Benefits and risks of hosting animal cells in the human brain

Lebanese medical students' knowledge on and attitude toward xenotransplantation and its ethical issues: A cross-sectional study

INTRODUCTION

Xenotransplantation is the transplantation of animal parts into humans. This study's aim is to assess the Lebanese medical students' attitude toward xenotransplantation and their knowledge of its related ethical issues.

METHODS

A questionnaire was answered by 120 Lebanese medical students. Statistical analysis and ethical reflections were performed on the results.

RESULTS

Out of the 120 participants, 100 already knew about xenotransplantation, and 61.7% of the students believe that it is applied clinically. Only 25% trust that the animals need to be genetically modified for the organs to be eligible for transplantation. About 25.8% and 46.7% of the students, respectively, strongly agree that the patients' and physicians' beliefs play an important role in agreeing to the procedure. Students were likely to accept the animal organ for transplantation if the transplant could completely cure or improve the course of an untreatable disease (86.6% and 75.0%) or if the outcomes and risks were the same as with a human transplant with a shorter waiting period (71.7%). Students who agreed that animal rights and welfare/wellbeing should be respected tended to disagree with the statement that it is acceptable to use animals for human ends (p = .026).

CONCLUSION

Medical students proved to be knowledgeable on the ethical dilemmas related to xenotransplantation and showed a favorable attitude toward this procedure, which is crucial in its establishment as an absolute clinical reality in the near future and in the development and strengthening of research in this field.

Gene Therapy Repairs for the Epileptic Brain: Potential for Treatment and Future Directions

Epilepsy is a syndrome specified by frequent seizures and is one of the most prevalent neurological conditions, and that one-third of people of epilepsy are resistant to available drugs. Surgery is supposed to be the main treatment for the remedy of multiple drug-resistant epilepsy, but it is a drastic procedure. Advancement in genomic technologies indicates that gene therapy can make such surgery unnecessary. The considerable number of new studies show the significance of mutation in mammalian target of rapamycin pathway, NMDA receptors, GABA receptors, potassium channels and G-protein coupled receptors. Illustration of the meticulous drug in epilepsy targeting new expression of mutations in SCN8A, GRIN2A, GRIN2D and KCNT1 are conferred. Various methods are utilized to express a gene in a precise area of the brain; Transplantation of cells in an ex vivo approach (fetal cells, fibroblasts, immortalized cells), nonviral vector delivery and viral vector delivery like retrovirus, herpes simplex virus adenovirus and adeno-related virus. Gene therapy has thus been explored to generate anti-epileptogenic, anti-seizure and disease-modifying effects. Specific targeting of the epileptogenic region is facilitated by gene therapy, hence sparing the adjacent healthy tissue and decreasing the adverse effects that frequently go hand in hand with antiepileptic medication.

Cell Replacement Therapy for Huntington's Disease

Huntington's disease (HD) is an inherited neurodegenerative disorder which is characterised by a triad of highly debilitating motor, cognitive, and psychiatric symptoms. While cell death occurs in many brain regions, GABAergic medium spiny neurons (MSNs) in the striatum experience preferential and extensive degeneration. Unlike most neurodegenerative disorders, HD is caused by a single genetic mutation resulting in a CAG repeat expansion and the production of a mutant Huntingtin protein (mHTT). Despite identifying the mutation causative of HD in 1993, there are currently no disease-modifying treatments for HD. One potential strategy for the treatment of HD is the development of cell-based therapies. Cell-based therapies aim to restore neuronal circuitry and function by replacing lost neurons, as well as providing neurotropic support to prevent further degeneration. In order to successfully restore basal ganglia functioning in HD, cell-based therapies would need to reconstitute the complex signalling network disrupted by extensive MSN degeneration. This chapter will discuss the potential use of foetal tissue grafts, pluripotent stem cells, neural stem cells, and somatic cell reprogramming to develop cell-based therapies for treating HD.

Regenerative medicine in Huntington's disease: Strengths and weaknesses of preclinical studies

Huntington's disease (HD) is an inherited neurodegenerative disorder, characterized by impairment in motor, cognitive and psychiatric domains. Currently, there is no specific therapy to act on the onset or progression of HD. The marked neuronal death observed in HD is a main argument in favour of stem cells (SCs) transplantation as a promising therapeutic perspective to replace the population of lost neurons and restore the functionality of the damaged circuitry. The availability of rodent models of HD encourages the investigation of the restorative potential of SCs transplantation longitudinally. However, the results of preclinical studies on SCs therapy in HD are so far largely inconsistent; this hampers the individuation of the more appropriate model and precludes the comparative analysis of transplant efficacy on behavioural end points. Thus, this review will describe the state of the art of in vivo research on SCs therapy in HD, analysing in a translational perspective the strengths and weaknesses of animal studies investigating the therapeutic potential of cell transplantation on HD progression.

Gene therapy for the nervous system: challenges and new strategies

Current clinical treatments for central nervous system (CNS) diseases, such as Parkinson's disease and glioblastoma do not halt disease progression and have significant treatment morbidities. Gene therapy has the potential to "permanently" correct disease by bringing in a normal gene to correct a mutant gene deficiency, knocking down mRNA of mutant alleles, and inducing cell-death in cancer cells using transgenes encoding apoptosis-inducing proteins. Promising results in clinical trials of eye disease (Leber's congenital aumorosis) and Parkinson's disease have shown that gene-based neurotherapeutics have great potential. The recent development of genome editing technology, such as zinc finger nucleases, TALENS, and CRISPR, has made the ultimate goal of gene correction a step closer. This review summarizes the challenges faced by gene-based neurotherapeutics and the current and recent strategies designed to overcome these barriers. We have chosen the following challenges to focus on in this review: (1) delivery vehicles (both virus and nonviral), (2) use of promoters for vector-mediated gene expression in CNS, and (3) delivery across the blood-brain barrier. The final section (4) focuses on promising pre-clinical/clinical studies of neurotherapeutics.

Stem cell-based therapy for Huntington's disease

Huntington's disease (HD) is a late-onset neurodegenerative disease characterized by a progressive loss of medium spiny neurons in the basal ganglia. The development of stem cell-based therapies for HD aims to replace lost neurons and/or to prevent cell death. This review will discuss pre-clinical studies which have utilized stem or progenitor cells for transplantation therapy using HD animal models. In several studies, neural stem and progenitor cells used as allotransplants and xenografts have been shown to be capable of surviving transplantation and differentiating into mature GABAergic neurons, resulting in behavioral improvements. Beneficial effects have also been reported for transplantation of stem cells derived from non-neural tissue, for example, mesenchymal- and adipose-derived stem cells, which have mainly been attributed to their secretion of growth and neurotrophic factors. Finally, we review studies using stem cells genetically engineered to over-express defined neurotrophic factors. While these studies prove the potential of stem cells for transplantation therapy in HD, it also becomes clear that technical and ethical issues regarding the availability of stem cells must be solved before human trials can be conducted.

Potential barriers to human hepatocyte transplantation in MUP-uPAtg(⁺/⁺)Rag2⁻/⁻γC⁻/⁻ mice

Primary human fetal and adult hepatocytes have been considered feasible donor cell sources for cell transplantation. We compared the engraftment efficiencies between adult human, fetal human, and adult porcine hepatocytes after transplantation into MUP-uPA(tg(+/+))Rag2(-/-)γC(-/-)mice. Transplantation of adult human hepatocytes yielded a 1,000-fold higher serum albumin level compared to transplantation of fetal human hepatocytes, while transplantation of adult porcine hepatocytes resulted in a 100-fold higher serum albumin level than adult human hepatocytes. These results suggest that adult liver cells are superior to fetal liver cells for transplantation, and caution should be applied if porcine hepatocytes are used for preclinical studies as a proof of concept for human hepatocytes.

Impact of cognitive dysfunction on drooling in Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is commonly characterized by its motor symptoms such as resting tremor, rigidity, bradykinesia and postural instability; however, some of the most debilitating symptoms of this disease are non-motor ones such as dementia and sialorrhea (drooling). Drooling is caused by impaired swallowing and it can have a significant impact on the quality of life. However, it is still unclear whether cognitive dysfunction could exacerbate drooling. We wanted to examine if any relationship existed between drooling and dementia in PD patients. Identifying the correlation will aid physicians to screen and initiate early management of drooling in the course of PD. This can possibly lead to improvements in the quality of life in these patients.

METHODS

In this retrospective study, we investigated the prevalence of drooling in 314 PD patients and further compared the difference in the prevalence of drooling in patients with dementia and without dementia. In addition, we studied the impact of gender on drooling in this patient population.

RESULTS

Our results show that a significant correlation exists between drooling and dementia in our sample of PD patients. Furthermore, in males, the correlation between the prevalence of drooling and dementia was found to be clinically significant as compared to the female population.

CONCLUSION

Our findings suggest that drooling is a major concern in the course of PD and should therefore be addressed early and more aggressively in patients with dementia.

Herpes simplex virus type 1/adeno-associated virus hybrid vectors

Herpes simplex virus type 1 (HSV-1) amplicons can accommodate foreign DNA of any size up to 150 kbp and, therefore, allow extensive combinations of genetic elements. Genomic sequences as well as cDNA, large transcriptional regulatory sequences for cell type-specific expression, multiple transgenes, and genetic elements from other viruses to create hybrid vectors may be inserted in a modular fashion. Hybrid amplicons use genetic elements from HSV-1 that allow replication and packaging of the vector DNA into HSV-1 virions, and genetic elements from other viruses that either direct integration of transgene sequences into the host genome or allow episomal maintenance of the vector. Thus, the advantages of the HSV-1 amplicon system, including large transgene capacity, broad host range, strong nuclear localization, and availability of helper virus-free packaging systems are retained and combined with those of heterologous viral elements that confer genetic stability to the vector DNA. Adeno-associated virus (AAV) has the unique capability of integrating its genome into a specific site, designated AAVS1, on human chromosome 19. The AAV rep gene and the inverted terminal repeats (ITRs) that flank the AAV genome are sufficient for this process. HSV-1 amplicons have thus been designed that contain the rep gene and a transgene cassette flanked by AAV ITRs. These HSV/AAV hybrid vectors direct site-specific integration of transgene sequences into AAVS1 and support long-term transgene expression.

Herpesviruses--a zoonotic threat?

Herpesviruses are highly host specific and share a long synchronous evolution with their hosts. Only in rare cases, species barriers fall and allow animal to human or human to animal transmission. Among the zoonotic herpesviruses, Cercopithecine herpesvirus 1 is the most significant and can be transmitted from macaques to human. Conversely, Human herpesvirus 1 is capable of causing severe disease in primates. Besides those two examples, there are several herpesviruses with a certainly limited or only suspected ability to cross species barriers. Those include Saimiriine herpesvirus 2, Phocid herpesvirus 2, Equid herpesvirus 1, Epstein-Barr Virus, Marek's disease virus, and Pseudorabies virus. Concerning xenotransplantations, porcine gammaherpesviruses must be considered as a zoonotic threat.

Vector therapies for malignant glioma: shifting the clinical paradigm

BACKGROUND

Malignant glioma represents one of the most aggressive and devastating forms of human cancer. At present, there exists no successful treatment for this disease. Gene therapy, or vector therapy, has emerged as a viable experimental treatment method for intracranial malignancies.

OBJECTIVE

Vector therapy paradigms that have entered the clinical arena have shown adequate safety; however, the majority of the studies failed to observe significant clinical benefits. As such, researchers have refocused their efforts on developing novel vectors as well as new delivery methods to enhance the therapeutic effect of a particular vector. In this review, we discuss common vector therapy approaches used in clinical trials, their drawbacks and potential ways of overcoming these challenges.

METHODS

We focus on the experimental evaluation of cell-based vector therapies and adenoviral and herpes simplex virus type 1 vectors in the treatment of malignant glioma.

CONCLUSION

Vector therapy remains a promising treatment strategy for malignant glioma. Although significant questions remain to be answered, early clinical data suggest safety of this approach and future studies will likely address the efficacy of the proposed therapy.

Gene therapy in epilepsy

Results from animal models suggest gene therapy is a promising new approach for the treatment of epilepsy. Several candidate genes such as neuropeptide Y and galanin have been demonstrated in preclinical studies to have a positive effect on seizure activity. For a successful gene therapy-based treatment, efficient delivery of a transgene to target neurons is also essential. To this end, advances have been made in the areas of cell transplantation and in the development of recombinant viral vectors for gene delivery. Recombinant adeno-associated viral (rAAV) vectors in particular show promise for gene therapy of neurological disorders due to their neuronal tropism, lack of toxicity, and stable persistence in neurons, which results in robust, long-term expression of the transgene. rAAV vectors have been recently used in phase I clinical trials of Parkinson's disease with an excellent safety profile. Prior to commencement of phase I trials for gene therapy of epilepsy, further preclinical studies are ongoing including evaluation of the therapeutic benefit in chronic models of epileptogenesis, as well as assessment of safety in toxicological studies.

Transient transmission of porcine endogenous retrovirus to fetal lambs after pig islet tissue xenotransplantation

Evidence for the in vivo transmission of porcine endogenous retrovirus (PERV) from porcine xenografts to various recipient animals has been inconsistent. To characterize the contribution of the host immune system to the potential for PERV transmission from pig islet tissue xenografts to host tissues, we examined two immunoincompetent animal models, thymectomizsed fetal lambs and NODscid mice. Pig proislets were grafted into fetal lambs or adult NODscid mice. Conventional, nested and real-time PCR/RT-PCR tests were used to search for PERV and pig cell-specific sequences (porcine mitochondrial cytochrome oxidase II (COII) or mitochondrial ribosomal 12S) in pig proislets, host liver and spleen at 5-84 days (lambs) or 96 days (mice) after transplantation. Xenografts were harvested at the same time points. The copy number of PERV sequences and host cell-specific nuclear (palmitoylcarnitine transferase) sequences was assessed by real-time PCR to estimate the proportion of PERV-infected host cells. Pig proislets were shown to be PERV+ve by PCR and immunohistochemistry (PERV B env protein p15E). PERV transmission (PERV A, B or C DNA in the absence of porcine COII or 12S sequences) was detected by nested PCR and real-time PCR in 4/12 fetal lamb liver samples 5-23 days after transplantation; the maximum copy number of PERV B env sequences was found at day 5 (700 copies/1 x 10(6) lamb cells). A total of 4/12 fetal lambs demonstrated both PERV and 12S porcine sequences in liver samples (days 5-84) by real-time PCR, suggesting that pig cells had migrated to those tissues and established microchimerism; nested PCR showed evidence for microchimerism (porcine COII sequences alone) in 2/12 lambs (day 5). The incidence of PERV transmission and frequency of microchimerism was similar in host spleen analysed by real-time PCR. Histological examination showed complete xenograft rejection by 23 days after transplantation to fetal lambs. In contrast, pig proislet xenografts survived long term (> or =day 96) in NODscid mice but no PERV transmission was found. Both nested and real-time PCR assays revealed that 2/3 mice had become microchimeric. Long-term expression of PERV A, B and C as well as porcine 12S or COII RNAs was found at the graft site (day 96) only, indicating that PERV transcription and possibly replication, continued in the donor pig islet tissue after transplantation. Overall, detection of PERV transmission and microchimerism was limited by the sensitivity of the PCR assay and the primers chosen. The absence of stable PERV transmission and microchimerism in fetal lambs and the rejection of pig proislet xenografts correlated in time with the establishment of host immunocompetence. We therefore suggest that the frequent failure to identify PERV transmission late after transplantation could be due to the immunological destruction of PERV-infected host cells. Recipient NODscid mice demonstrated long-term microchimerism and intragraft PERV expression, which was consistent with their stable immunoincompetence.

Development of DARPP-32-positive parts of fetal pig ganglionic eminence and ventral mesencephalon in organotypic slice co-cultures

Neurons from the fetal pig dopaminergic ventral mesencephalon (VM) and basal ganglia anlage (the ganglionic eminence) were co-cultured as organotypic slice cultures to study the development of the two interconnected brain areas. During a short developmental period (E35-E42), a groove separates the ganglionic eminence into a lateral and a medial part. This was used (a) to study the developmental expression of the striatal marker protein, dopamine and adenosine 3,5-monophosphate regulated phospho-protein (DARPP-32) in the two parts and (b) to compare innervations of the two parts by tyrosine hydroxylase (TH)-positive, dopaminergic fibers from co-cultured slices of the ventral mesencephalon. DARPP-32 expression was more extensive and dense in cultures of the lateral part of the striatal anlage than the medial part. The DARPP-32-positive areas moreover overlapped with areas rich in acetylcholine esterase (AChE) and were the preferred target areas for TH-positive fibers from the co-cultured VM.

Glioma therapy and real-time imaging of neural precursor cell migration and tumor regression

Despite many refinements in current therapeutic strategies, the overall prognosis for a patient with glioblastoma is dismal. Neural precursor cells (NPCs) are capable of tracking glioma tumors and thus could be used to deliver therapeutic molecules. We have engineered mouse NPCs to deliver a secreted form of tumor necrosis factor-related apoptosis-inducing ligand (S-TRAIL); S-TRAIL is optimized to selectively kill neoplastic cells. Furthermore, we have developed means to simultaneously monitor both the migration of NSCs toward gliomas and the changes in glioma burden in real time. Using a highly malignant human glioma model expressing Renilla luciferase (Rluc), intracranially implanted NPC-FL-sTRAIL expressing both firefly luciferase (Fluc) and S-TRAIL was shown to migrate into the tumors and have profound antitumor effects. These studies demonstrate the potential of NPCs as therapeutically effective delivery vehicles for the treatment of gliomas and also provide important tools to evaluate the migration of NPCs and changes in glioma burden in vivo.

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.

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.

Potentiated gene delivery to tumors using herpes simplex virus/Epstein-Barr virus/RV tribrid amplicon vectors

The development and use of gene transfer techniques creates an opportunity to achieve better treatment modalities for numerous disease entities. Promising results for treatment in tumor cells in culture and in small animal models have been reported. Nevertheless, the lack of widespread vector distribution throughout tumor tissue is one of the current limitations for successful clinical application of gene therapy paradigms. The use of migratory tumor cells themselves as vector delivery vehicles may allow wider vector distribution in tumors. In addition, continuous release of retrovirus vectors on-site could generate a high local virion concentration over an extended time period with consequent increases in transduction efficiency. In this paper, we present in culture and in vivo data of a herpes simplex virus-Epstein-Barr virus hybrid amplicon vector containing retrovirus vector components (tribrid vector) that allows conversion of tumor cells into retroviral producer cells. With this method, we were able to achieve a local fourfold amplification of stable transgene expression in tumors. The application of this system, which can integrate a transgene cassette into tumors with therapeutic bystander effects, could increase the local amplification effect to a level of clinical relevance.

Cellular replacement therapy for Parkinson's disease--where we are today?

The concept of replacing lost dopamine neurons in Parkinson's disease using mesencephalic brain cells from fetal cadavers has been supported by over 20 years of research in animals and over a decade of clinical studies. The ambitious goal of these studies was no less than a molecular and cellular "cure" for Parkinson's disease, other neurodegenerative diseases, and spinal cord injury. Much research has been done in rodents, and a few studies have been done in nonhuman primate models. Early uncontrolled clinical reports were enthusiastic, but the outcome of the first randomized, double blind, controlled study challenged the idea that dopamine replacement cells can cure Parkinson's disease, although there were some significant positive findings. Were the earlier animal studies and clinical reports wrong? Should we give up on the goal? Some aspects of the trial design and implantation methods may have led to lack of effects and to some side effects such as dyskinesias. But a detailed review of clinical neural transplants published to date still suggests that neural transplantation variably reverses some aspects of Parkinson's disease, although differing methods make exact comparisons difficult. While the randomized clinical studies have been in progress, new methods have shown promise for increasing transplant survival and distribution, reconstructing the circuits to provide dopamine to the appropriate targets and with normal regulation. Selected promising new strategies are reviewed that block apoptosis induced by tissue dissection, promote vascularization of grafts, reduce oxidant stress, provide key growth factors, and counteract adverse effects of increased age. New sources of replacement cells and stem cells may provide additional advantages for the future. Full recovery from parkinsonism appears not only to be possible, but a reliable cell replacement treatment may finally be near.

Generation of stable retrovirus packaging cell lines after transduction with herpes simplex virus hybrid amplicon vectors

BACKGROUND

A number of properties have relegated the use of Moloney murine leukemia virus (Mo-MLV)-based retrovirus vectors primarily to ex vivo protocols. Direct implantation of retrovirus producer cells can bypass some of the limitations, and in situ vector production may result in a large number of gene transfer events. However, the fibroblast nature of most retrovirus packaging cells does not provide for an effective distribution of vector producing foci in vivo, especially in the brain. Effective development of new retrovirus producer cells with enhanced biologic properties may require the testing of a large number of different cell types, and a quick and efficient method to generate them is needed.

METHODS

Moloney murine leukemia virus (Mo-MLV) gag-pol and env genes and retrovirus vector sequences carrying lacZ were cloned into different minimal HSV/AAV hybrid amplicons. Helper virus-free amplicon vectors were used to co-infect glioma cells in culture. Titers and stability of retrovirus vector production were assessed.

RESULTS

Simultaneous infection of two glioma lines, Gli-36 (human) and J3T (dog), with both types of amplicon vectors, generated stable packaging populations that produced retrovirus titers of 0.5-1.2 x 10(5) and 3.1-7.1 x 10(3) tu/ml, respectively. Alternatively, when cells were first infected with retrovirus vectors followed by infection with HyRMOVAmpho amplicon vector, stable retrovirus packaging populations were obtained from Gli-36 and J3T cells producing retrovirus titers comparable to those obtained with a traditional retrovirus packaging cell line, Psi CRIPlacZ.

CONCLUSIONS

This amplicon vector system should facilitate generation of new types of retrovirus producer cells. Conversion of cells with migratory or tumor/tissue homing properties could result in expansion of the spatial distribution or targeting capacity, respectively, of gene delivery by retrovirus vectors in vivo.

Cell implantation therapies for Parkinson's disease using neural stem, transgenic or xenogeneic donor cells

A new therapeutic neurological and neurosurgical methodology involves cell implantation into the living brain in order to replace intrinsic neuronal systems, that do not spontaneously regenerate after injury, such as the dopaminergic (DA) system affected in Parkinson's disease (PD) and aging. Current clinical data indicate proof of principle for this cell implantation therapy for PD. Furthermore, the disease process does not appear to negatively affect the transplanted cells, although the patient's endogenous DA system degeneration continues. However, the optimal cells for replacement, such as highly specialized human fetal dopaminergic cells capable of repairing an entire degenerated nigro-striatal system, cannot be reliably obtained or generated in sufficient numbers for a standardized medically effective intervention. Xenogeneic and transgenic cell sources of analogous DA cells have shown great utility in animal models and some promise in early pilot studies in PD patients. The cell implantation treatment discipline, using cell fate committed fetal allo- or xenogeneic dopamine neurons and glia, is currently complemented by research on potential stem cell derived DA neurons. Understanding the cell biological principles and developing methodology necessary to generate functional DA progenitors is currently our focus for obtaining DA cells in sufficient quantities for the unmet cell transplantation need for patients with PD and related disorders.

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.

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.

Visualization of antigen-specific T cell activation in vivo in response to intracerebral administration of a xenopeptide

Allogeneic or xenogenic tissues exhibit prolonged survival when grafted into the brain parenchyma in comparison to grafting into peripheral sites. The brain, therefore, has long been considered an immunologically privileged site. However, the immunological privilege of the brain is not absolute, and it cannot shield neural xenografts from rejection. In our laboratory, we are interested in determining how to prevent neural xenograft rejection. To do so, we need to first understand how the immune system responds to CNS antigens leading to graft rejection. In order to monitor immune system responses to CNS antigens an adoptive transfer system was used to directly track CNS antigen-specific CD4(+) T cell responses in vivo. This would then allow us to monitor changes in the number, activation state, and anatomic distribution of antigen-specific cells. We have found that, after intracerebral injection of xeno peptide antigens with adjuvant, antigen-specific cells accumulated in the cervical lymph node, proliferated there for several days, and then disappeared slowly from the nodes. Interestingly, peptide antigens given intracerebrally also stimulated a strong antigen-specific CD4(+) T cell response. Moreover, cells remaining in the lymph node 8 days after antigen stimulation produce IL-2 with secondary antigenic challenge. Previous studies have shown that the administration of antigens without adjuvant in a monomeric form via either the intraperitoneal or intravenous route has failed to induce cell-mediated immunity and resulted in antigen-specific T cell unresponsiveness. Our findings demonstrate that antigen delivered intracerebrally can activate immune responses in a manner different than antigen delivered to peripheral sites outside of the CNS.

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.

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.

Molecular remodelling of human CD46 for xenotransplantation: designing a potent complement regulator without measles virus receptor activity

In pig-to-human discordant xenotransplantation, human complement (C) is a major barrier to long survival of xenografts. The current idea on how to cope with this barrier is that human complement regulatory proteins are forcibly expressed on xenografts to serve as safeguards against host C-induced hyperacute rejection of xenografts. Co-expression of decay-accelerating factor (DAF) (CD55) and membrane cofactor protein (MCP) (CD46) would be the first choice for this trial, because most of the human cells are protected from C-mediated damage by two different modes with these two kinds of C-regulators. Many problems have arisen, however, for MCP expression on grafts. (i) MCP acts as a measles virus receptor, which may function to render donor pigs measles virus (MV) sensitive. (ii) MCP signals immune suppression which causes devastation of the recipient's immune responses. (iii) MCP exerts relatively low self-protective activity against C compared with other cofactors; development of more efficient forms is desirable. (iv) Grafts with a high expression level of MCP are difficult to produce. In this study, we made a number of cDNA constructs of MCP, expressed them on swine endothelial cell lines, and tested cell-protective potency and MV susceptibility. The short consensus repeat 1 (SCR1)-deleted MCP with glycosyl phosphatidylinositol (GPI)-anchored form (Delta1MCP-PI) of MCP was found to be most suitable for the purpose of overcoming these problems. However, it was also found that MV induces two modes of cytopathic effect (CPE) on swine endothelial cells, either MCP-dependent or -independent. Here, we discuss these two points which will be raised through study of MCP-transgenic animals.

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.

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.

Establishment and properties of a growth factor-dependent, perpetual neural stem cell line from the human CNS

The ready availability of unlimited quantities of neural stem cells derived from the human brain holds great interest for basic and applied neuroscience, including therapeutic cell replacement and gene transfer following transplantation. We report here the combination of epigenetic and genetic procedures for perpetuating human neural stem cell lines. Thus we tested various culture conditions and genes for those that optimally allow for the continuous, rapid expansion and passaging of human neural stem cells. Among them, v-myc (the p110 gag-myc fusion protein derived from the avian retroviral genome) seems to be the most effective gene; we have also identified a strict requirement for the presence of mitogens (FGF-2 and EGF) in the growth medium, in effect constituting a conditional perpetuality or immortalization. A monoclonal, nestin-positive, human neural stem cell line (HNSC.100) perpetuated in this way divides every 40 h and stops dividing upon mitogen removal, undergoing spontaneous morphological differentiation and upregulating markers of the three fundamental lineages in the CNS (neurons, astrocytes, and oligodendrocytes). HNSC.100 cells therefore retain basic features of epigenetically expanded human neural stem cells. Clonal analysis confirmed the stability, multipotency, and self-renewability of the cell line. Finally, HNSC.100 can be transfected and transduced using a variety of procedures and genes encoding proteins for marking purposes and of therapeutic interest (e.g., human tyrosine hydroxylase I).

Single-step conversion of cells to retrovirus vector producers with herpes simplex virus-Epstein-Barr virus hybrid amplicons

We report here on the development and characterization of a novel herpes simplex virus type 1 (HSV-1) amplicon-based vector system which takes advantage of the host range and retention properties of HSV-Epstein-Barr virus (EBV) hybrid amplicons to efficiently convert cells to retrovirus vector producer cells after single-step transduction. The retrovirus genes gag-pol and env (GPE) and retroviral vector sequences were modified to minimize sequence overlap and cloned into an HSV-EBV hybrid amplicon. Retrovirus expression cassettes were used to generate the HSV-EBV-retrovirus hybrid vectors, HERE and HERA, which code for the ecotropic and the amphotropic envelopes, respectively. Retrovirus vector sequences encoding lacZ were cloned downstream from the GPE expression unit. Transfection of 293T/17 cells with amplicon plasmids yielded retrovirus titers between 10(6) and 10(7) transducing units/ml, while infection of the same cells with amplicon vectors generated maximum titers 1 order of magnitude lower. Retrovirus titers were dependent on the extent of transduction by amplicon vectors for the same cell line, but different cell lines displayed varying capacities to produce retrovirus vectors even at the same transduction efficiencies. Infection of human and dog primary gliomas with this system resulted in the production of retrovirus vectors for more than 1 week and the long-term retention and increase in transgene activity over time in these cell populations. Although the efficiency of this system still has to be determined in vivo, many applications are foreseeable for this approach to gene delivery.

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.

HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part I. HSV-1 structure, replication and pathogenesis

The design of effective gene therapy strategies for brain tumors and other neurological disorders relies on the understanding of genetic and pathophysiological alterations associated with the disease, on the biological characteristics of the target tissue, and on the development of safe vectors and expression systems to achieve efficient, targeted and regulated, therapeutic gene expression. The herpes simplex virus type 1 (HSV-1) virion is one of the most efficient of all current gene transfer vehicles with regard to nuclear gene delivery in central nervous system-derived cells including brain tumors. HSV-1-related research over the past decades has provided excellent insight into the structure and function of this virus, which, in turn, facilitated the design of innovative vector systems. Here, we review aspects of HSV-1 structure, replication and pathogenesis, which are relevant for the engineering of HSV-1-based vectors.

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.

Prospects for new restorative and neuroprotective treatments in Parkinson's disease

The degeneration of forebrain dopamine systems in Parkinson's disease has been an effective target for pharmaceutical research over the past four decades. However, although dopamine replacement may alleviate the symptoms of the disease, it does not halt the underlying neuronal degeneration. The past decade has seen major advances in identifying discrete genetic and molecular causes of parkinsonism and mapping the events involved in nigral cell death. This new understanding of the pathogenesis of the disease now offers novel prospects for therapy based on targeted neuroprotection of vulnerable neurons and effective strategies for their replacement.

T cell response in xenorecognition and xenografts: a review

Xenotransplantation has recently become a subject of interest for the transplantation community due to the current organ shortage, which could be partially or even totally solved by the development of this strategy. The humoral response, which arises as a result of species disparities, is the major obstacle to the success of xenotransplantation. However, if the use of different strategies such as plasmapheresis, immunoadsorption, the utilization of organs from transgenic pigs for complement regulatory molecules and new immunosuppressive drugs, may allow to overcome or reduce the early antibody mediated rejections (hyperacute or acute vascular rejection), delayed responses based on cellular activations will still occur. In this review, despite the fact that different cell populations have been shown to be implicated in these phenomena (NK, granulocytes, macrophages), we will focus on recent published information concerning T cell response only, in xenorecognition.

Molecular remodeling of complement regulatory proteins for xenotransplantation

In pig-to-human discordant xenotransplantation, human complement is a major barrier against long survival of xenografts. Human complement regulatory proteins expressed on xenografts have been adapted as safeguards against host-induced hyperacute rejection of xenografts. For successful xenotransplantation, there have been many attempts to generate molecules with potent human complement regulatory activity but without activities related to harmful functions such as infection, immunosuppression and signal transduction devastating cellular homeostasis. Here, we summarize the strategy by which molecules for xenotransplantation should be designed and propose a GPI-anchored form of monomeric human C4bp as a candidate for efficient protection of swine xenografts from human complement attack.

Apoptosis in neuronal development and transplantation: role of caspases and trophic factors

Fetal ventral mesencephalic (VM) transplants have been studied in the context of dopaminergic (DA) replacement therapy for Parkinson's disease (PD). DA neurons from VM transplants will grow axons and form functional synapses in the adult host central nervous system (CNS). Recently, studies have demonstrated that most of the transplanted DA neurons die in grafts within the first week after implantation. An important feature of neural development, also in transplanted developing fetal neural tissue, is cell death. However, while about 50% of cells born in the CNS will die naturally, up to 99% of fetal cells die after neural transplantation. It has been shown that VM grafts contain many apoptotic cells even at 14 days after transplantation. The interleukin-1beta converting enzyme (ICE) cysteine protease and 11 other ICE-like-related proteases have been identified, now named caspases. Activation of caspases is one of the final steps before a neuron is committed to die by apoptosis. Here we review this cell death process in detail: Since the growth of fetal neural grafts placed in the adult brain in many ways mimics normal development, it is likely that the caspases also play a functional role in transplants. Pharmacological inhibitors of caspases and genetically modified mice are now available for the study of neuronal death in fetal neuronal transplants. Understanding cell death mechanisms involved in acute cellular injury, necrosis, and programmed cell death (PCD) is useful in improving future neuronal transplantation methodology, as well as in neuroprotection, for patients with neurodegenerative diseases.

This little piggy went to market: the xenotransplantation and xenozoonose debate

New technologies are changing our lives radically and quickly. New biotechnologies are moving to commercial uses faster than government regulators or private citizens can monitor. This tension manifests itself in the current debates over xenotransplantation technologies in medicine. The possibility of removing cells, tissues, and organs from animals and transplanting them into human beings is startling and unnerving. Natural immunesystem barriers between species, and even between individuals within a species, are formidable. Typically, transplantation results in violent rejection and death of the grafted organ. But despite the natural barriers to transplantation, xenotransplantation aims specifically to overcome them.

In this paper, I will discuss applications of xenograft technology, which raises clinical risks, ethical concerns, and policy issues. I conclude with a set of specific recommendations. As a recent letter to the journalNatureputs it, there is a “split between those who want to get it right, and those who want to get it right now.” No one knows what all the risks, benefits, and unintended consequences of xenotransplantation will be.

Zoonosis in xenotransplantation

Species barriers against microbial infection will be lowered to an unprecedented degree in xenotransplantation settings. Our knowledge about micro-organisms in donor animals is limited and it is difficult to predict the consequence of such cross-species infection.