Subretinal Transplantation of Human Embryonic Stem Cell Derived-retinal Pigment Epithelial Cells into a Large-eyed Model of Geographic Atrophy

Large animal model species in pluripotent stem cell therapy research and development for retinal diseases: a systematic review

Aim

Retinal cell therapy modalities, in the category of advanced therapy medicinal products (ATMPs), are being developed to target several retinal diseases. Testing in large animal models (LAMs) is a crucial step in translating retinal ATMPs into clinical practice. However, challenges including budgetary and infrastructure constraints can hinder LAM research design and execution. Here, to facilitate the comparison of the various LAMs in pluripotent retinal cell therapy research, we aimed to systematically evaluate the species distribution, reported scientific utility, and methodology of a range of LAMs.

Methods

A systematic search using the words retina, stem cell, transplantation, large animal, pig, rabbit, dog, and nonhuman primate was conducted in the PubMed, Embase, Science Direct and GoogleScholar databases in February 2023.

Results

We included 22 studies involving pluripotent stem cells (induced pluripotent stem cells or human embryonic stem cells) in LAMs, including non-human primates (NHP), pigs, dogs, and rabbits. Nearly half of the studies utilized wild-type animal models. In other studies, retinal degeneration features were simulated via laser, chemical, or genetic insult. Transplants were delivered subretinally, either as cell suspensions or pre-formed monolayers (with or without biodegradable scaffolding). The transplanted cells dose per eye varied widely (40,000 - 4,000,000 per dose). Cells were delivered via vitrectomy surgery in 15 studies and by an "ab externo" approach in one study. Structural outcomes were assessed using confocal scanning laser ophthalmoscopy imaging. Functional outcomes included multifocal electroretinogram and, in one case, a measure of visual acuity. Generally, cell suspension transplants exhibited low intraretinal incorporation, while monolayer transplants incorporated more efficiently. Immune responses posed challenges for allogeneic transplants, suggesting that autologous iPSC-derived transplants may be required to decrease the likelihood of rejection.

Conclusion

The use of appropriate LAMs helps to advance the development of retinal ATMPs. The anatomical similarity of LAM and human eyes allows the implementation of clinically-relevant surgical techniques. While the FDA Modernization Act 2.0 has provided a framework to consider alternative methods including tissue-on-a-chip and human cell culture models for pharmacologic studies, LAM testing remains useful for cell and tissue replacement studies to inform the development of clinical trial protocols.

Advancements in Human Embryonic Stem Cell Research: Clinical Applications and Ethical Issues

BACKGROUND

The development and use of human embryonic stem cells (hESCs) in regenerative medicine have been revolutionary, offering significant advancements in treating various diseases. These pluripotent cells, derived from early human embryos, are central to modern biomedical research. However, their application is mired in ethical and regulatory complexities related to the use of human embryos.

METHOD

This review utilized key databases such as ClinicalTrials.gov, EU Clinical Trials Register, PubMed, and Google Scholar to gather recent clinical trials and studies involving hESCs. The focus was on their clinical application in regenerative medicine, emphasizing clinical trials and research directly involving hESCs.

RESULTS

Preclinical studies and clinical trials in various areas like ophthalmology, neurology, endocrinology, and reproductive medicine have demonstrated the versatility of hESCs in regenerative medicine. These studies underscore the potential of hESCs in treating a wide array of conditions. However, the field faces ethical and regulatory challenges, with significant variations in policies and perspectives across different countries.

CONCLUSION

The potential of hESCs in regenerative medicine is immense, offering new avenues for treating previously incurable diseases. However, navigating the ethical, legal, and regulatory landscapes is crucial for the continued advancement and responsible application of hESC research in the medical field. Considering both scientific potential and ethical implications, a balanced approach is essential for successfully integrating hESCs into clinical practice.

Factors influencing mesenchymal stromal cells in in vitro cellular models to study retinal pigment epithelial cell rescue

Mesenchymal stromal cells (MSC) stop or slow retinal pigment epithelium (RPE) and neuroretina (NR) degeneration by paracrine activity in oxidative stress-induced retinal degenerative diseases. However, it is mandatory to develop adequate in vitro models that allow testing new treatment strategies against oxidative stress before performing in vivo studies. The viable double- and triple-layered setups are composed of separate layers of NR, MSC, and RPE (NR-MSC-RPE, NR-RPE, MSC-RPE) partially mimic in vivo retinal conditions. In this study, the paracrine neuroprotective effect of each setup's microenvironment on hydrogen peroxide (H₂O₂)-stressed was compared with unstressed RPE cells. RPE cell proliferation viability was assessed on day 1, 3, and 6 using Alamar Blue® (10%), MTT (10%) and a cell viability/cytotoxicity assay kit followed by data analysis. The results showed that RPE cells, highly viable (> 90%) in mixed medium of DMEM and neurobasal A (1:1), lost 50% viability on exposure to 400 µM of H₂O₂ (P < 0.05). The unexposed groups differed significantly from exposed groups for RPE cell growth (RPE and [Formula: see text]RPE (P < 0.0001), NR-MSC-RPE, and NR-MSC-[Formula: see text]RPE (P < 0.05), NR-RPE and NR-[Formula: see text]RPE (P < 0.01), and MSC-RPE and MSC-[Formula: see text]RPE (P < 0.01). NR-[Formula: see text]RPE and NR-RPE supported RPE cell proliferation viability better than other setups (P < 0.01) and RPE cells proliferated 0.49-fold more in NR-MSC-[Formula: see text]RPE than NR-MSC-RPE. Thus, NR and MSC presence improved significantly each setup's microenvironment for cell rescue, nevertheless, each setup also showed limitations for its use as an in vitro study tool. Health of microenvironment of such setups depends on many factors including cell-secreted trophic factors.

Large-Area Photoreceptor Degeneration Model in Rabbits by Photocoagulation and Oxidative Stress in the Retina

Photocoagulation is used for the treatment of retinal ischemic disease. However, due to the invasive nature of photocoagulation and variety of melanin concentrations between individuals, it is challenging to avoid damaging the adjacent photoreceptors and inducing several side effects. Previous studies indicate the role of laser power, duration, and spot size on retinal lesions, but the effect of interspot distance of the laser pulses needs to be considered in panretinal photocoagulation. In this study, we examine different parameters of photocoagulation on lesions of the retina in rabbit, finding that the lesion level of the outer nuclear layer of the retina depended on the pulse duration and laser spot size, and decreasing interspot distance could completely abolish the photoreceptor layer. The degeneration of the photoreceptor by photocoagulation occurred in 24 h and was not restored afterward. We then conducted panretinal photocoagulation in rabbit and found that oxidative stress was decreased in the inner nuclear layer of the retina, and pupillary light reflex and ERG signals were impaired. Our study could provide a rabbit model to explore the mechanism of photoreceptor degeneration and therapies for the side effects after photocoagulation.

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.

Identification of cell surface markers and establishment of monolayer differentiation to retinal pigment epithelial cells

In vitro differentiation of human pluripotent stem cells into functional retinal pigment epithelial (RPE) cells provides a potentially unlimited source for cell based reparative therapy of age-related macular degeneration. Although the inherent pigmentation of the RPE cells have been useful to grossly evaluate differentiation efficiency and allowed manual isolation of pigmented structures, accurate quantification and automated isolation has been challenging. To address this issue, here we perform a comprehensive antibody screening and identify cell surface markers for RPE cells. We show that these markers can be used to isolate RPE cells during in vitro differentiation and to track, quantify and improve differentiation efficiency. Finally, these surface markers aided to develop a robust, direct and scalable monolayer differentiation protocol on human recombinant laminin-111 and −521 without the need for manual isolation.

Whilst pigmentation has been used to identify retinal pigment epithelial (RPE) cells, surface markers for these cells remain unclear. Here, the authors define surface markers for the RPE including CD140b, which help produce hPSC-derived RPE cells at a large scale following a robust, direct and scalable monolayer differentiation protocol.

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.