Morphological and functional analysis of immortalized human corneal endothelial cells after transplantation

Fabrication of bioengineered corneal endothelial grafts using an allogeneic cornea-derived matrix

Corneal endothelial keratoplasty has been the primary treatment method of endothelial decompensation, but it is often limited in clinical practice due to global shortage of donor cornea. Here, we explored using an ultra-thin allogeneic cornea-derived matrix (uACM) films as a substrate for constructing bioengineered corneal endothelial grafts. We evaluated the films' optical, mechanical, and structural properties, and measured the composition of the extracellular matrix. The uACM was an ultrathin and curved cornea-shaped film with favorable optical and mechanical properties. The fabrication process efficiently preserved corneal extracellular matrix composition and significantly decreased cellular components. Moreover, human corneal endothelial cells and rabbit corneal endothelial cells (RCECs) can adhere and grow on the uACM films with a positive expression of the corneal endothelial functional markers Na+/K+-ATPase and ZO-1. The successful transplantation of uACM with RCECs grafts into the rabbit model of endothelial dysfunction via Descemet membrane endothelial keratoplasty resulted in prompt restoration of corneal transparency and thickness. During the four-week follow-up period, the uACM with RCECs implanted eyes exhibited comparable corneal transparency, central corneal thickness, and endothelial cell count to that of the healthy rabbit. Histologic examination revealed that the grafts were successfully attached and integrated onto the posterior surface of the corneal stroma. The uACM achieved biomimetic reconstruction in terms of both composition and structure, and can be used to construct the bioengineered corneal endothelial grafts. These results indicate that constructing bioengineered corneal endothelial grafts from discarded human corneal tissues may pave the way for generating high-quality corneal endothelial grafts for transplantation.

New developments in corneal endothelial cell replacement

Corneal transplantation is currently the most effective treatment to restore corneal clarity in patients with endothelial disorders. Endothelial transplantation, either by Descemet membrane endothelial keratoplasty (DMEK) or by Descemet stripping (automated) endothelial keratoplasty (DS(A)EK), is a surgical approach that replaces diseased Descemet membrane and endothelium with tissue from a healthy donor eye. Its application, however, is limited by the availability of healthy donor tissue. To increase the pool of endothelial grafts, research has focused on developing new treatment options as alternatives to conventional corneal transplantation. These treatment options can be considered as either 'surgery-based', that is tissue-efficient modifications of the current techniques (e.g. Descemet stripping only (DSO)/Descemetorhexis without endothelial keratoplasty (DWEK) and Quarter-DMEK), or 'cell-based' approaches, which rely on in vitro expansion of human corneal endothelial cells (hCEC) (i.e. cultured corneal endothelial cell sheet transplantation and cell injection). In this review, we will focus on the most recent developments in the field of the 'cell-based' approaches. Starting with the description of aspects involved in the isolation of hCEC from donor tissue, we then describe the different natural and bioengineered carriers currently used in endothelial cell sheet transplantation, and finally, we discuss the current 'state of the art' in novel therapeutic approaches such as endothelial cell injection.

Approaches for corneal endothelium regenerative medicine

The state of the art therapy for treating corneal endothelial disease is transplantation. Advances in the reproducibility and accessibility of surgical techniques are increasing the number of corneal transplants, thereby causing a global deficit of donor corneas and leaving 12.7 million patients with addressable visual impairment. Approaches to regenerate the corneal endothelium offer a solution to the current tissue scarcity and a treatment to those in need. Methods for generating corneal endothelial cells into numbers that could address the current tissue shortage and the possible strategies used to deliver them have now become a therapeutic reality with clinical trials taking place in Japan, Singapore and Mexico. Nevertheless, there is still a long way before such therapies are approved by regulatory bodies and become clinical practice. Moreover, acellular corneal endothelial graft equivalents and certain drugs could provide a treatment option for specific disease conditions without the need of donor tissue or cells. Finally, with the emergence of gene modulation therapies to treat corneal endothelial disease, it would be possible to treat presymptomatic patients or those presenting early symptoms, drastically reducing the need for donor tissue. It is necessary to understand the most recent developments in this rapidly evolving field to know which conditions could be treated with which approach. This article provides an overview of the current and developing regenerative medicine therapies to treat corneal endothelial disease and provides the necessary guidance and understanding towards the treatment of corneal endothelial disease.

Proliferation Increasing Genetic Engineering in Human Corneal Endothelial Cells: A Literature Review

The corneal endothelium is the inner layer of the cornea. Despite comprising only a monolayer of cells, dysfunction of this layer renders millions of people visually impaired worldwide. Currently, corneal endothelial transplantation is the only viable means of restoring vision for these patients. However, because the supply of corneal endothelial grafts does not meet the demand, many patients remain on waiting lists, or are not treated at all. Possible alternative treatment strategies include intracameral injection of human corneal endothelial cells (HCEnCs), biomedical engineering of endothelial grafts and increasing the HCEnC density on grafts that would otherwise have been unsuitable for transplantation. Unfortunately, the limited proliferative capacity of HCEnCs proves to be a major bottleneck to make these alternatives beneficial. To tackle this constraint, proliferation enhancing genetic engineering is being investigated. This review presents the diverse array of genes that have been targeted by different genetic engineering strategies to increase the proliferative capacity of HCEnCs and their relevance for clinical and research applications. Together these proliferation-related genes form the basis to obtain a stable and safe supply of HCEnCs that can tackle the corneal endothelial donor shortage.

Evolution of therapies for the corneal endothelium: past, present and future approaches

Corneal endothelial diseases are leading indications for corneal transplantations. With significant advancement in medical science and surgical techniques, corneal transplant surgeries are now increasingly effective at restoring vision in patients with corneal diseases. In the last 15 years, the introduction of endothelial keratoplasty (EK) procedures, where diseased corneal endothelium (CE) are selectively replaced, has significantly transformed the field of corneal transplantation. Compared to traditional penetrating keratoplasty, EK procedures, namely Descemet's stripping automated endothelial keratoplasty (DSAEK) and Descemet membrane endothelial keratoplasty (DMEK), offer faster visual recovery, lower immunological rejection rates, and improved graft survival. Although these modern techniques can achieve high success, there are fundamental impediments to conventional transplantations. A lack of suitable donor corneas worldwide restricts the number of transplants that can be performed. Other barriers include the need for specialized expertise, high cost, and risks of graft rejection or failure. Research is underway to develop alternative treatments for corneal endothelial diseases, which are less dependent on the availability of allogeneic tissues - regenerative medicine and cell-based therapies. In this review, an overview of past and present transplantation procedures used to treat corneal endothelial diseases are described. Potential novel therapies that may be translated into clinical practice will also be presented.

Experimental models of corneal endothelial cell therapy and translational challenges to clinical practice

The human corneal endothelium (CE) is a post-mitotic monolayer of endothelial cells, thought to be incapable of in vivo regeneration. Dysfunction of the CE is a commonly cited indication for corneal transplantation, with corneal blindness being the fifth most common cause of blindness globally. In 2012 alone 184,576 corneal transplants were performed in 116 countries (Gain et al., 2016). Presently, outcomes following human corneal transplantation have been reported to have over 97% success rate in restoring the recipient's vision (Patel et al., 2019). However, the continuing demand for cadaveric human corneas has driven research into alternative sources of CE and with the advent of protocols to produce cultured hCECs there is now the potential for cell therapy to regenerate the damaged CE. This review aims to examine the merits and limitations of different types of human and animal models used so far to test the concept of CE cell therapy.

Construction of a high cell density human corneal endothelial equivalent and its transplantation in primate models

BACKGROUND

Recently, many patients with corneal blindness caused by endothelial dysfunction have no opportunity to receive keratoplasty therapy because of the extremely limited number of donor corneas. Corneal tissue engineering opens a new path for in vitro reconstruction of tissue-engineered HCE which will cure the corneal endotheliopathy by clinical corneal transplantation. In this study, we construct a human corneal endothelium (HCE) equivalent with non-transfected monoclonal HCE (mcHCE) cells and modified denuded amniotic membrane (mdAM), and evaluate its functions in monkey models.

METHODS

Tissue-engineered HCE (TE-HCE) was constructed by culturing DiI-labeled mcHCE cells on mdAMs in 20% fetal bovine serum-containing DMEM/Ham's Nutrient Mixture F12 (1:1) medium and 5% CO₂ at 37°C on a 24-well culture plate. The constructed TE-HCE was transplanted into monkey corneas via penetrating keratoplasty with Descemet's membrane and endothelium stripped. The corneal transparency, thickness, and intraocular pressure were monitored in vivo, and the corneal morphology and histological structure were examined ex vivo 181 days after surgery.

RESULTS

The constructed TE-HCE, with an average density of 3602.22 ± 45.22 cells/mm² , mimicked its natural counterpart both in morphology and histological structure. In vivo, corneal transparency was maintained, and the corneal thickness gradually decreased to 567.33 ± 72.77 μm at day 181 after TE-HCE transplanted into monkey eyes, while intense corneal edema and turbid were found in mdAM-transplanted eyes with their corneal thicknesses maintained over 1000 μm during the monitoring period. Ex vivo, a monolayer of corneal endothelium, consisting of mcHCE cells at a density of 2795.65 ± 156.83 cells/mm² , was reconstructed in transplanted monkey eyes. The cells in the transplanted area had the hexagonal or polygonal morphology and normal ultrastructure, and established plenty of cell-cell and cell-stromal matrix junctions. Besides, huge membrane-bounded flat stacks with electric dense inclusions were found in mcHCE cells beneath the plasma membrane at the stromal side.

CONCLUSIONS

The constructed TE-HCE has normal histological property and functions well in monkey models. The TE-HCE could be used as a promising HCE equivalent in therapy of corneal endothelium dysfunction and corneal regenerative medicine.

A Human Corneal Equivalent Constructed from SV40-immortalised Corneal Cell Lines

Within the last decade, extensive research in the field of tissue and organ engineering has focused on the development of in vitro models of the cornea. The use of organotypic, three-dimensional corneal equivalents has several advantages over simple monolayer cultures. The aim of this study was to develop a corneal equivalent model composed of the same cell types as in the natural human tissue, but by using immortalised cell lines to ensure reproducibility and to minimise product variation. We report our success in the establishment of an SV40-immortalised human corneal keratocyte cell line (designated HCK). A collagen matrix, built up with these cells, displayed the morphological characteristics of the human stromal tissue and served as a biomatrix for the immortalised human corneal epithelial and endothelial cells. Histological cross-sections of the whole-cornea equivalents resemble human corneas in tissue structure. This organotypic in vitro model may serve as a research tool for the ophthalmic science community, as well as a model system for testing for eye irritancy and drug efficacy.

Descemet's Membrane Biomimetic Microtopography Differentiates Human Mesenchymal Stem Cells Into Corneal Endothelial-Like Cells

Supplemental Digital Content is Available in the Text.

Purpose:

Loss of corneal endothelial cells (CECs) bears disastrous consequences for the patient, including corneal clouding and blindness. Corneal transplantation is currently the only therapy for severe corneal disorders. However, the worldwide shortages of corneal donor material generate a strong demand for personalized stem cell–based alternative therapies. Because human mesenchymal stem cells are known to be sensitive to their mechanical environments, we investigated the mechanotransductive potential of Descemet membrane–like microtopography (DLT) to differentiate human mesenchymal stem cells into CEC-like cells.

Methods:

Master molds with inverted DLT were produced by 2-photon lithography (2-PL). To measure the mechanotransductive potential of DLT, mesenchymal stem cells were cultivated on silicone or collagen imprints with DLT. Changes in morphology were imaged, and changes in gene expression of CEC typical genes such as zonula occludens (ZO-1), sodium/potassium (Na/K)-ATPase, paired-like homeodomain 2 (PITX2), and collagen 8 (COL-8) were measured with real-time polymerase chain reaction. At least immunofluorescence analysis has been conducted to confirm gene data on the protein level.

Results:

Adhesion of MSCs to DLT molded in silicone and particularly in collagen initiates polygonal morphology and monolayer formation and enhances not only transcription of CEC typical genes such as ZO-1, Na/K-ATPase, PITX2, and COL-8 but also expression of the corresponding proteins.

Conclusions:

Artificial reproduction of Descemet membrane with respect to topography and similar stiffness offers a potential innovative way to bioengineer a functional CEC monolayer from autologous stem cells.

Feasibility Study of Human Corneal Endothelial Cell Transplantation Using an In Vitro Human Corneal Model

PURPOSE

To test the feasibility of a cell therapy approach to treat corneal endothelial (CE) disorders using an in vitro model of human corneal decompensation.

METHODS

A CE decompensation model was established by removal of the Descemet membrane/endothelium complex from cadaveric human corneas in an air interface organ culture system (group 2) and compared with normal corneas (group 1). The posterior stroma of decompensated corneas was seeded with immortalized human corneal endothelial cells (HCEC-12) in group 3 and passage 0 primary human CE cells in group 4 corneas. Functional effects on stromal thickness were determined with histological analysis 3 to 10 days after cell therapy treatment.

RESULTS

Removal of the Descemet membrane/endothelium complex in group 2 corneas resulted in a stromal thickness of 903 ± 86 μm at 12 hours compared with 557 ± 72 μm in group 1 corneas. Stromal thickness reduced from 1218 ± 153 μm to 458 ± 90 μm (63% ± 6%, P = 0.001) after cell transplantation in group 3 and from 1100 ± 86 μm to 489 ± 94 μm (55% ± 7%, P = 0.00004) in group 4. Posttransplantation histology demonstrated formation of a monolayer of corneal endothelium attached to the posterior stromal surface.

CONCLUSIONS

Direct transplantation of cultured human CE cells and immortalized HCEC-12 to bare posterior corneal stroma resulted in formation of an endothelial monolayer and restoration of stromal hydration to physiological thickness, demonstrating the feasibility of cell therapy in treatment of CE decompensation in a human in vitro model.

A microengineered human corneal epithelium-on-a-chip for eye drops mass transport evaluation

Animals are commonly used for pharmacokinetic studies which are the most frequent events tested during ocular drug development and preclinical evaluation. Inaccuracy, cost, and ethical criticism in these tests have created a need to construct an in vitro model for studying corneal constraints. In this work, a porous membrane embedded microfluidic platform is fabricated that separates a chip into an apical and basal side. After functionalizing the membrane surface with fibronectin, the membrane's mechanical and surface properties are measured to ensure correct modeling of in vivo characteristics. Immortalized human corneal epithelial cells are cultured on the membrane to create a microengineered corneal epithelium-on-a-chip (cornea chip) that is validated with experiments designed to test the barrier properties of the human corneal epithelium construct using model drugs. A pulsatile flow model is used that closely mimics the ocular precorneal constraints and is reasonable for permeability analysis that models in vivo conditions. This model can be used for preclinical evaluations of potential therapeutic drugs and to mimic the environment of the human cornea.

Human Corneal Endothelial Cells Expanded In Vitro Are a Powerful Resource for Tissue Engineering

Human corneal endothelial cells have two major functions: barrier function mediated by proteins such as ZO-1 and pump function mediated by Na-K-ATPase which help to maintain visual function. However, human corneal endothelial cells are notorious for their limited proliferative capability in vivo and are therefore prone to corneal endothelial dysfunction that eventually may lead to blindness. At present, the only method to cure corneal endothelial dysfunction is by transplantation of a cadaver donor cornea with normal corneal endothelial cells. Due to the global shortage of donor corneas, it is vital to engineer corneal tissue in vitro that could potentially be transplanted clinically. In this review, we summarize the advances in understanding the behavior of human corneal endothelial cells, their current engineering strategy in vitro and their potential applications.

Biodegradable and biocompatible poly(ethylene glycol)-based hydrogel films for the regeneration of corneal endothelium

Corneal endothelial cells (CECs) are responsible for maintaining the transparency of the human cornea. Loss of CECs results in blindness, requiring corneal transplantation. In this study, fabrication of biocompatible and biodegradable poly(ethylene glycol) (PEG)-based hydrogel films (PHFs) for the regeneration and transplantation of CECs is described. The 50-μm thin hydrogel films have similar or greater tensile strengths to human corneal tissue. Light transmission studies reveal that the films are >98% optically transparent, while in vitro degradation studies demonstrate their biodegradation characteristics. Cell culture studies demonstrate the regeneration of sheep corneal endothelium on the PHFs. Although sheep CECs do not regenerate in vivo, these cells proliferate on the films with natural morphology and become 100% confluent within 7 d. Implantation of the PHFs into live sheep corneas demonstrates the robustness of the films for surgical purposes. Regular slit lamp examinations and histology of the cornea after 28 d following surgery reveal minimal inflammatory responses and no toxicity, indicating that the films are benign. The results of this study suggest that PHFs are excellent candidates as platforms for the regeneration and transplantation of CECs as a result of their favorable biocompatibility, degradability, mechanical, and optical properties.

Tissue engineering of the corneal endothelium: a review of carrier materials

Functional impairment of the human corneal endothelium can lead to corneal blindness. In order to meet the high demand for transplants with an appropriate human corneal endothelial cell density as a prerequisite for corneal function, several tissue engineering techniques have been developed to generate transplantable endothelial cell sheets. These approaches range from the use of natural membranes, biological polymers and biosynthetic material compositions, to completely synthetic materials as matrices for corneal endothelial cell sheet generation. This review gives an overview about currently used materials for the generation of transplantable corneal endothelial cell sheets with a special focus on thermo-responsive polymer coatings.

Corneal endothelial regeneration and tissue engineering

Human corneal endothelial cells (HCECs) have a limited proliferative capacity. Descemet stripping with automated endothelial keratoplasty (DSAEK) has become the preferred method for the treatment of corneal endothelial deficiency, but it requires a donor cornea. To overcome the shortage of donor corneas, transplantation of cultured HCEC sheets has been attempted in experimental studies. This review summarizes current knowledge about the mechanisms of corneal endothelial wound healing and about tissue engineering for the corneal endothelium. We also discuss recent work on tissue engineering for DSAEK grafts using cultured HCECs and HCEC precursor cell isolation method (the sphere-forming assay). DSAEK grafts (HCEC sheets) were constructed by seeding cultured HCECs on human amniotic membrane, thin human corneal stroma, and collagen sheets. The pump function of the HCEC sheets thus obtained was approximately 75%-95% of that for human donor corneas. HCEC sheets were transplanted onto rabbit corneas after DSAEK. While the untransplanted control group displayed severe stromal edema, the transplanted group had clear corneas throughout the observation period. The sphere-forming assay using donor human corneal endothelium or cultured HCECs can achieved mass production of human corneal endothelial precursors. These findings indicate that cultured HCECs transplanted after DSAEK can perform effective corneal dehydration in vivo and suggest the feasibility of employing the transplantation of cultured HCECs to treat endothelial dysfunction. Additionally, corneal endothelial precursors may be an effective strategy for corneal endothelial regeneration.

Tissue engineering of corneal endothelium

Human corneal endothelial cells (HCECs) do not replicate after wounding. Therefore, corneal endothelial deficiency can result in irreversible corneal edema. Descemet stripping automated endothelial keratoplasty (DSAEK) allows selective replacement of the diseased corneal endothelium. However, DSAEK requires a donor cornea and the worldwide shortage of corneas limits its application. This review presents current knowledge on the tissue engineering of corneal endothelium using cultured HCECs. We also provide our recent work on tissue engineering for DSAEK grafts using cultured HCECs. We reconstructed DSAEK grafts by seeding cultured DiI-labelled HCECs on collagen sheets. Then HCEC sheets were transplanted onto the posterior stroma after descemetorhexis in the DSAEK group. Severe stromal edema was detected in the control group, but not in the DSAEK group throughout the observation period. Fluorescein microscopy one month after surgery showed numerous DiI-labelled cells on the posterior corneal surface in the DSAEK group. Frozen sections showed a monolayer of DiI-labelled cells on Descemet’s membrane. These findings indicate that cultured adult HCECs, transplanted with DSAEK surgery, maintain corneal transparency after transplantation and suggest the feasibility of performing DSAEK with HCECs to treat endothelial dysfunction.

Poloxamines for deswelling of organ-cultured corneas

BACKGROUND

Poloxamines are amphiphilic tetrofunctional block copolymers composed of four polyoxyethylene-polyoxypropylene arms joined to a central ethylene diamine bridge. Their safe profile allows diverse pharmaceutical and biomedical applications.

AIM

To assess their use for corneal deswelling using a porcine model of organ culture (OC).

METHODS

Five poloxamines (T90R4, T904, T908, T1107 and T1307) were dissolved in a standard commercial OC medium (control) to reach 350 mosm kg(-1). In vitro cytotoxicity was tested using MTT assay on human corneal epithelial and endothelial cell (EC) lines and on primary human corneal fibroblasts. Paired porcine corneas stored in OC for 3 days were assigned for 48 h to a poloxamine medium or to a standard deswelling medium containing 5% dextran T500. Corneal EC density, morphometry, mortality, stromal thickness and transparency were evaluated before and after deswelling. Post-deswelling, EC viability/mortality was determined using a fluorescent live/dead assay.

RESULTS

Besides similar corneal thickness reduction and transparency improvement, T908, T1107 and T1307 decreased EC loss (5.4 ± 1.7% vs. 9.9 ± 2.6% in controls (p < 0.001)) and mortality, improved EC morphometry and reduced endothelial lesions compared to dextran.

CONCLUSION

On this porcine model, poloxamines T908, T1107 and T1307 appear as good candidates to replace dextran for the deswelling. Experiments on human corneas are now necessary to confirm their efficiency and safety profile in OC.

Long-term cultivation of human corneal endothelial cells by telomerase expression

The objective of this study was to explore the potential role of human telomerase reverse transcriptase (TERT) in extending the proliferative lifespan of human corneal endothelial cells (HCECs) under long-term cultivation. A primary culture was initiated with a pure population of HCECs in DMEM/F12 media containing 10% fetal bovine serum and other various supplements. TERT gene was successfully transfected into normal HCECs. A stable HCECs cell line (TERT-HCECs) that expressed TERT was established. The cells could be subcultured for 36 passages. Within this line of cells, TERT not only extended proliferative lifespan and inhibited apoptosis but also enhanced the cell line remaining the normal characteristics similar to HCECs. There were no significantly differences in the expression of the pump function related proteins voltage dependent anion channel 3 (VDAC3), sodium bicarbonate cotransporter member 4 (SLC4A4), chloride channel protein 3 (CLCN3), Na(+)/K(+)-ATPase α1, and ZO-1 in the cell line TERT-HCECs and primary HCECs. TERT-HCECs formed a monolayer cell sheet, maintained similar cell junction formation and pump function with primary HCECs. Karyotype analysis exhibited normal chromosomal numbers. The soft agar colony assay and tumor formation in nude mice assay showed no malignant alterations in TERT-HCECs. Our findings indicated that we had established a cell line with its similar phenotype and properties to primary HCECs. Further study of the TERT-HCECs may be valuable in studying the function of the cells in vivo.

Proliferative capacity of corneal endothelial cells

The corneal endothelial monolayer helps maintain corneal transparency through its barrier and ionic "pump" functions. This transparency function can become compromised, resulting in a critical loss in endothelial cell density (ECD), corneal edema, bullous keratopathy, and loss of visual acuity. Although penetrating keratoplasty and various forms of endothelial keratoplasty are capable of restoring corneal clarity, they can also have complications requiring re-grafting or other treatments. With the increasing worldwide shortage of donor corneas to be used for keratoplasty, there is a greater need to find new therapies to restore corneal clarity that is lost due to endothelial dysfunction. As a result, researchers have been exploring alternative approaches that could result in the in vivo induction of transient corneal endothelial cell division or the in vitro expansion of healthy endothelial cells for corneal bioengineering as treatments to increase ECD and restore visual acuity. This review presents current information regarding the ability of human corneal endothelial cells (HCEC) to divide as a basis for the development of new therapies. Information will be presented on the positive and negative regulation of the cell cycle as background for the studies to be discussed. Results of studies exploring the proliferative capacity of HCEC will be presented and specific conditions that affect the ability of HCEC to divide will be discussed. Methods that have been tested to induce transient proliferation of HCEC will also be presented. This review will discuss the effect of donor age and endothelial topography on relative proliferative capacity of HCEC, as well as explore the role of nuclear oxidative DNA damage in decreasing the relative proliferative capacity of HCEC. Finally, potential new research directions will be discussed that could take advantage of and/or improve the proliferative capacity of these physiologically important cells in order to develop new treatments to restore corneal clarity.

Therapeutic efficiency of tissue-engineered human corneal endothelium transplants on rabbit primary corneal endotheliopathy

To evaluate the therapeutic efficiency of tissue-engineered human corneal endothelia (TE-HCEs) on rabbit primary corneal endotheliopathy (PCEP), TE-HCEs reconstructed with monoclonal human corneal endothelial cells (mcHCECs) and modified denuded amniotic membranes (mdAMs) were transplanted into PCEP models of New Zealand white rabbits using penetrating keratoplasty. The TE-HCEs were examined using diverse techniques including slit-lamp biomicroscopy observation and pachymeter and tonometer measurements in vivo, and fluorescent microscopy, alizarin red staining, paraffin sectioning, scanning and transmission electron microscopy observations in vitro. The corneas of transplanted eyes maintained transparency for as long as 200 d without obvious edema or immune rejection. The corneal thickness of transplanted eyes decreased gradually after transplanting, reaching almost the thickness of normal eyes after 156 d, while the TE-HCE non-transplanted eyes were turbid and showed obvious corneal edema. The polygonal corneal endothelial cells in the transplanted area originated from the TE-HCE transplant. An intact monolayer corneal endothelium had been reconstructed with the morphology, cell density and structure similar to those of normal rabbit corneal endothelium. In conclusion, the transplanted TE-HCE can reconstruct the integrality of corneal endothelium and restore corneal transparency and thickness in PCEP rabbits. The TE-HCE functions normally as an endothelial barrier and pump and promises to be an equivalent of HCE for clinical therapy of human PCEP.

Fabrication and characters of a corneal endothelial cells scaffold based on chitosan

A novel chitosan-based membrane that made of hydroxyethyl chitosan, gelatin and chondroitin sulfate was used as a carrier of corneal endothelial cells. The characteristics of the blend membrane including transparency, equilibrium water content, ion and glucose permeability were determined. The results showed that the optical transparency of the membrane was as good as the natural human cornea. The water content of this scaffold was 81.32% which was remarkably close to the native cornea. The membrane had a good ion permeability and its glucose permeability was even higher than natural human cornea. The cultured rabbit corneal endothelial cells formed a monolayer on the membrane. The results demonstrated that the membrane was suitable for corneal endothelial cells to attach and grow on it. In addition, the membranes in vivo could be degraded steadily with less inflammation and showed a good histocompatibility. These results demonstrated that the hydroxyethyl chitosan-chondroitin sulfate-gelatin blend membrane can potentially be used as a carrier for corneal endothelial cell transplantation.

Establishing a reproducible method for the culture of primary equine corneal cells

OBJECTIVE

To establish a reproducible method for the culture of primary equine corneal epithelial cells, keratocytes, and endothelial cells and to describe each cell's morphologic characteristics, immunocytochemical staining properties and conditions required for cryopreservation.

PROCEDURES

Corneas from eight horses recently euthanized for reasons unrelated to this study were collected aseptically and enzymatically separated into three individual layers for cell isolation. The cells were plated, grown in culture, and continued for several passages. Each cell type was characterized by morphology and immunocytochemical staining.

RESULTS

All three equine corneal cell types were successfully grown in culture. Cultured corneal endothelial cells were large, hexagonal cells with a moderate growth rate. Keratocytes were small, spindloid cells that grew rapidly. Epithelial cells had heterogeneous morphology and grew slowly. The endothelial cells and keratocytes stained positive for vimentin and were morphologically distinguishable from one another. The epithelial cells stained positive for cytokeratin. Keratocytes and endothelial cells were able to be cryopreserved and recovered. The cryopreserved cells maintained their morphological and immunocytochemical features after cryopreservation and recovery.

DISCUSSION

This work establishes reproducible methods for isolation and culture of equine corneal keratocytes and endothelial cells. Cell morphology and cytoskeletal element expression for equine corneal epithelial cells, keratocytes, and endothelial cells are also described. This has not previously been reported for equine corneal cells. This report also demonstrates the ability to preserve equine keratocytes and endothelial cells for extended periods of time and utilize them long after the primary-cell collection, a feature that has not been reported for veterinary corneal cell culture.

Using Basement Membrane of Human Amniotic Membrane as a Cell Carrier for Cultivated Cat Corneal Endothelial Cell Transplantation

PURPOSE

To study the feasibility of using basement membrane of human amniotic membrane (BMHAM) as a carrier for transplantation of cultivated cat corneal endothelial cells (cCCECs).

METHODS

BHMAM was obtained by enzymic digestion. cCCECs were seeded on the BMHAM and cultivated traditionally. The resulting continuous monolayer of cCCECs was transplanted onto the cat corneal graft stripped of the Descemet membrane with endothelium. To determine whether the transplanted cCCECs were vital and functional in vivo, the corneal grafts were examined by slit-lamp microscope every day for 6 weeks, and corneal thickness was measured by ultrasonic pachymetry. Either in vivo or in vitro, the cCCEC sheets on BMHAMs were examined morphologically by light and electron microscope, and the cell density was measured.

RESULTS

Seven to 10 days after seeding on the BMHAM, the cCCECs were confluent and formed a continuous monolayer with 3486 +/- 53 cells/mm(2) cell density. Like normal corneal endothelial cells, the cCCECs were almost hexagonal, squamous, and uniform in size. After transplantation, most cells were vital and functional nearly enough to maintain corneal graft thickness and transparency without rejection for at least 6 weeks. Six weeks after operation, the average thickness of the transplanted corneal grafts was only slightly greater than that before operation. Compared with that in vitro, after transplantation there was 5% to 8% reduction per week in cell density, which lasted for almost 3 weeks. After that, the average cCCEC density of corneal grafts was 2837 +/- 57 cells/mm(2) and quite stable maintained.

CONCLUSIONS

This study demonstrated that BMHAM would be an ideal alternative for corneal Descemet membrane and a cell carrier for cCCEC transplantation.

Necessary Prone Position Time for Human Corneal Endothelial Precursor Transplantation in a Rabbit Endothelial Deficiency Model

PURPOSE

We previously performed human corneal endothelial precursor cell transplantation into the anterior chamber and maintenance of the prone position for 24 hours in a bullous keratopathy model. This time, we investigated the necessary postoperative time in the prone position for clinical application of precursor cell transplantation.

METHODS

The sphere-forming assay was used to obtain precursors from cultured human corneal endothelial cells. Chloromethyl benzamidodialkylcarbocyanine (CM-DiI)-labeled precursor cells were injected into the anterior chamber of the eye in rabbits with corneal endothelial defects, and the prone position was maintained for 0, 1, 6, or 24 hours to allow attachment to Descemet's membrane. Rabbits maintained in the prone position for 24 hours without precursor cell transplantation were the controls. Each group was observed for 28 days after surgery, followed by histological examination and fluorescence microscopy.

RESULTS

The mean corneal thickness of the rabbits kept in the prone position for 1, 6, or 24 hours after precursor cell transplantation was significantly less than that of the rabbits without adoption of the prone position after transplantation or the untransplanted rabbits at 14 days (p<0.005), 21 days (p<0.0001), and 28 days (p<0.0001) after surgery. And there was no significant differences in corneal thickness between the two groups kept in the prone position for 6 hours and 24 hours throughout the observation DiI-positive human corneal endothelial-like hexagonal cells were detected on Descemet's membrane in the rabbits kept in the prone position for 1, 6, or 24 hours, but not in the control groups. Three of the six corneas in the 1-hour group showed focal edema and incomplete coverage of the endothelial defects.

CONCLUSIONS

Our findings demonstrated that transplantation of human corneal endothelial precursors into the anterior chamber with adoption of the prone position for 6 hours treated bullous keratopathy in rabbits with similar efficacy to maintenance of the prone position for 24 hours after surgery.

Preparation and properties of a chitosan-based carrier of corneal endothelial cells

A novel chitosan-based membrane that was made of hydroxypropyl chitosan, gelatin and chondroitin sulfate was used as a carrier of corneal endothelial cells. The characteristics of the blend membrane, such as transparency, equilibrium water content, permeability, mechanical properties, protein absorption ability, hydrophilicity and surface morphology, were determined. To study the effects of the membrane on cell attachment and growth, rabbit corneal endothelial cells were cultured on this artificial membrane. The biodegradability and biocompatibility of the blend membrane were in vivo evaluated by its implantation into the muscle of the rats. Glucose permeation results demonstrated that the blend membrane had higher glucose permeability than natural human cornea. Scanning electron microscopy (SEM) analysis of the membranes demonstrated that no fibrils were observed. As a result, the optical transparency of the membrane was as good as the natural human cornea. The average value of tensile strength of the membrane was 13.71 MPa for dry membrane and 1.48 MPa for wet membrane. The value of elongation at break of the wet was 45.64%. The cultured rabbit corneal endothelial cells formed a monolayer on the blend membrane which demonstrated that the membrane was suitable for corneal endothelial cells to attach and grow. In addition, the membranes in vivo showed a good bioabsorption property. The mild symptoms of inflammation at sites of treatment could be resolved as the implant was absorbed by the host. The results of this study demonstrated that the hydroxypropyl chitosan-chondroitin sulfate-gelatin blend membrane can potentially be used as a carrier for corneal endothelial cell transplantation.

Two clonal cell lines of immortalized human corneal endothelial cells show either differentiated or precursor cell characteristics

Access to primary human corneal endothelial cells (HCEC) is limited and donor-derived differences between cultures exacerbate the issue of data reproducibility, whereas cell lines can provide sufficient numbers of homogenous cells for multiple experiments. An immortalized HCEC population was adapted to serum-free culture medium and repeated cloning was performed. Clonally grown cells were propagated under serum-free conditions and growth curves were recorded. Cells were characterized immunocytochemically for junctional proteins, collagens, Na,K-ATPase and HCEC-specific 9.3.E-antigen. Ultrastructure was monitored by scanning and transmission electron microscopy. Two clonal cell lines, HCEC-B4G12 and HCEC-H9C1, could be isolated and expanded, which differed morphologically: B4G12 cells were polygonal, strongly adherent and formed a strict monolayer, H9C1 cells were less adherent and formed floating spheres. The generation time of B4G12 cells was 62.26 +/- 14.5 h and that of H9C1 cells 44.05 +/- 5.05 h. Scanning electron microscopy revealed that B4G12 cells had a smooth cell surface, while H9C1 cells had numerous thin filopodia. Both cell lines expressed ZO-1 and occludin adequately, and little but well detectable amounts of connexin-43. Expression of HCEC-specific 9.3.E-antigen was found commensurately in both cell lines, while expression of Na,K-ATPase alpha1 was higher in H9C1 cells than in B4G12 cells. B4G12 cells expressed collagen IV abundantly and almost no collagen III, while H9C1 cells expressed both collagens at reasonable amounts. It is concluded that the clonal cell line B4G12 represents an ideal model of differentiated HCEC, while H9C1 may reflect features of developing or transitional HCEC.

Establishment of a novel corneal endothelial cell line from domestic rabbit, Oryctolagus curiculus

To develop a rabbit corneal endothelial (RCE) cell line, in vitro culture of RCE cells was initiated from Oryctolagus curiculus corneas and a novel RCE cell line was established in this study. To initiate the primary culture of RCE cells, corneas from rabbit eyes were sliced and attached into glutin-coated wells with endothelial cell surface down. After being cultured at a time-gradient interval from 48 to 6 h, the corneal slices were detached and reattached into new wells, respectively. Cells in the wells containing only a pure population of RCE cells were collected and cultured in 20% FBS-DMEM/F12 medium containing chondroitin sulfate, ocular extract, epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), carboxymethyl-chitosan, N-acetylglucosamine hydrochloride, glucosamine hydrochloride, culture medium of rabbit corneal stromal cells and oxidation-degradation products of chondroitin sulfate at 37 degrees C, 5% CO(2). The cultured RCE cells, in quadrangle and polygonal shapes, proliferated to confluence 3 weeks later. During the subsequent subculture, the shape of RCE cells changed gradually from polygonal to more fibroblastic. A novel RCE cell line, growing at a steady rate, with a population doubling time of 53.8 h, has been established and subcultured to passage 67. Chromosome analysis showed that the RCE cells exhibited chromosomal aneuploidy with the modal chromosome number of 44. The results of immuno-cytochemical staining with neuron specific enolase (NSE) confirmed that the RCE cells were in neuroectodermal origin. Combined with the results of vascular endothelial growth factor (VEGF) treatment and endothelial cell morphology recovery, it can be concluded that the cell line established here is an RCE cell line. This RCE cell line may serve as a useful tool in theoretical researches of mammalian corneal endothelial cells, and may also have potential application in artificial corneal endothelium development.

Effects of interleukin-12p40 gene transfer on rat corneal allograft survival

PURPOSE

Despite the immunologically privileged nature of the cornea, graft rejection remains the major cause of human corneal allograft failure. Gene therapy is an interesting approach to introduce immunoregulatory molecules into the graft or the recipient to prevent rejection. In this study we investigated the immmunomodulatory effects of adenovirus-mediated gene transfer of a Th1 antagonist, interleukin-12p40 (IL-12p40), in vitro and on allogeneic graft survival in a rat experimental keratoplasty model.

METHODS

Donor corneas were transduced with an E1/E3 deleted adenoviral (Ad) vector encoding the IL-12p40 gene (AdIL-12p40) and assayed for the expression of the therapeutic gene. Cell culture supernatants containing IL-12p40 protein were generated by transducing human corneal endothelial cells with AdIL-12p40 and analysed for their capacity to inhibit production of IFN-gamma by naive T cells. The effect of both local (ex vivo Ad-mediated gene transfer) and systemic (i.p.-injection) over-expression of IL-12p40 was investigated by analysing the survival of corneal allografts transplanted from Wistar-Furth rats to fully MHC-class I/II incompatible Lewis rats. Moreover, the intra-graft mRNA-expression profile of cytokines and T cell markers was investigated at different time points after gene transfer.

RESULTS

Adenovirus-mediated gene transfer in cultured corneas led to significant IL-12p40 protein expression as determined by specific ELISA. Moreover we could show that IL-12p40 protein containing supernatants significantly inhibited the production of IFN-gamma by alloreactive naive T cells. Interestingly, neither ex vivo genetic modification of cultured corneas before transplantation nor systemic AdIL-12p40 treatment of recipients receiving allogeneic corneas did improve corneal allograft survival. Real-time RT-PCR analysis of ex vivo modified cornea allografts on day 7 after transplantation showed significantly higher IL-4 mRNA-expression levels in the AdIL-12p40 group compared to the control group. Other significant differences in mRNA-expression levels of intra-graft CD3, CD25, IFN-gamma, TNF-alpha, and IL-10 could not be detected, neither on day 7 nor on the day of rejection.

CONCLUSIONS

Despite the capacity of IL-12p40 protein to inhibit the production of IFN-gamma of naive T cells in vitro and some Th1/Th2 shift in vivo, no prolongation of allogeneic graft survival of both AdIL-12p40 modified rat corneas and systemically treated rats could be obtained after transplantation. The possible binding of Ad-mediated IL-12p40 with ubiquitously expressed IL-12p35 in vivo might therefore limit the application of IL-12p40 for the prevention of transplant rejection.

Gene transfer of cyto-protective molecules in corneal endothelial cells and cultured corneas: analysis of protective effects in vitro and in vivo

The loss of corneal endothelial cells plays a critical role in many corneal diseases and is a common phenomenon following cornea transplantation. In addition, the non-regenerative capacity of human corneal endothelial cells (HCEC) ultimately requires appropriate protection of corneal tissues during ex vivo storage to ensure vitality of the cells. However, only 70% of donor corneas can be used for grafting because of endothelial deficiencies. Corneal endothelial cell loss during storage is mainly induced by apoptotic cell death. This study was undertaken, for proof of principle, to investigate whether over-expression of cyto-protective molecules Bcl-x(L), Bag-1, and HO-1 prevents the loss of corneal endothelial cells both in vitro and in vivo. We demonstrate that gene transfer of both Bcl-x(L) and HO-1 has cyto-protective effects on HCEC in vitro. However, gene transfer of a single cyto-protective molecule does not prevent its rejection upon transplantation in a MHC class I/II disparate rat model.

The human corneal endothelium: new insights into electrophysiology and ion channels

The corneal endothelium is a monolayer that mediates the flux of solutes and water across the posterior corneal surface. Thereby, it plays an essential role to maintain the transparency of the cornea. Unlike the epithelium, the human endothelium is an amitotic cell layer with a critical cell density and the risk of corneal decompensation. The number of endothelial cells subsequently decreases with age. Moreover, the endothelial cell loss is accelerated after various impairments such as surgical trauma (e.g. cataract extraction) and following corneal transplantation. This cell loss is associated with programmed cell death (apoptosis) and changed ion channel activity. However, little is known about the electrophysiology and ion channel expression (in particular Ca2+ channels) in corneal endothelial cells. This article reviews our current knowledge about the electrophysiology of the corneal endothelium. It highlights ion channel expression, which may have a major role in corneal cell physiology and pathological events. A better understanding of the (electro)physiological function of the cornea may lead to the development of clinical relevant new therapeutic and preventive measures.

Structural characterization of bioengineered human corneal endothelial cell sheets fabricated on temperature-responsive culture dishes

For the purpose of corneal regenerative medicine, we fabricated human corneal endothelial cell sheets on temperature-responsive dishes, which could be non-invasively harvested as intact, transplantable sheets by simply reducing the culture temperature. Cells demonstrated hexagonal cell shape with numerous microvilli and cilia, and also exhibited abundant cytoplasmic organelles similar to these cells in vivo. Immunofluorescence for type IV collagen and fibronectin revealed that abundant extracellular matrix (ECM) was deposited on the basal surface throughout culture, and the deposited ECM was harvested along with the cell sheets by reducing culture temperature to 20 degrees C. Faint ECM remnants were observed on the dish surfaces after cell sheet detachment. Immunofluorescence for ZO-1 showed that tight junctions were established between cells, and immunoblotting indicated that intact ZO-1 was maintained during cell sheet harvest, while conventional proteolytic cell harvest methods resulted in the degradation of ZO-1. These results suggest that these transplantable corneal endothelial cell sheets can be applied to treat patients with damaged corneas.

Functional human corneal endothelial cell sheets harvested from temperature‐responsive culture surfaces

This study reports a new method for fabricating bioengineered human corneal endothelial cell sheets suitable for ocular surgery and repair. We have initially cultured human corneal endothelial cells on type IV collagen-coated dishes and, after several passages, expanded cells were then seeded onto novel temperature-responsive culture dishes. Four weeks after reaching confluence, these cultured endothelial cells were harvested as intact monolayer cell sheets by simple temperature reduction without enzymatic treatment. Scanning electron microscopy indicated that these cells were primarily hexagonal with numerous microvilli and cilia, similar to the native corneal endothelium. The Na+, K+-ATPase pump sites were located at the cell borders as in vivo. Moreover, cell densities and numbers of pump sites were identical to those of in vivo human corneal endothelium under optimized conditions. A 3H-ouabain binding analysis demonstrated a linear proportionality for cell pump density between confluent cell densities of 575 cells/mm2 and 3070 cells/mm2. We also confirmed Na+, K+-ATPase activity in the sheets in vitro. Xenograft transplantation results showed that the fabricated sheets retain their function of maintaining proper stromal hydration in vivo. We have established a regimen to culture and proliferate human corneal endothelial cells and fabricate endothelial sheets ex vivo morphologically and functionally similar to the native corneal endothelium. Our results support the value of harvested cell sheets for clinical applications in ocular reconstructive surgery in patients with ocular endothelial decompensation.

Long-term outcome of iron-endocytosing cultured corneal endothelial cell transplantation with magnetic attraction

The long-term efficacy and safety of transplanting iron-endocytosing cultured corneal endothelial cells (CECs) with magnetic attraction were evaluated. Rabbit corneas were subjected to cryo-injury to detach CECs. Cultured rabbit CECs (RCEC) were exposed to spherical iron powder and then injected into the anterior chamber, after which a neogium magnet was fixed on the eyelid for 24 hr to attract the cells to Descemet's membrane (RCEC-iron group, n=4). An RCEC group (cryo-injury and injection of normal cultured RCEC, n=4) and a Cryo group (cryo-injury without injection of RCEC, n=4) served as controls. Intraocular pressure was measured for 12 months after surgery. Corneal findings on slit lamp biomicroscopy, RCEC density, the electro-retinogram (ERG), and residual iron in the ocular tissues were evaluated at final assessment. Intraocular pressure did not increase in any group throughout 12 months of observation. At the final assessment, the average corneal edema score of the RCEC-iron group was significantly lower than that of the RCEC or Cryo groups (p=0.021). The average CEC density of the RCEC-iron group was 2581+/-230 cells mm(-2) (mean+/-SD), whereas no CECs were observed on the inner surface of the central cornea in the RCEC and Cryo groups. No significant differences of the ERG (a- and b-wave amplitudes, and b-wave/a-wave ratio) were detected among the groups. Iron powder was not detected by Berlin blue staining in the ocular tissues of the RCEC-iron group. Apoptotic cells were not observed in the endothelium by terminal transferase-mediated nick-end labeling. Transplanted iron-endocytosing RCEC remained viable for 12 months after surgery. There were no detectable ocular complications after the transplantation of iron-endocytosing cultured RCEC. Magnetic attachment of iron-endocytosing CECs can be an effective and safe method for corneal endothelial repair.

EGF suppresses hydrogen peroxide induced Ca2+ influx by inhibiting L-type channel activity in cultured human corneal endothelial cells

Endogenous generated hydrogen peroxide during eye bank storage limits viability. We determined in cultured human corneal endothelial cells (HCEC) whether: (1) this oxidant induces elevations in intracellular calcium concentration [Ca2+]i; (2) epidermal growth factor (EGF) medium supplementation has a protective effect against peroxide mediated rises in [Ca2+]i. Whereas pathophysiological concentrations of H2O2 (10 mM) induced irreversible large increases in [Ca2+]i, lower concentrations (up to 1 mM) had smaller effects, which were further reduced by exposure to either 5 microM nifedipine or EGF (10 ng ml(-1)). EGF had a larger protective effect against H2O2-induced rises in [Ca2+]i than nifedipine. In addition, icilin, the agonist for the temperature sensitive transient receptor potential protein, TRPM8, had complex dose-dependent effects (i.e. 10 and 50 microM) on [Ca2+]i. At 10 microM, it reversibly elevated [Ca2+]i whereas at 50 microM an opposite effect occurred suggesting complex effects of temperature on endothelial viability. Taken together, H2O2 induces rises in [Ca2+]i that occur through increases in Ca2+ permeation along plasma membrane pathways that include L-type Ca2+ channels as well as other EGF-sensitive pathways. As EGF overcomes H2O2-induced rises in [Ca2+]i, its presence during eye bank storage could improve the outcome of corneal transplant surgery.

Human corneal endothelial cell proliferation: potential for use in regenerative medicine

PURPOSE

To review and update the experience of our laboratory in culturing human corneal endothelial cells (HCEC) from young and older donors.

METHODS

Corneas were obtained from National Disease Research Interchange, Philadelphia, PA. Data from the past 3 years were reviewed to develop criteria for selecting donor corneas to be used for endothelial cell culture. Immunocytochemical localization using mAb 9.3.E identified endothelial cells, and Ki67 staining demonstrated actively cycling cells. Cell counts demonstrated the effect of growth-promoting agents on proliferation of cells from young (<30 years old) and older (>50 years old) donors. Phase-contrast microscopy documented morphologic characteristics of cells in primary culture and the effect of growth factors on cell morphology.

RESULTS

Exclusion criteria were developed to increase the chance of successful culture of HCEC. Isolation methods to remove Descemet membrane with attached endothelial cells avoided contamination with other corneal cell types. EDTA treatment combined with mechanical disruption facilitated isolation of cells. Culture medium containing FBS, EGF, NGF, and bovine pituitary extract stimulated maximal growth and facilitated normal monolayer formation. Age-related differences were detected in the density of confluent cells in primary culture and in the proliferative response to growth-promoting agents.

CONCLUSIONS

Untransformed HCEC can be successfully cultured from the corneas of both young and older donors by using care in the selection of donor material. Care must also be taken in the early phases of endothelial cell isolation to obtain maximal numbers of healthy cells for culture. There appear to be true age-related differences in overall proliferative capacity; however, the relative response to specific growth factors was similar in cells from young and older donors. Results of these studies provide guidelines for successful growth of untransformed HCEC for use in regenerative medicine.

Lipid-mediated gene transfer of acidic fibroblast growth factor into human corneal endothelial cells

The aim of this study was to optimize non-viral gene transfer conditions and investigate the effect of fibroblast growth factor-1 (FGF-1) gene transfer on human corneal endothelial cell (HCEC) proliferation. Five non-viral vectors (Lipofectin, DMRIE-C, DAC-30, Effectene, FuGene6) were used to transfect HCEC with plasmids coding for enhanced green fluorescent protein (EGFP) and FGF-1. Transfection efficiency and toxicity (n=6) were quantified and optimized using the EGFP construct by FACS-analysis. Using optimal conditions HCEC were transfected with the FGF-1 plasmid and cell proliferation as well as expression of FGF-1 were determined at days 4 and 7 by counting and western blotting, respectively. Lipofectin (17+/-2.02%) transfected HCEC more successfully than DMRIE-C (11+/-1.46%), Effectene (9+/-0.62%), FuGene (9+/-0.93%) and DAC-30 (7+/-0.59%). Toxicity of the lipids ranged from 2 to 4%. Optimal HCEC proliferation was achieved with DAC-30/FGF-1 (P<0.05), whereas all other vectors did not result in significantly increased cell proliferation. However, all of the transfected cells produced FGF-1 in different amounts as indicated by western blotting. Efficient and almost non-toxic transfer of the FGF-1 gene into HCEC can be successfully achieved by lipid-based techniques. Using optimal conditions significantly increased cell proliferation was independent on gene transfer efficiency. This may indicate that even a low transfection rate is sufficient to produce a concentration of FGF-1 that will have a stimulatory effect on HCECs.

Transplantation of corneas reconstructed with cultured adult human corneal endothelial cells in nude rats

PURPOSE

The feasibility of corneal reconstruction with cultured adult human corneal endothelial cells (HCEC) was examined in a nude rat model.

METHODS

Endothelial cells were removed from the corneas of Lewis rats using a sterile cotton swab. Cultured adult HCEC labelled with a fluorescent marker chloromethyl-benzamidodialkylcarbocyanine (CM-Dil) were seeded onto the denuded Descemet's membrane. Then the corneas were centrifuged, incubated for 2 days, and transplanted into the eyes of nude rats using the penetrating keratoplasty technique (HCEC group). Control nude received corneas denuded of endothelium and without HCEC. The operated eyes were observed for 28 days after transplantation, and then were subjected to histological and fluorescein microscopic examination.

RESULTS

The mean corneal thickness was significantly smaller in the HCEC group than in the control group throughout the observation period. The corneal endothelial cell density of the grafts at 28 days postoperatively ranged from 2425 to 3250 cells mm(-2) (mean+/-sd, 2744+/-337 cells mm(-2)). Fluorescein microscopy at 28 days after surgery showed numerous DiI-labelled cells on the posterior corneal surface in the HCEC group. Frozen sections showed a monolayer of DiI-labelled cells on Descemet's membrane.

CONCLUSIONS

Cultured adult HCEC function well and maintain corneal transparency for 1 month after transplantation in nude rats.

Prospects for endothelial transplantation

BACKGROUND

The human corneal endothelium has a limited proliferative capacity in vivo. Until now it has only been possible to replace damaged endothelium by transplantation of a donor cornea. After establishing methods for the isolation and in vitro cultivation of human corneal endothelial cells (HCEC), transplantation of these cells may be an alternative therapeutic option.

MATERIALS AND METHODS

In this review methods for the in vitro cultivation of HCEC and their transplantation onto the Descemet membrane of donor corneas are described.

RESULTS

In vitro proliferation of human adult corneal endothelial cells was achieved by the development of defined cell culture conditions, including supplementation of culture medium with specified growth factors. Dependent on the culture conditions, in vitro cultured endothelial cells showed phenotypic changes and different proliferative behaviour. The propagation of corneal endothelial cells in vitro offered the possibility of their transplantation onto donor corneas in an in vitro model. After transplantation, these cells formed a monolayer whose morphology and cell density depended on the differentiation status of the cells in vitro. Highest cell numbers up to 3000 cells/mm2 were achieved using a SV40-transformed HCEC-cell line. Monolayer integrity could be demonstrated by positive staining for integrins and light junction proteins, and pump function of the newly established endothelium was proven by perfusion studies.

CONCLUSIONS

Methods to transplant HCEC onto human denuded corneas have been successfully established to reconstruct human corneas. Recent developments in genetic manipulation of cells and tissue engineering will be of great help in constructing suitable corneas for keratoplasty. Thus corneal endothelial cell transplantation is one of the promising future possibilities to provide corneas of high quality for patients. Furthermore, improvement of the transplantation technique may lead to a method to directly manipulate the diseased endothelium of patients with corneal endothelial dystrophies.

Human corneal equivalent as cell culture model for in vitro drug permeation studies

AIMS

For the study of transcorneal in vitro permeation of ophthalmic drugs, excised animal cornea or corneal epithelial cell culture are frequently used as a replacement for the human cornea. The main purposes of this study were to reconstruct a complete human organotypic cornea equivalent, consisting of all three different cell types (epithelial, stromal, and endothelial); to test the barrier function of this bio-engineered human cornea using three different model drugs (pilocarpine hydrochloride (PHCl), befunolol hydrochloride (BHCl), and hydrocortisone (HC)); and to determine its usefulness as an in vitro model for prediction of ocular drug absorption into the human eye.

METHODS

A multilayer tissue construct was created step by step in Transwell cell culture insert using SV-40 immortalised human endothelial and epithelial cells and native stromal cells (fibroblasts). Morphology was characterised by light microscopy using routine H&E staining. Scanning electron microscopy was used to evaluate ultrastructural features. Ocular permeation of drugs across the human cornea construct was tested using modified Franz cells and compared with data obtained from excised porcine cornea and previously described porcine cornea constructs.

RESULTS

and conclusion: The cornea construct exhibited typical corneal structures such as a monolayer of hexagonally shaped endothelial cells and a multilayered epithelium consisting of seven to nine cell layers with flat superficial cells. The formation of microplicae and microvilli was also confirmed. The human cornea construct showed similar permeation behaviour for all substances compared with excised porcine cornea. However, permeability (permeation coefficients K(p)) of the human cornea equivalent (PHCl 13.4*10(-6) (SD 3.01*10(-6)); BHCl 9.88*10(-6) (SD 1.79*10(-6)); HC 5.41*10(-6) (SD 0.40*10(-6)) cm/s) was about 1.6-1.8 fold higher than excised porcine cornea. Compared with data from the porcine cornea construct the cultivated human equivalent showed a decreased permeability. The reconstructed human cornea could be appropriate to predict drug absorption into the human eye.

Immunocytochemical localization of Na+-HCO3- cotransporters and carbonic anhydrase dependence of fluid transport in corneal endothelial cells

In corneal endothelium, there is evidence for basolateral entry of HCO(3)(-) into corneal endothelial cells via Na(+)-HCO(3)(-) cotransporter (NBC) proteins and for net HCO(3)(-) flux from the basolateral to the apical side. However, how HCO(3)(-) exits the cells through the apical membrane is unclear. We determined that cultured corneal endothelial cells transport HCO(3)(-) similarly to fresh tissue. In addition, Cl(-) channel inhibitors decreased fluid transport by at most 16%, and inhibition of membrane-bound carbonic anhydrase IV by benzolamide or dextran-bound sulfonamide decreased fluid transport by at most 29%. Therefore, more than half of the fluid transport cannot be accounted for by anion transport through apical Cl(-) channels, CO(2) diffusion across the apical membrane, or a combination of these two mechanisms. However, immunocytochemistry using optical sectioning by confocal microscopy and cryosections revealed the presence of NBC transporters in both the basolateral and apical cell membranes of cultured bovine corneal endothelial cells and freshly isolated rabbit endothelia. This newly detected presence of an apical NBC transporter is consistent with its being the missing mechanism sought. We discuss discrepancies with other reports and provide a model that accounts for the experimental observations by assuming different stoichiometries of the NBC transport proteins at the basolateral and apical sides of the cells. Such functional differences might arise either from the expression of different isoforms or from regulatory factors affecting the stoichiometry of a single isoform.