Reconstruction of a human hemicornea through natural scaffolds compatible with the growth of corneal epithelial stem cells and stromal keratocytes

PURPOSE To reconstruct a human hemicornea in vitro by means of limbal stem cells cultured onto human keratoplasty lenticules (HKLs) and to obtain a natural corneal graft for clinical applications. METHODS Limbal stem cells were seeded onto HKLs with or without the presence of feeder layers of lethally irradiated 3T3-J2 cells and compared with the current "gold standard" scaffold, i.e., the fibrin glue. The effects of the scaffold on the preservation of stemness and/or induction of differentiation pathways were investigated through analysis of a variety of markers, including p63 and DeltaNp63alpha for stemness, 14-3-3sigma for early differentiation, keratins 3, 14, 12, and 19 to determine cell phenotype, and alpha6, beta1, and beta4 integrins to evaluate interactions with the stroma. Integrity of the stroma was assessed through analysis of keratan sulfate, CD-34 and aldehyde dehydrogenase 3A1 (ALDH3A1) (for keratocytes), visual system homeobox 1 (VSX1), and alpha-smooth muscle actin (alpha-SMA) (for fibroblasts and myofibroblasts). The structural properties of the reconstructed "hemicornea" were investigated through scanning electron microscopy. To evaluate the preservation of the stemness potential, cells were trypsinized from each scaffold and clonogenic/proliferative characteristics analyzed. RESULTS Limbal stem cells expanded onto HKLs gave rise to a stratified squamous keratinized epithelium morphologically similar to that of normal corneas. The resulting corneal epithelium was characterized by basal expression of p63 and DeltaNp63alpha, while expression of 14-3-3sigma, keratin 3, and keratin 12 was found in the upper cell layers. The basal cuboidal epithelial cells were anchored to the basement membrane and expressed keratin 14 and alpha6, beta1, and beta4 integrins. In the stroma of HKLs, keratocytes maintained the biosynthetic and phenotypic appearances typical of resting/quiescent cells and expressed keratan sulfate, CD-34, and ALDH3A1. Fibroblastic transformation was observed with the appearance of VSX1 and alpha-SMA. Scanning electron microscopy analysis showed that HKLs maintained their native conformation with collagen fibrils interconnected to the network and parallel to the corneal surface. HKLs did not alter the clonogenic/proliferative capacity of limbal stem cells. No differences were seen when HKL was compared to fibrin glue, one of the scaffolds currently used for limbal stem cell transplantation. CONCLUSIONS Our findings demonstrate that HKL could be a suitable scaffold for corneal epithelial stem cells as they were shown to proliferate, express differentiation markers, and bind to the underlying stroma with no alterations in clonogenic potential. HKLs have some advantages over currently used scaffolds, such as the possibility to allow cell growth with no feeder layers, to be freeze dried, and to preserve the integrity and viability of stromal keratocytes. The development of a tissue-engineered "hemicornea" might offer new therapeutic perspectives to patients affected by total limbal stem cell deficiency with stromal scarring.