Niche regulation of corneal epithelial stem cells at the limbus

Targeting limbal epithelial stem cells: master conductors of corneal epithelial regeneration from the bench to multilevel theranostics

The cornea is the outermost layer of the eye and plays an essential role in our visual system. Limbal epithelial stem cells (LESCs), which are localized to a highly regulated limbal niche, are the master conductors of corneal epithelial regeneration. Damage to LESCs and their niche may result in limbal stem cell deficiency (LSCD), a disease confused ophthalmologists so many years and can lead to corneal conjunctivalization, neovascularization, and even blindness. How to restore the LESCs function is the hot topic for ocular scientists and clinicians around the world. This review introduced LESCs and the niche microenvironment, outlined various techniques for isolating and culturing LESCs used in LSCD research, presented common diseases that cause LSCD, and provided a comprehensive overview of both the diagnosis and multiple treatments for LSCD from basic research to clinical therapies, especially the emerging cell therapies based on various stem cell sources. In addition, we also innovatively concluded the latest strategies in recent years, including exogenous drugs, tissue engineering, nanotechnology, exosome and gene therapy, as well as the ongoing clinical trials for treating LSCD in recent five years. Finally, we highlighted challenges from bench to bedside in LSCD and discussed cutting-edge areas in LSCD therapeutic research. We hope that this review could pave the way for future research and translation on treating LSCD, a crucial step in the field of ocular health.

Culturing Limbal Epithelial Cells of Long-term Stored Corneal Donors (Organ Culture) In Vitro - A Stepwise Linear Regression Algorithm

PURPOSE

To assess various potential factors on human limbal epithelial cell (LEC) outgrowth in vitro using corneal donor tissue following long-term storage (organ culture) and a stepwise linear regression algorithm.

METHODS

Of 215 donors, 304 corneoscleral rings were used for our experiments. For digestion of the limbal tissue and isolation of the limbal epithelial cells, the tissue pieces were incubated with 4.0 mg/mL collagenase A at 37 °C with 95% relative humidity and a 5% CO2 atmosphere overnight. Thereafter, limbal epithelial cells were separated from limbal keratocytes using a 20-µm CellTricks filter. The separated human LECs were cultured in keratinocyte serum-free medium medium, 1% penicillin/streptomycin (P/S), 0.02% epidermal growth factor (EGF), and 0.3% bovine pituitary extract (BPE). The potential effect of donor age (covariate), postmortem time (covariate), medium time (covariate), size of the used corneoscleral ring (360°, 270°180°, 120°, 90°, less than 90°) (covariate), endothelial cell density (ECD) (covariate), gender (factor), number of culture medium changes during organ culture (factor), and origin of the donor (donating institution and storing institution, factor) on the limbal epithelial cell outgrowth was analyzed with a stepwise linear regression algorithm.

RESULTS

The rate of successful human LEC outgrowth was 37.5%. From the stepwise linear regression algorithm, we found out that the relevant influencing parameters on the LEC growth were intercept (p < 0.001), donor age (p = 0.002), number of culture medium changes during organ culture (p < 0.001), total medium time (p = 0.181), and size of the used corneoscleral ring (p = 0.007), as well as medium time × size of the corneoscleral ring (p = 0.007).

CONCLUSIONS

The success of LEC outgrowth increases with lower donor age, lower number of organ culture medium changes during storage, shorter medium time in organ culture, and smaller corneoscleral ring size. Our stepwise linear regression algorithm may help us in optimizing LEC cultures in vitro.

A novel tissue-engineered corneal epithelium based on ultra-thin amniotic membrane and mesenchymal stem cells

Currently, in vitro cultured corneal epithelial transplantation is effective in treating limbal stem cell dysfunction (LSCD). Selecting carriers is crucial for constructing the corneal epithelium through tissue engineering. In this study, the traditional amniotic membrane (AM) was modified, and mesenchymal stem cells (MSCs) were inoculated into the ultra-thin amniotic membrane (UAM) stroma to construct a novel UAM-MSC tissue-engineered corneal epithelial carrier, that could effectively simulate the limbal stem cells (LSCs) microenvironment. The structure of different carriers cultured tissue-engineered corneal epithelium and the managed rabbit LSCD model corneas were observed through hematoxylin-eosin staining. Cell phenotypes were evaluated through fluorescence staining, Western blotting, and RT-qPCR. Additionally, cell junction genes and expression markers related to anti-neovascularization were evaluated using RT-qPCR. Corneal epithelium cell junctions were observed via an electron microscope. The tissue-engineered corneal epithelium culture medium was analyzed through mass spectrometry. Tissue-engineered corneal epithelial cells expanded by LSCs on UAM-MSCs had good transparency. Simultaneously, progenitor cell (K14, PNCA, p63) and corneal epithelial (PAX6) gene expression in tissue-engineered corneal epithelium constructed using UAM-MSCs was higher than that in corneal epithelial cells amplified by UAM and de-epithelialized amniotic membrane. Electron microscopy revealed that corneal epithelial cells grafted with UAM-MSCs were closely connected. In conclusion, the UAM-MSCs vector we constructed could better simulate the limbal microenvironment; the cultured tissue-engineered corneal epithelium had better transparency, anti-neovascularization properties, closer intercellular connections, and closer resemblance to the natural corneal epithelial tissue phenotype.

Focus on seed cells: stem cells in 3D bioprinting of corneal grafts

Corneal opacity is one of the leading causes of severe vision impairment. Corneal transplantation is the dominant therapy for irreversible corneal blindness. However, there is a worldwide shortage of donor grafts and consequently an urgent demand for alternatives. Three-dimensional (3D) bioprinting is an innovative additive manufacturing technology for high-resolution distribution of bioink to construct human tissues. The technology has shown great promise in the field of bone, cartilage and skin tissue construction. 3D bioprinting allows precise structural construction and functional cell printing, which makes it possible to print personalized full-thickness or lamellar corneal layers. Seed cells play an important role in producing corneal biological functions. And stem cells are potential seed cells for corneal tissue construction. In this review, the basic anatomy and physiology of the natural human cornea and the grafts for keratoplasties are introduced. Then, the applications of 3D bioprinting techniques and bioinks for corneal tissue construction and their interaction with seed cells are reviewed, and both the application and promising future of stem cells in corneal tissue engineering is discussed. Finally, the development trends requirements and challenges of using stem cells as seed cells in corneal graft construction are summarized, and future development directions are suggested.

Decoding cellular plasticity and niche regulation of limbal stem cells during corneal wound healing

BACKGROUND

Dysfunction or deficiency of corneal epithelium results in vision impairment or blindness in severe cases. The rapid and effective regeneration of corneal epithelial cells relies on the limbal stem cells (LSCs). However, the molecular and functional responses of LSCs and their niche cells to injury remain elusive.

METHODS

Single-cell RNA sequencing was performed on corneal tissues from normal mice and corneal epithelium defect models. Bioinformatics analysis was performed to confirm the distinct characteristics and cell fates of LSCs. Knockdown of Creb5 and OSM treatment experiment were performed to determine their roles of in corneal epithelial wound healing.

RESULTS

Our data defined the molecular signatures of LSCs and reconstructed the pseudotime trajectory of corneal epithelial cells. Gene network analyses characterized transcriptional landmarks that potentially regulate LSC dynamics, and identified a transcription factor Creb5, that was expressed in LSCs and significantly upregulated after injury. Loss-of-function experiments revealed that silencing Creb5 delayed the corneal epithelial healing and LSC mobilization. Through cell-cell communication analysis, we identified 609 candidate regeneration-associated ligand-receptor interaction pairs between LSCs and distinct niche cells, and discovered a unique subset of Arg1+ macrophages infiltrated after injury, which were present as the source of Oncostatin M (OSM), an IL-6 family cytokine, that were demonstrated to effectively accelerate the corneal epithelial wound healing.

CONCLUSIONS

This research provides a valuable single-cell resource and reference for the discovery of mechanisms and potential clinical interventions aimed at ocular surface reconstruction.

Mitigation of pesticide-mediated ocular toxicity via nanotechnology-based contact lenses: a review

The xenobiotic stress exerted by pesticides leads to the deterioration of human and animal health including ocular health. Acute or prolonged exposure to these agricultural toxicants has been implicated in a number of pathological conditions of the eye such as irritation, epiphora or hyper-lacrimation, abrasions on the ocular surface, and decreased visual acuity. The issue is compounded by the fact that tissues of the eye absorb pesticides faster than other organs of the body and are more susceptible to damage as well. However, there is a lacuna in our knowledge regarding the ways by which pesticide exposure-mediated ocular insult might be counteracted. Topical instillation of drugs known to combat the pesticide induced toxicity has been explored to mitigate the detrimental impact of pesticide exposure. However, topical eye drop solutions exhibit very low bioavailability and limited drug residence duration in the tear film decreasing their efficacy. Contact lenses have been explored in this respect to increase bioavailability of ocular drugs, while nanoparticles have lately been utilized to increase drug bioavailability and increase drug residence duration in different tissues. The current review focuses on drug delivery and futuristic aspects of corneal protection from ocular toxicity using contact lenses.

Design principles and therapeutic applications of novel synthetic WNT signaling agonists

Wingless-related integration site or Wingless and Int-1 or Wingless-Int (WNT) signaling is crucial for embryonic development, and adult tissue homeostasis and regeneration, through its essential roles in cell fate, patterning, and stem cell regulation. The biophysical characteristics of WNT ligands have hindered efforts to interrogate ligand activity in vivo and prevented their development as therapeutics. Recent breakthroughs have enabled the generation of synthetic WNT signaling molecules that possess characteristics of natural ligands and potently activate the pathway, while also providing distinct advantages for therapeutic development and manufacturing. This review provides a detailed discussion of the protein engineering of these molecular platforms for WNT signaling agonism. We discuss the importance of WNT signaling in several organs and share insights from the initial application of these new classes of molecules in vitro and in vivo. These molecules offer a unique opportunity to enhance our understanding of how WNT signaling agonism promotes tissue repair, enabling targeted development of tailored therapeutics.

The Utilization of Topical Insulin for Ocular Surface Diseases: A Narrative Review

Various etiologies, including diabetic keratopathy (DK), dry eye disease (DED), and neurotrophic keratopathy (NK), can disrupt corneal homeostasis, exacerbating corneal epithelial defects. Topical insulin has emerged as a promising therapy for promoting corneal wound healing and addressing underlying pathologies. This review systematically evaluates the efficacy of topical insulin across different corneal disorders. A literature review was conducted across the PubMed, Google Scholar, and Scopus research databases. The search resulted in a total of 19 articles, consisting of clinical trials, retrospective studies, and case reports. In DK, topical insulin accelerates corneal wound healing post-vitreoretinal surgery with lower concentrations showing higher outcomes when compared to conventional therapy, possibly due to improved epithelial stem cell migration. In comparison, the dry-eye disease results are inconclusive regarding patient-reported outcomes and corneal staining. For NK, topical insulin accelerates corneal wound healing and restores corneal nerve sensation. Other persistent epithelial defect (PED) etiologies that have been treated with topical insulin are infection, immune-mediated, mechanical and chemical trauma, and chronic ocular surface alterations. Although individual mechanisms for the benefits of topical insulin for each of these etiologies have not been studied, the literature demonstrates that topical insulin is efficacious for PEDs regardless of etiology. Future clinical trials need to be conducted to further evaluate optimal dosing, duration, and use of topical insulin for the restoration of the corneal surface.

Sox9 marks limbal stem cells and is required for asymmetric cell fate switch in the corneal epithelium

Adult tissues with high cellular turnover require a balance between stem cell renewal and differentiation, yet the mechanisms underlying this equilibrium are unclear. The cornea exhibits a polarized lateral flow of progenitors from the peripheral stem cell niche to the center; attributed to differences in cellular fate. To identify genes that are critical for regulating the asymmetric fates of limbal stem cells and their transient amplified progeny in the central cornea, we utilized an in vivo cell cycle reporter to isolate proliferating basal cells across the anterior ocular surface epithelium and performed single-cell transcriptional analysis. This strategy greatly increased the resolution and revealed distinct basal cell identities with unique expression profiles of structural genes and transcription factors. We focused on Sox9; a transcription factor implicated in stem cell regulation across various organs. Sox9 was found to be differentially expressed between limbal stem cells and their progeny in the central corneal. Lineage tracing analysis confirmed that Sox9 marks long-lived limbal stem cells and conditional deletion led to abnormal differentiation and squamous metaplasia in the central cornea. These data suggest a requirement for Sox9 for the switch to asymmetric fate and commitment toward differentiation, as transient cells exit the limbal niche. By inhibiting terminal differentiation of corneal progenitors and forcing them into perpetual symmetric divisions, we replicated the Sox9 loss-of-function phenotype. Our findings reveal an essential role for Sox9 for the spatial regulation of asymmetric fate in the corneal epithelium that is required to sustain tissue homeostasis.

Enrichment, Characterization, and Proteomic Profiling of Small Extracellular Vesicles Derived from Human Limbal Mesenchymal Stromal Cells and Melanocytes

Limbal epithelial progenitor cells (LEPC) rely on their niche environment for proper functionality and self-renewal. While extracellular vesicles (EV), specifically small EVs (sEV), have been proposed to support LEPC homeostasis, data on sEV derived from limbal niche cells like limbal mesenchymal stromal cells (LMSC) remain limited, and there are no studies on sEVs from limbal melanocytes (LM). In this study, we isolated sEV from conditioned media of LMSC and LM using a combination of tangential flow filtration and size exclusion chromatography and characterized them by nanoparticle tracking analysis, transmission electron microscopy, Western blot, multiplex bead arrays, and quantitative mass spectrometry. The internalization of sEV by LEPC was studied using flow cytometry and confocal microscopy. The isolated sEVs exhibited typical EV characteristics, including cell-specific markers such as CD90 for LMSC-sEV and Melan-A for LM-sEV. Bioinformatics analysis of the proteomic data suggested a significant role of sEVs in extracellular matrix deposition, with LMSC-derived sEV containing proteins involved in collagen remodeling and cell matrix adhesion, whereas LM-sEV proteins were implicated in other cellular bioprocesses such as cellular pigmentation and development. Moreover, fluorescently labeled LMSC-sEV and LM-sEV were taken up by LEPC and localized to their perinuclear compartment. These findings provide valuable insights into the complex role of sEV from niche cells in regulating the human limbal stem cell niche.

Animal Models for Limbal Stem Cell Deficiency: A Critical Narrative Literature Review

This literature review will provide a critical narrative overview of the highlights and potential pitfalls of the reported animal models for limbal stem cell deficiency (LSCD) and will identify the neglected aspects of this research area. There exists significant heterogeneity in the literature regarding the methodology used to create the model and the predefined duration after the insult when the model is supposedly fully fit for evaluations and/or for testing various therapeutic interventions. The literature is also replete with examples wherein the implementation of a specific model varies significantly across different studies. For example, the concentration of the chemical, as well as its duration and technique of exposure in a chemically induced LSCD model, has a great impact not only on the validity of the model but also on the severity of the complications. Furthermore, while some models induce a full-blown clinical picture of total LSCD, some are hindered by their ability to yield only partial LSCD. Another aspect to consider is the nature of the damage induced by a specific method. As thermal methods cause more stromal scarring, they may be better suited for assessing the anti-fibrotic properties of a particular treatment. On the other hand, since chemical burns cause more neovascularisation, they provide the opportunity to tap into the potential treatments for anti-neovascularisation. The animal species (i.e., rats, mice, rabbits, etc.) is also a crucial factor in the validity of the model and its potential for clinical translation, with each animal having its unique set of advantages and disadvantages. This review will also elaborate on other overlooked aspects, such as the anaesthetic(s) used during experiments, the gender of the animals, care after LSCD induction, and model validation. The review will conclude by providing future perspectives and suggestions for further developments in this rather important area of research.

Adult stem cells in the eye: Identification, characterisation, and therapeutic application in ocular regeneration - A review

Adult stem cells, present in various parts of the human body, are undifferentiated cells that can proliferate and differentiate to replace dying cells within tissues. Stem cells have specifically been identified in the cornea, trabecular meshwork, crystalline lens, iris, ciliary body, retina, choroid, sclera, conjunctiva, eyelid, lacrimal gland, and orbital fat. The identification of ocular stem cells broadens the potential therapeutic strategies for untreatable eye diseases. Currently, stem cell transplantation for corneal and conjunctival diseases remains the most common stem cell-based therapy in ocular clinical management. Lens epithelial stem cells have been applied in the treatment of paediatric cataracts. Several early-phase clinical trials for corneal and retinal regeneration using ocular stem cells are also underway. Extensive preclinical studies using ocular stem cells have been conducted, showing encouraging outcomes. Ocular stem cells currently demonstrate great promise in potential treatments of eye diseases. In this review, we focus on the identification, characterisation, and therapeutic application of adult stem cells in the eye.

Long-term ocular surface stability in conjunctivolimbal autograft and ocular surface safety in the donor eyes

PURPOSE

The current study is aimed to present the long-term results of the patients who underwent conjunctivolimbal autograft (CLAU) as the primary operation in unilateral limbal stem cell deficiency and the ocular surface safety of the donor eyes. The patients were followed up for five years or longer.

METHODS

The records of all patients who underwent CLAU as the primary operation were retrospectively analyzed. Additional ocular surface operations, ocular surface stability, best-corrected visual acuity (BCVA), and ocular surface status of the donor eyes were investigated.

RESULTS

The mean age of the patients at the time of transplantation was 35.07 ± 12.9 (12-60). Twenty-nine eyes of 29 patients were followed up for an average of 97.82 ± 34.45 (60-186) months. Additional ocular surface operation was required in 27.58% (8/29) of the eyes in order to achieve a stable ocular surface. Ocular surface stability was achieved in 82.75% (24/29) of the eyes at the end of the follow-up period. BCVA increased from 1.78 ± 0.82 to 0.91 ± 0.92 logMAR at the last visit (p < 0.001). Corneal ectasia and vascularization developed in one donor eye in the fifth postoperative year.

CONCLUSIONS

CLAU tissues provide ocular surface stability with a successful vision result in the long term. CLAU theoretically carries risks including limbal stem cell deficiency in the donor eye. In the long-term follow-up of donor eyes after CLAU, ectasia and limbal stem cell deficiency were observed in one eye.

Current Advances in Corneal Stromal Stem Cell Biology and Therapeutic Applications

Corneal stromal stem cells (CSSCs) are of particular interest in regenerative ophthalmology, offering a new therapeutic target for corneal injuries and diseases. This review provides a comprehensive examination of CSSCs, exploring their anatomy, functions, and role in maintaining corneal integrity. Molecular markers, wound healing mechanisms, and potential therapeutic applications are discussed. Global corneal blindness, especially in more resource-limited regions, underscores the need for innovative solutions. Challenges posed by corneal defects, emphasizing the urgent need for advanced therapeutic interventions, are discussed. The review places a spotlight on exosome therapy as a potential therapy. CSSC-derived exosomes exhibit significant potential for modulating inflammation, promoting tissue repair, and addressing corneal transparency. Additionally, the rejuvenation potential of CSSCs through epigenetic reprogramming adds to the evolving regenerative landscape. The imperative for clinical trials and human studies to seamlessly integrate these strategies into practice is emphasized. This points towards a future where CSSC-based therapies, particularly leveraging exosomes, play a central role in diversifying ophthalmic regenerative medicine.

In Vivo Confocal Microscopy in Limbal Stem Cell Deficiency After Mesenchymal Stem Cell Transplantation: A Sub-analysis from a Phase I-II Clinical Trial

INTRODUCTION

The aim of this work is to evaluate the effect of mesenchymal stem cell transplantation (MSCT) and cultivated limbal epithelial transplantation (CLET) therapies on the limbus of patients suffering from limbal stem cell deficiency (LSCD).

METHODS

A sub-analysis of a phase I-II randomized, controlled, and double-masked clinical trial was performed to assess the changes in the anatomical structures of the limbus. In vivo confocal microscopy (IVCM) analysis was carried out in LSCD eyes before and 12 months after allogeneic MSCT or CLET. Epithelial phenotype of the central cornea, as well as the presence of transition zones and palisades of Vogt in the limbus, were assessed using Wilcoxon test.

RESULTS

Twenty-three LSCD (14 MSCT and nine CLET) eyes were included. The epithelial phenotype of the central cornea improved significantly (p < 0.001) from 15 (eight MSCT, seven CLET) and eight (six MSCT, two CLET) LSCD eyes showing conjunctival and mixed phenotypes, respectively, to eight (five MSCT, three CLET), five (two MSCT, three CLET), and ten (seven MSCT, three CLET) eyes showing conjunctival, mixed, and corneal phenotypes, respectively. Transition areas and palisades of Vogt were observed in at least one quadrant in nine (five MSCT, four CLET) and 16 (nine MSCT, seven CLET), and in four (two MSCT, two CLET) and six (three MSCT, three CLET) LSCD eyes before and after surgery, respectively. Changes in the transition zones and palisades were solely significant (p = 0.046) for the nasal and inferior quadrants, respectively.

CONCLUSIONS

MSCT and CLET improved the central corneal epithelial phenotype despite only minor changes in the anatomical structures of the limbus, as detected by IVCM technology.

TRIAL REGISTRATION

ClinicalTrials.gov identifier, NCT01562002.

Limbal Stem Cell Dysfunction Induced by Severe Dry Eye via Activation of the p38 MAPK Signaling Pathway

Severe dry eye (SDE) can cause grievous damage to the ocular surface and result in vision impairment and even blindness. To investigate the fate of limbal stem cells in SDE and the underlying mechanism, the current study established an SDE rat model by removing the extraorbital and infraorbital lacrimal glands and maintaining them in a low-humidity environment. One month after the surgery, aqueous tear secretion was reduced dramatically, blood vessels invaded into the central cornea, and inflammatory cells infiltrated into the limbal stroma. The expressions of keratin 12 and paired box gene 6 were down-regulated dramatically, while those of keratin 10, small proline-rich protein 1b, and mucin 5AC were up-regulated in the corneal epithelium of the SDE rats. Cell proliferation in the limbal epithelium was up-regulated, while the stem/progenitor marker adenosine 5'-triphosphate-binding cassette member 2 and the limbal epithelial colony-forming efficiency were decreased in the SDE condition. Furthermore, the p38 mitogen-activated protein kinase signaling pathway was activated in the limbal corneal epithelium of SDE rats. The abnormal differentiation and stemness loss in the corneal epithelium could be reversed upon treatment with a p38 inhibitor in a SDE in vivo model and in vitro hyperosmolar corneal epithelial culture conditions. These data suggest that SDE can lead to limbal stem cell dysfunction, and p38 mitogen-activated protein kinase signaling pathway activation plays an essential role in this process.

The Role of Sensory Innervation in Homeostatic and Injury-Induced Corneal Epithelial Renewal

The cornea is the window through which we see the world. Corneal clarity is required for vision, and blindness occurs when the cornea becomes opaque. The cornea is covered by unique transparent epithelial cells that serve as an outermost cellular barrier bordering between the cornea and the external environment. Corneal sensory nerves protect the cornea from injury by triggering tearing and blink reflexes, and are also thought to regulate corneal epithelial renewal via unknown mechanism(s). When protective corneal sensory innervation is absent due to infection, trauma, intracranial tumors, surgery, or congenital causes, permanent blindness results from repetitive epithelial microtraumas and failure to heal. The condition is termed neurotrophic keratopathy (NK), with an incidence of 5:10,000 people worldwide. In this report, we review the currently available therapeutic solutions for NK and discuss the progress in our understanding of how the sensory nerves induce corneal epithelial renewal.

Mesenchymal Stem Cells and Exosomes: A Novel Therapeutic Approach for Corneal Diseases

The cornea, with its delicate structure, is vulnerable to damage from physical, chemical, and genetic factors. Corneal transplantation, including penetrating and lamellar keratoplasties, can restore the functions of the cornea in cases of severe damage. However, the process of corneal transplantation presents considerable obstacles, including a shortage of available donors, the risk of severe graft rejection, and potentially life-threatening complications. Over the past few decades, mesenchymal stem cell (MSC) therapy has become a novel alternative approach to corneal regeneration. Numerous studies have demonstrated the potential of MSCs to differentiate into different corneal cell types, such as keratocytes, epithelial cells, and endothelial cells. MSCs are considered a suitable candidate for corneal regeneration because of their promising therapeutic perspective and beneficial properties. MSCs compromise unique immunomodulation, anti-angiogenesis, and anti-inflammatory properties and secrete various growth factors, thus promoting corneal reconstruction. These effects in corneal engineering are mediated by MSCs differentiating into different lineages and paracrine action via exosomes. Early studies have proven the roles of MSC-derived exosomes in corneal regeneration by reducing inflammation, inhibiting neovascularization, and angiogenesis, and by promoting cell proliferation. This review highlights the contribution of MSCs and MSC-derived exosomes, their current usage status to overcome corneal disease, and their potential to restore different corneal layers as novel therapeutic agents. It also discusses feasible future possibilities, applications, challenges, and opportunities for future research in this field.

Single-cell RNA sequencing in cornea research: Insights into limbal stem cells and their niche regulation

The corneal epithelium is composed of stratified squamous epithelial cells on the outer surface of the eye, which acts as a protective barrier and is critical for clear and stable vision. Its continuous renewal or wound healing depends on the proliferation and differentiation of limbal stem cells (LSCs), a cell population that resides at the limbus in a highly regulated niche. Dysfunction of LSCs or their niche can cause limbal stem cell deficiency, a disease that is manifested by failed epithelial wound healing or even blindness. Nevertheless, compared to stem cells in other tissues, little is known about the LSCs and their niche. With the advent of single-cell RNA sequencing, our understanding of LSC characteristics and their microenvironment has grown considerably. In this review, we summarized the current findings from single-cell studies in the field of cornea research and focused on important advancements driven by this technology, including the heterogeneity of the LSC population, novel LSC markers and regulation of the LSC niche, which will provide a reference for clinical issues such as corneal epithelial wound healing, ocular surface reconstruction and interventions for related diseases.

Characterization of Porcine Ocular Surface Epithelial Microenvironment

The porcine ocular surface is used as a model of the human ocular surface; however, a detailed characterization of the porcine ocular surface has not been documented. This is due, in part, to the scarcity of antibodies produced specifically against the porcine ocular surface cell types or structures. We performed a histological and immunohistochemical investigation on frozen and formalin-fixed, paraffin-embedded ocular surface tissue from domestic pigs using a panel of 41 different antibodies related to epithelial progenitor/differentiation phenotypes, extracellular matrix and associated molecules, and various niche cell types. Our observations suggested that the Bowman's layer is not evident in the cornea; the deep invaginations of the limbal epithelium in the limbal zone are analogous to the limbal interpalisade crypts of human limbal tissue; and the presence of goblet cells in the bulbar conjunctiva. Immunohistochemistry analysis revealed that the epithelial progenitor markers cytokeratin (CK)15, CK14, p63α, and P-cadherin were expressed in both the limbal and conjunctival basal epithelium, whereas the basal cells of the limbal and conjunctival epithelium did not stain for CK3, CK12, E-cadherin, and CK13. Antibodies detecting marker proteins related to the extracellular matrix (collagen IV, Tenascin-C), cell-matrix adhesion (β-dystroglycan, integrin α3 and α6), mesenchymal cells (vimentin, CD90, CD44), neurons (neurofilament), immune cells (HLA-ABC; HLA-DR, CD1, CD4, CD14), vasculature (von Willebrand factor), and melanocytes (SRY-homeobox-10, human melanoma black-45, Tyrosinase) on the normal human ocular surface demonstrated similar immunoreactivity on the normal porcine ocular surface. Only a few antibodies (directed against N-cadherin, fibronectin, agrin, laminin α3 and α5, melan-A) appeared unreactive on porcine tissues. Our findings characterize the main immunohistochemical properties of the porcine ocular surface and provide a morphological and immunohistochemical basis useful to research using porcine models. Furthermore, the analyzed porcine ocular structures are similar to those of humans, confirming the potential usefulness of pig eyes to study ocular surface physiology and pathophysiology.

Derivation of Limbal Stem Cells from Human Adult Mesenchymal Stem Cells for the Treatment of Limbal Stem Cell Deficiency

Approximately 10 million individuals have blindness due to limbal stem cell (LSCs) deficiency, one of the most challenging problems in ophthalmology. To replenish the LSC pool, an autologous extraocular cell source is appropriate, thereby avoiding the risk of immune rejection, the need for immunosuppression and the risk of damaging the contralateral eye. In recent years, adipose-derived mesenchymal stem cells (ADSCs) have been a key element in ocular regenerative medicine. In this study, we developed a protocol for deriving human LSCs from ADSCs compatible with the standard carrier human amniotic membrane, helping provide a stem cell pool capable of maintaining proper corneal epithelial homeostasis. The best protocol included an ectodermal induction step by culturing ADSCs with media containing fetal bovine serum, transforming growth factor-β inhibitor SB-505124, Wnt inhibitor IWP-2 and FGF2 for 7 days, followed by an LSC induction step of culture in modified supplemental hormonal epithelial medium supplemented with pigment epithelium-derived factor and keratinocyte growth factor for 10 additional days. The optimal differentiation efficiency was achieved when cells were cultured in this manner over vitronectin coating, resulting in up to 50% double-positive αp63/BMI-1 cells. The results of this project will benefit patients with LSC deficiency, aiding the restoration of vision.

The progress in techniques for culturing human limbal epithelial stem cells

In vitro culture of human limbal epithelial stem cells (hLESCs) is crucial to cell therapy in the treatment of limbal stem cell deficiency, a potentially vision-threatening disease that is characterized by persistent corneal epithelial defects and corneal epithelium conjunctivalization. Traditionally, hLESCs are cultivated based on either limbal tissue explants or single-cell suspensions in culture media containing xenogenous components, such as fetal bovine serum and murine 3T3 feeder cells. Plastic culture dishes and human amniotic membranes are classical growth substrates used in conventional hLESC culture systems. The past few decades have witnessed considerable progress and innovations in hLESC culture techniques to ensure a higher level of biosafety and lower immunogenicity for further cell treatment, including complete removal of xenogenous components from culture media, the application of human-derived feeder cells, and the development of novel scaffolds. Three-dimensional artificial niches and three-dimensional culture techniques have also been established to simulate the real microenvironment of limbal crypts for better cell outgrowth and proliferation. All these progresses ensure that in vitro cultured hLESCs are more adaptable to translational stem cell therapy for limbal stem cell deficiency.

Influence of spiral topographies on human limbal-derived immortalized corneal epithelial cells

Cells migrate from the limbus to the corneal epithelium following a centripetal pathway. Corneal epithelial cells tend to orientate in spiral or vortex patterns. However, when cultured in-vitro, limbal derived corneal epithelia do not tend to align or migrate in a spiral pattern. Here, we used soft lithography to create silk fibroin substrates with spiral topographies that direct the human limbal-derived immortalized corneal epithelial cells (hTCEpi) to form a spiral orientation. The impact of this topography on the cells was then characterized. The spiral patterns affected cytoskeletal organization, cell spreading, and nuclei shapes. Spiral width and numbers had a significant impact on proliferation of cells, their focal adhesion, their chromatin condensation, and number of actin filaments. Immunocytochemical staining showed that the spiral pattern enhanced the expression of markers associated with limbal stem cells. The current work illustrates micro spiral patterns can serve to control the nature of limbal derived epithelial cells by providing relevant biophysical cues.

Profile of biological characterizations and clinical application of corneal stem/progenitor cells

Corneal stem/progenitor cells are typical adult stem/progenitor cells. The human cornea covers the front of the eyeball, which protects the eye from the outside environment while allowing vision. The location and function demand the cornea to maintain its transparency and to continuously renew its epithelial surface by replacing injured or aged cells through a rapid turnover process in which corneal stem/progenitor cells play an important role. Corneal stem/progenitor cells include mainly corneal epithelial stem cells, corneal endothelial cell progenitors and corneal stromal stem cells. Since the discovery of corneal epithelial stem cells (also known as limbal stem cells) in 1971, an increasing number of markers for corneal stem/progenitor cells have been proposed, but there is no consensus regarding the definitive markers for them. Therefore, the identification, isolation and cultivation of these cells remain challenging without a unified approach. In this review, we systematically introduce the profile of biological characterizations, such as anatomy, characteristics, isolation, cultivation and molecular markers, and clinical applications of the three categories of corneal stem/progenitor cells.

Construction of ceRNA network and identification of hub genes in aniridia-associated keratopathy using bioinformatics analysis

Aniridia-associated keratopathy (AAK) is characteristic at ocular surface of aniridia caused by haploinsufficiency of PAX6. Competing endogenous RNA (ceRNA) has been reported to play an important role in various diseases, whereas its function on AAK is unclear. The microarray data of 20 AAK patients and 20 healthy people were downloaded from the Gene Expression Omnibus (GEO) database. Differentially expressed lncRNAs, miRNAs, and mRNAs were analyzed using “limma” packages and weighted gene co-expression network analysis (WGCNA). A ceRNA network was constructed by Cytoscape 3.9.1, and miR-224-5p, miR-30a-5p, and miR-204-5p were at the center of the network. CIBERSORTx algorithm and ssGSEA analyses revealed that AAK was associated with immune cell infiltration, showing that activated Mast cells increased while resting Mast cells decreased and NK cells decreased in AAK. Type II INF Response, CCR, parainflammation, T cell co-stimulation, and APC co-stimulation of AAK patients differed from healthy individuals. Additionally, the ROC curve of five genes, MITF(AUC = 0.988), RHOB(AUC = 0.973), JUN(AUC = 0.953), PLAUR (AUC = 0.925), and ARG2 (AUC = 0.915) with high confidence in predicting AAK were identified. Gene set enrichment analysis (GSEA) analysis of hub genes enriched in the IL-17 signaling pathway.

Downregulation of p38 MAPK signaling pathway ameliorates tissue engineered corneal epithelium

Tissue engineered corneal epithelium transplantation is effective treatment for severe limbal stem cell deficiency (LSCD), while epithelial terminal differentiation, tans-differentiation and insufficient stem cell during construction affects the quality of tissue engineered corneal epithelium. In this study, we applied SB203580 in the culture medium to downregulate the P38 MAPK signaling pathway during construction of tissue engineered corneal epithelium. With application of SB203580, tissue engineered corneal epithelium showed enhanced strength and condensed structure. The expression of progenitor cell markers ABCG2, P63, K14, Wnt7a was increased, differentiation markers K12, Pax6, K10, K13, and trans-differentiation markers α-SMA and Snail1 was decreased, while cell junction markers Claudin-1 and E-cadherin was increased in the tissue engineered corneal epithelium. The wnt/β-catenin signaling pathway was upregulated in the epithelium after p38 MAPK inhibition. Transplantation of tissue engineered corneal epithelium treated with SB203580 to rabbit LSCD model showed faster wound healing and improved epithelial quality. We conclude that downregulation of p38 MAPK signaling pathway helps maintain the stemness, prevent terminal differentiation and abnormal differentiation of corneal epithelial cells during epithelium construction process, thus can improve the quality of tissue engineered corneal epithelium.

[Regenerative medicine for the corneal epithelium : Cell therapy from bench to bedside]

In the case of thermal or caustic burns of the ocular surface, loss of limbal epithelial stem cells leads to compromised self-renewal of the corneal epithelium. This results in permanent loss of vision. In these situations, transplantation of cultured limbal epithelial cells on an amniotic membrane or fibrin gel as substrate (Holoclar®) can help to regenerate the corneal surface. The required cells are obtained from the healthy partner eye, if available. Adult stem cells from other parts of the body potentially serve as alternative cell sources: hair follicles, oral mucosa, mesenchymal stromal cells, or induced pluripotent stem cells (originally, e.g., skin fibroblasts). The reprogramming of such cells can be achieved with the help of transcription factors. In addition, work is being done on biosynthetic or synthetic matrices, which not only serve as substrate material for the transplantation but also support the functional properties of these cells (self-renewal, corneal epithelial-typical phenotype).

Multiple roles of Pax6 in postnatal cornea development

Mammalian corneal development is a multistep process, including formation of the corneal epithelium (CE), endothelium and stroma during embryogenesis, followed by postnatal stratification of the epithelial layers and continuous renewal of the epithelium to replace the outermost corneal cells. Here, we employed the Cre-loxP system to conditionally deplete Pax6 proteins in two domains of ocular cells, i.e., the ocular surface epithelium (cornea, limbus and conjunctiva) (OSE) or postnatal CE via K14-cre or Aldh3-cre, respectively. Earlier and broader inactivation of Pax6 in the OSE resulted in thickened OSE with CE and limbal cells adopting the conjunctival keratin expression pattern. More restricted depletion of Pax6 in postnatal CE resulted in an abnormal cornea marked by reduced epithelial thickness despite increased epithelial cell proliferation. Immunofluorescence studies revealed loss of intermediate filament Cytokeratin 12 and diffused expression of adherens junction components, together with reduced tight junction protein, Zonula occludens-1. Furthermore, the expression of Cytokeratin 14, a basal cell marker in apical layers, indicates impaired differentiation of CE cells. Collectively, our data demonstrate that Pax6 is essential for maintaining proper differentiation and strong intercellular adhesion in postnatal CE cells, whereas limbal Pax6 is required to prevent the outgrowth of conjunctival cells to the cornea.

Identification, Isolation, and Characterization of Melanocyte Precursor Cells in the Human Limbal Stroma

Given their vital role in the homeostasis of the limbal stem cell niche, limbal melanocytes have emerged as promising candidates for tissue engineering applications. This study aimed to isolate and characterize a population of melanocyte precursors in the limbal stroma, compared with melanocytes originating from the limbal epithelium, using magnetic-activated cell sorting (MACS) with positive (CD117/c-Kit microbeads) or negative (CD326/EpCAM or anti-fibroblast microbeads) selection approaches. Both approaches enabled fast and easy isolation and cultivation of pure limbal epithelial and stromal melanocyte populations, which differed in phenotype and gene expression, but exhibited similar functional properties regarding proliferative potential, pigmentation, and support of clonal growth of limbal epithelial stem/progenitor cells (LEPCs). In both melanocyte populations, limbus-specific matrix (laminin 511-E8) and soluble factors (LEPC-derived conditioned medium) stimulated melanocyte adhesion, dendrite formation, melanogenesis, and expression of genes involved in UV protection and immune regulation. The findings provided not only a novel protocol for the enrichment of pure melanocyte populations from limbal tissue applying easy-to-use MACS technology, but also identified a population of stromal melanocyte precursors, which may serve as a reservoir for the replacement of damaged epithelial melanocytes and an alternative resource for tissue engineering applications.

WNT16B enhances the proliferation and self-renewal of limbal epithelial cells via CXCR4/MEK/ERK signaling

Culture of limbal epithelial cells (LECs) provides the principal source of transplanted limbal stem cells (LESCs) for treatment of limbal-stem-cell deficiency. Optimization of the culture conditions for in-vitro-expanded LECs will help to create a graft with an optimized quality and quantity of LESCs. This study aimed to investigate the effects of WNT16B on LECs and corneal wound healing and the underlying mechanism. Treatment with exogenous WNT16B increased the proliferative capacity and self-renewal of LECs in the cultures. We further revealed that C-X-C chemokine receptor type 4 (CXCR4) was vital for the effects of WNT16B, and activation of CXCR4/MEK/ERK signaling was pivotal in mediating the effects of WNT16B on LECs enriched for LESCs. The stimulatory effect of WNT16B on corneal epithelial repair was confirmed in a mouse corneal-wound-healing model. In summary, WNT16B enhances proliferation and self-renewal of LECs via the CXCR4/MEK/ERK signaling cascade and accelerates corneal-epithelial wound healing.

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The expression of WNT16B decreases with age

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WNT16B promotes the proliferation and self-renewal of limbal epitheial cells

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The effect of WNT16B on LEC is modulated through CXCR4/MEK/ERK signaling pathway

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WNT16B accelerates corneal-epithelial wound healing

Genetic Modification of Limbal Stem Cells to Decrease Allogeneic Immune Responses

Limbal stem cell (LSC) transplantation is the only efficient treatment for patients affected by LSC deficiency (LSCD). Allogeneic LSC transplantation is one of the most successful alternative for patients with bilateral LSCD. Nevertheless, the high variability of the human leukocyte antigens (HLA) remains a relevant obstacle to long-term allogeneic graft survival. This study characterized the immunologic properties of LSCs and proposed a genetic engineering strategy to reduce the immunogenicity of LSCs and of their derivatives. Hence, LSC HLA expression was silenced using lentiviral vectors encoding for short hairpin (sh) RNAs targeting β2-microglobulin (β2M) or class II major histocompatibility complex transactivator (CIITA) to silence HLA class I and II respectively. Beside the constitutive expression of HLA class I, LSCs showed the capability to upregulate HLA class II expression under inflammatory conditions. Furthermore, LSCs demonstrated the capability to induce T-cell mediated immune responses. LSCs phenotypical and functional characteristics are not disturbed after genetic modification. However, HLA silenced LSC showed to prevent T cell activation, proliferation and cytotoxicity in comparison to fully HLA-expressing LSCs. Additionally; HLA-silenced LSCs were protected against antibody-mediated cellular-dependent cytotoxicity. Our data is a proof-of-concept of the feasibility to generate low immunogenic human LSCs without affecting their typical features. The use of low immunogenic LSCs may support for long-term survival of LSCs and their derivatives after allogeneic transplantation.

Analysis of different conditioned media secreted by limbal progenitor cells in the modulation of corneal healing

Ex vivo cultivation and transplantation of limbal epithelial cells has been reported as an alternative source for ocular surface reconstruction. However, until now, the functional improvement of these patients is limited due to the low survival rate of the transplanted cells. Consequently, the clinical benefits of this therapeutic strategy are only temporary and can assign them to paracrine effects associated with the transplanted cells. With this background in mind, we aimed to analyze the effect of different conditioned media containing growth factors secreted by limbal progenitor cells on corneal epithelial healing, both in vitro and in vivo. Limbal tissue was used to obtain different conditioned media (CM). For the in vitro assay, corneal epithelial cells were treated with CM and the epithelial migration was analyzed. Growth factors in the CM were identified with ELISA and multiplex. For the in vivo assay in rats, total limbal stem cell deficiency (LSCD) was induced with an abrasive injury to the ocular surface, and the animals were treated with different CM. Clinical and histological analyses were performed. In the in vitro assay, treatment with limbal fibroblast (LF CM) was more effective compared to the other CM, and analysis revealed high concentrations of keratinocyte growth factor (KGF) and hepatocyte growth factor (HGF). In the in vivo assay, animals treated with LF CM showed epithelial defect improvement, maintenance of thickness, and decreased opacity and neovascularization. This treatment also allowed better ocular surface tissue organization when compared to the other treatments. The in vitro and in vivo experiments showed better outcomes in the corneal wound healing for the LF CM treatment. The high concentrations of KGF and HGF, linked to epithelial cell migration and proliferation, may correlate to the best results found in this treatment.

Regulation of Limbal Epithelial Stem Cells: Importance of the Niche

Limbal epithelial stem/progenitor cells (LSCs) reside in a niche that contains finely tuned balances of various signaling pathways including Wnt, Notch, BMP, Shh, YAP, and TGFβ. The activation or inhibition of these pathways is frequently dependent on the interactions of LSCs with various niche cell types and extracellular substrates. In addition to receiving molecular signals from growth factors, cytokines, and other soluble molecules, LSCs also respond to their surrounding physical structure via mechanotransduction, interaction with the ECM, and interactions with other cell types. Damage to LSCs or their niche leads to limbal stem cell deficiency (LSCD). The field of LSCD treatment would greatly benefit from an understanding of the molecular regulation of LSCs in vitro and in vivo. This review synthesizes current literature around the niche factors and signaling pathways that influence LSC function. Future development of LSCD therapies should consider all these niche factors to achieve improved long-term restoration of the LSC population.

Long-term survival of cultivated oral mucosal epithelial cells in human cornea: generating cell sheets using an animal product-free culture protocol

Previously, we reported a collagenase-based, animal product-free protocol for cultivated oral mucosal epithelial cell sheets for transplantation (COMET). Here, we reported the long-term outcomes of first 2 clinical cases. A 27-year-old man suffered from thermal burn, which resulted in symblepharon of lower fornix OD. COMET was performed, and the cornea remained clear with few peripheral NV and no more symblepharon 34 months postoperatively. Another 42-year-old man suffered from severe alkaline burn OD. He underwent COMET, followed by corneal transplantation half a year later. A biopsy taken two years after COMET showed stratified epithelium positive for keratin 4, 13, and 3 in the suprabasal layer. Staining for p63 and p75^NTR was both positive in the basal layer. The graft remained clear up to post-OP 4 years. Our study confirmed the long-term survival of the transplanted OMECs, suggesting that collagenase-based spheroidal suspension culture is a promising technique for COMET.

Trial registration ClinicalTrials.gov, ClinicalTrials.gov ID: NCT03943797 Registered 9 May 2019-Retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT03943797.

Biofabrication of Artificial Stem Cell Niches in the Anterior Ocular Segment

The anterior segment of the eye is a complex set of structures that collectively act to maintain the integrity of the globe and direct light towards the posteriorly located retina. The eye is exposed to numerous physical and environmental insults such as infection, UV radiation, physical or chemical injuries. Loss of transparency to the cornea or lens (cataract) and dysfunctional regulation of intra ocular pressure (glaucoma) are leading causes of worldwide blindness. Whilst traditional therapeutic approaches can improve vision, their effect often fails to control the multiple pathological events that lead to long-term vision loss. Regenerative medicine approaches in the eye have already had success with ocular stem cell therapy and ex vivo production of cornea and conjunctival tissue for transplant recovering patients' vision. However, advancements are required to increase the efficacy of these as well as develop other ocular cell therapies. One of the most important challenges that determines the success of regenerative approaches is the preservation of the stem cell properties during expansion culture in vitro. To achieve this, the environment must provide the physical, chemical and biological factors that ensure the maintenance of their undifferentiated state, as well as their proliferative capacity. This is likely to be accomplished by replicating the natural stem cell niche in vitro. Due to the complex nature of the cell microenvironment, the creation of such artificial niches requires the use of bioengineering techniques which can replicate the physico-chemical properties and the dynamic cell-extracellular matrix interactions that maintain the stem cell phenotype. This review discusses the progress made in the replication of stem cell niches from the anterior ocular segment by using bioengineering approaches and their therapeutic implications.

Goals and Challenges of Stem Cell-Based Therapy for Corneal Blindness Due to Limbal Deficiency

Corneal failure is a highly prevalent cause of blindness. One special cause of corneal failure occurs due to malfunction or destruction of the limbal stem cell niche, upon which the superficial cornea depends for homeostatic maintenance and wound healing. Failure of the limbal niche is referred to as limbal stem cell deficiency. As the corneal epithelial stem cell niche is easily accessible, limbal stem cell-based therapy and regenerative medicine applied to the ocular surface are among the most highly advanced forms of this novel approach to disease therapy. However, the challenges are still great, including the development of cell-based products and understanding how they work in the patient's eye. Advances are being made at the molecular, cellular, and tissue levels to alter disease processes and to reduce or eliminate blindness. Efforts must be coordinated from the most basic research to the most clinically oriented projects so that cell-based therapies can become an integrated part of the therapeutic armamentarium to fight corneal blindness. We undoubtedly are progressing along the right path because cell-based therapy for eye diseases is one of the most successful examples of global regenerative medicine.

Pathophysiology of aniridia-associated keratopathy: Developmental aspects and unanswered questions

Aniridia, a rare congenital disease, is often characterized by a progressive, pronounced limbal insufficiency and ocular surface pathology termed aniridia-associated keratopathy (AAK). Due to the characteristics of AAK and its bilateral nature, clinical management is challenging and complicated by the multiple coexisting ocular and systemic morbidities in aniridia. Although it is primarily assumed that AAK originates from a congenital limbal stem cell deficiency, in recent years AAK and its pathogenesis has been questioned in the light of new evidence and a refined understanding of ocular development and the biology of limbal stem cells (LSCs) and their niche. Here, by consolidating and comparing the latest clinical and preclinical evidence, we discuss key unanswered questions regarding ocular developmental aspects crucial to AAK. We also highlight hypotheses on the potential role of LSCs and the ocular surface microenvironment in AAK. The insights thus gained lead to a greater appreciation for the role of developmental and cellular processes in the emergence of AAK. They also highlight areas for future research to enable a deeper understanding of aniridia, and thereby the potential to develop new treatments for this rare but blinding ocular surface disease.

Melanocytes as emerging key players in niche regulation of limbal epithelial stem cells

PURPOSE

Limbal melanocytes (LMel) represent essential components of the corneal epithelial stem cell niche and are known to protect limbal epithelial stem/progenitor cells (LEPCs) from UV damage by transfer of melanosomes. Here, we explored additional functional roles for LMel in niche homeostasis, immune regulation and angiostasis.

METHODS

Human corneoscleral tissues were morphologically analyzed in normal, inflammatory and wound healing conditions. The effects of LMel on LEPCs were analyzed in direct and indirect co-culture models using electron microscopy, immunocytochemistry, qRT-PCR, Western blotting and functional assays; limbal mesenchymal stromal cells and murine embryonic 3T3 fibroblasts served as controls. The immunophenotype of LMel was assessed by flow cytometry before and after interferon-γ stimulation, and their immunomodulatory properties were analyzed by mixed lymphocytes reaction, monocyte adhesion assays and cytometric bead arrays. Their angiostatic effects on human umbilical cord endothelial cells (HUVECs) were evaluated by proliferation, migration, and tube formation assays.

RESULTS

LMel and LEPCs formed structural units in the human limbal stem cell niche in situ, which could be functionally replicated, including melanosome transfer, by co-cultivation in vitro. LMel supported LEPCs during clonal expansion and during epithelial wound healing by stimulating proliferation and migration, and suppressed their differentiation through direct contact and paracrine effects. Under inflammatory conditions, LMel were increased in numbers and upregulated expression of ICAM-1 and MHC II molecules (HLA-DR), but lacked expression of HLA-G, -DP, -DQ and costimulatory molecules CD80 and CD86. They were also found to be potent suppressors of alloreactive T- cell proliferation and cytokine secretion, which largely depended on direct cell-cell interaction. Moreover, the LMel secretome exerted angiostatic activity by inhibiting vascular endothelial cell proliferation and capillary network formation.

CONCLUSION

These findings suggest that LMel are not only professional melanin-producing cells, but exert various non-canonical functions in limbal niche homeostasis by regulating LEPC maintenance, immune responses, and angiostasis. Their potent regulatory, immunomodulatory and anti-angiogenic properties may have important implications for future regenerative cell therapies.

Application of mesenchymal stem cells in corneal regeneration

Due to delicate its structure, the cornea is susceptible to physical, chemical, and genetic damages. Corneal transplantation is the main treatment for serious corneal damage, but it faces significant challenges, including donor shortages and severe complications. In recent years, cell therapy is suggested as a novel alternative method for corneal regeneration. Regarding the unique characteristics of Mesenchymal stem cells including the potential to differentiate into discrete cell types, secretion of growth factors, mobilization potency, and availability from different sources; special attention has been paid to these cells in corneal engineering. Differentiation of MSCs into specialized corneal cells such as keratocytes, epithelial and endothelial cells is reported. Potential for Treatment of keratitis, reducing inflammation, and inhibition of neovascularization by MSCs, introducing them as novel agents for corneal repairing. In this review, various types of MSCs used to treat corneal injuries as well as their potential for restoring different corneal layers was investigated.

Novel Cell Culture Paradigm Prolongs Mouse Corneal Epithelial Cell Proliferative Activity in vitro and in vivo

It has been a long-standing challenge to obtain from cell cultures adequate amounts of mouse corneal epithelial cells (mCEC) to perform transplantation surgery. This limitation is attributable to the passage dependent declines in their proliferative activity. We describe here development of a novel 6C medium that contains six different modulators of different signaling pathways, which control proliferative mCEC activity. Its usage shortens the time and effort required to obtain epithelial sheets for hastening healing of an epithelial wound in an experimental animal model. This serum-free 6C medium contains:Y27632, forskolin, SB431542, DAPT, IWP-2, LDN-193189 and also DermaLife K keratinocyte calcium. Their inclusion inhibits rises in four specific markers of epithelial mesenchymal transdifferentiation:ZEB1/2, Snail, β-catenin and α-SMA. This medium is applied in a feeder-free air-lifted system to obtain sufficient populations of epithelial progenitor cells whose procurement is facilitated due to suppression of progenitor epithelial cell transdifferentiation into epithelial-mesenchymal cells. Diminution of this decline in transdifferentiation was confirmed based on the invariance of P63, K14, Pax6, and K12 gene expression levels. This cell culture technique is expected to facilitate ex vivo characterization of mechanisms underlying cell fate determination. Furthermore, its implementation will improve yields of progenitor mouse corneal epithelial cells, which increases the likelihood of using these cells as a source to generate epithelial sheets for performing transplantation surgery to treat limbal stem cell deficiency in a clinical setting. In addition, the novel insight obtainable from such studies is expected to improve the outcomes of corneal regenerative medicine.

Spectral-domain optical coherence tomography for evaluating palisades of Vogt in ocular surface disorders with limbal involvement

Spectral-domain optical coherence tomography (SD-OCT) has been used to observe the morphology of the palisades of Vogt (POV) with satisfactory resolutions. In this study, we used SD-OCT to examine the microstructure of the POV in ocular surface disorders with limbal involvement. We detect subclinical limbal pathologies based on five parameters, including (1) decreased epithelial thickness, (2) loss of the sharp stromal tip, (3) loss of the smooth epithelial-stromal interface, (4) dilated stromal vessels, and (5) decreased POV density. Eighteen eyes of 10 patients with advancing wavelike epitheliopathy (AWE) and 15 eyes of 9 patients with phlyctenular keratitis/ocular rosacea were recruited. SD-OCT could detect abnormal changes in the POV in 100% of the lesion sites. In presumed-healthy areas of the diseased eyes diagnosed by slit-lamp biomicroscopy, SD-OCT detected abnormal changes in the POV in 100% of the eyes in both groups. In patients with unilateral disease, abnormal changes in the POV were detected by SD-OCT in 50% and 100% of presumed-healthy eyes diagnosed by slit-lamp biomicroscopy in the AWE group and phlyctenular keratitis/ocular rosacea group, respectively. SD-OCT is powerful in detecting POV changes in ocular surface disorders and can provide useful information that cannot be provided by slit-lamp biomicroscopy.

A Comparison of Methods for Isolation of Limbal Niche Cells: Maintenance of Limbal Epithelial Stem/Progenitor Cells

Purpose

Limbal niche cells (LNCs) play a vital role in the maintenance of limbal epithelial stem/progenitor cells (LESCs). Four methods have been reported to isolate and expand LNCs: digestion by collagenase alone (C-LNC), collagenase following dispase removal of the limbal epithelium (DC-LNC), dissection of dispase-isolated limbal epithelial sheets (D-LNC), and explant cultures of limbal stromal tissues (Ex-LNC). This study aimed to isolate LNCs using those four methods and to compare their capacity to maintain LESCs.

Methods

LNCs were isolated from the rat corneal limbus by the following methods: C-LNC, DC-LNC, D-LNC, and Ex-LNC. Quantitative real-time PCR and immunofluorescence staining were used to analyze the expression of embryonic stem cell (ESC) markers. The ability to maintain LESCs was assessed on the basis of colony-forming capacity and the expression of progenitor, proliferation, and differentiation markers in three-dimensional (3D) Matrigel and Transwell systems. Notch signaling of LESCs supported by different LNCs in Transwell inserts was analyzed by quantitative real-time PCR.

Results

DC-LNCs exhibited lower expression of CK12 during isolation and expansion. Among P4-expanded LNCs, DC-LNCs expressed significantly higher levels of Sox2, Oct4, Nanog, and N-cadherin than C-LNCs, D-LNCs, and Ex-LNCs. Compared with other LNCs, DC-LNCs were more effective in maintaining LESCs with higher holoclone-forming efficiency, greater expression of ΔNp63α and Ki67, and lower expression of CK12. DC-LNCs were also more capable of downregulating Notch signaling of LESCs.

Conclusions

DC-LNCs were more effective in expressing ESC markers and maintaining LESCs compared to other LNCs. This study identifies an optimal method for the isolation of LNCs in tissue engineering and ocular surface reconstruction.

Molecular identity of human limbal heterogeneity involved in corneal homeostasis and privilege

PURPOSE

The corneal limbus maintains the homeostasis, immune and angiogenic privilege of cornea. This study aimed to depict the landscape of human limbal tissues by single-cell RNA sequencing (scRNA-seq).

METHODS

Single cells of human limbus collected from donor corneas were subjected to 10x scRNA-seq, followed by clustering cell types through the t-distributed stochastic neighbor embedding (t-SNE) and unbiased computational informatic analysis. Immunofluorescent staining was performed using human corneas to validate the analysis results.

RESULTS

47,627 cells acquired from six human limbal tissues were collected and subjected to scRNA-seq. 14 distinct clusters were identified and 8 cell types were annotated with representative markers. In-depth dissection revealed three limbal epithelial cell subtypes and refined the X-Y-Z hypothesis of corneal epithelial maintenance. We further unveiled two cell states with higher stemness (TP63⁺ and CCL20⁺ cells), and two other differentiated cell states (GPHA2⁺ and KRT6B ⁺ cells) in homeostatic limbal stem/progenitor cells (LSPCs) that differ in transcriptional profiles. Cell-cell communication analysis revealed the central role of LSPCs and their bidirectional regulation with various niche cells. Moreover, comparative analysis between limbus and skin deciphered the pivotal contribution of limbal immune cells, vascular and lymphatic endothelial cells to corneal immune and angiogenic privilege.

CONCLUSIONS

The human limbus atlas provided valuable resources and foundations for understanding corneal biology, disease and potential interventions.

The Use of Microfabrication Techniques for the Design and Manufacture of Artificial Stem Cell Microenvironments for Tissue Regeneration

The recapitulation of the stem cell microenvironment is an emerging area of research that has grown significantly in the last 10 to 15 years. Being able to understand the underlying mechanisms that relate stem cell behavior to the physical environment in which stem cells reside is currently a challenge that many groups are trying to unravel. Several approaches have attempted to mimic the biological components that constitute the native stem cell niche, however, this is a very intricate environment and, although promising advances have been made recently, it becomes clear that new strategies need to be explored to ensure a better understanding of the stem cell niche behavior. The second strand in stem cell niche research focuses on the use of manufacturing techniques to build simple but functional models; these models aim to mimic the physical features of the niche environment which have also been demonstrated to play a big role in directing cell responses. This second strand has involved a more engineering approach in which a wide set of microfabrication techniques have been explored in detail. This review aims to summarize the use of these microfabrication techniques and how they have approached the challenge of mimicking the native stem cell niche.

Timolol induces necroptosis, apoptosis and senescence concentration-dependently in rabbit Limbal stem cells in vitro

Limbal stem cells (LSCs) are crucial for corneal transparency and vision. Any damages to LSCs might lead to limbal stem cell deficiency resulting in corneal opacification and even blindness. Here, we investigated the cytotoxicity of timolol and its underlying mechanisms in rabbit LSCs (rLSCs) in vitro. High concentrations of 0.5% and 0.25% timolol induced necroptosis in rLSCs to upregulate receptor interacting protein kinase (RIPK)1, RIPK3, mixed lineage kinase domain-like (MLKL) and phosphorylated MLKL along with downregulation of caspase-8 and caspase-2 within 4 h. While, median concentrations of 0.125% to 0.0625% timolol induced apoptosis in the rLSCs within 28 h. The apoptotic mechanism in the median-concentration timolol-treated rLSCs is probably via extrinsic apoptosis pathway by activating caspase-2, caspase-8 and caspase-3 and intrinsic apoptosis pathway triggered by excessive generation of ROS and subsequent DNA damage to upregulate Bax and Bad, downregulate Bcl-2 and Bcl-xL, subsequently disrupt mitochondrial membrane potential, cytosolically translocate cytochrome c and apoptosis-inducing factor, and activate caspase-9. In addition, low concentration of 0.03125% timolol induced senescence in the rLSCs by elevating ROS level and increasing number of senescence associated β-galactosidase positive cells at 28 h. Our findings reveal that timolol induces necroptosis, apoptosis and senescence concentration-dependently in rLSCs in vitro.

Corneal stromal mesenchymal stem cells: reconstructing a bioactive cornea and repairing the corneal limbus and stromal microenvironment

Corneal stroma-derived mesenchymal stem cells (CS-MSCs) are mainly distributed in the anterior part of the corneal stroma near the corneal limbal stem cells (LSCs). CS-MSCs are stem cells with self-renewal and multidirectional differentiation potential. A large amount of data confirmed that CS-MSCs can be induced to differentiate into functional keratocytes in vitro, which is the motive force for maintaining corneal transparency and producing a normal corneal stroma. CS-MSCs are also an important component of the limbal microenvironment. Furthermore, they are of great significance in the reconstruction of ocular surface tissue and tissue engineering for active biocornea construction. In this paper, the localization and biological characteristics of CS-MSCs, the use of CS-MSCs to reconstruct a tissue-engineered active biocornea, and the repair of the limbal and matrix microenvironment by CS-MSCs are reviewed, and their application prospects are discussed.

Subconjunctival injection of mesenchymal stem cells for corneal failure due to limbal stem cell deficiency: state of the art

Mesenchymal stem cells (MSCs) have unique and beneficial properties and are currently used to treat a broad variety of diseases. These properties include the potential for differentiation into other cell types, secretion of different trophic factors that promote a regenerative microenvironment, anti-inflammatory actions, selective migration to damaged tissues, and non-immunogenicity. MSCs are effective for the treatment of ocular surface diseases such as dry eye, corneal burns, and limbal stem cell deficiency (LSCD), both in experimental models and in humans. LSCD is a pathological condition in which damage occurs to the limbal epithelial stem cells, or their niche, that are responsible for the continuous regeneration of the corneal epithelium. If LSCD is extensive and/or severe, it usually causes corneal epithelial defects, ulceration, and conjunctival overgrowth of the cornea. These changes can result in neovascularization and corneal opacity, severe inflammation, pain, and visual loss. The effectiveness of MSCs to reduce corneal opacity, neovascularization, and inflammation has been widely studied in different experimental models of LSCD and in some clinical trials; however, the methodological disparity used in the different studies makes it hard to compare outcomes among them. In this regard, the MSC route of administration used to treat LSCD and other ocular surface diseases is an important factor. It should be efficient, minimally invasive, and safe. So far, intravenous and intraperitoneal injections, topical administration, and MSC transplantation using carrier substrata like amniotic membrane (AM), fibrin, or synthetic biopolymers have been the most commonly used administration routes in experimental models. However, systemic administration carries the risk of potential side effects and transplantation requires surgical procedures that could complicate the process. Alternatively, subconjunctival injection is a minimally invasive and straightforward technique frequently used in ophthalmology. It enables performance of local treatments using high cell doses. In this review, we provide an overview of the current status of MSC administration by subconjunctival injection, analyzing the convenience, safety, and efficacy for treatment of corneal failure due to LSCD in different experimental models. We also provide a summary of the clinical trials that have been completed, are in progress, or being planned.

Characterization of the secretory profile and exosomes of limbal stem cells in the canine species

Limbal stem cells (LSCs) are a quiescent cell population responsible for the renewal of the corneal epithelium. Their deficiency is responsible for the conjunctivization of the cornea that is seen in different ocular pathologies, both in humans and in the canine species. The canine species represents an interesting preclinical animal model in ocular surface pathologies. However, the role of LSCs in physiological and pathological conditions in canine species is not well understood. Our objective was to characterize for the first time the soluble factors and the proteomic profile of the secretome and exosomes of canine LSCs (cLSCs). In addition, given the important role that fibroblasts play in the repair of the ocular surface, we evaluated the influence of the secretome and exosomes of cLSCs on their proliferation in vitro. Our results demonstrated a secretory profile of cLSCs with high concentrations of MCP-1, IL-8, VEGF-A, and IL-10, as well as significant production of exosomes. Regarding the proteomic profile, 646 total proteins in the secretome and 356 in exosomes were involved in different biological processes. Functionally, the cLSC secretome showed an inhibitory effect on the proliferation of fibroblasts in vitro, which the exosomes did not. These results open the door to new studies on the possible use of the cLSC secretome or some of its components to treat certain pathologies of the ocular surface in canine species.

Recent Advances in Stem Cell Therapy for Limbal Stem Cell Deficiency: A Narrative Review

Destruction of the limbus and depletion of limbal stem cells (LSCs), the adult progenitors of the corneal epithelium, leads to limbal stem cell deficiency (LSCD). LSCD is a rare, progressive ocular surface disorder which results in conjunctivalisation and neovascularisation of the corneal surface. Many strategies have been used in the treatment of LSCD, the common goal of which is to regenerate a self-renewing, transparent, and uniform epithelium on the corneal surface. The development of these techniques has frequently resulted from collaboration between stem cell translational scientists and ophthalmologists. Direct transplantation of autologous or allogeneic limbal tissue from a healthy donor eye is regarded by many as the technique of choice. Expansion of harvested LSCs in vitro allows smaller biopsies to be taken from the donor eye and is considered safer and more acceptable to patients. This technique may be utilised in unilateral cases (autologous) or bilateral cases (living related donor). Recently developed, simple limbal epithelial transplant (SLET) can be performed with equally small biopsies but does not require in vitro cell culture facilities. In the case of bilateral LSCD, where autologous limbal tissue is not available, autologous oral mucosa epithelium can be expanded in vitro and transplanted to the diseased eye. Data on long-term outcomes (over 5 years of follow-up) for many of these procedures is needed, and it remains unclear how they produce a self-renewing epithelium without recreating the vital stem cell niche. Bioengineering techniques offer the ability to re-create the physical characteristics of the stem cell niche, while induced pluripotent stem cells offer an unlimited supply of autologous LSCs. In vivo confocal microscopy and anterior segment OCT will complement impression cytology in the diagnosis, staging, and follow-up of LSCD. In this review we analyse recent advances in the pathology, diagnosis, and treatment of LSCD.