Limbal stem cell deficiency (LSCD) is caused by concomitant dysfunction or loss of the stem cell population that is responsible for continuous renewal of the corneal epithelium, and destruction of their niche, resulting from inherited (e.g., aniridia) or acquired (e.g., chemical burns) conditions. LSCD is a painful and potentially blinding condition, involving chronic epithelial defects, neovascularization, inflammation, ulceration, and corneal scarring. The prevalent conception is that corneal surface repair and visual rehabilitation can only be achieved by replenishing the depleted stem cell pool.
LSCD-targeted cell therapy is an emerging therapeutic strategy that aims to regenerate the damaged corneal surface by transplantation of cultured limbal epithelial stem cells (LESCs) or alternative autologous adult stem cell derived from tissues such as, the oral and nasal mucosal epithelium, conjunctival epithelium, epidermis and hair follicle.
LESCs are the preferred cell source for LSCD therapy. Ex vivo expansion of LESCs prior to transplantation is required, in order to reduce the size of the limbal biopsy taken from either the patient’s contralateral healthy eye or from the eye of an allogeneic donor. Cultured limbal epithelial transplantation (CLET), using human amniotic membrane or fibrin gel as a carrier, has been clinically validated and represents the only clinically applied stem cell–based ophthalmological therapy. While use of human amniotic membrane as a substrate for culture and transplantation is a gold standard technique for clinical applications, a number of different carriers have been tested in clinical (e.g., fibrin gels) or preclinical applications (e.g., collagen-based scaffolds). In addition, carrier-free methods, in which intact cell sheets are transplanted without an underlying supportive membrane, have been tested. One such carrier-free technique utilizes temperature-responsive synthetic polymers, which facilitate cell adhesion and growth at 370C, whereas lower temperatures induce complete cell detachment. In preclinical studies, carrier-free transplantation yielded superior success rates than control experiments using an amniotic membrane.
Oral and nasal mucosal epithelium is an easily accessible source of stem cells that quickly heals with minimal scarring after taking a biopsy, which contains mucin-secreting goblet cells, central to tear film stabilization, rendering them an ideal substitute for a cicatricial ocular surface.
Cultivated oral mucosal epithelial transplantation (COMET) has been shown successfully restoring the corneal surface in LSCD patients. However, in order to improve its therapeutic outcome, peripheral neovascularization, commonly observed following COMET, must be prevented. Thus, concomitant anti-angiogenic therapy has been proposed.
Nasal epithelium is another source of autologous stem cells, and has demonstrated promising results when applied directly to the ocular surface of the patients.
Cultivated conjunctival epithelial cells transplantation in 10 patients with LSCD induced improved clinical parameters (impression cytology, immunocytochemistry, and in vivo confocal analysis) within 1–2 years in more than 80% of the treated eyes.
Other autologous, nonepithelial stem cells that have been proposed as alternative sources for treatment of LSCD, include mesenchymal stem cells (MSCs), dental pulp and embryonic stem cells. These cell types have only been explored in preclinical studies, and have all shown promising results in both in vitro and animal corneal damage models. These cultured cells were generally applied as an intact cell sheet, with or without a carrier (e.g., amniotic membrane, fibrin).
The efficacy of bone marrow-derived MSCs in LSCD therapy has also been evaluated following administration via systemic injection and injection under transplanted amniotic membrane. MSC injection under an amniotic membrane did not improve corneal surface compared to controls with conjunctivalization and vascularization of the cornea occurring in treated eyes. In contrast, MSCs transplantation as an intact sheet led to ocular surface repair.
Embryonic stem cells application to the surface of the mouse eye demonstrated complete cornea re-epithelialization within 24 hours of cell application. Cells were delivered as a cell suspension and allowed to attach for one hour
Murine hair follicle–derived stem cells are easily accessible and have been demonstrated to transdifferentiate into a corneal epithelial phenotype in vitro. The resulting epithelial cell sheets transplanted to LESC-deficient mouse or rabbit eyes regenerated the corneal surface within 1 week of transplantation, highlighting their promising therapeutic potential.
In summary, cultured LESC-based therapy can effectively restore the corneal surface in LSCD patients. However, in cases of bilateral LSCD, alternative cell sources are required to enable autologous therapy, such as COMET, which have provided promising results. Alternative cell sources are still under investigation for LSCD therapy.