Epithelial cell characteristics of cultured human limbal explants

A Biosynthetic Alternative to Human Amniotic Membrane for Use in Ocular Surface Surgery

Purpose

The biosynthetic Symatix membrane (SM) was developed to replace fresh human amniotic membrane (hAM) in ocular surgical applications. The purpose of this study was to test the biocompatibility of the SM with human limbus-derived epithelial cells with regard to their physical and biological properties.

Methods

Different physical properties of SM were tested ex vivo by simulation on human corneas. In vitro, primary limbal epithelial cells from limbal explants were used to test biological properties such as cell migration, proliferation, metabolic activity, and limbal epithelial cell markers on the SM, hAM, and freeze-dried amniotic membrane (FDAM).

Results

The surgical handleability of the SM was equivalent to that of the hAM. Ultrastructural and histological studies demonstrated that epithelial cells on the SM had the typical tightly apposed, polygonal, corneal epithelial cell morphology. The epithelial cells were well stratified on the SM, unlike on the hAM and FDAM. Rapid wound healing occurred on the SM within 3 days. Immunofluorescence studies showed positive expression of CK-19, Col-1, laminin, ZO-1, FN, and p-63 on the SM, plastic, and FDAM compared to positive expression of ZO-1, Col-1, laminin, FN, and p63 and negative expression of CK-19 in the hAM.

Conclusions

These results indicate that the SM is a better substrate for limbal epithelial cell migration, proliferation, and tight junction formation. Altogether, the SM can provide a suitable alternative to the hAM for surgical application in sight-restoring operations.

Translational Relevance

The hAM, currently widely used in ocular surface surgery, has numerous variations and limitations. The biocompatibility of corneal epithelial cells with the SM demonstrated in this study suggests that it can be a viable substitute for the hAM.

Marginal Corneal Infiltration Following Treatment for Metastatic Breast Cancer with Triple Chemotherapy of Trastuzumab, Pertuzumab & Docetaxel

PURPOSE

To report a case of bilateral marginal corneal infiltration upon treatment with trastuzumab, pertuzumab, and docetaxel via novel proposed mechanisms.

CASE DESCRIPTION

A patient, diagnosed with metastatic breast cancer and positive for human epidermal growth factor receptor 2 (HER2) with high Ki67, presented with bilateral severe marginal corneal infiltration upon undergoing first cycle of triple chemotherapy: trastuzumab, pertuzumab, and docetaxel. Treatment with topical corticosteroids and antibiotics was unsuccessful and was replaced by allogeneic serum eye drops (SED). The case improved significantly 10 days upon starting allogeneic SED.

CONCLUSIONS

We propose that trastuzumab, pertuzumab, and docetaxel suppress HER2 and Ki67 in the cornea and lacrimal gland. To the best of our knowledge, our report is the first to highlight the potential impact of this triple chemotherapy on the lacrimal gland and cornea and the first to highlight the proposed role of Ki67 suppression in damaging corneal integrity.

PAX6 Expression Patterns in the Adult Human Limbal Stem Cell Niche

Paired box 6 (PAX6), a nuclear transcription factor, determines the fate of limbal epithelial progenitor cells (LEPC) and maintains epithelial cell identity. However, the expression of PAX6 in limbal niche cells, primarily mesenchymal stromal cells (LMSC), and melanocytes is scarce and not entirely clear. To distinctly assess the PAX6 expression in limbal niche cells, fresh and organ-cultured human corneoscleral tissues were stained immunohistochemically. Furthermore, the expression of PAX6 in cultured limbal cells was investigated. Immunostaining revealed the presence of PAX6-negative cells which were positive for vimentin and the melanocyte markers Melan-A and human melanoma black-45 in the basal layer of the limbal epithelium. PAX6 staining was not observed in the limbal stroma. Moreover, the expression of PAX6 was observed by Western blot in cultured LEPC but not in cultured LMSC or LM. These data indicate a restriction of PAX6 expression to limbal epithelial cells at the limbal stem cell niche. These observations warrant further studies for the presence of other PAX isoforms in the limbal stem cell niche.

Interleukin-13 increases the stemness of limbal epithelial stem cells cultures

This study aimed to determine the effect of interleukin-13 (IL13) on the stemness, differentiation, proliferation, clonogenicity, and morphology of cultured limbal epithelial cells (LECs). Human limbal explants were used to culture LECs up to the second passage (P0-P2) with or without IL13 (IL13+ and IL13-, respectively). Cells were analyzed by qPCR (for the expression of ΔNp63α, BMI-1, keratin (K) 3, K7, K12, K14, K17, mucin 4, and MKI67) and immunofluorescence staining for p63α. The clonogenic ability was determined by colony-forming assay (CFA), and their metabolic activity was measured by WST-1 assay. The results of the CFA showed a significantly increased clonogenic ability in P1 and P2 cultures when LECs were cultured with IL13. In addition, the expression of putative stem cell markers (ΔNp63α, K14, and K17) was significantly higher in all IL13+ cultures compared to IL13-. Similarly, immunofluorescence analysis showed a significantly higher percentage of p63α positive cells in P2 cultures with IL13 than without it. LECs cultures without IL13 lost their cuboidal morphology with a high nucleocytoplasmic ratio after P1. The use of IL13 also led to significantly higher proliferation in P2, which can be reflected by a higher ability to reach confluence in P2 cultures. On the other hand, IL13 had no effect on corneal epithelial cell differentiation (K3 and K12 expression), and the expression of the conjunctival marker K7 significantly increased in all IL13+ cultures compared to the respective cell culture without IL13. This study showed that IL13 enhanced the stemness of LECs by increasing the clonogenicity and the expression of putative stem cell markers of LECs while maintaining their stem cell morphology. We established IL13 as a culture supplement for LESCs, which increases their stemness potential in culture, even after the second passage, and may lead to the greater success of LESCs transplantation in patients with LSCD.

Characterisation of corneas following different time and storage methods for their use as a source of stem-like limbal epithelial cells

The transplantation of expansions of limbal epithelial stem cells (LESC) remains one of the most efficient therapies for the treatment of limbal stem cell deficiency (LSCD) to date. However, the available donor corneas are scarce, and the corneas conserved for long time, under hypothermic conditions (after 7 days) or in culture (more than 28 days), are usually discarded due to poor viability of the endothelial cells. To establish an objective criterion for the utilisation or discarding of corneas as a source of LESC, we characterized, by immunohistochemistry analysis, donor corneas conserved in different conditions and for different periods of time. We also studied the potency of LESCs isolated from these corneas and maintained in culture up to 3 cell passages. We hoped that the study of markers of LESCs present in both the corneoscleral histological sections and the cell cultures would show the adequacy of the methods used for cell isolation and how fit the LESC enrichment of the obtained cell populations to be expanded was. Thus, the expressions of markers of the cells residing in the human limbal and corneal epithelium (cytokeratin CK15 and CK12, vimentin, Collagen VII, p63α, ABCG2, Ki67, Integrin β4, ZO1, and melan A) were analysed in sections of corneoscleral tissues conserved in hypothermic conditions for 2-9 days with post-mortem time (pmt) < 8 h or for 1 day with pmt > 16 h, and in sclerocorneal rims maintained in an organ culture medium for 29 days. Cell populations isolated from donor corneoscleral tissues were also assessed based on these markers to verify the adequacy of isolation methods and the potential of expanding LESCs from these tissues. Positivity for several putative stem cell markers such as CK15 and p63α was detected in all corneoscleral tissues, although a decrease was recorded in the ones conserved for longer times. The barrier function and the ability to adhere to the extracellular matrix were maintained in all the analysed tissues. In limbal epithelial cell cultures, a simultaneous decrease in the melan A melanocyte marker and the putative stem cell markers was detected, suggesting a close relationship between the melanocytes and the limbal stem cells of the niche. Holoclones stained with putative stem cell markers were obtained from long-term, hypothermic, stored sclerocorneal rims. The results showed that the remaining sclerocorneal rims after corneal transplantation, which were conserved under hypothermic conditions for up to 7 days and would have been discarded at a first glance, still maintained their potential as a source of LESC cultures.

Process development and safety evaluation of ABCB5+ limbal stem cells as advanced-therapy medicinal product to treat limbal stem cell deficiency

Background

While therapeutic success of the limbal tissue or cell transplantation to treat severe cases of limbal stem cell (LSC) deficiency (LSCD) strongly depends on the percentage of LSCs within the transplanted cells, prospective LSC enrichment has been hampered by the intranuclear localization of the previously reported LSC marker p63. The recent identification of the ATP-binding cassette transporter ABCB5 as a plasma membrane-spanning marker of LSCs that are capable of restoring the cornea and the development of an antibody directed against an extracellular loop of the ABCB5 molecule stimulated us to develop a novel treatment strategy based on the utilization of in vitro expanded allogeneic ABCB5⁺ LSCs derived from human cadaveric limbal tissue.

Methods

We developed and validated a Good Manufacturing Practice- and European Pharmacopeia-conform production and quality-control process, by which ABCB5⁺ LSCs are derived from human corneal rims, expanded ex vivo, isolated as homogenous cell population, and manufactured as an advanced-therapy medicinal product (ATMP). This product was tested in a preclinical study program investigating the cells’ engraftment potential, biodistribution behavior, and safety.

Results

ABCB5⁺ LSCs were reliably expanded and manufactured as an ATMP that contains comparably high percentages of cells expressing transcription factors critical for LSC stemness maintenance (p63) and corneal epithelial differentiation (PAX6). Preclinical studies confirmed local engraftment potential of the cells and gave no signals of toxicity and tumorgenicity. These findings were sufficient for the product to be approved by the German Paul Ehrlich Institute and the U.S. Food & Drug Administration to be tested in an international multicenter phase I/IIa clinical trial (NCT03549299) to evaluate the safety and therapeutic efficacy in patients with LSCD.

Conclusion

Building upon these data in conjunction with the previously shown cornea-restoring capacity of human ABCB5⁺ LSCs in animal models of LSCD, we provide an advanced allogeneic LSC-based treatment strategy that shows promise for replenishment of the patient’s LSC pool, recreation of a functional barrier against invading conjunctival cells and restoration of a transparent, avascular cornea.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02272-2.

In vitro reconstructed 3D corneal tissue models for ocular toxicology and ophthalmic drug development

Testing of all manufactured products and their ingredients for eye irritation is a regulatory requirement. In the last two decades, the development of alternatives to the in vivo Draize eye irritation test method has substantially advanced due to the improvements in primary cell isolation, cell culture techniques, and media, which have led to improved in vitro corneal tissue models and test methods. Most in vitro models for ocular toxicology attempt to reproduce the corneal epithelial tissue which consists of 4-5 layers of non-keratinized corneal epithelial cells that form tight junctions, thereby limiting the penetration of chemicals, xenobiotics, and pharmaceuticals. Also, significant efforts have been directed toward the development of more complex three-dimensional (3D) equivalents to study wound healing, drug permeation, and bioavailability. This review focuses on in vitro reconstructed 3D corneal tissue models and their utilization in ocular toxicology as well as their application to pharmacology and ophthalmic research. Current human 3D corneal epithelial cell culture models have replaced in vivo animal eye irritation tests for many applications, and substantial validation efforts are in progress to verify and approve alternative eye irritation tests for widespread use. The validation of drug absorption models and further development of models and test methods for many ophthalmic and ocular disease applications is required.

Differentiation Induction of Human Stem Cells for Corneal Epithelial Regeneration

Deficiency of corneal epithelium causes vision impairment or blindness in severe cases. Transplantation of corneal epithelial cells is an effective treatment but the availability of the tissue source for those cells is inadequate. Stem cells can be induced to differentiate to corneal epithelial cells and used in the treatment. Multipotent stem cells (mesenchymal stem cells) and pluripotent stem cells (embryonic stem cells and induced pluripotent stem cells) are promising cells to address the problem. Various protocols have been developed to induce differentiation of the stem cells into corneal epithelial cells. The feasibility and efficacy of both human stem cells and animal stem cells have been investigated for corneal epithelium regeneration. However, some physiological aspects of animal stem cells are different from those of human stem cells, the protocols suited for animal stem cells might not be suitable for human stem cells. Therefore, in this review, only the investigations of corneal epithelial differentiation of human stem cells are taken into account. The available protocols for inducing the differentiation of human stem cells into corneal epithelial cells are gathered and compared. Also, the pathways involving in the differentiation are provided to elucidate the relevant mechanisms.

Histological study of the effect of granulocyte colony-stimulating factor on experimentally induced corneal burn in adult male albino rats

Chemical injuries to the eye represent one of the true ophthalmic emergencies that require immediate and intensive intervention to minimize severe complications and visual loss. Granulocyte colony-stimulating factor (G-CSF) is a potent hematopoietic cytokine that influences the proliferation, survival, maturation, and the functional activation of granulocytes. The present work was performed to evaluate the histological effect of G-CSF in treating rat corneal alkali burn model. Thirty adult male albino rats were divided equally into three main groups: Group I was served as a control group, and in Group II and III, their corneas of the right eyes were injured by applying a piece of filter paper soaked in 1M NaOH. Group II (alkali burn-induced group) was left without any treatment, while Group III (G-CSF-treated group) was injected subcutaneously by 100 µg/kg of G-CSF for 5 consecutive days. All animals were sacrificed after 3 weeks. Cornea specimens were processed for histological and immunohistochemical staining for P63 followed by morphometry. Microscopic examination of Group II revealed marked alterations in the corneal epithelium, inflammatory cellular infiltration, and neovascularization. Treatment with G-CSF showed great improvement of the corneal structure, disappearance of the neovascularization and the inflammatory cells, and decreased p63 reaction of the basal layers.

Temporal Regulation of Notch Signaling and Its Influence on the Differentiation of Ex Vivo Cultured Limbal Epithelial Cells

Purpose: Notch signaling plays a vital role in the differentiation and proliferation of corneal epithelial cells from limbal stem cells. The temporal regulation of Notch signaling during this differentiation remains unknown. Hence, we investigated the importance of temporal activation/blockage of Notch signaling during corneal differentiation.Methods: Human limbal epithelial cultures were established with and without Notch activators (rec-Human Jagged1 Fc chimera) and pharmacological blockers (LY-411575). The modulation of Notch signaling was done at different time points during cell differentiation, which were collected on Day 14 for further analysis of differentiation, proliferation, maturation and apoptosis using RT-qPCR and immunofluorescence staining.Results: The activation of Notch signaling at Day 8 resulted in the highest number of mature corneal epithelial cells (p = .008) and pro-apoptosis marker BAX (p = .0001) with no increase in the number of corneal progenitors, and proliferation marker Ki67 compared to untreated controls. Cultures grown in the presence of Notch signaling blockers showed a significantly higher number of corneal progenitors (p = .0001) and proliferation marker Ki67 (p = .02) but lower corneal epithelial marker CK3/CK12 (p = .0007) and no difference in the pro-apoptotic marker BAX compared to untreated controls.Conclusion: During the differentiation of limbal epithelial cells to the corneal epithelial cell type, Day 8 seems to be a crucial window to modulate Notch signaling for a customized outcome.

Nanofiber-reinforced decellularized amniotic membrane improves limbal stem cell transplantation in a rabbit model of corneal epithelial defect

Human amniotic membrane (AM) offers unique advantages as a matrix to support the transplantation of limbal stem cells (LSCs) due to its inherent pro-regenerative and anti-inflammatory properties. However, the widespread use of AM in clinical treatments of ocular surface disorders is limited by its weak mechanical strength and fast degradation, and high cost associated with preserving freshly isolated AM. Here we constructed a composite membrane consisting of an electrospun bioabsorbable poly(ε-caprolactone) (PCL) nanofiber mesh to significantly improve the ultimate tensile strength, toughness, and suture retention strength by 4-10-fold in comparison with decellularized AM sheet. The composite membrane showed extended stability and conferred longer-lasting coverage on wounded cornea surface compared with dAM. The composite membrane maintained the pro-regenerative and immunomodulatory properties of dAM, promoted LSC survival, retention, and organization, improved re-epithelialization of the defect area, and reduced inflammation and neovascularization. This study demonstrates the translational potential of our composite membrane for stem cell-based treatment of ocular surface damage. STATEMENT OF SIGNIFICANCE: Human decellularized amniotic membrane (dAM) has been widely shown as a biodegradable and bioactive matrix for regenerative tissue repair. However, the weak mechanical property has limited its widespread use in the clinic. Here we constructed a composite membrane using a layer of electrospun poly(ε-caprolactone) (PCL) nanofiber mesh to reinforce the dAM sheet through covalent interfacial bonding, while retaining the unique bioactivity of dAM. In a rabbit model of limbal stem cell (LSC) deficiency induced by alkaline burn, we demonstrated the superior property of this PCL-dAM composite membrane for repairing damaged cornea through promoting LSC transplantation, improving re-epithelialization, and reducing inflammation and neovascularization. This new composite membrane offers great translational potential in supporting stem cell-based treatment of ocular surface damage.

Effects of explant size on epithelial outgrowth, thickness, stratification, ultrastructure and phenotype of cultured limbal epithelial cells

Purpose

Transplantation of limbal stem cells is a promising therapy for limbal stem cell deficiency. Limbal cells can be harvested from either a healthy part of the patient’s eye or the eye of a donor. Small explants are less likely to inflict injury to the donor site. We investigated the effects of limbal explant size on multiple characteristics known to be important for transplant function.

Methods

Human limbal epithelial cells were expanded from large versus small explants (3 versus 1 mm of the corneal circumference) for 3 weeks and characterized by light microscopy, immunohistochemistry, and transmission electron microscopy. Epithelial thickness, stratification, outgrowth, ultrastructure and phenotype were assessed.

Results

Epithelial thickness and stratification were similar between the groups. Outgrowth size correlated positively with explant size (r = 0.37; P = 0.01), whereas fold growth correlated negatively with explant size (r = –0.55; P < 0.0001). Percentage of cells expressing the limbal epithelial cell marker K19 was higher in cells derived from large explants (99.1±1.2%) compared to cells derived from small explants (93.2±13.6%, P = 0.024). The percentage of cells expressing ABCG2, integrin β1, p63, and p63α that are markers suggestive of an immature phenotype; Keratin 3, Connexin 43, and E-Cadherin that are markers of differentiation; and Ki67 and PCNA that indicate cell proliferation were equal in both groups. Desmosome and hemidesmosome densities were equal between the groups.

Conclusion

For donor- and culture conditions used in the present study, large explants are preferable to small in terms of outgrowth area. As regards limbal epithelial cell thickness, stratification, mechanical strength, and the attainment of a predominantly immature phenotype, both large and small explants are sufficient.

Conditional reprogrammed human limbal epithelial cells represent a novel in vitro cell model for drug responses

In this study, we established human limbal epithelial cells (LECs) from normal limbal tissues by using Conditional Reprogramming (CR) technology (refer to CR-LEC cells in this study). We have successfully established CR-LEC cell strains from three human donors (3 out of 3), and normal rabbits (2 out of 2) and pig (1 out of 1) as well. CR-LEC cells sustained a continuous and stable proliferation status with a normal karyotype, normal response to DNA damage, well-defined structured spheres in matrigel 3D culture. Responses of CR-LEC cells to IFN α2b, Ganciclovir and 5-Fluorouracil were different, suggesting that these drugs had different toxicities to these cells as expected. More important, there was no significant difference of responses to drugs between early and late passages of CR-LEC cells (p>0.05), indicating CR-LEC cells can serve a stable normal human cell model for toxicity assessment. Toxicity tests with monolayer cultures of CR-LEC cells were measured by staining the F-actin and Dsg-1 expression. Toxicity of three drugs at LD50 concentration resulted in a gradually increased destruction of monolayer, which is, in accordance with the irritation grade of three drugs on human cornea epithelium. Therefore, CR-LEC cells provide a novel and reliable in vitro physiological cell model for corneal toxicity assessment.

Feeder Cells Free Rabbit Oral Mucosa Epithelial Cell Sheet Engineering

The optimal cell culture method of autologous oral mucosal epithelial cell sheet is not well established for a safe transplantation on to the patients' ocular surface. Animal serum and 3T3 mouse feeder cells are currently being used to stimulate the growth of the epithelial cells. However, the use of animal compounds can have potential side effects for the patient after transplantation of the engineered cell sheet. In the present study, we focused on engineering a rabbit oral mucosal epithelial cell sheet without 3T3 mouse feeder cells using a mix of Dulbecco's Modified Eagle Medium/Bronchial Epithelial Cell Growth Medium culture media (DMEM/BEGM). Autologous oral mucosal epithelial cell sheets, engineered with DMEM/BEGM feeder cell free culture media, were compared to those cultured in presence of serum and feeder cells. Using a DMEM/BEGM mix culture media, feeder cell free culture condition, autologous oral mucosal epithelial cells reached confluence and formed a multilayered sheet. The phenotype of engineered cell sheets cultured with DMEM/BEGM were characterized and compared to those cultured with serum and feeder. Hematoxylin and eosin staining showed the formation of a similar stratified multilayer cell sheets, in both culture conditions. The expression of deltaN-p63, ABCG2, PCNA, E-cadherin, Beta-catenin, CK3, CK4, CK13, Muc5AC, was similar in both culture conditions. We demonstrated that rabbit autologous oral mucosal epithelial cell sheet can be engineered, in feeder cell free conditions. The use of the DMEM/BEGM culture media to engineer culture autologous oral mucosa epithelial cell sheet will help to identify key factors involved in the growth and differentiation of oral mucosal epithelial cells.

Comparison of culture media indicates a role for autologous serum in enhancing phenotypic preservation of rabbit limbal stem cells in explant culture

In this study, we aimed to compare the effects of six different cell culture media and autologous serum (AS) on the phenotypic characteristics of rabbit limbal epithelial stem cells (LESC) cultivated on porous polyethylene terephthalate (PET) membranes. Limbal explants from rabbit corneas were grown on PET membrane inserts in five different media: DMEM-F12 with fetal bovine serum (FBS) (DMEM-F12-FBS), with pluripotin (DMEM-F12-pluripotin) and with autologous serum (DMEM-F12-AS), Epilife, Keratinocyte Serum Free Medium (KSFM) and Defined-Keratinocyte Serum Free Medium. The effects of different media were evaluated by total cell yield from explants, measuring the expression of proteins by immunofluorescence and gene expression by Real Time PCR. In all five media tested, most of the limbal epithelial cells (LEC) which proliferated from explants were positive for cytokeratin (CK) 14 (85-90%), indicating that all five media support the growth of LESC from explants. The expression of differentiation markers; CK 3 and 12 was highest in DMEM-F12-FBS (56%), was lower in Epilife and KSFM (26 and 19%, respectively), with the lowest values (13%) obtained in DMEM-F12-AS. Gene expression of limbal cultures on PET membrane inserts was compared to fresh limbal tissue. In DMEM-F12-FBS, DMEM-F12-pluripotin, and DMEM-F12-AS, expression of potential LESC markers CXCR4 and polycomb complex protein BMI-1 were similar to limbal tissue. DMEM-F12 with 10% AS maintained a higher percentage of potential stem cell marker genes and lower expression of genes involved in differentiation compared to Epilife or KSFM. Our study shows that rabbit LEC can be cultivated on PET inserts using DMEM-F12 with autologous serum without a requirement for amniotic membrane or feeder cells.

Cellular Stiffness as a Novel Stemness Marker in the Corneal Limbus

Healthy eyes contain a population of limbal stem cells (LSCs) that continuously renew the corneal epithelium. However, each year, 1 million Americans are afflicted with severely reduced visual acuity caused by corneal damage or disease, including LSC deficiency (LSCD). Recent advances in corneal transplant technology promise to repair the cornea by implanting healthy LSCs to encourage regeneration; however, success is limited to transplanted tissues that contain a sufficiently high percentage of LSCs. Attempts to screen limbal tissues for suitable implants using molecular stemness markers are confounded by the poorly understood signature of the LSC phenotype. For cells derived from the corneal limbus, we show that the performance of cell stiffness as a stemness indicator is on par with the performance of ΔNP63α, a common molecular marker. In combination with recent methods for sorting cells on a biophysical basis, the biomechanical stemness markers presented here may enable the rapid purification of LSCs from a heterogeneous population of corneal cells, thus potentially enabling clinicians and researchers to generate corneal transplants with sufficiently high fractions of LSCs, regardless of the LSC percentage in the donor tissue.

Limbal Stem Cell Deficiency: Current Treatment Options and Emerging Therapies

Severe ocular surface disease can result in limbal stem cell deficiency (LSCD), a condition leading to decreased visual acuity, photophobia, and ocular pain. To restore the ocular surface in advanced stem cell deficient corneas, an autologous or allogenic limbal stem cell transplantation is performed. In recent years, the risk of secondary LSCD due to removal of large limbal grafts has been significantly reduced by the optimization of cultivated limbal epithelial transplantation (CLET). Despite the great successes of CLET, there still is room for improvement as overall success rate is 70% and visual acuity often remains suboptimal after successful transplantation. Simple limbal epithelial transplantation reports higher success rates but has not been performed in as many patients yet. This review focuses on limbal epithelial stem cells and the pathophysiology of LSCD. State-of-the-art therapeutic management of LSCD is described, and new and evolving techniques in ocular surface regeneration are being discussed, in particular, advantages and disadvantages of alternative cell scaffolds and cell sources for cell based ocular surface reconstruction.

Long-Term Cultures of Human Cornea Limbal Explants Form 3D Structures Ex Vivo - Implications for Tissue Engineering and Clinical Applications

Long-term cultures of cornea limbal epithelial stem cells (LESCs) were developed and characterized for future tissue engineering and clinical applications. The limbal tissue explants were cultivated and expanded for more than 3 months in medium containing serum as the only growth supplement and without use of scaffolds. Viable 3D cell outgrowth from the explants was observed within 4 weeks of cultivation. The outgrowing cells were examined by immunofluorescent staining for putative markers of stemness (ABCG2, CK15, CK19 and Vimentin), proliferation (p63α, Ki-67), limbal basal epithelial cells (CK8/18) and differentiated cornea epithelial cells (CK3 and CK12). Morphological and immunostaining analyses revealed that long-term culturing can form stratified 3D tissue layers with a clear extracellular matrix deposition and organization (collagen I, IV and V). The LESCs showed robust expression of p63α, ABCG2, and their surface marker fingerprint (CD117/c-kit, CXCR4, CD146/MCAM, CD166/ALCAM) changed over time compared to short-term LESC cultures. Overall, we provide a model for generating stem cell-rich, long-standing 3D cultures from LESCs which can be used for further research purposes and clinical transplantation.

Isolation of adult stem cell populations from the human cornea

Corneal blindness is a leading cause of vision loss globally. From a tissue engineering perspective, the cornea represents specific challenges in respect to isolating, stably expanding, banking, and effectively manipulating the various cell types required for effective corneal regeneration. The current research trend in this area focuses on a combined stem cell component with a biological or synthetic carrier or engineering scaffold. Corneal derived stem cells play an important role in such strategies as they represent an available supply of cells with specific abilities to further generate corneal cells in the long term. This chapter describes the isolation protocols of the epithelial stromal and endothelial stem cell populations.

Limbal epithelial stem-microenvironmental alteration leads to pterygium development

Maintenance of tissue homeostasis relies on the accurate regulation of tissue specific stem cell activity which is governed by the dynamic interaction between the positive and negative feedback modulating mechanism of stem cell microenvironmental niche. Alteration or deregulation of the "stem-microenvironmental networking" provokes disease development. Limbal epithelial stem cells (LESC) are the initiator hierarchy that maintains corneal integrity. Compartmentalization of LESC within the limbal vicinity provides an opportunity to understand the stem-microenvironmental relationship. The purpose of this study was to determine the microenvironmental alteration associated with LESCs fate in pterygium condition in comparison with healthy state. Clinical observations evaluated the ocular surface disorder with respect to corneal vascularization, tear film abnormality, and thickening of limbal area in pterygium patients. Structural alteration of limbal stem/progenitor cells and its neighboring niche components were observed using histology and scanning electron microscopy. Receptor overexpression of TGFβ-R1, EGF-R1, and IL6-Rα and alteration of IL2-Rα expression pointed toward aberration of "stem-microenvironmental networking" in the limbal vicinity during disease development. Increased cell proliferation index along with TERT, Cyclin-D1, and PCNA over-expression in limbal part of pterygium epithelial cells indicated increased cellular proliferation and disturbed homeostatic equilibrium. We postulate that pterygium is associated with limbal microenvironmental anomaly where the resident epithelial cells became hyperproliferative.

Human limbal mesenchymal cells support the growth of human corneal epithelial stem/progenitor cells

PURPOSE

We tested the viability of human limbal mesenchymal cells (LMCs) to support the expansion of human corneal epithelial stem/progenitor cells (LSCs).

METHODS

Human LMCs were isolated from sclerocorneal tissue using collagenase A. Primary limbal epithelial cells (LECs) in the form of single cell suspension or cell clusters were cocultured on a monolayer of either 3T3 cells (control) or LMCs (SC-LMC culture). The LEC clusters also were grown directly on LMCs (CC-LMC culture) and in an optimized 3-dimensional culture method (3D CC-LMC culture). Colony-forming efficiency (CFE) and LEC proliferation were analyzed. The phenotype of the cultured LECs was assessed by their expression level of putative stem cell markers and a differentiation marker by qRT-PCR and immunocytochemistry.

RESULTS

The LECs in the SC-LMC culture had a very limited growth and the stem/progenitor phenotype was lost compared to the control. Growth and cell morphology improved using the CC-LMC culture. The 3D CC-LMC culture method was the best to support the growth of the LSC population. Expression of ATP-binding cassette family G2 and ΔNp63 at the mRNA level was maintained or increased in CC-LMCs and 3D CC-LMC cultures compared to the control. The percentage of the K14(+) and K12(+) cells was comparable in these three cultures. There was no significant difference in the percentage of p63α high expressing cells in the control (21%) and 3D CC-LMC culture (17%, P > 0.05).

CONCLUSIONS

Human LMCs can substitute 3T3 cells in the expansion of LSCs using the 3-dimensional culture system.

Maintenance of the corneal epithelium is carried out by germinative cells of its basal stratum and not by presumed stem cells of the limbus

The purpose of this investigation was to analyze the proliferative behavior of rabbit corneal epithelium and establish if any particular region was preferentially involved in epithelial maintenance. [³H]-thymidine was injected intravitreally into both normal eyes and eyes with partially scraped corneal epithelium. Semithin sections of the anterior segment were evaluated by quantitative autoradiography. Segments with active replication (on) and those with no cell division (off) were intermingled in all regions of the tissue, suggesting that the renewal of the epithelial surface of the cornea followed an on/off alternating pattern. In the limbus, heavy labeling of the outermost layers was observed, coupled with a few or no labeled nuclei in the basal stratum. This suggests that this region is a site of rapid cell differentiation and does not contain many slow-cycling cells. The conspicuous and protracted labeling of the basal layer of the corneal epithelium suggests that its cells undergo repeated cycles of replication before being sent to the suprabasal strata. This replication model is prone to generate label-retaining cells. Thus, if these are adult stem cells, one must conclude that they reside in the corneal basal layer and not the limbal basal layer. One may also infer that the basal cells of the cornea and not of the limbus are the ones with the main burden of renewing the corneal epithelium. No particular role in this process could be assigned to the cells of the basal layer of the limbal epithelium.

Heterogeneity of limbal basal epithelial progenitor cells

Although corneal epithelial stem cells (SCs) are located at the limbus between the cornea and the conjunctiva, not all limbal basal epithelial cells are SCs. Using 2 dispase digestions to remove different amounts of limbal basal epithelial cells for cross-sections, flat mounts, and cytospin preparations, double immunostaining to pancytokeratins (PCK) and vimentin (Vim) identified 3 p63+ epithelial progenitors such as PCK-/Vim+, PCK/Vim, and PCK-/Vim+ and 1 p63+ mesenchymal cell, PCK-/Vim+. PCK-/Vim- progenitors had the smallest cell size were 10-20 times more enriched on collagen I-coated dishes in the 5-minute rapid adherent fraction that contained the highest percentage of p63+ cells but the lowest percentage of cytokeratin12+ cells, and gave rise to high Ki67 labeling and vivid clonal growth. In contrast, PCK+/Vim+ and PCK+/Vim- progenitors were found more in the slow-adherent fraction and yielded poor clonal growth. PCK/Vim progenitors and clusters of PCK-/Vim+ mesenchymal cells, which were neither melanocytes nor Langerhans cells, were located in the limbal basal region. Therefore, differential expression of PCK and Vim helps identify small PCK-/Vim- cells as the most likely candidate for SCs among a hierarchy of heterogeneous limbal basal progenitors, and their close association with PCK-/Vim+ presumed "niche" cells.

The effects of actin cytoskeleton perturbation on keratin intermediate filament formation in mesenchymal stem/stromal cells

F-actin plays a crucial role in composing the three-dimensional cytoskeleton and F-actin depolymerization alters fate choice of mesenchymal stem/stromal cells (MSCs). Here, we investigated differential gene expression and subsequent physiological changes in response to F-actin perturbation by latrunculin B in MSCs. Nineteen genes were down-regulated and 27 genes were up-regulated in the first 15 min after F-actin depolymerization. Functional enrichment analysis revealed that five genes involved in keratin (KRT) intermediate filaments clustering in the chromosome 17q21.2 region, i.e., KRT14, KRT19, KRT34, KRT-associated protein (KRTAP) 1-5, and KRTAP2-3, were strongly up-regulated. Transcription factor prediction identified NKX2.5 as the potential transcription factor to control KRT19, KRT34, KRTAP1-5, and KRTAP2-3; and indeed, the protein level of NKX2.5 was markedly increased in the nuclear fraction within 15 min of F-actin depolymerization. The peak of keratin intermediate filament formation was 1 h after actin perturbation, and the morphological changes showed by decrease in the ratio of long-axis to short-axis diameter in MSCs was observed after 4 h. Together, F-actin depolymerization rapidly triggers keratin intermediate filament formation by turning on keratin-related genes on chromosome 17q21.2. Such findings offer new insight in lineage commitment of MSCs and further scaffold design in MSC-based tissue engineering.

Topical administration of orbital fat-derived stem cells promotes corneal tissue regeneration

INTRODUCTION

Topical administration of eye drops is the major route for drug delivery to the cornea. Orbital fat-derived stem cells (OFSCs) possess an in vitro corneal epithelial differentiation capacity. Both the safety and immunomodulatory ability of systemic OFSC transplantation were demonstrated in our previous work. In this study, we investigated the safety, therapeutic effect, and mechanism(s) of topical OFSC administration in an extensive alkali-induced corneal wound.

METHODS

Corneal injury was created by contact of a piece of 0.5 N NaOH-containing filter paper on the corneal surface of a male Balb/c mouse for 30 s. The area of the filter paper covered the central 70% or 100% of the corneal surface. OFSCs (2 × 10(5)) in 5 μl phosphate-buffered saline (PBS) were given by topical administration (T) twice a day or by two intralimbal (IL) injections in the right cornea, while 5 μl of PBS in the left cornea served as the control.

RESULTS

Topical OFSCs promoted corneal re-epithelialization of both the limbal-sparing and limbal-involved corneal wounds. In the first three days, topical OFSCs significantly reduced alkali-induced corneal edema and stromal infiltration according to a histopathological examination. Immunohistochemistry and immunofluorescence staining revealed that transplanted cells were easily detectable in the corneal epithelium, limbal epithelium and stroma, but only some of transplanted cells at the limbal epithelium had differentiated into cytokeratin 3-expressing cells. OFSCs did not alter neutrophil (Ly6G) levels in the cornea, but significantly reduced macrophage (CD68) infiltration and inducible nitrous oxide synthetase (iNOS) production during acute corneal injury as quantified by a Western blot analysis. Continuous topical administration of OFSCs for seven days improved corneal transparency, and this was accompanied by diffuse stromal engraftment of transplanted cells and differentiation into p63-expressing cells at the limbal area. The therapeutic effect of the topical administration of OFSCs was superior to that of the IL injection. OFSCs from the IL injection clustered in the limbal area and central corneal epithelium, which was associated with a persistent corneal haze.

CONCLUSIONS

Topical OFSC administration is a simple, non-surgical route for stem cell delivery to promote corneal tissue regeneration through ameliorating acute inflammation and corneal epithelial differentiation. The limbal area serves as a niche for OFSCs differentiating into corneal epithelial cells in the first week, while the stroma is a potential site for anti-inflammation of OFSCs. Inhibition of corneal inflammation is related to corneal transparency.

Donor cornea transfer from Optisol GS to organ culture storage: a two-step procedure to increase donor tissue lifespan

Purpose

Storage time for donor corneas in Optisol GS is limited compared to Eye Bank Organ Culture (EBOC). We here examine the epithelium on donor corneoscleral rims after primary storage in Optisol GS and subsequent incubation in EBOC.

Methods

Morphology was monitored by light and electron microscopy, expression of phenotypic and genotypic markers by immunohistochemistry and RT-PCR and changes in oxidative lipid and DNA damage by ELISA and COMET assay.

Results

A prominent loss of cells was observed after storage in Optisol GS. After maintenance in EBOC, spreading apical cells were Occludin⁺, while the staining for E-cadherin and Connexin-43 was less intense. There were an upregulation of Occludin and a downregulation of E-cadherin and Connexin-43. Eye Bank Organ Culture was associated with an ongoing proliferative activity and a downregulation of putative progenitor/stem cell marker ABCG2 and p63. Staining for 8-OHdG and Caspase-3 did not increase, while levels of malondialdehyde and number of DNA strand breaks and oxidized bases increased.

Conclusions

This dual procedure should be pursued as an option to increase the storage time and the pool of available donor corneas. The observed downregulation of markers associated with stemness during EBOC is relevant considering the potential use of donor epithelium in the treatment of ocular surface disorders.

Pathological changes of the anatomical structure and markers of the limbal stem cell niche due to inflammation

PURPOSE

The corneoscleral limbus is the site of corneal epithelial stem cells (SC). The aim of this study is to evaluate the expression of different SC markers in the normal human limbus and to determine how this is affected by inflammation.

METHODS

Corneoscleral specimens from healthy and inflamed donor eyes were examined by immunohistochemistry/immunofluorescence for p63, vimentin, laminin 5, integrin α6, β1, β4, ABCG2, desmoglein 3, connexin 43, N-cadherin, and cytokeratins 12 and 15. The distribution and anatomic structure of the limbal crypts and the percentage of SC marker antigens in healthy donors were analyzed. In inflamed tissues, we evaluated the anatomic structure of the limbal epithelial crypt (LEC) and the positivity for SC markers.

RESULTS

In normal limbus, the niche structures were distributed differently. The variability of their number correlated with the percentage of p63 positivity. Integrin β1 staining directly correlated with p63 positivity while the remaining proteins were variably and widely distributed. Double staining for p63 and vimentin did not reveal any co-localization. In inflamed eyes, the basal cells in the crypts were "stretched" and surrounded by inflammatory cells, and only a few SC markers were still present.

CONCLUSIONS

Diseases involving the limbus may result in marked changes of expression of SC markers within the LEC and also alter the crypt structure.

The culture of limbal epithelial cells

The transplantation of cultured limbal epithelial cells (LEC) has since its first application in 1997 emerged as a promising technique for treating limbal stem cell deficiency. The culture methods hitherto used vary with respect to preparation of the harvested tissue, choice of culture medium, culture time, culture substrates, and supplementary techniques. In this chapter, we describe a procedure for establishing human LEC cultures using a feeder-free explant culture technique with human amniotic membrane (AM) as the culture substrate.

Phenotypic alteration of limbal niche-associated limbal epithelial stem cell deficiency by ultraviolet-B exposure-induced phototoxicity in mice

Good vision requires a healthy cornea, and a healthy cornea needs healthy stem cells. Limbal epithelial stem cells (LESCs) are a traditional source of corneal epithelial cells and are recruited for the continuous production of epithelium without seizing throughout an animal's life, which maintains corneal transparency. Like the maintenance of other adult somatic stem cells, the maintenance of LESCs depends on the specific microenvironmental niche in which they reside. The purpose of this study was to determine the microenvironmental damage associated with LESCs fate due to ultraviolet (UV)-B exposure in a mouse model. Structural alteration and deregulation of the stem cell and its neighboring niche components were observed by using clinical, morphological, explant culture study, and flowcytometric analysis, which demonstrated that the limbal microenvironment plays an important role in cornea-related disease development. In UV-exposed mice, overexpression of vascular endothelial growth factor receptor 2 indicated neovascularization, decreased CD38 expression signified the alteration of limbal epithelial superficial cells, and the loss of limbal stem cell marker p63 indicated limbal stem cell deficiency in the limbal vicinity. We concluded that LESC deficiency diseases (LESCDDs) are associated with pathophysiological changes in the LESC niche, with some inhibitory interception such as UV-B irradiation, which results in corneal defects.

Oxidative stress gradient in a medium during human corneal organ culture

PURPOSE

Lipid peroxidation content was measured in an organ culture medium after one-week storage of human donor corneas. Moreover, the effects of the medium on oxidative stress, antioxidant capacity, and the proliferation of cultured human corneal cells were studied.

METHODS

The medium was sampled from the upper and lower halves of storage vials and from controls (n=42). Malondialdehyde (MDA) was measured by high pressure liquid chromatography (HPLC). Cultured human corneal epithelium (CRL-11515) was exposed to different medium samples and monitored for changes in MDA (enzyme-linked immunosorbent assay [ELISA]), total antioxidant capacity (antioxidant assay kit), and proliferation (Ki-67).

RESULTS

A significant increase in MDA was observed in the organ culture medium in the lower level of storage vials. The addition of this fraction to cultured cells increased MDA significantly after 3 days, and the medium from both levels significantly increased MDA after 7 days. The medium from both levels significantly decreased the total antioxidant capacity of the cells but did not affect proliferative activity.

CONCLUSIONS

An oxidative gradient with an evident biologic effect is established in the medium in vials during organ culture of human donor corneas. Donor tissue stored at the bottom or in lower levels of such vials is exposed to a significant amount of oxidative stress.

Effect of biopsy location and size on proliferative capacity of ex vivo expanded conjunctival tissue

PURPOSE

To evaluate the effect of location and size of biopsy on phenotype and proliferative capacity of cultured rat conjunctival epithelial cells.

METHODS

Pieces of conjunctiva were used from six areas: superior and inferior areas of bulbus, fornix, and tarsus of male Sprague-Dawley rats (n = 6). Explants were grown in RPMI 1640 with 10% fetal bovine serum on coverslips for 8 days or assayed for colony-forming efficiency (n = 9). Analysis included immunofluorescence microscopy and outgrowth measurements with ImageJ software. The Mann-Whitney test and Spearman's rank-order correlation test were used.

RESULTS

Superior (23.9 ± 2.9-fold growth) and inferior (22.4 ± 1.2-fold growth) forniceal tissues yielded significantly more outgrowth with respect to explant size than superior bulbar (13.4 ± 1.9-fold growth; P < 0.05 and P < 0.01, respectively), inferior bulbar (13.6 ± 1.6-fold growth; P = 0.01 and P < 0.01, respectively), and inferior tarsal tissues (14.0 ± 1.3-fold growth; P = 0.01). Outgrowth size correlated positively with explant size (r(s) = 0.54; P < 0.001), whereas explant size correlated negatively with fold growth (r(s) = 0.36; P < 0.001). Superior forniceal cells displayed higher colony-forming efficiency (3.6% ± 0.9%) than superior bulbar (1.1% ± 0.3%; P < 0.05) and inferior bulbar cells (1.6% ± 0.8%; P < 0.05). Percentage of p63+ and PCNA+ cells correlated positively with explant and outgrowth size.

CONCLUSIONS

Small forniceal conjunctival explants grow the most effectively; however, for transplantation purposes, the loss of p63+ and PCNA+ cells with small explants must be considered.

Ex vivo expanded autologous limbal epithelial cells on amniotic membrane using a culture medium with human serum as single supplement

In patients with limbal stem cell deficiency (LSCD), transplantation of ex vivo expanded human limbal epithelial cells (HLECs) can restore the structural and functional integrity of the corneal surface. However, the protocol for cultivation and transplantation of HLECs differ significantly, and in most protocols growth additives such as cholera toxins, exogenous growth factors, hormones and fetal calf serum are used. In the present article, we compare for the first time human limbal epithelial cells (HLECs) cultivated on human amniotic membrane (HAM) in a complex medium (COM) including fetal bovine serum to a medium with human serum as single growth supplement (HSM), and report on our first examinations of HLECs expanded in autologous HSM and used for transplant procedures in patients with LSCD. Expanded HLECs were examined by genome-wide microarray, RT-PCR, Western blotting, and for cell viability, morphology, expression of immunohistochemical markers and colony forming efficiency. Cultivation of HLECs in HSM produced a multilayered epithelium where cells with markers associated with LESCs were detected in the basal layers. There were few transcriptional differences and comparable cell viability between cells cultivated in HSM and COM. The p63 gene associated with LESCs were expressed 3.5 fold more in HSM compared to COM, and Western blotting confirmed a stronger p63α band in HSM cultures. The cornea-specific keratin CK12 was equally found in both culture conditions, while there were significantly more CK3 positive cells in HSM. Cells in epithelial sheets on HAM remaining after transplant surgery of patients with LSCD expressed central epithelial characteristics, and dissociated cells cultured at low density on growth-arrested fibroblasts produced clones containing 21 ± 12% cells positive for p63α (n = 3). In conclusion, a culture medium without growth additives derived from animals or from animal cell cultures and with human serum as single growth supplement may serve as an equivalent replacement for the commonly used complex medium for ex vivo expansion of HLECs on HAM.

Human epithelial cell cultures from superficial limbal explants

PURPOSE

To study the kinetics of growth and the phenotype of cells cultured from human limbal explants in a cholera toxin-free medium with no feeder cell layer.

METHODS

Human organ-cultured corneas were used to prepare limbal explants (full-thickness and superficial limbal explants) and corneal stromal explants. Cell growth kinetics and phenotypes were assessed by cultivating explants in cholera toxin-free Green medium. Epithelial and progenitor cell markers were assessed by immunocytochemistry, flow cytometry, and Reverse Transcription and Polymerase Chain Reaction (RT-PCR).

RESULTS

The successful epithelial cell growth rates from full thickness limbal explant and superficial limbal explant tissues were 41 and 86%, respectively (p=0.0001). The mean cell area and the percentage of small cells in superficial and full-thickness explant cultures were, respectively, 317 µm(2) and 429 µm(2), and 8.9% and 1.7% (p<0.001). The percentage of positive cells in superficial and full-thickness limbal explant cultures as assessed by immunocytochemistry were the following: broad spectrum cytokeratins (cytokeratins 4, 5, 6, 8, 10, 13, and 18 [MNF116]), 82%/37% (p=0.01); cytokeratin 3 (CK3), 74%/25% (p=0.009); cytokeratin 19 (CK19), 46%/25% (p=0.19); vimentin, 56%/53% (p=0.48); delta N p63α, 54%/0% (p<0.001); and ABCG2, 5%/0% (p=0.1). Flow cytometry showed a higher percentage of small cells, a higher percentage of MNF116+ cells, and stronger expression of progenitor-associated markers in superficial than in full-thickness explant cultures. For superficial limbal explant cultures, analysis of the expression profiles for various mRNAs at the end of 21 days of culture showed high levels of expression of the mRNAs encoding CK3, vimentin, and CK19. The expression of mRNA of delta N p63α and ABCG2 was weaker. Cultures obtained from full-thickness limbal explants featured no expression of mRNA of CK19, delta N p63α, and ABCG2, whereas mRNAs encoding CK3 and vimentin were detected. Human corneal stromal explants cultured with the same medium featured late cell growth, large mean cell area (2,529 µm(2)), no expression of cytokeratins, delta N p63α, and ABCG2, and high expression of vimentin.

CONCLUSIONS

Superficial limbal explants appear to be superior to full-thickness limbal explants for growing human limbal epithelial cells. Preparation of explants using surgical facilities (i.e., operating microscope and microsurgical blades) led to a dramatic increase in the percentage of successful cultures, higher epithelial cell growth, decreased fibroblast contamination, and better preservation of limbal epithelial progenitors.

Induction of corneal epithelium-like cells from cynomolgus monkey embryonic stem cells and their experimental transplantation to damaged cornea

PURPOSE

We previously reported the successful transplantation of corneal epithelium-like cells derived from mouse embryonic stem (ES) cells onto injured mouse cornea. Here, we tested whether nonhuman primate ES cells have ability to differentiate into corneal epithelial cells and whether monkey ES cell-derived corneal epithelium-like cells were applicable for the experimental transplantation to damaged cornea.

METHODS

Monkey ES cells were cultivated on type IV collagen-coated dishes for various days to induce differentiation into corneal epithelium-like cells. The differentiation was evaluated by reverse transcription-polymerase chain reaction and immunostaining. The corneal epithelium-like cells were transplanted to the injured mouse cornea. Reconstitution of the corneal epithelium was evaluated by immunostaining.

RESULTS

The cells cultured on type IV collagen showed cobblestone-like appearance resembling epithelial cells. They expressed messenger RNA of pax6, p63, E-cadherin, CD44, proliferating cell nuclear antigen, keratin 3, and keratin 12. Protein expressions of pax6, keratin 3/12, p63, proliferating cell nuclear antigen, E-cadherin, and CD44 were confirmed by immunostaining. When the corneal epithelium-like cells were transplanted, they adhered to the corneal stroma, leading to formation of multiple cell layers. The grafted cells were stained with anti-human nuclear protein antibody, which cross-reacted with nuclei of monkey cells but not with those of mouse cells. They retained the expressions of keratin 3/12, E-cadherin, and CD44.

CONCLUSIONS

We induced corneal epithelium-like cells from monkey ES cells with moderate efficiency. The cells were successfully transplanted onto the injured mouse cornea. This is the first demonstration that nonhuman primate ES cells were induced to differentiate into corneal epithelium-like cells, which were applicable for transplantation to an animal model of corneal injury.

Corneal Epithelial Cultures Generated from Organ-Cultured Limbal Tissue: Factors Influencing Epithelial Cell Growth

PURPOSE

To explore the in vitro proliferative potential of human limbal epithelial cells after 31 degrees C organ-culture storage and to investigate putative factors influencing it.

METHODS

185 cultures of limbal explants were carried-out either from full-thickness explants (n = 102) or from enzymatically dissociated cells (n = 83) seeded on a feeder layer of human keratocytes. Epithelial outgrowth was assessed by phase contrast microscopy using a computerized image analysis software. Cell phenotype was evaluated by transmission electron microscopy and immunocytology. Univariate and multivariate analysis were performed to determine factors influencing epithelial growth in culture.

RESULTS

An epithelial outgrowth of 100 square mm or more was observed in 52% of cultures, (average growth area: 440 +/- 256 mm at three weeks). Corneal epithelial phenotype was confirmed by transmission electron microscopy, and cytokeratin pattern. Cytokeratine 19, deltaNp63, nestin and vimentin positive staining revealed undifferentiated epithelial cells in both explant and cell suspension cultures at three weeks. Short death to cornea retrieval time (p < 0.03) and female donors (p < 0.01) were associated with higher cell growth. Enzymatic treatment of explants by trypsin, but not dispase, decreased cell proliferation at two (p < 0.03) and three weeks (p < 0.04). Donor age, duration of corneal storage, and source of the explant did not influence the cell growth.

CONCLUSION

Organ-culture conditions can preserve limbal cell mitotic potential if limbal tissue is excised early after circulatory arrest. Human keratocytes can be used as a feeder layer allowing epithelial cells to maintain poorly differentiated phenotype in culture. Further investigations are needed to explain the influence of the donor sex on epithelial cell growth in culture.

Living-related conjunctival-limbal allograft for chronic or delayed-onset mustard gas keratopathy

PURPOSE

To determine the long-term outcomes of living-related stem cell transplantation in patients with delayed or chronic mustard gas keratopathy (MGK).

MATERIALS AND METHODS

In this noncomparative interventional case series, 21 consecutive patients with advanced delayed or chronic MGK received living-related conjunctival-limbal allograft and were followed up for at least 1 year. All subjects received immunosuppression with systemic cyclosporine. Main outcome measures were reduction of subjective complaints, corneal epithelial healing, and regression of corneal neovascularization adjacent to the transplant area.

RESULTS

Twenty-five eyes of 21 patients (all male), including 4 patients who received bilateral grafts, were operated. Mean age at the time of surgery was 35.8 +/- 3.8 years, mean interval between mustard gas exposure and surgery was 12.2 +/- 3.5 years, and mean follow-up was 37.2 +/- 18.5 months. Average size of the donor lenticule was 71.16 +/- 17.34 degrees. Simultaneous penetrating and lamellar keratoplasty were performed in 5 and 2 eyes, respectively. All patients consistently reported marked subjective improvement. Mean time for epithelial healing was 7.76 +/- 3.2 days. Visual acuity was 1.35 +/- 0.81 LogMAR before surgery, which improved to 0.59 +/- 0.34 LogMAR 3 months after the procedure (P < 0.001). Mean visual acuity at final examination was 0.82 +/- 0.49 LogMAR (P = 0.001). Acute stem cell rejection was observed in 10 (40%) eyes, which improved by increasing the dose of topical and systemic steroids. Chronic stem cell rejection was diagnosed in 8 (32%) eyes, which led to failure in 5 (20%) eyes.

CONCLUSIONS

Living-related conjunctival-limbal allograft is effective in stabilizing the ocular surface in patients with delayed or chronic MGK.

A novel isolation technique of progenitor cells in human corneal epithelium using non-tissue culture dishes

The existence of adult stem cells or progenitor cells in the human corneal epithelium (i.e., self-renewing squamous cells) has long been suggested, but these cells have not yet been isolated. Here we describe a novel isolation technique using non-tissue culture dishes to enrich progenitor cells, which are able to reconstitute a three-dimensional human corneal epithelial equivalent from single cells in serum-, feeder-, and bovine pituitary extract-free medium. These cells showed original tissue-committed differentiation, a high proliferative capacity, and limited self-renewal. Laminin-5 was measured by mass spectrometric analysis. Pretreatment of cells with anti-laminin-5 antibody demonstrated that laminin-5 was important in allowing corneal epithelial progenitor cells to adhere to non-tissue culture dishes. Hydrophilic tubes (used for cell collection throughout this study) are essential for efficient isolation of adherent corneal epithelial progenitor cells expressing laminin-5. These findings indicate that our new technique using non-tissue culture dishes allows the isolation of progenitor cells from human corneal limbal epithelium and that laminin-5 has a critical role in the adhesion of these cells.

Differences in the protein expression in limbal versus central human corneal epithelium--a search for stem cell markers

In the search for potential limbal stem cell protein markers, the purpose of this study was to characterize differences in protein expression between human central and limbal corneal epithelium by a proteomic approach using two-dimensional polyacrylamide gel electrophoresis (2D PAGE) combined with mass spectrometry (LC-MS/MS). The results were subsequently confirmed by Western blotting and immunohistochemistry. We detected more than 1000 protein spots in each gel. Thirty-two spots were significantly over-expressed in the central part and 70 spots were significantly over-expressed in the limbal part. We identified 25 different proteins. Among these 11 proteins representing different cellular locations and functions were selected for further investigations. Most interestingly, superoxide dismutase 2 (SOD2), was expressed in clusters of cells in the basal limbal epithelium. Heat shock protein 70 protein 1 (HSP70.1) and annexin I were highly abundant in limbal epithelium, although they were also present in the central epithelium to a minor extent. Among the proteins primarily expressed in the limbal fraction we further identified cytokeratin (CK) 15, CK19 and alpha enolase, which have been reported previously to be related to the limbal basal epithelium. The basal limbal epithelium consists of clusters of slow cycling limbal stem cells and rapid cycling transient amplifying cells. Ideally, proteins exclusively expressed in the limbal part of the epithelium may serve as markers for the basal limbal cells. SOD2 and CK15 identify clusters of limbal basal cells and therefore they may serve as markers for limbal stem cells in conjunction with the earliest transient amplifying cells.

Characterization of DeltaNp63 isoforms in normal cornea and telomerase-immortalized human corneal epithelial cells

Previous reports have suggested that specific isoforms of the potential stem cell marker p63 may regulate corneal epithelial homeostatic renewal through control of cell proliferation. In this study, we characterized the presence of DeltaNp63 isoforms in telomerase-immortalized human corneal epithelial cells (hTCEpi) in comparison to normal human corneal epithelium to validate the hTCEpi cell line as a viable model for the study of p63 isoforms. We further examined roles for DeltaNp63 in proliferation and differentiation. For in vitro studies, hTCEpi cells were cultured in serum-free culture media and grown under 0.15 mM calcium or sequential 1.15 mM calcium/air-lifted culture. Fresh donor human corneal tissue was used to assess expression and localization in situ. mRNA and protein levels were assessed by real-time PCR, Immunofluorescence (IF) and Western blotting (WB). DeltaNp63 expression levels throughout the cell cycle were assessed by double-labeling with DeltaNp63 and Ki-67. In situ, DeltaNp63 localized to nuclei throughout the human corneal epithelium and was lost only in superficial cells. WB confirmed the presence of all three DeltaNp63 isoforms in the central corneal epithelium and in hTCEpi cells. DeltaNp63 mRNA levels decreased when grown on collagen substrate and under increased calcium/air-lifted culture. mRNA and protein levels increased as cells approached confluence, with a significant decrease in post-confluent culture. DeltaNp63 expression levels did not vary with the cell cycle, as assessed by Ki-67 labeling. Collectively, the presence of all three DeltaNp63 isoforms in hTCEpi cells and in intact cornea validates the use of this cell line for the study of individual isoforms in the corneal epithelium; and these data suggest that expression of DeltaNp63 isoforms are not altered as a function of the cell cycle or cell division in subconfluent hTCEpi cells cultured in serum-free media, but demonstrate reduced expression upon contact-inhibited growth down-regulation and differentiation. Significantly, the localization of DeltaNp63 in central corneal epithelial cells with a loss of expression in superficial cells suggests that DeltaNp63 may play a role in mediating desquamative events at the ocular surface.

Corneal epithelial stem cells: characterization, culture and transplantation

The epithelium covering the cornea at the front of the eye is maintained by stem cells located at its periphery, in a region known as the limbus. A lack or dysfunction of these so-called limbal stem cells (LSCs) results in the painful and blinding disease of LSC deficiency. In this review, current knowledge regarding the biology of these particular stem cells will be outlined, including recent advances that are enabling the gene expression analysis of these cells. The use of LSCs in therapeutic interventions for LSC deficiency will also be discussed, including the role for ex vivo expansion. In particular, the translation of basic science advances in LSC biology into therapeutic strategies will be highlighted.

Human amniotic epithelial cells as novel feeder layers for promoting ex vivo expansion of limbal epithelial progenitor cells

Human amniotic epithelial cells (HAECs) are a unique embryonic cell source that potentially can be used as feeder layers for expanding different types of stem cells. In vivo, HAECs uniformly expressed pan-cytokeratins (pan-CK) and heterogeneously expressed vimentin (Vim). The two phenotypes expressing either pan-CK(+)/Vim(+) or pan-CK(+)/Vim(-) were maintained in serum-free media with high calcium. In contrast, all HAECs became pan-CK(+)/Vim(+) in serum-containing media, which also promoted HAEC proliferation for at least eight passages, especially supplemented with epidermal growth factor and insulin. Mitomycin C-arrested HAEC feeder layers were more effective in promoting clonal growth of human limbal epithelial progenitors than conventional 3T3 murine feeder layers. Cells in HAEC-supported clones were uniformly smaller, sustained more proliferation, and expressed less CK12 and connexin 43 but higher levels of stem cell-associated markers such as p63, Musashi-1, and ATP-binding cassette subfamily G2 than those of 3T3-supported clones. Subculturing of clonally expanded limbal progenitors from HAEC feeder layers, but not from 3T3 feeder layers, gave rise to uniformly p63-positive epithelial progenitor cells as well as nestin-positive neuronal-like progenitors. Collectively, these results indicated that HAECs can be used as a human feeder layer equivalent for more effective ex vivo expansion of adult epithelial stem cells from the human limbus. Disclosure of potential conflicts of interest is found at the end of this article.

Identification of Notch-1 expression in the limbal basal epithelium

PURPOSE

To determine whether Notch-1, a ligand-activated transmembrane receptor known to maintain cells in an undifferentiated state, primarily progenitor cells in other systems, could be used as a stem cell marker in human limbal epithelium.

METHODS

Human corneoscleral tissues obtained from the Doheny Eye & Tissue Transplant Bank were prepared for cross section and whole mount analysis. Tissue for whole mount was incubated in dispase; the epithelial sheet was removed and fixed in 4% paraformaldehyde. Sections and whole mount were stained with antibodies against Notch-1, Notch-2, beta-1 integrin, alpha-6, and the G2 subtype member of the ATP binding cassette transporter (ABCG2). Specificity of the Notch-1 antibody was determined by western blot analysis with Cos-7 cells transfected with Notch-1. Explant culture was performed and only primary cultures were used in this experiment.

RESULTS

Notch-1 was found to be expressed in the limbal basal region where stem cells reside. Notch-1 antigenicity was more pronounced in cell clusters, mainly in the palisades of Vogt. The central cornea was almost devoid of Notch-1. The intensity of Notch-1 staining in cultured cells from the limbal explants was high in only a few cells. The Notch-1 signal was diminished in dividing cells. Expression in cultured cells was more cytoplasmic; few cells showed additional nuclear staining. The Notch-1-stained whole mount showed only a few cells in the limbal region. A 300 kDa and a 110 kDa band confirmed the specificity of the antibody in Cos-7 cells transfected with Notch-1. Double staining for ABCG2 and Notch-1 showed some ABCG2-positive cells co-expressing Notch-1 in the limbal basal epithelium, indicating that Notch-1-expressing cells might be a unique subpopulation of cells with stem cell properties.

CONCLUSIONS

Immunofluorescence data shows that Notch-1 could be a possible marker for the stem cells in the limbal basal epithelium. Further studies and characterization of the Notch pathway in corneal development will provide valuable clues for the identification of stem cells.

Effect of limbal explant orientation on the histology, phenotype, ultrastructure and barrier function of cultured limbal epithelial cells

PURPOSE

To compare the histology, phenotype, ultrastructure and barrier function of cultured limbal epithelial cells using two explant culture protocols.

METHODS

Epithelial cells were cultured for 16 days from limbal explants, positioned with either the stromal side (stromal group) or the epithelial side (epithelial group) on intact amniotic membranes. The cultured epithelium (n = 56) was examined using light microscopy, immunohistochemistry for K3, Cx43, ABCG2 and p63 expression, Western blot analysis of DeltaNp63alpha, transmission electron microscopy, a horseradish peroxidase (HRP) permeability assay and scanning electron microscopy.

RESULTS

The epithelial group demonstrated a significantly higher expression of p63-positive cells (85.7 +/- 4.2%) than the stromal group (75.3 +/- 8.9%), and Western blots showed a stronger band of DeltaNp63alpha. K3 and ABCG2 were not detected in either group, whereas Cx43 displayed moderate immunostaining in the suprabasal layer. The number of cell layers, the desmosome number and the undulation length in the epithelial group were not significantly different from those in the stromal group. In both groups, HRP accumulated on the apical surface of the superficial cells, and scanning electron microscopy demonstrated tightly apposed superficial cells.

CONCLUSIONS

Our findings indicate that limbal explants positioned epithelial side down may give rise to cultured epithelia with higher expression of p63 and DeltaNp63alpha.

A novel method for preserving cultured limbal epithelial cells

AIM

To investigate organ culture preservation of cultured limbal epithelial cells in order to enhance the availability of tissue-engineered epithelia that are used to treat patients with limbal stem cell deficiency.

METHODS

Limbal epithelial cells were cultured for 3 weeks on intact amniotic membrane fastened to a polyester membrane carrier. The cultured epithelia were stored for 1 week at 23 degrees C in organ culture medium. The preserved epithelia were then examined using a colorimetric cell viability assay, light microscopy and immunohistochemistry.

RESULTS

The viability of the preserved epithelia was 84% (20%), and no statistically significant difference was found compared with non-preserved epithelia. In general, the cell borders were maintained, the nuclei showed no sign of degeneration, and the original layered structure was preserved. Mild intercellular oedema was occasionally observed. Expression of p63, K19 and vimentin was maintained.

CONCLUSIONS

Cultured limbal epithelial cells can be preserved in organ culture medium for 1 week at room temperature, while maintaining the original layered structure and undifferentiated phenotype.