A Step by Step Protocol for Subretinal Surgery in Rabbits

Biomaterials used for tissue engineering of barrier-forming cell monolayers in the eye

Cell monolayers that form a barrier between two structures play an important role for the maintenance of tissue functionality. In the anterior portion of the eye, the corneal endothelium forms a barrier that controls fluid exchange between the aqueous humor of the anterior chamber and the corneal stroma. This monolayer is central in the pathogenesis of Fuchs endothelial corneal dystrophy (FECD). FECD is a common corneal disease, in which corneal endothelial cells deposit extracellular matrix that increases the thickness of its basal membrane (Descemet's membrane), and forms excrescences (guttae). With time, there is a decrease in endothelial cell density that generates vision loss. Transplantation of a monolayer of healthy corneal endothelial cells on a Descemet membrane substitute could become an interesting alternative for the treatment of this pathology. In the back of the eye, the retinal pigment epithelium (RPE) forms the blood-retinal barrier, controlling fluid exchange between the choriocapillaris and the photoreceptors of the outer retina. In the retinal disease dry age-related macular degeneration (dry AMD), deposits (drusen) form between the RPE and its basal membrane (Bruch's membrane). These deposits hinder fluid exchange, resulting in progressive RPE cell death, which in turn generates photoreceptor cell death, and vision loss. Transplantation of a RPE monolayer on a Bruch's membrane/choroidal stromal substitute to replace the RPE before photoreceptor cell death could become a treatment alternative for this eye disease. This review will present the different biomaterials that are proposed for the engineering of a monolayer of corneal endothelium for the treatment of FECD, and a RPE monolayer for the treatment of dry AMD.

Optimizing Retinal Thermofusion in Retinal Detachment Repair: Achieving Instant Adhesion without Air Tamponade

PURPOSE

Rhegmatogenous retinal detachment repair by intraoperative sealing of the tear without a tamponade agent should enable faster restoration of vision and resumption of normal activities. It avoids the need for further surgery in the case of silicone oil endotamponade. This study evaluated the retinal thermofusion (RTF) retinopexy method of subretinal space dehydration before photocoagulation to create an instantaneous intraoperative retina reattachment in a preclinical model.

DESIGN

Preclinical study.

PARTICIPANTS

Twenty Dutch Belt, pigmented rabbits that underwent RTF repair after experimental retinal detachment.

METHODS

This ex vivo model quantified adhesion force between the retina and underlying retinal pigment epithelium and choroid after treatment of 1 retinal edge using postmortem porcine or human retina (6 × 12 mm). We compared (1) control, (2) laser photocoagulation alone, (3) dehydration alone, and (4) dehydration followed by photocoagulation (RTF). Optimized parameters for RTF were then applied in the in vivo rabbit model of retinal detachment. Animals were followed up for 14 days.

MAIN OUTCOME MEASURES

For this ex vivo model, we measured adhesion force and related this to tissue temperature. For the in vivo study, we assessed retinal attachment using funduscopy and histologic analysis.

RESULTS

The ex vivo model showed that RTF repair produced significantly higher adhesion force than photocoagulation alone independent of dehydration method: warm (60° C) high airflow (50-70 ml/minute) or using laser wavelengths targeting water absorption peaks (1470 or 1940 nm) with coaxial low airflow (10-20 ml/minute). The latter approach produced a smaller footprint of dehydration. Application of RTF (1940-nm laser with coaxial airflow) in an in vivo retinal detachment model in rabbit eyes resulted in immediate retinal adhesion, achieving forces similar to those in the ex vivo experiments. Retinal thermofusion repair resulted in stable reattachment of the retina over the 2-week follow-up period.

CONCLUSIONS

We showed that a short preliminary dehydrating laser treatment of a retinal tear margin before traditional laser photocoagulation creates an immediate intraoperative waterproof retinopexy adhesion independent of tamponade and a wound-healing response. This approach potentially will allow rapid postoperative recovery regardless of the tear location and improved vision.

The complement C3a‑C3aR and C5a‑C5aR pathways promote viability and inflammation of human retinal pigment epithelium cells by targeting NF‑κB signaling

Retinal detachment (RD) and its special form of rhegmatogenous RD associated with choroidal detachment (RRDCD) feature similar pathological alterations, including enhanced retinal cell inflammation. Although the importance of the complement components C3a and C5a and their corresponding receptors in retinal maintenance has been demonstrated, the relevance of these molecules to the pathogenesis of RD or RRDCD remains to be investigated. The contents of C3a, C5a and inflammatory factors, such as TNF-α, IL-1β, IL-6 and prostaglandin (PG)E2, in related clinical samples were examined by ELISA. Subsequently, human retinal pigment epithelial (HRPE) cells were subjected to challenge with the C3a and C5a recombinant proteins with or without C3a and C5a antagonists and NF-κB inhibitor, and the cell viability and inflammatory cytokines were then determined by a Cell Counting Kit-8 assay and ELISA, respectively. In addition, reverse transcription-quantitative PCR and western blot analyses were utilized to examine the mRNA or/and protein levels of C3a and its receptor C3aR, as well as C5a and its receptor C5aR, and NF-κB. In addition, the correlation of C3a and C5a with the aforementioned inflammatory factors was analyzed. The inflammatory factor levels of C3a and C5a were considerably elevated in patients with RRDCD compared to those in the controls. Consistently, C3a and C5a treatment led to increased cell viability and aggravated inflammation in HRPE cells. Accordingly, C3a and C5a induced upregulation of their corresponding receptors C3aR and C5aR, which was in turn observed to be linked to the activation of the NF-κB signaling pathway. Furthermore, there was a positive correlation of the complements C3a and C5a with individual TNF-α, IL-1β, IL-6 and PGE2. Taken together, the C3a-C3aR and C5a-C5aR pathways were indicated to promote cell viability and inflammation of HRPE cells by targeting the NF-κB signaling pathway.

A bio-functional polymer that prevents retinal scarring through modulation of NRF2 signalling pathway

One common cause of vision loss after retinal detachment surgery is the formation of proliferative and contractile fibrocellular membranes. This aberrant wound healing process is mediated by epithelial-mesenchymal transition (EMT) and hyper-proliferation of retinal pigment epithelial (RPE) cells. Current treatment relies primarily on surgical removal of these membranes. Here, we demonstrate that a bio-functional polymer by itself is able to prevent retinal scarring in an experimental rabbit model of proliferative vitreoretinopathy. This is mediated primarily via clathrin-dependent internalisation of polymeric micelles, downstream suppression of canonical EMT transcription factors, reduction of RPE cell hyper-proliferation and migration. Nuclear factor erythroid 2-related factor 2 signalling pathway was identified in a genome-wide transcriptomic profiling as a key sensor and effector. This study highlights the potential of using synthetic bio-functional polymer to modulate RPE cellular behaviour and offers a potential therapy for retinal scarring prevention.

Subretinal Implantation of Human Primary RPE Cells Cultured on Nanofibrous Membranes in Minipigs

Purpose: The development of primary human retinal pigmented epithelium (hRPE) for clinical transplantation purposes on biodegradable scaffolds is indispensable. We hereby report the results of the subretinal implantation of hRPE cells on nanofibrous membranes in minipigs. Methods: The hRPEs were collected from human cadaver donor eyes and cultivated on ultrathin nanofibrous carriers prepared via the electrospinning of poly(L-lactide-co-DL-lactide) (PDLLA). “Libechov” minipigs (12–36 months old) were used in the study, supported by preoperative tacrolimus immunosuppressive therapy. The subretinal implantation of the hRPE-nanofibrous carrier was conducted using general anesthesia via a custom-made injector during standard three-port 23-gauge vitrectomy, followed by silicone oil endotamponade. The observational period lasted 1, 2, 6 and 8 weeks, and included in vivo optical coherence tomography (OCT) of the retina, as well as post mortem immunohistochemistry using the following antibodies: HNAA and STEM121 (human cell markers); Bestrophin and CRALBP (hRPE cell markers); peanut agglutining (PNA) (cone photoreceptor marker); PKCα (rod bipolar marker); Vimentin, GFAP (macroglial markers); and Iba1 (microglial marker). Results: The hRPEs assumed cobblestone morphology, persistent pigmentation and measurable trans-epithelial electrical resistance on the nanofibrous PDLLA carrier. The surgical delivery of the implants in the subretinal space of the immunosuppressed minipigs was successfully achieved and monitored by fundus imaging and OCT. The implanted hRPEs were positive for HNAA and STEM121 and were located between the minipig’s neuroretina and RPE layers at week 2 post-implantation, which was gradually attenuated until week 8. The neuroretina over the implants showed rosette or hypertrophic reaction at week 6. The implanted cells expressed the typical RPE marker bestrophin throughout the whole observation period, and a gradual diminishing of the CRALBP expression in the area of implantation at week 8 post-implantation was observed. The transplanted hRPEs appeared not to form a confluent layer and were less capable of keeping the inner and outer retinal segments intact. The cone photoreceptors adjacent to the implant scaffold were unchanged initially, but underwent a gradual change in structure after hRPE implantation; the retina above and below the implant appeared relatively healthy. The glial reaction of the transplanted and host retina showed Vimentin and GFAP positivity from week 1 onward. Microglial activation appeared in the retinal area of the transplant early after the surgery, which seemed to move into the transplant area over time. Conclusions: The differentiated hRPEs can serve as an alternative cell source for RPE replacement in animal studies. These cells can be cultivated on nanofibrous PDLLA and implanted subretinally into minipigs using standard 23-gauge vitrectomy and implantation injector. The hRPE-laden scaffolds demonstrated relatively good incorporation into the host retina over an eight-week observation period, with some indication of a gliotic scar formation, and a likely neuroinflammatory response in the transplanted area despite the use of immunosuppression.

Selective Large-Area Retinal Pigment Epithelial Removal by Microsecond Laser in Preparation for Cell Therapy

Purpose

Cell therapy is a promising treatment for retinal pigment epithelium (RPE)-associated eye diseases such as age-related macular degeneration. Herein, selective microsecond laser irradiation targeting RPE cells was used for minimally invasive, large-area RPE removal in preparation for delivery of retinal cell therapeutics.

Methods

Ten rabbit eyes were exposed to laser pulses 8, 12, 16, and 20 µs in duration (wavelength, 532 nm; top-hat beam profile, 223 × 223 µm²). Post-irradiation retinal changes were assessed with fluorescein angiography (FA), indocyanine green angiography (ICGA), and optical coherence tomography (OCT). RPE viability was evaluated with an angiographic probit model. Following vitrectomy, a subretinal injection of balanced salt solution was performed over a lasered (maximum 13.6 mm2) and untreated control area. Bleb retinal detachment (bRD) morphology was then evaluated by intraoperative OCT.

Results

Within 1 hour after irradiation, laser lesions showed FA and ICGA leakage. OCT revealed that large-area laser damage was limited to the RPE. The angiographic median effective dose irradiation thresholds (ED50) were 45 µJ (90 mJ/cm2) at 8 µs, 52 µJ (104 mJ/cm2) at 12 µs, 59 µJ (118 mJ/cm2) at 16 µs, and 71 µJ (142 mJ/cm2) at 20 µs. Subretinal injection over the lasered area resulted in a controlled, shallow bRD rise, whereas control blebs were convex in shape, with less predictable spread.

Conclusions

Large-area, laser-based removal of host RPE without visible photoreceptor damage is possible and facilitates surgical retinal detachment.

Translational Relevance

Selective microsecond laser-based, large-area RPE removal prior to retinal cell therapy may reduce iatrogenic trauma.

Migration of pre-induced human peripheral blood mononuclear cells from the transplanted to contralateral eye in mice

Background

Retina diseases may lead to blindness as they often afflict both eyes. Stem cell transplantation into the affected eye(s) is a promising therapeutic strategy for certain retinal diseases. Human peripheral blood mononuclear cells (hPBMCs) are a good source of stem cells, but it is unclear whether pre-induced hPBMCs can migrate from the injected eye to the contralateral eye for bilateral treatment. We examine the possibility of bilateral cell transplantation from unilateral cell injection.

Methods

One hundred and sixty-one 3-month-old retinal degeneration 1 (rd1) mice were divided randomly into 3 groups: an untreated group (n = 45), a control group receiving serum-free Dulbecco’s modified Eagle’s medium (DMEM) injection into the right subretina (n = 45), and a treatment group receiving injection of pre-induced hPBMCs into the right subretina (n = 71). Both eyes were examined by full-field electroretinogram (ERG), immunofluorescence, flow cytometry, and quantitative real-time polymerase chain reaction (qRT-PCR) at 1 and 3 months post-injection.

Results

At both 1 and 3 months post-injection, labeled pre-induced hPBMCs were observed in the retinal inner nuclear layer of the contralateral (left untreated) eye as well as the treated eye as evidenced by immunofluorescence staining for a human antigen. Flow cytometry of fluorescently label cells and qRT-PCR of hPBMCs genes confirmed that transplanted hPBMCs migrated from the treated to the contralateral untreated eye and remained viable for up to 3 months. Further, full-field ERG showed clear light-evoked a and b waves in both treated and untreated eyes at 3 months post-transplantation. Labeled pre-induced hPBMCs were also observed in the contralateral optic nerve but not in the blood circulation, suggesting migration via the optic chiasm.

Conclusion

It may be possible to treat binocular eye diseases by unilateral stem cell injection.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02180-5.

Surgical Transplantation of Human RPE Stem Cell-Derived RPE Monolayers into Non-Human Primates with Immunosuppression

Recent trials of retinal pigment epithelium (RPE) transplantation for the treatment of disorders such as age-related macular degeneration have been promising. However, limitations of existing strategies include the uncertain survival of RPE cells delivered by cell suspension and the inherent risk of uncontrolled cell proliferation in the vitreous cavity. Human RPE stem cell-derived RPE (hRPESC-RPE) transplantation can rescue vision in a rat model of retinal dystrophy and survive in the rabbit retina for at least 1 month. The present study placed hRPESC-RPE monolayers under the macula of a non-human primate model for 3 months. The transplant was able to recover in vivo and maintained healthy photoreceptors. Importantly, there was no evidence that subretinally transplanted monolayers underwent an epithelial-mesenchymal transition. Neither gliosis in adjacent retina nor epiretinal membranes were observed. These findings suggest that hRPESC-RPE monolayers are safe and may be a useful source for RPE cell replacement therapy.

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hRPESC-RPE monolayer transplanted under macula of non-human primates

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Transplanted hRPESC-RPE recovers in vivo and maintains healthy photoreceptors

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Transplanted cells did not undergo epithelial-mesenchymal transition

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Gliosis was not observed in adjacent retina for up to at least 3 months

Hints for Gentle Submacular Injection in Non-Human Primates Based on Intraoperative OCT Guidance

Purpose

Delivery of Advanced Therapy Medicinal Products to the submacular space is increasingly evolving into a therapeutic modality. Cell replacement for age-related macular degeneration (AMD) and gene therapy for RPE65 are recent successful examples. Herein, a nonhuman primate (NHP) model was used to investigate surgical means to detach the macula.

Methods

Sixteen eyes of 13 healthy macaques underwent a 25-gauge vitrectomy and subretinal injection of balanced salt solution monitored by microscope-integrated intraoperative optical coherence tomography (miOCT). The animals were followed with OCT and histology.

Results

The miOCT monitoring allowed a more precise definition of surgical trauma ranging from an initial full-thickness foveal tear, or induction of a cystoid macular edema (CME), until no foveal defect was discernible, as the technique improved. However, as the subretinal fluid wave detached the fovea, the aforementioned lesions formed, whereas persistent retinal adhesion reproducibly proved to remain in the distal parafoveal semi-annulus. Measures to reduce foveal trauma during submacular fluid injection included reducing intraocular pressure, injection volume, and velocity, as well as the retinal location for bleb initiation, use of a vitreous tamponade, and a dual-bore subretinal cannula.

Conclusions

A stable very low intraocular pressure and careful subretinal injection may avoid tangential macular stretching or mechanical CME formation, while vitreous tamponade may facilitate a more lamellar subretinal flow, all thereby reducing foveal trauma during submacular injection in NHP.

Translational Relevance

These results can be relevant to any submacular surgery procedure used today, as they synergistically reduce the risk of compromising foveal integrity.

Possibilities of an experimental damaging effect on the retinal pigment epithelium

PURPOSE

To simulate the damaging effect on retinal pigment epithelium (RPE) in an experiment studying the effect of human neuronal precursors (NPs).

MATERIAL AND METHODS

The study was carried out on 31 rabbits (31 eyes) of the Chinchilla breed, which were divided into 3 groups: the 1^st group received a subretinal injection of balanced saline solution (BSS); the 2^nd group - subretinal injection of BSS with vitrectomy, displacement of the injection bladder away from the injection site using a perfluororganic compound (PFOC) and laser coagulation; the 3^rd group - subretinal injection of a culture of NPs using the same method as in the group 2. All rabbits were observed for 21 days using ophthalmoscopy, optical coherence tomography (OCT) and autofluorescence (AF).

RESULTS

In the 1^st group, 4 out of 5 rabbits were observed to have total retinal detachment and vitreoretinal proliferative processes in the early postoperative period after subretinal injection of the BSS. In the 2^nd group, OCT and AF revealed atrophy of the outer and inner layers of the retina as well as disorganization of the photoreceptors-RPE-Bruch's membrane complex in the area of injection on the 21 day after the operation. In the 3^rd group, the OCT data obtained during the 21 days of observation showed that a hyperreflective zone at the level of the RPE-Bruch's membrane complex corresponding to the NPs injection site was preserved, while there was a partial loss of the outer retinal layers - but of a smaller volume compared to the BSS injection. The suggested method of subretinal injection led to a reduced number of complications: in the 1^st group, postoperative complications amounted to 80%, while in the 2^nd and 3^rd groups - 45%.

CONCLUSION

The study proposes a new method for retinal injection of BSS, which can help reduce RPE degeneration patterns and possible postoperative complications, thus increasing research efficiency. Subretinal injection of a culture of neuronal precursors derived from human induced pluripotent stem cells (iPSCs) in an experiment can serve as a universal model for studying the survival and integration of stem cells.

Biotechnology and Biomaterial-Based Therapeutic Strategies for Age-Related Macular Degeneration. Part II: Cell and Tissue Engineering Therapies

Age-related Macular Degeneration (AMD) is an up-to-date untreatable chronic neurodegenerative eye disease of multifactorial origin, and the main causes of blindness in over 65 y.o. people. It is characterized by a slow progression and the presence of a multitude of factors, highlighting those related to diet, genetic heritage and environmental conditions, present throughout each of the stages of the illness. Current therapeutic approaches, mainly consisting on intraocular drug delivery, are only used for symptoms relief and/or to decelerate the progression of the disease. Furthermore, they are overly simplistic and ignore the complexity of the disease and the enormous differences in the symptomatology between patients. Due to the wide impact of the AMD and the up-to-date absence of clinical solutions, Due to the wide impact of the AMD and the up-to-date absence of clinical solutions, different treatment options have to be considered. Cell therapy is a very promising alternative to drug-based approaches for AMD treatment. Cells delivered to the affected tissue as a suspension have shown poor retention and low survival rate. A solution to these inconveniences has been the encapsulation of these cells on biomaterials, which contrive to their protection, gives them support, and favor their retention of the desired area. We offer a two-papers critical review of the available and under development AMD therapeutic approaches, from a biomaterials and biotechnological point of view. We highlight benefits and limitations and we forecast forthcoming alternatives based on novel biomaterials and biotechnology methods. In this second part we review the preclinical and clinical cell-replacement approaches aiming at the development of efficient AMD-therapies, the employed cell types, as well as the cell-encapsulation and cell-implant systems. We discuss their advantages and disadvantages and how they could improve the survival and integration of the implanted cells.

First steps for the development of silk fibroin-based 3D biohybrid retina for age-related macular degeneration (AMD)

Age-related macular degeneration is an incurable chronic neurodegenerative disease, causing progressive loss of the central vision and even blindness. Up-to-date therapeutic approaches can only slow down he progression of the disease.

OBJECTIVE

Feasibility study for a multilayered, silk fibroin-based, 3D biohybrid retina.

APPROACH

Fabrication of silk fibroin-based biofilms; culture of different types of cells: retinal pigment epithelium, retinal neurons, Müller and mesenchymal stem cells ; creation of a layered structure glued with silk fibroin hydrogel.

MAIN RESULTS

In vitro evidence for the feasibility of layered 3D biohybrid retinas; primary culture neurons grow and develop neurites on silk fibroin biofilms, either alone or in presence of other cells cultivated on the same biomaterial; cell organization and cellular phenotypes are maintained in vitro for the seven days of the experiment.

SIGNIFICANCE

3D biohybrid retina can be built using silk silkworm fibroin films and hydrogels to be used in cell replacement therapy for AMD and similar retinal neurodegenerative diseases.

[Closure of Persisting Full Thickness Macular Holes by Subretinal Fluid Application: Technical Approach and Surgical Considerations]

INTRODUCTION

Firm adhesions between the retina and adjacent retinal pigment epithelium (RPE) may prevent the closure of macular holes (MH) after chromovitrectomy. Controlled application of subretinal (SR) fluid with BSS may release these adhesions leading to closure of the retracted retina in large and or refractory macular holes.

METHODS

For a standardized procedure, it is recommended to exclude residues of epiretinal membranes on the retinal surface preoperatively at OCT or intraoperatively by means of vital dyes. Intraoperatively, a perfluorocarbon (PFO) bubble is placed above the MH and lowers the infusion bottle of 20 mmHg. Subsequently, SR-fluid blebs are applied in the upper, temporal and inferior quadrants with a subretinal 41-gauge cannula. After removing decalin bubble, the SR-detachment is enlarged toward the foveal center. This is essential to achieve a complete detachment of the outer macular edges from the RPE. The MH can be closed by a temporary gas endotamponade.

RESULTS

With a standardized procedure, the operation can be carried out safely and with minimal effort. Additional measures, such as care for bubble-free SR-fluid sands or machine assistance, were added. In a pilot study, experienced VR surgeons performed the SR-fluid application safely and without complications. The preoperative diameter of the MH was 1150 µm (651 - 2350 µm). The secondary closure rate for our PMH was 80.9%.

CONCLUSION

SR-adhesions seem to have a previously unnoticed component in persistent macular holes. An SR-fluid application can be carried out quickly, safely and with minimal material effort. The initial results show a high secondary closure rate.

[Morphological and functional indicators of retinal pigment epithelium and photoreceptor apparatus in inherited retinal diseases]

PURPOSE

To evaluate the relationship between the morphological and functional parameters of retinal pigment epithelium (RPE) and photoreceptors (PR) in inherited retinal diseases (IRD).

MATERIAL AND METHODS

The study included 52 patients (104 eyes), 23 of them with Stargardt Disease (STGD), 19 with cone-rod dystrophy (CRD), 10 with retinitis pigmentosa/pigmentary abiotrophy (RP) of comparable disease durations. All patients underwent standard and additional ophthalmological examination: fundus autofluorescence (AF), spectral optical coherence tomography (OCT), computer perimetry (CP), electro-oculography (EOG), Ganzfeld electroretinography (gERG).

RESULTS

Comparison of the groups of IRD patients and groups according to the degree of RPE damage with the control group revealed an increase in differences in the EOG and gERG indicators as the area and depth of damage to the RPE and PR progressed. The patterns of changes in RPE and PR, the frequency of their occurrence with IRD in this patient sample are described. A moderate correlation was found between the amount of RPE loss and EOG light rise, as well as between the defect of the ellipsoid zone and the amplitude of α- and β-waves, the latency of β-wave of the gERG. Some patients showed a mismatch between a small defect of the ellipsoid zone and RPE with significant damage to the visual field and reduction of the EOG and gERG indicators. The obtained electrophysiological indicators revealed pathological changes in RPE and PR, more significant and widespread in some cases than it was shown with visualization methods. Weak and moderate correlations between visual acuity, and RPE damage and light sensitivity index with loss of ellipsoid zone were calculated.

CONCLUSIONS

Modern methods of retinal examination can help obtain complete and versatile picture of morphological and functional state of the retina in IDR that supplement each other. EOG and gERG have capability to determine the degree of RPE and PR functions impairment including those cases when morphological studies are not sufficiently informative.

[Possibilities of treating retinal diseases in patients with damaged retinal pigment epithelium]

Retinal diseases associated with damage to retinal pigment epithelium (PPE) are the most frequent causes of irreversible loss of vision in adults. Since there is no therapeutic treatment available that could repair RPE and its connections with the adjacent photoreceptors, the review focuses on various methods of surgical treatment. One of the most promising methods at present is the use of stem cells derivatives. Results of numerous experimental and clinical trials show that use of human induced pluripotent stem cells in the treatment of degenerative diseases of the retina can be considered effective and promising.

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.

Visual evoked potential in rabbits' eyes with subretinal implantation by vitrectomy of Okayama University-type retinal prosthesis (OURePTM)

Okayama University-type retinal prosthesis (OUReP^TM) is a photoelectric dye-coupled polyethylene film which generates electric potential in response to light and stimulates nearby neurons. This study aims to test surgical feasibility for subretinal film implantation and to examine functional durability of films in subretinal space. Dye-coupled films were implanted subretinally by vitrectomy in the right eye of normal white rabbits: 8 rabbits for 1 month and 8 rabbits for 6 months. The implanted films were removed by vitrectomy in 4 of these 8 rabbits in 1-month or 6-month implantation group. The films were also implanted in 4 rhodopsin-transgenic retinal dystrophic rabbits. Visual evoked potential was measured before film implantation as well as 1 or 6 months after film implantation, or 1 month after film removal. The films were successfully implanted in subretinal space of retinal detachment induced by subretinal fluid injection with a 38G polyimide tip. The retina was reattached by fluid-air exchange in vitreous cavity, retinal laser coagulation, and silicone oil injection. The ratios of P₂ amplitudes of visual evoked potential in the implanted right eye over control left eye did not show significant changes between pre-implantation and post-implantation or post-removal (paired t-test). In Kelvin probe measurements, 4 pieces each of removed films which were implanted for 1 or 6 months showed proportional increase of surface electric potential in response to increasing light intensity. The film implantation was safe and implanted films were capable of responding to light.

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.