vitro fertilization (IVF-ESCs) or by nuclear transfer (NT-ESCs), in heart transplant mouse models. IVF-ESCs did not prolong the survival of fully allogeneic cardiac transplants but significantly prolonged the survival of semiallogeneic hearts from the same ESC donor strain for >100 d in 44% of the animals. However, 28% of transplanted animals infused with IVF-ESCs experienced development of a teratoma. NT-ESCs similarly prolonged semiallogeneic heart graft survival (>100 d in 40% of the animals) but were less teratogenic. By in vitro studies, IVF-ESC and NT-ESC immunoregulation was mediated both by cell contact-dependent mechanisms and by the release of soluble factors. By adding specific inhibitors, we identified PGE(2) as a soluble mediator of ESC immunoregulation. Expansion of regulatory T cells was found in lymphoid organs and in the grafts of IVF-ESC- and NT-ESC-tolerized mice. Our study demonstrates that both IVF-ESCs and NT-ESCs modulate recipient immune response toward tolerance to solid organ transplantation, and that NT-ESCs exhibit a lower tendency for teratoma formation. Because NT-ESCs are obtained by NT of a somatic cell from living individuals into an enucleated oocyte, they could represent a source of donor-derived stem cells to induce tolerance to solid organ allograft.

Human embryonic stem cell-extracts inhibit the differentiation and function of monocyte-derived dendritic cells

Embryonic stem cells (ESC) possess inherent properties of immune privilege with the capacity to evade allogeneic immune responses. Moreover, ESCs have been shown to prevent immune activation in response to third party antigen presenting cells in vitro and have the capacity to promote allograft survival in vivo. However, clinical use of human ESCs to treat immunological disorders may risk teratoma or ectopic tissue formation. Here, we show that cellular extracts from both human and mouse ESCs retain the immune modulatory properties of intact cells. ESC-extracts that contained 12-24 μg of total protein effectively prevented T cell proliferation in allogeneic mixed lymphocyte reactions (MLR), whereas control fibroblast extracts did not affect proliferation. Cellular mechanisms underlying hESC extract-mediated immune modulation involve the maturation of monocyte derived dendritic cells (mDC). hESC extract-treated mDCs had reduced surface expression of co-stimulatory and maturation markers CD80, HLA-DR and CD83 and secreted lower levels of IL12p40. Accordingly, hESC extract-treated DCs were found to be poor stimulators of purified allogeneic T cells compared to those DCs treated with vehicle or fibroblast extracts. Our results demonstrate that ESC extracts retain the immune modulatory properties of ESCs and for the first time demonstrates that ESC derived factors can inhibit human mDC maturation and function.

Major histocompatibility complex-I expression on embryonic stem cell-derived vascular progenitor cells is critical for syngeneic transplant survival

Donor-recipient cell interactions are essential for functional engraftment after nonautologous cell transplantation. During this process, transplant engraftment is characterized and defined by interactions between transplanted cells with local and recruited inflammatory cells. The outcome of these interactions determines donor cell fate. Here, we provide evidence that lineage-committed embryonic stem cell (ESC)-derived vascular progenitor cells are the target of major histocompatibility complex (MHC) class I-dependent, natural killer (NK) cell-mediated elimination in vitro and in vivo. Treatment with interferon γ was found to significantly upregulate MHC class I expression on ESC-derived vascular progenitor cells, rendering them less susceptible to syngeneic NK cell-mediated killing in vitro and enhancing their survival and differentiation potential in vivo. Furthermore, in vivo ablation of NK cells led to enhanced progenitor cell survival after transplantation into a syngeneic murine ischemic hindlimb model, providing additional evidence that NK cells mediate ESC-derived progenitor cell transplant rejection. These data highlight the importance of recipient immune-donor cell interactions, and indicate a functional role for MHC-I antigen expression during successful ESC-derived syngeneic transplant engraftment.

Transgenic expression of P1A induced thymic tumor: a role for onco-fetal antigens in tumorigenesis

P1A is the first known tumor rejection antigen. It is expressed in embryonic stem cells and multiple tumors but is silent in adult tissues except for the testis and placenta. Therefore, P1A represents a prototype for onco-fetal antigens. To test the potential function of P1A in tumorigenesis, we used a transgenic mouse expressing P1A in lymphoid cells. We observed that immunodeficient host P1A transgenic mice developed thymic tumors after 7 months of age and had shorter survival rates compared to control groups. Most of the 7 examined tumors displayed B cell lineage markers. The P1A transgenic bone marrow cells had higher proliferation ability and more potential progenitors compared to control bone marrow cells. To our knowledge, our data provided the first example that onco-fetal antigen can promote tumorigenesis.

Approaches for immunological tolerance induction to stem cell-derived cell replacement therapies

The shortage of donors for organ transplantation and also to treat degenerative diseases has led to the development of the new field of regenerative medicine. One aim of this field, in addition to in vivo induction of endogenous tissue regeneration, is to utilize stem cells as a supplementary source of cells to repair or replace tissues or organs that have ceased to function owing to ageing or autoimmunity. Embryonic stem cells hold promise in this respect because of their developmental capacity to generate all tissues within the body. More recently, the discovery of induced pluripotent stem cells, somatic cells reprogrammed to a primitive embryonic-like state by the introduction of pluripotency factors, may also act as an important cell source for cell replacement therapy. However, before cell replacement therapy can become a reality, one must consider how to overcome the potential transplant rejection of stem cell-derived products. There are several potential ways to circumvent the hurdles presented by the immune system in this setting, not least the induction of immunological tolerance in the host. In this review, we consider this and other approaches for engendering acceptance of stem cell-derived tissues.

Characteristics of the early immune response following transplantation of mouse ES cell derived insulin-producing cell clusters

BACKGROUND

The fully differentiated progeny of ES cells (ESC) may eventually be used for cell replacement therapy (CRT). However, elements of the innate immune system may contribute to damage or destruction of these tissues when transplanted.

METHODOLOGY/PRINCIPAL FINDINGS

Herein, we assessed the hitherto ill-defined contribution of the early innate immune response in CRT after transplantation of either ESC derived insulin producing cell clusters (IPCCs) or adult pancreatic islets. Ingress of neutrophil or macrophage cells was noted immediately at the site of IPCC transplantation, but this infiltration was attenuated by day three. Gene profiling identified specific inflammatory cytokines and chemokines that were either absent or sharply reduced by three days after IPCC transplantation. Thus, IPCC transplantation provoked less of an early immune response than pancreatic islet transplantation.

CONCLUSIONS/SIGNIFICANCE

Our study offers insights into the characteristics of the immune response of an ESC derived tissue in the incipient stages following transplantation and suggests potential strategies to inhibit cell damage to ensure their long-term perpetuation and functionality in CRT.

The immunosuppressive effect of embryonic stem cells and mesenchymal stem cells on both primary and secondary alloimmune responses

Recently, both embryonic stem cells and mesenchymal stem cells have been demonstrated to have immunosuppressive effects. The purpose of this study was to elucidate whether the embryonic stem cells and/or mesenchymal stem cells modulate both primary and secondary alloimmune responses. Both stem cells suppressed in vitro proliferation and cytokine production in primary alloimmune responses. They also suppressed in vitro proliferation and cytokine production of the allosensitized CD44+ memory T cells. However, they failed to prolong skin graft survival across both a major mismatch barrier (BALB/C, C57BL6/J) and a minor mismatch barrier (male to female). In conclusion, both embryonic stem cells and mesenchymal stem cells can suppress secondary alloimmune response in vitro as well as primary alloimmune responses; however, neither embryonic stem cells nor mesenchymal stem cells suppressed allograft rejection in stringent murine skin transplantation models.

Roadblocks en route to the clinical application of induced pluripotent stem cells

Since the first studies of human embryonic stem cells (hESCs) and, more recently, human induced pluripotent stem cells (hiPSCs), the stem-cell field has been abuzz with the promise that these pluripotent populations will one day be a powerful therapeutic tool. Although it has been proposed that hiPSCs will supersede hESCs with respect to their research and/or clinical potential because of the ease of their derivation and the ability to create immunologically matched iPSCs for each individual patient, recent evidence suggests that iPSCs in fact have several underappreciated characteristics that might mean they are less suitable for clinical application. Continuing research is revealing the similarities, differences and deficiencies of various pluripotent stem-cell populations, and suggests that many years will pass before the clinical utility of hESCs and hiPSCs is realized. There are a plethora of ethical, logistical and technical roadblocks on the route to the clinical application of pluripotent stem cells, particularly of iPSCs. In this Essay, we discuss what we believe are important issues that should be considered when attempting to bring hiPSC-based technology to the clinic.

Human embryonic stem cells suppress T cell responses via arginase I-dependent mechanism

Human embryonic stem cells (hESCs) can proliferate extensively in culture and give rise to progeny of the three germ layers. Several reports suggested that mouse and hESCs may attenuate immune responses. In this study, we focused on the mechanism by which hESCs inhibit T cell responses. Using coculture experiments, we demonstrate that hESCs inhibit cytokine secretion and T cell proliferation in response to potent T cell activators. Furthermore, we show that hESCs downmodulate the TCR-associated CD3-zeta chain. These effects are maintained when hESCs are replaced by their conditioned media and can be restored by the addition of L-arginine to hESC-conditioned media or by treatment of hESCs with a specific arginase inhibitor. Moreover, we show arginase-I expression and activity in hESCs. We further demonstrate that mouse ESCs (mESCs) similarly inhibit T cell activation via arginase I, suggesting an evolutionary conserved mechanism of T cell suppression by ESCs. In addition, we demonstrate that arginase I expression is not limited to ESCs in culture, but can also be detected in the inner cell mass and the trophectoderm of preimplantation mouse embryos and hESC-derived trophectoderm cells. Finally, T cells infiltrating ESC-derived teratomas have significantly lower levels of CD3-zeta chain. Collectively, the data indicate a role for ESC-arginase I activity in the attenuation of T cell activation.

Multipotent adult germ-line stem cells, like other pluripotent stem cells, can be killed by cytotoxic T lymphocytes despite low expression of major histocompatibility complex class I molecules

Background

Multipotent adult germ-line stem cells (maGSCs) represent a new pluripotent cell type that can be derived without genetic manipulation from spermatogonial stem cells (SSCs) present in adult testis. Similarly to induced pluripotent stem cells (iPSCs), they could provide a source of cellular grafts for new transplantation therapies of a broad variety of diseases. To test whether these stem cells can be rejected by the recipients, we have analyzed whether maGSCs and iPSCs can become targets for cytotoxic T lymphocytes (CTL) or whether they are protected, as previously proposed for embryonic stem cells (ESCs).

Results

We have observed that maGSCs can be maintained in prolonged culture with or without leukemia inhibitory factor and/or feeder cells and still retain the capacity to form teratomas in immunodeficient recipients. They were, however, rejected in immunocompetent allogeneic recipients, and the immune response controlled teratoma growth. We analyzed the susceptibility of three maGSC lines to CTL in comparison to ESCs, iPSCs, and F9 teratocarcinoma cells. Major histocompatibility complex (MHC) class I molecules were not detectable by flow cytometry on these stem cell lines, apart from low levels on one maGSC line (maGSC Stra8 SSC5). However, using a quantitative real time PCR analysis H2K and B2m transcripts were detected in all pluripotent stem cell lines. All pluripotent stem cell lines were killed in a peptide-dependent manner by activated CTLs derived from T cell receptor transgenic OT-I mice after pulsing of the targets with the SIINFEKL peptide.

Conclusion

Pluripotent stem cells, including maGSCs, ESCs, and iPSCs can become targets for CTLs, even if the expression level of MHC class I molecules is below the detection limit of flow cytometry. Thus they are not protected against CTL-mediated cytotoxicity. Therefore, pluripotent cells might be rejected after transplantation by this mechanism if specific antigens are presented and if specific activated CTLs are present. Our results show that the adaptive immune system has in principle the capacity to kill pluripotent and teratoma forming stem cells. This finding might help to develop new strategies to increase the safety of future transplantations of in vitro differentiated cells by exploiting a selective immune response against contaminating undifferentiated cells.

Reviewers

This article was reviewed by Bhagirath Singh, Etienne Joly and Lutz Walter.

Intravenous mesenchymal stem cell therapy for traumatic brain injury

OBJECT

Cell therapy has shown preclinical promise in the treatment of many diseases, and its application is being translated to the clinical arena. Intravenous mesenchymal stem cell (MSC) therapy has been shown to improve functional recovery after traumatic brain injury (TBI). Herein, the authors report on their attempts to reproduce such observations, including detailed characterizations of the MSC population, non-bromodeoxyuridine-based cell labeling, macroscopic and microscopic cell tracking, quantification of cells traversing the pulmonary microvasculature, and well-validated measurement of motor and cognitive function recovery.

METHODS

Rat MSCs were isolated, expanded in vitro, immunophenotyped, and labeled. Four million MSCs were intravenously infused into Sprague-Dawley rats 24 hours after receiving a moderate, unilateral controlled cortical impact TBI. Infrared macroscopic cell tracking was used to identify cell distribution. Immunohistochemical analysis of brain and lung tissues 48 hours and 2 weeks postinfusion revealed transplanted cells in these locations, and these cells were quantified. Intraarterial blood sampling and flow cytometry were used to quantify the number of transplanted cells reaching the arterial circulation. Motor and cognitive behavioral testing was performed to evaluate functional recovery.

RESULTS

At 48 hours post-MSC infusion, the majority of cells were localized to the lungs. Between 1.5 and 3.7% of the infused cells were estimated to traverse the lungs and reach the arterial circulation, 0.295% reached the carotid artery, and a very small percentage reached the cerebral parenchyma (0.0005%) and remained there. Almost no cells were identified in the brain tissue at 2 weeks postinfusion. No motor or cognitive functional improvements in recovery were identified.

CONCLUSIONS

The intravenous infusion of MSCs appeared neither to result in significant acute or prolonged cerebral engraftment of cells nor to modify the recovery of motor or cognitive function. Less than 4% of the infused cells were likely to traverse the pulmonary microvasculature and reach the arterial circulation, a phenomenon termed the "pulmonary first-pass effect," which may limit the efficacy of this therapeutic approach. The data in this study contradict the findings of previous reports and highlight the potential shortcomings of acute, single-dose, intravenous MSC therapy for TBI.

Treatment of Parkinson's disease model mice with allogeneic embryonic stem cells: necessity of immunosuppressive treatment for sustained improvement

OBJECTIVE

The purpose of the present study was to examine the efficacy of transplantation of mouse embryonic stem (ES) into Parkinson's disease (PD) model mice as well as the necessity of immunosuppression in allogeneic donor-host combinations.

MATERIALS AND METHODS

ES cells, derived from SvJ129 strain mice, were differentiated into tyrosine hydroxylase (TH)-positive neurons in vitro by an embryoid body (EB)-based multistep differentiation method and used as graft cells for PD mice, which were prepared by injection of 6-hydroxydopamine (OHDA) into C57BL/6, BALB/c and C3H/HeN strains. Mice from each strain were divided into Groups 1-3. Four weeks after the 6-OHDA injection, Group 1 received phosphate-buffered saline in the striatum wounds, while Group 2 received 2 x 10(4) graft cells, and Group 3 mice received 2 x 10(4) graft cells and were also treated with cyclosporine A.

RESULTS

Apomorphine-induced rotational behavior was improved in Groups 2 and 3, but not in Group 1. However, the behavioral improvement ceased later in Group 2, whereas sustained improvement was observed in Group 3 throughout the 8 week observation period after transplantation. ES-derived TH(+) cells were found at the grafted sites at the end of the experiment in Groups 2 and 3, and tended to be more abundant in Group 3.

CONCLUSION

Intra-striatum transplantation of ES-derived dopaminergic neurons was effective in treating PD mice, even in allogeneic donor-host combinations. Immunosuppressive treatment did not have an effect on initial behavioral restoration after transplantation; however, it was necessary for sustained improvement over a prolonged period.

Variation in MHC expression between undifferentiated mouse ES cells and ES cell-derived insulin-producing cell clusters

BACKGROUND

The progeny of embryonic stem (ES) cells may eventually be used to replace damaged tissues in transplantation, yet their immunogenicity remains ill-defined. The major histocompatibility complex (MHC) is a determinant of immunogenicity in transplantation.

METHODS AND RESULTS

Herein, we show differences in MHC expression between mouse ES cells and ES cell derived insulin producing cell clusters (IPCCs), including a relatively higher expression of MHC Class I in IPCCs and a faster, more dramatic induction of MHC Class I in IPCCs following challenge with interferon-gamma (IFN-gamma). MHC Class II was induced on IPCCs, but not ES cells, after exposure to IFN-gamma. Transplantation of syngeneic or allogeneic IPCCs was insufficient to trigger up-regulation of MHC class I within three days after transplantation.

DISCUSSION

These data highlight differences in MHC expression between ES cells and a fully differentiated ES cell derived tissue and suggest how the progeny of ES cells may be susceptible to rejection after transplantation.

Gender-dependent survival of allogeneic trophoblast stem cells in liver

In view of the well-known phenomenon of trophoblast immune privilege, trophoblast stem cells (TSCs) might be expected to be immune privileged, which could be of interest for cell or gene therapies. Yet in the ectopic sites tested so far, TSC transplants fail to show noticeable immune privilege and seem to lack physiological support. However, we show here that after portal venous injection, green fluorescent protein (GFP)-labeled TSCs survive for several months in the livers of allogeneic female but not male mice. Gonadectomy experiments revealed that this survival does not require the presence of ovarian hormones but does require the absence of testicular factors. By contrast, GFP-labeled allogeneic embryonic stem cells (ESCs) are reliably rejected; however, these same ESCs survive when mixed with unlabeled TSCs. The protective effect does not require immunological compatibility between ESCs and TSCs. Tumors were not observed in animals with either successfully engrafted TSCs or coinjected ESCs. We conclude that in a suitable hormonal context and location, ectopic TSCs can exhibit and confer immune privilege. These findings suggest applications in cell and gene therapy as well as a new model for studying trophoblast immunology and physiology.

Human embryonic stem cells hemangioblast express HLA-antigens

Background

It has been suggested that the initial differentiation of endothelial and hematopoietic cells during embryogenesis occurs from a common progenitor, called hemangioblast (hB). We hypothesized that these cells with dual hematopoietic/endothelial potential could be used in future regenerative medicine.

Methods

We used the two-step differentiation technology to generate bipotential blast cells from human embryonic stem cells (hES). This involved short differentiation in our in vitro EB system followed by differentiation in semisolid culture medium supplemented with mixture of cytokines.

Results

The occurrence of blast-colony-forming cells (BL-CFC) during EB differentiation (day 0–6) was transient and peaked on day 3. The emergence of this event was associated with expression of mesoderm gene T, and inversely correlated with expression of endoderm gene FoxA2. Similarly, the highest BL-CFC number was associated with increase in expression of early hematopoietic/endothelial genes: CD34, CD31 and KDR. The derived colonies were composed of 30–50 blast cells on day 6 in culture. These cells had homogenous appearance in Wright-Giemsa stain, but to a different extent expressed markers of immature hematopoietic and endothelial cells (CD31, CD34, VE-cadherin, Flt-1) and mature differentiated cells (CD45, CD33, CD146). We found that some of them expressed fetal and embryonic globin genes. Interestingly, these cells expressed also HLA class I molecules, however at very low levels compared to endothelial and hematopoietic cells. The blast cells could be successfully differentiated to hematopoietic cells in a CFU assay. In these conditions, blast cells formed CFU-M colonies (63.4 ± 0.8%) containing macrophages, BFU-E colonies (19.5 ± 3.5%) containing nucleated red blood cells, and CFU-EM colonies (17.1 ± 2.7%) composed of macrophages and nucleated erythrocytes. Cells of CFU-EM and BFU-E colonies expressed both ε – and γ- globin genes, but not adult-type γ-globin. When in endothelial cell culture conditions, blast cells differentiated to endothelial cells which had the ability to take up Dil-Ac-LDL and to form complex vascular networks in Matrigel.

Conclusion

1) Hematoendothelial precursors exist transiently in early embryonic development and form single cell-derived colonies; 2) their differentiation can be tracked by the use of chosen molecular markers; 3) blast colonies consist of cells having properties of endothelial and hematopoietic precursors, however the issue of their ability to maintain dual properties over time needs to be further explored; 4) blast cells can potentially be used in regenerative medicine due to their low expression of HLA molecules.

In vivo imaging of embryonic stem cells reveals patterns of survival and immune rejection following transplantation

Embryonic stem cell (ESC)-based transplantation is considered a promising novel therapy for a variety of diseases. This is bolstered by the suggested immune-privileged properties of ESCs. In this study, we used in vivo bioluminescent imaging (BLI) to non-invasively track the fate of transplanted murine ESCs (mESCs), which are stably transduced with a double fusion reporter gene consisting of firefly luciferase (FLuc) and enhanced green fluorescent protein (eGFP). Following syngeneic intramuscular transplantation of 1 x 10(6) mESCs, the cells survived and differentiated into teratomas. In contrast, allogeneic mESC transplants were infiltrated by a variety of inflammatory cells, leading to rejection within 28 days. Acceleration of rejection was observed when mESCs were allotransplanted following prior sensitization of the host. Finally, we demonstrate that the mESC derivatives were more rapidly rejected compared to undifferentiated mESCs. These data show that mESCs do not retain immune-privileged properties in vivo and are subject to immunological rejection as assessed by novel molecular imaging approaches.

Expression of major histocompatibility complex class I proteins and their antigen processing chaperones in mouse embryonic stem cells from fertilized and parthenogenetic embryos

Embryonic stem (ES) cells are pluripotent cells with the potential to differentiate into cells or tissues that may be used for transplantation therapy. Parthenogenetic ES (pES) cells have been recently derived from both mouse and human oocytes and hold promise as a cell source that is histocompatible to the oocyte donor. Because of the importance of major histocompatibility complex (MHC) antigens in mediating tissue rejection or acceptance, we examined levels of mRNA and protein expression of MHC class I proteins, as well as several MHC class I antigen processing and presentation chaperones in mouse ES cells derived from both fertilized (fES) and parthenogenetic (pES) embryos. We found that H-2K, Qa-2, TAP1, TAP2, and tapasin mRNAs were all expressed at low levels in undifferentiated and differentiating ES cells and were significantly upregulated in response to interferon-gamma (IFN-gamma) treatment following 14 days of differentiation. Likewise, expression of H-2K(b) and H-2K(k) proteins were upregulated to detectable levels by IFN-gamma after 14 days of differentiation, but Qa-2 protein expression remained low or absent. We also found that MHC class I, TAP1, TAP2, and tapasin mRNAs were all expressed at very low levels in ES cells compared with T cells, suggesting transcriptional regulation of these genes in ES cells. Calnexin, a chaperone molecule involved in other pathways than MHC expression, had mRNA levels that were similar in ES cells and T cells and was not upregulated by IFN-gamma in ES cells. Overall, ES cells derived from fertilized embryos and parthenogenetic embryos displayed remarkably similar patterns of gene expression at the mRNA and protein levels. The similarity between the fES and pES cell lines with regard to expression of MHC class I and antigen-processing machinery provides evidence for the potential usefulness of pES cells in transplantation therapy.

Immune or inflammatory response by the host brain suppresses neuronal differentiation of transplanted ES cell-derived neural precursor cells

Embryonic stem (ES) cells are a promising donor source for transplantation therapy, but several problems must be solved before they can be clinically useful. One of these is the host immune reaction to allogeneic grafts. In this article, we examine the effect of the host immune reaction on survival and differentiation of grafted ES cell-derived neural precursor cells (NPCs). We induced NPCs from mouse ES cells by stromal cell-derived inducing activity and then transplanted them into mouse brains with or without administering the immunosuppressant cyclosporine A (CsA). Two and 8 weeks following transplantation, the accumulation of host-derived microglia/macrophages and lymphocytes was observed around the graft. This effect was reduced by CsA treatment, although no significant difference in graft volume was observed. These data suggest that an immune response occurs in allografts of ES cell-derived NPCs. Intriguingly, however, the ratio of neurons to astrocytes in the graft was higher in immunosuppressed mice. Because inflammatory or immune cells produce various cytokines, we examined the effect of IL-1beta, IL-6, IFN-gamma, and TNF-alpha on the differentiation of NPCs in vitro. Only IL-6 promoted glial cell fate, and this effect could be reversed by the addition of an IL-6 neutralizing antibody. These results suggest that allogeneic ES cell-derived NPCs can cause an immune response by the host brain, but it is not strong enough to reject the graft. More important, activated microglia and lymphocytes can suppress neuronal differentiation of grafted NPCs in vivo by producing cytokines such as IL-6.

The tumorigenicity of mouse embryonic stem cells and in vitro differentiated neuronal cells is controlled by the recipients' immune response

Embryonic stem (ES) cells have the potential to differentiate into all cell types and are considered as a valuable source of cells for transplantation therapies. A critical issue, however, is the risk of teratoma formation after transplantation. The effect of the immune response on the tumorigenicity of transplanted cells is poorly understood. We have systematically compared the tumorigenicity of mouse ES cells and in vitro differentiated neuronal cells in various recipients. Subcutaneous injection of 1×10⁶ ES or differentiated cells into syngeneic or allogeneic immunodeficient mice resulted in teratomas in about 95% of the recipients. Both cell types did not give rise to tumors in immunocompetent allogeneic mice or xenogeneic rats. However, in 61% of cyclosporine A-treated rats teratomas developed after injection of differentiated cells. Undifferentiated ES cells did not give rise to tumors in these rats. ES cells turned out to be highly susceptible to killing by rat natural killer (NK) cells due to the expression of ligands of the activating NK receptor NKG2D on ES cells. These ligands were down-regulated on differentiated cells. The activity of NK cells which is not suppressed by cyclosporine A might contribute to the prevention of teratomas after injection of ES cells but not after inoculation of differentiated cells. These findings clearly point to the importance of the immune response in this process. Interestingly, the differentiated cells must contain a tumorigenic cell population that is not present among ES cells and which might be resistant to NK cell-mediated killing.

Immunological barriers to embryonic stem cell-derived therapies

Replacement of diseased tissues with healthy cells derived from embryonic stem (ES) cells has a potential to become, in the future, a better alternative to current treatments of a number of conditions characterized by irreversible tissue injury, such as heart and liver failure, diabetes mellitus and neurodegeneration. However, several obstacles have to be overcome before this new therapeutic modality becomes part of a standard clinical practice. First of all, ethical and safety issues have to be resolved, the methodologies must be developed to enable obtaining sufficient amounts of differentiated cells, and the immune rejection of allogeneic cells must be prevented in order to ensure their long-term engraftment and function. Data on immunological properties of human and murine ES cells and their differentiated derivatives are controversial, ranging from those claiming unique immune-privileged properties for ES cells to those which refute these conclusions. This indicates that much more research is required to definitively understand the immunological features and engraftment capacity of ES cell derivatives. We review here the current state of the art in this new and exciting field of ES cell immunology and discuss the implications of these findings for the development of ES cell-based therapies.