Histological evidence of fetal pig neural cell survival after transplantation into a patient with Parkinson's disease

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

Neural transplantation holds promise for focal CNS repair. Owing to the shortage of human donor material, which is derived from aborted embryos, and ethical concerns over its use, animal donor tissue is now considered an appropriate alternative. In the USA, individuals suffering from Parkinson's disease, Huntington's disease, focal epilepsy or stroke have already received neural grafts from pig embryos. However, in animal models, neural tissue transplanted between species is usually promptly rejected, even when implanted in the brain. Some of the immunological mechanisms that underlie neural xenograft rejection have recently been elucidated, but others remain to be determined and controlled before individuals with neurological disorders can benefit from xenotransplantation.

Xenotransplantation of the liver

Xenotransplantation of the liver, in its broadest conception, might involve the transplantation of an intact organ or xenogeneic hepatocytes, or the use of an intact xenogeneic liver or cells as an ex vivo "device." The indications for xenotransplantation include not only hepatic failure but also, potentially, the treatment of metabolic diseases. The hurdles to xenotransplantation include immune, physiologic, and infectious complications. New information and progress in experimental systems are bringing xenotransplantation closer to clinical application.

Nonviral glial cell-derived neurotrophic factor gene transfer enhances survival of cultured dopaminergic neurons and improves their function after transplantation in a rat model of Parkinson's disease

Transplantation of dopaminergic fetal mesencephalic tissue into the striatum is currently being developed for treatment of patients with advanced Parkinson's disease. Ethical concerns regarding the use of human fetal tissue, and the limited availability as well as poor survival and differentiation of dopaminergic neurons after transplantation have reduced the extent and outcome of this approach so far. With the purpose of finding means to increase the yield of dopaminergic neurons in transplants, and to reduce the amount of fetal tissue needed for each transplanted patient, we transfected rat fetal ventral mesencephalic (VM) tissue grown as organotypic free-floating roller tube (FFRT) cultures with a vector encoding human glial cell-derived neurotrophic factor (hGDNF). For transfer of an episomal expression vector (pRep7-GDNF8) a nonviral, nonliposomal cationic transfection technique was applied and optimized. Recombinant hGDNF expression resulted in a higher number of TH-positive neurons in the cultures as measured 6 days after transfection. Ventral mesencephalic cultures expressing hGDNF were then grafted into the striatum of unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. Grafting of genetically modified VM cultures resulted in earlier functional recovery compared with grafting nontransfected cultures. We conclude that organotypic free-floating roller tube cultures can be successfully transfected to produce hGDNF with effects on TH-expressing neurons in vitro and functional effects after grafting in a rat Parkinson's disease model.

Improved survival of embryonic porcine dopaminergic neurons in coculture with a conditionally immortalized GDNF-producing hippocampal cell line

Transplantation of embryonic nigral tissue is used as an experimental therapy for patients with Parkinson's disease but is hampered by a limited survival rate of dopaminergic neurons. Glial cell line-derived neurotrophic factor (GDNF) is a potent survival factor for nigrostriatal dopaminergic neurons, and the present in vitro study aimed at improving the survival of dopaminergic neurons in porcine mesencephalic brain slice cultures by adding transfected, immortalized, temperature-sensitive GDNF-releasing HiB5 cells (HiB5-GDNF). Embryonic (E27/28) porcine ventral mesencephalic brain slices were placed on membrane inserts in six-well plates with serum-containing medium, and HiB5-GDNF, nontransfected HiB5 cells (HiB5-control), or green fluorescent protein-producing HiB5 cells (HiB5-GFP) were seeded onto each tissue slice. The concentration of GDNF in the coculture medium was 0.49 +/- 0.13 ng/ml at day 9 and 0. 22 +/- 0.05 ng/ml at day 19 (mean +/- SEM) as measured by GDNF ELISA. The decrease in release of GDNF over time was paralleled by a gradual reduction in the number of HiB5-GFP cells expressing the reporter gene (EGFP). At day 12, HPLC analysis revealed that medium from HiB5-GDNF cocultures contained 2.0 times more dopamine than medium from HiB5-control cocultures. At day 21 there was 1.6 times more dopamine. Similar results were obtained for the dopamine metabolite 3,4-dihydroxyphenylacetic acid. At day 21, cell counts showed that HiB5-GDNF cocultures contained 1.5 times more tyrosine hydroxylase immunoreactive neurons than HiB5-control cocultures, which must be compared with a 1.8 fold increase after chronic treatment with rhGDNF (10 ng/ml). In conclusion, the better survival of HiB5-GDNF cocultures is promising for the generation of effective cell lines for local delivery of neurotrophic factors to intracerebral nigral grafts.

Prospects for the treatment of Parkinson's disease using neural grafts

Parkinson's disease (PD) is an incurable neurodegenerative condition of the central nervous system (CNS) that typically presents in the fifth to seventh decade of life, with a movement disorder that consists of a resting tremor, bradykinesia and rigidity. It is a disease that can only be diagnosed with certainty at postmortem when the pathological hallmark is loss of the dopaminergic nigrostriatal pathway and presence of Lewy bodies in the substantia nigra. However, pathological changes, including Lewy body formation, are found outside of the nigrostriatal system and involve other neurotransmitters, which may also account for some of the cognitive, psychiatric and autonomic abnormalities in these patients. To date, the mainstay of treatment for patients with PD has been drugs that activate the dopaminergic network, namely the dopamine precursor L-dopa and dopamine receptor agonists. However, recently interest has turned towards more curative therapies, including the use of grafts of neural tissue to replace dopaminergic neurones that have been lost. This approach has now entered clinical trials and this review will analyse the therapeutic approach of neural grafting in PD.

A role for complement in the rejection of porcine ventral mesencephalic xenografts in a rat model of Parkinson's disease

Vascularized whole organ discordant xenografts placed in the periphery are rejected by a rapid "hyperacute" process that involves preformed antibody binding to the xeno-antigens on the donor endothelial cells with complement activation. In the CNS, xenografts are classically thought to be rejected more slowly by a T-cell-dependent process. We now report that xenografts of embryonic porcine ventral mesencephalic tissue in the 6-hydroxydopamine-lesioned, nonimmunosuppressed rat induce both a humoral and a cell-mediated response. Over the first 10 d after implantation, the xenografts matured with identifiable TH neurons and pig-specific neurofilament fibers extending along host white matter tracts. During this period of time, IgM and complement binding were observed within the graft, as well as a CD8 cellular infiltrate, leading to rejection of the transplant over the next 25 d. These intracerebral xenografts were not associated with an early systemic antibody response. A role for complement in this rejection process was further investigated using cobra venom factor (CVF), which systemically depleted the rats of complement for 7 d. CVF treatment, when given in the period immediately before and after grafting, delayed but did not prevent the cellular immune response induced by the graft, demonstrating that xenografted neural tissue can activate the humoral arm of the rejection process, in particular the complement cascade. This suggests that interventions targeting this aspect of the immune rejection process may be of great importance for the future development of xenotransplantation for neurodegenerative conditions.

Establishment and characterization of molecular clones of porcine endogenous retroviruses replicating on human cells

The use of pig xenografts is being considered to alleviate the shortage of allogeneic organs for transplantation. In addition to the problems overcoming immunological and physiological barriers, the existence of numerous porcine microorganisms poses the risk of initiating a xenozoonosis. Recently, different classes of type C porcine endogenous retoviruses (PERV) which are infectious for human cells in vitro have been partially described. We therefore examined whether completely intact proviruses exist that produce infectious and replication-competent virions. Several proviral PERV sequences were cloned and characterized. One molecular PERV class B clone, PERV-B(43), generated infectious particles after transfection into human 293 cells. A second clone, PERV-B(33), which was highly homologous to PERV-B(43), showed a G-to-A mutation in the first start codon (Met to Ile) of the env gene, preventing this provirus from replicating. However, a genetic recombinant, PERV-B(33)/ATG, carrying a restored env start codon, became infectious and could be serially passaged on 293 cells similar to virus clone PERV-B(43). PERV protein expression was detected 24 to 48 h posttransfection (p. t.) using cross-reacting antiserum, and reverse transcriptase activity was found at 12 to 14 days p.t. The transcriptional start and stop sites as well as the splice donor and splice acceptor sites of PERV mRNA were mapped, yielding a subgenomic env transcript of 3. 1 kb. PERV-B(33) and PERV-B(43) differ in the number of copies of a 39-bp segment in the U3 region of the long terminal repeat. Strategies to identify and to specifically suppress or eliminate those proviruses from the pig genome might help in the production of PERV-free animals.

Intrastriatal and intranigral grafting of hNT neurons in the 6-OHDA rat model of Parkinson's disease

The clinical findings on neural transplantation for Parkinson's disease (PD) reported thus far are promising but many issues must be addressed before neural transplantation can be considered a routine therapeutic option for PD. The future of neural transplantation for the treatment of neurological disorders may rest in the discovery of a suitable alternative cell type for fetal tissue. One such alternative may be neurons derived from a human teratocarcinoma (hNT). hNT neurons have been shown to survive and integrate within the host brain following transplantation and provide functional recovery in animal models of stroke and Huntington's disease. In this study, we describe the transplantation of hNT neurons in the substantia nigra (SN) and striatum of the rat model for PD. Twenty-seven rats were grafted with one of three hNT neuronal products; hNT neurons, hNT-DA neurons, or lithium chloride (LiCl) pretreated hNT-DA neurons. Robust hNT grafts could be seen with anti-neural cell adhesion molecule and anti-neuron-specific enolase immunostaining. Immunostaining for tyrosine hydroxylase (TH) expression revealed no TH-immunoreactive (THir) neurons in any animals with hNT neuronal grafts. THir cells were observed in 43% of animals with hNT-DA neuronal grafts and all animals with LiCl pretreated hNT-DA neuronal grafts (100%). The number of THir neurons in these animals was low and not sufficient to produce significant functional recovery. In summary, this study has demonstrated that hNT neurons survive transplantation and express TH in the striatum and SN. Although hNT neurons are promising as an alternative to fetal tissue and may have potential clinical applications in the future, further improvements in enhancing TH expression are needed.

Porcine xenografts in Parkinson's disease and Huntington's disease patients: preliminary results

The observation that fetal neurons are able to survive and function when transplanted into the adult brain fostered the development of cellular therapy as a promising approach to achieve neuronal replacement for treatment of diseases of the adult central nervous system. This approach has been demonstrated to be efficacious in patients with Parkinson's disease after transplantation of human fetal neurons. The use of human fetal tissue is limited by ethical, infectious, regulatory, and practical concerns. Other mammalian fetal neural tissue could serve as an alternative cell source. Pigs are a reasonable source of fetal neuronal tissue because of their brain size, large litters, and the extensive experience in rearing them in captivity under controlled conditions. In Phase I studies porcine fetal neural cells grafted unilaterally into Parkinson's disease (PD) and Huntington's disease (HD) patients are being evaluated for safety and efficacy. Clinical improvement of 19% has been observed in the Unified Parkinson's Disease Rating Scale "off" state scores in 10 PD patients assessed 12 months after unilateral striatal transplantation of 12 million fetal porcine ventral mesencephalic (VM) cells. Several patients have improved more than 30%. In a single autopsied PD patient some porcine fetal VM cells were observed to survive 7 months after transplantation. Twelve HD patients have shown a favorable safety profile and no change in total functional capacity score 1 year after unilateral striatal placement of up to 24 million fetal porcine striatal cells. Xenotransplantation of fetal porcine neurons is a promising approach to delivery of healthy neurons to the CNS. The major challenges to the successful use of xenogeneic fetal neuronal cells in neurodegenerative diseases appear to be minimizing immune-mediated rejection, management of the risk of xenotic (cross-species) infections, and the accurate assessment of clinical outcome of diseases that are slowly progressive.

Conditionally immortalized, multipotential and multifunctional neural stem cell lines as an approach to clinical transplantation

Experiments are described using rats with two kinds of brain damage and consequent cognitive deficit (in the Morris water maze, three-door runway, and radial maze): 1) ischemic damage to the CA1 hippocampal cell field after four-vessel occlusion (4VO), and 2) damage to the forebrain cholinergic projection system by local injection of excitotoxins to the nuclei of origin or prolonged ethanol administration. Cell suspension grafts derived from primary fetal brain tissue display a stringent requirement for homotypical cell replacement in the 4VO model: cells from the embryonic day (E)18-19 CA1 hippocampal subfield, but not from CA3 or dentate gyrus or from E16 basal forebrain (cholinergic rich) led to recovery of cognitive function. After damage to the cholinergic system, conversely, recovery of function was seen with cell suspension grafts from E16 basal forebrain or cholinergic-rich E14 ventral mesencephalon, but not with implants of hippocampal tissue. These two models therefore provided a test of multifunctionality for a clonal line of conditionally immortalized neural stem cells, MHP36, derived from the E14 "immortomouse" hippocampal anlage. Implanted above the damaged CA1 cell field in 4VO-treated adult rats, these cells (multipotential in vitro) migrated to the damaged area, reconstituted the gross morphology of the CA1 pyramidal layer, took up both neuronal and glial phenotypes, and gave rise to cognitive recovery. Similar recovery of function and restoration of species-typical morphology was observed when MHP36 cells were implanted into marmosets with excitotoxic CAI damage. MHP36 implants led to recovery of cognitive function also in two experiments with rats with excitotoxic damage to the cholinergic system damage, either unilaterally in the nucleus basalis or bilaterally in both the nucleus basalis and the medial septal area. Thus, MHP36 cells are both multipotent (able to take up multiple cellular phenotypes) and multifunctional (able to repair diverse types of brain damage).

Neural transplantation cannula and microinjector system: experimental and clinical experience. Technical note

The authors present a simple, reliable, and safe system for performing neural transplantation in the human brain. The device consists of a transplantation cannula and microinjector system that has been specifically designed to reduce implantation-related trauma and to maximize the number of graft deposits per injection. The system was evaluated first in an experimental rat model of Parkinson's disease (PD). Animals in which transplantation with this system had been performed showed excellent graft survival with minimal trauma to the brain. Following this experimental stage, the cannula and microinjector system were used in eight patients with PD enrolled in the Halifax Neural Transplantation Program who received bilateral putaminal transplants of fetal ventral mesencephalic tissue. A total of 16 transplantation operations and 64 trajectories were performed in the eight patients, and there were no intraoperative or perioperative complications. Magnetic resonance imaging studies obtained 24 hours after surgery revealed no evidence of tissue damage or hemorrhage. Transplant survival was confirmed by fluorodopa positron emission tomography scans obtained 6 and 12 months after surgery. As neural transplantation procedures for the treatment of neurological conditions evolve, the ability to deliver viable grafts safely will become critically important. The device presented here has proved to be of value in maximizing the number of graft deposits while minimizing implantation-related trauma to the host brain.

The DaNeX study of embryonic mesencephalic, dopaminergic tissue grafted to a minipig model of Parkinson's disease: preliminary findings of effect of MPTP poisoning on striatal dopaminergic markers

A multicenter study is under way to investigate the efficacy of allografting of embryonic mesencephalic neurons in a pig model of Parkinson's disease. We have first established that a stable parkinsonian syndrome can be established by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication of adult male Göttingen minipigs. We are now using positron emission tomography (PET) methods for testing the physiological responses to MPTP intoxication and the time course of the response to several treatment strategies. We now report preliminary results obtained in 11 pigs employed in the initial phase of the study; the completed study shall ultimately include 30 pigs. Animals were randomly assigned to one of five groups: 1) Control, 2) MPTP intoxication, 3) MPTP intoxication followed by allograft, 4) MPTP intoxication followed by allograft with immunosuppression, and 5) MPTP intoxication followed by allograft with immunosuppression and co-grafting of immortalized HiB5 cells, which had been manipulated to secrete glia cell line-derived neurotrophic factor (GDNF) (approximately 2 ng GDNF/h/10(5) cells). MPTP was administered (1 mg/kg/day, SC) for 7-10 days until the pigs had developed mild parkinsonian symptoms of muscle rigidity, hypokinesia, and impaired coordination, especially of the hind limbs. Approximately 2 weeks after the last MPTP dose, animals received a T1-weighted magnetic resonance imaging (MRI) scan, and a series of dynamic PET recordings. After the first series of PET scans, four grafts of porcine embryonic mesencephalic tissue (E28 days) were placed in each striatum of some MPTP-intoxicated pigs, using MRI-based stereotactic techniques. Immunosuppression of some animals with cyclosporin and prednisolone began just prior to surgery. Two more series of PET scans were performed at 4-month intervals after surgery. After the last scans, pigs were killed and the brains were perfused for unbiased stereological examination of cytological and histochemical markers in striatum and substantial nigra. The behavioral impairment of the animals (the "Parkinson's score") had been evaluated throughout the 8-month period. Kinetic analysis of the first set of PET scans has indicated that the rate constant for the decarboxylation of FDOPA in catecholamine fibers was reduced by 33% in striatum of the mildly parkinsonian pigs. The rate of association of [11C]NS-2214 to catecholamine uptake sites was reduced by 62% in the same groups of pigs. No significant difference was found in the binding potential of [11C]raclopride to the dopamine D2-like receptors in striatum of the MPTP-intoxicated versus control pigs. These preliminary results are suggestive that the activity of DOPA decarboxylase may be upregulated in the partially denervated pig striatum.

Intrastriatal ventral mesencephalic xenografts of porcine tissue in rats: immune responses and functional effects

Transplantation of neural tissue from other species has the potential to improve function in patients with neurodegenerative disorders. We investigated the functional effects of embryonic porcine dopaminergic neurons transplanted in a rat model of Parkinson's disease and the immune responses to the grafts in immunosuppressed and nonimmunosuppressed hosts. Twenty-three rats with unilateral 6-hydroxydopamine lesions received dissociated, 27-day-old embryonic porcine ventral mesencephalic tissue in the right striatum. Eighteen rats received cyclosporine (10 mg/kg, IP, daily) during the whole period of 14 weeks, in combination with prednisolone (20 mg/kg, IP, daily) the first 4 days. Five rats served as nonimmunosuppressed controls. All rats were tested for amphetamine-induced rotational behavior at 3-week intervals. Two immunosuppressed rats were excluded due to severe side effects of the treatment. Functional recovery was seen in 9 of 16 immunosuppressed rats at 12 weeks. Six animals remained functionally recovered at 14 weeks and contained an average of 5750+/-1450 (SEM) dopaminergic neurons. Between 9 and 14 weeks, three immunosuppressed rats rejected their grafts, based on rotation scores and immunohistochemical demonstration of cell infiltrates. One additional immunosuppressed rat showed evidence of ongoing rejection at 14 weeks. The striata in animals with ongoing or recent rejection contained large numbers of CD4- and CD8-positive lymphocytes, NK cells, macrophages, and microglia cells, whereas scar tissue was found in rats with grafts rejected at earlier time points (n = 11). Embryonic porcine ventral mesencephalic tissue matures in the adult rat striatum, reinnervates the host brain, and restores behavioral defects. Immunosuppressive treatment was necessary for long-term graft survival and functional recovery, but did not sufficiently protect from rejection mechanisms. Porcine neural tissue is an interesting alternative to embryonic human tissue for intracerebral transplantation in neurodegenerative diseases. However, to achieve stable graft survival in discordant xenogeneic combinations, an appropriate immunosuppressive treatment or donor tissue modifications are needed.

Neural tissue xenotransplantation: what is needed prior to clinical trials in Parkinson's disease? Neural Tissue Xenografting Project

Embryonic allografted human tissue in patients with Parkinson's disease has been shown to survive and ameliorate many of the symptoms of this disease. Despite this success, the practical problems of using this tissue coupled to the ethical restrictions of using aborted human fetal tissue have lead to an exploration for alternative sources of suitable material for grafting, including xenogeneic embryonic dopaminergic-rich neural tissue. Nevertheless, xenografted neural tissue itself generates a number of practical, ethical, safety, and immunological issues that have to be addressed prior to any clinical xenotransplant program. In this article we review these critical issues and set out the criteria that we consider need to be met in the development of our clinical xenotransplantation research programs. We advocate that these, or similar, criteria should be adopted and made explicit by other centers contemplating similar clinical trials.

Xenotransplantation

BACKGROUND

The success of clinical transplantation has led to a large discrepancy between donor organ availability and demand; considerable pressure exists to develop an alternative source of organs. The use of animal organs for donation is a possible solution that is not yet clinically applicable.

METHODS AND RESULTS

A literature review was performed based on a Medline search to find articles on xenotransplantation. Keywords included hyperacute, acute vascular, xenograft rejection combined with concordant and discordant. Additional references cited in these articles from journals not included in Medline were obtained from the British Library. Limited information on unpublished, preliminary work has been included from sources known to the authors, based on their research work in the field. One hundred and forty-six references and four personal communications have been included in this review article.

CONCLUSION

A greater understanding of the pathogenesis of xenograft rejection is developing rapidly. Strategies to abrogate hyperacute rejection have proved successful, but control of antibody-driven acute vascular rejection has not yet been achieved. The safety and viability of xenotransplantation as a therapeutic modality are still unproven.

Reduced xenograft rejection in rat striatum after pretransplant photodynamic therapy of murine neural xenografts

OBJECT

The goal of this study was to develop a method of reducing neural xenograft rejection by pretreating the graft with photodynamic therapy (PDT).

METHODS

Xenograft cell suspensions were prepared from fetal mouse mesencephalon, after which they were incubated for 30 minutes with various concentrations of a photosensitizer, verteporfin for injection, and light exposure. The xenograft cell suspensions were injected into the dopamine-depleted striata of 40 hemiparkinsonian rats assigned to different treatment groups. Four weeks after transplantation, xenograft function (determined by methamphetamine-induced rotation) and survival (determined by immunohistochemical staining for murine neurons) were compared. Group 1 animals (xenografts pretreated with 25 ng/ml verteporfin) and Group 3 animals (no verteporfin pretreatment, but daily administration of cyclosporin A) had significantly better xenograft survival and function compared with control animals (no pretreatment with verteporfin). Group 2 animals (xenografts pretreated with 250 ng/ml verteporfin) had no significant improvement.

CONCLUSIONS

This work demonstrates improved neural xenograft survival and function when using pretransplant PDT of the graft in a rodent model. The potential benefits of this new therapy are its convenience (one pretransplant treatment) and its compatibility with host immunosuppression.

Xenotransplantation

BACKGROUND

Over the past 10 years xenotransplantation has generated much interest in the hope that it will enable us to overcome the current lack of human organ donors. This review examines the evolution and current therapeutic strategies that have been developed to overcome the predominant problem of graft rejection.

METHODS

A literature review was undertaken using a Medline search from January 1966 to August 1999.

RESULTS AND CONCLUSION

Despite the considerable advances that have been made in molecular biological techniques, xenograft rejection cannot be prevented without significant immunosuppression and toxic side-effects. The problem of delayed rejection, in particular, will probably be very difficult to overcome, although some of the difficulties associated with hyperacute rejection have been resolved. The potential risk of porcine endogenous retrovirus transmission has generated much debate recently, but it is likely that some of the important issues relating to xenotransplantation will never be resolved until carefully regulated clinical trials are allowed to begin.

Functional integration of neural grafts in Parkinson's disease

Functional imaging in a Parkinson's patient with a neural transplant shows that the graft is still functional after ten years, and that dopamine from the graft can bind to postsynaptic sites.

Xenogeneic infections and public health

1. The scarcity of available human donor organs for use in allotransplantation has fuelled interest in xenotransplantation, the therapeutic use of living animal tissue in humans. The use of living animal tissue for therapeutic purposes in humans has raised concerns that xenotransplantation clinical trials may pose a presently unquantifiable but undeniable risk to public health. 2. Xenotransplantation has the potential to introduce new infections to the human community by infecting human recipients with agents that were not previously endemic in human populations (xenogeneic infections). 3. Manipulations intended to prevent xenograft rejection may also facilitate the transmission of agents that rarely or never infect humans under natural circumstances. 4. The US Food and Drug Administration (the government agency responsible for monitoring drug safety) has chosen to allow limited numbers of xenotransplantation clinical trials to proceed under carefully monitored conditions outlined in the Public Health Service (PHS) Guideline on Infectious Disease Issues in Xenotransplantation. 5. This PHS guideline particularly emphasizes the importance of pretransplantation screening and post-transplantation surveillance for safety monitoring. 6. Laboratory based surveillance for endogenous retroviruses and other identifiable agents that cannot be removed from the xenograft can augment clinical surveillance. 7. Laboratory based studies of xenograft survivors increase our ability to quantify xenotransplant-associated risks and, thereby, expand our capacity to make science-based assessments of appropriate public policy.

Current status of xenotransplantation

1. The transplantation of organs and tissues from animals into humans (i.e. xenotransplantation) has been a long sought objective to allow xenotransplantation to achieve its full impact in the clinical practice of medicine. 2. The main hurdles to the application of xenotransplantation are the immunological reaction of the recipient against the transplant, the functional limitations of tissues and organs in biogenetically disparate recipients and the possibility of transferring infectious organisms from the graft into the recipient. 3. Advances in a variety of fields have shed new light on these hurdles and have given rise to potential solutions and prospects for the clinical application of xenotransplant and are summarized in the report that follows.

Fetal porcine dopaminergic cell survival in vitro and its relationship to embryonic age

One of the critical factors in the survival of embryonic neural grafts is the age at which the population of donor neurons is harvested. This is especially the case for the developing dopaminergic neurons in the embryonic ventral mesencephalon, which are used for neural grafts in Parkinson's disease (PD). The donor age for optimal harvesting of these cells has been well characterized in the mouse, rat, and marmoset, and to a lesser extent in humans. However, the best donor age for porcine ventral mesencephalic tissue has not been ascertained, even though the use of this tissue for xenografts has been explored both experimentally and clinically. In this study the effect of donor age on dopaminergic cell survival was assessed in vitro, from a range of fetal pigs aged from E24 to E35. The number of tyrosine hydroxylase (TH)-positive cells per ventral mesencephalon was then calculated after 1 and 7 days in culture. E26-E27 embryos gave the highest yield of such cells at both survival time points, suggesting that this will be the optimal age for harvesting tissues whether for experimental or clinical nigral xenograft programs.

Porcine embryonic brain cell cytotoxicity mediated by human natural killer cells

Intracerebral transplantation of porcine embryonic dopamine-producing neurons has been suggested as a method to treat patients with Parkinson's disease. Even though the brain is an immunologically privileged site, neuronal xenografts are usually rejected within a few weeks. T cells are important for this process, but the exact cellular events leading to rejection are poorly characterized. Brain cells from ventral mesencephalon of 26-27-day-old pig embryos were used as target cells in flow cytometry-assessed cytotoxicity assays using non- and IL-2-activated CD3- CD16+ CD56+ human natural killer (NK) cells as effector cells. The ability of human NK cells to kill pig embryonic brain cells by antibody-dependent cellular cytotoxicity (ADCC) in the presence of nondepleted and anti-Gal alpha1,3Gal antibody-depleted human blood group AB serum (AB serum) was evaluated using the same assay. Both nondepleted and anti-Gal alpha1,3Gal antibody-depleted AB serum could mediate ADCC of pig embryonic VM cells when human NK cells were used as effector cells. Nonactivated NK cells did not show any direct cytotoxic effect on freshly isolated VM cells, whereas IL-2-activated NK cells killed approximately 50% of the VM cells at an effector-to-target ratio of 50:1 in a 4-h cytotoxicity assay. Activation of VM cells by TNF-alpha did not change their sensitivity to human NK cell cytotoxicity. Human NK cells may thus contribute to a cellular rejection of pig neuronal xenografts by ADCC, or following IL-2 activation, by a direct cytotoxic effect.

Xenogeneic transplantation of porcine hepatocytes into the CCl4 cirrhotic rat model

Liver support using extracorporeal devices and hepatocyte transplantation has received renewed interest for the management of acute and chronic liver failure. The aim of this study was to determine whether xenogeneic porcine hepatocytes could integrate into the liver parenchyma of cirrhotic Lewis rats when administered by an intrasplenic route. Cirrhosis was induced by carbon tetrachloride (CCl4) inhalation and confirmed histologically. Freshly isolated porcine hepatocytes were infused directly into the splenic pulp at laparotomy over a 5-15-min interval. Using (111)In-labeled hepatocytes, the degree of localization of porcine hepatocytes to the spleen and liver was found to be greater than 60% in both control and cirrhotic rats. Integration of porcine hepatocytes into the rat liver parenchyma was determined by immunohistochemical staining for porcine albumin in rat liver sections. Further confirmation was provided by in situ hybridization using a porcine-specific probe that binds to a distinct repetitive element (PRE) in porcine DNA. Evidence of integrated porcine hepatocytes was seen for over 50 days in animals under cyclosporine immunosuppression. These data demonstrate the integration of xenogeneic porcine hepatocytes into the liver of the cirrhotic rat and their ability to produce porcine albumin for up to 50 days.

Development of fetal neural transplantation as a treatment for Parkinson's disease

Since 1988, patients with Parkinson's disease have participated in clinical trials evaluating the efficacy of transplantation of human fetal dopamine cells into the caudate and putamen. Transplantation of fetal tissue leads to clinical benefits in some patients which is associated with a reduction of the amount of LDOPA administered. Major issues in transplant research need to be addressed before this technique can be widely applied. In this review, a pool of 35 patients was generated from the published cases of human fetal tissue transplantation. This group of transplant recipients was examined for motor improvement and reduction in L-DOPA dosage at one year post-transplant. Issues addressed in this review include the benefits of unilateral vs bilateral transplantation, age of the transplant recipient, solid vs suspensions of fetal mesencephalon and the number of fetal donors per recipient.

Therapeutic strategies in multiple sclerosis. II. Long-term repair

Spontaneous myelin repair in multiple sclerosis (MS) provides a striking example of the brain's inherent capacity for sustained and stable regenerative tissue repair--but also clearly emphasizes the limitations of this capacity; remyelination ultimately fails widely in many patients, and disability and handicap accumulate. The observation of endogenous partial myelin repair has raised the possibility that therapeutic interventions designed to supplement or promote remyelination might have a useful and significant impact both in the short term, in restoring conduction, and in the long term, in safeguarding axons. Therapeutic remyelination interventions must involve manipulations to either the molecular or the cellular environment within lesions; both depend crucially on a detailed understanding of the biology of the repair process and of those glia implicated in spontaneous repair, or capable of contributing to exogenous repair. Here we explore the biology of myelin repair in MS, examining the glia responsible for successful remyelination, oligodendrocytes and Schwann cells, their 'target' cells, neurons and the roles of astrocytes. Options for therapeutic remyelinating strategies are reviewed, including glial cell transplantation and treatment with growth factors or other soluble molecules. Clinical aspects of remyelination therapies are considered--which patients, which lesions, which stage of the disease, and how to monitor an intervention--and the remaining obstacles and hazards to these approaches are discussed.

Gene transfer to the nigrostriatal system by hybrid herpes simplex virus/adeno-associated virus amplicon vectors

To improve gene transfer to CNS neurons, critical elements of herpes simplex virus 1 (HSV-1) amplicons and recombinant adeno-associated virus (AAV) vectors were combined to construct a hybrid amplicon vector, and then packaged via a helper virus-free system. We tested the HSV/AAV hybrid amplicon vectors for transduction efficiency and stability of transgene expression (green fluorescent protein) in primary neuronal cultures from rat fetal ventral mesencephalon, in comparison with traditional HSV amplicon, AAV, or adenovirus (Ad) vectors at the same multiplicity of infection. The HSA/AAV hybrid vectors transduced the highest number of primary neurons in culture 2 days after infection. As compared with all other vectors tested, only hybrid vectors containing the AAV rep gene maintained the 2-day level of transgene expression over 12 days in culture. This rep-containing hybrid vector was then tested for efficiency and safety in the brain. One month after injection into adult rat striatum (1 x 10(6) transducing units injected), transgene expression was observed within the striatum (ranging from 564 to 8610 cells) and the substantia nigra (via retrograde transport, ranging from 130 to 809 neurons). The HSV/AAV hybrid amplicon vectors transduced predominantly neurons within the striatum, and showed transduction efficacy similar to and in many cases higher than that of HSV amplicon vectors. No immune response was observed in the HSA/AAV hybrid vector-injected brains, as determined by immune markers specific for helper T lymphocytes, cytotoxic T lymphocytes, and microglia. This HSV/AAV hybrid system shows high transduction efficiency and stability in culture. The effective and safe transgene delivery into the nigrostriatal system illustrates its potential for therapeutic application for neurologic disorders, such as Parkinson and Huntington disease.

Expression of the human alpha1,2-fucosyltransferase in transgenic pigs modifies the cell surface carbohydrate phenotype and confers resistance to human serum-mediated cytolysis

Hyperacute rejection (HAR) is the first critical immunological hurdle that must be addressed in order to develop xenogeneic organs for human transplantation. In the area of cell-based xenotransplant therapies, natural antibodies (XNA) and complement have also been considered barriers to successful engraftment. Transgenic expression of human complement inhibitors in donor cells and organs has significantly prolonged the survival of xenografts. However, expression of complement inhibitors without eliminating xenogeneic natural antibody (XNA) reactivity may provide insufficient protection for clinical application. An approach designed to prevent XNA reactivity during HAR is the expression of human alpha1, 2-fucosyltransferase (H-transferase, HT). H-transferase expression modifies the cell surface carbohydrate phenotype of the xenogeneic cell, resulting in the expression of the universal donor O antigen and a concomitant reduction in the expression of the antigenic Galalpha1,3-Gal epitope. We have engineered various transgenic pig lines that express HT in different cells and tissues, including the vascular endothelium. We demonstrate that in two different HT transgenic lines containing two different HT promoter constructs, expression can be differentially regulated in a constitutive and cytokine-inducible manner. The transgenic expression of HT results in a significant reduction in the expression of the Galalpha1,3-Gal epitope, reduced XNA reactivity, and an increased resistance to human serum-mediated cytolysis. Transgenic pigs that express H-transferase promise to become key components for the development of xenogeneic cells and organs for human transplantation.

Xenotransplantation and public health: identifying the legal issues

The debate over the acceptability of clinical trials for xenotransplantation has focussed primarily on analyses of: 1) the medical benefits that might accrue to individual patients in need of organ replacement therapy; 2) the risk of introducing new infectious disease(s) into the population; and 3) the ability of public safety measures to minimize that risk. It is now generally accepted that if we are to proceed with xenobiotechnology, sufficient public safety measures must first be adopted. Despite the growing consensus as to the indispensability of scientific safeguards, few authors have questioned the ability of current or novel legal frameworks to sustain and enforce such safeguards. A legal analysis of the public health concerns must be incorporated into the debate if we are to ensure a thorough and responsible decision-making process.

Expression of major histocompatibility complex antigens and induction of human T-lymphocyte proliferation by astrocytes and macrophages from porcine fetal brain

Porcine fetal brain cells are of potential use as donor cells for transplantation therapies of neurodegenerative diseases in humans. Our aim was to determine the immunestimulatory properties of astrocytes and macrophages from porcine fetal brain in vitro. By flow cytometry, freshly isolated porcine fetal brain cells were nonautofluorescent, while primary cultures of these cells, prepared to favor growth of astrocytes and macrophages/microglia, consisted of both an autofluorescent and a nonautofluorescent cell population. The cultured autofluorescent cells had qualities typical of macrophages: CD18 (beta(2) integrin subunit) expression, high granularity, and phagocytic activity. The cultured nonautofluorescent cells stained positive for the astrocyte marker glial fibrillary acidic protein and CD56 (NCAM isoform). While freshly isolated porcine fetal brain cells expressed very low levels of major histocompatibility complex (MHC) class I and no MHC class II antigens, primary culture of these cells resulted in upregulation of MHC class I antigens on astrocytes and macrophages and MHC class II antigens on a subpopulation of the macrophages. Single-cell suspensions prepared from the primary cultures were flow sorted into astrocyte and macrophage populations on the basis of cell granularity and autofluorescence or on the basis of CD56 expression. Pure suspensions (>98%) of astrocytes induced a low proliferative response in human T lymphocytes, as determined by [(3)H]thymidine incorporation after 4 days of coculture. A suspension of 91% macrophages was a strong inducer of human T-cell proliferation, even stronger than allogeneic mononuclear blood cells. For neural xenotransplantation, our findings suggest that depletion of macrophages from the donor-cell suspensions may enhance graft survival by reducing cell-mediated rejection.

Human natural antibodies cytotoxic to pig embryonic brain cells recognize novel non-Galalpha1,3Gal-based xenoantigens

Transplantation of porcine embryonic brain cells, including dopaminergic neurons, from ventral mesencephalon (VM) is considered a potential treatment for patients with Parkinson's disease. In the present study, we characterized the distribution among VM cells of the major porcine endothelial xenoantigen, the Galalpha1,3Gal epitope, and evaluated the cytotoxic effect of anti-Galalpha1,3Gal antibody-depleted and nondepleted human AB serum on VM cells. Overall levels of Galalpha1,3Gal-epitope expression was very low on the VM cell population using Bandeiraea simplicifolia IB(4) lectin staining of resuspended VM cells in flow cytometric analyses or staining of SDS-PAGE-separated, solubilized VM cell membrane proteins in Western blot analyses. Lectin-histochemical staining of sections of pig embryonal VM regions with BSA IB(4) lectin showed staining restricted to endothelial cells and microglia. In the presence of complement, both nondepleted and anti-Galalpha1,3Gal antibody-depleted AB sera were shown to be cytotoxic to VM cells as assessed in microcytotoxicity- and flow cytometry-based cytotoxicity assays. Purified IgM and IgG were both cytotoxic in the presence of complement. Three major VM cell membrane antigens of approximately 210, 105, and 50 kDa were reactive with natural IgM antibodies present in pooled human AB sera. Thus, antibody-dependent cytotoxicity may contribute to pig to human brain cell xenorejection, necessitating donor tissue modifications prior to a more widespread utilization of neural tissue xenografting.

Genetic therapies and xenotransplantation

There is an acute shortage of human organs available for transplantation. Transplanting animal organs or tissues is a potential solution and could also provide a novel means of gene delivery. Xenotransplantation faces many challenges, including the immune response of the recipient against the transplant, the physiological limitations of the transplant and the possibility of introducing infectious organisms into the recipient. A number of medical centres around the world are addressing these issues. Much of this research focuses on the application of genetic therapies because of the unique opportunity this represents for genetic engineering to be used at the source of organ tissue, rather than to the recipient.

Cerebral 6-[(18)F]fluoro-L-DOPA (FDOPA) metabolism in pig studied by positron emission tomography

We measured 6-[(18)F]fluoro-L-DOPA (FDOPA) uptake and metabolism in the brain of 4-month-old female pigs (n = 8) using a high-resolution positron emission tomograph (PET) in 3D mode. The mean net blood-brain clearance of FDOPA (K(i)(D)) to striatum was 0.011 ml g(-1) min(-1). Correcting for the elimination of decarboxylated metabolites from striatum (k(loss) = 0.004 min(-1)) increased the apparent magnitude of the estimate of K(i)(D) by 50%, at the expense of doubling the variance of the mean estimate. The mean decarboxylation rate of FDOPA in striatum relative to the cerebellum input (k(3)(s)) was 0.008 min(-1). For multicompartmental analyses, the FDOPA partition volume (V(e)(D)) was constrained to the individual value observed in cerebellum (mean = 0.53 ml g(-1)), with correction for the presence in brain of the plasma metabolite 3-O-methyl-FDOPA (OMFD). Using the first 60 min of the dynamic PET scans, the rate constant of FDOPA decarboxylation (k(3)(D)) was estimated to be 0.037 min(-1 )in striatum, but was not significantly different than zero in frontal cortex. Fitting of a compartmental model correcting for elimination of decarboxylated metabolites to the complete PET frame-sequence (120 min) increased the variance of the estimate of k(3)(D) in striatum. The magnitude of k(3)(D) in striatum of young pig was less than values estimated previously in neonatal piglet, adult monkey, and human. MRI-based simulations predicted that recovery of radioactivity from pig striatum was highly sensitive to the volume of interest. We conclude that the spatial resolution of our tomograph reduces the apparent magnitude of k(3)(D) in striatum. However, anaesthetised pigs are an appropriate experimental model for PET studies of DOPA decarboxylation in striatum.

A neural transplantation cannula and microinjector system: experimental and clinical experience

The authors present a simple, reliable, and safe system for performing neural transplantation in the human brain. The device consists of a transplantation cannula and microinjector system that has been specifically designed to reduce implantation-related trauma and to maximize the number of graft deposits for each injection. The system was evaluated first in an experimental rat model of Parkinson's disease (PD). Animal transplantation with this system showed excellent graft survival with minimal trauma to the brain. Following this experimental stage, the cannula and microinjector system was used in eight patients with PD enrolled in the Halifax Neural Transplantation Program who received bilateral putaminal transplants of fetal ventral mesencephalic tissue. A total of 16 transplantation operations and 64 trajectories were performed in the eight patients, and there were no intra- or perioperative complications. Magnetic resonance imaging studies obtained 24 hours after surgery revealed no evidence of tissue damage or hemorrhage. Transplant survival was confirmed on fluorodopa positron emission tomography scans 6 and 12 months after surgery.

As neural transplantation procedures for the treatment of neurological conditions evolve, the ability to deliver viable grafts safely will become of critical importance. The device presented here has been proven to be of value in maximizing the number of graft deposits while minimizing implantation-related trauma to the host brain.

Evidence of absence of porcine endogenous retrovirus (PERV) infection in patients treated with a bioartificial liver support system

Porcine endogenous retrovirus (PERV) genomes are present in all pig cells. In this retrospective study, we assessed PERV infectivity in 28 patients treated with an extracorporeal bioartificial liver (HepatAssist system) that includes a membrane device containing porcine hepatocytes. All patients tested negative for PERV using polymerase chain reaction analysis of peripheral blood mononuclear cells (PBMC) collected up to 5 years after treatment. In vitro results showed that the membrane decreased the risk of PERV transmission by a factor of 105, and porcine hepatocytes did not produce infectious PERV in co-cultures with human cell line 293. Our results do not support the presence of PERV infection in patients treated with this porcine hepatocyte-based bioartificial liver.

An overview of central nervous system transplantation in human disease

Although its roots date back over a century, the field of neurotransplantation has been shaped mostly by advances over the past 30 years. Animal models of nigrostriatal disconnection in the 1970s allowed investigators to explore the feasibility of neural grafting. By the end of that decade, functional and behavioral effects had been demonstrated using fetal tissue grafts. In the 1980s, animal experimentation continued, as did clinical trials involving patients with idiopathic Parkinson's disease. Both autologous adrenal medullary tissue and fetal allografts were tested in the clinical setting, with the latter proving to yield superior results. Animal models of striatal cell loss provided the impetus for limited clinical trials in patients with Huntington's disease by the early 1990s, and work with both diseases continues today. Although much has been learned, neural grafting remains experimental. Broader applications are being explored even now, though, as transplant techniques are applied to animal models of dementia, spinal cord injury, cortical injury, and pain. Some very limited human trials have already begun in some of these areas. In this review some of the advances in the field are highlighted.

Prospects for the clinical application of neural transplantation with the use of conditionally immortalized neuroepithelial stem cells

Although neural transplantation has made a relatively successful transition from the animal laboratory to human neurosurgery for the treatment of Parkinson's disease, the use of human embryonic brain tissue as the source of transplants raises difficult ethical and practical problems. These are likely to impede the widespread use of this otherwise promising therapy across the range of types of brain damage to which the results of animal experiments suggest its potential applicability. Various alternative approaches are reviewed briefly, aimed at developing sources of tissue for transplantation that can be maintained in vitro until needed, so obviating the requirement for fresh embryonic tissue at each occasion of surgery. Particularly promising are conditionally immortalized neuroepithelial stem cell lines in which the immortalizing gene is downregulated upon transplantation into a host brain. We describe experiments from our laboratory with the use of cells of this kind, the multipotent MHP clonal cell lines, derived from the developing hippocampus of a transgenic mouse harbouring a temperature-sensitive oncogene. Implanted into the hippocampus of rats and marmosets with damage to the CA1 cell field, the MHP36 line gave rise to healthy surviving grafts and to essentially complete recovery of cognitive function. Postmortem study of the implanted rat brains indicated that MHP36 cells migrate to the region of damage, adopt both neuronal (pyramidal) and glial phenotypes in vivo, and reconstitute the normal laminated appearance of the CA1 cell field. We have previously shown that, when primary differentiated foetal tissue is used as the source of grafts in rats with CA1 damage, there is a stringent requirement for replacement with homotypic CA1 cells. We interpret our results as showing that the MHP36 cell line responds to putative signals associated with damage to the hippocampus and takes up a phenotype appropriate for the repair of this damage; they therefore open the way to the development of a novel strategy with widespread applicability to the treatment of the diseased or damaged human brain.