Sequential intracerebral transplantation of allogeneic and syngeneic fetal dopamine-rich neuronal tissue in adult rats: will the first graft be rejected?

Immunology of cell and gene therapy approaches for neurologic diseases

Repair and replacement strategies using cell replacement or viral gene transfer for neurologic diseases are becoming increasingly efficacious with clinically meaningful benefits in several conditions. An increased understanding of disease processes opens up opportunities for genetic therapies and precision medicine methods aiming at disease modification or repair of lesioned neurologic structures. However, such therapeutic effects may be limited or rendered ineffective by immune responses against gene products or cells used for the intended treatments. When introducing therapeutic agents into the nervous system, a set of biologic responses are inevitably triggered, which may lead to host responses that limit the intended therapeutic goals. Factors of importance include the type of vector used and origin of cells, the mode of introduction, the degree of host immunization, and any prior exposure to the agents used. It is possible to apply specific treatments that interfere with many of these steps and factors in order to limit host immunization and to reduce or eliminate host effector reactions against the therapeutic agents. This includes immune-evading design measures of the advanced therapeutic medicinal products and various immunosuppressive processes. Limited duration of specific immune modulations may be possible under carefully monitored programs.

A Sequential Intrastriatal Dopaminergic Graft Strategy in the Rodent Model for Parkinson's Disease: Implications for Graft Survival and Targeting

Optimal placement of intrastriatal dopaminergic grafts is likely crucial to optimize clinical recovery in Parkinson's disease (PD). The target sites of dopaminergic grafts vary among clinical trials and may partially explain the variable results in clinical efficacy reported thus far. In this study we hypothesized that a subsequent dopaminergic graft may promote functional recovery following a suboptimal initial graft. To test this hypothesis, rats with unilateral 6-hydroxydopamine lesions of the right nigrostriatal pathway were randomly divided into three groups. The first group received 900,000 fetal nigral cells in the medial striatum only (n = 6). The second group received 900,000 cells in both the medial and lateral striatum simultaneously (1.8 million total; n = 8). The final group received a second graft of 900,000 cells in the lateral striatum 6 weeks following initial transplantation of a medial graft (n = 6). Amphetamine-induced circling behavior was significantly reduced in both simultaneous and sequential graft groups at 9 and 12 weeks following transplantation of the initial graft. However, no recovery was noted in the single medial graft group at those time points. Furthermore, increased survival of dopaminergic cells was observed in the lateral graft of sequentially grafted animals compared with the medial graft. We conclude that a well-positioned subsequent graft can restore function in animals with a suboptimal initial graft and that the initial graft may improve survival of the second graft. These results are further discussed in relation to their important clinical implication for neural transplantation in PD.

Fetal Grafting for Parkinson's Disease: Expression of Immune Markers in Two Patients with Functional Fetal Nigral Implants

In a number of centers throughout the world, fetal nigral transplantation is being performed for the treatment of Parkinson's disease (PD). Clinical results have been inconsistent. One parameter that differs among transplant studies is the degree and manner by which patients are immunosuppressed following transplantation. Indeed, the role of the immune system following fetal grafting in humans is not well understood. Recently, two patients from our open label trial that received fetal nigral implants have come to autopsy. These patients were immunosuppressed with cyclosporin for 6 mo posttransplantation and survived for a total of 18 mo postgrafting. Robust survival of grafted dopamine-containing cells was observed in both cases. Immunostaining for HLA-DR revealed a dense collection of cells within grafts from both cases. HLA-DR staining was rarely observed within the host including non-grafted regions of the striatum. A more detailed analysis of immune markers was performed in Case 2. Numerous pan macrophages, T-cells, and B-cells were observed within graft sites located in the postcommissural putamen. In contrast, staining for these immune cells was not observed within the ungrafted anterior putamen. These findings suggest that even in healthy appearing functional nigral implants, grafts are invaded by host immune cells that could compromise their long-term viability and function. Alternatively, immune cells are known to secrete trophic factors, which may ultimately favor graft survival and function. Further work is needed to understand the role of the immune system in fetal grafting.

Development of histocompatible primate-induced pluripotent stem cells for neural transplantation

Immune rejection and risk of tumor formation are perhaps the greatest hurdles in the field of stem cell transplantation. Here, we report the generation of several lines of induced pluripotent stem cells (iPSCs) from cynomolgus macaque (CM) skin fibroblasts carrying specific major histocompatibility complex (MHC) haplotypes. To develop a collection of MHC-matched iPSCs, we genotyped the MHC locus of 25 CMs by microsatellite polymerase chain reaction analysis. Using retroviral infection of dermal skin fibroblasts, we generated several CM-iPSC lines carrying different haplotypes. We characterized the immunological properties of CM-iPSCs and demonstrated that CM-iPSCs can be induced to differentiate in vitro along specific neuronal populations, such as midbrain dopaminergic (DA) neurons. Midbrain-like DA neurons generated from CM-iPSCs integrated into the striatum of a rodent model of Parkinson's disease and promoted behavioral recovery. Importantly, neither tumor formation nor inflammatory reactions were observed in the transplanted animals up to 6 months after transplantation. We believe that the generation and characterization of such histocompatible iPSCs will allow the preclinical validation of safety and efficacy of iPSCs for neurodegenerative diseases and several other human conditions in the field of regenerative medicine.

MHC mismatch inhibits neurogenesis and neuron maturation in stem cell allografts

Background

The role of histocompatibility and immune recognition in stem cell transplant therapy has been controversial, with many reports arguing that undifferentiated stem cells are protected from immune recognition due to the absence of major histocompatibility complex (MHC) markers. This argument is even more persuasive in transplantation into the central nervous system (CNS) where the graft rejection response is minimal.

Methodology/Principal Findings

In this study, we evaluate graft survival and neuron production in perfectly matched vs. strongly mismatched neural stem cells transplanted into the hippocampus in mice. Although allogeneic cells survive, we observe that MHC-mismatch decreases surviving cell numbers and strongly inhibits the differentiation and retention of graft-derived as well as endogenously produced new neurons. Immune suppression with cyclosporine-A did not improve outcome but non-steroidal anti-inflammatory drugs, indomethacin or rosiglitazone, were able to restore allogeneic neuron production, integration and retention to the level of syngeneic grafts.

Conclusions/Significance

These results suggest an important but unsuspected role for innate, rather than adaptive, immunity in the survival and function of MHC-mismatched cellular grafts in the CNS.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

The more you have, the less you get: the functional role of inflammation on neuronal differentiation of endogenous and transplanted neural stem cells in the adult brain

The differentiation of neural stem cells toward a neuronal phenotype is determined by the extracellular and intracellular factors that form the neurogenic niche. In this review, we discuss the available data on the functional role of inflammation and in particular, pro- and anti-inflammatory cytokines, on neuronal differentiation from endogenous and transplanted neural stem/progenitor cells. In addition, we discuss the role of microglial cell activation on these processes and the fact that microglial cell activation is not univocally associated with a pro-inflammatory milieu. We conclude that brain cytokines could be regarded as part of the endogenous neurogenic niche. In addition, we propose that accumulating evidence suggests that pro-inflammatory cytokines have a negative effect on neuronal differentiation, while anti-inflammatory cytokines exert an opposite effect. The clarification of the functional role of cytokines on neuronal differentiation will be relevant not only to better understand adult neurogenesis, but also to envisage complementary treatments to modulate cytokine action that could increase the therapeutic benefit of future progenitor/stem cell-based therapies.

Embryonic stem cell-derived neural precursor cells improve memory dysfunction in Abeta(1-40) injured rats

The versatility of neural precursor cells (NPCs) derived from mouse embryonic stem cells (ESCs) has recently rekindled interests in cell replacement strategies aimed at neurodegenerative diseases. We observed the survival, migration, differentiation and functional recovery of NPCs transplanted into the hippocampus of aggregated beta-amyloid (Abeta) peptide injured rats. Congo Red plaques, Fluro-jade B positive degenerating neurons and neuronal loss were observed in the Abeta-injured hippocampus of rats, accompanied with significant increases in escape latency and decrease in the ratio of exploratory time in a Morris water maze test. EGFP-expressing mouse ES cells were induced into Nestin-positive NPCs before transplantation into the Abeta-injured hippocampus. A marked decrease in escape latency and exploratory time were observed at least 16 weeks after transplantation compared to Abeta-injured animals without grafts. Grafted EGFP-expressing NPCs spread away from the injection tract and about 12.01+/-0.67% and 9.41+/-0.78% of NPCs differentiated into, respectively, GFAP- and NF200-positive cells 4 W after transplantation. These ratios gradually increased to 40.25+/-0.57% and 19.35+/-0.84% by 16 W. The restoration of hippocampal function by ESCs suggests that cell transplantation may be the effective choice to improve the cognitive function caused by Abeta injured.

The high integration and differentiation potential of autologous neural stem cell transplantation compared with allogeneic transplantation in adult rat hippocampus

Cell therapy is thought to have a central role in restorative therapy, which aims to restore function to the damaged nervous system. The purpose of this study was to establish an autologous neural stem cell (NSC) transplantation model using adult rats and to compare survival, migration, and differentiation between this system and allogeneic NSC transplantation. Furthermore, we compared the immunologic response of the host tissue between autologous and allogeneic transplantation. NSCs were removed from the subventricular zone of adult Fischer 344 rats using stereotactic methods. NSCs were expanded and microinjected into normal hippocampus in the autologous brain. Allogeneic NSC (derived from adult Wistar rats) transplantation was performed using the same procedure, and hippocampal sections were analyzed immunohistologically 3 weeks post-transplantation. The cell survival and migration rate were higher for autologous transplantation than for allogeneic transplantation, and the neuronal differentiation rate in the autologous transplanted cells far exceeded that of allogeneic transplantation. Furthermore, there was less astrocyte and microglia reactivity in the host tissue of the autologous transplantation compared with allogeneic transplantation. These findings demonstrate that immunoreactivity of the host tissue strongly influences cell transplantation in the CNS as the autologous transplantation did not induce host tissue immunoreactivity; the microenvironment was essentially maintained in an optimal condition for the transplanted cells.

Immune problems in central nervous system cell therapy

Transplantation of cells and tissues to the mammalian brain and CNS has revived the interest in the immunological status of brain and its response to grafted tissue. The previously held view that the brain was an absolute "immunologically privileged site" allowing indefinite survival without rejection of grafts of cells has proven to be wrong. Thus, the brain should be regarded as a site where immune responses can occur, albeit in a modified form, and under certain circumstances these are as vigorous as those seen in other peripheral sites. Clinical cell transplant trials have now been performed in Parkinson's disease, Huntington's disease, demyelinating diseases, retinal disorders, stroke, epilepsy, and even deafness, and normally are designed as cell replacement strategies, although implantation of genetically modified cells for supplementation of growth factors has also been tried. In addition, some disorders of the CNS for which cell therapies are being considered have an immunological basis, such as multiple sclerosis, which further complicates the situation. Embryonic neural tissue allografted into the CNS of animals and patients with neurodegenerative conditions survives, makes and receives synapses, and ameliorates behavioral deficits. The use of aborted human tissue is logistically and ethically complicated, which has lead to the search for alternative sources of cells, including xenogeneic tissue, genetically modified cells, and stem cells, all of which can and will induce some level of immune reaction. We review some of the immunological factors involved in transplantation of cells to CNS.

Intracerebral cytokine profiles in adult rats grafted with neural tissue of different immunological disparity

To understand graft rejection in cell based therapies for brain repair we have quantified IL-1beta, IL-2, IL-4, IL-10, IL-12p40, IFN-gamma and TNF-alpha mRNA levels using real-time PCR, at days 4, 14, and 42 post-transplantation, in rats engrafted with syngeneic, allogeneic, concordant and discordant xenogeneic neural tissues. In addition, in the discordant xenografts immunohistochemistry and in situ hybridization were applied to detect local expression of IFN-gamma, TNF-alpha, IL-10 and TGF-beta. Allografts remained non-rejected but expressed IL-1beta, TNF-alpha and IL-4 transcripts but not IL-12p40 and IFN-gamma. Xenografts demonstrated distinct cytokine profiles that differed from syngeneic and allogeneic grafts. Non-rejected discordant xenografts contained higher levels of TNF-alpha transcripts and lower levels of IL-2 transcripts than the rejected ones at day 42. Discordant xenografts displayed a stronger and earlier expression of IL-1beta and TNF-alpha, followed by T-helper 1 and T-helper 2 associated cytokine expression. The number of cells expressing mRNA encoding TNF-alpha and TGF-beta was significantly increased over time in the discordant group. In conclusion, the immunological disparity of the implanted tissue explains survival rates and is associated with different cytokine profiles. In allografts, a chronic inflammatory reaction was detected and in xenogeneic grafts a delayed hypersensitivity like reaction may be involved in rejection.

Different mechanisms mediate the rejection of porcine neurons and endothelial cells transplanted into the rat brain

In order to investigate the early cellular responses mediating xenograft rejection in the brain, porcine aortic endothelial cells (PAEC) or porcine fetal mesencephalic neurons (PNEU) were transplanted into the striatum of LEW.1A rats. PAEC were detected with a specific anti-beta1 integrin antibody, and PNEU with an anti-porcine neurofilament antibody, or an antibody recognizing the NeuN antigen. PAEC grafts were massively infiltrated within 24 h by OX42-positive cells, which may correspond to polymorphonuclear (PMN) cells or macrophages. At that moment, the graft contained numerous cells expressing the inducible isoform of NO-synthase (iNOS). Infiltration by ED1-positive macrophages was effective after three days. The beta1-integrin labeling decreased from that time-point to day 7 post-implantation, and vanished after 11 days. Although some OX8-positive cells were present around the graft as soon as 3 days after transplantation, cells expressing the T-cell receptor (TCR)-beta chain infiltrated the graft after 7 days and their number remained low. A strong, diffuse OX8-and ED1-positive immunoreactive material remained in the scar up to the third week. In striking contrast, PNEU grafts remained poorly infiltrated by OX42- or ED1-positive cells during the first two weeks. A massive infiltration by macrophages and TCRbeta-positive lymphocytes occurred after 3 weeks. Natural killer (NK) cells were more scarce. The inflammation territory enlarged, and blood vessels were overloaded with macrophages or lymphocytes. Nevertheless, the graft contained NeuN-positive nuclei and neurites harbouring the porcine neurofilament protein. Hence, rejection was not completed at this time-point. These results suggest that the rapid rejection of PAEC is mainly driven by macrophages and possibly PMN cells, unlike PNEU, whose rejection is delayed and also involves lymphocytes. Differences in immunogenicity of grafted cells and/or patterns of production of pro-inflammatory cytokines may account for these contrasted rejection kinetics.

Discordant neural tissue xenografts survive longer in immunoglobulin deficient mice

BACKGROUND

The immune response against discordant xenografts in the brain is incompletely understood and remains a major obstacle for future clinical applications of xenogeneic neural tissue transplants in neurodegenerative disorders. To determine the role of antibodies in the rejection process, we compared graft survival and immune reactions between immunoglobulin deficient (IgKO) and normal mice.

METHODS

A cell suspension of embryonic porcine ventral mesencephalon was injected into the striatum of adult normal and IgKO mice. Graft sizes and number of infiltrating CD4- and CD8-positive lymphocytes were determined by stereological methods at 4 days and 2, 4, and 6 weeks after the transplants. Microglial accumulation was determined using the optical densitometrical method. Intraparenchymal deposition of IgG was investigated at 4 days and 2 weeks.

RESULTS

The majority of IgKO mice had surviving grafts for up to 4 weeks, whereas survival was minimal in control mice beyond 4 days. Graft sizes differed significantly between IgKO and control mice at 2 weeks (P<0.01, Kruskal Wallis ANOVA, followed by Mann Whitney test). The majority of infiltrating lymphocytes were CD4-positive in control mice but CD8-positive in IgKO mice. Microglial accumulation was strong around surviving grafts in IgKO mice at 4 weeks. Prominent staining of IgG, diffuse in the transplanted hemisphere and specific on grafted neurons, was found in control mice.

CONCLUSIONS

Our results suggest that immunoglobulins play an initiating role in rejection of discordant neural xenografts. After a prolonged graft survival of approximately 4 weeks, a cellular response with a large proportion CD8-positive cells leads to rejection in IgKO mice.

Discordant xenografts: different outcome after mouse and rat neural tissue transplantation to guinea-pigs

Embryonic neural tissue obtained from other species has been considered as a donor tissue source in repair strategies for human neurodegenerative disorders. The neuro- and immunobiology of distantly related species combinations, discordant xenografts, need to be characterised. For this purpose, a small animal model would be an important research tool. Adult guinea-pigs, and adult rats as controls, received intrastriatal grafts of either mouse or rat embryonic ventral mesencephalic tissue. The survival rates and types of host immune response were assessed at 2 weeks after grafting using stereological techniques and semi-quantitative evaluations. In the mouse-to-guinea-pig group, all transplants were rejected and no tyrosine hydroxylase-immuno reactive (TH-IR) cells remained. In the rat-to-guinea-pig group, there was good survival of TH-IR cells (5050 SEM+/-1550), similar to that in the rat-to-rat group (4900 SEM+/-1540). In the mouse-to-rat group, half of the animals had no surviving TH-IR cells (520 SEM+/-230 for the whole group). These species combinations offer inexpensive, efficient, and suitable conditions to study important survival factors for discordant xenogeneic neural tissue transplants. The factors responsible for the divergent graft outcomes between the two combinations might provide clues on how to manipulate xenogeneic tissue to increase survival rates in the future.

Quinolinic acid-induced inflammation in the striatum does not impair the survival of neural allografts in the rat

It has been suggested that inflammation related to intracerebral transplantation surgery can affect the survival of intrastriatal neural allografts. To test this hypothesis, we transplanted dissociated embryonic mesencephalic tissue from one of two rat strains, Lewis (allogeneic grafts) or Sprague-Dawley (syngeneic grafts), to the striatum of Sprague-Dawley rats. The target striatum was either intact or had received a local injection of quinolinic acid 9 days earlier, in order to induce a marked inflammation. At 6 or 12 weeks after transplantation, there was no significant difference between the different groups regarding the number of surviving grafted tyrosine hydroxylase immunoreactive neurons. However, the graft volume of both the syngeneic and allogeneic implants was significantly larger in the quinolinate-lesioned than in the intact striatum. There were dramatically increased levels of expression of major histocompatibility complex class I and II antigens, marked infiltrates of macrophages, activated microglia and astrocytes, and accumulation of large numbers of CD4 and CD8 positive T-lymphocytes in the quinolinate-lesioned striatum. In contrast, these immunological markers were much less abundant around both syngeneic and allogeneic grafts placed in intact striatum. We conclude that severe inflammation caused by quinolinic acid does not lead to rejection of intrastriatal neural allografts.

Rat intrastriatal neural allografts challenged with skin allografts at different time points

The present study was designed to address two questions. First, can an intrastriatal neural allograft exhibit long-term survival (18 weeks) if the host is immunized by an orthotopic skin graft 6 weeks after neural transplantation (the 6w-Long group)? Second, can an intrastriatal neural allograft survive when the host is challenged by an orthotopic skin allograft either simultaneously (Sim) with the intracerebral graft surgery or 2 (2w) weeks later? Dissociated embryonic ventral mesencephalic tissue from Lewis rats was stereotaxically injected into the striatum of Sprague-Dawley rats with unilateral 6-hydroxydopamine lesions. Six weeks after neural grafting, no reduction in amphetamine-induced motor asymmetry was observed in the Sim and 2w groups. At 6 weeks after skin grafting, the mean motor asymmetry scores had returned to the initial pretransplantation levels in the 6w-Long group. All the neural allografts in the Sim group were completely rejected, and the mean number of tyrosine hydroxylase immunoreactivity neurons in the grafts was significantly reduced in the 2w and the 6w-Long group, when compared to the no-skin control group. There were very high levels of expression of MHC class I and II antigens, marked cellular infiltrates containing macrophages and T-lymphocytes, and several activated microglia and astrocytes in and around the surviving intracerebral transplants in the 2w and the 6w-Long groups. The results suggest that intrastriatal neural allografts are more likely to be rejected rapidly if the host is efficiently immunized with the same alloantigens simultaneously or soon after the neural transplantation than at a later time point. When established neural allografts are subjected to a strong immunological challenge, they undergo protracted rejection.

Fetal grafting for Parkinson's disease: expression of immune markers in two patients with functional fetal nigral implants

In a number of centers throughout the world, fetal nigral transplantation is being performed for the treatment of Parkinson's disease (PD). Clinical results have been inconsistent. One parameter that differs among transplant studies is the degree and manner by which patients are immunosuppressed following transplantation. Indeed, the role of the immune system following fetal grafting in humans is not well understood. Recently, two patients from our open label trial that received fetal nigral implants have come to autopsy. These patients were immunosuppressed with cyclosporin for 6 mo posttransplantation and survived for a total of 18 mo postgrafting. Robust survival of grafted dopamine-containing cells was observed in both cases. Immunostaining for HLA-DR revealed a dense collection of cells within grafts from both cases. HLA-DR staining was rarely observed within the host including nongrafted regions of the striatum. A more detailed analysis of immune markers was performed in Case 2. Numerous pan macrophages, T-cells, and B-cells were observed within graft sites located in the postcommissural putamen. In contrast, staining for these immune cells was not observed within the ungrafted anterior putamen. These findings suggest that even in healthy appearing functional nigral implants, grafts are invaded by host immune cells that could compromise their long-term viability and function. Alternatively, immune cells are known to secrete trophic factors, which may ultimately favor graft survival and function. Further work is needed to understand the role of the immune system in fetal grafting.

Addition of allogeneic spleen cells causes rejection of intrastriatal embryonic mesencephalic allografts in the rat

To address the importance of antigen-presenting cells for the survival of intracerebral neural allografts, allogeneic spleen cells were added to the graft tissue before transplantation. Dissociated embryonic, dopamine-rich mesencephalic and adult spleen tissues were prepared from either inbred Lewis or Sprague-Dawley rats. A mixture of neural and spleen cells was sterotaxically transplanted into the right striatum of adult Sprague-Dawley rats. Controls were neural allografts without addition of allogeneic spleen cells and syngeneic neural grafts with or without the addition of syngeneic spleen cells. Six weeks after transplantation, brain sections were processed immunocytochemically for tyrosine hydroxylase, specific for grafted dopamine neurons, and a bank of markers for various components in the immune and inflammatory responses. The neural allografts which were mixed with allogeneic spleen cells were rejected. In these rats, there were high levels of expression of major histocompatibility complex class I and II antigens, intense cellular infiltration including macrophages and activated microglial cells, and a presence of cluster of differentiation 4- and 8-immunoreactive cells in the graft sites. Moreover, there were increased levels of intercellular adhesion molecule-1, tumour necrosis factor-alpha and interleukin-6 in and around the grafts which were undergoing rejection. In contrast, syngeneic neural grafts survived well regardless of whether they were mixed with syngeneic spleen cells or not, and control neural allografts also exhibited unimpaired survival. No significant difference was observed in the number of grafted dopamine neurons among these three latter groups. The levels of expression of the different markers for inflammation and rejection were generally lower in these grafts than in implants of combined allogeneic neural and spleen cells. In summary, intrastriatal neural allografts, which normally survive well in our animal model, were rejected if allogeneic spleen cells from the same donor were added to the graft tissue. The added spleen cells caused strong host immune and inflammatory responses. The study gave support to the notion that immunological privilege of the brain does not provide absolute protection to immunogenetically histoincompatible neural grafts.

Allogeneic grafts of fetal dopamine neurons: immunological reactions following active and adoptive immunizations

To define the importance of adoptive sensitization and duration of graft residence on transplant alloimmunization, behavioral and histochemical parameters were examined in unilaterally 6-OHDA-lesioned F344 rat hosts which received fetal ventral mesencephalic (VM) grafts from Wistar-Furth (WF) donors. In all animals which showed increased rotations after alloimmunization, increased numbers of T cell receptor (TcR) positive, CD8+ lymphocytes were detected in the grafts. In addition, an increased density of class I MHC antigens was seen in the graft and in the adjacent host brain. Lesser numbers of CD4+, CD11b+, and MHCII+ positive elements were also seen. Perivascular cuffing was often found in actively immunized animals. An increase in TcR+ and MHC class I+ elements was also seen in animals only adoptively immunized. The tyrosine hydroxylase positive graft area was also markedly reduced in actively immunized animals and the extent of reduction correlated with the number of cells used for immunization. These studies indicate that established allografts can evade rejection as long as host lymphocytes are not activated against graft alloantigens. In addition, increasing graft residence time in the host and adoptive immunization render the graft more susceptible to subsequent rejection.

The spinal cord as an alternative model for nerve tissue graft

The spinal cord provides an alternative model for nerve tissue grafting experiments. Anatomo-functional correlations are easier to make here than in any other region of the CNS because of a direct implication of spinal cord neurons in sensorimotor activities. Lesions can be easily performed to isolate spinal cord neurons from descending inputs. The anatomy of descending monoaminergic systems is well defined and these systems offer a favourable paradigm for lesion-graft experiments.

Multiple obstacles to gene therapy in the brain

Neuwelt et al. have proposed gene-transfer experiments utilizing an animal model that offers many important advantages for investigating the feasibility of gene therapy in the human brain. A variety of tissues concerning the viral vector and mode of delivery of the corrective genes need to be resolved, however, before such therapy is scientifically supportable.

Principles of brain tissue engineering

It is often presumed that effects of neural tissue transplants are due to release of neurotransmitter. In many cases, however, effects attributed to transplants may be related to phenomena such as trophic effects mediated by glial cells or even tissue reactions to injury. Any conclusion regarding causation of graft effects must be based on the control groups or other comparisons used. In human clinical studies, for example, comparing the same subject before and after transplantation allows for many interpretations of the causes of clinical changes.

Lessons on transplant survival from a successful model system

Studies on the snailMelampusreveal that connectivity is crucial to the survival of transplanted ganglia. Transplanted CNS ganglia can innervate targets or induce supernumerary structures. Neuron survival is optimized by the neural incorporation that occurs when a transplanted ganglion is substituted for an excised ganglion. Better provision for the trophic requirements of neurons will improve the success of mammalian fetal transplants.

Repairing the brain: Trophic factor or transplant?

Three experiments on neural grafting with adult rat hosts are described. Working memory impairments were produced by lesioning the hippocampus or severing its connections with the septum by ablating the fimbria-fornix. The results suggest that the survival and growth of a neural graft, whether an autograft or a xenograft, is not a necessary condition for functional recovery on a task tapping working memory.

Will brain tissue grafts become an important therapy to restore visual function in cerebrally blind patients?

Grafting embryonic brain tissue into the brain of patients with visual field loss due to cerebral lesions may become a method to restore visual function. This method is not without risk, however, and will only be considered in cases of complete blindness after bilateral occipital lesions, when other, risk-free neuropsychological methods fail.

Difficulties inherent in the restoration of dynamically reactive brain systems

The responses displayed by an injured or diseased nervous system are complex. Some of the responses may effect a functional reorganization of the affected neural circuitry. Strategies aimed at the restoration of function, whether or not these involve transplantation, need to recognize the innate reactive capacity of the nervous system to damage. More successful strategies will probably incorporate, rather than ignore, the adaptive responses of the compromised neural systems.

Elegant studies of transplant-derived repair of cognitive performance

Cholinergic-rich grafts have been shown to be effective in restoring maze-learning deficits in rats with lesions of the forebrain cholinergic projection system. However, the relevance of those studies to developing novel therapies for Alzheimer's disease is questioned.

Neural transplants are grey matters

The lesion and transplantation data cited by Sinden et al., when considered in tandem, seem to harbor an internal inconsistency, raising questions of false localization of function. The extrapolation of such data to cognitive impairment and potential treatment strategies in Alzheimer's disease is problematic. Patients with focal basal forebrain lesions (e.g., anterior communicating artery aneurysm rupture) might be a more appropriate target population.

Immunobiology of neural transplants and functional incorporation of grafted dopamine neurons

In contrast to the views put forth by Stein & Glasier, we support the use of inbred strains of rodents in studies of the immunobiology of neural transplants. Inbred strains demonstrate homology of the major histocompatibility complex (MHC). Virtually all experimental work in transplantation immunology is performed using inbred strains, yet very few published studies of immune rejection in intracerebral grafts have used inbred animals.

Local and global gene therapy in the central nervous system

For focal neurodegenerative diseases or brain tumors, localized delivery of protein or genetic vectors may be sufficient to alleviate symptoms, halt disease progression, or even cure the disease. One may circumvent the limitation imposed by the blood-brain barrier by transplantation of genetically altered cell grafts or focal inoculation of virus or protein. However, permanent gene replacement therapy for diseases affecting the entire brain will require global delivery of genetic vectors. The neurotoxicity of currently available viral vectors and the transient nature of transgene expression invivomust be overcome before their use in human gene therapy becomes clinically applicable.

Neural grafting in human disease versus animal models: Cautionary notes

Over the past two decades, research on neural transplantation in animal models of neurodegeneration has provided provocative in sights into the therapeutic use of grafted tissue for various neurological diseases. Although great strides have been made and functional benefits gained in these animal models, much information is still needed with regard to transplantation in human patients. Several factors are unique to human disease, for example, age of the recipient, duration of disease, and drug interaction with grafted cells; these need to be explored before grafting can be considered a safe and effective therapeutic tool.

Building a rational foundation for neural transplantation

The neural transplantation research described by Sinden and colleagues provides part of the rationale for the clinical application of neural transplantation. The authors are asked to clarify their view of the role of the cholinergic system in cognition, to address extrahippocampal damage caused by transient forebrain ischemia, and to consider the effects of delayed neural degeneration in their structure-function analysis.

Intraretrosplenial grafts of cholinergic neurons and spatial memory function

The transplantation of cholinergic neurons into the hippocampal formation has been well characterized. We describe our studies on the effects of cholinergic transplants in the retrosplenial cortex. These transplants were capable of ameliorating spatial navigation deficits in rats with septohippocampal lesions. In addition, we provide evidence for the modulation of transplanted neurons by the host brain.

Gene therapy and neural grafting: Keeping the message switched on

A major problem in developing an effective gene therapy for the nervous system lies in understanding the principles that maintain or turn off the expression of genes following their transfer into the CNS.

Therapeutic neural transplantation: Boon or boondoggle?

Despite reports of recovery of function after neural transplantation, the biological interactions between transplanted neurons and the host brain that are necessary to mediate recovery are unclear at present. One source of confusion is in the variety of models and protocols used in these studies. It is suggested that multisite experimentation using standard protocols, models, and recovery criteria would be helpful in moving neural transplantation from the laboratory to the clinic.

The ethics of fetal tissue grafting should be considered along with the science

In addition to the scientific and medical issues surrounding the use of fetal tissue transplants, the ethical implications should be considered. Two major ethical issues are relevant. The first of these is whether this experimental procedure can be justified on the basis of potential benefit to the patient. The second is whether the use of tissue obtained from intentionally aborted fetuses can be justified in the context of historical and existing guidelines for the protection of human subjects. The separation of ethical decisions from medical practice and scientific research is necessary to prevent the exploitation of innocent human life.

Gene therapy for neurodegenerative disorders and malignant brain tumors

Gene therapy approaches have great promise in the treatment of neurodegenerative disorders and malignant brain tumors. Neuwelt et al. review available viral-mediated gene therapy methods and their blood-brain-barrier (BBB) disruption delivery technique, briefly mentioning nonviral mediated gene therapy methods. This commentary discussed the BBB disruption delivery technique, viral and nonviral mediated gene therapy approaches to Parkinson's disease, and the potential use of antisense oligo to suppress malignant brain tumors.

Behavioral effects of neural grafts: Action still in search of a mechanism

This commentary reviews data supporting circuitry reconstruction, replacement neurotransmitters, and trophic action as mechanisms whereby transplants promote recovery of function. Issue is taken with the thesis of Sinden et al. that adequate data exist to indicate that reconstruction of hippocampal circuitry damaged by hypoxia with CA1 transplants is a confirmed mechanism whereby these transplants produce recovery. Sinden et al.'s and Stein & Glasier's proposal that there is definitive evidence showing that all transplants produce trophic effects is also questioned.

Neural transplantation, cognitive aging and speech

Research on neural transplantation has great potential societal importance in part because of the expanding proportion of the population that is elderly. Transplantation studies can benefit from the guidance of research on cognitive aging, especially in connection with the assessment of behavioral outcomes. Speech for example, might be explored using avian models.

Pathway rewiring with neural transplantation

A lesion to the brain is not necessary for a successful neural transplantation. Embryonic Purkinje cells placed on the surface of an uninjured adult cerebellum can develop and migrate into the host molecular layer. Both the Purkinje cells that migrated into the host cerebellum and those that remained in the graft were innervated by collateral sprouting of adult intact climbing fibers.

Multiple potential mechanisms of graft action is not a new idea

It is well established that neural grafts can exert functional effects on the host animal by a multiplicity of different mechanisms – by diffuse release of trophic molecules, neurohormones, and deficient neurotransmitters, as well as by growth and reformation of neural circuits. Our challenge is to understand how these different mechanisms complement each other.

Grafts and the art of mind's reconstruction

The use of neural transplantation to alleviate cognitive deficits is still in its infancy. We have an inadequate understanding of the deficits induced by different types of brain damage and their homologies in animal models against which to assess graft-induced recovery, and of the ways in which graft growth and function are influenced by factors within the host brain and the environment in which the host is operating. Further, use of fetal tissue may only be a transitory phase in the search for appropriate donor sources. Nevertheless, findings from our laboratory and elsewhere have made aprima faciecase for successful cognitive reconstruction by graft methods.