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

Clinically compliant cryopreservation of differentiated retinal pigment epithelial cells

BACKGROUND AIMS

Age-related macular degeneration (AMD) is the most common cause of blindness in elderly patients within developed countries, affecting more than 190 million worldwide. In AMD, the retinal pigment epithelial (RPE) cell layer progressively degenerates, resulting in subsequent loss of photoreceptors and ultimately vision. There is currently no cure for AMD, but therapeutic strategies targeting the complement system are being developed to slow the progression of the disease.

METHODS

Replacement therapy with pluripotent stem cell-derived (hPSC) RPEs is an alternative treatment strategy. A cell therapy product must be produced in accordance with Good Manufacturing Practices at a sufficient scale to facilitate extensive pre-clinical and clinical testing. Cryopreservation of the final cell product is therefore highly beneficial, as the manufacturing, pre-clinical and clinical testing can be separated in time and location.

RESULTS

We found that mature hPSC-RPE cells do not survive conventional cryopreservation techniques. However, replating the cells 2-5 days before cryopreservation facilitates freezing. The replated and cryopreserved hPSC-RPE cells maintained their identity, purity and functionality as characteristic RPEs, shown by cobblestone morphology, pigmentation, transcriptional profile, RPE markers, transepithelial resistance and pigment epithelium-derived factor secretion. Finally, we showed that the optimal replating time window can be tracked noninvasively by following the change in cobblestone morphology.

CONCLUSIONS

The possibility of cryopreserving the hPSC-RPE product has been instrumental in our efforts in manufacturing and performing pre-clinical testing with the aim for clinical translation.

Comparison of retinal degeneration treatment with four types of different mesenchymal stem cells, human induced pluripotent stem cells and RPE cells in a rat retinal degeneration model

BACKGROUND

Retinal degeneration (RD) is a group of disorders on irreversible vision loss. Multiple types of stem cells were used in clinical trials for RD treatment. However, it remains unknown what kinds of stem cells are most effective for the treatment. Therefore, we investigated the subretinal transplantation of several types of stem cells, human adipose-derived stem cells (hADSCs), amniotic fluid stem cells (hAFSCs), bone marrow stem cells (hBMSCs), dental pulp stem cells (hDPSCs), induced pluripotent stem cell (hiPSC), and hiPSC-derived retinal pigment epithelium (RPE) cells for protection effects, paracrine effects and treatment efficiency in an RD disease model rats.

METHODS

The generation and characterization of these stem cells and hiPSC-derived RPE cells were performed before transplantation. The stem cells or hiPSC-derived RPE cell suspension labelled with CellTracker Green to detect transplanted cells were delivered into the subretinal space of 3-week-old RCS rats. The control group received subretinal PBS injection or non-injection. A series of detections including fundus photography, optomotor response (OMR) evaluations, light-dark box testing, electroretinography (ERG), and hematoxylin and eosin (HE) staining of retinal sections were conducted after subretinal injection of the cells.

RESULTS

Each stem cell, hiPSC-derived RPE cell or PBS (blank experiment) was successfully transplanted into at least six RCS rats subretinally. Compared with the control rats, RCS rats subjected to subretinal transplantation of any stem cells except hiPSCs showed higher ERG waves (p < 0.05) and quantitative OMR (qOMR) index values (hADSCs: 1.166, hAFSCs: 1.249, hBMSCs: 1.098, hDPSCs: 1.238, hiPSCs: 1.208, hiPSC-RPE cells: 1.294, non-injection: 1.03, PBS: 1.06), which indicated better visual function, at 4 weeks post-injection. However, only rats that received hiPSC-derived RPE cells maintained their visual function at 8 weeks post-injection (p < 0.05). The outer nuclear layer thickness observed in histological sections after HE staining showed the same pattern as the ERG and qOMR results.

CONCLUSIONS

Compared to hiPSC-derived RPE cells, adult and fetal stem cells yielded improvements in visual function for up to 4 weeks post-injection; this outcome was mainly based on the paracrine effects of several types of growth factors secreted by the stem cells. Patients with RD will benefit from the stem cell therapy.

A treatment within sight: challenges in the development of stem cell-derived photoreceptor therapies for retinal degenerative diseases

Stem cell therapies can potentially treat various retinal degenerative diseases, including age-related macular degeneration (AMD) and inherited retinal diseases like retinitis pigmentosa. For these diseases, transplanted cells may include stem cell-derived retinal pigmented epithelial (RPE) cells, photoreceptors, or a combination of both. Although stem cell-derived RPE cells have progressed to human clinical trials, therapies using photoreceptors and other retinal cell types are lagging. In this review, we discuss the potential use of human pluripotent stem cell (hPSC)-derived photoreceptors for the treatment of retinal degeneration and highlight the progress and challenges for their efficient production and clinical application in regenerative medicine.

hPSC-derived RPE transplantation for the treatment of macular degeneration

Macular degeneration (MD) is a group of diseases characterized by irreversible and progressive vision loss. Patients with MD suffer from severely impaired central vision, especially elderly people. Currently, only one type of MD, wet age-related macular degeneration (AMD), can be treated with anti-vascular endothelium growth factor (VEGF) drugs. Other types of MD remain difficult to treat. With the advent of human pluripotent stem cells (hPSCs) and their differentiation into retinal pigmented epithelium (RPE), it is promising to treat patients with MD by transplantation of hPSC-derived RPE into the subretinal space. In this review, the current progress in hPSC-derived RPE transplantation for the treatment of patients with MD is described from bench to bedside, including hPSC differentiation into RPE and the characterization and usage of hPSC-derived RPE for transplantation into patients with MD.

Artificial intelligence method based on multi-feature fusion for automatic macular edema (ME) classification on spectral-domain optical coherence tomography (SD-OCT) images

Purpose

A common ocular manifestation, macular edema (ME) is the primary cause of visual deterioration. In this study, an artificial intelligence method based on multi-feature fusion was introduced to enable automatic ME classification on spectral-domain optical coherence tomography (SD-OCT) images, to provide a convenient method of clinical diagnosis.

Methods

First, 1,213 two-dimensional (2D) cross-sectional OCT images of ME were collected from the Jiangxi Provincial People's Hospital between 2016 and 2021. According to OCT reports of senior ophthalmologists, there were 300 images with diabetic (DME), 303 images with age-related macular degeneration (AMD), 304 images with retinal-vein occlusion (RVO), and 306 images with central serous chorioretinopathy (CSC). Then, traditional omics features of the images were extracted based on the first-order statistics, shape, size, and texture. After extraction by the alexnet, inception_v3, resnet34, and vgg13 models and selected by dimensionality reduction using principal components analysis (PCA), the deep-learning features were fused. Next, the gradient-weighted class-activation map (Grad-CAM) was used to visualize the-deep-learning process. Finally, the fusion features set, which was fused from the traditional omics features and the deep-fusion features, was used to establish the final classification models. The performance of the final models was evaluated by accuracy, confusion matrix, and the receiver operating characteristic (ROC) curve.

Results

Compared with other classification models, the performance of the support vector machine (SVM) model was best, with an accuracy of 93.8%. The area under curves AUC of micro- and macro-averages were 99%, and the AUC of the AMD, DME, RVO, and CSC groups were 100, 99, 98, and 100%, respectively.

Conclusion

The artificial intelligence model in this study could be used to classify DME, AME, RVO, and CSC accurately from SD-OCT images.

100 plus years of stem cell research-20 years of ISSCR

The International Society for Stem Cell Research (ISSCR) celebrates its 20^th anniversary in 2022. This review looks back at some of the key developments in stem cell research as well as the evolution of the ISSCR as part of that field. Important discoveries from stem cell research are described, and how the improved understanding of basic stem cell biology translates into new clinical therapies and insights into disease mechanisms is discussed. Finally, the birth and growth of ISSCR into a leading stem cell society and a respected voice for ethics, advocacy, education and policy in stem cell research are described.

Pluripotent stem cell therapy for retinal diseases

Pluripotent stem cells (PSCs), which include human embryonic stem cells (hESCs) and induced pluripotent stem cell (iPSC), have been used to study development of disease processes, and as potential therapies in multiple organ systems. In recent years, there has been increasing interest in the use of PSC-based transplantation to treat disorders of the retina in which retinal cells have been functionally damaged or lost through degeneration. The retina, which consists of neuronal tissue, provides an excellent system to test the therapeutic utility of PSC-based transplantation due to its accessibility and the availability of high-resolution imaging technology to evaluate effects. Preclinical trials in animal models of retinal diseases have shown improvement in visual outcomes following subretinal transplantation of PSC-derived photoreceptors or retinal pigment epithelium (RPE) cells. This review focuses on preclinical studies and clinical trials exploring the use of PSCs for retinal diseases. To date, several phase I/II clinical trials in patients with age-related macular degeneration (AMD) and Stargardt disease (STGD1) have demonstrated the safety and feasibility of PSC-derived RPE transplantation. Additional phase I/II clinical trials using PSC-derived RPE or photoreceptor cells for the treatment of AMD, STGD1, and also retinitis pigmentosa (RP) are currently in the pipeline. As this field continues to evolve, additional technologies may enhance PSC-derived cell transplantation through gene-editing of autologous cells, transplantation of more complex cellular structures such as organoids, and monitoring of transplanted cells through novel imaging technologies.

Improvement of human embryonic stem cell-derived retinal pigment epithelium cell adhesion, maturation, and function through coating with truncated recombinant human vitronectin

AIM

To explore an xeno-free and defined coating substrate suitable for the culture of H9 human embryonic stem cell-derived retinal pigment epithelial (hES-RPE) cells in vitro, and compare the behaviors and functions of hES-RPE cells on two culture substrates, laminin521 (LN-521) and truncated recombinant human vitronectin (VTN-N).

METHODS

hES-RPE cells were used in the experiment. The abilities of LN-521 and VTN-N at different concentrations to adhere to hES-RPE cells were compared with a high-content imaging system. Quantitative real-time polymerase chain reaction was used to evaluate RPE-specific gene expression levels midway (day 10) and at the end (day 20) of the time course. Cell polarity was observed by immunofluorescent staining for apical and basal markers of the RPE. The phagocytic ability of hES-RPE cells was identified by flow cytometry and immunofluorescence.

RESULTS

The cell adhesion assay showed that the ability of LN-521 to adhere to hES-RPE cells was dose-dependent. With increasing coating concentration, an increasing number of cells attached to the surface of LN-521-coated wells. In contrast, VTN-N presented a strong adhesive ability even at a low concentration. The optimal concentration of LN-521 and VTN-N required to coat and adhesion to hES-RPE cells were 2 and 0.25 µg/cm², respectively. Furthermore, both LN-521 and VTN-N could facilitate adoption of the desired cobblestone cellular morphology with tight junction and showed polarity by the hES-RPE cells. However, hES-RPE cells cultivated in VTN-N had a greater phagocytic ability, and it took less time for these hES-RPE cells to mature.

CONCLUSION

VTN-N is a more suitable coating substrate for cultivating hES-RPE cells.

Pluripotent Stem Cell-Based Organoid Technologies for Developing Next-Generation Vision Restoration Therapies of Blindness

Blindness, associated with death of retinal cells at the back of the eye, is caused by a number of conditions with high prevalence such as glaucoma, age-related macular degeneration, and diabetic retinopathy. In addition, a large number of orphan inherited (mostly monogenic) conditions, such as retinitis pigmentosa and Leber Congenital Amaurosis, add to the overall number of patients with blinding retinal degenerative diseases. Blindness caused by deterioration and loss of retina is so far incurable. Modern biomedical research leveraging molecular and regenerative medicine approaches had a number of groundbreaking discoveries and proof-of-principle treatments of blindness in animals. However, these methods are slow to be standardized and commercialized as therapies to benefit people losing their eyesight due to retinal degenerative conditions. In this review, we will outline major regenerative medicine approaches, which are emerging as promising for preserving or/and restoring vision. We discuss the potential of each of these approaches to reach commercialization step and be converted to treatments, which could at least ameliorate blindness caused by retinal cell death.

Xeno-free cryopreservation of adherent retinal pigmented epithelium yields viable and functional cells in vitro and in vivo

Age-related macular degeneration (AMD) is the primary cause of blindness in adults over 60 years of age, and clinical trials are currently assessing the therapeutic potential of retinal pigmented epithelial (RPE) cell monolayers on implantable scaffolds to treat this disease. However, challenges related to the culture, long-term storage, and long-distance transport of such implants currently limit the widespread use of adherent RPE cells as therapeutics. Here we report a xeno-free protocol to cryopreserve a confluent monolayer of clinical-grade, human embryonic stem cell-derived RPE cells on a parylene scaffold (REPS) that yields viable, polarized, and functional RPE cells post-thaw. Thawed cells exhibit ≥ 95% viability, have morphology, pigmentation, and gene expression characteristic of mature RPE cells, and secrete the neuroprotective protein, pigment epithelium-derived factor (PEDF). Stability under liquid nitrogen (LN₂) storage has been confirmed through one year. REPS were administered immediately post-thaw into the subretinal space of a mammalian model, the Royal College of Surgeons (RCS)/nude rat. Implanted REPS were assessed at 30, 60, and 90 days post-implantation, and thawed cells demonstrate survival as an intact monolayer on the parylene scaffold. Furthermore, immunoreactivity for the maturation marker, RPE65, significantly increased over the post-implantation period in vivo, and cells demonstrated functional attributes similar to non-cryopreserved controls. The capacity to cryopreserve adherent cellular therapeutics permits extended storage and stable transport to surgical sites, enabling broad distribution for the treatment of prevalent diseases such as AMD.

Transplantation of retinal pigment epithelium and photoreceptors generated concomitantly via small molecule-mediated differentiation rescues visual function in rodent models of retinal degeneration

Background

Age-related macular degeneration (AMD) is a result of degeneration/damage of the retinal pigment epithelium (RPE) while retinitis pigmentosa (RP), an inherited early-onset disease, results from premature loss of photoreceptors. A promising therapeutic approach for both is the replacement of lost/damaged cells with human induced pluripotent stem cell (hiPSC)-derived retinal cells.

Methods

The aim of this study was to investigate the in vivo functionality of RPE and photoreceptor progenitor (PRP) cells derived from a clinical-grade hiPSC line through a unified protocol. De novo-generated RPE and PRP were characterized extensively to validate their identity, purity, and potency.

Results

RPE expressed tight junction proteins, showed pigmentation and ciliation, and secreted polarization-related factors vascular endothelial growth factor (VEGF) and pigment epithelium-derived factor (PEDF). PRP expressed neural retina proteins and cone and rod markers, and responded to KCl-induced polarization. Transcriptomic analysis demonstrated an increase in the expression of mature retinal tissue-specific genes coupled with concomitant downregulation of genes from undesired lineages. RPE transplantation rescued visual function in RCS rats shown via optokinetic tracking and photoreceptor rescue. PRP transplantation improved light perception in NOD.SCID-rd1 mice, and positive electroretinography signals indicated functional photoreceptor activity in the host’s outer nuclear layer. Graft survival and integration were confirmed using immunohistochemistry, and no animals showed teratoma formation or any kind of ectopic growth in the eye.

Conclusions

To our knowledge, this is the first demonstration of a unified, scalable, and GMP-adaptable protocol indicating strong animal efficacy and safety data with hiPSC-derived RPE and PRP cells. These findings provide robust proof-of-principle results for IND-enabling studies to test these potential regenerative cell therapies in patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02134-x.

Dorsal-Ventral Differences in Retinal Structure in the Pigmented Royal College of Surgeons Model of Retinal Degeneration

Retinitis pigmentosa is a family of inherited retinal degenerations associated with gradual loss of photoreceptors, that ultimately leads to irreversible vision loss. The Royal College of Surgeon's (RCS) rat carries a recessive mutation affecting mer proto-oncogene tyrosine kinase (merTK), that models autosomal recessive disease. The aim of this study was to understand the glial, microglial, and photoreceptor changes that occur in different retinal locations with advancing disease. Pigmented RCS rats (RCS-p^+/LAV) and age-matched isogenic control rdy (RCS-rdy ⁺p⁺/LAV) rats aged postnatal day 18 to 6 months were evaluated for in vivo retinal structure and function using optical coherence tomography and electroretinography. Retinal tissues were assessed using high resolution immunohistochemistry to evaluate changes in photoreceptors, glia and microglia in the dorsal, and ventral retina. Photoreceptor dysfunction and death occurred from 1 month of age. There was a striking difference in loss of photoreceptors between the dorsal and ventral retina, with a greater number of photoreceptors surviving in the dorsal retina, despite being adjacent a layer of photoreceptor debris within the subretinal space. Loss of photoreceptors in the ventral retina was associated with fragmentation of the outer limiting membrane, extension of glial processes into the subretinal space that was accompanied by possible adhesion and migration of mononuclear phagocytes in the subretinal space. Overall, these findings highlight that breakdown of the outer limiting membrane could play an important role in exacerbating photoreceptor loss in the ventral retina. Our results also highlight the value of using the RCS rat to model sectorial retinitis pigmentosa, a disease known to predominantly effect the inferior retina.

Cell transplantation to replace retinal ganglion cells faces challenges - the Switchboard Dilemma

The mammalian retina displays incomplete intrinsic regenerative capacities; therefore, retina degeneration is a major cause of irreversible blindness such as glaucoma, age-related macular degeneration and diabetic retinopathy. These diseases lead to the loss of retinal cells and serious vision loss in the late stage. Stem cell transplantation is a great promising novel treatment for these incurable retinal degenerative diseases and represents an exciting area of regenerative neurotherapy. Several suitable stem cell sources for transplantation including human embryonic stem cells, induced pluripotent stem cells and adult stem cells have been identified as promising target populations. However, the retina is an elegant neuronal complex composed of various types of cells with different functions. The replacement of these different types of cells by transplantation should be addressed separately. So far, retinal pigment epithelium transplantation has achieved the most advanced stage of clinical trials, while transplantation of retinal neurons such as retinal ganglion cells and photoreceptors has been mostly studied in pre-clinical animal models. In this review, we opine on the key problems that need to be addressed before stem cells transplantation, especially for replacing injured retinal ganglion cells, may be used practically for treatment. A key problem we have called the Switchboard Dilemma is a major block to have functional retinal ganglion cell replacement. We use the public switchboard telephone network as an example to illustrate different difficulties for replacing damaged components in the retina that allow for visual signaling. Retinal ganglion cell transplantation is confronted by significant hurdles, because retinal ganglion cells receive signals from different interneurons, integrate and send signals to the correct targets of the visual system, which functions similar to the switchboard in a telephone network – therefore the Switchboard Dilemma.

Retinal Pigment Epithelium Replacement Therapy for Age-Related Macular Degeneration: Are We There Yet?

Pluripotent stem cells (PSCs) are a potential replacement tissue source for degenerative diseases. Age-related macular degeneration (AMD) is a blinding disease triggered by degeneration of the retinal pigment epithelium (RPE), a monolayer tissue that functionally supports retinal photoreceptors. Recently published clinical and preclinical studies have tested PSC-derived RPE as a potential treatment for AMD. Multiple approaches have been used to manufacture RPE cells, to validate them functionally, to confirm their safety profile, and to deliver them to patients either as suspension or as a monolayer patch. Since most of these studies are at an early regulatory approval stage, the primary outcome has been to determine the safety of RPE transplants in patients. However, preliminary signs of efficacy were observed in a few patients. Here, we review the current progress in the PSC-derived RPE transplantation field and provide a comparative assessment of various approaches under development as potential therapeutics for AMD.

Implantable anti-angiogenic scaffolds for treatment of neovascular ocular pathologies

The retinal physiology can accrue oxidative damage and inflammatory insults due to age and metabolic irregularities. Two notable diseases that involve retinal and choroidal neovascularization are proliferative diabetic retinopathy and wet age-related macular degeneration. Currently, these diseases are mainly treated with anti-VEGF drugs (VEGF = vascular endothelial growth factor), generally on a monthly dosage scheme. We discuss recent developments for the treatment of these diseases, including bioactive tissue-engineered materials, which may reduce frequency of dosage and propose a path forward for improving patient outcomes. Graphical abstract Development of materials for long-term intravitreal delivery for management of posterior segment diseases.

Limitations and Promise of Retinal Tissue From Human Pluripotent Stem Cells for Developing Therapies of Blindness

The self-formation of retinal tissue from pluripotent stem cells generated a tremendous promise for developing new therapies of retinal degenerative diseases, which previously seemed unattainable. Together with use of induced pluripotent stem cells or/and CRISPR-based recombineering the retinal organoid technology provided an avenue for developing models of human retinal degenerative diseases "in a dish" for studying the pathology, delineating the mechanisms and also establishing a platform for large-scale drug screening. At the same time, retinal organoids, highly resembling developing human fetal retinal tissue, are viewed as source of multipotential retinal progenitors, young photoreceptors and just the whole retinal tissue, which may be transplanted into the subretinal space with a goal of replacing patient's degenerated retina with a new retinal "patch." Both approaches (transplantation and modeling/drug screening) were projected when Yoshiki Sasai demonstrated the feasibility of deriving mammalian retinal tissue from pluripotent stem cells, and generated a lot of excitement. With further work and testing of both approaches in vitro and in vivo, a major implicit limitation has become apparent pretty quickly: the absence of the uniform layer of Retinal Pigment Epithelium (RPE) cells, which is normally present in mammalian retina, surrounds photoreceptor layer and develops and matures first. The RPE layer polarize into apical and basal sides during development and establish microvilli on the apical side, interacting with photoreceptors, nurturing photoreceptor outer segments and participating in the visual cycle by recycling 11-trans retinal (bleached pigment) back to 11-cis retinal. Retinal organoids, however, either do not have RPE layer or carry patches of RPE mostly on one side, thus directly exposing most photoreceptors in the developing organoids to neural medium. Recreation of the critical retinal niche between the apical RPE and photoreceptors, where many retinal disease mechanisms originate, is so far unattainable, imposes clear limitations on both modeling/drug screening and transplantation approaches and is a focus of investigation in many labs. Here we dissect different retinal degenerative diseases and analyze how and where retinal organoid technology can contribute the most to developing therapies even with a current limitation and absence of long and functional outer segments, supported by RPE.

Pluripotent Stem Cells for the Treatment of Retinal Degeneration: Current Strategies and Future Directions

Stem cells have been part of the biomedical landscape since the early 1960s. However, the translation of stem cells to effective therapeutics have met significant challenges, especially for retinal diseases. The retina is a delicate and complex architecture of interconnected cells that are steadfastly interdependent. Degenerative mechanisms caused by acquired or inherited diseases disrupt this interconnectivity, devastating the retina and causing severe vision loss in many patients. Consequently, retinal differentiation of exogenous and endogenous stem cells is currently being explored as replacement therapies in the debilitating diseases. In this review, we will examine the mechanisms involved in exogenous stem cells differentiation and the challenges of effective integration to the host retina. Furthermore, we will explore the current advancements in trans-differentiation of endogenous stem cells, primarily Müller glia.

Recent developments in regenerative ophthalmology

Regenerative medicine (RM) is one of the most promising disciplines for advancements in modern medicine, and regenerative ophthalmology (RO) is one of the most active fields of regenerative medicine. This review aims to provide an overview of regenerative ophthalmology, including the range of tools and materials being used, and to describe its application in ophthalmologic subspecialties, with the exception of surgical implantation of artificial tissues or organs (e.g., contact lens, artificial cornea, intraocular lens, artificial retina, and bionic eyes) due to space limitations. In addition, current challenges and limitations of regenerative ophthalmology are discussed and future directions are highlighted.

Preclinical safety studies of human embryonic stem cell-derived retinal pigment epithelial cells for the treatment of age-related macular degeneration

As pluripotent stem cell (PSC)-based reparative cell therapies are reaching the bedside, there is a growing need for the standardization of studies concerning safety of the derived products. Clinical trials using these promising strategies are in development, and treatment for age-related macular degeneration is one of the first that has reached patients. We have previously established a xeno-free and defined differentiation protocol to generate functional human embryonic stem cells (hESCs)-derived retinal pigment epithelial (RPE) cells. In this study, we perform preclinical safety studies including karyotype and whole-genome sequencing (WGS) to assess genome stability, single-cell RNA sequencing to ensure cell purity, and biodistribution and tumorigenicity analysis to rule out potential migratory or tumorigenic properties of these cells. WGS analysis illustrates that existing germline variants load is higher than the introduced variants acquired through in vitro culture or differentiation, and enforces the importance to examine the genome integrity at a deeper level than just karyotype. Altogether, we provide a strategy for preclinical evaluation of PSC-based therapies and the data support safety of the hESC-RPE cells generated through our in vitro differentiation methodology.

A Systematic Review on Transplantation Studies of the Retinal Pigment Epithelium in Animal Models

The retinal pigment epithelium (RPE) and the adjacent light-sensitive photoreceptors form a single functional unit lining the back of the eye. Both cell layers are essential for normal vision. RPE degeneration is usually followed by photoreceptor degeneration and vice versa. There are currently almost no effective therapies available for RPE disorders such as Stargardt disease, specific types of retinitis pigmentosa, and age-related macular degeneration. RPE replacement for these disorders, especially in later stages of the disease, may be one of the most promising future therapies. There is, however, no consensus regarding the optimal RPE source, delivery strategy, or the optimal experimental host in which to test RPE replacement therapy. Multiple RPE sources, delivery methods, and recipient animal models have been investigated, with variable results. So far, a systematic evaluation of the (variables influencing) efficacy of experimental RPE replacement parameters is lacking. Here we investigate the effect of RPE transplantation on vision and vision-based behavior in animal models of retinal degenerated diseases. In addition, we aim to explore the effect of RPE source used for transplantation, the method of intervention, and the animal model which is used.

METHODS

In this study, we systematically identified all publications concerning transplantation of RPE in experimental animal models targeting the improvement of vision (e.g., outcome measurements related to the morphology or function of the eye). A variety of characteristics, such as species, gender, and age of the animals but also cell type, number of cells, and other intervention characteristics were extracted from all studies. A risk of bias analysis was performed as well. Subsequently, all references describing one of the following outcomes were analyzed in depth in this systematic review: a-, b-, and c-wave amplitudes, vision-based, thickness analyses based on optical coherence tomography (OCT) data, and transplant survival based on scanning laser ophthalmoscopy (SLO) data. Meta-analyses were performed on the a- and b-wave amplitudes from electroretinography (ERG) data as well as data from vision-based behavioral assays.

RESULTS

original research articles met the inclusion criteria after two screening rounds. Overall, most studies were categorized as unclear regarding the risk of bias, because many experimental details were poorly reported. Twenty-three studies reporting one or more of the outcome measures of interest were eligible for either descriptive (thickness analyses based on OCT data; n = 2) or meta-analyses. RPE transplantation significantly increased ERG a-wave (Hedges' g 1.181 (0.471-1.892), n = 6) and b-wave (Hedges' g 1.734 (1.295-2.172), n = 42) amplitudes and improved vision-based behavior (Hedges' g 1.018 (0.826-1.209), n = 96). Subgroup analyses revealed a significantly increased effect of the use of young and adolescent animals compared to adult animals. Moreover, transplanting more cells (in the range of 105 versus in the range of 104) resulted in a significantly increased effect on vision-based behavior as well. The origin of cells mattered as well. A significantly increased effect was found on vision-based behavior when using ARPE-19 and OpRegen® RPE.

CONCLUSIONS

This systematic review shows that RPE transplantation in animal models for retinal degeneration significantly increases a- and b- wave amplitudes and improves vision-related behavior. These effects appear to be more pronounced in young animals, when the number of transplanted cells is larger and when ARPE-19 and OpRegen® RPE cells are used. We further emphasize that there is an urgent need for improving the reporting and methodological quality of animal experiments, to make such studies more comparable.

Effective Differentiation and Biological Characterization of Retinal Pigment Epithelium Derived from Human Induced Pluripotent Stem Cells

PURPOSE

Human induced pluripotent stem cells (hiPSC)-derived retinal pigment epithelium (RPE) cells are therapeutic cells that have been shown to be promising in the rescue of lost photoreceptors. In this study, we generated hiPSC from human epidermal keratinocytes and subsequently differentiated them into RPE cells to investigate their ability to influence the retinal functions of the Royal College of Surgeon (RCS) rats.

METHODS

Keratinocytes were reprogrammed to hiPSC using a non-integrating Sendai reprogramming system. Established hiPSCs were differentiated into RPE cells, and complete characterization was performed. Next, the suspension of hiPSC-RPE cells was transplanted into the subretinal space of 3-week-old RCS rats (n = 12). Posttransplantation evaluations were performed using optical coherence tomography (OCT), electroretinography, and immunohistochemical analysis.

RESULTS

The hiPSC colonies were identical to embryonic stem-like cells that revealed the expression of pluripotency markers and retention of the normal genome. These cells exhibited the ability to differentiate into an amalgam of germ layers and produce RPE cells. The differentiated RPE cells exhibited an identical pigmented morphology that expressed RPE-specific markers, such as CRALBP, BESTROPHIN, RPE65, and MERTK. At 8 weeks of longitudinal culture, the RPE cells exhibited maximum pigmentation with in vitro phagocytotic activity. Furthermore, transplantation data showed improved retinal function till week 12 post-transplantation and a significantly higher number of rod/cone ratios in transplanted eyes compared to non-surgery control eyes.

CONCLUSION

hiPSC-derived RPE cells exhibited naïve RPE cell properties and functionality that provided trophic support and the transient rescue of photoreceptor cells.

Transplantation of human embryonic stem cell-derived retinal pigment epithelial cells (MA09-hRPE) in macular degeneration

The use of human embryonic stem cell (hESC)-derived Retinal Pigment Epithelium (RPE) transplants has advanced dramatically in different forms for clinical application in macular degeneration. This review focuses on the first generation of hESC-RPE cell line, named as “MA09-hRPE” by Astellas Institute of Regenerative Medicine (AIRM), and its therapeutic application in human, which evaluated the safety and efficacy of MA09-hRPE cell line transplanted in patients with macular degeneration. This project marks the first milestone in overcoming ethical hurdles and oncogenic safety concerns associated with the use of an embryonic stem cell-derived line. Through in-depth, evidence-based analysis of the MA09-hRPE cell line, along with other hESC-RPE cell lines, this review aims to draw attention to the key technical challenges pertinent to the generation of a biologically competent hESC-RPE cell line and distill the four key prognostic factors residing in the host retina, which concurrently determine the outcomes of clinical efficacy and visual benefits. Given that the technology is still at its infancy for human use, a new clinical regulatory path could aid in cell line validation through small cohort, adaptive clinical trials to accelerate product development toward commercialization. These strategic insights will be invaluable to help both academia and industry, collaboratively shorten the steep learning curve, and reduce large development expenditures spent on unnecessary lengthy clinical trials.

Subretinal Transplantation of Human Central Nervous System Stem Cells Stimulates Controlled Proliferation of Endogenous Retinal Pigment Epithelium

Purpose

The loss of retinal pigment epithelial (RPE) cells is a feature common to age-related macular degeneration (AMD) and retinitis pigmentosa (RP) and multiple early phase clinical trials are underway testing the safety of RPE cell replacement for these diseases. We examined whether transplantation of human neural stem cells into the subretinal space could enhance the endogenous proliferative capacity of the host RPE cell to regenerate.

Methods

Human central nervous system stem cells (HuCNS-SC) were isolated from enzymatically treated brain tissue using flow cytometry. Pigmented dystrophic Royal College of Surgeons (RCS) and S334ter-4 rats treated with oral bromodeoxyuridine (BrdU) received a unilateral subretinal injection of 1.0 × 10⁵ HuCNS-SC cells at either postnatal day 21 or 60. Animals were sacrificed at 90, 120, and 150 days of age. Eyes were fixed processed for cryostat sectioning. Sections were immunostained with Stem101, Ku80, RPE65, OTX1/2, BrdU, and CRALBP antibodies and analyzed via confocal microscopy.

Results

RCS rats that received transplantation of HuCNS-SC had significantly more (approximately 3-fold) Ki67-positive or BrdU-labelled host RPE cells adjacent to the HuCNS-SC graft than controls. Significantly increased host RPE cell proliferation as a result of HuCNS-SC transplantation also was confirmed in S334ter-line 4 transgenic rats with higher proliferation observed in animals with longer posttransplantation periods.

Conclusions

These results suggest that controlled proliferation of endogenous RPE by HuCNS-SC may provide another mechanism by which RPE cell diseases could be treated.

Translational Relevance

Engaging the capacity for endogenous RPE cell regeneration in atrophic diseases may be a novel therapeutic strategy for degenerative diseases of the RPE and retina.

Evaluation of visual function in Royal College of Surgeon rats using a depth perception visual cliff test

Preserving of vision is the main goal in vision research. The presented research evaluates the preservation of visual function in Royal College of Surgeon (RCS) rats using a depth perception test. Rats were placed on a stage with one side containing an illusory steep drop ("cliff") and another side with a minimal drop ("table"). Latency of stage dismounting and the percentage of rats that set their first foot on the "cliff" side were determined. Nondystrophic Long-Evans (LE) rats were tested as control. Electroretinogram and histology analysis were used to determine retinal function and structure. Four-week-old RCS rats presented a significantly shorter mean latency to dismount the stage compared with 6-week-old rats (mean ± standard error, 13.7 ± 1.68 vs. 20.85 ± 6.5 s, P = 0.018). Longer latencies were recorded as rats aged, reaching 45.72 s in 15-week-old rats (P &lt; 0.00001 compared with 4-week-old rats). All rats at the age of 4 weeks placed their first foot on the table side. By contrast, at the age of 8 weeks, 28.6% rats dismounted on the cliff side and at the age of 10 and 15 weeks, rats randomly dismounted the stage to either table or cliff side. LE rats dismounted the stage faster than 4-week-old RCS rats, but the difference was not statistically significant (7 ± 1.58 s, P = 0.057) and all LE rats dismounted on the table side. The latency to dismount the stage in RCS rats correlated with maximal electroretinogram b-wave under dark and light adaptation (Spearman's rho test = -0.603 and -0.534, respectively, all P &lt; 0.0001), outer nuclear layer thickness (Spearman's rho test = -0.764, P = 0.002), and number of S- and M-cones (Spearman's rho test = -0.763 [P = 0.002], and -0.733 [P = 0.004], respectively). The cliff avoidance test is an objective, quick, and readily available method for the determination of RCS rats' visual function.

Clinical-grade stem cell-derived retinal pigment epithelium patch rescues retinal degeneration in rodents and pigs

Considerable progress has been made in testing stem cell-derived retinal pigment epithelium (RPE) as a potential therapy for age-related macular degeneration (AMD). However, the recent reports of oncogenic mutations in induced pluripotent stem cells (iPSCs) underlie the need for robust manufacturing and functional validation of clinical-grade iPSC-derived RPE before transplantation. Here, we developed oncogenic mutation-free clinical-grade iPSCs from three AMD patients and differentiated them into clinical-grade iPSC-RPE patches on biodegradable scaffolds. Functional validation of clinical-grade iPSC-RPE patches revealed specific features that distinguished transplantable from nontransplantable patches. Compared to RPE cells in suspension, our biodegradable scaffold approach improved integration and functionality of RPE patches in rats and in a porcine laser-induced RPE injury model that mimics AMD-like eye conditions. Our results suggest that the in vitro and in vivo preclinical functional validation of iPSC-RPE patches developed here might ultimately be useful for evaluation and optimization of autologous iPSC-based therapies.

Immunological Considerations for Retinal Stem Cell Therapy

There is an increasing effort toward generating replacement cells for neuronal application due to the nonregenerative nature of these tissues. While much progress has been made toward developing methodologies to generate these cells, there have been limited improvements in functional restoration. Some of these are linked to the degenerative and often nonreceptive microenvironment that the new cells need to integrate into. In this chapter, we will focus on the status and role of the immune microenvironment of the retina during homeostasis and disease states. We will review changes in both innate and adaptive immunity as well as the role of immune rejection in stem cell replacement therapies. The chapter will end with a discussion of immune-modulatory strategies that have helped to ameliorate these effects and could potentially improve functional outcome for cell replacement therapies for the eye.

Pluripotent Stem Cells for Retinal Tissue Engineering: Current Status and Future Prospects

The retina is a very fine and layered neural tissue, which vitally depends on the preservation of cells, structure, connectivity and vasculature to maintain vision. There is an urgent need to find technical and biological solutions to major challenges associated with functional replacement of retinal cells. The major unmet challenges include generating sufficient numbers of specific cell types, achieving functional integration of transplanted cells, especially photoreceptors, and surgical delivery of retinal cells or tissue without triggering immune responses, inflammation and/or remodeling. The advances of regenerative medicine enabled generation of three-dimensional tissues (organoids), partially recreating the anatomical structure, biological complexity and physiology of several tissues, which are important targets for stem cell replacement therapies. Derivation of retinal tissue in a dish creates new opportunities for cell replacement therapies of blindness and addresses the need to preserve retinal architecture to restore vision. Retinal cell therapies aimed at preserving and improving vision have achieved many improvements in the past ten years. Retinal organoid technologies provide a number of solutions to technical and biological challenges associated with functional replacement of retinal cells to achieve long-term vision restoration. Our review summarizes the progress in cell therapies of retina, with focus on human pluripotent stem cell-derived retinal tissue, and critically evaluates the potential of retinal organoid approaches to solve a major unmet clinical need—retinal repair and vision restoration in conditions caused by retinal degeneration and traumatic ocular injuries. We also analyze obstacles in commercialization of retinal organoid technology for clinical application.

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.

Immunological Properties of Human Embryonic Stem Cell-Derived Retinal Pigment Epithelial Cells

Age-related macular degeneration is caused by dysfunction and loss of retinal pigment epithelium (RPE) cells, and their transplantation may rescue visual functions and delay disease progression. Human embryonic stem cells (hESCs) may be an unlimited source of RPE cells for allotransplantation. We analyzed the immunomodulatory properties of hESC-derived RPE (hESC-RPE) cells, and showed that they inhibited T cell responses. Co-culture experiments showed that RPE cells inhibited interfon-γ secretion and proliferation of activated T cells. Furthermore, hESC-RPE cells enhanced T cell apoptosis and secretion of the anti-inflammatory cytokine interleukin-10 (IL-10). In addition, RPE cells altered the expression of T cell activation markers, CD69 and CD25. RPE cells transplanted into RCS rats without immunosuppression survived, provided retinal rescue, and enhanced IL-10 blood levels. Our data suggest that hESC-RPE cells have immunosuppressive properties. Further studies will determine if these properties are sufficient to alleviate the need for immunosuppression therapy after their clinical allotransplantation.

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T cells proliferation and IFN-γ secretion are inhibited by hESC-RPE cells

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T cells apoptosis and secretion of IL-10 are enhanced by hESC-RPE cells

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RPE cells survive, provide retinal rescue, and enhance IL-10 blood levels in vivo

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These findings are relevant to immunosuppressive regimens for RPE cell therapies

Long-term Rescue of Photoreceptors in a Rodent Model of Retinitis Pigmentosa Associated with MERTK Mutation

MERTK mutation reduces the ability of retinal pigment epithelial (RPE) cells to phagocytize the photoreceptor outer segments, which leads to accumulation of debris separating photoreceptors from RPE cells, resulting in their degeneration and loss of vision. In a rat model of Retinitis Pigmentosa due to MERTK mutation, we demonstrate that surgical removal of debris performed when about half of photoreceptors are lost (P38), allows the remaining photoreceptor cells to renew their outer segments and survive for at least 6 months - 3 times longer than in untreated eyes. In another set of experiments, patterned laser photocoagulation was performed before the debris formation (P19-25) to destroy a fraction of photoreceptors and thereby reduce the phagocytic load of shed outer segment fragments. This treatment also delayed the degeneration of the remaining photoreceptors. Both approaches were assessed functionally and morphologically, using electroretinography, optical coherence tomography, and histology. The long-term preservation of photoreceptors we observed indicates that MERTK-related form of inherited retinal degeneration, which has currently no cure, could be amenable to laser therapy or subretinal surgery, to extend the visual function, potentially for life.

CRISPR-Cas9 genome engineering: Treating inherited retinal degeneration

Gene correction is a valuable strategy for treating inherited retinal degenerative diseases, a major cause of irreversible blindness worldwide. Single gene defects cause the majority of these retinal dystrophies. Gene augmentation holds great promise if delivered early in the course of the disease, however, many patients carry mutations in genes too large to be packaged into adeno-associated viral vectors and some, when overexpressed via heterologous promoters, induce retinal toxicity. In addition to the aforementioned challenges, some patients have sustained significant photoreceptor cell loss at the time of diagnosis, rendering gene replacement therapy insufficient to treat the disease. These patients will require cell replacement to restore useful vision. Fortunately, the advent of induced pluripotent stem cell and CRISPR-Cas9 gene editing technologies affords researchers and clinicians a powerful means by which to develop strategies to treat patients with inherited retinal dystrophies. In this review we will discuss the current developments in CRISPR-Cas9 gene editing in vivo in animal models and in vitro in patient-derived cells to study and treat inherited retinal degenerative diseases.

Regenerating Eye Tissues to Preserve and Restore Vision

Ocular regenerative therapies are on track to revolutionize treatment of numerous blinding disorders, including corneal disease, cataract, glaucoma, retinitis pigmentosa, and age-related macular degeneration. A variety of transplantable products, delivered as cell suspensions or as preformed 3D structures combining cells and natural or artificial substrates, are in the pipeline. Here we review the status of clinical and preclinical studies for stem cell-based repair, covering key eye tissues from front to back, from cornea to retina, and including bioengineering approaches that advance cell product manufacturing. While recognizing the challenges, we look forward to a deep portfolio of sight-restoring, stem cell-based medicine. VIDEO ABSTRACT.

Xeno- and feeder-free differentiation of human pluripotent stem cells to two distinct ocular epithelial cell types using simple modifications of one method

BACKGROUND

Human pluripotent stem cells (hPSCs) provide a promising cell source for ocular cell replacement therapy, but often lack standardized and xenogeneic-free culture and differentiation protocols. We aimed to develop a xeno- and feeder cell-free culture system for undifferentiated hPSCs along with efficient methods to derive ocular therapy target cells: retinal pigment epithelial (RPE) cells and corneal limbal epithelial stem cells (LESCs).

METHODS

Multiple genetically distinct hPSC lines were adapted to a defined, xeno-, and feeder-free culture system of Essential 8™ medium and laminin-521 matrix. Thereafter, two-stage differentiation methods toward ocular epithelial cells were established utilizing xeno-free media and a combination of extracellular matrix proteins. Both differentiation methods shared the same basal elements, using only minor inductive modifications during early differentiation towards desired cell lineages. The resulting RPE cells and LESCs were characterized after several independent differentiation experiments and recovery after xeno-free cryopreservation.

RESULTS

The defined, xeno-, and feeder-free culture system provided a robust means to generate high-quality hPSCs with chromosomal stability limited to early passages. Inductive cues introduced during the first week of differentiation had a substantial effect on lineage specification, cell survival, and even mature RPE properties. Derivative RPE formed functional epithelial monolayers with mature tight junctions and expression of RPE genes and proteins, as well as phagocytosis and key growth factor secretion capacity after 9 weeks of maturation on inserts. Efficient LESC differentiation led to cell populations expressing LESC markers such as p40/p63α by day 24. Finally, we established xeno-free cryobanking protocols for pluripotent hPSCs, hPSC-RPE cells, and hPSC-LESCs, and demonstrated successful recovery after thawing.

CONCLUSIONS

We propose methods for efficient and scalable, directed differentiation of high-quality RPE cells and LESCs. The two clinically relevant cell types are generated with simple inductive modification of the same basal method, followed by adherent culture, passaging, and cryobanking.