Human RPE stem cells grown into polarized RPE monolayers on a polyester matrix are maintained after grafting into rabbit subretinal space

Advances in the study of tissue-engineered retinal pigment epithelial cell sheets

Regarded as the most promising treatment modality for retinal degenerative diseases, retinal pigment epithelium cell replacement therapy holds significant potential. Common retinal degenerative diseases, including Age-related Macular Degeneration, are frequently characterized by damage to the unit comprising photoreceptors, retinal pigment epithelium, and Bruch's membrane. The selection of appropriate tissue engineering materials, in conjunction with retinal pigment epithelial cells, for graft preparation, can offer an effective treatment for retinal degenerative diseases. This article presents an overview of the research conducted on retinal pigment epithelial cell tissue engineering, outlining the challenges and future prospects.

Electrospun poly(l-lactide-co-dl-lactide) nanofibrous scaffold as substrate for ex vivo limbal epithelial cell cultivation

Ultrathin electrospun poly (l-lactide-co-dl-lactide) nanofibrous membranes coated with fibronectin were explored as scaffolds for the ex vivo cultivation of limbal epithelial cells (LECs) for the treatment of limbal stem cell deficiency. The developed scaffolds were compared with the "gold-standard" fibrin gel. The resulting membranes composed of nanofibers possessed a very low thickness of 4 μm and allowed very good optical transparency in the wet state. The fibronectin-coated nanofibrous scaffolds demonstrated LEC expansion and successful cultivation similar to that on fibrin gel. Unlike the regular cobblestone epithelial cell morphology on the fibrin gel, the nanofibrous scaffold presented a mostly irregular epithelial morphology with a shift to a mesenchymal phenotype, as confirmed by the upregulation of profibroblastic genes: ACTA2 (p = 0.023), FBLN1 (p < 0.001), and THY1 (p < 0.001). Both culture conditions revealed comparable expression of stem cell markers, including KLF4, ΔNp63α and ABCG2, emphasizing the promise of polylactide-based nanofibrous membranes for further investigations.

The Effects of the Coating and Aging of Biodegradable Polylactic Acid Membranes on In Vitro Primary Human Retinal Pigment Epithelium Cells

Age-related macular degeneration (AMD) is the most frequent cause of blindness in developed countries. The replacement of dysfunctional human retinal pigment epithelium (hRPE) cells by the transplantation of in vitro-cultivated hRPE cells to the affected area emerges as a feasible strategy for regenerative therapy. Synthetic biomimetic membranes arise as powerful hRPE cell carriers, but as biodegradability is a requirement, it also poses a challenge due to its limited durability. hRPE cells exhibit several characteristics that putatively respond to the type of membrane carrier, and they can be used as biomarkers to evaluate and further optimize such membranes. Here, we analyze the pigmentation, transepithelial resistance, genome integrity, and maturation markers of hRPE cells plated on commercial polycarbonate (PC) versus in-house electrospun polylactide-based (PLA) membranes, both enabling separate apical/basolateral compartments. Our results show that PLA is superior to PC-based membranes for the cultivation of hRPEs, and the BEST1/RPE65 maturation markers emerge as the best biomarkers for addressing the quality of hRPE cultivated in vitro. The stability of the cultures was observed to be affected by PLA aging, which is an effect that could be partially palliated by the coating of the PLA membranes.

Hydrogel-Based Therapy for Age-Related Macular Degeneration: Current Innovations, Impediments, and Future Perspectives

Age-related macular degeneration (AMD) is an ocular disease that leads to progressive photoreceptor death and visual impairment. Currently, the most common therapeutic strategy is to deliver anti-vascular endothelial growth factor (anti-VEGF) agents into the eyes of patients with wet AMD. However, this treatment method requires repeated injections, which potentially results in surgical complications and unwanted side effects for patients. An effective therapeutic approach for dry AMD also remains elusive. Therefore, there is a surge of enthusiasm for the developing the biodegradable drug delivery systems with sustained release capability and develop a promising therapeutic strategy. Notably, the strides made in hydrogels which possess intricate three-dimensional polymer networks have profoundly facilitated the treatments of AMD. Researchers have established diverse hydrogel-based delivery systems with marvelous biocompatibility and efficacy. Advantageously, these hydrogel-based transplantation therapies provide promising opportunities for vision restoration. Herein, we provide an overview of the properties and potential of hydrogels for ocular delivery. We introduce recent advances in the utilization of hydrogels for the delivery of anti-VEGF and in cell implantation. Further refinements of these findings would lay the basis for developing more rational and curative therapies for AMD.

Recent Achievements in the Heterogeneity of Mammalian and Human Retinal Pigment Epithelium: In Search of a Stem Cell

Retinal pigment epithelium (RPE) cells are important fundamentally for the development and function of the retina. In this regard, the study of the morphological and molecular properties of RPE cells, as well as their regenerative capabilities, is of particular importance for biomedicine. However, these studies are complicated by the fact that, despite the external morphological similarity of RPE cells, the RPE is a population of heterogeneous cells, the molecular genetic properties of which have begun to be revealed by sequencing methods only in recent years. This review carries out an analysis of the data from morphological and molecular genetic studies of the heterogeneity of RPE cells in mammals and humans, which reveals the individual differences in the subpopulations of RPE cells and the possible specificity of their functions. Particular attention is paid to discussing the properties of "stemness," proliferation, and plasticity in the RPE, which may be useful for uncovering the mechanisms of retinal diseases associated with pathologies of the RPE and finding new ways of treating them.

Advances in the engineering of the outer blood-retina barrier: From in-vitro modelling to cellular therapy

The outer blood-retina barrier (oBRB), crucial for the survival and the proper functioning of the overlying retinal layers, is disrupted in numerous diseases affecting the retina, leading to the loss of the photoreceptors and ultimately of vision. To study the oBRB and/or its degeneration, many in vitro oBRB models have been developed, notably to investigate potential therapeutic strategies against retinal diseases. Indeed, to this day, most of these pathologies are untreatable, especially once the first signs of degeneration are observed. To cure those patients, a current strategy is to cultivate in vitro a mature oBRB epithelium on a custom membrane that is further implanted to replace the damaged native tissue. After a description of the oBRB and the related diseases, this review presents an overview of the oBRB models, from the simplest to the most complex. Then, we propose a discussion over the used cell types, for their relevance to study or treat the oBRB. Models designed for in vitro applications are then examined, by paying particular attention to the design evolution in the last years, the development of pathological models and the benefits of co-culture models, including both the retinal pigment epithelium and the choroid. Lastly, this review focuses on the models developed for in vivo implantation, with special emphasis on the choice of the material, its processing and its characterization, before discussing the reported pre-clinical and clinical trials.

Human and Pig Pluripotent Stem Cells: From Cellular Products to Organogenesis and Beyond

Pluripotent stem cells (PSCs) are important for studying development and hold great promise in regenerative medicine due to their ability to differentiate into various cell types. In this review, we comprehensively discuss the potential applications of both human and pig PSCs and provide an overview of the current progress and challenges in this field. In addition to exploring the therapeutic uses of PSC-derived cellular products, we also shed light on their significance in the study of interspecies chimeras, which has led to the creation of transplantable human or humanized pig organs. Moreover, we emphasize the importance of pig PSCs as an ideal cell source for genetic engineering, facilitating the development of genetically modified pigs for pig-to-human xenotransplantation. Despite the achievements that have been made, further investigations and refinement of PSC technologies are necessary to unlock their full potential in regenerative medicine and effectively address critical healthcare challenges.

Progress in Stem Cells-Based Replacement Therapy for Retinal Pigment Epithelium: In Vitro Differentiation to In Vivo Delivery

Retinal pigment epithelium (RPE) is a critical cell monolayer forming the blood-retina-barrier (BRB) and a permeable bridge between the choriocapillaris and the retina. RPE is also crucial in maintaining photoreceptor function and for completing the visual cycle. Loss of the RPE is associated with the development of degenerative diseases like age-related macular degeneration (AMD). To treat diseases like AMD, pluripotent stem cell-derived RPE (pRPE) has been recently explored extensively as a regenerative module. pRPE like other ectodermal tissues requires specific lineage differentiation and long-term in vitro culturing for maturation. Therefore, understanding the differentiation process of RPE could be useful for stem cell-based RPE derivation. Developing pRPE-based transplants and delivering them into the subretinal space is another aspect that has garnered interest in the last decade. In this review, we discuss the basic strategies currently employed for stem cell-based RPE derivation, their delivery, and recent clinical studies related to pRPE transplantation in patients. We have also discussed a few limitations with in vitro RPE culture and potential solutions to overcome such problems which can be helpful in developing functional RPE tissue.

A rabbit model for outer retinal atrophy caused by surgical RPE removal

PURPOSE

We aimed to establish a rabbit model with retinal atrophy induced by an iatrogenic retinal pigment epithelium (RPE) removal, for future testing of the efficacy and safety of cell therapy strategies.

METHODS

A localized detachment of the retina from the RPE/choroid layer was created in 18 pigmented rabbits. The RPE was removed by scraping with a custom-made extendable loop instrument. The resulting RPE wound was observed over a time course of 12 weeks with optical coherence tomography and angiography. After 4 days (group 1) and 12 weeks (group 2), histology was done and staining with hematoxylin and eosin, as well as immunofluorescence performed to further investigate the effects of debridement on the RPE and the overlying retina.

RESULTS

Already after 4 days, we observed a closure of the RPE wound by proliferating RPE and microglia/macrophage cells forming a multilayered clump. This pattern continued over the observation time course of 12 weeks, whereby the inner and outer nuclear layer of the retina became atrophic. No neovascularization was observed in the angiograms or histology. The observed changes were limited to the site of the former RPE wound.

CONCLUSIONS

Localized surgical RPE removal induced an adjacent progressive retinal atrophy. Altering the natural course of this model may serve as a basis to test RPE cell therapeutics.

Hydrogel assisted photoreceptor delivery inhibits material transfer

Cell therapy holds tremendous promise for vision restoration; yet donor cell survival and integration continue to limit efficacy of these strategies. Transplanted photoreceptors, which mediate light sensitivity in the retina, transfer cytoplasmic components to host photoreceptors instead of integrating into the tissue. Donor cell material transfer could, therefore, function as a protein augmentation strategy to restore photoreceptor function. Biomaterials, such as hyaluronan-based hydrogels, can support donor cell survival but have not been evaluated for effects on material transfer. With increased survival, we hypothesized that we would achieve greater material transfer; however, the opposite occurred. Photoreceptors delivered to the subretinal space in mice in a hyaluronan and methylcellulose (HAMC) hydrogel showed reduced material transfer. We examined mitochondria transfer in vitro and cytosolic protein transfer in vivo and demonstrate that HAMC significantly reduced transfer in both contexts, which we ascribe to reduced cell-cell contact. Nanotube-like donor cell protrusions were significantly reduced in the hydrogel-transplanted photoreceptors compared to the saline control group, which suggests that HAMC limits the contact required to the host retina for transfer. Thus, HAMC can be used to manipulate the behaviour of transplanted donor cells in cell therapy strategies.

Neural differentiation of human retinal pigment epithelial cells on alginate/gelatin substrate

Purpose

The development of biomaterials provides potent promise for the regeneration of neuroretinal cells in degenerative eye diseases and retinal tissue engineering. Biomimetic three-dimensional (3D) microenvironments and specific growth factors motivate the differentiation of human retinal pigment epithelial (hRPE) cells toward a retinal neural lineage. In this study, we evaluated alginate/gelatin (A/G) as a substrate for the culture of hRPE cells.

Methods

hRPE cells were isolated from neonatal human cadaver globes and cultivated on A/G substrate under different culture conditions, including 30% human amniotic fluid (HAF), 10% fetal bovine serum (FBS), and serum-free Dulbecco's modified Eagle's medium/nutrient mixture F-12 (DMEM/F12). The proliferation of cells in different culture conditions was determined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and a cell proliferation assay. Immunocytochemistry and real-time PCR were performed to evaluate the effect of the substrate on hRPE cell differentiation.

Results

A significant increase in the cell proliferation rate was observed in hRPE cells cultivated on an A/G substrate. Continuous observations demonstrated that hRPE cells formed densely packed, suspended spheroids in DMEM/F12 culture conditions, with dominant transdifferentiation into amacrine cells. Small adherent clusters of hRPE cells in HAF- and FBS-treated cultures represented dedifferentiation toward retinal progenitor cells. These cultures generated amacrine, rod photoreceptors, and bipolar cells.

Conclusions

These findings indicated that A/G substrate induced neural retinal cell propagation in cultures and would therefore be promising for RPE-based tissue engineering studies.

Induced retinal pigment epithelial cells with anti-epithelial-to-mesenchymal transition ability delay retinal degeneration

The hostile microenvironment of the retina in patients with age-related macular degeneration (AMD) may trigger epithelial-to-mesenchymal transition (EMT) of grafted retinal pigment epithelial (RPE) cells, thus attenuating the therapeutic outcome. Here, we transformed human dedifferentiated induced pluripotent stem cell-derived RPE (iPSC-RPE) cells into induced RPE (iRPE) cells using a cocktail of four transcription factors (TFs)—CRX, MITF-A, NR2E1, and C-MYC. These critical TFs maintained the epithelial property of iRPE cells by regulating the expression of bmp7, forkhead box f2, lin7a, and pard6b, and conferred resistance to TGF-β-induced EMT in iRPE cells by targeting ppm1a. The iRPE cells with Tet-on system-regulated c-myc expression exhibited EMT resistance and better therapeutic function compared with iPSC-RPE cells in rat AMD model. Our study demonstrates that endowing RPE cells with anti-EMT property avoids the risk of EMT after cells are grafted into the subretinal space, and it may provide a suitable candidate for AMD treatment.

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CRX, MITF-A, NR2E1, and C-MYC transform De-iPSC-RPE cells into iRPE cells

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iRPE cells have resistance to TGF-β-induced EMT

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BMP7, FOXF2, LIN7A, PARD6B, and PPM1A mediate the functions of TFs in iRPE cells

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iRPE cells have better retinal protective function than iPSC-RPE cells

New Prospects for Retinal Pigment Epithelium Transplantation

ABSTRACT

Retinal pigment epithelium (RPE) transplants rescue photoreceptors in selected animal models of retinal degenerative disease. Early clinical studies of RPE transplants as treatment for age-related macular degeneration (AMD) included autologous and allogeneic transplants of RPE suspensions and RPE sheets for atrophic and neovascular complications of AMD. Subsequent studies explored autologous RPE-Bruch membrane-choroid transplants in patients with neovascular AMD with occasional marked visual benefit, which establishes a rationale for RPE transplants in late-stage AMD. More recent work has involved transplantation of autologous and allogeneic stem cell-derived RPE for patients with AMD and those with Stargardt disease. These early-stage clinical trials have employed RPE suspensions and RPE monolayers on biocompatible scaffolds. Safety has been well documented, but evidence of efficacy is variable. Current research involves development of better scaffolds, improved modulation of immune surveillance, and modification of the extracellular milieu to improve RPE survival and integration with host retina.

Biomaterials to enhance stem cell transplantation

The successful transplantation of stem cells has the potential to transform regenerative medicine approaches and open promising avenues to repair, replace, and regenerate diseased, damaged, or aged tissues. However, pre-/post-transplantation issues of poor cell survival, retention, cell fate regulation, and insufficient integration with host tissues constitute significant challenges. The success of stem cell transplantation depends upon the coordinated sequence of stem cell renewal, specific lineage differentiation, assembly, and maintenance of long-term function. Advances in biomaterials can improve pre-/post-transplantation outcomes by integrating biophysiochemical cues and emulating tissue microenvironments. This review highlights leading biomaterials-based approaches for enhancing stem cell transplantation.

Scaffold free retinal pigment epithelium sheet engineering using modified alginate-RGD hydrogel

Tissue-specific extracellular matrix (ECM) plays a critical role in cell survival and homeostasis, which are particularly essential for directing differentiation of different complex tissues such as retina. However, ECM maintenance should be considered to design an effective therapeutic strategy for retina regeneration. To achieve this, cell sheet engineering has emerged as a growing approach to closely reconstruct basal membrane of cells through a scaffold-free manner. Several irreversible sight-threatening diseases are characterized by the dysfunction and lose of retinal pigment epithelium (RPE), leading to vision loss and eventually total blindness in patients. According to impressive developments in achievement of RPE from human embryonic stem cells (hESCs), we obtained RPE cells without any extrinsic factors in a co-culture system, and cultured them on a temporary alginate hydrogel substrate. Subsequently, Arg-Gly-Asp (RGD) peptide was superficially immobilized on the upper layer of hydrogel to improve cell attachment before harvesting sheet layer. RPE cell sheet layer was released by treating pre-seeded hydrogels with sodium citrate as a calcium chelating agent and characterized in both in vitro and in vivo models. RPE sheets formed tight junction and expressed high levels of retina structural markers such as ZO-1, Bestrophin and Collagen type IV. One week after in vivo transplantation of RPE sheet, cells survived in the subretinal space, indicating that our harvesting method is non-invasive. To sum up, we introduced a unique scaffold-free method for RPE cell sheet engineering, which can find potential use for future therapeutic purposes.

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.

Advantages of nanofibrous membranes for culturing of primary RPE cells compared to commercial scaffolds

PURPOSE

Dysfunction of the retinal pigment epithelium (RPE) causes numerous forms of retinal degeneration. RPE replacement is a modern option to save vision. We aimed to test the results of transplanting cultured RPEs on biocompatible membranes.

METHODS

We cultivated porcine primary RPE cells isolated from cadaver eyes from the slaughterhouse on two types of membranes: commercial polyester scaffolds Transwell (Corning Inc., Kenneburg, ME, USA) with 0.4 µm pore size and prepared Poly (L-lactide-co-DL-lactide) (PDLLA) nanofibrous membranes with an average pore size of 0.4 µm.

RESULTS

Five types of assays were used for the analysis: immunocytochemistry (ICC), phagocytosis assay, Western blotting, real-time qPCR (RT-qPCR) and electron microscopy. RT-qPCR demonstrated that RPEs cultured on nanofibrous membranes have higher expressions of BEST1 (bestrophin 1), RLBP1 (retinaldehyde-binding protein 1), RPE65 (retinal pigment epithelium-specific 65 kDa protein), PAX6 (transcription factor PAX6), SOX9 (transcription factor SOX9), DCT (dopachrome tautomerase) and MITF (microphthalmia-associated transcription factor). ICC of the RPEs cultured on nanofibrous membranes showed more intensive staining of markers such as BEST1, MCT1 (monocarboxylate transporter 1), Na⁺ /K⁺ ATPase, RPE65 and acetylated tubulin in comparison with commercial ones. Additionally, the absence of α-SMA proved the stability of the RPE polarization state and the absence of epithelial-to-mesenchymal transition. RPE possessed high phagocytic activity. Electron microscopy of both membranes confirmed a confluent layer of RPE cells and their genuine morphological structure, which was comparable to native RPEs.

CONCLUSIONS

Retinal pigment epitheliums cultured on polylactide nanofibrous membranes improved the final quality of the cell product by having better maturation and long-term survival of the RPE monolayer compared to those cultured on commercial polyester scaffolds. PDLLA-cultured RPEs are a plausible source for the replacement of non-functioning RPEs during cell therapy.

Scaffold-Free Retinal Pigment Epithelium Microtissues Exhibit Increased Release of PEDF

The retinal pigmented epithelium (RPE) plays a critical role in photoreceptor survival and function. RPE deficits are implicated in a wide range of diseases that result in vision loss, including age-related macular degeneration (AMD) and Stargardt disease, affecting millions worldwide. Subretinal delivery of RPE cells is considered a promising avenue for treatment, and encouraging results from animal trials have supported recent progression into the clinic. However, the limited survival and engraftment of transplanted RPE cells delivered as a suspension continues to be a major challenge. While RPE delivery as epithelial sheets exhibits improved outcomes, this comes at the price of increased complexity at both the production and transplant stages. In order to combine the benefits of both approaches, we have developed size-controlled, scaffold-free RPE microtissues (RPE-µTs) that are suitable for scalable production and delivery via injection. RPE-µTs retain key RPE molecular markers, and interestingly, in comparison to conventional monolayer cultures, they show significant increases in the transcription and secretion of pigment-epithelium-derived factor (PEDF), which is a key trophic factor known to enhance the survival and function of photoreceptors. Furthermore, these microtissues readily spread in vitro on a substrate analogous to Bruch’s membrane, suggesting that RPE-µTs may collapse into a sheet upon transplantation. We anticipate that this approach may provide an alternative cell delivery system to improve the survival and integration of RPE transplants, while also retaining the benefits of low complexity in production and delivery.

Localized Structural and Functional Deficits in a Nonhuman Primate Model of Outer Retinal Atrophy

Purpose

Cell-based therapy development for geographic atrophy (GA) in age-related macular degeneration (AMD) is hampered by the paucity of models of localized photoreceptor and retinal pigment epithelium (RPE) degeneration. We aimed to characterize the structural and functional deficits in a laser-induced nonhuman primate model, including an analysis of the choroid.

Methods

Macular laser photocoagulation was applied in four macaques. Fundus photography, optical coherence tomography (OCT), dye angiography, and OCT-angiography were conducted over 4.5 months, with histological correlation. Longitudinal changes in spatially resolved macular dysfunction were measured using multifocal electroretinography (MFERG).

Results

Lesion features, depending on laser settings, included photoreceptor layer degeneration, inner retinal sparing, skip lesions, RPE elevation, and neovascularization. The intralesional choroid was degenerated. The normalized mean MFERG amplitude within lesions was consistently lower than control regions (0.94 ± 0.35 vs. 1.10 ± 0.27, P = 0.032 at month 1, 0.67 ± 0.22 vs. 0.83 ± 0.15, P = 0.0002 at month 2, and 0.97 ± 0.31 vs. 1.20 ± 0.21, P < 0.0001 at month 3.5). The intertest variation of mean MFERG amplitudes in rings 1 to 5 ranged from 13.0% to 26.0% in normal eyes.

Conclusions

Laser application in this model caused localized outer retinal, RPE, and choriocapillaris loss. Localized dysfunction was apparent by MFERG in the first month after lesion induction. Correlative structure-function testing may be useful for research on the functional effects of stem cell-based therapy for GA. MFERG amplitude data should be interpreted in the context of relatively high intertest variability of the rings that correspond to the central macula. Sustained choroidal insufficiency may limit long-term subretinal graft viability in this model.

Oxidative stress in retinal pigment epithelium impairs stem cells: a vicious cycle in age-related macular degeneration

Aging, chronic oxidative stress, and inflammation are major pathogenic factors in the development and progression of age-related macular degeneration (AMD) with the loss of retinal pigment epithelium (RPE). The human RPE contains a subpopulation of progenitors (i.e., RPE stem cells-RPESCs) whose role in the RPE homeostasis is under investigation. We evaluated the paracrine effects of mature RPE cells exposed to oxidative stress (H₂O₂) on RPESCs behavior through co-cultural, morphofunctional, and bioinformatic approaches. RPESCs showed a decline in proliferation, an increase of the senescence-associated β-galactosidase activity, the acquisition of a senescent-like secretory phenotype (SASP), and the reduction of their stemness and differentiation competencies. IL-6 and Superoxide Dismutase 2 (SOD2) seem to be key molecules in RPESCs response to oxidative stress. Our results get insight into stress-induced senescent-associated molecular mechanisms implicated in AMD pathogenesis. The presence of chronic oxidative stress in the microenvironment reduces the RPESCs abilities, inducing and/or maintaining a pro-inflammatory retinal milieu that in turn could affect AMD onset and progression.

An insight on established retinal injury mechanisms and prevalent retinal stem cell activation pathways in vertebrate models

Implementing different tools and injury mechanisms in multiple animal models of retina regeneration, researchers have discovered the existence of retinal stem/progenitor cells. Although they appear to be distributed uniformly across the vertebrate lineage, the reparative potential of the retina is mainly restricted to lower vertebrates. Regenerative repair post-injury requires the creation of a proliferative niche, vital for proper stem cell activation, propagation, and lineage differentiation. This seems to be lacking in mammals. Hence, in this review, we first discuss the many forms of retinal injuries that have been generated using animal models. Next, we discuss how they are utilized to stimulate regeneration and mimic eye disease pathologies. The key to driving stem cell activation in mammals relies on the information we can gather from these models. Lastly, we present a brief update about the genes, growth factors, and signaling pathways that have been brought to light using these models.

Submacular integration of hESC-RPE monolayer xenografts in a surgical non-human primate model

Background

Human pluripotent stem cells (hPSCs) provide a promising cell source for retinal cell replacement therapy but often lack standardized cell production and live-cell shipment logistics as well as rigorous analyses of surgical procedures for cell transplantation in the delicate macula area. We have previously established a xeno- and feeder cell-free production system for hPSC differentiated retinal pigment epithelial (RPE) cells, and herein, a novel immunosuppressed non-human primate (NHP) model with a disrupted ocular immune privilege is presented for transplanting human embryonic stem cell (hESC)-derived RPE on a scaffold, and the safety and submacular graft integration are assessed. Furthermore, the feasibility of intercontinental shipment of live hESC-RPE is examined.

Methods

Cynomolgus monkeys were systemically immunosuppressed and implanted with a hESC-RPE monolayer on a permeable polyester-terephthalate (PET) scaffold. Microscope-integrated intraoperative optical coherence tomography (miOCT)-guided surgery, postoperative follow-up incorporated scanning laser ophthalmoscopy, spectral domain (SD-) OCT, and full-field electroretinography (ERG) were used as outcome measures. In addition, histology was performed after a 28-day follow-up.

Results

Intercontinental cell shipment, which took >30 h from the manufacturing to the transplantation site, did not alter the hESC-RPE quality. The submacular hESC-RPE xenotransplantation was performed in 11 macaques. The miOCT typically revealed foveal disruption. ERG showed amplitude and peak time preservation in cases with favorable surgical outcomes. Histology confirmed photoreceptor preservation above the grafts and in vivo phagocytosis by hESC-RPE, albeit evidence of cytoplasmic redistribution of opsin in photoreceptors and glia hypertrophy. The immunosuppression protocol efficiently suppressed retinal T cell infiltration and microglia activation.

Conclusion

These results suggest both structural and functional submacular integrations of hESC-RPE xenografts. It is anticipated that surgical technique refinement will further improve the engraftment of macular cell therapeutics with significant translational relevance to improve future clinical trials.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02395-6.

Immunologic Rejection of Transplanted Retinal Pigmented Epithelium: Mechanisms and Strategies for Prevention

Replacement of dysfunctional retinal pigmented epithelium (RPE) with grafts derived from stem cells has the potential to improve vision for patients with retinal disorders. In fact, the potential is such that a great number of groups are attempting to realize this therapy through individual strategies with a variety of stem cell products, hosts, immunomodulatory regimen, and techniques to assess the success of their design. Comparing the findings of different investigators is complicated by a number of factors. The immune response varies greatly between xenogeneic and allogeneic transplantation. A unique immunologic environment is created in the subretinal space, the target of RPE grafts. Both functional assessment and imaging techniques used to evaluate transplants are susceptible to erroneous conclusions. Lastly, the pharmacologic regimens used in RPE transplant trials are as numerous and variable as the trials themselves, making it difficult to determine useful results. This review will discuss the causes of these complicating factors, digest the strategies and results from clinical and preclinical studies, and suggest places for improvement in the design of future transplants and investigations.

The Future of Regenerative Medicine: Cell Therapy Using Pluripotent Stem Cells and Acellular Therapies Based on Extracellular Vesicles

The rapid progress in the field of stem cell research has laid strong foundations for their use in regenerative medicine applications of injured or diseased tissues. Growing evidences indicate that some observed therapeutic outcomes of stem cell-based therapy are due to paracrine effects rather than long-term engraftment and survival of transplanted cells. Given their ability to cross biological barriers and mediate intercellular information transfer of bioactive molecules, extracellular vesicles are being explored as potential cell-free therapeutic agents. In this review, we first discuss the state of the art of regenerative medicine and its current limitations and challenges, with particular attention on pluripotent stem cell-derived products to repair organs like the eye, heart, skeletal muscle and skin. We then focus on emerging beneficial roles of extracellular vesicles to alleviate these pathological conditions and address hurdles and operational issues of this acellular strategy. Finally, we discuss future directions and examine how careful integration of different approaches presented in this review could help to potentiate therapeutic results in preclinical models and their good manufacturing practice (GMP) implementation for future clinical trials.

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

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.

Advancing a Stem Cell Therapy for Age-Related Macular Degeneration

The retinal pigment epithelium (RPE) is a multifunctional monolayer located at the back of the eye required for the survival and function of the light-sensing photoreceptors. In Age-related Macular Degeneration (AMD), the loss of RPE cells leads to photoreceptor death and permanent blindness. RPE cell transplantation aims to halt or reverse vision loss by preventing the death of photoreceptor cells and is considered one of the most viable applications of stem cell therapy in the field of regenerative medicine. Proof-of-concept of RPE cell transplantation for treating retinal degenerative disease, such as AMD, has long been established in animal models and humans using primary RPE cells, while recent research has focused on the transplantation of RPE cells derived from human pluripotent stem cells (hPSC). Early results from clinical trials indicate that transplantation of hPSC-derived RPE cells is safe and can improve vision in AMD patients. Current hPSC-RPE cell production protocols used in clinical trials are nevertheless inefficient. Treatment of large numbers of AMD patients using stem cellderived products may be dependent on the ability to generate functional cells from multiple hPSC lines using robust and clinically-compliant methods. Transplantation outcomes may be improved by delivering RPE cells on a thin porous membrane for better integration into the retina, and by manipulation of the outcome through control of immune rejection and inflammatory responses.

Plasma polymer surface modified expanded polytetrafluoroethylene promotes epithelial monolayer formation in vitro and can be transplanted into the dystrophic rat subretinal space

The aim of this study was to evaluate whether the surface modification of expanded polytetrafluoroethylene (ePTFE) using an n-heptylamine (HA) plasma polymer would allow for functional epithelial monolayer formation suitable for subretinal transplant into a non-dystrophic rat model. Freshly isolated iris pigment epithelial (IPE) cells from two rat strains (Long Evans [LE] and Dark Agouti [DA]) were seeded onto HA, fibronectin-coated n-heptylamine modified (F-HA) and unmodified ePFTE and fibronectin-coated tissue culture (F-TCPS) substrates. Both F-HA ePTFE and F-TCPS substrates enabled functional monolayer formation with both strains of rat. Without fibronectin coating, only LE IPE formed a monolayer on HA-treated ePTFE. Functional assessment of both IPE strains on F-HA ePTFE demonstrated uptake of POS that increased significantly with time that was greater than control F-TCPS. Surgical optimization using Healon GV and mixtures of Healon GV: phosphate buffered saline (PBS) to induce retinal detachment demonstrated that only Healon GV:PBS allowed F-HA ePTFE substrates to be successfully transplanted into the subretinal space of Royal College of Surgeons rats, where they remained flat beneath the neural retina for up to 4 weeks. No apparent substrate-induced inflammatory response was observed by fundus microscopy or immunohistochemical analysis, indicating the potential of this substrate for future clinical applications.

The choice of biopolymer is crucial to trigger angiogenesis with vascular endothelial growth factor releasing coatings

Bio-based coatings and release systems for pro-angiogenic growth factors are of interest to overcome insufficient vascularization and bio-integration of implants. This study compares different biopolymer-based coatings on polyethylene terephthalate (PET) membranes in terms of coating homogeneity and stability, coating thickness in the swollen state, endothelial cell adhesion, vascular endothelial growth factor (VEGF) release and pro-angiogenic properties. Coatings consisted of carbodiimide cross-linked gelatin type A (GelA), type B (GelB) or albumin (Alb), and heparin (Hep), or they consisted of radically cross-linked gelatin methacryloyl-acetyl (GM5A5) and heparin methacrylate (HepM5). We prepared films with thicknesses of 8-10 µm and found that all coatings were homogeneous after washing. All gelatin-based coatings enhanced the adhesion of primary human endothelial cells compared to the uncoated membrane. The VEGF release was tunable with the loading concentration and dependent on the isoelectric points and hydrophilicities of the biopolymers used for coating: GelA-Hep showed the highest releases, while releases were indistinguishable for GelB-Hep and Alb-Hep, and lowest for GM5A5-HepM5. Interestingly, not only the amount of VEGF released from the coatings determined whether angiogenesis was induced, but a combination of VEGF release, metabolic activity and adhesion of endothelial cells. VEGF releasing GelA-Hep and GelB-Hep coatings induced angiogenesis in a chorioallantoic membrane assay, so that these coatings should be considered for further in vivo testing.

Functional assessment of cryopreserved clinical grade hESC-RPE cells as a qualified cell source for stem cell therapy of retinal degenerative diseases

The biosafety and efficiency of transplanting retinal pigment epithelial (RPE) cells derived from both human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have been evaluated in phase I and phase II clinical trials. For further large-scale application, cryopreserved RPE cells must be used; thus, it is highly important to investigate the influence of cryopreservation and thawing on the biological characteristics of hESC-RPE cells and their post-transplantation vision-restoring function. Here, via immunofluorescence, qPCR, transmission electron microscopy, transepithelial electrical resistance, and enzyme-linked immunosorbent assays (ELISAs), we showed that cryopreserved hESC-RPE cells retained the specific gene expression profile, morphology, ultrastructure, and maturity-related functions of induced RPE cells. Additionally, cryopreserved hESC-RPE cells exhibited a polarized monolayer, tight junction, and gap junction structure and an in vitro nanoparticle phagocytosis capability similar to those of induced hESC-RPE cells. However, the level of pigment epithelium-derived factor (PEDF) secretion was significantly decreased in cryopreserved hESC-RPE cells. Royal College of Surgeons rats with cryopreserved hESC-RPE cells engrafted into the subretinal space exhibited a significant decrease in the b-wave amplitude compared with rats engrafted with induced hESC-RPE cells at 4 weeks post transplantation. However, the difference disappeared at 8 weeks and 12 weeks post operation. No significant difference in the outer nuclear layer (ONL) thickness was observed between the two groups. Our data showed that even after cryopreservation and thawing, cryopreserved hESC-RPE cells are still qualified as a donor cell source for cell-based therapy of retinal degenerative diseases.

Embryonic stem cell microenvironment enhances proliferation of human retinal pigment epithelium cells by activating the PI3K signaling pathway

BACKGROUND

Retinal pigment epithelium (RPE) replacement has been proposed as an efficacious treatment for age-related macular degeneration (AMD), which is the primary cause of vision loss in the elderly worldwide. The embryonic stem cell (ESC) microenvironment has been demonstrated to enable mature cells to gain a powerful proliferative ability and even enhance the stem/progenitor phenotype via activation of the phosphoinositide 3-kinase (PI3K) signaling pathway. As the PI3K signaling pathway plays a pivotal role in proliferation and homeostasis of RPE, we hypothesize that the stemness and proliferative capability of RPE can be enhanced by the ESC microenvironment via activation of the PI3K signaling pathway.

METHODS

To investigate whether the ESC microenvironment improves the stem cell phenotype and proliferation properties of human RPE (hRPE) cells by regulating the PI3K signaling pathway, primary hRPE cells were cocultured with either ESCs or human corneal epithelial cells (CECs) for 72 h, after which their proliferation, apoptosis, cell cycle progression, and colony formation were assayed to evaluate changes in their biological characteristics. Gene expression was detected by real-time PCR and protein levels were determined by western blotting or immunofluorescence. LY294002, an antagonist of the PI3K signaling pathway, was used to further confirm the mechanism involved.

RESULTS

In comparison to hRPE cells cultured alone, hRPE cells cocultured with ESCs had an increased proliferative capacity, reduced apoptotic rate, and higher colony-forming efficiency. The expression of the stem cell-associated marker KLF4 and the differentiation marker CRALBP increased and decreased, respectively, in hRPE cells isolated from the ESC coculture. Furthermore, PI3K pathway-related genes were significantly upregulated in hRPE cells after exposure to ESCs. LY294002 reversed the pro-proliferative effect of ESCs on hRPE cells. In contrast, CECs did not share the ability of ESCs to influence the biological behavior and gene expression of hRPE cells.

CONCLUSIONS

Our findings indicate that the ESC microenvironment enhances stemness and proliferation of hRPE cells, partially via activation of the PI3K signaling pathway. This study may have a significant impact and clinical implication on cell therapy in regenerative medicine, specifically for age-related macular degeneration.

Human pluripotent stem cells: A toolbox to understand and treat retinal degeneration

Age-related Macular Degeneration (AMD) and Retinitis Pigmentosa (RP) are retinal degenerative disorders that dramatically damage the retina. As there is no therapeutic option for the majority of patients, vision is progressively and irremediably lost. Owing to their unlimited renewal and potency to give rise to any cell type of the human adult body, human pluripotent stem cells (hPSCs) have been extensively studied in recent years to develop more physiologically relevant in vitro cellular models. Such models open new perspectives to investigate the pathological molecular mechanisms of AMD and RP but also in drug screening. Moreover, proof-of-concept of hPSC-derived retinal cell therapy in animal models have led to first clinical trials. This review outlines the recent advances in the use of hPSCs in pathological modeling of retinal degeneration and their use in regenerative medicine. We also address the associated limitations and challenges that need to be overcome when using hPSCs.

Microphysiological Systems: Design, Fabrication, and Applications

Microphysiological systems, including organoids, 3-D printed tissue constructs and organ-on-a-chips (organ chips), are physiologically relevant in vitro models and have experienced explosive growth in the past decades. Different from conventional, tissue culture plastic-based in vitro models or animal models, microphysiological systems recapitulate key microenvironmental characteristics of human organs and mimic their primary functions. The advent of microphysiological systems is attributed to evolving biomaterials, micro-/nanotechnologies and stem cell biology, which enable the precise control over the matrix properties and the interactions between cells, tissues and organs in physiological conditions. As such, microphysiological systems have been developed to model a broad spectrum of organs from microvasculature, eye, to lung and many others to understand human organ development and disease pathology and facilitate drug discovery. Multiorgans-on-a-chip systems have also been developed by integrating multiple associated organ chips in a single platform, which allows to study and employ the organ function in a systematic approach. Here we first discuss the design principles of microphysiological systems with a focus on the anatomy and physiology of organs, and then review the commonly used fabrication techniques and biomaterials for microphysiological systems. Subsequently, we discuss the recent development of microphysiological systems, and provide our perspectives on advancing microphysiological systems for preclinical investigation and drug discovery of human disease.

Quantifiable In Vivo Imaging Biomarkers of Retinal Regeneration by Photoreceptor Cell Transplantation

Purpose

Short-term improvements in retinal anatomy are known to occur in preclinical models of photoreceptor transplantation. However, correlative changes over the long term are poorly understood. We aimed to develop a quantifiable imaging biomarker grading scheme, using noninvasive multimodal confocal scanning laser ophthalmoscopy (cSLO) imaging, to enable serial evaluation of photoreceptor transplantation over the long term.

Methods

Photoreceptor cell suspensions or sheets from rhodopsin-green fluorescent protein mice were transplanted subretinally, into either NOD.CB17-Prkdc^scid/J or C3H/HeJ-Pde6b^rd1 mice. Multimodal cSLO imaging was performed serially for up to three months after transplantation. Imaging biomarkers were scored, and a grade was defined for each eye by integrating the scores. Image grades were correlated with immunohistochemistry (IHC) data.

Results

Multimodal imaging enabled the extraction of quantitative imaging biomarkers including graft size, GFP intensity, graft length, on-target graft placement, intra-graft lamination, hemorrhage, retinal atrophy, and periretinal proliferation. Migration of transplanted material was observed. Changes in biomarker scores and grades were detected in 14/16 and 7/16 eyes, respectively. A high correlation was found between image grades and IHC parameters.

Conclusions

Serial evaluation of multiple imaging biomarkers, when integrated into a per-eye grading scheme, enabled comprehensive tracking of longitudinal changes in photoreceptor cell grafts over time. The application of systematic multimodal in vivo imaging could be useful in increasing the efficiency of preclinical retinal cell transplantation studies in rodents and other animal models.

Translational Relevance

By allowing longitudinal evaluation of the same animal over time, and providing quantifiable biomarkers, non-invasive multimodal imaging improves the efficiency of retinal transplantation studies in animal models. Such assays will facilitate the development of cell therapy for retinal diseases.

A Strategy for Personalized Treatment of iPS-Retinal Immune Rejections Assessed in Cynomolgus Monkey Models

Recently, we successfully transplanted an autograft, or major histocompatibility complex (MHC)-matched allografts, from induced-pluripotent-stem-cell-derived retinal pigment epithelial (iPSC-RPE) cells in patients with age-related macular degeneration. However, there was an issue regarding immune rejection after transplantation. In this study, we established a preoperational in vitro "drug-lymphocytes-grafts immune reaction (Drug-LGIR)" test to determine the medication for immune rejection using host immunocompetent cells (lymphocytes) and transplant cells (target iPSC-RPE cells) together with different medications. The adequacy of the test was assessed by in vivo transplantation in monkey models together with medication based on in vitro data. In the results of Drug-LGIR tests, some drugs exhibited significant suppression of RPE cell-related allogeneic reactions, while other drugs did not, and the efficacy of each drug differed among the recipient monkeys. Based on the results of Drug-LGIR, we applied cyclosporine A or local steroid (triamcinolone) therapy to two monkeys, and successfully suppressed RPE-related immune rejections with RPE grafts, which survived without any signs of rejection under drug administration. We propose that our new preoperational in vitro Drug-LGIR test, which specifies the most efficacious medication for each recipient, is useful for controlling immune attacks with personalized treatment for each patient after retinal transplantation.

Generation of Retinal Pigment Epithelial Cells Derived from Human Embryonic Stem Cells Lacking Human Leukocyte Antigen Class I and II

Human embryonic stem cell-derived retinal pigment epithelial (hESC-RPE) cells could serve as a replacement therapy in advanced stages of age-related macular degeneration. However, allogenic hESC-RPE transplants trigger immune rejection, supporting a strategy to evade their immune recognition. We established single-knockout beta-2 microglobulin (SKO-B2M), class II major histocompatibility complex transactivator (SKO-CIITA) and double-knockout (DKO) hESC lines that were further differentiated into corresponding hESC-RPE lines lacking either surface human leukocyte antigen class I (HLA-I) or HLA-II, or both. Activation of CD4+ and CD8+ T-cells was markedly lower by hESC-RPE DKO cells, while natural killer cell cytotoxic response was not increased. After transplantation of SKO-B2M, SKO-CIITA, or DKO hESC-RPEs in a preclinical rabbit model, donor cell rejection was reduced and delayed. In conclusion, we have developed cell lines that lack both HLA-I and -II antigens, which evoke reduced T-cell responses in vitro together with reduced rejection in a large-eyed animal model.

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hESC-RPEs are immunosuppressive but can elicit T-cell and NK cell responses in vitro

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hESC-RPEs lacking HLA-I and -II evade T-cell response

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hESC-RPEs lacking HLA-I and -II do not increase NK cell cytotoxic activity

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When xeno-transplanted, these immune-modified hESC-RPEs show reduced rejection

Retinal stem cell transplantation: Balancing safety and potential

Stem cell transplantation holds great promise as a potential treatment for currently incurable retinal degenerative diseases that cause poor vision and blindness. Recently, safety data have emerged from several Phase I/II clinical trials of retinal stem cell transplantation. These clinical trials, usually run in partnership with academic institutions, are based on sound preclinical studies and are focused on patient safety. However, reports of serious adverse events arising from cell therapy in other poorly regulated centers have now emerged in the lay and scientific press. While progress in stem cell research for blindness has been greeted with great enthusiasm by patients, scientists, doctors and industry alike, these adverse events have raised concerns about the safety of retinal stem cell transplantation and whether patients are truly protected from undue harm. The aim of this review is to summarize and appraise the safety of human retinal stem cell transplantation in the context of its potential to be developed into an effective treatment for retinal degenerative diseases.

Bio-inspired human in vitro outer retinal models: Bruch's membrane and its cellular interactions

Retinal degenerative disorders, such as age-related macular degeneration (AMD), are one of the leading causes of blindness worldwide, however, treatments to completely stop the progression of these debilitating conditions are non-existent. Researchers require sophisticated models that can accurately represent the native structure of human retinal tissue to study these disorders. Current in vitro models used to study the retina are limited in their ability to fully recapitulate the structure and function of the retina, Bruch's membrane and the underlying choroid. Recent developments in the field of induced pluripotent stem cell technology has demonstrated the capability of retinal pigment epithelial cells to recapitulate AMD-like pathology. However, such studies utilise unsophisticated, bio-inert membranes to act as Bruch's membrane and support iPSC-derived retinal cells. This review presents a concise summary of the properties and function of the Bruch's membrane-retinal pigment epithelium complex, the initial pathogenic site of AMD as well as the current status for materials and fabrication approaches used to generate in vitro models of this complex tissue. Finally, this review explores required advances in the field of in vitro retinal modelling. STATEMENT OF SIGNIFICANCE: Retinal degenerative disorders such as age-related macular degeneration are worldwide leading causes of blindness. Previous attempts to model the Bruch's membrane-retinal pigment epithelial complex, the initial pathogenic site of age-related macular degeneration, have lacked the sophistication to elucidate valuable insights into disease mechanisms. Here we provide a detailed account of the morphological, physical and chemical properties of Bruch's membrane which may aid the fabrication of more sophisticated and physiologically accurate in vitro models of the retina, as well as various fabrication techniques to recreate this structure. This review also further highlights some recent advances in some additional challenging aspects of retinal tissue modelling including integrated fluid flow and photoreceptor alignment.

Sphere-induced reprogramming of RPE cells into dual-potential RPE stem-like cells

Background

The retinal pigment epithelium (RPE) has the potential to regenerate the entire neuroretina upon retinal injury in amphibians. In contrast, this regenerative capacity has been lost in mammals. The reprogramming of differentiated somatic cells into induced pluripotent stem cells (iPSCs) by viral transduction of exogenous stem cell factors has triggered a revolution in regenerative medicine. However, the risks of potential mutation(s) caused by random viral vector insertion in host genomes and tumor formation in recipients hamper its clinical application. One alternative is to immortalize adult stem cells with limited potential or to partially reprogram differentiated somatic cells into progenitor-like cells through non-integration protocols.

Methods

Sphere-induced RPE stem cells (iRPESCs) were generated from adult mouse RPE cells. Their stem cell functionality was studied in a mouse model of retinal degeneration. The molecular mechanism underlying the sphere-induced reprogramming was investigated using microarray and loss-of-function approaches.

Findings

We provide evidence that our sphere-induced reprogramming protocol can immortalize and transform mouse RPE cells into iRPESCs with dual potential to differentiate into cells that express either RPE or photoreceptor markers both in vitro and in vivo. When subretinally transplanted into mice with retinal degeneration, iRPESCs can integrate to the RPE and neuroretina, thereby delaying retinal degeneration in the model animals. Our molecular analyses indicate that the Hippo signaling pathway is important in iRPESC reprogramming.

Interpretation

The Hippo factor Yap1 is activated in the nuclei of cells at the borders of spheres. The factors Zeb1 and P300 downstream of the Hippo pathway are shown to bind to the promoters of the stemness genes Oct4, Klf4 and Sox2, thereby likely transactivate them to reprogram RPE cells into iRPESCs.

Fund

National Natural Science Foundation of China and the National Institute of Health USA.

Interleukin-13 Gene Modification Enhances Grafted Mesenchymal Stem Cells Survival After Subretinal Transplantation

Mesenchymal stem cells (MSCs) hold great potential for cell- and gene-based therapies for retinal degeneration. Limited survival is the main obstacle in achieving successful subretinal transplantation of MSCs. The present study sought to evaluate the effect of interleukin-13 (IL-13) gene modification on the phenotypic alteration of retinal microglia (RMG) and the survival of MSCs following subretinal grafting. In this study, LPS-activated RMG were cocultured with MSCs or IL-13-expressing MSCs (IL-13-MSCs) for 24 h, and activated phenotypes were detected in vitro. Western blotting was performed to quantify cytokine secretion by light-injured retinas following subretinal transplantation. The numbers of activated RMG and surviving grafted cells were analysed, and the integrity of the blood-retinal barrier (BRB) was examined in vivo. We found that, compared with normal MSCs, cocultured IL-13-MSCs suppressed the expression of pro-inflammatory factors and major histocompatibility complex II, promoted the expression of anti-inflammatory cytokines by activated RMG and simultaneously inhibited the proliferation of and phagocytosis by RMG. The subretinal transplantation of IL-13-MSCs increased the expression of neurotrophic factors, IL-13 and tight junction proteins in the host retina, decreased the number of phagocytic RMG and improved the survival of grafted cells. Furthermore, IL-13-MSCs alleviated BRB breakdown induced by subretinal injection. Our results demonstrate that IL-13-MSCs can polarize activated RMG to the neuroprotective M2 phenotype and enhance the survival of grafted MSCs against the damage stress induced by subretinal transplantation.

Regenerative medicine as a novel strategy for AMD treatment: a review

Age-related macular degeneration (AMD) is known as a major cause of irreversible blindness in elderly adults. The segment of the retina responsible for central vision damages in the disease process. Degeneration of retinal pigmented epithelium (RPE) cells, photoreceptors, and choriocapillaris associated with aging participate for visual loss. In 2010, AMD involved 6.6% of all blindness cases around the world. Some of the researches have evaluated the replacing of damaged RPE in AMD patients by using the cells from various sources. Today, the advancement of RPE differentiation or generation from stem cells has been gained, and currently, clinical trials are testing the efficiency and safety of replacing degenerated RPE with healthy RPE. However, the therapeutic success of RPE transplantation may be restricted unless the transplanted cells can be adhered, distributed and survive for long-term in the transplanted site without any infections. In recent years a variety of scaffold types were used as a carrier for RPE transplantation and AMD treatment. In this review, we have discussed types of scaffolds; natural or synthetic, solid or hydrogel and their results in RPE replacement. Eventually, our aim is highlighting the novel and best scaffold carriers that may have potentially promoting the efficacy of RPE transplantation.

Clinical-grade production and safe delivery of human ESC derived RPE sheets in primates and rodents

Age-related macular degeneration as well as some forms of Retinitis Pigmentosa (RP) are characterized by a retinal degeneration involving the retinal pigment epithelium (RPE). Various strategies were proposed to cure these disorders including the replacement of RPE cells using human pluripotent stem cells (hPSCs), an unlimited source material to generate in vitro RPE cells. The formulation strategy of the cell therapy (either a reconstructed sheet or a cell suspension) is crucial to achieve an efficient and long lasting therapeutic effect. We previously developed a hPSC-RPE sheet disposed on human amniotic membrane that sustained the vision of rodents with retinal degeneration compared to the same cells injected as a suspension. However, the transplantation strategy was difficult to implement in large animals. Herein we developed two medical devices for the preparation, conservation and implantation of the hPSC-RPE sheet in nonhuman primates. The surgery was safe and well tolerated during the 7-week follow up. The graft integrity was preserved in primates. Moreover, the hPSC-RPE sheet did not induce teratoma or grafted cell dispersion to other organs in rodent models. This work clears the way for the first cell therapy for RP patients carrying RPE gene mutations (LRAT, RPE65 and MERTK).

Survival and functionality of xeno-free human embryonic stem cell-derived retinal pigment epithelial cells on polyester substrate after transplantation in rabbits

PURPOSE

To study immunogenic properties of human embryonic stem cell-derived retinal pigment epithelium (hESC-RPE) and to evaluate subretinal xenotransplantation of hESC-RPE on porous polyethylene terephthalate (PET) in rabbits.

METHODS

Human ESC-RPE cells were characterized by morphology, transepithelial electrical resistance (TER), protein expression and photoreceptor outer segment phagocytosis in vitro. Expression of major histocompatibility complex (MHC) proteins was assessed in conventionally or xeno-free produced hESC-RPE ± interferon-gamma (IFN-γ) stimulation (n = 1). Xeno-free hESC-RPE on PET with TER < 200 Ω·cm²  > or PET alone were transplanted into 18 rabbits with short-term triamcinolone ± extended tacrolimus immunosuppression. Rabbits were monitored by spectral domain optical coherence tomography. After 4 weeks, the eyes were processed for histology and transmission electron microscopy.

RESULTS

Upon in vitro IFN-γ stimulation, xeno-free hESC-RPE expressed lower level of MHC-II proteins compared to the conventional cells. Outer nuclear layer (ONL) atrophy was observed over the graft in most cases 4 weeks post-transplantation. In 3/4 animals with high TER hESC-RPE, but only in 1/3 animals with low TER hESC-RPE, ONL atrophy was observed already within 1 week. Retinal cell infiltrations were more frequent in animals with high TER hESC-RPE. However, the difference was not statistically significant. In three animals, preservation of ONL was observed. Weekly intravitreal tacrolimus did not affect ONL preservation. In all animals, hESC-RPE cells survived for 4 weeks, but without tacrolimus, enlarged vacuoles accumulated in hESC-RPE (n = 1).

CONCLUSIONS

Xenografted xeno-free hESC-RPE monolayers can survive and retain some functionality for 4 weeks following short-term immunosuppression. The preliminary findings of this study suggest that further investigations to improve transplantation success of hESC-RPE xenografts in rabbits should be addressed especially toward the roles of hESC-RPE maturation stage and extended intravitreal immunosuppression.

Human iPSC-Derived Retinal Pigment Epithelium: A Model System for Prioritizing and Functionally Characterizing Causal Variants at AMD Risk Loci

We evaluate whether human induced pluripotent stem cell-derived retinal pigment epithelium (iPSC-RPE) cells can be used to prioritize and functionally characterize causal variants at age-related macular degeneration (AMD) risk loci. We generated iPSC-RPE from six subjects and show that they have morphological and molecular characteristics similar to those of native RPE. We generated RNA-seq, ATAC-seq, and H3K27ac ChIP-seq data and observed high similarity in gene expression and enriched transcription factor motif profiles between iPSC-RPE and human fetal RPE. We performed fine mapping of AMD risk loci by integrating molecular data from the iPSC-RPE, adult retina, and adult RPE, which identified rs943080 as the probable causal variant at VEGFA. We show that rs943080 is associated with altered chromatin accessibility of a distal ATAC-seq peak, decreased overall gene expression of VEGFA, and allele-specific expression of a non-coding transcript. Our study thus provides a potential mechanism underlying the association of the VEGFA locus with AMD.

Characterization of Human iPSC-RPE on a Prosthetic Bruch's Membrane Manufactured From Silk Fibroin

Purpose

RPE cell transplantation as a potential treatment for AMD has been extensively investigated; however, in AMD, ultrastructural damage affects both the RPE and its underlying matrix support, the Bruch's membrane (BrM). An RPE monolayer supported by a surrogate scaffold could thus provide a more effective approach to cell-based therapy for AMD. Toward this goal, we aimed to establish a functional human induced pluripotent stem cell-derived (hiPSC)-RPE monolayer on a Bombyx mori silk fibroin (BMSF) scaffold.

Methods

RPE differentiated from five distinct hiPSC lines were cultured on BMSF membrane coated with extracellular matrix (ECM, COL1), and either regular tissue culture plastic or Transwell coated with ECM (LAM-TCP). Morphologic, gene and protein expression, and functional characteristics of the hiPSC-RPE cultured on different membranes were compared in longitudinal experiments spanning 1 day to ≥3 months.

Results

The hiPSC-RPE monolayers on ECM-coated BMSF and TCP could be maintained in culture for ≥3 months and displayed RPE-characteristic morphology, pigmentation, polarity, and expression of RPE signature genes and proteins. Furthermore, hiPSC-RPE on both ECM-coated BMSF and TCP displayed robust expression and secretion of several basement membrane proteins. Importantly, hiPSC-RPE cells on COL1-BMSF and LAM-TCP showed similar efficacy in the phagocytosis and degradation of photoreceptor outer segments.

Conclusions

A biomaterial scaffold manufactured from silk fibroin supports the maturation and long-term survival of a functional hiPSC-RPE monolayer. This has significant implications for both in vitro disease modeling and in vivo cell replacement therapy.

Adult Stem Cells, Tools for Repairing the Retina

Purpose

Retinal degenerative diseases lead to the death of retinal neurons causing visual impairment and blindness. In lower order vertebrates, the retina and its surrounding tissue contain stem cell niches capable of regenerating damaged tissue. Here we examine these niches and review their capacity to be used as retinal stem/progenitor cells (RSC/RPCs) for retinal repair.

Recent Findings

Exogenous factors can control the in vitro activation of RSCs/PCs found in several niches within the adult eye including cells in the ciliary margin, the retinal pigment epithelium, iris pigment epithelium as well as the inducement of Müller and amacrine cells within the neural retina itself. Recently, factors have been identified for the activation of adult mammalian Müller cells to a RPC state in vivo.

Summary

Whereas cell transplantation still holds potential for retinal repair, activation of the dormant native regeneration process may lead to a more successful process including greater integration efficiency and proper synaptic targeting.