Easy xeno-free and feeder-free method for isolating and growing limbal stromal and epithelial stem cells of the human cornea

Expansion and characterization of human limbus-derived stromal/mesenchymal stem cells in xeno-free medium for therapeutic applications

BACKGROUND

Mesenchymal stem cells (MSCs) have been proven to prevent and clear corneal scarring and limbal stem cell deficiency. However, using animal-derived serum in a culture medium raises the ethical and regulatory bar. This study aims to expand and characterize human limbus-derived stromal/mesenchymal stem cells (hLMSCs) for the first time in vitro in the xeno-free medium.

METHODS

Limbal tissue was obtained from therapeutic grade corneoscleral rims and subjected to explant culture till tertiary passage in media with and without serum (STEM MACS XF; SM), to obtain pure hLMSCs. Population doubling time, cell proliferation, expression of phenotypic markers, tri-lineage differentiation, colony-forming potential and gene expression analysis were carried out to assess the retention of phenotypic and genotypic characteristics of hLMSCs.

RESULTS

The serum-free medium supported the growth of hLMSCs, retaining similar morphology but a significantly lower doubling time of 23 h (*p < 0.01) compared to the control medium. FACS analysis demonstrated ≥ 90% hLMSCs were positive for CD90⁺, CD73⁺, CD105⁺, and ≤ 6% were positive for CD45^-, CD34^- and HLA-DR^-. Immunofluorescence analysis confirmed similar expression of Pax6⁺, COL IV⁺, ABCG2⁺, ABCB5⁺, VIM⁺, CD90⁺, CD105⁺, CD73⁺, HLA-DR^- and CD45^-, αSMA^- in both the media. Tri-lineage differentiation potential and gene expression of hLMSCs were retained similarly to that of the control medium.

CONCLUSION

The findings of this study demonstrate successful isolation, characterization and culture optimization of hLMSCs for the first time in vitro in a serum-free environment. This will help in the future pre-clinical and clinical applications of MSCs in translational research.

The progress in techniques for culturing human limbal epithelial stem cells

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

Mechanotransduction Regulates Reprogramming Enhancement in Adherent 3D Keratocyte Cultures

Suspended spheroid culture using ultralow attachment plates (ULAPs) is reported to effect corneal fibroblast reprogramming. Polydimethylsiloxane (PDMS), with hydrophobic and soft substrate properties, facilitates adherent spheroid formation that promotes cellular physical reprogramming into stem-like cells without using transcription factors. However, it is still unknown whether the biophysical properties of PDMS have the same effect on adult human corneal keratocyte reprogramming. Here, PDMS and essential 8 (E8) medium were utilized to culture keratocyte spheroids and fibroblast spheroids, and the reprogramming results were compared. We provide insights into the probable mechanisms of the PDMS effect on spheroids. qPCR analysis showed that the expression of some stem cell marker genes (OCT4, NANOG, SOX2, KLF4, CMYC, ABCG2 and PAX6) was significantly greater in keratocyte spheroids than in fibroblast spheroids. The endogenous level of stemness transcription factors (OCT4, NANOG, SOX2, KLF4 and CMYC) was higher in keratocytes than in fibroblasts. Immunofluorescence staining revealed Klf4, Nanog, Sox2, ABCG2 and Pax6 were positively stained in adherent 3D spheroids but weakly or negatively stained in adherent 2D cells. Furthermore, OCT4, NANOG, SOX2, KLF4, HNK1, ABCG2 and PAX6 gene expression was significantly higher in adherent 3D spheroids than in adherent 2D cells. Meanwhile, SOX2, ABCG2 and PAX6 were more upregulated in adherent 3D spheroids than in suspended 3D spheroids. The RNA-seq analysis suggested that regulation of the actin cytoskeleton, TGFβ/BMP and HIF-1 signaling pathways induced changes in mechanotransduction, the mesenchymal-to-epithelial transition and hypoxia, which might be responsible for the effect of PDMS on facilitating reprogramming. In conclusion, compared to corneal fibroblasts, keratocytes were more susceptible to reprogramming due to higher levels of endogenous stemness transcription factors. Spheroid culture of keratocytes using PDMS had a positive impact on promoting the expression of some stem cell markers. PDMS, as a substrate to form spheroids, was better able to promote reprogramming than ULAPs. These results indicated that the physiological cells and culture conditions herein enhance reprogramming. Therefore, adherent spheroid culture of keratocytes using PDMS is a promising strategy to more safely promote reprogramming, suggesting its potential application for developing clinical implants in tissue engineering and regenerative medicine.

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

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

Niche regulation of limbal epithelial stem cells: HC-HA/PTX3 as surrogate matrix niche

Homeostasis of the corneal epithelium is ultimately maintained by stem cells that reside in a specialized microenvironment within the corneal limbus termed palisades of Vogt. This limbal niche nourishes, protects, and regulates quiescence, self-renewal, and fate decision of limbal epithelial stem/progenitor cells (LEPCs) toward corneal epithelial differentiation. This review focuses on our current understanding of the mechanism by which limbal (stromal) niche cells (LNCs) regulate the aforementioned functions of LEPCs. Based on our discovery and characterization of a unique extracellular matrix termed HC-HA/PTX3 (Heavy chain (HC1)-hyaluronan (HA)/pentraxin 3 (PTX3) complex, "-" denotes covalent linkage; "/" denotes non-covalent binding) in the birth tissue, i.e., amniotic membrane and umbilical cord, we put forth a new paradigm that HC-HA/PTX3 serves as a surrogate matrix niche by maintaining the in vivo nuclear Pax6+ neural crest progenitor phenotype to support quiescence and self-renewal but prevent corneal fate decision of LEPCs. This new paradigm helps explain how limbal stem cell deficiency (LSCD) develops in aniridia due to Pax6-haplotype deficiency and further explains why transplantation of HC-HA/PTX3-containing amniotic membrane prevents LSCD in acute chemical burns and Stevens Johnson syndrome, augments the success of autologous LEPCs transplantation in patients suffering from partial or total LSCD, and assists ex vivo expansion (engineering) of a graft containing LEPCs. We thus envisage that this new paradigm based on regenerative matrix HC-HA/PTX3 as a surrogate niche can set a new standard for regenerative medicine in and beyond ophthalmology.

Corneal stromal stem cells restore transparency after N2 injury in mice

Corneal scarring associated with various corneal conditions is a leading cause of blindness worldwide. The present study aimed to test the hypothesis that corneal stromal stem cells have a therapeutic effect and are able to restore the extracellular matrix organization and corneal transparency in vivo. We first developed a mouse model of corneal stromal scar induced by liquid nitrogen (N₂) application. We then reversed stromal scarring by injecting mouse or human corneal stromal stem cells in injured cornea. To characterize the mouse model developed in this study and the therapeutic effect of corneal stromal stem cells, we used a combination of in vivo (slit lamp, optical coherence tomography, in vivo confocal microscopy, optical coherence tomography shear wave elastography, and optokinetic tracking response) and ex vivo (full field optical coherence microscopy, flow cytometry, transmission electron microscopy, and histology) techniques. The mouse model obtained features early inflammation, keratocyte apoptosis, keratocyte transformation into myofibroblasts, collagen type III synthesis, impaired stromal ultrastructure, corneal stromal haze formation, increased corneal rigidity, and impaired visual acuity. Injection of stromal stem cells in N₂‐injured cornea resulted in improved corneal transparency associated with corneal stromal stem cell migration and growth in the recipient stroma, absence of inflammatory response, recipient corneal epithelial cell growth, decreased collagen type III stromal content, restored stromal ultrastructure, decreased stromal haze, decreased corneal rigidity, and improved vision. Our study demonstrates the ability of corneal stromal stem cells to promote regeneration of transparent stromal tissue after corneal scarring induced by liquid nitrogen.

Selecting Appropriate Reference Genes for Quantitative Real-Time Polymerase Chain Reaction Studies in Isolated and Cultured Ocular Surface Epithelia

The introduction of tissue engineering has allowed scientists to push the boundaries and treat seriously damaged ocular surface epithelia. They have managed to do this through the development of biological substitutes that restore, maintain or improve tissue function. To ensure the generation of a therapeutically safe and effective graft, knowledge on the transcriptional profile of native and cultured ocular surface epithelia is of undeniable value. Gene expression studies are, however, only as reliable as their proper selection of internal reaction controls or reference genes. In this study, we determined the expression stability of a number of reference genes: 18s rRNA, ACTB, ATP5B, CyC1, EIF4A2, GAPDH, RPL13A, SDHA, TOP1, UBC, and YWHAZ in primary isolates as well as in ex vivo cultured ocular surface epithelia explants (day 0 and/or day 14). Expression stability of the reference genes was assessed with both the geNorm and NormFinder software that use a pairwise comparison and a model-based approach, respectively. Our results extend the general recommendation of using multiple reference genes for normalization purposes to our model systems and provide an overview of several references genes that are likely to be stable in similar culture protocols.

The Neural Differentiation Potential of Limbal Stem Cells: A Model for Studying the Multipotency of Limbal Stem Cells

PURPOSE

To investigate the multipotency, especially the neural differentiation potential, of limbal stem cells (LSCs) using a coculture system and to provide a relevant literature review.

METHODS

Limbal tissue was harvested from adult New Zealand white rabbits and treated with collagenase A. Small pieces of the resulting limbal epithelial sheets were cocultured with a neuroblastoma cell line (Neuro-2A) in transwells. Morphological observation of the growing epithelial sheets was accomplished by microscopy, and marker expression was detected by immunocytochemistry for ßIII-tubulin and microtubule-associated protein 2. A literature review of associated studies was performed.

RESULTS

In the coculture group, directly adherent colonies of neuron-like (DACN) cells were observed among the growing limbal epithelial sheets from day 3. The DACN cells exhibited neuron-like morphology. The control group comprising limbal cell sheets cultured alone showed a very small number of DACN cells at the end of the culture period (day 14). Immunocytochemical staining revealed that the DACN cells were positive for ßIII-tubulin and microtubule-associated protein 2, confirming the neuronal phenotype of the neuron-like cells. By contrast, the DACN cells in the control group produced negative results. In previous reports, LSCs and niches exhibited neural potential, but most differentiated neural cells were observed as floating spheres, in contrast to the DACN cells observed in the present study.

CONCLUSIONS

We developed a coculture system of LSCs and Neuro-2A neuroblastoma cells and obtained DACN cells with neural differentiation potential. Our findings confirm the neural potential of LSCs, consistent with previous reports, but in a form other than floating spheres.

Production of a Double-Layer Scaffold for the "On-Demand" Release of Fibroblast-like Limbal Stem Cells

The production and characterization of a double-layer scaffold, to be used as a system for the "on-demand" release of corneal limbal stem cells, are reported here. The devices used in the clinics and proposed so far in the scientific literature, for the release of corneal stem cells in the treatment of limbal stem cell deficiency, cannot control the in vivo space-time release of cells as the biomaterial of which they are composed is devoid of the stimuli-responsive feature. Our approach was to produce a scaffold composed of two different polymeric layers that give the device the appropriate mechanical properties to be placed on the ocular surface and the possibility of releasing the stem cells following a noninvasive and cell-friendly treatment. This device consists of an electrospun microfibrillar scaffold of poly-l-lactic acid coated by a polymeric film based on an amphiphilic derivative of hyaluronic acid sensitive to the ionic strength of the external medium and to the presence of a complexing agent. The latter represents the "sacrificial" cell containing layer of the scaffold that can be dissolved "on demand" by the treatment with a solution of cyclodextrins. The rapid removal of the external polymeric film from the device is exploited to control the space-time release of the cells. In vitro and ex vivo experiments showed that fibroblast-like limbal stem cells cultured on the scaffold without the use of the feeder layer maintained their characteristics of stem cells and can be released "on demand" on the culture well coated with Matrigel or on the decellularized bovine cornea, respectively.