Limbal Stem Cells from Aged Donors Are a Suitable Source for Clinical Application

Limbal stem cells (LSC) are the progenitor cells that maintain the transparency of the cornea. Limbal stem cell deficiency (LSCD) leads to corneal opacity, inflammation, scarring, and blindness. A clinical approach to treat this condition consists in LSC transplantation (LSCT) after ex vivo expansion of LSC. In unilateral LSCD, an autologous transplant is possible, but cases of bilateral LSCD require allogenic LSCT. Cadaveric donors represent the most important source of LSC allografts for treatment of bilateral LSCD when living relative donors are not available. To evaluate the suitability of aged cadaveric donors for LSCT, we compared three pools of LSC from donors of different ages (<60 years, 60–75 years, and >75 years). We evaluated graft quality in terms of percent of p63-positive (p63+) cells by immunofluorescence, colony forming efficiency, and mRNA and protein expression of p63, PAX6, Wnt7a, E-cadherin, and cytokeratin (CK) 12, CK3, and CK19. The results showed that LSC cultures from aged donors can express ≥3% of p63+ cells—considered as the minimum value for predicting favorable clinical outcomes after LSCT—suggesting that these cells could be a suitable source of LSC for transplantation. Our results also indicate the need to evaluate LSC graft quality criteria for each donor.

Preservation of epithelial progenitor cells from collagenase-digested oral mucosa during ex vivo cultivation

To avoid xenogeneic infection, we report a novel protocol for producing animal-derived component-free oral mucosal epithelial cells (OMECs) sheet for transplantation, in which collagenase was used to replace dispase II/trypsin-EDTA for digesting oral mucosal tissue, and human platelet-derived PLTMax to replace fetal bovine serum. The resulting epithelial aggregates were expanded on de-epithelialized amniotic membranes without 3T3 feeder cells, and serum-free EpiLife was used to reduce contamination by submucosal mesenchymal cells. The OMEC sheets thus generated showed similar positive keratin 3/76-positive and keratin 8-negative staining patterns compared with those generated by the original protocol. Colony formation efficiency assay, BrdU label retention assay, and p63 and p75^NTR immunostaining results indicated that higher proliferative potentials and more progenitor cells were preserved by the modified protocol. TaqMan array analysis revealed that the transcription of integrin-linked kinase (ILK) was up-regulated along with an increase in β-catenin signaling and its downstream cell cycle modulators, cyclin D1 and p27^KIP1. Furthermore, ILK silencing led to the inhibition of nuclear β-catenin accumulation, suppressed p63 expression, and reduced the expression of cyclin D1 and p27^KIP1; these observations suggest that ILK/β-catenin pathway may be involved in cell proliferation regulation during the ex vivo expansion of OMECs for transplantation purposes.

Contact Lenses: A Delivery Device for Stem Cells to Treat Corneal Blindness

: Worldwide, 45 million people are blind. Corneal blindness is a major cause of visual loss, estimated to affect 10 million. For the most difficult to treat patients, including those with a disease called limbal stem cell deficiency, a donor corneal graft is not a viable option; thus, patients are treated with specialized stem cell grafts, which fail in a significant proportion (30 to 50%) of subjects. This unacceptable failure rate means there is a pressing need to develop minimally invasive, long-lasting, cost-effective therapies to improve patient quality of life and lessen the economic burden. Restoring vision in patients with severe corneal disease is the main focus of our research program; however, to achieve our goals and deliver the best quality stem cell therapy, we must first understand the basic biology of these cells, including their residence, the factors that support their long-term existence, markers to identify and isolate them, and carriers that facilitate expansion, delivery, and protection during engraftment. We recently achieved some of these goals through the discovery of stem cell markers and the development of a novel and innovative contact lens-based cell transfer technique that has been successfully trialed on patients with corneal blindness. Although several popular methodologies are currently available to nurture and transfer stem cells to the patients' ocular surface, contact lenses provide many advantages that will be discussed in this review article. The job for clinician-researchers will be to map precisely how these cells contribute to restoring ocular health and whether improvements in the quality of cells and the cell delivery system can be developed to reduce disease burden.

In vivo Confocal Microscopic Analysis of Limbal Stroma in Patients With Limbal Stem Cell Deficiency

PURPOSE

Using in vivo confocal microscopy, we established that unique hyperreflective structures in the anterior limbal stroma of healthy individuals represent the limbal stromal niche. The aim of this study was to characterize the limbal stromal microarchitecture in patients with limbal stem cell deficiency (LSCD).

METHODS

After obtaining informed consent, 10 patients with LSCD and 3 with macular corneal dystrophy were recruited. In vivo confocal imaging of the limbus and cornea of the affected and normal eyes was performed using an HRT III laser scanning microscope, beyond the epithelium deep into the stroma.

RESULTS

In the case of LSCD, the limbal epithelium was replaced by conjunctival epithelium. A large number of inflammatory and dendritic cells were identified along with blood vessels from the epithelium to deep stromal layers. The unique hyperreflective niche structures were replaced by homogenously bright fibrous structures in all eyes with total LSCD. In patients with partial LSCD, even the clinically defined normal limbus had fibrotic stroma. In a patient with focal LSCD, only the affected limbal stroma remained fibrotic, whereas the adjacent clinically normal limbus had the unique hyperreflective structures. Although the opaque corneal stroma appeared bright because of proteoglycan deposition, it was possible to identify the normal limbal epithelial and stromal architecture in macular corneal dystrophy.

CONCLUSIONS

In the case of LSCD, the limbal stromal niche was replaced by bright fibrotic structures indicating persistence of damage several months after injury. Further studies are required to characterize the sequential events occurring in the anterior limbal stroma after injury using this noninvasive method.

Stem Cell Therapies in Clinical Trials: Progress and Challenges

Clinical investigations using stem cell products in regenerative medicine are addressing a wide spectrum of conditions using a variety of stem cell types. To date, there have been few reports of safety issues arising from autologous or allogeneic transplants. Many cells administered show transient presence for a few days with trophic influences on immune or inflammatory responses. Limbal stem cells have been registered as a product for eye burns in Europe and mesenchymal stem cells have been approved for pediatric graft versus host disease in Canada and New Zealand. Many other applications are progressing in trials, some with early benefits to patients.

From discovery to approval of an advanced therapy medicinal product-containing stem cells, in the EU

In 1997, the human corneal epithelium was reconstructed in vitro and transplanted on patients. Later, it became a routine treatment, before regulations considered advanced therapy medicinal products and drugs on the same lines. Manufacturing, before and after good manufacturing practice setting, was established in different facilities and the clinical application in several hospitals. Advanced therapy medicinal products, including stem cells, are unique products with different challenges than other drugs: some uncertainties, in addition to benefit, cannot be avoided. This review will focus on all recent developments in the stem cell-based corneal therapy.

Immunological Properties of Corneal Epithelial-Like Cells Derived from Human Embryonic Stem Cells

Transplantation of ex vivo expanded corneal limbal stem cells (LSCs) has been the main treatment for limbal stem cell deficiency, although the shortage of donor corneal tissues remains a major concern for its wide application. Due to the development of tissue engineering, embryonic stem cells (ESCs)-derived corneal epithelial-like cells (ESC-CECs) become a new direction for this issue. However, the immunogenicity of ESC-CECs is a critical matter to be solved. In the present study, we explored the immunological properties of ESC-CECs, which were differentiated from ESCs. The results showed that ESC-CECs had a similar character and function with LSCs both in vitro and in vivo. In ESC-CECs, a large number of genes related with immune response were down-regulated. The expressions of MHC-I, MHC-II, and co-stimulatory molecules were low, but the expression of HLA-G was high. The ESC-CECs were less responsible for T cell proliferation and NK cell lysis in vitro, and there was less immune cell infiltration after transplantation in vivo compared with LSCs. Moreover, the immunological properties were not affected by interferon-γ. All these results indicated a low immunogenicity of ESC-CECs, and they can be promising in clinical use.

An Ultra-thin Amniotic Membrane as Carrier in Corneal Epithelium Tissue-Engineering

Amniotic membranes (AMs) are widely used as a corneal epithelial tissue carrier in reconstruction surgery. However, the engineered tissue transparency is low due to the translucent thick underlying AM stroma. To overcome this drawback, we developed an ultra-thin AM (UAM) by using collagenase IV to strip away from the epithelial denuded AM (DAM) some of the stroma. By thinning the stroma to about 30 μm, its moist and dry forms were rendered acellular, optically clear and its collagen framework became compacted and inerratic. Engineered rabbit corneal epithelial cell (RCEC) sheets generated through expansion of limbal epithelial cells on UAM were more transparent and thicker than those expanded on DAM. Moreover, ΔNp63 and ABCG2 gene expression was greater in tissue engineered cell sheets expanded on UAM than on DAM. Furthermore, 2 weeks after surgery, the cornea grafted with UAM based cell sheets showed higher transparency and more stratified epithelium than the cornea grafted with DAM based cell sheets. Taken together, tissue engineered corneal epithelium generated on UAM has a preferable outcome because the transplanted tissue is more transparent and better resembles the phenotype of the native tissue than that obtained by using DAM for this procedure. UAM preserves compact layer of the amniotic membrane and maybe an ideal substrate for corneal epithelial tissue engineering.

Comparative Analysis of Substrate-Free Cultured Oral Mucosal Epithelial Cell Sheets from Cells of Subjects with and without Stevens-Johnson Syndrome for Use in Ocular Surface Reconstruction

Purpose

To compare the regenerative potential of cultured oral mucosal epithelial cells sheets (COMECs) from Stevens-Johnson syndrome (SJS) subjects with those from non-SJS subjects.

Methods

Human oral mucosal epithelial cells from SJS and non-SJS subjects were cultured, and colony-forming efficiency (CFE), proliferative and migration potential, expression of cytokines/growth factors and stem cells were compared. COMECs from SJS and non-SJS subjects were transplanted into 12 limbal stem cell-deficient rabbits, and their regenerative potential was analyzed at 1 week after transplantation.

Results

CFE (p>0.05, student’s t test), cell proliferation potential (p>0.05, two-way ANOVA) and expression of the cytokeratins (K3, K4, K13, K19) in the oral mucosal epithelial cells from SJS subjects were similar to those of the cells from non-SJS subjects. The initial migratory potential of SJS cells was delayed compared to that of non-SJS cells (p <0.05, RM two-way ANOVA). The SJS cells expressed lower levels of EGF and higher levels of VEGF compared to that of non-SJS cells (p<0.05, one-way ANOVA). In vivo transplanted SJS-COMECs showed similar expression of K3, K4, and K13, proliferation markers (Ki-67; p>0.05, Mann-Whitney U test), and stem cell markers (p63; p>0.05, Mann-Whitney U test) compared to non-SJS COMECs. The initial epithelial defects in vivo were larger in the eyes treated with SJS-COMECs on day 3 (p<0.01, RM two-way ANOVA), but no differences were observed by day 7 between SJS- and non-SJS-COMECs.

Conclusions

These results suggest that, aside from differences in migratory potential, oral mucosal epithelial cells from SJS and non-SJS subjects are comparable in their regeneration potential in treating limbal stem cell deficiency.

Human pluripotent stem cell-derived limbal epithelial stem cells on bioengineered matrices for corneal reconstruction

Corneal epithelium is renewed by limbal epithelial stem cells (LESCs), a type of tissue-specific stem cells located in the limbal palisades of Vogt at the corneo-scleral junction. Acute trauma or inflammatory disorders of the ocular surface can destroy these stem cells, leading to limbal stem cell deficiency (LSCD) - a painful and vision-threatening condition. Treating these disorders is often challenging and complex, especially in bilateral cases with extensive damage. Human pluripotent stem cells (hPSCs) provide new opportunities for corneal reconstruction using cell-based therapy. Here, we investigated the use of hPSC-derived LESC-like cells on bioengineered collagen matrices in serum-free conditions, aiming for clinical applications to reconstruct the corneal epithelium and partially replace the damaged stroma. Differentiation of hPSCs towards LESC-like cells was directed using small-molecule induction followed by maturation in corneal epithelium culture medium. After four to five weeks of culture, differentiated cells were seeded onto bioengineered matrices fabricated as transparent membranes of uniform thickness, using medical-grade porcine collagen type I and a hybrid cross-linking technology. The bioengineered matrices were fully transparent, with high water content and swelling capacity, and parallel lamellar microstructure. Cell proliferation of hPSC-LESCs was significantly higher on bioengineered matrices than on collagen-coated control wells after two weeks of culture, and LESC markers p63 and cytokeratin 15, along with proliferation marker Ki67 were expressed even after 30 days in culture. Overall, hPSC-LESCs retained their capacity to self-renew and proliferate, but were also able to terminally differentiate upon stimulation, as suggested by protein expression of cytokeratins 3 and 12. We propose the use of bioengineered collagen matrices as carriers for the clinically-relevant hPSC-derived LESC-like cells, as a novel tissue engineering approach for corneal 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.

Stem Cell Therapy for Corneal Epithelium Regeneration following Good Manufacturing and Clinical Procedures

Objective. To evaluate outcomes of cultivated limbal epithelial transplantation (CLET) for management of ocular surface failure due to limbal stem cell deficiency (LSCD). Design. Prospective, noncomparative, interventional case series and extensive comparison with recent similar studies. Participants. Twenty eyes with LSCD underwent CLET (11 autologous; 9 allogeneic) and were followed up for 3 years. Etiologies were divided into 3 prognostic categories: Group 1, chemical injuries (7 eyes); Group 2, immune-based inflammation (4 eyes); and Group 3, noninflammatory diseases (9 eyes). Intervention. Autologous and allogeneic limbal epithelial cells were cultivated on amniotic membranes and transplanted. Evaluations were based on clinical parameters, survival analysis, and in vivo confocal microscopy (IVCM). European Union Tissues/Cells Directive and good manufacturing procedures were followed. Main Outcome Measures. Improved clinical parameters, absence of epithelial defects, and improved central corneal epithelial phenotype. Results. Success rate was 80% at 1-2 years and 75% at 3 years. Autografts and allografts had similar survival. Success rate was significantly lower in prognostic Group 1 (42.9%) than in Groups 2-3 (100% each). All clinical parameters improved substantially. By IVCM, 80% of cases improved in epithelial status. Conclusions. CLET improved corneal epithelium quality, with subsequent improvement in symptoms, quality of life, and vision. These results confirm that CLET is a valid therapy for ocular surface failure.

Hybrid Microgels with Thermo-Tunable Elasticity for Controllable Cell Confinement

Stimuli-responsive hydrogels are able to change their physical properties such as their elastic moduli in response to changes in their environment. If biocompatible polymers are used to prepare such materials and if living cells are encapsulated within these networks, their switchability allows the cell-matrix interactions to be investigated with unprecedented consistency. In this paper, thermo-responsive macro- and microscopic hydrogels are presented based on azide-functionalized copolymers of poly(N-(2-hydroxypropyl)-methacrylamide) and poly(hydroxyethyl methacrylate) grafted with poly(N-isopropylacrylamide) side chains. Crosslinking of these comb polymers is realized by bio-orthogonal strain-promoted azide-alkyne cycloaddition with cyclooctyne-functionalized poly(ethylene glycol). The resulting hybrid hydrogels exhibit thermo-tunable elasticity tailored by the polymer chain length and grafting density. This bio-orthogonal polymer crosslinking strategy is combined with droplet-based microfluidics to encapsulate living cells into stimuli-responsive microgels, proving them to be a suitable platform for future systematic stem-cell research.

Progress in corneal wound healing

Corneal wound healing is a complex process involving cell death, migration, proliferation, differentiation, and extracellular matrix remodeling. Many similarities are observed in the healing processes of corneal epithelial, stromal and endothelial cells, as well as cell-specific differences. Corneal epithelial healing largely depends on limbal stem cells and remodeling of the basement membrane. During stromal healing, keratocytes get transformed to motile and contractile myofibroblasts largely due to activation of transforming growth factor-β (TGF-β) system. Endothelial cells heal mostly by migration and spreading, with cell proliferation playing a secondary role. In the last decade, many aspects of wound healing process in different parts of the cornea have been elucidated, and some new therapeutic approaches have emerged. The concept of limbal stem cells received rigorous experimental corroboration, with new markers uncovered and new treatment options including gene and microRNA therapy tested in experimental systems. Transplantation of limbal stem cell-enriched cultures for efficient re-epithelialization in stem cell deficiency and corneal injuries has become reality in clinical setting. Mediators and course of events during stromal healing have been detailed, and new treatment regimens including gene (decorin) and stem cell therapy for excessive healing have been designed. This is a very important advance given the popularity of various refractive surgeries entailing stromal wound healing. Successful surgical ways of replacing the diseased endothelium have been clinically tested, and new approaches to accelerate endothelial healing and suppress endothelial-mesenchymal transformation have been proposed including Rho kinase (ROCK) inhibitor eye drops and gene therapy to activate TGF-β inhibitor SMAD7. Promising new technologies with potential for corneal wound healing manipulation including microRNA, induced pluripotent stem cells to generate corneal epithelium, and nanocarriers for corneal drug delivery are discussed. Attention is also paid to problems in wound healing understanding and treatment, such as lack of specific epithelial stem cell markers, reliable identification of stem cells, efficient prevention of haze and stromal scar formation, lack of data on wound regulating microRNAs in keratocytes and endothelial cells, as well as virtual lack of targeted systems for drug and gene delivery to select corneal cells.

A Comparative Study of the Therapeutic Potential of Mesenchymal Stem Cells and Limbal Epithelial Stem Cells for Ocular Surface Reconstruction

Stem cell-based therapy has become an attractive and promising approach for the treatment of severe injuries or thus-far incurable diseases. However, the use of stem cells is often limited by a shortage of available tissue-specific stem cells; therefore, other sources of stem cells are being investigated and tested. In this respect, mesenchymal stromal/stem cells (MSCs) have proven to be a promising stem cell type. In the present study, we prepared MSCs from bone marrow (BM-MSCs) or adipose tissue (Ad-MSCs) as well as limbal epithelial stem cells (LSCs), and their growth, differentiation, and secretory properties were compared. The cells were grown on nanofiber scaffolds and transferred onto the alkali-injured eye in a rabbit model, and their therapeutic potential was characterized. We found that BM-MSCs and tissue-specific LSCs had similar therapeutic effects. Clinical characterization of the healing process, as well as the evaluation of corneal thickness, re-epithelialization, neovascularization, and the suppression of a local inflammatory reaction, were comparable in the BM-MSC- and LSC-treated eyes, but results were significantly better than in injured, untreated eyes or in eyes treated with a nanofiber scaffold alone or with a nanofiber scaffold seeded with Ad-MSCs. Taken together, the results show that BM-MSCs' therapeutic effect on healing of injured corneal surface is comparable to that of tissue-specific LSCs. We suggest that BM-MSCs can be used for ocular surface regeneration in cases when autologous LSCs are absent or difficult to obtain.

SIGNIFICANCE

Damage of ocular surface represents one of the most common causes of impaired vision or even blindness. Cell therapy, based on transplantation of stem cells, is an optimal treatment. However, if limbal stem cells (LSCs) are not available, other sources of stem cells are tested. Mesenchymal stem cells (MSCs) are a convenient type of cell for stem cell therapy. The therapeutic potential of LSCs and MSCs was compared in an experimental model of corneal injury, and healing was observed following chemical injury. MSCs and tissue-specific LSCs had similar therapeutic effects. The results suggest that bone marrow-derived MSCs can be used for ocular surface regeneration in cases when autologous LSCs are absent or difficult to obtain.

Potential Role of Induced Pluripotent Stem Cells (IPSCs) for Cell-Based Therapy of the Ocular Surface

The integrity and normal function of the corneal epithelium are crucial for maintaining the cornea’s transparency and vision. The existence of a cell population with progenitor characteristics in the limbus maintains a dynamic of constant epithelial repair and renewal. Currently, cell-based therapies for bio replacement—cultured limbal epithelial transplantation (CLET) and cultured oral mucosal epithelial transplantation (COMET)—present very encouraging clinical results for treating limbal stem cell deficiency (LSCD) and restoring vision. Another emerging therapeutic approach consists of obtaining and implementing human progenitor cells of different origins in association with tissue engineering methods. The development of cell-based therapies using stem cells, such as human adult mesenchymal or induced pluripotent stem cells (IPSCs), represent a significant breakthrough in the treatment of certain eye diseases, offering a more rational, less invasive, and better physiological treatment option in regenerative medicine for the ocular surface. This review will focus on the main concepts of cell-based therapies for the ocular surface and the future use of IPSCs to treat LSCD.

iPS Cells for Modelling and Treatment of Retinal Diseases

For many decades, we have relied on immortalised retinal cell lines, histology of enucleated human eyes, animal models, clinical observation, genetic studies and human clinical trials to learn more about the pathogenesis of retinal diseases and explore treatment options. The recent availability of patient-specific induced pluripotent stem cells (iPSC) for deriving retinal lineages has added a powerful alternative tool for discovering new disease-causing mutations, studying genotype-phenotype relationships, performing therapeutics-toxicity screening and developing personalised cell therapy. This review article provides a clinical perspective on the current and potential benefits of iPSC for managing the most common blinding diseases of the eye: inherited retinal diseases and age-related macular degeneration.

Normalization of wound healing and stem cell marker patterns in organ-cultured human diabetic corneas by gene therapy of limbal cells

Overexpression of c-met and suppression of matrix metalloproteinase-10 (MMP-10) and cathepsin F genes was previously shown to normalize wound healing, epithelial and stem cell marker patterns in organ-cultured human diabetic corneas. We now examined if gene therapy of limbal cells only would produce similar effects. Eight pairs of organ-cultured autopsy human diabetic corneas were used. One cornea of each pair was treated for 48 h with adenoviruses (Ad) harboring full-length c-met mRNA or a mixture (combo) of Ad with c-met and shRNA to MMP-10 and cathepsin F genes. Medium was kept at the limbal level to avoid transduction of central corneal epithelium. Fellow corneas received control Ad with EGFP gene. After additional 5 (c-met) or 10 days (combo) incubation, central corneal epithelial debridement with n-heptanol was performed, and wound healing times were determined microscopically. Corneal cryostat sections were immunostained for diabetic and putative limbal stem cell markers, α3β1 integrin, nidogen-1, fibronectin, laminin γ3 chain, ΔNp63α, keratins 14, 15, and 17, as well as for activated signaling intermediates, phosphorylated EGFR, Akt, and p38. Limbal c-met overexpression significantly accelerated healing of 8.5-mm epithelial wounds over EGFP controls (6.3 days vs. 9.5 days, p < 0.02). Combo treatment produced a similar result (6.75 days vs. 13.5 days, p < 0.03). Increased immunostaining vs. EGFP controls for most markers and signaling intermediates accompanied c-met gene or combo transduction. Gene therapy of limbal epithelial stem cell compartment has a beneficial effect on the diabetic corneal wound healing and on diabetic and stem cell marker expression, and shows potential for alleviating symptoms of diabetic keratopathy.

Modulating the stem cell niche for tissue regeneration

The field of regenerative medicine holds considerable promise for treating diseases that are currently intractable. Although many researchers are adopting the strategy of cell transplantation for tissue repair, an alternative approach to therapy is to manipulate the stem cell microenvironment, or niche, to facilitate repair by endogenous stem cells. The niche is highly dynamic, with multiple opportunities for intervention. These include administration of small molecules, biologics or biomaterials that target specific aspects of the niche, such as cell-cell and cell-extracellular matrix interactions, to stimulate expansion or differentiation of stem cells, or to cause reversion of differentiated cells to stem cells. Nevertheless, there are several challenges in targeting the niche therapeutically, not least that of achieving specificity of delivery and responses. We envisage that successful treatments in regenerative medicine will involve different combinations of factors to target stem cells and niche cells, applied at different times to effect recovery according to the dynamics of stem cell-niche interactions.

Differentiation of human limbal-derived induced pluripotent stem cells into limbal-like epithelium

Limbal epithelial stem cell (LESC) deficiency (LSCD) leads to corneal abnormalities resulting in compromised vision and blindness. LSCD can be potentially treated by transplantation of appropriate cells, which should be easily expandable and bankable. Induced pluripotent stem cells (iPSCs) are a promising source of transplantable LESCs. The purpose of this study was to generate human iPSCs and direct them to limbal differentiation by maintaining them on natural substrata mimicking the native LESC niche, including feederless denuded human amniotic membrane (HAM) and de-epithelialized corneas. These iPSCs were generated with nonintegrating vectors from human primary limbal epithelial cells. This choice of parent cells was supposed to enhance limbal cell differentiation from iPSCs by partial retention of parental epigenetic signatures in iPSCs. When the gene methylation patterns were compared in iPSCs to parental LESCs using Illumina global methylation arrays, limbal-derived iPSCs had fewer unique methylation changes than fibroblast-derived iPSCs, suggesting retention of epigenetic memory during reprogramming. Limbal iPSCs cultured for 2 weeks on HAM developed markedly higher expression of putative LESC markers ABCG2, ΔNp63α, keratins 14, 15, and 17, N-cadherin, and TrkA than did fibroblast iPSCs. On HAM culture, the methylation profiles of select limbal iPSC genes (including NTRK1, coding for TrkA protein) became closer to the parental cells, but fibroblast iPSCs remained closer to parental fibroblasts. On denuded air-lifted corneas, limbal iPSCs even upregulated differentiated corneal keratins 3 and 12. These data emphasize the importance of the natural niche and limbal tissue of origin in generating iPSCs as a LESC source with translational potential for LSCD treatment.

Report on ISN Forefronts, Florence, Italy, 12-15 September 2013: Stem cells and kidney regeneration

In recent years it has become clear that most organs and tissues, including kidney, contain resident stem/progenitor cells. Stem cells are undifferentiated, long-lived cells that are unique in their ability to produce differentiated daughter cells and to retain their stem cell identity by self-renewal. A primary goal of this meeting was to review the current understanding of kidney stem cells and mechanisms of kidney regeneration in both lower vertebrates and mammals. Presenters covered a broad range of topics including stem cell quiescence, epigenetics, transcriptional control circuits, dedifferentiation, pluripotent stem cells, renal progenitors, and novel imaging approaches in kidney regeneration. By the end of this highly interactive conference it was clear we are entering into very exciting times for regenerative medicine and the kidney.

Culture of human limbal epithelial stem cells on tenon's fibroblast feeder-layers: a translational approach

The coculture technique is the standard method to expand ex vivo limbal stem cells (LSCs) by using inactivated embryonic murine feeder layers (3T3). Although alternative techniques such as amniotic membranes or scaffolds have been proposed, feeder layers are still considered to be the best method, due to their ability to preserve some critical properties of LSCs such as cell growth and viability, stemness phenotype, and clonogenic potential. Furthermore, clinical applications of LSCs cultured on 3T3 have taken place. Nevertheless, for an improved Good Manufacturing Practice (GMP) compliance, the use of human feeder-layers as well as a fine standardization of the process is strictly encouraged. Here, we describe a translational approach in accordance with GMP regulations to culture LSCs onto human Tenon's fibroblasts (TFs). In this chapter, based on our experience we identify and analyze issues that often are encountered by researchers and discuss solutions to common problems.