The cornea of the eye is overlaid by a stratified epithelium that is essential for vision. At the limbus—an anatomical zone situated at the edge of the cornea where it adjoins the sclera (the white of the eye)—the corneal epithelium contains a reservoir of stem cells, which are located basally.¹ These cells proliferate to provide a continuous supply of epithelial cells to the central cornea, maintaining its healthy state. Corneal limbal epithelial stem cells are highly proliferative, express p63 transcription factor, and exhibit holoclone-forming capabilities.² In their absence, or when insufficient numbers are present, patients suffer from a condition known as a limbal stem-cell deficiency (LSCD).³ A unilateral LSCD is often associated with acquired non-immune-mediated aetiologies such as trauma to the eye caused by thermal or chemical burns. Bilateral LSCDs can be caused by acquired primary immune-mediated aetiologies (for example, Stevens-Johnson syndrome or ocular mucous membrane pemphigoid) or idiopathic or hereditary disease such as congenital aniridia.³ Whatever its origin, an LSCD typically leads to the enveloping of the corneal surface by fibrotic conjunctival tissue and a consequent loss of vision.

 

The management of an LSCD necessitates the optimisation of the ocular surface and subsequent surgical removal of conjunctival scar tissue from the corneal surface, followed by a graft of functional corneal epithelial tissue.⁴ The choice of graft material depends on the type of disease. For patients with a unilateral LSCD, autologous transplant procedures should initially be considered because they tend to engender better long-term graft survival. Such surgeries include keratolimbal autografts,⁵ autologous cultivated limbal epithelial cell transplantation,^6–8 and autologous simple limbal transplantation,⁹ in which tissue biopsies are obtained from the patient's unaffected eye. If a patient suffers from a bilateral LSCD, however, no autologous ocular tissue is available, so candidate procedures include cadaveric limbal stem-cell transplantation,¹⁰ allogeneic cultivated limbal epithelial cell transplantation (including in situations in which the biopsy can be obtained from a living related donor),¹¹ allogeneic cultivated limbal epithelial cell transplantation,¹² and autologous cultivated oral mucosal epithelial cell sheet transplantation.^13,14 Despite the value of these treatments, the autologous and allogeneic approaches both come with drawbacks. These include the requirement for a biopsy of healthy limbus or oral mucosal epithelial tissue for autologous procedures, with the variability of autologous cell sources and individual fabrication regimes contributing to a sometimes uncertain outcome.¹⁵ In addition, postoperative neovascularisation following cultivated oral mucosal epithelial cell sheet transplantation is inevitable, variable in its severity, and very difficult to manage.^13,14 Allogeneic therapies are accompanied by problems that include the risk of immunological rejction.¹⁰

Here, we report a novel regenerative therapy for an LSCD that uses human induced pluripotent stem cells (iPSCs; figure 1A). The approach is based on our development of a self-formed, ectodermal, autonomous, multizone (SEAM) cultivation protocol that effectively produces precursor cells of ocular tissues from human iPSCs.¹⁶ This groundbreaking method partly mimics whole-eye development, and from SEAMs we have successfully fabricated functional ocular tissues including conjunctiva and lacrimal gland.^17,18 The SEAM technology also facilitates the generation of human iPSC-derived corneal epithelial cell sheets (iCEPSs), which were able to reconstruct the cornea in an experimentally induced animal corneal injury model.¹⁶