Strategies for reconstructing the limbal stem cell niche

Advances in the Diagnosis and Management of Limbal Stem Cell Deficiency

ABSTRACT

This concise review focuses on the latest advancements in the diagnosis and management of limbal stem cell deficiency (LSCD). Ensuring the standard of care for individuals affected by LSCD involves the crucial task for physicians to meticulously and accurately diagnose the condition and determine its specific stage. A standardized diagnostic approach forms the foundation for formulating and delivering customized therapeutic interventions to maximize treatment outcomes for each patient. In this review, we introduce a systematic diagnostic algorithm to guide the assessment of LSCD. In addition, the current management algorithm and emerging therapies for LSCD are summarized.

Advances in Regenerative Medicine, Cell Therapy, and 3D Bioprinting for Corneal, Oculoplastic, and Orbital Surgery

Advances in regenerative medicine, cell therapy, and 3D bioprinting are reshaping the landscape of ocular surgery, offering innovative approaches to address complex conditions affecting the cornea, ocular adnexal structures, and the orbit. These technologies hold the potential to enhance treatment precision, improve functional outcomes, and address limitations in traditional surgical and therapeutic interventions.The cornea, as the eye's primary refractive and protective barrier, is particularly well-suited for regenerative approaches due to its avascular and immune-privileged nature. Cell-based therapies, including limbal stem cell transplantation as well as stromal keratocyte and corneal endothelial cell regeneration, are being investigated for their potential to restore corneal clarity and function in conditions such as limbal stem cell deficiency, keratoconus, and endothelial dysfunction. Simultaneously, 3D bioprinting technologies are enabling the development of biomimetic corneal constructs, potentially addressing the global shortage of donor tissues and facilitating personalized surgical solutions.In oculoplastic and orbital surgery, regenerative strategies and cell therapies are emerging as possible alternatives to conventional approaches for conditions such as eyelid defects, meibomian gland dysfunction, and Graves' orbitopathy. Stem cell-based therapies and bioengineered scaffolds are showing potential in restoring lacrimal glands' function as well as reconstructing complex ocular adnexal and orbital structures. Moreover, 3D-printed orbital implants and scaffolds offer innovative solutions for repairing traumatic, post-tumor resection, and congenital defects, with the potential for improved biocompatibility and precision.Molecular and gene-based therapies, including exosome delivery systems, nanoparticle-based interventions, and gene-editing techniques, are expanding the therapeutic arsenal for ophthalmic disorders. These approaches aim to enhance the efficacy of regenerative treatments by addressing underlying pathophysiological mechanisms of diseases. This chapter provides an overview of these advancements and the challenges of translating laboratory discoveries into effective therapies in clinical practice.

The Spatial Transcriptomic Atlas of Human Limbus and Vital Niche Microenvironment Regulating the Fate of Limbal Epithelial Stem Cells

Purpose

This study aimed to generate the spatial atlas of the human limbus using spatial transcriptomic technology and reveal the deep interaction among the niche microenvironment.

Methods

The spatial transcriptomic atlas of human limbus was performed using 10× Genomics Space Ranger software platform. Single-cell RNA sequencing data of human limbal epithelial stem cells (LESCs) were downloaded for integrating analysis.

Results

We profiled more than 400 spots within each sample and spatially located major cell types within the limbus area. LESCs were localized mainly in the basement membrane, and limbal niche cells were situated predominantly within the stromal area. Next, the limbus was divided into four regions based on histological structure, and the differential expressed genes among the four regions were analyzed. Notably, GPHB5 was highly expressed in the epithelium of the middle region and co-staining with deltaNp63 suggested it might be a novel potential biomarker of LESCs. Subsequently, limbal mesenchymal stem cells were found to exhibit the greatest amounts of ligands associated with LESCs. The widespread activity of COL6A2/CD44 signaling among limbal mesenchymal stem cells, melanocytes, immune cells, and LESCs indicate its essential role in mediating bidirectional communication via the collagen pathway.

Conclusions

This research mapped the spatial positioning of key cells within the limbal niche and detailed interactions between major cell types. These findings provide a foundation for further LESC research and enhance our understanding of corneal biology.

Midterm results after allogeneic simple limbal epithelial transplantation from deceased-donor eyes in patients with persistent corneal epithelial defects due to limbal stem cell deficiency

BACKGROUND

This study aims to characterize the clinical outcomes after allogeneic simple limbal epithelial transplantation (alloSLET) utilizing tissue from cadaveric donor eyes to address persistent corneal epithelial defects caused by limbal stem cell deficiency.

METHODS

We conducted a retrospective analysis of medical records from 20 patients, encompassing 24 eyes, who underwent alloSLET at least 2 years prior. The primary endpoint was the achievement of complete epithelialization of the corneal surface by corneal epithelium. Secondary endpoints included corrected distance visual acuity (CDVA) and postoperative adverse events.

RESULTS

The median postoperative follow-up period was 36 months (range, 24-74 months). At 1, 3 and 6 months post-surgery, 96% of eyes demonstrated epithelialized corneal surfaces, which declined to 71% at 12 months, to 54% at 24 and 36 months after surgery, and to 50% thereafter. There were no significant differences in graft survival between alloSLET performed alone versus in combination with penetrating keratoplasty. However, instances of graft failure were associated with postoperative elevated intraocular pressure (IOP) and a history of multiple amniotic membrane and corneal graft transplants.

CONCLUSIONS

AlloSLET emerges as a viable mid-term intervention for limbal stem cell deficiency-associated non-healing corneal epithelial defects in the absence of autologous limbal tissue. Our findings underscore the increased risk of graft failure in patients with elevated IOP and a background of multiple previous amniotic membrane and corneal graft procedures.

Stiffness Regulates the Morphology and Stemness of Limbal Niche Cells Through Unique nYAP/cYAP Translocation

Purpose

To investigate the effect of matrix stiffness on the morphology and stem characters of maintenance and differentiation of limbal niche cells (LNCs) and the mechanisms involved.

Methods

Human LNCs were isolated, cultured, and identified based on published literature, and LNCs from passages 4 to 6 (P4-P6) were used in this study. They were coated with hydrogels of different concentrations to prepare matrices with different stiffnesses, and non-coated plate were used for the control group. Elastic modulus values were determined by atomic force microscopy (AFM). The expression of putative stem cell markers (SOX2, OCT4, PAX6) and fibrosis markers (α-SMA, COL1A1, S100A4) was analyzed by immunofluorescence and quantitative reverse-transcription PCR (RT-qPCR). The intracellular distribution and expression of Yes-associated protein (YAP) and drosophila mothers against decapentaplegic protein family members 2 and 3 (SMAD2/3) accordingly were analyzed using immunofluorescence and western blot.

Results

The elastic modulus values of plastic, low-concentration hydrogel-coated surfaces, and high-concentration hydrogel-coated surfaces were 3261.05 ± 172.78 MPa, 30.39 ± 5.84 kPa, and 6.99 ± 4.04 kPa, respectively; thus, they were referred to as the dish, stiff, and soft groups. Using an in vitro model to explore the effect of matrix stiffness on LNCs, we found that a soft substrate could activate YAP to change the morphology and elevate the stemness of LNCs, whereas activation of SMAD2/3 on a stiff substrate decreased nuclear YAP (nYAP) levels, leading to myofibroblast phenotype. Inhibition of SMAD2/3 on stiff substrates partially restored LNC stemness by promoting YAP nuclear translocation.

Conclusions

Our findings confirm that matrix stiffness regulates the stemness and differentiation of LNCs through the YAP/SMAD signaling pathway, indicating a potential strategy for the treatment of limbal stem cell deficiency based on LNCs.

Posterior Limbal Mesenchymal Stromal Cells Promote Proliferation and Stemness of Transition Zone Cells: A Novel Insight Into Corneal Endothelial Rejuvenation

Purpose

Progenitors for the corneal endothelium have been identified in the transition zone (TZ), but their cellular interactions remain undefined. Posterior limbal mesenchymal stromal cells (P-LMSCs) may support TZ cells in the posterior limbus. This study aims to characterize P-LMSCs and investigate their effects on TZ cells.

Methods

Human P-LMSCs and TZ cells were isolated by explant culture. P-LMSCs were characterized by comparing with anterior limbal mesenchymal stromal cells (A-LMSCs) using immunocytochemistry. TZ cells were cocultured with P-LMSCs in a Transwell, with TZ cell and A-LMSC coculture and TZ cells only as the control groups. The proliferation and wound healing capacity of TZ cells were assessed by EdU assay and scratch wound assay. Colony forming assay, droplet digital PCR, Western blotting, and immunocytochemistry were used to compare the stemness of TZ cells. The effect of P-LMSC conditioned medium on endothelial wound healing was evaluated in organ-cultured mouse corneas. Endothelial regeneration was measured by trypan blue staining.

Results

P-LMSCs expressed similar proteins (vimentin, Nestin, TRA-1-60, Oct3/4) as A-LMSCs. TZ cells cocultured with P-LMSCs had significantly higher proliferation, wound healing speed, and colony-forming efficiency than TZ cells only. TZ cells supported by P-LMSCs expressed higher levels of stem/progenitor markers (Nestin, Sox9, AP-2α, Pitx2) than the control groups. P-LMSC conditioned medium stimulated regeneration of mouse corneal endothelium from the TZ region.

Conclusions

The proliferation and stemness of TZ cells were enhanced by P-LMSCs in both cell and organ culture models. Our study provides an innovative strategy for corneal endothelial rejuvenation.

Progress in Transdifferentiation of Autologous Alternative Cell Sources into Corneal Epithelial Cells

Corneal limbal epithelial stem cells (LESCs) play a crucial role in corneal epithelium regeneration. Severe damage to these cells can result in limbal stem cell deficiency (LSCD), characterized by repeated corneal conjunctivalization, leading to corneal turbidity and scar formation. Restoring functional LESCs and their ecological location are essential for treating LSCD. The goal of this review is to provide researchers and clinicians with key insights into LESCs biology and to conclude the current cell-based therapies advancement in LSCD treatments. Therapeutic cell resources mainly include mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), skin keratinocyte stem cells (SKCs), and oral mucosal epithelial cells (OMECs).

Biomolecule-based hydrogels as delivery systems for limbal stem cell transplantation: A review

Limbal stem cell deficiency (LSCD) is a complex disease of the cornea resulting from dysfunction and/or loss of limbal stem cells (LSCs) and their niche. Most patients with LSCD cannot be treated by conventional corneal transplants because the donor tissue lacks the LSCs necessary for corneal epithelial regeneration. Successful treatment of LSCD depends on effective stem cell transplantation to the ocular surface for replenishment of the LSC reservoir. Thus, stem cell therapies employing carrier substrates for LSCs have been widely explored. Hydrogel biomaterials have many favorable characteristics, including hydrophilicity, flexibility, cytocompatibility, and optical properties suitable for the transplantation of LSCs. Therefore, due to these properties, along with the necessary signals for stem cell proliferation and differentiation, hydrogels are ideal carrier substrates for LSCD treatment. This review summarizes the use of different medical-type hydrogels in LSC transplantation from 2001 to 2024. First, a brief background of LSCD is provided. Then, studies that employed various hydrogel scaffolds as LSC carriers are highlighted to provide a multimodal strategic reference for LSCD treatment. Finally, an analysis of prospective future developments and challenges in the field of hydrogels as LSC carriers for treating LSCD is presented.

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.

Immunophenotypical Characterization of Limbal Mesenchymal Stromal Cell Subsets during In Vitro Expansion

Limbal mesenchymal stromal cells (LMSCs) reside in the limbal niche, supporting corneal integrity and facilitating regeneration. While mesenchymal stem/stromal cells (MSCs) are used in regenerative therapies, there is limited knowledge about LMSC subpopulations and their characteristics. This study characterized human LMSC subpopulations through the flow cytometric assessment of fifteen cell surface markers, including MSC, wound healing, immune regulation, ASC, endothelial, and differentiation markers. Primary LMSCs were established from remnant human corneal transplant specimens and passaged eight times to observe changes during subculture. The results showed the consistent expression of typical MSC markers and distinct subpopulations with the passage-dependent expression of wound healing, immune regulation, and differentiation markers. High CD166 and CD248 expressions indicated a crucial role in ocular surface repair. CD29 expression suggested an immunoregulatory role. Comparable pigment-epithelial-derived factor (PEDF) expression supported anti-inflammatory and anti-angiogenic roles. Sustained CD201 expression indicated maintained differentiation capability, while VEGFR2 expression suggested potential endothelial differentiation. LMSCs showed higher VEGF expression than fibroblasts and endothelial cells, suggesting a potential contribution to ocular surface regeneration through the modulation of angiogenesis and inflammation. These findings highlight the heterogeneity and multipotent potential of LMSC subpopulations during in vitro expansion, informing the development of standardized protocols for regenerative therapies and improving treatments for ocular surface disorders.

Corneal Neurotization: Essentials for The Facial Paralysis Surgeon

Deficits in corneal innervation lead to neurotrophic keratopathy (NK). NK is frequently associated with facial palsy, and corneal damage can be accelerated by facial palsy deficits. Corneal nerves are important regulators of limbal stem cells, which play a critical role in epithelial maintenance and healing. Nonsurgical treatments of NK have undergone recent innovation, and growth factors implicated in corneal epithelial renewal are a promising therapeutic avenue. However, surgical intervention with corneal neurotization (CN) remains the only definitive treatment of NK. CN involves the transfer of unaffected sensory donor nerve branches to the affected cornea, and a variety of donor nerves and approaches have been described. CN can be performed in a direct or indirect manner; employ the supraorbital, supratrochlear, infraorbital, or great auricular nerves; and utilize autograft, allograft, or nerve transfer alone. Unfortunately, comparative studies of these factors are limited due to the procedure's novelty and varied recovery timelines after CN. Regardless of the chosen approach, CN has been shown to be a safe and effective procedure to restore corneal sensation and improve visual acuity in patients with NK.

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.

Cellular structural and functional imaging of donor and pathological corneas with label-free dual-mode full-field optical coherence tomography

In this study, a dual-mode full-field optical coherence tomography (FFOCT) was customized for label-free static and dynamic imaging of corneal tissues, including donor grafts and pathological specimens. Static images effectively depict relatively stable structures such as stroma, scar, and nerve fibers, while dynamic images highlight cells with active intracellular metabolism, specifically for corneal epithelial cells. The dual-mode images complementarily demonstrate the 3D microstructural features of the cornea and limbus. Dual-modal imaging reveals morphological and functional changes in corneal epithelial cells without labeling, indicating cellular apoptosis, swelling, deformation, dynamic signal alterations, and distinctive features of inflammatory cells in keratoconus and corneal leukoplakia. These findings propose dual-mode FFOCT as a promising technique for cellular-level cornea and limbus imaging.

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.

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.

Advances in Cell Transplantation Therapy for Limbal Stem Cell Deficiency

BACKGROUND

Limbal stem cells (LSCs) are essential for maintaining corneal transparency and ocular surface integrity. Many external factors or genetic diseases can lead to corneal limbal stem cell deficiency (LSCD), resulting in the loss of barrier and corneal epithelial cell renewal functions. Stem cell transplantation is one of the primary treatments for LSCD, including limbal transplantation and cultivated limbal epithelial transplantation. In addition, a variety of non-limbal stem cell lines have been experimented with for LSCD treatment. Biological scaffolds are also used to support in vitro stem cell culture and transplantation. Here, we review the mechanisms of corneal maintenance by LSCs, the clinical stage and surgical treatment of LSCD, the source of stem cells, and the biological scaffolds required for in vitro culture.

METHODS

This study is a narrative retrospective study aimed at collecting available information on various aspects of surgical treatments for LSCD. Relevant literature was searched in a range of online databases, including Web of Science, Scopus, and PubMed from 2005 to March, 2023.

RESULTS

A total of 397 relevant articles were found, and 49 articles with strong relevance to the studies in this paper were obtained and analyzed. Moreover, 11 of these articles were on the concept of LSCD and the mechanism of LESCs maintaining the corneal epithelium, 3 articles on the staging and grading of LSCD, 17 articles on cell transplantation methods and donor cell sources, and 18 articles on scaffolds for delivering stem cells. We also summarized the advantages and disadvantages of different cell transplantation methods and the benefits and limitations of scaffolds based on the above literature.

CONCLUSION

The treatment of LSCD is determined by the clinical stage and whether it involves monocular or binocular eyes. Appropriate surgical techniques should be taken for LSCD patients in order to reconstruct the ocular surface, relieve symptoms, and restore visual function. Meanwhile, biological scaffolds assist in the ex vivo culture and implantation of stem cells.

Corneal Vascularization Associated With a Novel PDGFRB Variant

Purpose

The purpose of this study was to identify the genetic cause of aggressive corneal vascularization in otherwise healthy children in one family. Further, to study molecular consequences associated with the identified variant and implications for possible treatment.

Methods

Exome sequencing was performed in affected individuals. HeLa cells were transduced with the identified c.1643C>A, p.(Ser548Tyr) variant in the platelet-derived growth factor receptor beta gene (PDGFRB) or wild-type PDGFRB. ELISA and immunoblot analysis were used to detect the phosphorylation levels of PDGFRβ and downstream signaling proteins in untreated and ligand-stimulated cells. Sensitivity to various receptor tyrosine kinase inhibitors (TKIs) was determined.

Results

A novel c.1643C>A, p.(Ser548Tyr) PDGFRB variant was found in affected family members. HeLa cells transduced with this variant did not have increased baseline levels of phosphorylated PDGFRβ. However, upon stimulation with ligand, excessive activation of PDGFRβ was observed compared to cells transduced with the wild-type variant. PDGFRβ with the p.(Ser548Tyr) amino acid substitution was successfully inhibited with tyrosine kinase inhibitors (axitinib, dasatinib, imatinib, and sunitinib) in vitro.

Conclusions

A novel c.1643C>A, p.(Ser548Tyr) PDGFRB variant was found in family members with isolated corneal vascularization. Cells transduced with the newly identified variant showed increased phosphorylation of PDGFRβ upon ligand stimulation. This suggests that PDGF-PDGFRβ signaling in these patients leads to overactivation of PDGFRβ, which could lead to abnormal wound healing of the cornea. The examined TKIs prevented such overactivation, introducing the possibility for targeted treatment in these patients.

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.

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.

Single mRNA detection of Wnt signaling pathway in the human limbus

Limbal epithelial stem/progenitor cells (LSCs) are adult stem cells located at the limbus, tightly regulated by their close microenvironment. It has been shown that Wnt signaling pathway is crucial for LSCs regulation. Previous differential gene profiling studies confirmed the preferential expression of specific Wnt ligands (WNT2, WNT6, WNT11, WNT16) and Wnt inhibitors (DKK1, SFRP5, WIF1, FRZB) in the limbal region compared to the cornea. Among all frizzled receptors, frizzled7 (Fzd7) was found to be preferentially expressed in the basal limbal epithelium. However, the exact localization of Wnt signaling molecules-producing cells in the limbus remains unknown. The current study aims to evaluate the in situ spatial expression of these 4 Wnt ligands, 4 Wnt inhibitors, and Fzd7. Wnt ligands, DKK1, and Fzd7 expression were scattered within the limbal epithelium, at a higher abundance in the basal layer than the superficial layer. SFRP5 expression was diffuse among the limbal epithelium, whereas WIF1 and FRZB expression was clustered at the basal limbal epithelial layer corresponding to the areas of high levels of Fzd7 expression. Quantitation of the fluorescence intensity showed that all 4 Wnt ligands, 3 Wnt inhibitors (WIF1, DKK1, FRZB), and Fzd7 were highly expressed at the basal layer of the limbus, then in a decreasing gradient toward the superficial layer (P < 0.05). The expression levels of all 4 Wnt ligands, FRZB, and Fzd7 in the basal epithelial layer were higher in the limbus than the central cornea (P < 0.05). All 4 Wnt ligands, 4 Wnt inhibitors, and Fzd7 were also highly expressed in the limbal stroma immediately below the epithelium but not in the corneal stroma (P < 0.05). In addition, Fzd7 had a preferential expression in the superior limbus compared to other limbal quadrants (P < 0.05). Taken together, the unique expression patterns of the Wnt molecules in the limbus suggests the involvement of both paracrine and autocrine effects in LSCs regulation, and a fine balance between Wnt activators and inhibitors to govern LSC fate.

Recent Approaches to the Modification of Collagen Biomatrix as a Corneal Biomatrix and Its Cellular Interaction

Over the last several decades, numerous modifications and advancements have been made to design the optimal corneal biomatrix for corneal epithelial cell (CECs) or limbal epithelial stem cell (LESC) carriers. However, researchers have yet to discover the ideal optimization strategies for corneal biomatrix design and its effects on cultured CECs or LESCs. This review discusses and summarizes recent optimization strategies for developing an ideal collagen biomatrix and its interactions with CECs and LESCs. Using PRISMA guidelines, articles published from June 2012 to June 2022 were systematically searched using Web of Science (WoS), Scopus, PubMed, Wiley, and EBSCOhost databases. The literature search identified 444 potential relevant published articles, with 29 relevant articles selected based on inclusion and exclusion criteria following screening and appraising processes. Physicochemical and biocompatibility (in vitro and in vivo) characterization methods are highlighted, which are inconsistent throughout various studies. Despite the variability in the methodology approach, it is postulated that the modification of the collagen biomatrix improves its mechanical and biocompatibility properties toward CECs and LESCs. All findings are discussed in this review, which provides a general view of recent trends in this field.

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.

Concise Review: Bioengineering of Limbal Stem Cell Niche

The corneal epithelium is composed of nonkeratinized stratified squamous cells and has a significant turnover rate. Limbal integrity is vital to maintain the clarity and avascularity of the cornea as well as regeneration of the corneal epithelium. Limbal epithelial stem cells (LESCs) are located in the basal epithelial layer of the limbus and preserve this homeostasis. Proper functioning of LESCs is dependent on a specific microenvironment, known as the limbal stem cell niche (LSCN). This structure is made up of various cells, an extracellular matrix (ECM), and signaling molecules. Different etiologies may damage the LSCN, leading to limbal stem cell deficiency (LSCD), which is characterized by conjunctivalization of the cornea. In this review, we first summarize the basics of the LSCN and then focus on current and emerging bioengineering strategies for LSCN restoration to combat LSCD.

Corneal stem cells niche and homeostasis impacts in regenerative medicine; concise review

The limbal stem cells niche (LSCN) is an optimal microenvironment that provides the limbal epithelial stem cells (LESCs) and strictly regulates their proliferation and differentiation. Disturbing the LSCN homeostasis can lead to limbal stem cell dysfunction (LSCD) and subsequent ocular surface aberrations, such as corneal stromal inflammation, persistent epithelial defects, corneal neovascularisation, lymphangiogenesis, corneal opacification, and conjunctivalization. As ocular surface disorders are considered the second main cause of blindness, it becomes crucial to explore different therapeutic strategies for restoring the functions of the LSCN. A major limitation of corneal transplantation is the current shortage of donor tissue to meet the requirements worldwide. In this context, it becomes mandatory to find an alternative regenerative medicine, such as using cultured limbal epithelial/stromal stem cells, inducing the production of corneal like cells by using other sources of stem cells, and using tissue engineering methods aiming to produce the three-dimensional (3D) printed cornea. Limbal epithelial stem cells have been considered the magic potion for eye treatment. Epithelial and stromal stem cells in the limbal niche hold the responsibility of replenishing the corneal epithelium. These stem cells are being used for transplantation to maintain corneal epithelial integrity and ultimately sustain optimal vision. In this review, we summarised the characteristics of the LSCN and their current and future roles in restoring corneal homeostasis in eyes with LSCD.

Applications of mesenchymal stem cells in ocular surface diseases: sources and routes of delivery

INTRODUCTION

Mesenchymal stem cells (MSCs) are novel, promising agents for treating ocular surface disorders. MSCs can be isolated from several tissues and delivered by local or systemic routes. They produce several trophic factors and cytokines, which affect immunomodulatory, transdifferentiating, angiogenic, and pro-survival pathways in their local microenvironment via paracrine secretion. Moreover, they exert their therapeutic effect through a contact-dependent manner.

AREAS COVERED

In this review, we discuss the characteristics, sources, delivery methods, and applications of MSCs in ocular surface disorders. We also explore the potential application of MSCs to inhibit senescence at the ocular surface.

EXPERT OPINION

Therapeutic application of MSCs in ocular surface disorders are currently under investigation. One major research area is corneal epitheliopathies, including chemical or thermal burns, limbal stem cell deficiency, neurotrophic keratopathy, and infectious keratitis. MSCs can promote corneal epithelial repair and prevent visually devastating sequelae of non-healing wounds. However, the optimal dosages and delivery routes have yet to be determined and further clinical trials are needed to address these fundamental questions.

Hyaluronan Modulates the Biomechanical Properties of the Cornea

Purpose

Hyaluronan (HA) is a major constituent of the extracellular matrix (ECM) that has high viscosity and is essential for maintaining tissue hydration. In the cornea, HA is enriched in the limbal region and is a key component of the limbal epithelial stem cell niche. HA is upregulated after injury participating in the formation of the provisional matrix, and has a key role in regulating the wound healing process. This study investigated whether changes in the distribution of HA before and after injury affects the biomechanical properties of the cornea in vivo.

Methods

Corneas of wild-type (wt) mice and mice lacking enzymes involved in the biosynthesis of HA were analyzed before, immediately after, and 7 and 14 days after a corneal alkali burn (AB). The corneas were evaluated using both a ring light and fluorescein stain by in vivo confocal microscopy, optical coherence elastography (OCE), and immunostaining of corneal whole mounts.

Results

Our results show that wt mice and mice lacking HA synthase (Has)1 and 3 present an increase in corneal stiffness 7 and 14 days after AB without a significant increase in HA expression and absence of scarring at 14 days after AB. In contrast, mice lacking Has2 present a significant decrease in corneal stiffness, with a significant increase in HA expression and scarring at 14 days after AB.

Conclusions

Our findings show that the mechanical properties of the cornea are significantly modulated by changes in HA distribution following alkali burn.

Corneal regeneration: insights in epithelial stem cell heterogeneity and dynamics

The discovery of slow-cycling cells at the corneal periphery three decades ago established the limbus as the putative corneal stem cell niche. Since then, studies have underscored the importance of the limbal stem cells in maintaining the health and function of the ocular surface. Advancements in our understanding of stem cell biology have been successfully translated into stem cell therapies for corneal diseases. Here, we review recent developments in mouse genetics, intravital imaging, and single-cell genomics that have revealed an underappreciated complexity of the limbal stem cells, from their molecular identity, function, and interactions with their niche environment. Continued efforts to elucidate stem cell dynamics of this extraordinary tissue are critical for not only understanding stem cell biology but also for advancing therapeutic innovation and development.

Mini-Conjunctival Limbal Autograft (Mini-CLAU) Using Platelet-Rich Plasma Eye Drops (E-PRP): A Case Series

PURPOSE

The purpose of this study was to introduce a new method of limbal stem cell transplantation using autologous platelet-rich plasma (E-PRP) eye drops for unilateral total limbal stem cell deficiency.

METHODS

Patients with total unilateral limbal stem cell deficiency due to chemical burn underwent mini-conjunctival limbal autograft using autologous E-PRP drops. One small limbal block, measuring 2 × 2 mm, was harvested from the patients' contralateral healthy eye and transplanted to the diseased eye. All patients received E-PRP drops until achieving complete corneal epithelialization. Subsequent corneal transplantation was performed in eyes with significant stromal opacification. Corneal buttons obtained during corneal transplantation underwent immunohistochemistry for the evaluation of limbal stem cell markers (ABCG2 and P63). Visual acuity, epithelial healing, corneal clarity, and regression of corneal conjunctivalization/vascularization were evaluated after surgery.

RESULTS

Ten patients with acid (n = 7) or alkali (n = 3) burn were included. The mean follow-up period was 21.7 ± 5.8 months (range, 12-32 months). Corneas were completely reepithelialized within 14.9 ± 3.5 days (range, 11-21 days). Corneal conjunctivalization/vascularization dramatically regressed 1 to 2 months after surgery in all cases, and corneal clarity considerably improved in 7 patients. In the 3 eyes with significant stromal opacification, subsequent optical penetrating keratoplasty was performed. The ocular surface was stable throughout the follow-up period in all eyes. BSCVA improved to 0.60 ± 0.0.32 and 0.46 ± 0.0.25 logMAR in eyes with and without corneal transplantation, respectively, at the final follow-up visit. ABCG2 and P63 markers were detected on corneal buttons after keratoplasty.

CONCLUSIONS

Based on our clinical and laboratory findings, mini-conjunctival limbal autograft using E-PRP can be considered as a promising alternative to ocular surface reconstruction.

Therapeutic Strategies for Restoring Perturbed Corneal Epithelial Homeostasis in Limbal Stem Cell Deficiency: Current Trends and Future Directions

Limbal stem cells constitute an important cell population required for regeneration of the corneal epithelium. If insults to limbal stem cells or their niche are sufficiently severe, a disease known as limbal stem cell deficiency occurs. In the absence of functioning limbal stem cells, vision-compromising conjunctivalization of the corneal epithelium occurs, leading to opacification, inflammation, neovascularization, and chronic scarring. Limbal stem cell transplantation is the standard treatment for unilateral cases of limbal stem cell deficiency, but bilateral cases require allogeneic transplantation. Herein we review the current therapeutic utilization of limbal stem cells. We also describe several limbal stem cell markers that impact their phenotype and function and discuss the possibility of modulating limbal stem cells and other sources of stem cells to facilitate the development of novel therapeutic interventions. We finally consider several hurdles for widespread adoption of these proposed methodologies and discuss how they can be overcome to realize vision-restoring interventions.

Bone marrow mesenchymal stromal cells in a 3D system produce higher concentration of extracellular vesicles (EVs) with increased complexity and enhanced neuronal growth properties

PURPOSE

Extracellular vesicles (EVs) derived from mesenchymal stromal cells (MSCs) have been demonstrated to possess great potential in preclinical models. An efficient biomanufacturing platform is necessary for scale up production for clinical therapeutic applications. The aim of this study is to investigate the potential differences in neuro-regenerative properties of MSC-derived EVs generated in 2D versus 3D culture systems.

METHOD

Human bone marrow MSCs (BM-MSCs) were cultured in 2D monolayer and 3D bioreactor systems. EVs were isolated using ultracentrifugation followed by size and concentration measurements utilizing dynamic light scattering (NanoSight) and by fluorescence staining (ExoView). Mouse trigeminal ganglia (TG) neurons were isolated from BALB/c mice and cultured in the presence or absence of EVs derived from 2D or 3D culture systems. Neuronal growth and morphology were monitored over 5 days followed by immunostaining for β3 tubulin. Confocal images were analyzed by Neurolucida software to obtain the density and length of the neurites.

RESULTS

The NanoSight tracking analysis revealed a remarkable increase (24-fold change) in the concentration of EVs obtained from the 3D versus 2D culture condition. ExoView analysis showed a significantly higher concentration of CD63, CD81, and CD9 markers in the EVs derived from 3D versus 2D conditions. Furthermore, a notable shift toward a more heterogeneous phenotype was observed in the 3D-derived EVs compared to those from 2D culture systems. EVs derived from both culture conditions remarkably induced neurite growth and elongation after 5 days in culture compared to untreated control. Neurolucida analysis of the immunostaining images (β3 tubulin) showed a significant increase in neurite length in TG neurons treated with 3D- versus 2D-derived EVs (3301.5 μm vs. 1860.5 μm, P < 0.05). Finally, Sholl analysis demonstrated a significant increase in complexity of the neuronal growth in neurons treated with 3D- versus 2D-derived EVs (P < 0.05).

CONCLUSION

This study highlights considerable differences in EVs obtained from different culture microenvironments, which could have implications for their therapeutic effects and potency. The 3D culture system seems to provide a preferred environment that modulates the paracrine function of the cells and the release of a higher number of EVs with enhanced biophysical properties and functions in the context of neurite elongation and growth.

Cotransplantation of Limbal Epithelial and Stromal Cells for Ocular Surface Reconstruction

Purpose

To propose an improved stem cell-based strategy for limbal stem cell deficiency (LSCD) treatment.

Design

Experimental randomized or parallel-group animal study.

Subjects

Fifty adult male New Zealand white rabbits.

Methods

Human limbal stem/progenitor cells (LSCs) and limbal stromal stem/progenitor cells (LSSCs) were cultured in serum-free conditions and further differentiated into corneal epithelial cells and keratocytes, respectively. All cell types were characterized with lineage-specific markers. Gene expression analysis was performed to identify the potential function of LSSCs in corneal regeneration. Two LSCD models of rabbits for transplantations were used: transplantation performed at the time of limbal and corneal epithelial excision (LSCD model) and transplantation performed after clinical signs were induced in an LSCD model (pLSCD model). The pLSCD model better mimics the pathologic changes and symptoms of human LSCD. Rabbit models received LSC or LSC plus LSSC treatment. Corneal epithelial defects, neovascularization, and opacity were assessed every 3 weeks for 24 weeks. ZsGreen-labeled LSSCs were used for short-term tracking in vivo.

Main Outcome Measures

Rates of corneal epithelial defect area, corneal neovascularization and opacity scores, graft survival rate, and immunofluorescence staining of specific markers.

Results

Both LSC transplantation and LSC plus LSSC cotransplantation effectively repaired the corneal surface in the LSCD model. These 2 strategies showed no significant differences in terms of graft survival rate or epithelial repair. However, corneal opacity was observed in the LSC group (in 3 of 8 rabbits), but not in the LSC plus LSSC group. Notably, when treating LSCD rabbits with distinguishable stromal opacification and neovascularization, cotransplantation of LSCs and LSSCs exhibited significantly better therapeutic effects than transplantation of LSCs alone, with graft survival rates of 87.5% and 37.5%, respectively. The implanted LSSCs could differentiate into keratocytes during the wound-healing process. RNA sequencing analysis showed that the stromal cells produced not only a collagen-rich extracellular matrix to facilitate reconstruction of the lamellar structure, but also niche factors that accelerated epithelial cell growth and inhibited angiogenesis and inflammation.

Conclusions

These findings highlight the support of stromal cells in niche homeostasis and tissue regeneration, providing LSC plus LSSC cotransplantation as a new treatment strategy for corneal blindness.

Induction of Corneal Epithelial Differentiation of Induced Pluripotent and Orbital Fat-Derived Stem Cells Seeded on Decellularized Human Corneas

Up to 40% of donor corneas are deemed unsuitable for transplantation, aggravating the shortage of graft tissue. In most cases, the corneal extracellular matrix is intact. Therefore, their decellularization followed by repopulation with autologous cells may constitute an efficient alternative to reduce the amount of discarded tissue and the risk of immune rejection after transplantation. Although induced pluripotent (hiPSCs) and orbital fat-derived stem cells (OFSCs) hold great promise for corneal epithelial (CE) reconstruction, no study to date has evaluated the capacity of decellularized corneas (DCs) to support the attachment and differentiation of these cells into CE-like cells. Here, we recellularize DCs with hiPSCs and OFSCs and evaluate their differentiation potential into CE-like cells using animal serum-free culture conditions. Cell viability and adhesion on DCs were assessed by calcein-AM staining and scanning electron microscopy. Cell differentiation was evaluated by RT-qPCR and immunofluorescence analyses. DCs successfully supported the adhesion and survival of hiPSCs and OFSCs. The OFSCs cultured under differentiation conditions could not express the CE markers, TP63, KRT3, PAX6, and KRT12, while the hiPSCs gave rise to cells expressing high levels of these markers. RT-qPCR data suggested that the DCs provided an inductive environment for CE differentiation of hiPSCs, supporting the expression of PAX6 and KRT12 without the need for any soluble induction factors. Our results open the avenue for future studies regarding the in vivo effects of DCs as carriers for autologous cell transplantation for ocular surface reconstruction.

Limbal Bio-engineered Tissue Employing 3D Nanofiber-Aerogel Scaffold to Facilitate LSCs Growth and Migration

Constrained by the existing scaffold inability to mimic limbal niche, limbal bio-engineered tissue constructed in vitro is challenging to be widely used in clinical practice. Here, 3D nanofiber-aerogel scaffold was fabricated by employing thermal cross-linking electrospinned film Polycaprolactone (PCL) and gelatin (GEL) as the precursor. Benefiting from the cross-linked (160°C, vacuum) structure, the homogenized and lyophilized 3D nanofiber-aerogel scaffold with preferable mechanical strength was capable of refraining the volume collapse in humid vitro. Intriguingly, compared with traditional electrospinning scaffolds, our 3D nanofiber-aerogel scaffolds possessed enhanced water absorption (1100%-1300%), controllable aperture (50-100 μm) and excellent biocompatibility (optical density value, 0.953 ± 0.021). The well-matched aperture and nanostructure of the scaffolds with cells enabled the construction of limbal bio-engineered tissue. It is foreseen that the proposed general method could be extended to various aerogels, providing new opportunities for the development of novel limbal bio-engineered tissue. This article is protected by copyright. All rights reserved.

OUP accepted manuscript

The corneal epithelium serves to protect the underlying cornea from the external environment and is essential for corneal transparency and optimal visual function. Regeneration of this epithelium is dependent on a population of stem cells residing in the basal layer of the limbus, the junction between the cornea and the sclera. The limbus provides the limbal epithelial stem cells (LESCs) with an optimal microenvironment, the limbal niche, which strictly regulates their proliferation and differentiation. Disturbances to the LESCs and/or their niche can lead to the pathologic condition known as limbal stem cell deficiency (LSCD) whereby the corneal epithelium is not generated effectively. This has deleterious effects on the corneal and visual function, due to impaired healing and secondary corneal opacification. In this concise review, we summarize the characteristics of LESCs and their niche, and present the current and future perspectives in the management of LSCD with an emphasis on restoring the function of the limbal niche.

The Limbal Niche and Regenerative Strategies

The protective function and transparency provided by the corneal epithelium are dependent on and maintained by the regenerative capacity of limbal epithelial stem cells (LESCs). These LESCs are supported by the limbal niche, a specialized microenvironment consisting of cellular and non-cellular components. Disruption of the limbal niche, primarily from injuries or inflammatory processes, can negatively impact the regenerative ability of LESCs. Limbal stem cell deficiency (LSCD) directly hampers the regenerative ability of the corneal epithelium and allows the conjunctival epithelium to invade the cornea, which results in severe visual impairment. Treatment involves restoring the LESC population and functionality; however, few clinically practiced therapies currently exist. This review outlines the current understanding of the limbal niche, its pathology and the emerging approaches targeted at restoring the limbal niche. Most emerging approaches are in developmental phases but show promise for treating LSCD and accelerating corneal regeneration. Specifically, we examine cell-based therapies, bio-active extracellular matrices and soluble factor therapies in considerable depth.

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.

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.

Preclinical Evaluation of the Safety and Efficacy of Cryopreserved Bone Marrow Mesenchymal Stromal Cells for Corneal Repair

Purpose

Mesenchymal stromal cells (MSCs) have been shown to enhance tissue repair as a cell-based therapy. In preparation for a phase I clinical study, we evaluated the safety, dosing, and efficacy of bone marrow–derived MSCs after subconjunctival injection in preclinical animal models of mice, rats, and rabbits.

Methods

Human bone marrow–derived MSCs were expanded to passage 4 and cryopreserved. Viability of MSCs after thawing and injection through small-gauge needles was evaluated by vital dye staining. The in vivo safety of human and rabbit MSCs was studied by subconjunctivally injecting MSCs in rabbits with follow-up to 90 days. The potency of MSCs on accelerating wound healing was evaluated in vitro using a scratch assay and in vivo using 2-mm corneal epithelial debridement wounds in mice. Human MSCs were tracked after subconjunctival injection in rat and rabbit eyes.

Results

The viability of MSCs after thawing and immediate injection through 27- and 30-gauge needles was 93.1% ± 2.1% and 94.9% ± 1.3%, respectively. Rabbit eyes demonstrated mild self-limiting conjunctival inflammation at the site of injection with human but not rabbit MSCs. In scratch assay, the mean wound healing area was 93.5% ± 12.1% in epithelial cells co-cultured with MSCs compared with 40.8% ± 23.1% in controls. At 24 hours after wounding, all MSC-injected murine eyes had 100% corneal wound closure compared with 79.9% ± 5.5% in controls. Human MSCs were detectable in the subconjunctival area and peripheral cornea at 14 days after injection.

Conclusions

Subconjunctival administration of MSCs is safe and effective in promoting corneal epithelial wound healing in animal models.

Translational Relevance

These results provide preclinical data to support a phase I clinical study.

Current perspectives of limbal-derived stem cells and its application in ocular surface regeneration and limbal stem cell transplantation

Limbal stem cells are involved in replenishing and maintaining the epithelium of the cornea. Damage to the limbus due to chemical/physical injury, infections, or genetic disorders leads to limbal stem cell deficiency (LSCD) with partial or total vision loss. Presently, LSCD is treated by transplanting limbal stem cells from the healthy eye of the recipient, living-related, or cadaveric donors. This review discusses limbal-derived stem cells, the importance of extracellular matrix in stem cell niche maintenance, the historical perspective of treating LSCD, including related advantages and limitations, and our experience of limbal stem cell transplantation over the decades.

Allogeneic Limbal Epithelial Transplantation Modified With Solid Platelet-Rich Plasma for Bilateral Limbal Stem Cell Deficiency

PURPOSE

To present successful management of bilateral limbal stem cell deficiency (LSCD) by using an allogeneic limbal epithelial stem cell transplantation together with solid activated platelet-rich plasma (PRP).

METHODS

A 59-year-old man with a history of bilateral LSCD due to penicillin-induced Stevens-Johnson Syndrome suffered from a lime corneal burn in his right eye, leading to a total LSCD with severely reduced visual acuity. After stabilizing the ocular surface, we performed an allogeneic limbal epithelial transplantation from a cadaveric donor using an autologous clot of PRP to cover the limbal grafts to nourish the ocular surface microenvironment.

RESULTS

At the first week after the procedure, the corneal epithelium had fully reepithelized. At month 3, visual acuity improved from hand motion to 20/70.

CONCLUSIONS

In this case, this new modified procedure was a promising, easy-to-perform, apparently safe, and effective treatment option to enhanced epithelial wound healing in ocular surface diseases. To our knowledge, this is the first report describing the incorporation of solid PRP in limbal transplantation procedures.

Current and future therapies for persistent corneal epithelial defects and neurotrophic keratopathy

PURPOSE OF REVIEW

The corneal epithelium is a crucial barrier against pathogens, and when disrupted in the setting of certain underlying risk factors such as neurotrophic keratopathy (NK), may result in persistent epithelial defects (PEDs) of the cornea. Management is challenging and may require a variety of different approaches ranging from conservative medical therapy to surgical intervention. The purpose of this review is to provide an update on current and potential future therapeutic options for PEDs and NK.

RECENT FINDINGS

Recent research has yielded promising results for numerous novel therapies aimed at treating PEDs. Many of these attempt to stimulate healing at the cellular level, via signaling of corneal epithelial differentiation, migration, and proliferation. Considerable advances have also been made regarding medical and surgical promotion of corneal re-innervation and restoration of corneal sensitivity to directly address the underlying NK condition.

SUMMARY

Together with the current well established therapeutic options available for PEDs and NK, growing research on newer alternatives suggest increasing potential for both more effective and more convenient therapies for these difficult situations.

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.

Surgical Management and Recent Advances in Chemical Injury: A 5-year Review

Purpose: To review the surgical management options in ocular chemical burn, including newer advances.Methods: Online literature search of published articles over last 5 years on surgical management of ocular chemical burn and newer advances were performed on December 30, 2020.Results: Following literature search and screening using adequate filters, 67 review articles on surgical management of ocular chemical burns were retrieved. The review talks about the surgical management options starting from Debridement in acute stage to various visual rehabilitative procedures in the chronic stage. The review also highlights the evolving surgical advances in this field.Conclusion: It is imperative to choose adequate surgical tool wherever applicable; current review discusses the role of each surgical option at different clinical stages in detail.

In-situ porcine corneal matrix hydrogel as ocular surface bandage

PURPOSE

Bioactive substrates can be used therapeutically to enhance wound healing. Here, we evaluated the effect of an in-situ thermoresponsive hydrogel from decellularized porcine cornea ECM, COMatrix (COrnea Matrix), for application as an ocular surface bandage for corneal epithelial defects.

METHODS

COMatrix hydrogel was fabricated from decellularized porcine corneas. The effects of COMatrix hydrogel on attachment and proliferation of human corneal epithelial cells (HCECs) were evaluated in vitro. The effect of COMatrix on the expressions of the inflammatory genes, IL-1β, TNF-α, and IL-6 was assessed by RT-PCR. The in-situ application and also repairing effects of COMatrix hydrogel as an ocular bandage was studied in a murine model of corneal epithelial wound. The eyes were examined by optical coherence tomography (OCT) and slit-lamp microscopy in vivo and by histology and immunofluorescence post-mortem.

RESULTS

In vitro, COMatrix hydrogel significantly enhanced the attachment and proliferation of HCECs relative to control. HCECs exposed to COMatrix had less induced expression of TNF-α (P < 0.05). In vivo, COMatrix formed a uniform hydrogel that adhered to the murine ocular surface after in-situ curing. Corneal epithelial wound closure was significantly accelerated by COMatrix hydrogel compared to control (P < 0.01). There was significant increase in the expression of proliferation marker Ki-67 in wounded corneal epithelium by COMatrix hydrogel compared to control (P < 0.05).

CONCLUSIONS

COMatrix hydrogel is a naturally derived bioactive material with potential application as an ocular surface bandage to enhance epithelial wound healing.

Efficacy of Modified Amnion-Assisted Conjunctival Epithelial Redirection (ACER) for Partial Limbal Stem Cell Deficiency

Background and objectives: the aim of this study was to analyze the efficacy of a modified “amnion-assisted conjunctival epithelial redirection (ACER)” technique for the treatment of partial limbal stem cell deficiency (LSCD). Materials and methods: the medical records of three patients with partial LSCD who underwent corneal surface reconstruction with modified ACER following superficial keratectomy were retrospectively studied. Briefly, in this technique, an inner amniotic membrane (AM) layer was applied on the corneal surface to promote corneal re-epithelialization. The outer AM layer was applied as a barrier to prevent the invasion of conjunctival epithelial cells into the cornea before the corneal surface was completely covered by corneal epithelial cells derived from the remaining intact limbal stem cells. Results: in all three cases, the outer AM layer successfully kept the conjunctival epithelium away from the corneal surface and prevented an admixture of conjunctival epithelial cells with corneal epithelial cells. In all three patients, the cornea was completely re-epithelized with epithelial cells derived from the remaining healthy limbal stem cells, and a clear visual axis was maintained without recurrence for a mean follow-up period of 37.3 ± 8.6 months. Conclusions: the preliminary results suggest that modified ACER appears to be a viable option for patients with partial LSCD.

The Effect of SPARC on the Proliferation and Migration of Limbal Epithelial Stem Cells During the Corneal Epithelial Wound Healing

Secreted protein acidic and rich in cysteine (SPARC) shows a specific colocalization with limbal epithelial stem cells (LESCs) in vivo; however, the inherent relationship between SPARC and LESCs is still unclear. This study investigated the effects of SPARC on the maintenance of LESC stemness and corneal wound healing. To test the influence of different concentration of exogenous SPARC on the proliferation of LESCs, cell counting kit-8 assay and 5-ethynyl-2'-deoxyuridine staining were performed and the results indicated that 1 μg/mL SPARC was the optimum concentration for enhanced LESC proliferation. Compared with a control group, SPARC-treated group showed a higher expression of LESC-positive markers p63α, ABCG-2, and Bmi-1, and a lower level of differentiation marker cytokeratin-3 (CK3), thereby suggesting that SPARC could maintain LESC characteristic phenotype and suppress spontaneous epithelial differentiation in vitro. In vivo, exogenous SPARC accelerated the wound-healing process by both the enhancement of LESC proliferation and promoting the migration of the proliferating cells. However, the intact epithelium impaired this function of SPARC by contact inhibition.

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.

The Ocular Neural Crest: Specification, Migration, and Then What?

During vertebrate embryonic development, a population of dorsal neural tube-derived stem cells, termed the neural crest (NC), undergo a series of morphogenetic changes and extensive migration to become a diverse array of cell types. Around the developing eye, this multipotent ocular NC cell population, called the periocular mesenchyme (POM), comprises migratory mesenchymal cells that eventually give rise to many of the elements in the anterior of the eye, such as the cornea, sclera, trabecular meshwork, and iris. Molecular cell biology and genetic analyses of congenital eye diseases have provided important information on the regulation of NC contributions to this area of the eye. Nevertheless, a complete understanding of the NC as a contributor to ocular development remains elusive. In addition, positional information during ocular NC migration and the molecular pathways that regulate end tissue differentiation have yet to be fully elucidated. Further, the clinical challenges of ocular diseases, such as Axenfeld-Rieger syndrome (ARS), Peters anomaly (PA) and primary congenital glaucoma (PCG), strongly suggest the need for better treatments. While several aspects of NC evolution have recently been reviewed, this discussion will consolidate the most recent current knowledge on the specification, migration, and contributions of the NC to ocular development, highlighting the anterior segment and the knowledge obtained from the clinical manifestations of its associated diseases. Ultimately, this knowledge can inform translational discoveries with potential for sorely needed regenerative therapies.

Opening the Soul Window Manually: Limbal Tissue Scaffolds with Electrospun Polycaprolactone/Gelatin Nanocomposites

Restricted by the difficulty in fabricating scaffolds suitable for cell proliferation, the use of ex vivo expanded limbal stem cell (LSC) for LSC transplantation, an effective treatment method for patients with limb stem cell deficiency (LSCD), is hard to be widely used in clinical practice. To tackle these challenges, a novel electrospun polycaprolactone (PCL)/gelatin nanocomposite is proposed to make 3D scaffolds for limbal niche cells (LNC) proliferation in vitro, which is a milestone in the treatment of diseases such as LSCD. PCL and gelatin in different weight ratios are dissolved in a mixed solvent, and then electrospinning and cross-linking are performed to prepare a scaffold for cell proliferation. The characterizations of the nanocomposites indicate that the gelatin content has a significant effect on its micro-morphology, thermal properties, crystallinity, degradation temperature, hydrophilicity, and mechanical properties. P8G2-C (PCL: gelatin = 80: 20, cross-linked), with smooth fibers and homogeneous pores, has better hydrophilicity, mechanical properties, and flexibility, so it can support LNC as cell proliferation assays revealed. This detailed investigation presented here demonstrates the feasibility of using PCL/gelatin nanocomposites electrospun fiber membranes as a limbus tissue engineering scaffold, which undoubtedly provide a new perspective for the development of tissue engineering field.