Subretinal transplantation of forebrain progenitor cells in nonhuman primates: survival and intact retinal function

GMP-grade human neural progenitors delivered subretinally protect vision in rat model of retinal degeneration and survive in minipigs

BACKGROUND

Stem cell products are increasingly entering early stage clinical trials for treating retinal degeneration. The field is learning from experience about comparability of cells proposed for preclinical and clinical use. Without this, preclinical data supporting translation to a clinical study might not adequately reflect the performance of subsequent clinical-grade cells in patients.

METHODS

Research-grade human neural progenitor cells (hNPC) and clinical-grade hNPC (termed CNS10-NPC) were injected into the subretinal space of the Royal College of Surgeons (RCS) rat, a rodent model of retinal degeneration such as retinitis pigmentosa. An investigational new drug (IND)-enabling study with CNS10-NPC was performed in the same rodent model. Finally, surgical methodology for subretinal cell delivery in the clinic was optimized in a large animal model with Yucatan minipigs.

RESULTS

Both research-grade hNPC and clinical-grade hNPC can survive and provide functional and morphological protection in a dose-dependent fashion in RCS rats and the optimal cell dose was defined and used in IND-enabling studies. Grafted CNS10-NPC migrated from the injection site without differentiation into retinal cell phenotypes. Additionally, CNS10-NPC showed long-term survival, safety and efficacy in a good laboratory practice (GLP) toxicity and tumorigenicity study, with no observed cell overgrowth even at the maximum deliverable dose. Finally, using a large animal model with the Yucatan minipig, which has an eye size comparable to the human, we optimized the surgical methodology for subretinal cell delivery in the clinic.

CONCLUSIONS

These extensive studies supported an approved IND and the translation of CNS10-NPC to an ongoing Phase 1/2a clinical trial (NCT04284293) for the treatment of retinitis pigmentosa.

A stem cell JOURNEY IN OPHTHALMOLOGY: From the bench to the clinic

Debilitating diseases of the eye represent a large unmet medical need potentially addressable with stem cell-based approaches. Over the past decade, the California Institute for Regenerative Medicine (CIRM) has funded and supported the translation, from early research concepts to human trials, of therapeutic stem cell approaches for dry age-related macular degeneration, retinitis pigmentosa, and limbal stem cell deficiency. This article chronicles CIRM's journey in the ophthalmology field and discusses some key challenges and questions that were addressed along the way as well as questions that remain.

Outer Retinal Cell Replacement: Putting the Pieces Together

Retinal degenerative diseases (RDDs) affecting photoreceptors (PRs) are one of the most prevalent sources of incurable blindness worldwide. Due to a lack of endogenous repair mechanisms, functional cell replacement of PRs and/or retinal pigmented epithelium (RPE) cells are among the most anticipated approaches for restoring vision in advanced RDD. Human pluripotent stem cell (hPSC) technologies have accelerated development of outer retinal cell therapies as they provide a theoretically unlimited source of donor cells. Human PSC-RPE replacement therapies have progressed rapidly, with several completed and ongoing clinical trials. Although potentially more promising, hPSC-PR replacement therapies are still in their infancy. A first-in-human trial of hPSC-derived neuroretinal transplantation has recently begun, but a number of questions regarding survival, reproducibility, functional integration, and mechanism of action remain. The discovery of biomaterial transfer between donor and PR cells has highlighted the need for rigorous safety and efficacy studies of PR replacement. In this review, we briefly discuss the history of neuroretinal and PR cell transplantation to identify remaining challenges and outline a stepwise approach to address specific pieces of the outer retinal cell replacement puzzle.

Interaction Between Mesenchymal Stem Cells and Retinal Degenerative Microenvironment

Retinal degenerative diseases (RDDs) are a group of diseases contributing to irreversible vision loss with yet limited therapies. Stem cell-based therapy is a promising novel therapeutic approach in RDD treatment. Mesenchymal stromal/stem cells (MSCs) have emerged as a leading cell source due to their neurotrophic and immunomodulatory capabilities, limited ethical concerns, and low risk of tumor formation. Several pre-clinical studies have shown that MSCs have the potential to delay retinal degeneration, and recent clinical trials have demonstrated promising safety profiles for the application of MSCs in retinal disease. However, some of the clinical-stage MSC therapies have been unable to meet primary efficacy end points, and severe side effects were reported in some retinal “stem cell” clinics. In this review, we provide an update of the interaction between MSCs and the RDD microenvironment and discuss how to balance the therapeutic potential and safety concerns of MSCs' ocular application.

Cell Therapy for Retinal Dystrophies: From Cell Suspension Formulation to Complex Retinal Tissue Bioengineering

Retinal degeneration is an irreversible phenomenon caused by various disease conditions including age-related macular degeneration (AMD) and retinitis pigmentosa (RP). During the course of these diseases, photoreceptors (PRs) are susceptible to degeneration due to their malfunctions or to a primary dysfunction of the retinal pigment epithelium (RPE). Once lost, these cells could not be endogenously regenerated in humans, and cell therapy to replace the lost cells is one of the promising strategies to recover vision. Depending on the nature of the primary defect and the stage of the disease, RPE cells, PRs, or both might be transplanted to achieve therapeutic effects. We describe in this review the current knowledge and recent progress to develop such approaches. The different cell sources proposed for cell therapy including human pluripotent stem cells are presented with their advantages and limits. Another critical aspect described herein is the pharmaceutical formulation of the end product to be delivered into the eye of patients. Finally, we also outline the future research directions in order to develop a complex multilayered retinal tissue for end-stage patients.

Surgical Approaches for Cell Therapeutics Delivery to the Retinal Pigment Epithelium and Retina

Developing successful surgical strategies to deliver cell therapeutics to the back of the eye is an essential pillar to success for stem cell-based applications in blinding retinal diseases. Within this chapter, we have attempted to gather all key considerations during preclinical animal trials.Guidance is provided for choices on animal models, options for immunosuppression, as well as anesthesia. Subsequently we cover surgical strategies for RPE graft delivery, both as suspension as well as in monolayers in small rodents, rabbits, pigs, and nonhuman primate. A detailed account is given in particular on animal variations in vitrectomy and subretinal surgery, which requires a considerable learning curve, when transiting from human to animal. In turn, however, many essential subretinal implantation techniques in large-eyed animals are directly transferrable to human clinical trial protocols.A dedicated subchapter on photoreceptor replacement provides insights on preparation of suspension as well as sheet grafts, to subsequently outline the basics of subretinal delivery via both the transscleral and transvitreal route. In closing, a future outlook on vision restoration through retinal cell-based therapeutics is presented.

Allogeneic iPSC-Derived RPE Cell Graft Failure Following Transplantation Into the Subretinal Space in Nonhuman Primates

Purpose

To characterize the intraocular immune response following transplantation of iPS-derived allogeneic RPE cells into the subretinal space of non–immune-suppressed rhesus macaques.

Methods

GFP-labeled allogeneic iPS-derived RPE cells were transplanted into the subretinal space of one eye (n = 6), and into the contralateral eye 1 day to 4 weeks later, using a two-stage transretinal and transscleral approach. Retinas were examined pre- and post-surgery by color fundus photography, fundus autofluorescence, and optical coherence tomography (OCT) imaging. Animals were euthanized between 2 hours and 7 weeks following transplantation. T-cell (CD3), B-cell (CD20), and microglial (Iba1) responses were assessed immunohistochemically.

Results

Cells were delivered into the subretinal space in all eyes without leakage into the vitreous. Transplanted RPE cells were clearly visible at 4 days after surgery but were no longer detectable by 3 weeks. In localized areas within the bleb containing transplanted cells, T- and B-cell infiltrates and microglia were observed in the subretinal space and underlying choroid. A T-cell response predominated at 4 days, but converted to a B-cell response at 3 weeks. By 7 weeks, few infiltrates or microglia remained. Host RPE and choroid were disrupted in the immediate vicinity of the graft, with fibrosis in the subretinal space.

Conclusions

Engraftment of allogeneic RPE cells failed following transplantation into the subretinal space of rhesus macaques, likely due to rejection by the immune system. These data underscore the need for autologous cell sources and/or confirmation of adequate immune suppression to ensure survival of transplanted RPE cells.

Intravitreal, Subretinal, and Suprachoroidal Injections: Evolution of Microneedles for Drug Delivery

Even though the very thought of an injection into the eye may be frightening, an estimated 6 million intravitreal (IVT) injections were made in the USA during 2016. With the introduction of new therapeutic agents, this number is expected to increase. In addition, drug products that are injectable in ocular compartments other than the vitreous humor are expected to enter the back of the eye market in the not so distant future. Besides the IVT route, some of the most actively investigated routes of invasive administration to the eye include periocular, subretinal, and suprachoroidal (SC) routes. While clinical efficacy is the driving force behind new injectable drug product development for the eye, safety is also being improved with time. In the case of IVT injections, the procedural guidelines have evolved over the years to improve patient comfort and reduce injection-related injury and infection. Similar advances are anticipated for other routes of administration of injectable products to the eye. In addition to procedural improvements, the design of needles, particularly those with smaller diameters, length, and controlled bevel angles are expected to improve overall safety and acceptance of injected ophthalmic drug products. A key development in this area is the introduction of microneedles of a length less than a millimeter that can target the SC space. In the future, needles with smaller diameters and lengths, potentially approaching nanodimensions, are expected to revolutionize ophthalmic disease management.

Assessment of Safety and Functional Efficacy of Stem Cell-Based Therapeutic Approaches Using Retinal Degenerative Animal Models

Dysfunction and death of retinal pigment epithelium (RPE) and or photoreceptors can lead to irreversible vision loss. The eye represents an ideal microenvironment for stem cell-based therapy. It is considered an “immune privileged” site, and the number of cells needed for therapy is relatively low for the area of focused vision (macula). Further, surgical placement of stem cell-derived grafts (RPE, retinal progenitors, and photoreceptor precursors) into the vitreous cavity or subretinal space has been well established. For preclinical tests, assessments of stem cell-derived graft survival and functionality are conducted in animal models by various noninvasive approaches and imaging modalities. In vivo experiments conducted in animal models based on replacing photoreceptors and/or RPE cells have shown survival and functionality of the transplanted cells, rescue of the host retina, and improvement of visual function. Based on the positive results obtained from these animal experiments, human clinical trials are being initiated. Despite such progress in stem cell research, ethical, regulatory, safety, and technical difficulties still remain a challenge for the transformation of this technique into a standard clinical approach. In this review, the current status of preclinical safety and efficacy studies for retinal cell replacement therapies conducted in animal models will be discussed.

Long-Term Efficacy of GMP Grade Xeno-Free hESC-Derived RPE Cells Following Transplantation

Purpose

Retinal pigment epithelium (RPE) dysfunction underlies the retinal degenerative process in age-related macular degeneration (AMD), and thus RPE cell replacement provides an optimal treatment target. We characterized longitudinally the efficacy of RPE cells derived under xeno-free conditions from clinical and xeno-free grade human embryonic stem cells (OpRegen) following transplantation into the subretinal space of Royal College of Surgeons (RCS) rats.

Methods

Postnatal (P) day 20 to 25 RCS rats (n = 242) received a single subretinal injection of 25,000 (low)-, 100,000 (mid)-, or 200,000 (high)-dose xeno-free RPE cells. BSS+ (balanced salt solution) (vehicle) and unoperated eyes served as controls. Optomotor tracking (OKT) behavior was used to quantify functional efficacy. Histology and immunohistochemistry were used to evaluate photoreceptor rescue and transplanted cell survival at 60, 100, 150, and 200 days of age.

Results

OKT was rescued in a dose-dependent manner. Outer nuclear layer (ONL) was significantly thicker in cell-treated eyes than controls up to P150. Transplanted RPE cells were identified in both the subretinal space and integrated into the host RPE monolayer in animals of all age groups, and often contained internalized photoreceptor outer segments. No pathology was observed.

Conclusions

OpRegen RPE cells survived, rescued visual function, preserved rod and cone photoreceptors long-term in the RCS rat. Thus, these data support the use of OpRegen RPE cells for the treatment of human RPE cell disorders including AMD.

Translational Relevance

Our novel xeno-free RPE cells minimize concerns of animal derived contaminants while providing a promising prospective therapy to the diseased retina.

Compound anisodine affects the proliferation and calcium overload of hypoxia-induced rat retinal progenitor cells and brain neural stem cells via the p-ERK1/2/HIF-1α/VEGF pathway

As a Traditional Chinese Medicine, compound anisodine (CA) has previously been shown to regulate the vegetative nervous system, improve microcirculation and scavenge reactive oxygen species, and has been commonly utilized as a neuroprotective agent to treat ischemic optic neuropathy and choroidoretinopathy. The present study aimed to investigate the neuroprotective effects of CA on the proliferation and calcium overload of hypoxia-induced rat retinal progenitor cells (RPCs) and brain neural stem cells (BNSCs) harvested from neonatal Sprague-Dawley rats. Cells were treated with CA at 0.126, 0.252, 0.505 or 1.010 g/l for four hours prior to or after hypoxia (<1% oxygen) for four h, followed by re-oxygenation for four hours; a normal control group and a CA-untreated hypoxia model group were also included. An MTT assay demonstrated that the cell viability was markedly improved following treatment with 0.126–1.010 g/l CA, compared with that in the hypoxia model group (P<0.05). Bromodeoxyuridine (BrdU) immunocytochemical staining and flow cytometry indicated that after culture in hypoxia for 4 h, the number of BrdU⁺ RPCs and BNSCs was significant decreased, as well as the cell population in S+G2 phase of the cell cycle, which was significantly attenuated by treatment with 1.010 g/l CA for 4 h prior to hypoxia (P<0.05). Furthermore, laser scanning confocal microscopy showed that the intracellular calcium concentration in hypoxia-cultured RPCs and BNSCs was markedly increased, which was attenuated by 0.126–1.010 g/l CA in a concentration-dependent manner (P<0.05). Furthermore, western blot analysis demonstrated that after hypoxia, the protein levels of hypoxia-inducible factor (HIF)-1α and vascular endothelial growth factor (VEGF) were upregulated in RPCs and BNSCs, whereas phosphorylated extracellular signal-regulated kinase (phospho-ERK 1/2^{Thr202/Tyr204}) and Cyclin D1 were downregulated; of note, treatment with 1.010 g/l CA significantly attenuated these changes (P<0.05). The results of the present study suggested that CA may improve the proliferation and inhibit calcium overload in hypoxia-induced RPCs and BNSCs by altering the protein levels of Cyclin D1 as well as signaling through the p-ERK1/2/HIF-1α/VEGF pathway.

Transplantation of Human Embryonic Stem Cell-Derived Retinal Cells into the Subretinal Space of a Non-Human Primate

PURPOSE

Previous studies have demonstrated the ability of retinal cells derived from human embryonic stem cells (hESCs) to survive, integrate into the host retina, and mediate light responses in murine mouse models. Our aim is to determine whether these cells can also survive and integrate into the retina of a nonhuman primate, Saimiri sciureus, following transplantation into the subretinal space.

METHODS

hESCs were differentiated toward retinal neuronal fates using our previously published technique and cultured for 60 to 70 days. Differentiated cells were further treated with 20 μM N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT) for a period of 5 days immediately prior to subretinal transplantation. Differentiated cells were labeled with a lentivirus expressing GFP. One million cells (10,000 cells/μL) were injected into the submacular space into a squirrel monkey eye, using an ab externo technique.

RESULTS

RetCam imaging demonstrated the presence and survival of human donor cells 3 months after transplantation in the S. sciureus eye. Injected cells consolidated in the temporal macula. GFP⁺ axonal projections were observed to emanate from the central consolidation of cells at 1 month, with some projecting into the optic nerve by 3 months after transplantation.

CONCLUSIONS

Human ES cell-derived retinal neurons injected into the submacular space of a squirrel monkey survive at least 3 months postinjection without immunosuppression. Some donor cells appeared to integrate into the host inner retina, and numerous donor axonal projections were noted throughout, with some projecting into the optic nerve.

TRANSLATIONAL RELEVANCE

These data illustrate the feasibility of hESC-derived retinal cell replacement in the nonhuman primate eye.

Cell-based therapeutic strategies for replacement and preservation in retinal degenerative diseases

Cell-based therapeutics offer diverse options for treating retinal degenerative diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). AMD is characterized by both genetic and environmental risks factors, whereas RP is mainly a monogenic disorder. Though treatments exist for some patients with neovascular AMD, a majority of retinal degenerative patients have no effective therapeutics, thus indicating a need for universal therapies to target diverse patient populations. Two main cell-based mechanistic approaches are being tested in clinical trials. Replacement therapies utilize cell-derived retinal pigment epithelial (RPE) cells to supplant lost or defective host RPE cells. These cells are similar in morphology and function to native RPE cells and can potentially supplant the responsibilities of RPE in vivo. Preservation therapies utilize supportive cells to aid in visual function and photoreceptor preservation partially by neurotrophic mechanisms. The goal of preservation strategies is to halt or slow the progression of disease and maintain remaining visual function. A number of clinical trials are testing the safety of replacement and preservation cell therapies in patients; however, measures of efficacy will need to be further evaluated. In addition, a number of prevailing concerns with regards to the immune-related response, longevity, and functionality of the grafted cells will need to be addressed in future trials. This review will summarize the current status of cell-based preclinical and clinical studies with a focus on replacement and preservation strategies and the obstacles that remain regarding these types of treatments.

Human limbal neurospheres prevent photoreceptor cell death in a rat model of retinal degeneration

BACKGROUND

The culture of retinal progenitors from an accessible adult stem cell source such as the limbus could provide a useful autologous source of retinal cell therapies. The human corneoscleral limbus contains multipotent stem cells that can be cultured as floating neurospheres. Previous work in rodents has demonstrated neuronal and photoreceptor differentiation from limbal neurosphere cultures. Here, this study has examined undifferentiated cultured adult human limbal neurospheres as donor cells for retinal cell therapies by transplantation into a rat model of retinal degeneration.

METHODS

Gene expression in limbal neurospheres was examined by immunostaining and western blot. Human limbal neurospheres were transplanted into the subretinal space of Royal College of Surgeon's rats. Rats were monitored by optical coherence tomography for 6 weeks then processed for retinal histology.

RESULTS

Human limbal neurospheres expressed the neural lineage markers, Nestin, sex determining region box-2 and N-cadherin, and the retinal transcription factors microphthalmia-associated transcription factor, sex determining region box-2 and orthodentical homeobox-2. Human limbal neurospheres could be cultured to express NeuN, neurofilament and rhodopsin. Rats receiving saline or no injection underwent complete degeneration of the retinal outer nuclear layer after 3 weeks. In contrast, rats injected with human limbal neurospheres or retinal pigment epithelial cells maintained the outer nuclear layer for up to 6 weeks. Gene expression in transplanted limbal neurospheres was inconsistent with the production of mature retinal pigment epithelial or photoreceptor cells.

CONCLUSIONS

Human limbal neurospheres represent an accessible source of autologous donor cells for the treatment of retinal diseases.

Gene expression changes in the retina following subretinal injection of human neural progenitor cells into a rodent model for retinal degeneration

PURPOSE

Retinal degenerative diseases (RDDs) affect millions of people and are the leading cause of vision loss. Although treatment options for RDDs are limited, stem and progenitor cell-based therapies have great potential to halt or slow the progression of vision loss. Our previous studies have shown that a single subretinal injection of human forebrain derived neural progenitor cells (hNPCs) into the Royal College of Surgeons (RCS) retinal degenerate rat offers long-term preservation of photoreceptors and visual function. Furthermore, neural progenitor cells are currently in clinical trials for treating age-related macular degeneration; however, the molecular mechanisms of stem cell-based therapies are largely unknown. This is the first study to analyze gene expression changes in the retina of RCS rats following subretinal injection of hNPCs using high-throughput sequencing.

METHODS

RNA-seq data of retinas from RCS rats injected with hNPCs (RCS(hNPCs)) were compared to sham surgery in RCS (RCS(sham)) and wild-type Long Evans (LE(sham)) rats. Differential gene expression patterns were determined with in silico analysis and confirmed with qRT-PCR. Function, biologic, cellular component, and pathway analyses were performed on differentially expressed genes and investigated with immunofluorescent staining experiments.

RESULTS

Analysis of the gene expression data sets identified 1,215 genes that were differentially expressed between RCS(sham) and LE(sham) samples. Additionally, 283 genes were differentially expressed between the RCS(hNPCs) and RCS(sham) samples. Comparison of these two gene sets identified 68 genes with inverse expression (termed rescue genes), including Pdc, Rp1, and Cdc42ep5. Functional, biologic, and cellular component analyses indicate that the immune response is enhanced in RCS(sham). Pathway analysis of the differential expression gene sets identified three affected pathways in RCS(hNPCs), which all play roles in phagocytosis signaling. Immunofluorescent staining detected the increased presence of macrophages and microglia in RCS(sham) retinas, which decreased in RCS(hNPCs) retinas similar to the patterns detected in LE(sham).

CONCLUSIONS

The results from this study provide evidence of the gene expression changes that occur following treatment with hNPCs in the degenerating retina. This information can be used in future studies to potentially enhance or predict responses to hNPC and other stem cell therapies for retinal degenerative diseases.

The immune response of stem cells in subretinal transplantation

Stem cell transplantation is a potential curative treatment for degenerative diseases of the retina. Among cell injection sites, the subretinal space (SRS) is particularly advantageous as it is maintained as an immune privileged site by the retinal pigment epithelium (RPE) layer. Thus, the success of subretinal transplantation depends on maintenance of RPE integrity. Moreover, both embryonic stem cells (ESCs) and mesenchymal stem cells (MSCs) have negligible immunogenicity and in fact are immunosuppressive. Indeed, many studies have demonstrated that immunosuppressive drugs are not necessary for subretinal transplantation of stem cells if the blood-retinal barrier is not breached during surgery. The immunogenicity of induced pluripotent stem cells (iPSCs) appears more complex, and requires careful study before clinical application. Despite low rates of graft rejection in animal models, survival rates for ESCs, MSCs, and iPSCs in retina are generally poor, possibly due to resident microglia activated by cell transplantation. To improve graft survival in SRS transplantation, damage to the blood-retinal barrier must be minimized using appropriate surgical techniques. In addition, agents that inhibit microglial activation may be required. Finally, immunosuppressants may be required, at least temporarily, until the blood-retinal barrier heals. We review surgical methods and drug regimens to enhance the likelihood of graft survival after SRS transplantation.

The Effect of Transient Local Anti-inflammatory Treatment on the Survival of Pig Retinal Progenitor Cell Allotransplants

PURPOSE

The development of photoreceptor replacement therapy for retinal degenerative disorders requires the identification of the optimal cell source and immunosuppressive regimen in a large animal model. Allotransplants are not acutely rejected in swine subretinal space, although it is not known if survival can be improved with immunosuppression. Here we investigated the survival and integration of expanded pig retinal progenitor cells (pRPCs) in normal recipients with and without transient anti-inflammatory suppression.

METHODS

pRPCs were derived from the neural retina of E60 GFP transgenic pigs, expanded for six passages, characterized, and transplanted into the subretinal space of 12 pigs. Six recipients received a single intravitreal injection of rapamycin and dexamethasone.

RESULTS

pRPCs expressed the photoreceptor development genes Sox2, Pax6, Lhx2, Crx, Nrl, and Recoverin in vitro. Transplanted cells were identified in 9 out of 12 recipients 4 weeks after the injection. pRPCs integrated primarily into the photoreceptor inner segment layer and outer nuclear layer with single cells present in the inner nuclear layer. Donor cells remained recoverin-positive and acquired rhodopsin. We did not observe any signs of graft proliferation. The immunosuppression did not affect the survival or distribution of grafts. No macrophage infiltration or loss of retinal structure was observed in either group.

CONCLUSIONS

Local immunosuppression with rapamycin and dexamethasone does not improve the outcome of pRPC allotransplantation into the subretinal space.

TRANSLATIONAL RELEVANCE

Survival and integration of pRPC together with the lack of graft proliferation suggests that allogeneic RPC transplantation without transient immunosuppression is a favorable approach for photoreceptor cell replacement.

Human iPSC-Derived Neural Progenitors Preserve Vision in an AMD-Like Model

Pluripotent stem cell-derived retinal pigment epithelial (RPE) cells are currently being tested for cell replacement in late-stage age-related macular degeneration (AMD). However, preserving vision at early-stages may also be possible. Here, we demonstrate that transplantation of neural progenitor cells (NPCs) derived from induced pluripotent stem cells (iNPCs) limits disease progression in the Royal College of Surgeons rat, a preclinical model of AMD. Grafted-iNPCs survived, remained undifferentiated, and distributed extensively in a laminar fashion in the subretinal space. Retinal pathology resulting from the accumulation of undigested photoreceptor outer segments (POS) was significantly reduced in iNPC-injected rats compared with controls. Phagosomes within grafted-iNPCs contained POS, suggesting that iNPCs had compensated for defective POS phagocytosis by host-RPE. The iNPC-treated eyes contained six to eight rows of photoreceptor nuclei that spanned up to 5 mm in length in transverse retinal sections, compared with only one row of photoreceptors in controls. iNPC treatment fully preserved visual acuity measured by optokinetic response. Electrophysiological recordings revealed that retina with the best iNPC-protected areas were 140-fold more sensitive to light stimulation than equivalent areas of contralateral eyes. The results described here support the therapeutic utility of iNPCs as autologous grafts for early-stage of AMD.

A Subsequent Human Neural Progenitor Transplant into the Degenerate Retina Does Not Compromise Initial Graft Survival or Therapeutic Efficacy

PURPOSE

Stem and progenitor cell transplantation provides a promising clinical application for treating degenerative retinal diseases, including age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Our previous studies have shown that a single subretinal injection of human cortical-derived neural progenitor cells (hNPCctx) into cyclosporine-treated Royal College of Surgeons (RCS) rats preserved both photoreceptors and visual function. However, it is still unknown whether nonautologous progenitor cell readministration for sustained vision is efficacious and safe in terms of the initial graft initiating an immune response to a subsequent graft.

METHODS

A cell suspension containing 3×104 hNPCctx into one eye of cyclosporine-treated RCS rats at postnatal day 21 (P21), followed by a second transplantation at P95 into the previously untreated fellow eye.

RESULTS

hNPCctx delayed photoreceptor degeneration and preserved visual function, as measured by electroretinography (ERG), optokinetic response (OKR), and luminance threshold recordings (LTRs). Visual function and photoreceptors of the initially treated eye were still preserved 6 weeks after hNPCctx were injected into the second eye. Antibodies against T-cell markers showed that CD3, CD4, and CD8 T cells were not detected at P90 and P140 in most cases. No detectable level of anti-nestin antibody was found in serum by enzyme-linked immunosorbent assay (ELISA).

CONCLUSIONS

This xenograft study with cyclosporine-treated animals demonstrates that readministration of hNPCctx into the fellow eye did not induce anti-graft immune responses or lower therapeutic efficacy of hNPCctx in preserving vision. Thus, readministration of progenitor cells to sustain long-term efficacy may be an option for long-term therapies of retinal degeneration.

TRANSLATIONAL RELEVANCE

Redosing neural progenitors do not affect the efficacy of the initial grafts in protecting vision or induce unwanted immune responses.

Functional and morphological analysis of the subretinal injection of human retinal progenitor cells under Cyclosporin A treatment

PURPOSE

The purpose of this study is to evaluate the functional and morphological changes in subretinal xenografts of human retinal progenitor cells (hRPCs) in B6 mice treated with Cyclosporin A (CsA; 210 mg/l in drinking water).

METHODS

The hRPCs from human fetal eyes were isolated and expanded for transplantation. These cells, with green fluorescent protein (GFP) at 11 passages, were transplanted into the subretinal space in B6 mice. A combination of invasive and noninvasive approaches was used to analyze the structural and functional consequences of the subretinal injection of the hRPCs. The process of change was monitored using spectral domain optical coherence tomography (SDOCT), histology, and electroretinography (ERG) at 3 days, 1 week, and 3 weeks after transplantation. Cell counts were used to evaluate the survival rate with a confocal microscope. ERGs were performed to evaluate the physiologic changes, and the structural changes were evaluated using SDOCT and histological examination.

RESULTS

The results of the histological examination showed that the hRPCs gained a better survival rate in the mice treated with CsA. The SDOCT showed that the bleb size of the retinal detachment was significantly decreased, and the retinal reattachment was nearly complete by 3 weeks. The ERG response amplitudes in the CsA group were less decreased after the injection, when compared with the control group, in the dark-adapted and light-adapted conditions. However, the cone-mediated function in both groups was less affected by the transplantation after 3 weeks than the rod-mediated function.

CONCLUSIONS

Although significant functional and structural recovery was observed after the subretinal injection of the hRPCs, the effectiveness of CsA in xenotransplantation may be a novel and potential approach for increasing retinal progenitor cell survival.

Stem cell therapy for glaucoma: science or snake oil?

In recent years there has been substantial progress in developing stem cell treatments for glaucoma. As a downstream approach that targets the underlying susceptibility of retinal ganglion and trabecular meshwork cells, stem cell therapy has the potential to both replace lost, and protect damaged, cells by secreting neurotrophic factors. A variety of sources, including embryonic cells, adult cells derived from the central nervous system, and induced pluripotent stem cells show promise as therapeutic approaches. Even though safety concerns and ethical controversies have limited clinical implementation, some institutions have already commercialized stem cell therapy and are using direct-to-consumer advertising to attract patients with glaucoma. We review the progress of stem cell therapy and its current commercial availability.

Multipotent adult hippocampal progenitor cells maintained as neurospheres favor differentiation toward glial lineages

Adult hippocampal progenitor cells (AHPCs) are generally maintained as a dispersed monolayer population of multipotent neural progenitors. To better understand cell-cell interactions among neural progenitors and their influences on cellular characteristics, we generated free-floating cellular aggregates, or neurospheres, from the adherent monolayer population of AHPCs. Results from in vitro analyses demonstrated that both populations of AHPCs were highly proliferative under maintenance conditions, but AHPCs formed in neurospheres favored differentiation along a glial lineage and displayed greater migrational activity than the traditionally cultured AHPCs. To study the plasticity of AHPCs from both populations in vivo, we transplanted green fluorescent protein (GFP)-expressing AHPCs via intraocular injection into the developing rat eyes. Both AHPC populations were capable of surviving and integrating into developing host central nervous system, but considerably more GFP-positive cells were observed in the retinas transplanted with neurosphere AHPCs, compared to adherent AHPCs. These results suggest that the culture configuration during maintenance for neural progenitor cells (NPCs) influences cell fate and motility in vitro as well as in vivo. Our findings have implication for understanding different cellular characteristics of NPCs according to distinct intercellular architectures and for developing cell-based therapeutic strategies using lineage-committed NPCs.

Human pluripotent stem cells: applications and challenges in neurological diseases

The ability to generate human pluripotent stem cells (hPSCs) holds great promise for the understanding and the treatment of human neurological diseases in modern medicine. The hPSCs are considered for their in vitro use as research tools to provide relevant cellular model for human diseases, drug discovery, and toxicity assays and for their in vivo use in regenerative medicine applications. In this review, we highlight recent progress, promises, and challenges of hPSC applications in human neurological disease modeling and therapies.

Long-term survival and differentiation of retinal neurons derived from human embryonic stem cell lines in un-immunosuppressed mouse retina

PURPOSE

To examine the potential of NIH-maintained human embryonic stem cell (hESC) lines TE03 and UC06 to differentiate into retinal progenitor cells (hESC-RPCs) using the noggin/Dkk-1/IGF-1/FGF9 protocol. An additional goal is to examine the in vivo dynamics of maturation and retinal integration of subretinal and epiretinal (vitreous space) hESC-RPC grafts without immunosuppression.

METHODS

hESCs were neuralized in vitro with noggin for 2 weeks and expanded to derive neuroepithelial cells (hESC-neural precursors, NPs). Wnt (Integration 1 and wingless) blocking morphogens Dickkopf-1 (Dkk-1) and Insulin-like growth factor 1 (IGF-1) were used to direct NPs to a rostral neural fate, and fibroblast growth factor 9 (FGF9)/fibroblast growth factor-basic (bFGF) were added to bias the differentiation of developing anterior neuroectoderm cells to neural retina (NR) rather than retinal pigment epithelium (RPE). Cells were dissociated and grafted into the subretinal and epiretinal space of young adult (4-6-week-old) mice (C57BL/6J x129/Sv mixed background). Remaining cells were replated for (i) immunocytochemical analysis and (ii) used for quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis. Mice were sacrificed 3 weeks or 3 months after grafting, and the grafts were examined by histology and immunohistochemistry for survival of hESC-RPCs, presence of mature neuronal and retinal markers, and the dynamics of in vivo maturation and integration into the host retina.

RESULTS

At the time of grafting, hESC-RPCs exhibited immature neural/neuronal immunophenotypes represented by nestin and neuronal class III β-tubulin, with about half of the cells positive for cell proliferation marker Kiel University -raised antibody number 67 (Ki67), and no recoverin-positive (recoverin [+]) cells. The grafted cells expressed eye field markers paired box 6 (PAX6), retina and anterior neural fold homeobox (RAX), sine oculis homeobox homolog 6 (SIX6), LIM homeobox 2 (LHX2), early NR markers (Ceh-10 homeodomain containing homolog [CHX10], achaete-scute complex homolog 1 [MASH1], mouse atonal homolog 5 [MATH5], neurogenic differentiation 1 [NEUROD1]), and some retinal cell fate markers (brain-specific homeobox/POU domain transcription factor 3B [BRN3B], prospero homeobox 1 [PROX1], and recoverin). The cells in the subretinal grafts matured to predominantly recoverin [+] phenotype by 3 months and survived in a xenogenic environment without immunosuppression as long as the blood-retinal barrier was not breached by the transplantation procedure. The epiretinal grafts survived but did not express markers of mature retinal cells. Retinal integration into the retinal ganglion cell (RGC) layer and the inner nuclear layer (INL) was efficient from the epiretinal but not subretinal grafts. The subretinal grafts showed limited ability to structurally integrate into the host retina and only in cases when NR was damaged during grafting. Only limited synaptogenesis and no tumorigenicity was observed in grafts.

CONCLUSIONS

Our studies show that (i) immunosuppression is not mandatory to xenogenic graft survival in the retina, (ii) the subretinal but not the epiretinal niche can promote maturation of hESC-RPCs to photoreceptors, and (iii) the hESC-RPCs from epiretinal but not subretinal grafts can efficiently integrate into the RGC layer and INL. The latter could be of value for long-lasting neuroprotection of retina in some degenerative conditions and glaucoma. Overall, our results provide new insights into the technical aspects associated with cell-based therapy in the retina.

Stem cells for retinal replacement therapy

Retinal degenerative disease has limited therapeutic options and the possibility of stem cell-mediated regenerative treatments is being actively explored for these blinding retinal conditions. The relative accessibility of this central nervous system tissue and the ability to visually monitor changes after transplantation make the retina and adjacent retinal pigment epithelium prime targets for pioneering stem cell therapeutics. Prior work conducted for several decades indicated the promise of cell transplantation for retinal disease, and new strategies that combine these established surgical approaches with stem cell-derived donor cells is ongoing. A variety of tissue-specific and pluripotent-derived donor cells are being advanced to replace lost or damaged retinal cells and/or to slow the disease processes by providing neuroprotective factors, with the ultimate aim of long-term improvement in visual function. Clinical trials are in the early stages, and data on safety and efficacy are widely anticipated. Positive outcomes from these stem cell-based clinical studies would radically change the way that blinding disorders are approached in the clinic.

Induced pluripotent stem cell therapies for geographic atrophy of age-related macular degeneration

There is currently no FDA-approved therapy for treating patients with geographic atrophy (GA), a late stage of age-related macular degeneration (AMD). Cell transplantation has the potential to restore vision in these patients. This review discusses how recent advancement in induced pluripotent stem (iPS) cells provides a promising therapy for GA treatment. Recent advances in stem cell biology have demonstrated that it is possible to derive iPS cells from human somatic cells by introducing reprogramming factors. Human retinal pigment epithelium (RPE) cells and photoreceptors can be derived from iPS cells by defined factors. Studies show that transplanting these cells can stabilize or recover vision in animal models. However, cell derivation protocols and transplantation procedures still need to be optimized. Much validation has to be done before clinical-grade, patient-derived iPS can be applied for human therapy. For now, RPE cells and photoreceptors derived from patient-specific iPS cells can serve as a valuable tool in elucidating the mechanism of pathogenesis and drug discovery for GA.

Xenotransplantation of human neural progenitor cells to the subretinal space of nonimmunosuppressed pigs

To investigate the feasibility of transplanting human neural progenitor cells (hNPCs) to the retina of nonimmunosuppressed pigs, cultured hNPCs were injected into the subretinal space of 5 adult pigs after laser burns were applied to promote donor cell integration. Postoperatively, the retinal vessels appeared normal without signs of exudation, bleeding, or subretinal elevation. Eyes were harvested at 10–28 days. H&E consistently showed mild retinal vasculitis, depigmentation of the RPE, and marked mononuclear cell infiltrate in the choroid adjacent to the site of transplantation. Human-specific antibodies revealed donor cells in the subretinal space at 10–13 days and smaller numbers within the retina on days 12 and 13, with evidence suggesting a limited degree of morphological integration; however, no cells remained at 4 weeks. The strong mononuclear cell reaction and loss of donor cells indicate that modulation of host immunity is likely necessary for prolonged xenograft survival in this model.

Transplantation of photoreceptor and total neural retina preserves cone function in P23H rhodopsin transgenic rat

Background

Transplantation as a therapeutic strategy for inherited retinal degeneration has been historically viewed to restore vision as a method by replacing the lost retinal cells and attempting to reconstruct the neural circuitry with stem cells, progenitor cells and mature neural retinal cells.

Methods and Findings

We present evidence for an alternative strategy aimed at preventing the secondary loss of cones, the most crucial photoreceptors for vision, by transplanting normal photoreceptors cells into the eye of the P23H rat, a model of dominant retinitis pigmentosa. We carried out transplantation of photoreceptors or total neural retina in 3-month-old P23H rats and evaluated the function and cell counts 6 months after surgery. In both groups, cone loss was significantly reduced (10%) in the transplanted eyes where the cone outer segments were found to be considerably longer. This morphological effect correlated with maintenance of the visual function of cones as scored by photopic ERG recording, but more precisely with an increase in the photopic b-wave amplitudes by 100% and 78% for photoreceptor transplantation and whole retinal transplantation respectively.

Conclusions

We demonstrate here that the transplanted tissue prevents the loss of cone function, which is further translated into cone survival.

Efficient transduction of feline neural progenitor cells for delivery of glial cell line-derived neurotrophic factor using a feline immunodeficiency virus-based lentiviral construct

Work has shown that stem cell transplantation can rescue or replace neurons in models of retinal degenerative disease. Neural progenitor cells (NPCs) modified to overexpress neurotrophic factors are one means of providing sustained delivery of therapeutic gene products in vivo. To develop a nonrodent animal model of this therapeutic strategy, we previously derived NPCs from the fetal cat brain (cNPCs). Here we use bicistronic feline lentiviral vectors to transduce cNPCs with glial cell-derived neurotrophic factor (GDNF) together with a GFP reporter gene. Transduction efficacy is assessed, together with transgene expression level and stability during induction of cellular differentiation, together with the influence of GDNF transduction on growth and gene expression profile. We show that GDNF overexpressing cNPCs expand in vitro, coexpress GFP, and secrete high levels of GDNF protein—before and after differentiation—all qualities advantageous for use as a cell-based approach in feline models of neural degenerative disease.

Protective effects of human iPS-derived retinal pigment epithelium cell transplantation in the retinal dystrophic rat

Transformation of somatic cells with a set of embryonic transcription factors produces cells with the pluripotent properties of embryonic stem cells (ESCs). These induced pluripotent stem (iPS) cells have the potential to differentiate into any cell type, making them a potential source from which to produce cells as a therapeutic platform for the treatment of a wide range of diseases. In many forms of human retinal disease, including age-related macular degeneration (AMD), the underlying pathogenesis resides within the support cells of the retina, the retinal pigment epithelium (RPE). As a monolayer of cells critical to photoreceptor function and survival, the RPE is an ideally accessible target for cellular therapy. Here we report the differentiation of human iPS cells into RPE. We found that differentiated iPS-RPE cells were morphologically similar to, and expressed numerous markers of developing and mature RPE cells. iPS-RPE are capable of phagocytosing photoreceptor material, in vitro and in vivo following transplantation into the Royal College of Surgeons (RCS) dystrophic rat. Our results demonstrate that iPS cells can be differentiated into functional iPS-RPE and that transplantation of these cells can facilitate the short-term maintenance of photoreceptors through phagocytosis of photoreceptor outer segments. Long-term visual function is maintained in this model of retinal disease even though the xenografted cells are eventually lost, suggesting a secondary protective host cellular response. These findings have identified an alternative source of replacement tissue for use in human retinal cellular therapies, and provide a new in vitro cellular model system in which to study RPE diseases affecting human patients.