Characterisation of the immune response in a neural xenograft rejection paradigm

Stem Cell Therapies for Restorative Treatments of Central Nervous System Ischemia-Reperfusion Injury

Ischemic damage to the central nervous system (CNS) is a catastrophic postoperative complication of aortic occlusion subsequent to cardiovascular surgery that can cause brain impairment and sometimes even paraplegia. Over recent years, numerous studies have investigated techniques for protecting and revascularizing the nervous system during intraoperative ischemia; however, owing to a lack of knowledge of the physiological distinctions between the brain and spinal cord, as well as the limited availability of testing techniques and treatments for ischemia-reperfusion injury, the cause of brain and spinal cord ischemia-reperfusion injury remains poorly understood, and no adequate response steps are currently available in the clinic. Given the limited ability of the CNS to repair itself, it is of great clinical value to make full use of the proliferative and differentiation potential of stem cells to repair nerves in degenerated and necrotic regions by stem cell transplantation or mobilization, thereby introducing a novel concept for the treatment of severe CNS ischemia-reperfusion injury. This review summarizes the most recent advances in stem cell therapy for ischemia-reperfusion injury in the brain and spinal cord, aiming to advance basic research and the clinical use of stem cell therapy as a promising treatment for this condition.

Stem cell transplantation therapy for multifaceted therapeutic benefits after stroke

One of the exciting advances in modern medicine and life science is cell-based neurovascular regeneration of damaged brain tissues and repair of neuronal structures. The progress in stem cell biology and creation of adult induced pluripotent stem (iPS) cells has significantly improved basic and pre-clinical research in disease mechanisms and generated enthusiasm for potential applications in the treatment of central nervous system (CNS) diseases including stroke. Endogenous neural stem cells and cultured stem cells are capable of self-renewal and give rise to virtually all types of cells essential for the makeup of neuronal structures. Meanwhile, stem cells and neural progenitor cells are well-known for their potential for trophic support after transplantation into the ischemic brain. Thus, stem cell-based therapies provide an attractive future for protecting and repairing damaged brain tissues after injury and in various disease states. Moreover, basic research on naïve and differentiated stem cells including iPS cells has markedly improved our understanding of cellular and molecular mechanisms of neurological disorders, and provides a platform for the discovery of novel drug targets. The latest advances indicate that combinatorial approaches using cell based therapy with additional treatments such as protective reagents, preconditioning strategies and rehabilitation therapy can significantly improve therapeutic benefits. In this review, we will discuss the characteristics of cell therapy in different ischemic models and the application of stem cells and progenitor cells as regenerative medicine for the treatment of stroke.

Remyelination of Demyelinated CNS Axons by Transplanted Human Schwann Cells: The Deleterious Effect of Contaminating Fibroblasts

Areas of demyelination can be remyelinated by transplanting myelin-forming cells. Schwann cells are the naturally remyelinating cells of the peripheral nervous system and have a number of features that may make them attractive for cell implantation therapies in multiple sclerosis, in which spontaneous but limited Schwann cell remyelination has been well documented. Schwann cells can be expanded in vitro, potentially affording the opportunity of autologous transplantation; and they might also be spared the demyelinating process in multiple sclerosis. Although rat, cat, and monkey Schwann cells have been transplanted into rodent demyelinating lesions, the behavior of transplanted human Schwann cells has not been evaluated. In this study we examined the consequences of injecting human Schwann cells into areas of acute demyelination in the spinal cords of adult rats. We found that transplants containing significant fibroblast contamination resulted in deposition of large amounts of collagen and extensive axonal degeneration. However, Schwann cell preparations that had been purified by positive immunoselection using antibodies to human low-affinity nerve growth factor receptor containing less than 10% fibroblasts were associated with remyelination. This result indicates that fibroblast contamination of human Schwann cells represents a greater problem than would have been appreciated from previous studies.

Intracerebellar transplantation of neural stem cells into mice with neurodegeneration improves neuronal networks with functional synaptic transmission

Recent studies have shown that many kinds of stem cells are beneficial for patients suffering with neurodegenerative diseases. We investigated the effects of neural stem cell (NSC), Maudsley hippocampal clone 36 (MHP36) in the Niemann-Pick disease type C (NP-C) model mice. Herein, we demonstrate that MHP36 transplantation improves the neuropathological features without acute immune response and promotes neuronal networks with functional synaptic transmission. The number of surviving Purkinje neurons substantially increased in MHP36 transplanted NP-C mice compared with sham-transplanted NP-C mice. MHP36 significantly reduced both of astrocytic and microglial activations. We also found that these surviving Purkinje neurons have normal functional synapses with parallel fibers that have normal glutamate release probability in MHP36 transplanted NP-C mice. Furthermore, real-time PCR analysis revealed up-regulation of genes involved in both excitatory and inhibitory neurotransmission encoding subunits of the ionotropic glutamate receptors GluR2, 3 and GABAA receptor beta2. These findings suggest that NSC, MHP36 transplantation may have therapeutic effects in the treatment of NP-C and other neurodegenerative diseases.

Expression and regulation of major histocompatibility complex on neural stem cells and their lineages

The expression of major histocompatibility complex (MHC) antigens on neural stem cells (NSCs) and their lineages is tightly related to the fate of these cells as grafts in allogenic transplantation. In this study, we observed that NSCs derived from embryonic rat forebrain expressed MHC class I and class II molecules at a low level, whereas the cells differentiated from NSCs, including neurons, astrocytes, and oligodendrocytes, lost their MHC expression. However, a proinflammatory factor, interferon-gamma (IFN-gamma), could induce and up-regulate the expression of MHC in both NSCs and their differentiated lineages in vitro. These results suggest that predifferentiating NSCs into lineage-limited cells prior to transplantation combined with controlling the local production of proinflammatory cytokines moderately may potentially benefit the survival of transplants.

Neuroretinal xenotransplantation to immunocompetent hosts in a discordant species combination

In spite of its immune privileged state, xenotransplantation within the CNS is associated with rapid graft destruction in immunocompetent hosts. Efforts to enhance graft survival have mostly focused on host immune response, whereas relatively little attention has been paid to donor tissue characteristics. In the present paper, we explore long-term survival of xenogeneic full-thickness neuroretinal transplants in immunocompetent hosts and investigate the significance of tissue integrity in relation to graft survival. Adult rabbits receiving no immunosuppression were used as hosts and fetal Sprague-Dawley rat neuroretina as donors. Using vitreoretinal surgical techniques, rabbits received either a full thickness or a fragmented neuroretinal graft to the subretinal space of one eye. Eyes receiving full-thickness grafts were examined morphologically after 91 days and fragmented grafts after 7-14 days. Surviving full thickness grafts were found in six of eight eyes, four of which displayed the normal laminated appearance. Major histocompatibility complex (MHC) up-regulation in surviving grafts was minimal and they contained a well-organized photoreceptor layer, protein kinase C (PKC) labeled rod bipolar cells, parvalbumin labeled AII amacrine cells and glial fibrillary acidic protein (GFAP) labeled Müller cells. Fragmented grafts (n=6) were all destroyed or showed severe signs of rejection. A mass of inflammatory cells derived from the choroid was evident in these specimens, and no labeling of retina-specific cells was seen. We conclude that full-thickness rat neuroretina can survive for several months after subretinal transplantation to the subretinal space of immunocompetent rabbits, while fragmented counterparts are rapidly rejected. Surviving full-thickness grafts can develop many of the normal retinal morphological characteristics, indicating a thriving relationship between the initially immature donor tissue and its foreign host. Our results strongly indicate that donor tissue integrity is a crucial factor for graft survival in CNS xenotransplantation.

Cyclosporine affects the proliferation and differentiation of neural stem cells in culture

Cyclosporine is one of the foremost immunosuppressive agents for cell, tissue, and organ transplantation. Cyclosporine is, however, associated with significant side effects in the host, and may also affect the fate of the donor cells. This study was performed to test whether cyclosporine may change the fate of neural stem cells, as neural stem cell transplant has become a potential treatment for neurological disorders and damage. Results of this study showed that cyclosporine inhibited the proliferation significantly in a dosage-dependent manner. Cyclosporine also affected the differentiation of neural stem cells, which mainly increased astrocyte genesis and decreased neuron differentiation.

Neural stem cell transplant survival in brains of mice: assessing the effect of immunity and ischemia by using real-time bioluminescent imaging

PURPOSE

To use bioluminescent imaging in a murine transplant model to monitor the in vivo responses of transplanted luciferase-gene-positive neural progenitor cells (NPCs) to host immunity and ischemia.

MATERIALS AND METHODS

All animal studies were conducted according to institutional guidelines, with approval of the Subcommittee on Research Animal Care. Cranial windows were created in all animals, and all animals underwent NPC (C17.2-Luc-GFP-gal) transplantation into the right basal ganglia. An observational study was performed on C57 BL/6 (n = 5), nude (n = 4), and CD-1 (n = 4) mice, with bioluminescent imaging performed at days 7, 11, and 14 after transplantation. A study on the effects of ischemia was performed in a similar manner, but with the following differences: On day 9 after transplantation, the C57 BL/6 mice underwent 18 minutes of transient forebrain ischemia by means of temporary bilateral carotid occlusions (n = 6). A control group of C57 BL/6 mice underwent sham surgery (n = 6). Bioluminescent imaging was performed on the ischemic animals and control animals at days 7, 9, 11, and 14. Repeated-measures analysis of variance or Student t test was used to compare the means of the luciferase activities.

RESULTS

In vivo cell tracking demonstrated that (a) C17.2-Luc-GFP-gal NPCs survived and proliferated better in the T-cell deficient nude mice than in the immunocompetent C57 BL/6 or CD-1 mice, in which progressive immune mediated cell loss was shown, and (b) transient forebrain ischemia appeared, unexpectedly, to act as a short-term stimulus to transplanted NPC growth and survival in immunocompetent mice.

CONCLUSION

Immune status and host immunity can have an influence on NPC graft survival, and these changes can be noninvasively assessed with bioluminescent imaging in this experimental model.

[From bench to bedside: should we believe in the efficacy of stem cells in cerebral ischaemia?]

Stroke is the third cause of mortality and the leading cause of morbidity in industrialized countries. At the present time, ischaemic stroke is treated at the acute phase by thrombolysis with a recombinant of the tissular-plasminogen activator, which must be administered within the first 3 hours. Cell therapy, while using the self-renewal and differentiation potentials of stem cells, brings new hope for the long-term care of ischaemic stroke. Animal studies show that stem cells improve functional deficit without reduction of infarct volume and with very rare differentiation of the stem cell. These experimental studies suggest that stem cells would support cerebral plasticity via growth factor production and stimulation of endogenous mechanisms of local repair. Assessment of effectiveness and safety in the use of stem cells in cerebral ischaemia still require thorough investigation before clinical trials in humans can be developed.

MHC expression in a human adult stem cell line and its down-regulation by hCMV US gene transfection

Due to their unique capacity to self-renew and for multiple differentiation, stem cells are considered promising candidates for cell replacement therapy in many devastating diseases. However, studies on immune rejection, which is a major problem facing successful stem cell therapy, are rare. In this study, we examined MHC expression in the M13SV1 cell line, which has previously been shown to have stem cell properties and to be non-tumorigenic, in order to determine whether human adult stem cells might be rejected after transplantation. Our results show low expression levels of MHC class I molecules on the surface of these cells. An induction of MHC class I expression was observed when the cells were treated with IFN-gamma. Maximal induction of MHC class protein expression was observed at 48 h after treatment with concentrations above 5 ng/ml of IFN-gamma. Elevated MHC class I levels were sustained for 72 h after withdrawing IFN-gamma. Therefore, we introduced human cytomegalovirus (hCMV) US genes, which are known to be able to reduce MHC class I expression on the cell surface after infection, into M13SV1 cells. Cells transfected with the hCMV US2, US3, US6 or US11 genes exhibited a reduction (40-60%) of MHC class I expression compared with mock-transfected cells. These results suggest that human adult stem cells are capable of expressing high levels of MHC class I proteins, and thus may be rejected on transplantation unless they are modified. In addition, viral stealth mechanisms can be exploited for stem cell transplantation.

Down-regulation of MHC class I expression in human neuronal stem cells using viral stealth mechanism

Due to their unique capacity for self-renewal in addition to their ability to differentiate into cells of all neuronal lineages, neuronal stem cells (NSCs) are promising candidates for cell replacement therapy in neuronal injury and neurodegenerative diseases. However, there are few studies on immune rejection, which is one of the main problems facing successful stem cell therapy. In order to determine if human NSC might be rejected after transplantation the MHC expression level was examined in the HB1.F3 cell line, which has previously been shown to exhibit NSC properties. The results showed low expression levels of the MHC class I molecules on the surfaces of these cells. A dramatic increase in the MHC class I expression level was observed when the cells were treated with IFN-gamma, TNF-alpha, and IL-1beta, alone or in combination. The maximum induction of MHC class I protein expression was observed at above 20ng/ml IFN-gamma 48h after the treatment. The apparent additive effects of TNF-alpha and IL-1beta in combination on the maximum induction of MHC class I expression exerted by IFN-gamma treatment were not observed. The MHC class I levels elevated by IFN-gamma were sustained for 72h after withdrawing the IFN-gamma. Therefore, this study introduced human cytomegalovirus (hCMV) US genes, which are known to be able to reduce the MHC class I expression level on the cell surface after infection, into HB1.F3 cells. The cells transfected with the hCMV US2, US3, US6 or US11 genes showed 20-50% reduction in the MHC class I expression level compared with the mock-transfected cells. These results suggest that NSC expresses high levels of the MHC class I proteins, and unless they are modified, might be rejected upon transplantation. In addition, the various viral stealth mechanisms can be exploited for stem cell transplantation.

In vitro expression of major histocompatibility class I and class II antigens by conditionally immortalized murine neural stem cells

The expression of major histocompatibility complex (MHC) antigens on the surface of cells is intimately linked to in vivo graft survival. It has been previously shown that the conditionally immortalized temperature-sensitive Maudsley hippocampal clone 36 (MHP36) neural stem cells show good long-term graft survival and do not elicit an acute immunological response following transplantation. Here we report that MHP36 cells express both MHC class I and class II antigens when grown in culture under proliferative conditions (33 degrees C), whereas cells with a differentiated morphology in the non-proliferative (37-39 degrees C) condition express low to undetectable levels of either MHC molecules. However, morphologically undifferentiated cells persisting under non-proliferating conditions continued to express both MHC antigens. The downregulation of MHC antigens upon differentiation following cell transplantation could therefore contribute to the graft survival of MHP36 cells.

Transplantation of neural stem cells in a rat model of stroke: assessment of short-term graft survival and acute host immunological response

The use of progenitors and stem cells for neural grafting is promising, as these not only have the potential to be maintained in vitro until use, but may also prove less likely to evoke an immunogenic response in the host, when compared to primary (fetal) grafts. We investigated whether the short-term survival of a grafted conditionally immortalised murine neuroepithelial stem cell line (MHP36) (2 weeks post-implantation, 4 weeks post-ischaemia) is influenced by: (i) immunosuppression (cyclosporin A (CSA) vs. no CSA), (ii) the local (intact vs. lesioned hemisphere), or (iii) global (lesioned vs. sham) brain environment. MHP36 cells were transplanted ipsi- and contralateral to the lesion in rats with middle cerebral artery occlusion (MCAo) or sham controls. Animals were either administered CSA or received no immunosuppressive treatment. A proliferation assay of lymphocytes dissociated from cervical lymph nodes, grading of the survival of the grafted cells, and histological evaluation of the immune response revealed no significant difference between animals treated with or without CSA. There was no difference in survival or immunological response to cells grafted ipsi- or contralateral to the lesion. Although a local upregulation of immunological markers (MHC class I, MHC class II, CD45, CD11b) was detected around the injection site and the ischaemic lesion, these were not specifically upregulated in response to transplanted cells. These results provide evidence for the low immunogenic properties of MHP36 cells during the initial period following implantation, known to be associated with an acute host immune response and ensuing graft rejection.

Porcine neural xenografts in the immunocompetent rat: immune response following grafting of expanded neural precursor cells

Intracerebral neural xenografts elicit a host immune response that results in their rapid rejection. This forms a key barrier to the therapeutic use of xenogeneic tissue transplantation for conditions such as Parkinson's disease. The current study sought to provide insight into the cellular components of donor cell suspensions that are important in stimulating the host rejection response and thereby to suggest rational manipulations of xenogeneic donor tissue that might ultimately enhance its clinical utility. The neural stem cell mitogens, epidermal growth factor and fibroblast growth factor-2, have been used to isolate and expand populations of primordial neural precursor cells from the embryonic pig brain. The immune response elicited by these cells on transplantation into the non-immunosuppressed rat has been fully characterised. In the first experiments, expanded neural precursors were grafted into the hemi-parkinsonian, non-immunosuppressed Sprague-Dawley rat and graft status and host response examined 10, 21, 35 and 60 days post-transplantation. While equivalent primary tissue grafts were completely eliminated at 35 days, grafts of expanded neural precursors with healthy neurofilament-positive projections were present at all time-points, and two large grafts remained even at 60 days. Some grafts appeared to elicit minimal host immune responses at the time-points they were examined, although most did appear to be undergoing a rejection process since a co-ordinated response involving host cytotoxic T-lymphocytes, microglia/macrophages, immunoglobulin M and complement could be demonstrated to varying degrees. Subsequent experiments went on to demonstrate further that expanded precursor populations and primary tissue suspensions differed in their immunogenic profile. Firstly, when primary tissue was injected intraperitoneally into immunocompetent rats a vigorous primary humoral response was generated. No such response was detected following injection of expanded neural precursors. Secondly, flow cytometric analysis revealed small but significant levels of class II porcine major histocompatibility complex expression in primary cell suspensions but no such expression in expanded precursor populations.The results of this study therefore demonstrate that the immunogenicity of porcine neural cell suspensions used for intracerebral grafting is reduced when neural stem cell mitogens are used to expand precursor cells. The implications of these findings in the development of novel xenogeneic cellular therapies for neurodegenerative conditions such as Parkinson's disease are discussed.

Rejection of wild-type and genetically engineered major histocompatibility complex-deficient glial cell xenografts in the central nervous system results in bystander demyelination and Wallerian degeneration

Mixed glial cell cultures prepared from neonatal wild type and mutant male mice lacking either major histocompatibility complex class I, class II or both class I and II molecules (major histocompatibility complex class I(o/o)II(o/o)), and from syngeneic male rats were transplanted into female rat spinal cord white matter. Graft survival was monitored using DNA probes specific to the Y chromosome. Survival of major histocompatibility complex class-deficient grafts was not prolonged compared to wild-type grafts and in most cases grafts could not be detected at 28 days post-transplantation, at which time syngeneic grafts were still present. However, rejection of xenografts resulted in significant bystander damage to host tissue. In recipients of wild-type and major histocompatibility complex class I(o/o) xenografts the predominant pathology was demyelination. Demyelination was also observed in recipients of major histocompatibility complex class II(o/o) and major histocompatibility complex class I(o/o)II(o/o) xenografts, however in addition there was marked collagen deposition and meningeal cell invasion. Significantly more axons had undergone Wallerian degeneration in recipients of major histocompatibility complex class II(o/o) and major histocompatibility complex class I(o/o)II(o/o) xenografts than recipients of wild-type and major histocompatibility complex class I(o/o) xenografts. These findings were interpreted as evidence of a more destructive immune response associated with rejection of grafts lacking major histocompatibility complex class II molecules. It was proposed that the difference in the severity of bystander damage may be related to the previously demonstrated ability of xenogeneic major histocompatibility complex class II molecules to activate host T cells directly, whereas xenografts lacking major histocompatibility complex class II molecules were capable of activating host T cells only by the indirect pathway.