The immunological challenges of cell transplantation for the treatment of Parkinson's disease

Are Cell-Based Therapies Safe and Effective in the Treatment of Neurodegenerative Diseases? A Systematic Review with Meta-Analysis

Over the past two decades, significant advances have been made in the field of regenerative medicine. However, despite being of the utmost clinical urgency, there remains a paucity of therapeutic strategies for conditions with substantial neurodegeneration such as (progressive) multiple sclerosis (MS), spinal cord injury (SCI), Parkinson's disease (PD) and Alzheimer's disease (AD). Different cell types, such as mesenchymal stromal cells (MSC), neuronal stem cells (NSC), olfactory ensheathing cells (OEC), neurons and a variety of others, already demonstrated safety and regenerative or neuroprotective properties in the central nervous system during the preclinical phase. As a result of these promising findings, in recent years, these necessary types of cell therapies have been intensively tested in clinical trials to establish whether these results could be confirmed in patients. However, extensive research is still needed regarding elucidating the exact mechanism of action, possible immune rejection, functionality and survival of the administered cells, dose, frequency and administration route. To summarize the current state of knowledge, we conducted a systematic review with meta-analysis. A total of 27,043 records were reviewed by two independent assessors and 71 records were included in the final quantitative analysis. These results show that the overall frequency of serious adverse events was low: 0.03 (95% CI: 0.01-0.08). In addition, several trials in MS and SCI reported efficacy data, demonstrating some promising results on clinical outcomes. All randomized controlled studies were at a low risk of bias due to appropriate blinding of the treatment, including assessors and patients. In conclusion, cell-based therapies in neurodegenerative disease are safe and feasible while showing promising clinical improvements. Nevertheless, given their high heterogeneity, the results require a cautious approach. We advocate for the harmonization of study protocols of trials investigating cell-based therapies in neurodegenerative diseases, adverse event reporting and investigation of clinical outcomes.

From psychiatry to neurology: Psychedelics as prospective therapeutics for neurodegenerative disorders

The studies of psychedelics, especially psychedelic tryptamines like psilocybin, are rapidly gaining interest in neuroscience research. Much of this interest stems from recent clinical studies demonstrating that they have a unique ability to improve the debilitating symptoms of major depressive disorder (MDD) long-term after only a single treatment. Indeed, the Food and Drug Administration (FDA) has recently designated two Phase III clinical trials studying the ability of psilocybin to treat forms of MDD with "Breakthrough Therapy" status. If successful, the use of psychedelics to treat psychiatric diseases like depression would be revolutionary. As more evidence appears in the scientific literature to support their use in psychiatry to treat MDD on and substance use disorders (SUD), recent studies with rodents revealed that their therapeutic effects might extend beyond treating MDD and SUD. For example, psychedelics may have efficacy in the treatment and prevention of brain injury and neurodegenerative diseases such as Alzheimer's Disease. Preclinical work has highlighted psychedelics' ability to induce neuroplasticity and synaptogenesis, and neural progenitor cell proliferation. Psychedelics may also act as immunomodulators by reducing levels of proinflammatory biomarkers, including IL-1β, IL-6, and tumor necrosis factor-α (TNF-α). Their exact molecular mechanisms, and induction of cellular interactions, especially between neural and glial cells, leading to therapeutic efficacy, remain to be determined. In this review, we discuss recent findings and information on how psychedelics may act therapeutically on cells within the central nervous system (CNS) during brain injuries and neurodegenerative diseases.

Lipocalin-2 mediates the rejection of neural transplants

Lipocalin-2 (LCN2) has been implicated in promoting apoptosis and neuroinflammation in neurological disorders; however, its role in neural transplantation remains unknown. In this study, we cultured and differentiated Lund human mesencephalic (LUHMES) cells into human dopaminergic-like neurons and found that LCN2 mRNA was progressively induced in mouse brain after the intrastriatal transplantation of human dopaminergic-like neurons. The induction of LCN2 protein was detected in a subset of astrocytes and neutrophils infiltrating the core of the engrafted sites, but not in neurons and microglia. LCN2-immunoreactive astrocytes within the engrafted sites expressed lower levels of A1 and A2 astrocytic markers. Recruitment of microglia, neutrophils, and monocytes after transplantation was attenuated in LCN2 deficiency mice. The expression of M2 microglial markers was significantly elevated and survival of engrafted neurons was markedly improved after transplantation in LCN2 deficiency mice. Brain type organic cation transporter (BOCT), the cell surface receptor for LCN2, was induced in dopaminergic-like neurons after differentiation, and treatment with recombinant LCN2 protein directly induced apoptosis in dopaminergic-like neurons in a dose-dependent manner. Our results, therefore, suggested that LCN2 is a neurotoxic factor for the engrafted neurons and a modulator of neuroinflammation. LCN2 inhibition may be useful in reducing rejection after neural transplantation.

Challenges and translational considerations of mesenchymal stem/stromal cell therapy for Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta and the presence of Lewy bodies, which gives rise to motor and non-motor symptoms. Unfortunately, current therapeutic strategies for PD merely treat the symptoms of the disease, only temporarily improve the patients' quality of life, and are not sufficient for completely alleviating the symptoms. Therefore, cell-based therapies have emerged as a novel promising therapeutic approach in PD treatment. Mesenchymal stem/stromal cells (MSCs) have arisen as a leading contender for cell sources due to their regenerative and immunomodulatory capabilities, limited ethical concerns, and low risk of tumor formation. Although several studies have shown that MSCs have the potential to mitigate the neurodegenerative pathology of PD, variabilities in preclinical and clinical trials have resulted in inconsistent therapeutic outcomes. In this review, we strive to highlight the sources of variability in studies using MSCs in PD therapy, including MSC sources, the use of autologous or allogenic MSCs, dose, delivery methods, patient factors, and measures of clinical outcome. Available evidence indicates that while the use of MSCs in PD has largely been promising, conditions need to be standardized so that studies can be effectively compared with one another and experimental designs can be improved upon, such that this body of science can continue to move forward.

Gut Microbiota Approach-A New Strategy to Treat Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by neuronal loss and dysfunction of dopaminergic neurons located in the substantia nigra, which contain a variety of misfolded α-synuclein (α-syn). Medications that increase or substitute for dopamine can be used for the treatment of PD. Recently, numerous studies have shown gut microbiota plays a crucial role in regulating and maintaining multiple aspects of host physiology including host metabolism and neurodevelopment. In this review article, the role of gut microbiota in the etiological mechanism of PD will be reviewed. Furthermore, we discussed current pharmaceutical medicine-based methods to prevent and treat PD, followed by describing specific strains that affect the host brain function through the gut-brain axis. We explained in detail how gut microbiota directly produces neurotransmitters or regulate the host biosynthesis of neurotransmitters. The neurotransmitters secreted by the intestinal lumen bacteria may induce epithelial cells to release molecules that, in turn, can regulate neural signaling in the enteric nervous system and subsequently control brain function and behavior through the brain-gut axis. Finally, we proved that the microbial regulation of the host neuronal system. Endogenous α-syn can be transmitted long distance and bidirectional between ENS and brain through the circulatory system which gives us a new option that the possibility of altering the community of gut microbiota in completely new medication option for treating PD.

Is the Immunological Response a Bottleneck for Cell Therapy in Neurodegenerative Diseases?

Neurodegenerative disorders such as Parkinson's (PD) and Huntington's disease (HD) are characterized by a selective detrimental impact on neurons in a specific brain area. Currently, these diseases have no cures, although some promising trials of therapies that may be able to slow the loss of brain cells are underway. Cell therapy is distinguished by its potential to replace cells to compensate for those lost to the degenerative process and has shown a great potential to replace degenerated neurons in animal models and in clinical trials in PD and HD patients. Fetal-derived neural progenitor cells, embryonic stem cells or induced pluripotent stem cells are the main cell sources that have been tested in cell therapy approaches. Furthermore, new strategies are emerging, such as the use of adult stem cells, encapsulated cell lines releasing trophic factors or cell-free products, containing an enriched secretome, which have shown beneficial preclinical outcomes. One of the major challenges for these potential new treatments is to overcome the host immune response to the transplanted cells. Immune rejection can cause significant alterations in transplanted and endogenous tissue and requires immunosuppressive drugs that may produce adverse effects. T-, B-lymphocytes and microglia have been recognized as the main effectors in striatal graft rejection. This review aims to summarize the preclinical and clinical studies of cell therapies in PD and HD. In addition, the precautions and strategies to ensure the highest quality of cell grafts, the lowest risk during transplantation and the reduction of a possible immune rejection will be outlined. Altogether, the wide-ranging possibilities of advanced therapy medicinal products (ATMPs) could make therapeutic treatment of these incurable diseases possible in the near future.

Glial cells in intracerebral transplantation for Parkinson's disease

In the last few decades, intracerebral transplantation has grown from a dubious neuroscientific topic to a plausible modality for treatment of neurological disorders. The possibility for cell replacement opens a new field of perspectives in the therapy of neurodegenerative disorders, ischemia, and neurotrauma, with the most lessons learned from intracerebral transplantation in Parkinson's disease. Multiple animal studies and a few small-scale clinical trials have proven the concept of intracerebral grafting, but still have to provide a uniform and highly efficient approach to the procedure, suitable for clinical application. The success of intracerebral transplantation is highly dependent on the integration of the grafted cells with the host brain. In this process, glial cells are clearly more than passive bystanders. They provide transplanted cells with mechanical support, trophics, mediate synapse formation, and participate in graft vascularization. At the same time, glial cells mediate scarring, graft rejection, and neuroinflammation, which can be detrimental. We can use this information to try to understand the mechanisms behind the glial reaction to intracerebral transplantation. Recognizing and utilizing glial reactivity can move translational research forward and provide an insight not only to post-transplantation events but also to mechanisms of neuronal death and degeneration. Knowledge about glial reactivity to transplanted cells could also be a key for optimization of transplantation protocols, which ultimately should contribute to greater patient benefit.

Olfactory ensheathing cells improve the survival of porcine neural xenografts in a Parkinsonian rat model

BACKGROUND

Parkinson's disease (PD) features the motor control deficits resulting from irreversible, progressive degeneration of dopaminergic (DA) neurons of the nigrostriatal pathway. Although intracerebral transplantation of human fetal ventral mesencephalon (hfVM) has been proven effective at reviving DA function in the PD patients, this treatment is clinically limited by availability of hfVM and the related ethical issues. Homologous tissues to hfVM, such as porcine fetal ventral mesencephalon (pfVM) thus present a strong clinical potential if immune response following xenotransplantation could be tamed. Olfactory ensheathing cells (OECs) are glial cells showing immunomodulatory properties. It is unclear but intriuging whether these properties can be applied to reducing immune response following neural xenotransplantation of PD.

METHODS

To determine whether OECs may benefit neural xenografts for PD, different compositions of grafting cells were transplanted into striatum of the PD model rats. We used apomorphine-induced rotational behavior to evaluate effectiveness of the neural grafts on reviving DA function. Immunohistochemistry was applied to investigate the effect of OECs on the survival of neuroxenografts and underlying mechanisms of this effect.

RESULTS

Four weeks following the xenotransplantation, we found that the PD rats receiving pfVM + OECs co-graft exhibited a better improvement in apomorphine-induced rotational behavior compared with those receiving only pfVM cells. This result can be explained by higher survival of DA neurons (tyrosine hydroxylase immunoreactivity) in grafted striatum of pfVM + OECs group. Furthermore, pfVM + OECs group has less immune response (CD3⁺ T cells and OX-6⁺ microglia) around the grafted area compared with pfVM only group. These results suggest that OECs may enhance the survival of the striatal xenografts via dampening the immune response at the grafted sites.

CONCLUSIONS

Using allogeneic OECs as a co-graft material for xenogeneic neural grafts could be a feasible therapeutic strategy to enhance results and applicability of the cell replacement therapy for PD.

Antagonization of the Nogo-Receptor 1 Enhances Dopaminergic Fiber Outgrowth of Transplants in a Rat Model of Parkinson's Disease

Intrastriatal transplantation of fetal human ventral mesencephalic dopaminergic neurons is an experimental therapy for patients suffering from Parkinson’s disease. The success of this approach depends on several host brain parameters including neurotrophic factors and growth inhibitors that guide survival and integration of transplanted neurons. While the potential of neurotrophic factors has been extensively investigated, repression of growth inhibitors has been neglected, despite the significant effects reported in various CNS injury models. Recently, we demonstrated that infusion of neutralizing antibodies against Nogo-A into the lateral ventricles of hemi-parkinsonian rats significantly enhanced graft function. Since the Nogo-receptor 1 also interacts with other neurite growth inhibitors, we investigated whether a direct antagonization of the receptor would result in more robust effects. Therefore, rats with unilateral striatal 6-hydroxydopamine lesions were grafted with ventral mesencephalic tissue in combination with intraventricular infusions of the Nogo-receptor 1 antagonist NEP1-40. Transplanted rats receiving saline infusions served as controls. To test whether NEP1-40 treatment alone affects the remaining dopaminergic striatal fibers, rats with unilateral striatal 6-hydroxydopamine lesions were infused with NEP1-40 or saline without receiving a transplant. Motor behavior was assessed prior to the lesion as well as prior and 1, 3, and 5 weeks after the transplantations. At the end of the experimental period the number of graft-derived dopaminergic fibers growing into the host brain, the number of surviving dopaminergic neurons and graft volume were analyzed. In rats without a transplant, the density of dopaminergic fibers in the striatum was analyzed. We detected that NEP1-40 treatment significantly enhanced graft-derived dopaminergic fiber outgrowth as compared to controls while no effects were detected for graft volume and survival of grafted dopaminergic neurons. Notably, the enhanced dopaminergic fiber outgrowth was not sufficient to improve the functional recovery as compared to controls. Moreover, NEP1-40 infusions in hemi-parkinsonian rats without a transplant did not result in enhanced striatal dopaminergic fiber densities and consequently did not improve behavior. In sum, our findings demonstrate that antagonization of the Nogo-receptor 1 has the capacity to support the engraftment of transplanted mesencephalic tissue in an animal model of Parkinson’s disease.

Engineered hydrogels increase the post-transplantation survival of encapsulated hESC-derived midbrain dopaminergic neurons

Cell replacement therapies have broad biomedical potential; however, low cell survival and poor functional integration post-transplantation are major hurdles that hamper clinical benefit. For example, following striatal transplantation of midbrain dopaminergic (mDA) neurons for the treatment of Parkinson's disease (PD), only 1-5% of the neurons typically survive in preclinical models and in clinical trials. In general, resource-intensive generation and implantation of larger numbers of cells are used to compensate for the low post-transplantation cell-survival. Poor graft survival is often attributed to adverse biochemical, mechanical, and/or immunological stress that cells experience during and after implantation. To address these challenges, we developed a functionalized hyaluronic acid (HA)-based hydrogel for in vitro maturation and central nervous system (CNS) transplantation of human pluripotent stem cell (hPSC)-derived neural progenitors. Specifically, we functionalized the HA hydrogel with RGD and heparin (hep) via click-chemistry and tailored its stiffness to encourage neuronal maturation, survival, and long-term maintenance of the desired mDA phenotype. Importantly, ∼5 times more hydrogel-encapsulated mDA neurons survived after transplantation in the rat striatum, compared to unencapsulated neurons harvested from commonly used 2D surfaces. This engineered biomaterial may therefore increase the therapeutic potential and reduce the manufacturing burden for successful neuronal implantation.

Cell Therapy for Parkinson's Disease: A Translational Approach to Assess the Role of Local and Systemic Immunosuppression

Neural transplantation is a promising therapeutic approach for neurodegenerative diseases; however, many patients receiving intracerebral fetal allografts exhibit signs of immunization to donor antigens that could compromise the graft. In this context, we intracerebrally transplanted mesencephalic pig xenografts into primates to identify a suitable strategy to enable long-term cell survival, maturation, and differentiation. Parkinsonian primates received WT or CTLA4-Ig transgenic porcine xenografts and different durations of peripheral immunosuppression to test whether systemic plus graft-mediated local immunosuppression might avoid rejection. A striking recovery of spontaneous locomotion was observed in primates receiving systemic plus local immunosuppression for 6 mo. Recovery was associated with restoration of dopaminergic activity detected both by positron emission tomography imaging and histological examination. Local infiltration by T cells and CD80/86+ microglial cells expressing indoleamine 2,3-dioxigenase were observed only in CTLA4-Ig recipients. Results suggest that in this primate neurotransplantation model, peripheral immunosuppression is indispensable to achieve the long-term survival of porcine neuronal xenografts that is required to study the beneficial immunomodulatory effect of local blockade of T cell costimulation.

RNAi-mediated silencing of HLA A2 suppressed acute rejection against human fibroblast xenografts in the striatum of 6-OHDA lesioned rats

Major histocompatibility complex class l (MHC I) molecules play a role in determining whether transplanted cells will be accepted or rejected, and masking of MHC I on donor cells has been found useful for immunoprotection of neural xenografts. In the present study, primary human embryonic lung fibroblasts (HELF), HELF treated with lentivirus-mediated small interfering RNAs (siRNAs) targeting human leukocyte antigen A2 (HLA A2, MHC I in humans) (siHELF), and rat embryonic lung fibroblasts (RELF) were stereotaxically grafted into the striatum of 6-hydroxydopamine lesioned rats to explore whether knockdown of HLA A2 could reduce host immune responses against xenografts. Before lentiviral infection, the cells were transduced with retroviruses harboring tyrosine hydroxylase cDNA. Knockdown of HLA A2 protein was examined by Western blotting. The immune responses (the number of CD4 and CD8 T-cells in the brain and peripheral blood), glial reaction, and survival of human fibroblasts were quantitatively evaluated by flow cytometry and immunohistochemistry at 4d, 2w, and 6w post-graft. Animal behaviors were assessed by counting apomorphine-induced rotations pre- and post-grafts. It was shown that a lower level of HLA A2 was observed in siHELF grafts than in HELF grafts, and knockdown of HLA A2 decreased rat immune responses, as indicated by less remarkable increases in the number of CD8 and CD4 T-cells in the brain and the ratio of CD4:CD8 T-cells in the peripheral blood in rats grafted with siHELF. Rats grafted with siHELF exhibited a significant improvement in motor asymmetry post-transplantation and a better survival of human fibroblasts at 2w. The increasing number of activated microglia and the decreasing number of astrocytes were found in three groups of rats post-implantation. These data suggested that RNAi-mediated knockdown of HLA A2 could suppress acute rejection against xenogeneic human cell transplants in the rat brain.

Foetal Cell Transplantation for Parkinson's Disease: Focus on Graft-Induced Dyskinesia

Transplantation of dopamine- (DA-) rich foetal ventral mesencephalic cells emerged as a promising therapy for Parkinson's disease (PD), as it allowed significant improvement of motor symptoms in several PD patients in open-label studies. However, double-blind clinical trials have been largely disappointing. The general agreement in the field is that the lack of standardization of tissue collection and preparation, together with the absence of postsurgical immunosuppression, played a key role in the failure of these studies. Moreover, a further complication that emerged in previous studies is the appearance of the so-called graft-induced dyskinesia (GID), in a subset of grafted patients, which resembles dyskinesia induced by L-DOPA but in the absence of medication. Preclinical evidence pointed to the serotonin neurons as possible players in the appearance of GID. In agreement, clinical investigations have shown that grafted tissue may contain a large number of serotonin neurons, in the order of half of the DA cells; moreover, the serotonin 5-HT1A receptor agonist buspirone has been found to produce significant dampening of GID in grafted patients. In this paper, we will review the recent preclinical and clinical studies focusing on cell transplantation for PD and on the mechanisms underlying GID.

Transplantation of human retinal pigment epithelial cells in the nucleus accumbens of cocaine self-administering rats provides protection from seeking

Chronic exposure to drugs and alcohol leads to damage to dopaminergic neurons and their projections in the 'reward pathway' that originate in the ventral tegmental area (VTA) and terminate in the nucleus accumbens (NAc). This damage is thought to contribute to the signature symptom of addiction: chronic relapse. In this study we show that bilateral transplants of human retinal pigment epithelial cells (RPECs), a cell mediated dopaminergic and trophic neuromodulator, into the medial shell of the NAc, rescue rats with a history of high rates of cocaine self-administration from drug-seeking when returned, after 2 weeks of abstinence, to the drug-associated chamber under extinction conditions (i.e., with no drug available). Excellent survival was noted for the transplant of RPECs in the shell and/or the core of the NAc bilaterally in all rats that showed behavioral recovery from cocaine seeking. Design based unbiased stereology of tyrosine hydroxylase (TH) positive cell bodies in the VTA showed better preservation (p<0.035) in transplanted animals compared to control animals. This experiment shows that the RPEC graft provides beneficial effects to prevent drug seeking in drug addiction via its effects directly on the NAc and its neural network with the VTA.

Cell therapy for Parkinson's disease: Functional role of the host immune response on survival and differentiation of dopaminergic neuroblasts

Parkinson's disease (PD) is a neurodegenerative disorder, whose cardinal pathology is the loss of dopaminergic neurons in the substantia nigra. Current treatments for PD have side effects in the long term and do not halt disease progression or regenerate dopaminergic cell loss. Attempts to compensate neuronal cell loss by transplantation of dopamine-producing cells started more than 30 years ago, leading to several clinical trials. These trials showed safety and variable efficacy among patients. In addition to variability in efficacy, several patients developed graft-induced dyskinesia. Nevertheless, they have provided a proof of concept that motor symptoms could be improved by cell transplantation. Cell transplantation in the brain presents several immunological challenges. The adaptive immune response should be abolished to avoid graft rejection by the host. In addition, the innate immune response will always be present after transplanting cells into the brain. Remarkably, the innate immune response can have dramatic effects on the survival, differentiation and proliferation of the transplanted cells, but has been hardly investigated. In this review, we analyze data on the functional effects of signals from the innate immune system on dopaminergic differentiation, survival and proliferation. Then, we discussed efforts on cell transplantation in animal models and PD patients, highlighting the immune response and the immunomodulatory treatment strategies performed. The analysis of the available data lead us to conclude that the modulation of the innate immune response after transplantation can increase the success of future clinical trials in PD by enhancing cell differentiation and survival. This article is part of a Special Issue entitled SI: PSC and the brain.

Cell reprogramming and neuronal differentiation applied to neurodegenerative diseases: Focus on Parkinson's disease

Adult cells from patients can be reprogrammed to induced pluripotent stem cells (iPSCs) which successively can be used to obtain specific cells such as neurons. This remarkable breakthrough represents a new way of studying diseases and brought new therapeutic perspectives in the field of regenerative medicine. This is particular true in the neurology field, where few techniques are amenable to study the affected tissue of the patient during illness progression, in addition to the lack of neuroprotective therapies for many diseases. In this review we discuss the advantages and unresolved issues of cell reprogramming and neuronal differentiation. We reviewed evidence using iPSCs-derived neurons from neurological patients. Focusing on data obtained from Parkinson's disease (PD) patients, we show that iPSC-derived neurons possess morphological and functional characteristics of this disease and build a case for the use of this technology to study PD and other neuropathologies while disease is in progress. These data show the enormous impact that this new technology starts to have on different purposes such as the study and design of future therapies of neurological disease, especially PD.

PET Imaging of Serotonin Transporters With 4-[(18)F]-ADAM in a Parkinsonian Rat Model With Porcine Neural Xenografts

Parkinson's disease (PD) is a neurodegenerative disease characterized by a loss of dopaminergic neurons in the nigrostriatal pathway. Apart from effective strategies to halt the underlying neuronal degeneration, cell replacement now offers novel prospects for PD therapy. Porcine embryonic neural tissue has been considered an alternative source to human fetal grafts in neurodegenerative disorders because its use avoids major practical and ethical issues. This study was undertaken to evaluate the effects of embryonic day 27 (E27) porcine mesencephalic tissue transplantation in a PD rat model using animal positron emission tomography (PET) coupled with 4-[(18)F]-ADAM, a serotonin transporter (SERT) imaging agent. The parkinsonian rat was induced by injecting 6-hydroxydopamine into the medial forebrain bundle (MFB) of the right nigrostriatal pathway. The apomorphine-induced rotation behavioral test and 4-[(18)F]-ADAM/animal PET scanning were carried out following 6-OHDA lesioning. At the second week following 6-OHDA lesioning, the parkinsonian rat rotates substantially on apomorphine-induced contralateral turning. In addition, the mean striatal-specific uptake ratio (SUR) of 4-[(18)F]-ADAM decreased by 44%. After transplantation, the number of drug-induced rotations decreased markedly, and the mean SUR of 4-[(18)F]-ADAM and the level of SERT immunoreactivity (SERT-ir) in striatum were partially restored. The mean SUR level was restored to 71% compared to that for the contralateral intact side, which together with the abundant survival of tyrosine hydroxylase (TH) neurons accounted for functional recovery at the fourth week postgraft. In regard to the extent of donor-derived cells, we found the neurons of the xenografts from E27 transgenic pigs harboring red fluorescent protein (RFP) localized with TH-ir cells and SERT-ir in the grafted area. Thus, transplanted E27 porcine mesencephalic tissue may restore dopaminergic and serotonergic systems in the parkinsonian rat. The 4-[(18)F]-ADAM/animal PET can be used to detect serotonergic neuron loss in PD and monitor the efficacy of therapy.