Feasibility of a cryopreservation of cultured human corneal endothelial cells

Current state of endothelial cell therapy

PURPOSE OF REVIEW

Currently, there is heightened interest surrounding endothelial cell therapy for the treatment of corneal edema. The purpose of this review article is to describe and summarize the background information as well as the research surrounding the emerging treatment modalities for endothelial cell therapy.

RECENT FINDINGS

Marked advancements have been made in the translational research in this area, and increasing refinements have been demonstrated in the treatment protocols for cell therapy. Human clinical trials in this field are ongoing, specifically, in the area of injected human corneal endothelial cells (HCECs), with early results showing favorable safety and efficacy profiles.

SUMMARY

Efficient and effective delivery of HCECs to patients with corneal edema and dysfunction now appears feasible, and the results from ongoing human clinical trials are much anticipated. Adjunct therapeutics-in the form of pharmacological agents and/or surgical techniques, such as descemetorhexis-will likely continue to play an important role in defining the future of endothelial cell therapy.

Cryopreservation-induced delayed injury and cell-type-specific responses during the cryopreservation of endothelial cell monolayers

The cryopreservation of endothelial cell monolayers is an important step that bridges the cryopreservation of cells in suspension to that of tissues. Previous studies have identified clear distinctions in freezing mechanisms between cells in suspension and in monolayers, as well as developed novel protocols for monolayer cryopreservation. Recently, our group has shown that human umbilical vein endothelial cell (HUVEC) and porcine corneal endothelial cell (PCEC) monolayers grown on Rinzl plastic substrate can be cryopreserved in 5% dimethyl sulfoxide, 6% hydroxyethyl starch, and 2% chondroitin sulfate, following a slow-cooling protocol (-1 °C/min) with rapid plunge into liquid nitrogen from -40 °C. However, membrane integrity assessments were done immediately post thaw, which may result in an overestimation of cell viability due to possible delayed injury responses. Here, we show that for the optimal protocol condition of plunge at the -40 °C interrupt temperature, HUVEC and PCEC monolayers exhibited no significant immediate post-thaw injuries nor delayed injury responses during the 24-h post-thaw overnight culture period. HUVEC monolayers experienced no significant impact to their natural growth rate during the post-thaw culture, while PCEC monolayers experienced significantly higher growth than the unfrozen controls. The difference in the low-temperature responses between HUVEC and PCEC monolayers was further shown under high temperature plunge conditions. At these suboptimal plunge temperatures, HUVEC monolayers exhibited moderate immediate membrane injury but a pronounced delayed injury response during the 24-h post-thaw culture, while PCEC monolayers showed significant immediate membrane injury but no additional delayed injury response during the same period. Therefore, we provide further validation of our group's previously designed endothelial monolayer cryopreservation protocol for HUVEC and PCEC monolayers, and we identify several cell-type-specific responses to the freezing process.

Ex vivo expansion and characterization of human corneal endothelium for transplantation: a review

The corneal endothelium plays a key role in maintaining corneal transparency. Its dysfunction is currently treated with penetrating or lamellar keratoplasty. Advanced cell therapy methods seek to address the persistent global deficiency of donor corneas by enabling the renewal of the endothelial monolayer with tissue-engineered grafts. This review provides an overview of recently published literature on the preparation of endothelial grafts for transplantation derived from cadaveric corneas that have developed over the last decade (2010–2021). Factors such as the most suitable donor parameters, culture substrates and media, endothelial graft storage conditions, and transplantation methods are discussed. Despite efforts to utilize alternative cellular sources, such as induced pluripotent cells, cadaveric corneas appear to be the best source of cells for graft preparation to date. However, native endothelial cells have a limited natural proliferative capacity, and they often undergo rapid phenotype changes in ex vivo culture. This is the main reason why no culture protocol for a clinical-grade endothelial graft prepared from cadaveric corneas has been standardized so far. Currently, the most established ex vivo culture protocol involves the peel-and-digest method of cell isolation and cell culture by the dual media method, including the repeated alternation of high and low mitogenic conditions. Culture media are enriched by additional substances, such as signaling pathway (Rho-associated protein kinase, TGF-β, etc.) inhibitors, to stimulate proliferation and inhibit unwanted morphological changes, particularly the endothelial-to-mesenchymal transition. To date, this promising approach has led to the development of endothelial grafts for the first in-human clinical trial in Japan. In addition to the lack of a standard culture protocol, endothelial-specific markers are still missing to confirm the endothelial phenotype in a graft ready for clinical use. Because the corneal endothelium appears to comprise phenotypically heterogeneous populations of cells, the genomic and proteomic expression of recently proposed endothelial-specific markers, such as Cadherin-2, CD166, or SLC4A11, must be confirmed by additional studies. The preparation of endothelial grafts is still challenging today, but advances in tissue engineering and surgery over the past decade hold promise for the successful treatment of endothelial dysfunctions in more patients worldwide.

Challenges in corneal endothelial cell culture

Corneal endothelial cells (CECs) facilitate the function of maintaining the transparency of the cornea. Damage or dysfunction of CECs can lead to blindness, and the primary treatment is corneal transplantation. However, the shortage of cornea donors is a significant problem worldwide. Thus, cultured CEC therapy has been proposed and found to be a promising approach to overcome the lack of tissue supply. Unfortunately, CECs in humans rarely proliferate in vivo and, therefore, can be extremely challenging to culture in vitro. Several promising cell isolation and culture techniques have been proposed. Multiple factors affecting the success of cell expansion including donor characteristics, preservation and isolation methods, plating density, media preparation, transdifferentiation and biomarkers have been evaluated. However, there is no consensus on standard technique for CEC culture. This review aimed to determine the challenges and investigate potential options that would facilitate the standardization of CEC culture for research and therapeutic application.

Xeno-free cryopreservation of adherent retinal pigmented epithelium yields viable and functional cells in vitro and in vivo

Age-related macular degeneration (AMD) is the primary cause of blindness in adults over 60 years of age, and clinical trials are currently assessing the therapeutic potential of retinal pigmented epithelial (RPE) cell monolayers on implantable scaffolds to treat this disease. However, challenges related to the culture, long-term storage, and long-distance transport of such implants currently limit the widespread use of adherent RPE cells as therapeutics. Here we report a xeno-free protocol to cryopreserve a confluent monolayer of clinical-grade, human embryonic stem cell-derived RPE cells on a parylene scaffold (REPS) that yields viable, polarized, and functional RPE cells post-thaw. Thawed cells exhibit ≥ 95% viability, have morphology, pigmentation, and gene expression characteristic of mature RPE cells, and secrete the neuroprotective protein, pigment epithelium-derived factor (PEDF). Stability under liquid nitrogen (LN₂) storage has been confirmed through one year. REPS were administered immediately post-thaw into the subretinal space of a mammalian model, the Royal College of Surgeons (RCS)/nude rat. Implanted REPS were assessed at 30, 60, and 90 days post-implantation, and thawed cells demonstrate survival as an intact monolayer on the parylene scaffold. Furthermore, immunoreactivity for the maturation marker, RPE65, significantly increased over the post-implantation period in vivo, and cells demonstrated functional attributes similar to non-cryopreserved controls. The capacity to cryopreserve adherent cellular therapeutics permits extended storage and stable transport to surgical sites, enabling broad distribution for the treatment of prevalent diseases such as AMD.

Extracellular matrix changes in corneal opacification vary depending on etiology

Purpose

The purpose of this study is to examine the expression of tenascin-C and matrilin-2 in three different disorders, which frequently require corneal transplantation. These pathological conditions include bullous keratopathy (BK), Fuchs' endothelial corneal dystrophy (FECD), and corneal scarring in herpetic keratitis.

Methods

Histological sections of corneal buttons removed during keratoplasty were analyzed in BK (n = 20), FECD (n = 9), herpetic keratitis (n = 12), and cadaveric control (n = 10) groups with light microscopy following chromogenic immunohistochemistry. The sections were evaluated by three investigators, and semiquantitative scoring (0 to 3+) was applied according to standardized methods at 400X magnification. Each layer of the cornea was investigated; moreover, the stroma was subdivided into subepithelial, middle, and pre-Descemet's membrane areas for more detailed analysis.

Results

Excessive epithelial and stromal expression of tenascin-C was identified in all investigated conditions; the results were most pronounced in the pre-Descemet's membrane. Regarding matrilin-2, when examined in BK, there was increased labeling intensity in the epithelium (p<0.001) and stromal layers (p<0.05), and a decrease in the endothelium (p<0.001). In the other investigated conditions, only a low degree of stromal localization (p<0.05) of matrilin-2 was detected.

Conclusions

The expression of tenascin-C and matrilin-2 differs when examined in various corneal pathologies resulting in opacification. Both molecules seem to be involved in regeneration and wound healing of the corneal matrix in these diseases.

Translational and Regulatory Challenges of Corneal Endothelial Cell Therapy: A Global Perspective

Cell therapies are emerging as a unique class of clinical therapeutics in medicine. In 2015, Holoclar (ex vivo expanded autologous human corneal epithelial cells containing stem cells) gained the regulatory approval for treating limbal stem cell deficiency after chemical eye burn. This has set a precedent in ophthalmology and in medicine, reinforcing the therapeutic promise of cell therapy. However, to generalize and commercialize cell therapies on a global scale, stringent translational and regulatory requirements need to be fulfilled at both local and international levels. Over the past decade, the Singapore group has taken significant steps in developing human corneal endothelial cell (HCEnC) therapy for treating corneal endothelial diseases, which are currently the leading indication for corneal transplantation in many countries. Successful development of HCEnC therapy may serve as a novel solution to the current global shortage of donor corneas. Based on the experience in Singapore, this review aims to provide a global perspective on the translational and regulatory challenges for bench-to-bedside translation of cell therapy. Specifically, we discussed about the characterization of the critical quality attributes (CQA), the challenges that can affect the CQA, and the variations in the regulatory framework embedded within different regions, including Singapore, Europe, and the United States. Impact statement Functional corneal endothelium is critical to normal vision. Corneal endothelial disease-secondary to trauma, surgery, or pathology-represents an important cause of visual impairment and blindness in both developed and developing countries. Currently, corneal transplantation serves as the current gold standard for treating visually significant corneal endothelial diseases, although limited by the shortage of donor corneas. Over the past decade, human corneal endothelial cell therapy has emerged as a promising treatment option for treating corneal endothelial diseases. To allow widespread application of this therapy, significant regulatory challenges will need to be systematically overcome.

Corneal endothelial dysfunction: Evolving understanding and treatment options

The cornea is exquisitely designed to protect the eye while transmitting and focusing incoming light. Precise control of corneal hydration by the endothelial cell layer that lines the inner surface of the cornea is required for optimal transparency, and endothelial dysfunction or damage can result in corneal edema and visual impairment. Advances in corneal transplantation now allow selective replacement of dysfunctional corneal endothelium, providing rapid visual rehabilitation. A series of technique improvements have minimized complications and various adaptations allow use even in eyes with complicated anatomy. While selective endothelial keratoplasty sets a very high standard for safety and efficacy, a shortage of donor corneas in many parts of the world restricts access, prompting a search for alternatives. Clinical trials are underway to evaluate the potential for self-recovery after removal of dysfunctional central endothelium in patients with healthy peripheral endothelium. Various approaches to using cultured human corneal endothelial cells are also in clinical trials; these aim to multiply cells from a single donor cornea for use in potentially hundreds of patients. Pre-clinical studies are underway with induced pluripotent stem cells, endothelial stem cell regeneration, gene therapy, anti-sense oligonucleotides, and various biologic/pharmacologic approaches designed to treat, prevent, or retard corneal endothelial dysfunction. The availability of more therapeutic options will hopefully expand access around the world while also allowing treatment to be more precisely tailored to each individual patient.

A review of long-term corneal preservation techniques: Relevance and renewed interests in the COVID-19 era

The growth of eye banking in India was showing positive trends until the nation was hit by unprecedented times as a result of the COVID-19 pandemic. The impact of this has led to a downward spiraling in eye banking activities globally. Several measures had to be implemented to tide over the crisis and strategies planned for future to prepare for the needs of corneal transplantation. While eye banks in India have been practicing short- term and intermediate storage preservation media, there is a definite need to evolve other methods of very long-term preservation. This review discusses various methods of long term corneal preservation, their relevance and applications in the present times. We reviewed relevant medical literature in English from PUBMED with the key words “Corneal preservation”, “Cryopreservation”, “Glycerol preservation”, Gamma Irradiation”, “Eye Banking” ,”COVID-19”.