Xenotransplantation for CNS repair: immunological barriers and strategies to overcome them

Transplantation of Embryonic Porcine Neocortical Tissue into Newborn Rats

Several previous studies, suggesting the potential use of embryonic xenografts in the treatment of neurological disorders, indicate that neural growth and axonal guidance factors may function across species. In this light, blocks of fetal porcine neocortex were grafted into small cortical lesion cavities made in newborn rats. Sacrifice at 3–12.5 weeks posttransplantation revealed healthy looking grafts in several animals. Apparent graft rejection evidenced by areas of necrosis and OX1 reactivity was observed in some of the older transplants. Treatment of nursing mothers or of postweaning newborns with cyclosporin A did not appear to promote graft survival. Some transplants grew to extremely large proportions and were characterized by bands of cells and bundles of axons as observed using immunohistochemical staining for pig neurofilament. Neurofilament-positive axons projected from several of the grafts to course through the corpus callosum to the contralateral cortex or to course ipsilaterally within the subcortical white matter, where labeled fibers could be traced to the midbrain crus cerebri in older transplants. Bundles of axons were also observed coursing within the ipsilateral caudate putamen where terminal branching was apparent. The normal course of transplant efferents within the host brain indicates that growing pig axons can respond to rodent axonal guidance factors.

Quantitative [18F]Fluorodopa/PET and Histology of Fetal Mesencephalic Dopaminergic Grafts to the Striatum of MPTP-Poisoned Minipigs

The functional restoration of the dopamine innervation of striatum in MPTP-poisoned Göttingen minipigs was assessed for 6 months following grafting of fetal pig mesencephalic neurons. Pigs were assigned to a normal control group and a MPTP-poisoned group, members of which received no further treatment, or which received bilateral grafts to the striatum of tissue blocks harvested from E28 fetal pig mesencephalon with and without immunosuppressive treatment after grafting, or with additional co-grafting with immortalized rat neural cells transfected to produce GDNF. In the baseline condition, and again at 3 and 6 months postsurgery, all animals were subjected to quantitative [18F]fluorodopa PET scans and testing for motor impairment. At the end of 6 months, tyrosine hydroxylase (TH)-containing neurons were counted in the grafts by stereological methods. The MPTP poisoning persistently reduced the magnitude of k3D, the relative activity of DOPA decarboxylase in striatum, by 60%. Grafting restored the rate of [18F]fluorodopa decarboxylation to the normal range, and normalized the scores in motor function. The biochemical and functional recovery was associated with survival of approximately 100,000 TH-positive graft neurons in each hemisphere. Immunosuppression did not impart a greater recovery of [18F]fluorodopa uptake, nor were the number of TH-positive graft neurons or the volumes of the grafts increased in the immunosuppressed group. Contrary to expectation, co-grafting of transfected GDNF-expressing HiB5 cells, a rat-derived neural cell line, tended to impair the survival of the grafts with the lowest values for graft volumes, TH-positive cell numbers, behavioral scores, and relative DOPA decarboxylase activity. From the results we conclude that pig ventral mesencephalic allografts can restore functional dopamine innervation in adult MPTP-lesioned minipigs.

Concise Review: Innate and Adaptive Immune Recognition of Allogeneic and Xenogeneic Cell Transplants in the Central Nervous System

Over the last 30 years, numerous allogeneic and xenogeneic cell grafts have been transplanted into the central nervous system (CNS) of mice and men in an attempt to cure neurological diseases. In the early studies, human or porcine embryonic neural cells were grafted in the striatum of animals or patients in an attempt to replace lost neurons. Although the immune-privileged status of the brain as a recipient organ was widely accepted, it rapidly became evident that CNS-grafted allogeneic and xenogeneic cells could be recognized and rejected by the immune system, resulting in poor neural graft survival and limited functional recovery. Since then, the CNS transplantation field has witnessed a sharp rise in the number of studies in which allogeneic and xenogeneic neural or mesenchymal stem cells (NSCs or MSCs, respectively) are transplanted, predominantly aiming at providing trophic stimulation and promoting endogenous repair of the brain. Interestingly, in many recent NSC and MSC-based publications functional improvement was used as the principal measure to evaluate the success of cell transplantation, while the fate of transplanted cells remained largely unreported. In this review, we first attempt to understand why primary neural cell isolates were largely substituted for NSCs and MSCs in cell grafting studies. Next, we review the current knowledge on the immune mechanisms involved in the recognition and rejection of allogeneic and xenogeneic cellular grafts in the CNS. Finally, we propose strategies to reduce graft immunogenicity and to improve graft survival in order to design improved cell-based CNS therapies. Stem Cells Translational Medicine 2017;6:1434-1441.

IgG response to intracerebral xenotransplantation: specificity and role in the rejection of porcine neurons

Xenogenic fetal neuroblasts are considered as a potential source of transplantable cells for the treatment of neurodegenerative diseases, but immunological barriers limit their use in the clinic. While considerable work has been performed to decipher the role of the cellular immune response in the rejection of intracerebral xenotransplants, there is much still to learn about the humoral reaction. To this end, the IgG response to the transplantation of fetal porcine neural cells (PNC) into the rat brain was analyzed. Rat sera did not contain preformed antibodies against PNC, but elicited anti-porcine IgG was clearly detected in the host blood once the graft was rejected. Only the IgG1 and IgG2a subclasses were up-regulated, suggesting a T-helper 2 immune response. The main target of these elicited IgG antibodies was porcine neurons, as determined by double labeling in vitro and in vivo. Complement and anti-porcine IgG were present in the rejecting grafts, suggesting an active role of the host humoral response in graft rejection. This hypothesis was confirmed by the prolonged survival of fetal porcine neurons in the striatum of immunoglobulin-deficient rats. These data suggest that the prolonged survival of intracerebral xenotransplants relies on the control of both cell-mediated and humoral immune responses.

The influence of immunosuppressive drugs on neural stem/progenitor cell fate in vitro

In allogenic and xenogenic transplantation, adequate immunosuppression plays a major role in graft survival, especially over the long term. The effect of immunosuppressive drugs on neural stem/progenitor cell fate has not been sufficiently explored. The focus of this study is to systematically investigate the effects of the following four different immunotherapeutic strategies on human neural progenitor cell survival/death, proliferation, metabolic activity, differentiation and migration in vitro: (1) cyclosporine A (CsA), a calcineurin inhibitor; (2) everolimus (RAD001), an mTOR-inhibitor; (3) mycophenolic acid (MPA, mycophenolate), an inhibitor of inosine monophosphate dehydrogenase and (4) prednisolone, a steroid. At the minimum effective concentration (MEC), we found a prominent decrease in hNPCs' proliferative capacity (BrdU incorporation), especially for CsA and MPA, and an alteration of the NAD(P)H-dependent metabolic activity. Cell death rate, neurogenesis, gliogenesis and cell migration remained mostly unaffected under these conditions for all four immunosuppressants, except for apoptotic cell death, which was significantly increased by MPA treatment.

Approaches to repairing the damaged spinal cord: overview

Affecting young people during the most productive period of their lives, spinal cord injury (SCI) is a devastating problem for modern society. In the past, treating SCI seemed frustrating and hopeless because of the tremendous morbidity and mortality, life-shattering impact, and limited therapeutic options associated with the condition. Today, however, an understanding of the underlying pathophysiological mechanisms, the development of neuroprotective interventions, and progress toward regenerative interventions are increasing hope for functional restoration. In this chapter, we provide an overview of various repair strategies for the injured spinal cord. Special attention will be paid to strategies that promote spontaneous regeneration, including functional electrical stimulation, cell replacement, neuroprotection, and remyelination. The concept that limited rebuilding can provide a disproportionate improvement in quality of life is emphasized throughout. New surgical procedures, pharmacological treatments, and functional neuromuscular stimulation methods have evolved over the last decades and can improve functional outcomes after spinal cord injury; however, limiting secondary injury remains the primary goal. Tissue replacement strategies, including the use of embryonic stem cells, become an important tool and can restore function in animal models. Controlled clinical trials are now required to confirm these observations. The ultimate goal is to harness the body's own potential to replace lost central nervous system cells by activation of endogenous progenitor cell repair mechanisms.

Delayed transplantation of human neural precursor cells improves outcome from focal cerebral ischemia in aged rats

Neural precursor cell (NPC) transplantation may have a role in restoring brain function after stroke, but how aging might affect the brain's receptivity to such transplants is unknown. We reported previously that transplantation of human embryonic stem cell (hESC)-derived NPCs together with biomaterial (Matrigel) scaffolding into the brains of young adult Sprague-Dawley rats 3 weeks after distal middle cerebral artery occlusion (MCAO) reduced infarct volume and improved neurobehavioral performance. In this study, we compared the effect of NPC and Matrigel transplants in young adult (3-month-old) and aged (24-month-old) Fisher 344 rats from the National Institute on Aging's aged rodent colony. Distal MCAO was induced by electrocoagulation, and hESC-derived NPCs were transplanted into the infarct cavity 3 weeks later. Aged rats developed larger infarcts, but infarct volume and performance on the cylinder and elevated body swing tests, measured 6-8 weeks post-transplant, were improved by transplantation. We conclude that advanced age does not preclude a beneficial response to NPC transplantation following experimental stroke.

Benefits and risks of intranigral transplantation of GABA-producing cells subsequent to the establishment of kindling-induced seizures

Neural transplantation has been investigated experimentally and clinically for the purpose of developing new treatment options for intractable epilepsy. In the present study we assessed the anticonvulsant efficacy and safety of bilateral allotransplantation of genetically engineered striatal GABAergic rat cell lines into the substantia nigra pars reticulata (SNr). Rats with previously-established seizures, induced by amygdala kindling, were used as a model of temporal lobe epilepsy. Three cell lines were transplanted: (1) immortalized GABAergic cells (M213-2O) derived from embryonic rat striatum; (2) M213-2O cells (CL4) transfected with human GAD67 cDNA to obtain higher GABA synthesis than the parent cell line; and (3) control cells (121-1I), also derived from embryonic rat striatum, but which did not show GAD expression. A second control group received injections of medium alone. Transplantation of M213-2O cells into the SNr of kindled rats resulted in significant but transient anticonvulsant effects. Neither control cells nor medium induced anticonvulsant effects. Strong tissue reactions were, however, induced in the host brain of kindled but not of non-kindled rats, and only in animals that received grafts of genetically modified CL4 cells. These tissue reactions included graft rejection, massive infiltration of inflammatory immune cells, and gliosis. The anticonvulsant effect of M213-2O cells emphasizes the feasibility of local manipulations of seizures by intranigral transplantation of GABA-producing cells. On the other hand, the present data suggest that kindling-induced activation of microglia in the SNr can enhance immune reactions to transplanted cells. In this case, under conditions of further immunological stimulation by CL4 cells, transfected with a human cDNA, substantial immune reactions occurred. Thus, it appears that the condition of the host brain and the production of foreign proteins by transplanted cells have to be considered in estimating the risks of rejection of transplants into the brain.

Effect of inflammatory cytokines on major histocompatibility complex expression and differentiation of human neural stem/progenitor cells

To develop transplantation of neural stem/progenitor cells (NSPCs) as a successful treatment of neurodegenerative disorders, the possible induction of an inflammatory response following implantation needs to be taken into consideration. Inflammatory cytokines can upregulate major histocompatibility complex (MHC) expression on transplanted cells, thereby rendering them more susceptible to graft rejection. Furthermore, cytokines also have a profound effect on cell differentiation, migration, and proliferation, which can greatly affect the outcome of transplantation. Here we studied the effect of three inflammatory cytokines, interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and interleukin-6 (IL-6), from three different species (human, monkey, rat) on expression of MHC molecules and differentiation of two human NSPC lines derived from striatum and hippocampus. Human and monkey IFN-gamma strongly upregulate MHC expression in both NSPC lines in a dose-dependent manner, whereas rat IFN-gamma has an effect on MHC expression only in hippocampal cells. Furthermore, TNF-alpha, but not IL-6, upregulates MHC expression in both NSPC lines. Differentiation of NSPCs in the presence of cytokines showed that IFN-gamma increased the neuronal yield threefold in striatal NSPC cultures and increased the number of oligodendrocytes twofold in hippocampal NSPC cultures. Addition of TNF-alpha enhanced gliogenesis in both cell lines, whereas IL-6 stimulated neurogenesis. Human NSPC lines' response to cytokines is therefore species specific and also dependent on the NSPCs' region of origin. The successful translation of different cell lines from animal models to clinical trials could be substantially influenced by the species-specific regulation of MHC and differentiation as reported here. Disclosure of potential conflicts of interest is found at the end of this article.

Transplantation of neural progenitors enhances production of endogenous cells in the impaired brain

Grafting of neural progenitors has been shown to reverse a wide variety of neurobehavioral defects. While their role of replacing injured cells and restoring damaged circuitries has been shown, it is widely accepted that this cannot be the only mechanism, as therapy can occur even when an insufficient number of transplanted cells are found. We hypothesized that one major mechanism by which transplanted neural progenitors exert their therapeutic effect is by enhancing endogenous cells production. Consequently, in an allographic model of transplantation, prenatally heroin-exposed genetically heterogeneous (HS) mice were made defective in their hippocampal neurobehavioral function by exposing their mothers to heroin (10 mg kg(-1) heroin on gestation days 9-18). Hippocampal damage was confirmed by deficient performance in the Morris maze (P<0.009), and decreased production of endogenous cells in the dentate gyrus by 39% was observed. On postnatal day 35, they received an HS-derived neural progenitors transplant followed by repeated bromodeoxyuridine injections. The transplant returned endogenous cells production to normal levels (P<0.006) and reversed the behavioral defects (P<0.03), despite the fact that only 0.0334% of the transplanted neural progenitors survived and that they differentiated mainly to astrocytes. An immunological study demonstrated the presence of macrophages and T cells as a possible explanation for the paucity of the transplanted cells. This study suggests one mechanism for the therapeutic action of neural progenitors, the enhancement of the production of endogenous cells, pointing to future clinical applications in this direction by use of neural progenitors or by analogous cell-inducing techniques.

The immunobiological development of human bone marrow mesenchymal stem cells in the course of neuronal differentiation

The central nervous system (CNS) has been referred to as the "immunological privileged site". However, it is now clear that the privileged status of the CNS is a result of a balance between immune privilege and effective response. In vitro, human bone marrow mesenchymal stem cells (MSCs) have the ability to differentiate into neurons. Based on this biological attribute we gain the possibility by means of using MSCs as the donors to develop a future cell therapy in clinical application. But using MSCs as donor cells inevitably raises the question as to whether these donor cells would be immunogenic, and if so, would they be rejected after transplantation. To investigate this, human MSCs were cultured in vitro and induced to differentiate along neuronal lineage. The expression of human leukocyte antigen (HLA) class I and class II molecules and the co-stimulatory protein CD80 were increased on the surface of MSCs in the course of neuronal differentiation. But neither of the co-stimulatory proteins, CD40 or CD86, was expressed. After IFN-gamma exposure, the expression of the HLA molecules was further enhanced, but the co-stimulatory proteins were unaffected. MSCs that had been differentiated along neuronal lineage were not capable of inducing the proliferation of peripheral blood lymphocytes (PBLs). Even after IFN-gamma exposure, PBLs remained unresponsive. Furthermore, MSCs differentiated along neuronal lineage suppressed the proliferation of PBLs induced by allogeneic PBLs and mitogens. The mechanisms involved in the immunosuppression may be related to the effect of soluble factors and cell-cell interactions of neuronal differentiated MSCs and PBLs. From the above data we suggested that the low immunogenicity and immunomodulatory function of MSCs in the course of neuronal differentiation in vitro, which will be helpful to further investigation in order to establish the new way for future medical application.

Sustained survival of xenografted human neural stem/progenitor cells in experimental brain trauma despite discontinuation of immunosuppression

Neural stem cells have emerged as a promising therapeutic tool in CNS disease and injuries. In the clinical setting, cultured human neural stem/progenitor cells (hNSC) are an attractive possibility for transplantation to the damaged brain. However, transplantation of hNSC requires toxic immunosuppressive treatment to avoid rejection. The aim of the current study was to evaluate if shortening the duration of immunosuppression by cyclosporin A would affect hNSC survival and differentiation after transplantation to the site of a focal brain injury in the rat. hNSC were xenografted to the hippocampus and the medial limit of an experimentally induced cortical contusion. The animals received immunosuppression for either 6 or 3 weeks or no immunosuppression. The status of the grafted human cells was analysed by immunohistochemistry. No statistically significant differences were observed between the two immunosuppressed groups regarding graft survival, migration or proliferation at 6 weeks post-transplantation. In contrast, the graft survival was extremely poor in the non-immunosuppressed group. Furthermore, the expression of the differentiation markers nestin, neuronal nuclei (NeuN) and glial fibrillary acidic protein (GFAP) in the transplanted cells did not differ significantly between the two immunosuppressed groups. Moreover, a fourth group of eight animals that were immunosuppressed for 3 weeks were allowed to survive for 6 months. Five of these rats demonstrated robust graft survival in the hippocampus and scattered cells in the cortex. This study demonstrates the importance of immunosuppression but also the possibility of shortening immunosuppression without impacting on the phenotype of the grafted hNSC.

Combined immunosuppressive agents or CD4 antibodies prolong survival of human neural stem cell grafts and improve disease outcomes in amyotrophic lateral sclerosis transgenic mice

Amyotrophic lateral sclerosis (ALS) is a target for cell-replacement therapies, including therapies based on human neural stem cells (NSCs). These therapies must be first tested in the appropriate animal models, including transgenic rodents harboring superoxide dismutase (SOD1) mutations linked to familial ALS. However, these rodent subjects reject discordant xenografts. In the present investigation, we grafted NSCs from human embryonic spinal cord into the ventral lumbar cord of 2-month-old SOD1-G93A transgenic mice. Animals were immunosuppressed with FK506, FK506 plus rapamycin, FK506 plus rapamycin plus mycophenolate mofetil, or CD4 antibodies. With FK506 monotherapy, human NSC grafts were rejected within 1 week, whereas combinations of FK506 with one or two of the other agents or CD4 antibodies protected grafts into end-stage illness (i.e., more than 2 months after grafting). The combination of FK506 with rapamycin appeared to be optimal with respect to efficacy and simplicity of administration. Graft protection was achieved via the blockade of CD4- and CD8-cell infiltration and attenuation of the microglial phagocytic response from the host. Surviving NSCs differentiated extensively into neurons that began to establish networks with host nerve cells, including alpha-motor neurons. Immunosuppressed animals with live cells showed later onset and a slower progression of motor neuron disease and lived longer compared with immunosuppressed control animals with dead NSC grafts. Our findings indicate that combined immunosuppression promotes the survival of human NSCs grafted in the spinal cord of SOD1-G93A mice and, in doing so, allows the differentiation of NSCs into neurons and leads to the improvement of key parameters of motor neuron disease.

Pig xenografts to the immunocompetent rat brain: Survival rates using distinct neurotoxic lesions in the nigrostriatal pathway and two rat strains

Porcine foetal neurons for xenotransplantation in Parkinson's disease (PD) is an alternative source to human fetuses. One of the obstacles facing brain xenotransplantation is the existence of an immune response, which prevents long-term graft survival. Experimental results concerning the survival time of porcine foetal neurons implanted into the brain of immunocompetent rats have been quite different from one study to another, suggesting an effect on graft survival of uncontrolled experimental parameters. To identify such parameters, we have first analyzed the survival of porcine foetal nigral neurons at 5 and 10 weeks after implantation into the striatum of immunocompetent rats having different types of brain lesion affecting cells (quinolinic acid) or projections to the striatum (MPP+, 6-OHDA). In a second experiment, graft survival was analyzed in two strains of recipient rats (female Sprague-Dawley and male Lewis rats) in conditions of ipsilateral dopaminergic denervation using 6-OHDA. The characteristics of surviving grafts were assessed by measuring the graft volume, the number of TH+ neurons, the size of TH+ neurons soma, and CD5+ cell infiltration. Long-term survival (> or = 10 weeks) of porcine neurons could be observed in all experimental models. However, there was no significant difference in graft survival rates and characteristics of the surviving grafts between the lesioned groups, or between Sprague-Dawley and Lewis rats. Altogether, results were highly variable within groups of grafts exposed to similar experimental procedures at both 5 and 10 weeks post-grafting. We conclude that the distinct neurotoxins and host rat strains used in our experimental design are not major factors influencing the rejection time-course of primary neural xenografts.

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.

Transgenic expression of CTLA4-Ig by fetal pig neurons for xenotransplantation

The transplantation of fetal porcine neurons is a potential therapeutic strategy for the treatment of human neurodegenerative disorders. A major obstacle to xenotransplantation, however, is the immune-mediated rejection that is resistant to conventional immunosuppression. To determine whether genetically modified donor pig neurons could be used to deliver immunosuppressive proteins locally in the brain, transgenic pigs were developed that express the human T cell inhibitory molecule hCTLA4-Ig under the control of the neuron-specific enolase promoter. Expression was found in various areas of the brain of transgenic pigs, including the mesencephalon, hippocampus and cortex. Neurons from 28-day old embryos secreted hCTLA4-Ig in vitro and this resulted in a 50% reduction of the proliferative response of human T lymphocytes in xenogenic proliferation assays. Transgenic embryonic neurons also secreted hCTLA4-Ig and had developed normally in vivo several weeks after transplantation into the striatum of immunosuppressed rats that were used here to study the engraftment in the absence of immunity. In conclusion, these data show that neurons from our transgenic pigs express hCTLA4-Ig in situ and support the use of this material in future pre-clinical trials in neuron xenotransplantation.

Biological pacemaker created by fetal cardiomyocyte transplantation

BACKGROUND

The aim of this study was to investigate the feasibility of an alternative approach to electronic pacemaker by using spontaneously excitable cell grafts as a biological pacemaker in a large animal model of complete atrioventricular block.

METHODS AND RESULTS

Dissociated male human atrial cardiomyocytes including sinus nodal cells were grafted into the free wall of the left ventricle in five female pigs. Three weeks after the injection of cell-grafted solution/control medium the pigs underwent catheter ablation of the atrioventricular node (AV-node). After complete AV block was created, the idioventricular beat rate was more rapid in cell-grafted pigs than that in control pigs (86+/-21 vs. 30+/-10 bpm; P<0.001). Administering of isoprenalin significantly increased idioventricular rate from 86+/-21 to 117+/-18 bpm in the cell-grafted animals (P<0.01). Electrophysiological mapping studies demonstrated that the idioventricular rhythm originated from the cell-injection site. Polymerase chain reaction verifying the existence of SRY DNA in the cell injection site indicated that the grafted male cells were survived. Furthermore, the connexin-43 and N-cadherin positive junctions between donor cardiomyocytes and host cells were identified.

CONCLUSION

Xenografted fetal human atrial cardiomyocytes are able to survive and integrate into the host myocardium, and show a pacing function that can be modulated by autonomic agents.

MHC expression after human neural stem cell transplantation to brain contused rats

Human neural stem cells survive and improve motor function after transplantation to the contused brain. However, the transplants might be rejected and that depends on the graft immunogenicity, the host immunological status and the immunosuppression strategy. We transplanted human neural stem cells to rats with brain contusion and analyzed the donor and host MHC antigen expression and the effect of a short-term immunosuppression with cyclosporine. In vitro human neural stem cells expressed only MHC-II antigens. This expression was down-regulated 6 weeks after transplantation. The host response was characterized by an increased MHC-II expression which was down-regulated by a longer term of immunosuppression. These findings are novel and necessary in order to understand the immunogenicity of human neural stem cell grafts.

Genetic and Cellular Therapies for Cerebral Infarction

Neurosurgeons, working as surgical scientists, can have a prominent role in developing and implementing genetic and cellular therapies for cerebral ischemia. The rapid emergence of both genetic and cellular therapies for neural regeneration warrants a careful analysis before implementation of human studies to understand the pitfalls and promises of this strategy. In this article, we review the topic of genetic and cellular therapy for stroke to provide a foundation for practicing neurosurgeons and clinical scientists who may become involved in this type of work. In Part 1, we review preclinical approaches with gene transfer, such as 1) improved energy delivery, 2) reduction of intracellular calcium availability, 3) abrogation of effects of reactive oxygen species, 4) reduction of proinflammatory cytokine signaling, 5) inhibition of apoptosis mediators, and 6) restorative gene therapy, that are paving the way to develop new strategies to treat cerebral infarction. In Part 2, we discuss the results of studies that address the possibility of using cellular therapies for stroke in animal models and in human trials by reviewing 1) the basics of stem cell biology, 2) exogenous and 3) and endogenous cell sources for therapy, and 4) clinical considerations in cell therapy applications. These emerging technologies based on the advancements made in recent years in the fields of genetics, therapeutic cloning, neuroscience, stem cell biology, and gene therapy provide significant potential for new therapies for stroke.

Activated porcine embryonic brain endothelial cells induce a proliferative human T-lymphocyte response

Transplantation of allogeneic embryonic neural tissue is a potential treatment for patients with Parkinson's and Huntington's diseases. The supply of human donor tissue is limited, and alternatives such as the use of animal (e.g., porcine) donor tissue are currently being evaluated. Before porcine grafts can be used clinically, strategies to prevent neural xenograft rejection must be developed. Knowledge on how human T lymphocytes recognize porcine embryonic neural tissue would facilitate the development of such strategies. To investigate the ability of porcine embryonic brain microvascular endothelial cells (PBMEC) to stimulate human T-cell proliferation, PBMEC were immuno-magnetically isolated and cocultured with purified human CD4 or CD8 single-positive T cells. PBMEC had a cobblestone-like growth pattern and expressed the endothelial cell markers CD31 and CD106. PBMEC stimulated with the supernatant of phytohemagglutinin-activated porcine peripheral blood mononuclear cells or porcine IFN-gamma, but not nonstimulated PBMEC, induced proliferation of both CD8 and CD4 T cells as assessed by [3H]thymidine incorporation. Flow cytometric analyses showed that the degree of CD8 and CD4 T cell proliferation correlated with the expression levels of class I and II major histocompatibility complex (MHC) antigens, respectively. PBMEC expressed a CTLA-4/Fc-reactive molecule, most likely CD86, suggesting that these cells are able to deliver a costimulatory signal to the T cells. Human TNF-alpha, but not human IFN-gamma, induced class I, but not class II, MHC expression on PBMEC. Within a neural graft or the regional lymph nodes, PBMEC might stimulate human T cells via the direct pathway, and should therefore be removed from the donor tissue prior to transplantation.

The cellular repair of the brain in Parkinson's disease—past, present and future

Damage to the central nervous system was once considered irreparable. However, there is now growing optimism that neural transplant therapies may one day enable complete circuit reconstruction and thus functional benefit for patients with neurodegenerative conditions such as Parkinson's disease (PD), and perhaps even those with more widespread damage such as stroke patients. Indeed, since the late 1980s hundreds of patients with Parkinson's disease have received allografts of dopamine-rich embryonic human neural tissue. The grafted tissue has been shown to survive and ameliorate many of the symptoms of the disease, both in the clinical setting and in animal models of the disease. However, practical problems associated with tissue procurement and storage, and ethical concerns over using aborted human fetal tissue have fuelled a search for alternative sources of suitable material for grafting. In particular, stem cells and xenogeneic embryonic dopamine-rich neural tissue are being explored, both of which bring their own practical and ethical dilemmas. Here we review the progress made in neural transplantation, both in the laboratory and in the clinic with particular attention to the development of stem cell and xenogeneic tissue based therapy.

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.

CNS regeneration: clinical possibility or basic science fantasy?

Following injury to the CNS, severed axons undergo a phase of abortive sprouting in the vicinity of the wound, but do not spontaneously re-grow or regenerate. From a long history of attempts to stimulate regeneraion, a major strategy that has been developed clinically is the implantation of tissue into denervated target regions. Unfortunately trials have so far not borne out the promise that this would prove a useful therapy for disorders such as Parkinson's disease. Many strategies have also been developed to stimulate the regeneration of axons across sites of injury, particularly in the spinal cord. Animal data have demonstrated that some of these approaches hold promise and that the spinal cord has a remarkable degree of intrinsic plasticity. Attempts are now being made to utilize experimental techniques in spinal patients.

Induction of operational tolerance to discordant dopaminergic porcine xenografts

BACKGROUND

Porcine embryonic neural tissue transplanted intracerebrally could potentially relieve the symptoms of Parkinson's disease if the immune response toward the graft could be overcome. However, conventional immunosuppressive treatments have proven inefficient in preventing rejection. An alternative is blocking the costimulatory signals for lymphocyte activation. Treatment with cytotoxic T-lymphocyte antigen 4 immunoglobulin (CTLA4Ig) and anti-CD40L has been successful in preventing rejection of xenografts in some experimental studies, but not all. Lymphocyte function antigen (LFA)-1 is an important costimulatory molecule for CD8+ T cells, and we hypothesize that blockade with anti-LFA-1 may enhance the efficacy of CTLA4Ig and anti-CD40L therapy.

METHODS

C57BL/6 mice received intracerebral transplants of ventral mesencephalic tissue from embryonic porcine donors. CTLA4Ig, anti-CD40L, and anti-LFA-1 were administered every other day on days 0 to 8, and the transplants were studied after 4 to 6 weeks. Grafts were histologically analyzed for size, survival of dopaminergic nerve cells, and immune responses. Recipients were challenged with cultured glia cells of donor origin or an allogeneic skin graft to evaluate tolerance induction.

RESULTS

Mice treated with all three substances had large grafts containing high amounts of dopamine cells but a low degree of immune response. Grafts in recipients challenged with glial cells showed an increased immunologic activity but were not rejected. Triple-treated mice showed a normal rejection process of the allogeneic skin grafts.

CONCLUSION

After a short course of costimulation blocking therapy, discordant neural xenografts demonstrate long-term survival, withstand immunologic challenge, yet maintain host-versus-graft reactivity. Anti-LFA-1 complements CTLA4Ig and anti-CD40L in the induction of operational tolerance to these xenografts.

The immunological properties of adult hippocampal progenitor cells

Adult hippocampal progenitor cells (AHPCs) derived from mature rats were studied in mixed co-cultures and shown not to elicit a proliferative response from human peripheral blood mononuclear cells (PBMCs) or allogeneic spleen cells. FACS analysis revealed low class I and no detectable class II (Ia) MHC expression by these cells. RT-PCR showed that AHPCs express the anti-inflammatory cytokine TGF-beta1. AHPCs did not, however, significantly impede the proliferation of OKT3- or PHA-stimulated PBMCs. Taken together, these results indicate that AHPCs are non-immunogenic in vitro. This is consistent with their pattern of MHC expression and does not require an active immunosuppressive mechanism.

Clinical prospects for neural grafting therapy for hippocampal lesions and epilepsy

OBJECTIVE

Hippocampal lesions and epilepsy may be potential clinical targets for neural grafting. We hypothesized that neural grafting could be a restorative therapy either acutely, adding unformed neural elements, or chronically, treating postlesioning epilepsy. The goal of this review was to assess the clinical reality of this hypothesis of neural grafting and to determine the problems that remain to be resolved before grafting can be applied clinically.

METHODS

We quantitatively defined graft integration within the host, on a cellular basis, by directly assessing survival of the transplanted neurons, graft cell dispersion and migration, neuronal differentiation and development, and establishment of appropriate local and long-distance synaptic connectivity.

RESULTS

Embryonic hippocampal suspension grafts demonstrate excellent survival rates (20-80%). Embryonic axons exhibit extensive, appropriate, local and long-distance connectivity, can facilitate reconstruction of excitatory and inhibitory cortical circuitry, and can prevent the formation of aberrant circuitry. Immature neural stem cells demonstrate lesser degrees of integration, likely because of a paucity of positional cues in the lesioned brain for the differentiation of stem cells into region-specific neuronal phenotypes. Labeled grafted cells may be selectively and noninvasively removed from the host with triggerable stealth toxins, for the late treatment of unanticipated graft problems.

CONCLUSION

Neural grafting with appropriate embryonic neurons may provide significant clinical benefits. However, embryonic cell availability is severely limited, and alternative sources of cells, such as stem cells, require significant additional research into the induction and maintenance of neuronal commitment and the ability of the cells to form functional synaptic connections in vivo.

Restoring function after spinal cord injury

BACKGROUND

By affecting young people during the most productive period of their lives, spinal cord injury is a devastating problem for modern society. A decade ago, treating SCI seemed frustrating and hopeless because of the tremendous morbidity and mortality, life-shattering impact, and limited therapeutic options associated with the condition. Today, however, an understanding of the underlying pathophysiological mechanisms, the development of neuroprotective interventions, and progress toward regenerative interventions are increasing hope for functional restoration.

REVIEW SUMMARY

This study addresses the present understanding of SCI, including the etiology, pathophysiology, treatment, and scientific advances. The discussion of treatment options includes a critical review of high-dose methylprednisolone and GM-1 ganglioside therapy. The concept that limited rebuilding can provide a disproportionate improvement in quality of life is emphasized throughout.

CONCLUSIONS

New surgical procedures, pharmacologic treatments, and functional neuromuscular stimulation methods have evolved over the last decades that can improve functional outcomes after spinal cord injury, but limiting secondary injury remains the primary goal. Tissue replacement strategies, including the use of embryonic stem cells, become an important tool and can restore function in animal models. Controlled clinical trials are now required to confirm these observations. The ultimate goal is to harness the body's own potential to replace lost central nervous system cells by activation of endogenous progenitor cell repair mechanisms.

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.

Potential new methods for antiepileptic drug delivery

Use of novel drug delivery methods could enhance the efficacy and reduce the toxicity of antiepileptic drugs (AEDs). Slow-release oral forms of medication or depot drugs such as skin patches might improve compliance and therefore seizure control. In emergency situations, administration via rectal, nasal or buccal mucosa can deliver the drug more quickly than can oral administration. Slow-release oral forms and rectal forms of AEDs are already approved for use, nasal and buccal administration is currently off-label and skin patches for AEDs are an attractive but currently hypothetical option. Therapies under development may result in the delivery of AEDs directly to the regions of the brain involved in seizures. Experimental protocols are underway to allow continuous infusion of potent excitatory amino acid antagonists into the CSF. In experiments with animal models of epilepsy, AEDs have been delivered successfully to seizure foci in the brain by programmed infusion pumps, acting in response to computerised EEG seizure detection. Inactive prodrugs can be given systemically and activated at the site of the seizure focus by locally released compounds. One such drug under development is DP-VPA (or DP16), which is cleaved to valproic acid (sodium valproate) by phospholipases at the seizure focus. Liposomes and nanoparticles are engineered micro-reservoirs of a drug, with attached antibodies or receptor-specific binding agents designed to target the particles to a specific region of the body. Liposomes in theory could deliver a high concentration of an AED to a seizure focus. Penetration of the blood-brain barrier can be accomplished by linking large particles to iron transferrin or biological toxins that can cross the barrier. In the near future, it is likely that cell transplants that generate neurotransmitters and neuromodulators will accomplish renewable endogenous drug delivery. However, the survival and viability of transplanted cells have yet to be demonstrated in the clinical setting. Gene therapy also may play a role in local drug delivery with the use of adenovirus, adeno-associated virus, herpesvirus or other delivery vectors to induce brain cells to produce local modulatory substances. New delivery systems should significantly improve the therapeutic/toxic ratio of AEDs.

Lipid-mediated glial cell line-derived neurotrophic factor gene transfer to cultured porcine ventral mesencephalic tissue

Transplantation of dopaminergic ventral mesencephalic (VM) tissue into the basal ganglia of patients with Parkinson's disease (PD) shows at best moderate symptomatic relief in some of the treated cases. Experimental animal studies and clinical trials with allogenic and xenogenic pig-derived VM tissue grafts to PD patients indicate that one reason for the poor outcome of neural transplantation is the low survival and differentiation of grafted dopaminergic neurons. To improve dopaminergic cell survival through a gene-therapeutic approach we have established and report here results of lipid-mediated transfer of the gene for human glial cell line-derived neurotrophic factor (GDNF) to embryonic (E27/28) porcine VM tissue kept as organotypic explant cultures. Treatment of the developing VM with two mitogens, basic fibroblast growth factor and epidermal growth factor, prior to transfection significantly increased transfection yields. Expression of human GDNF via an episomal vector could be detected by in situ hybridization and by the measuring of GDNF protein secreted into the culture medium. When compared to mock-transfected controls, VM tissue expressing recombinant GDNF contained significantly higher numbers of tyrosine hydroxylase-positive neurons in the cultured VM tissue. We conclude that lipid-mediated gene transfer employed on embryonic pig VM explant cultures is a safe and effective method to improve survival of dopaminergic neurons and may become a valuable tool to improve allo- and xenotransplantation treatment in Parkinson's disease.

Life duration: a new systemic hypothesis

A new hypothesis of ageing based on morphologic evidence is presented. The developmental process is determined by a kinetic curve of population growth which is typical for every association of cells. Every multicellular organism is considered as a system that consists of various cellular associations in symbiotic interaction. One of these associations is dominant and determines the developmental kinetics of the whole organism. The nervous system is the dominant cell association in mammals. The duration of its development is restricted by the lack of capability of neurons to regenerate in adult organisms. This may determine the duration of life and persistence of various pathologic processes.

Spinal-cord injury

More than a decade ago, spinal-cord injury meant confinement to a wheelchair and a lifetime of medical comorbidity. The physician's armamentarium of treatments was very limited, and provision of care for individuals with spinal-cord injury was usually met with frustration. Advances in the neurosciences have drawn attention to research into spinal-cord injury. Nowadays, advanced interventions provide high hope for regeneration and functional restoration. As scientific advances become more frequent, scepticism is giving way to the ideas that spinal-cord injury will eventually be repairable and that strategies to restore function are within our grasp. We address the present understanding of spinal-cord injury, its cause, pathophysiology, diagnosis, and treatment, and look at promising research avenues. We also discuss new treatment options, including functional electric stimulation and part-weight-supported walking.

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.

Simultaneous inhibition of B7 and LFA-1 signaling prevents rejection of discordant neural xenografts in mice lacking CD40L

Transplantation of embryonic human neural tissue can restore dopamine neurotransmission and improve neurological function in patients with Parkinson's disease. Logistical and ethical factors limit the availability of human embryonic allogeneic tissue. Embryonic xenogeneic neural tissue from porcine donors is an alternative form of donor tissue, but effective immunomodulatory techniques are warranted for neural xenotransplantation to become clinically feasible. We transplanted embryonic porcine ventral mesencephalic tissue into the brains of adult untreated C57BL/6 mice, untreated CD40L-/-mice and CD40L-/-mice that received injections of anti-LFA-1, CTLA41g or both compounds. Double-treated CD40L-/-mice had large grafts with high numbers of dopaminergic neurons 4 wk after transplantation. The grafts were completely devoid of lymphocytes, macrophages and activated microglia. Untreated C57BL/6 mice had rejected their grafts. Untreated CD40L-/-mice and CD40L-/-mice treated with monotherapy of anti-LFA-1 or CTLA41g had smaller grafts and more microglial and lymphocytic infiltration than double-treated CD40L-/-mice. We conclude that immunomodulation with concomitant inhibition of LFA-1 and B7 signaling in the perioperative period in CD40L-/-mice prevented the rejection of discordant neural xenografts. The treatment most likely reduced antigen presenting capacity and interfered with the costimulatory signaling needed for T cell activation to occur.

The in vitro activity and specificity of human endothelial cell-specific promoters in porcine cells

The chronic shortage of human organs, tissues and cells for transplantation has inspired research on the possibility of using animal donor tissue instead. Transplantation over a species barrier is associated with rejections which are difficult to control. Therefore, it is generally agreed that successful pig to human xenotransplantation requires donor pigs to be genetically modified. Vascular endothelium is the most immediate barrier between the xenogeneic donor organ and host immune and nonimmune defense systems. Thus, these cells are the prime targets for such genetic modifications. Luciferase assays were used to evaluate the activity and specificity of human endothelial-cell specific promoters in porcine aortic-, microvascular- and nonendothelial cells. The promoters for human Flk-1 (fetal liver kinase-1), Flt-1 (fms-like tyrosine kinase), ICAM-2 (intercellular adhesion molecule-2), thrombomodulin and vWf (von Willebrand factor) supported similar levels of luciferase expression in human and porcine aortic endothelial cells, with the Flk-1 promoter being the strongest followed by the thrombomodulin promoter. Relative to the activity of the CMV promoter, the human endothelial cell-specific promoters all showed less activity in porcine kidney microvascular endothelial cells than in liver or brain microvascular endothelial cells. The thrombomodulin and Flk-1 promoters exhibited similar activity in liver and kidney microvascular endothelial cells, whereas the Flk-1 promoter was stronger in aortic and brain microvascular endothelial cells. Human endothelial cell-specific promoters also showed some degree of specificity in pig, because they supported less luciferase activity in porcine nonendothelial cell lines. Based on the in vitro data and previously published in vivo data, the human Flk-1 and thrombomodulin promoters are good candidate promoters for strong endothelial cell-specific gene expression in transgenic pigs.

Xenotransplantation for brain repair: reduction of porcine donor tissue immunogenicity by treatment with anti-Gal antibodies and complement

BACKGROUND

Transplantation of embryonic neural tissue is a potential treatment for Parkinson's disease. Because human donor material is in short supply, porcine xenografts are considered a useful alternative. Current immunosuppressive therapies fail, however, to protect intracerebral neural xenografts from host CD4 T lymphocytes. To reduce the immunogenicity of porcine donor tissue, we attempted to remove microglial cells with antibodies against the alpha-galactosyl epitope (Galalpha1,3Galbeta1,4GlcNAc-R), or anti-Gal, and complement, and studied whether this pretreatment can reduce direct and indirect T-cell responses to the tissue.

METHODS

Brain tissue from 27-day-old pig embryos was dissociated and treated with human anti-Gal and rabbit complement. The microglial content was analyzed by flow cytometry. [3H]thymidine incorporation in cocultures of the brain cells and purified human CD4 T cells was used to determine direct T-cell responses. Indirect T-cell responses were studied by grafting pretreated and control-pretreated (no anti-Gal) nigral tissue into the lesioned striatum of immunocompetent rats with 6-hydroxydopamine-induced hemiparkinsonism. Amphetamine-induced circling behavior was used to measure graft function.

RESULTS

Anti-Gal and complement reduced the microglial content to 11-24% of control and abolished the ability of the brain cells to induce human CD4 T-cell proliferation. Pretreated nigral tissue reduced hemiparkinsonism by more than 50% in five of eight rats at some point during the 10-week follow-up. Rats receiving control-pretreated nigral tissue did not display this degree of improvement.

CONCLUSIONS

Pretreatment with anti-Gal and complement can reduce the immunogenicity of porcine neural tissue, and might, therefore, be a valuable alternative or supplement to immunosuppression in neural xenotransplantation.

Effects of immunosuppressive treatment on host responses against intracerebral porcine neural tissue xenografts in rats

BACKGROUND

Embryonic xenogeneic neural tissue is an alternative for transplantation in Parkinson's disease, but immune responses limit the application. The aims of this study were to enhance the in vitro viability rates by donor tissue pretreatment; to compare the efficacy of cyclosporine A (CsA) and tacrolimus (FK) in inhibiting xenograft rejection in rats; to evaluate additional inductive therapy with prednisolone (PRE) or mycophenolate mofetil (MMF).

METHODS

Tirilazad (a lipid peroxidase inhibitor) or FK and acYVAD-cmk (a caspase inhibitor), were added to embryonic porcine ventral mesencephalic tissue and viability was assessed in vitro. Tirilazad-treated tissue was grafted to the striatum of rats that were either left untreated or immunosuppressed with FK (1 mg/kg) or CsA (15 mg/kg) alone or in combination with a 2-week PRE (20 mg/kg) or MMF (40 mg/kg) induction course. Xenograft survival and host responses were determined using immunohistochemistry.

RESULTS

Pretreatment with tirilazad enhanced tissue survival in vitro. After transplantation into untreated controls, there was no graft survival at twelve weeks. Neural cell counts were significantly improved in immunosuppressed recipients, but there were no differences between the treatment groups. Additional inductive treatment reduced the infiltration with CD4+ and CD8+ cells, and macrophage infiltration was reduced compared with animals given CsA or FK alone.

CONCLUSION

Pretreatment of the donor tissue with free-radical scavengers reduces cell loss caused by tissue trauma. Porcine neural tissue xenografts survive significantly better in animals immunosuppressed with either FK or CsA. Additional inductive treatment with PRE or MMF reduced the infiltration of host cells into the xenografts.