Transplantation of Human Neural Progenitor Cells to the Vitreous Cavity of the Royal College of Surgeons Rat

Dexamethasone Provides Effective Immunosuppression for Improved Survival of Retinal Organoids after Epiretinal Transplantation

We investigated the efficacy of the immunosuppressants rapamycin (RAP) and dexamethasone (DEX) in improving the survival of retinal organoids after epiretinal transplantation. We first compared the immunosuppressive abilities of DEX and RAP in activated microglia in an in vitro setting. Following this, we used immunofluorescence, real-time polymerase chain reaction, and flow cytometry to investigate the effects of DEX and RAP on cells in the retinal organoids. Retinal organoids were then seeded onto poly(lactic-co-glycolic) acid (PLGA) scaffolds and implanted into rhesus monkey eyes (including a healthy individual and three monkeys with chronic ocular hypertension (OHT) induction) and subjected to different post-operative immunosuppressant treatments; 8 weeks after the experiment, histological examinations were carried out to assess the success of the different treatments. Our in vitro experiments indicated that both DEX and RAP treatments were equally effective in suppressing microglial activity. Although both immunosuppressants altered the morphologies of cells in the retinal organoids and caused a slight decrease in the differentiation of cells into retinal ganglion cells, the organoid cells retained their capacity to grow and differentiate into retinal tissues. Our in vivo experiments indicate that the retinal organoid can survive and differentiate into retinal tissues in a healthy rhesus monkey eye without immunosuppressive treatment. However, the survival and differentiation of these organoids in OHT eyes was successful only with the DEX treatment. RAP treatment was ineffective in preventing immunological rejection, and the retinal organoid failed to survive until the end of 8 weeks. DEX is likely a promising immunosuppressant to enhance the survival of epiretinal implants.

Enhanced functional integration of human photoreceptor precursors into human and rodent retina in an ex vivo retinal explant model system

Retinal disease is the major cause of irreversible blindness in developed countries. Transplantation of photoreceptor precursor cells (PPCs) derived from human embryonic stem cells (hESCs) is a promising and widely applicable approach for the treatment of these blinding conditions. Previously, it has been shown that after transplantation into the degenerating retina, the percentage of PPCs that undergo functional integration is low. The factors that inhibit PPC engraftment remain largely unknown, in part, because so many adverse factors could be at play during in vivo experiments. To advance our knowledge in overcoming potential adverse effects and optimize PPC transplantation, we have developed a novel ex vivo system. Harvested neural retina was placed directly on top of cultured retinal pigment epithelial (RPE) cells from a number of different sources. To mimic PPC transplantation into the subretinal space, hESC-derived PPCs were inserted between the retinal explant and underlying RPE. Explants cocultured with hESC-derived RPE maintained normal gross morphology and viability for up to 2 weeks, whereas the explants cultured on ARPE19 and RPE-J failed by 7 days. Furthermore, the proportion of PPCs expressing ribbon synapse-specific proteins BASSOON and RIBEYE was significantly higher when cocultured with hESC-derived RPE (20% and 10%, respectively), than when cocultured with ARPE19 (only 6% and 2%, respectively). In the presence of the synaptogenic factor thrombospondin-1 (TSP-1), the proportion of BASSOON-positive and RIBEYE-positive PPCs cocultured with hESC-derived RPE increased to ∼30% and 15%, respectively. These data demonstrate the utility of an ex vivo model system to define factors, such as TSP-1, which could influence integration efficiency in future in vivo experiments in models of retinal degeneration.

Chondroitin sulfate proteoglycans and microglia prevent migration and integration of grafted Müller stem cells into degenerating retina

At present, there are severe limitations to the successful migration and integration of stem cells transplanted into the degenerated retina to restore visual function. This study investigated the potential role of chondroitin sulfate proteoglycans (CSPGs) and microglia in the migration of human Müller glia with neural stem cell characteristics following subretinal injection into the Lister hooded (LH) and Royal College of Surgeons (RCS) rat retinae. Neonate LH rat retina showed minimal baseline microglial accumulation (CD68-positive cells) that increased significantly 2 weeks after transplantation (p < .001), particularly in the ganglion cell layer (GCL) and inner plexiform layer. In contrast, nontransplanted 5-week-old RCS rat retina showed considerable baseline microglial accumulation in the outer nuclear layer (ONL) and photoreceptor outer segment debris zone (DZ) that further increased (p < .05) throughout the retina 2 weeks after transplantation. Marked deposition of the N-terminal fragment of CSPGs, as well as neurocan and versican, was observed in the DZ of 5-week-old RCS rat retinae, which contrasted with the limited expression of these proteins in the GCL of the adult and neonate LH rat retinae. Staining for CSPGs and CD68 revealed colocalization of these two molecules in cells infiltrating the ONL and DZ of the degenerating RCS rat retina. Enhanced immune suppression with oral prednisolone and intraperitoneal injections of indomethacin caused a reduction in the number of microglia but did not facilitate Müller stem cell migration. However, injection of cells with chondroitinase ABC combined with enhanced immune suppression caused a dramatic increase in the migration of Müller stem cells into all the retinal cell layers. These observations suggest that both microglia and CSPGs constitute a barrier for stem cell migration following transplantation into experimental models of retinal degeneration and that control of matrix deposition and the innate microglial response to neural retina degeneration may need to be addressed when translating cell-based therapies to treat human retinal disease.

Embryonic stem cells and retinal repair

In this review we examine the potential of embryonic stem cells (ESCs) for use in the treatment of retinal diseases involving photoreceptors and retinal pigment epithelium (RPE). We outline the ontogenesis of target retinal cell types (RPE, rods and cones) and discuss how an understanding of developmental processes can inform our manipulation of ESCs in vitro. Due to their potential for cellular therapy, special emphasis is placed upon the derivation and culture of human embryonic stem cells (HESCs) and their differentiation towards a retinal phenotype. In terms of achieving this goal, we suggest that much of the success to date reflects permissive in vitro environments provided by established protocols for HESC derivation, propagation and neural differentiation. In addition, we summarise key factors that may be important for enhancing efficiency of retinal cell-type derivation from HESCs. The retina is an amenable component of the central nervous system (CNS) and as such, diseases of this structure provide a realistic target for the application of HESC-derived cellular therapy to the CNS. In order to further this goal, the second component of our review focuses on the cellular and molecular cues within retinal environments that may influence the survival and behaviour of transplanted cells. Our analysis considers both the potential barriers to transplant integration in the retina itself together with the remodelling in host visual centres that is known to accompany retinal dystrophy.

Protection of visual functions by human neural progenitors in a rat model of retinal disease

Background

A promising clinical application for stem and progenitor cell transplantation is in rescue therapy for degenerative diseases. This strategy seeks to preserve rather than restore host tissue function by taking advantage of unique properties often displayed by these versatile cells. In studies using different neurodegenerative disease models, transplanted human neural progenitor cells (hNPC) protected dying host neurons within both the brain and spinal cord. Based on these reports, we explored the potential of hNPC transplantation to rescue visual function in an animal model of retinal degeneration, the Royal College of Surgeons rat.

Methodology/Principal Findings

Animals received unilateral subretinal injections of hNPC or medium alone at an age preceding major photoreceptor loss. Principal outcomes were quantified using electroretinography, visual acuity measurements and luminance threshold recordings from the superior colliculus. At 90–100 days postnatal, a time point when untreated rats exhibit little or no retinal or visual function, hNPC-treated eyes retained substantial retinal electrical activity and visual field with near-normal visual acuity. Functional efficacy was further enhanced when hNPC were genetically engineered to secrete glial cell line-derived neurotrophic factor. Histological examination at 150 days postnatal showed hNPC had formed a nearly continuous pigmented layer between the neural retina and retinal pigment epithelium, as well as distributed within the inner retina. A concomitant preservation of host cone photoreceptors was also observed.

Conclusions/Significance

Wild type and genetically modified human neural progenitor cells survive for prolonged periods, migrate extensively, secrete growth factors and rescue visual functions following subretinal transplantation in the Royal College of Surgeons rat. These results underscore the potential therapeutic utility of hNPC in the treatment of retinal degenerative diseases and suggest potential mechanisms underlying their effect in vivo.

[Stem cell-based therapies for retinal disorders]

In the context of cell-based therapies for hereditary retinal dystrophies and other retinal disorders, interest has focussed on the therapeutic potential of embryonic and tissue-specific stem cells. Stem cells are characterised by their capacity for self-renewal and by their multipotentiality. Because of these properties, they can be expanded in vitro and eventually differentiated into "desired" specialized cell types. Stem cells are not only candidate cells for the development of cell replacement strategies, but are also interesting cells for the establishment of ex vivo gene therapies. Here, we discuss recent experimental work performed to evaluate the therapeutic potential of embryonic, mesenchymal, hematopoietic, neural and retinal stem cells for the treatment of inherited retinal dystrophies and other retinal diseases.

Stem cells and retinal repair

Retinal stem cells (RSCs) are multipotent central nervous system (CNS) precursors that give rise to the retina during the course of development. RSCs are present in the embryonic eyecup of all vertebrate species and remain active in lower vertebrates throughout life. Mammals, however, exhibit little RSC activity in adulthood and thus little capacity for retinal growth or regeneration. Because CNS precursors can now be isolated from immature and mature mammals and expanded ex vivo, it is possible to study these cells in culture as well as following transplantation to the diseased retina. Such experiments have revealed a wealth of unanticipated findings, both in terms of the instructive cues present in the mature mammalian retina as well as the ability of grafted CNS precursors to respond to them. This review examines current knowledge regarding RSCs, together with other CNS precursors, from the perspective of investigators who wish to isolate, propagate, genetically modify, and transplant these cells as a regenerative strategy with application to retinal disease.

Stem cell therapies in reparative medicine

The future implementation of stem cell therapies to treat conditions thus far considered incurable has been envisioned as logical consequence of the fast-paced progress in stem cell research over the last few years. Still, many practical obstacles stand in the way to the routine application of these novel technologies in medicine. The conference "Stem Cell Therapies in Reparative Medicine," held aboard the cruise vessel Majesty of the Seas (Miami, USA-Nassau, Bahamas, April 19-22, 2002), focused on the analysis of these problems from different perspectives, including developmental biology (cell proliferation, fate determination, and enrichment), immunology (allorejection and prevention of autoimmunity recurrence), and clinical therapy, emphasizing the impact of stem cell technologies on the emerging field of tissue engineering and the treatment of alpha-1 antitrypsin deficiency.

Fate of multipotent neural precursor cells transplanted into mouse retina selectively depleted of retinal ganglion cells

In some parts of the CNS, depletion of a particular class of neuron might induce changes in the microenvironment that influence the differentiation of newly grafted neural precursor cells. This hypothesis was tested in the retina by inducing apoptotic retinal ganglion cell (RGC) death in neonatal and adult female mice and examining whether intravitreally grafted male neural precursor cells (C17.2), a neural stem cell (NSC)-like clonal line, become incorporated into these selectively depleted retinae. In neonates, rapid RGC death was induced by removal of the contralateral superior colliculus (SC), in adults, delayed RGC death was induced by unilateral optic nerve (ON) transection. Cells were injected intravitreally 6-48 h after SC ablation (neonates) or 0-7 days after ON injury (adults). Cells were also injected into non-RGC depleted neonatal and adult retinae. At 4 or 8 weeks, transplanted cells were identified using a Y-chromosome marker and in situ hybridisation or by their expression of the lacZ reporter gene product Escherichia coli beta-galactosidase (beta-gal). No C17.2 cells were identified in axotomised adult-injected eyes undergoing delayed RGC apoptosis (n = 16). Donor cells were however stably integrated within the retina in 29% (15/55) of mice that received C17.2 cell injections 24 h after neonatal SC ablation; 6-31% of surviving cells were found in the RGC layer (GCL). These NSC-like cells were also present in intact retinae, but on average, there were fewer cells in GCL. In SC-ablated mice, most grafted cells did not express retinal-specific markers, although occasional donor cells in the GCL were immunopositive for beta-III tubulin, a protein highly expressed by, but not specific to, developing RGCs. Targeted rapid RGC depletion thus increased cell incorporation into the GCL, but grafted C17.2 cells did not appear to differentiate into an RGC phenotype.

Migratory capacity of the cell line RN33B and the host glial cell response after subretinal transplantation to normal adult rats

As previously reported, the brain-derived precursor cell line RN33B has a great capacity to migrate when transplanted to adult brain or retina. This cell line is immortalized with the SV40 large T-antigen and carries the reporter gene LacZ and the green fluorescent protein GFP. In the present study, the precursor cells were transplanted to the subretinal space of adult rats and investigated early after grafting. The purpose was to demonstrate the migration of the grafted cells from the subretinal space into the retina and the glial cell response of the host retina. Detachment caused by the transplantation method was persistent up to 4 days after transplantation, and then reattachment occurred. The grafted cells were shown to migrate in between the photoreceptor cells before entering into the plexiform layers. Molecules involved in migration of immature neuronal cells as the polysialylated neural cell adhesion molecule (PSA-NCAM) and the collapsing response-mediated protein 4 (TUC-4) was found in the plexiform layers of the host retina, but not in the grafted cells. The expression of the intermediate filaments GFAP, vimentin, and nestin was intensely upregulated immediately after transplantation. A less pronounced upregulation was observed on sham-operated animals. In summary, the RN33B cell line migrated promptly posttransplantation and settled preferably into the plexiform layers of the retina, the same layers where the migration cues PSA-NCAM and TUC-4 were established. In addition, both the transplantation method per se and the implanted cells caused an intense glial cell response by the host retina.

Survival and Long Distance Migration of Brain‐Derived Precursor Cells Transplanted to Adult Rat Retina

Neural precursor cells transplanted to adult retina can integrate into the host. This is especially true when the neural precursor rat cell line RN33B is used. This cell line carries the reporter genes LacZ and green fluorescent protein (GFP). In grafted rat eyes, RN33B cells are localized from one eccentricity to the other of the host retina. In the present study, whole-mounted retinas were analyzed to obtain a more appropriate evaluation of the amount of transgene-expressing cells and the migratory capacity of these cells 3 and 8 weeks post-transplantation. Quantification was made of the number of beta-galactosidase- and GFP-expressing cells with a semiautomatized stereological cell counting system. With the same system, delineation of the distribution area of the grafted cells was also performed. At 3 weeks, 68% of the grafted eyes contained marker-expressing cells, whereas at 8 weeks only 35% of the eyes contained such cells. Counting of marker-expressing cells demonstrated a lower number of transgene-expressing cells at 3 weeks compared with 8 weeks post-transplantation. The distribution pattern of marker gene-expressing cells revealed cells occupying up to 21% at 3 weeks and up to 68% at 8 weeks of the entire host retina post-grafting. The precursor cells survived well in the adult retina although the most striking feature of the RN33B cell line was its extraordinary migratory capacity. This capability could be useful if precursor cells are used to deliver necessary genes or gene products that need to be distributed over a large diseased area.

Retinal transplantation of neural progenitor cells derived from the brain of GFP transgenic mice

Neural progenitor cells isolated from the brains of neonatal GFP transgenic mice were grafted to the retina of RCS rats and rds and B6 mice. Expression of GFP and differentiation markers was evaluated at 1-4 weeks post-transplantation. Grafted cells maintained transgene expression throughout the 4-week period. At 1 week there was widespread migration of GFP+cells within the host retina and at 2 weeks evidence of neuronal differentiation (as shown by both marker expression and cell morphology), although integration at 4 weeks was limited to syngeneic recipients. Because brain-derived neural progenitor cells exhibit both neuronal and astrocytic differentiation in diseased and normal host retina, these cells provide a useful tool for studies of retinal regeneration.

Differentiation of rhesus embryonic stem cells to neural progenitors and neurons

Embryonic stem (ES) cells are pluripotent cells capable of differentiating into cell lineages derived from all primary germ layers including neural cells. In this study we describe an efficient method for differentiating rhesus monkey ES cells to neural lineages and the subsequent isolation of an enriched population of Nestin and Musashi positive neural progenitor (NP) cells. Upon differentiation, these cells exhibit electrophysiological characteristics resembling cultured primary neurons. Embryoid bodies (EBs) were formed in ES growth medium supplemented with 50% MEDII. After 7 days in suspension culture, EBs were transferred to adherent culture and either differentiated in serum containing medium or expanded in serum free medium. Immunocytochemistry on differentiating cells derived from EBs revealed large networks of MAP-2 and NF200 positive neurons. DAPI staining showed that the center of the MEDII-treated EBs was filled with rosettes. NPs isolated from adherent EB cultures expanded in serum free medium were passaged and maintained in an undifferentiated state by culture in serum free N2 with 50% MEDII and bFGF. Differentiating neurons derived from NPs fired action potentials in response to depolarizing current injection and expressed functional ionotropic receptors for the neurotransmitters glutamate and gamma-aminobutyric acid (GABA). NPs derived in this way could serve as models for cellular replacement therapy in primate models of neurodegenerative disease, a source of neural cells for toxicity and drug testing, and as a model of the developing primate nervous system.

Expression of cytokines by multipotent neural progenitor cells

Recent work with mammalian neural stem cells has highlighted the role of cytokine signaling in the proliferation and differentiation of these multipotent cells. While the responsiveness of neural progenitors to exogenously applied growth factors has been demonstrated in vivo as well as in vitro, little attention has been given to the production of cytokines by these cells. Here we use immunocytochemistry, RT-PCR, and ELISA to show that under standard growth conditions multipotent neural progenitor cells from humans express multiple cytokines including IL-1alpha, IL-1beta, IL-6, TGF-beta1, TGF-beta2, TNF-alpha, but not IL-2, IL-4, or IFN-gamma. Neural progenitor cells from rat and mouse express some, but not all, of these cytokines under similar conditions. While the function of cytokine expression by neural progenitor cells remains to be elucidated, these signaling molecules are known to be involved in neural development and may play a role in the activation of quiescent stem cells by a variety of pathological processes.

The immunological properties of adult hippocampal progenitor cells

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

Differential lineage restriction of rat retinal progenitor cells in vitro and in vivo

To identify and characterize the lineage potential of rat neural retina progenitor cells (NRPCs) in vitro and engrafted into rats with retinal degeneration, NRPCs were isolated from neural retinas of embryonic day 17 Long Evans rats and cultured in serum-free or serum-containing media with fibroblast growth factor 2 and neurotrophin 3. After expansion, cellular differentiation was initiated by the withdrawal of these growth factors. Despite forming primary neurospheres, NRPCs cultured in serum-free medium survived poorly after passage. In contrast, NRPCs cultured in serum-containing medium could be expanded for up to 12 passages and differentiated into glial fibrillary acidic protein-positive glial cells and retina-specific neurons expressing rhodopsin, S-antigen, calbindin, recoverin, and calretinin. For in vivo analysis, passage 1 (P1) undifferentiated NRPCs were labeled with bromodeoxyuridine (BrdU), implanted into the subretinal space of Royal College of Surgeons (RCS) rats, and analyzed immunohistochemically 4 weeks postgrafting. The grafted NRPCs showed extensive glial differentiation, irrespective of their topographic localization. A few BrdU-labeled grafted NRPCs expressed protein kinase C, a marker for bipolar and amacrine interneuron-specific differentiation. Other retina-specific or oligodendrocytic differentiation was not detected in the grafted cells. Although NRPCs are capable of self-renewal and multilineage differentiation in vitro, they developed mostly into glial cells following engraftment into the adult retina. These data suggest that the adult retina retains epigenetic signals that are either restrictive for neuronal differentiation or instructive for glial differentiation. Induction of lineage-specific cell differentiation of engrafted NRPCs to facilitate retinal repair will likely require initiation of specific differentiation in vitro prior to grafting and/or modification of the host environment concomitantly with NRPC grafting.

Behavior of human neural progenitor cells transplanted to rat brain

Human neural stem/progenitor cells provide a useful tool for studies of neural development and differentiation, as well as a potential means for neuroreplacement therapeutic needs in the human CNS. Stem cells isolated from developing human central nervous system of 8-12-week fetuses were transplanted to the forebrain and cerebellum of young and adult rats after 14 days of in vitro expansion. Cells were labeled by bisbenzimide prior to transplantation without immunosuppression. Recipient brains were examined 10 and 20 days after transplantation. Labeled stem cells were found in the neocortex, lateral ventricle and caudate nucleus in the forebrain, and in the molecular layer, Purkinje cell layer, and granular layer of the cerebellum. Mitotically dividing stem cells were observed in graft core, confirming their proliferative potential in new microenvironment. Engrafted cells migrate through the parenchyme of striatum, along the ventricular ependymal layer and callosal fibers, some of them reaching the opposite hemisphere. Some cells migrating along the capillaries express glial acid fibrillary protein, demonstrating their differentiation into astrocytes. Grafted cells expressing calbindin were found in the Purkinje cell layer, suggesting their differentiation into the Purkinje cells. At the same time, some grafted cells were undifferentiated and expressed vimentin. Our results demonstrate that cultured human neural stem/progenitor cells migrate and differentiate into both neurons and astrocytes after transplantation to the rat forebrain or cerebellum of young and adult rats.

Surface markers expressed by multipotent human and mouse neural progenitor cells include tetraspanins and non-protein epitopes

Surface molecules play important roles in a wide range of cellular functions, yet little has been reported regarding the expression of such markers by neural stem cells. Here, multipotent human neural progenitor cells (hNPCs) were expanded as a monolayer in the presence of fibroblast/epidermal growth factor, harvested, labeled with monoclonal antibodies, and analyzed by flow cytometry. Positive markers included CD9, CD15, CD81, CD95 (Fas), GD(2) ganglioside, and major histocompatibility complex class I and beta2 microglobulin, as well as low levels of the hematopoietic stem cell marker CD34. Of these, mouse NPCs were positive for CD9, CD15, CD81, and GD(2) ganglioside. The markers reported here have been implicated in a wide range of cellular functions including proliferation, migration, differentiation, apoptosis, and immune recognition.