There are numerous diseases and disorders that can effect corneal tissue and which can, as a result, adversely effect or eliminate vision. For example, allergies, conjunctivitis, corneal infections, Fuchs' dystrophy (deterioration of corneal endothelial cells), varicella-zoster virus, iridocomeal endothelial syndrome, keratoconus, ocular herpes and a number of other conditions, as well as congenital corneal abnormalities can be responsible for corneal damage or irregularity that may affect vision. In an endeavour to restore sight or improve vision in people suffering from corneal abnormalities it has become particularly common to perform corneal transplant operations where the abnormal corneal tissue is removed and replaced using fine sutures with normal corneal tissue obtained from a donor. Although corneal transplant operations enjoy a high rate of success there are nonetheless some problems which can occur, such as rejection of the replacement cornea and ocular fibrosis or scarring. Even in the case of a successful corneal transplantation it is necessary for subsequent administration of immunomodulatory agents. Non-compliance by the patient with prescribed dosing regime of immunomodulating agents may give rise to tissue rejection. There is, accordingly, a need for improved means of prolonging corneal graft survival and preventing tissue rejection as well as for the provision of approaches for therapy of ocular infection, wounds and fibrosis and for therapy of other ocular disorders, for example.
The cornea is a highly organised group of cells and proteins which unlike most tissue is clear, and does not contain blood vessels to nourish or protect against infection. The cornea receives nutritional supply from tears and the aqueous humor found in the anterior chamber located behind it. The cornea is composed of five basic layers, namely the protective external epithelium, Bowman's layer which is located below the epithelial basement membrane and is composed of collagen fibers, the stroma which consists primarily of water and collagen and is located beneath Bowman's layer; and Descemet's membrane located beneath the stroma, which is composed of collagen fibers produced by the endothelial cells located in the lower endothelium. The endothelial cells are essential in maintaining clarity of the cornea by removing excess fluid from the stroma.
Irreversible immunological rejection is the major cause of human corneal graft failure (1), despite the immunologically privileged nature of the eye (2). The histological correlates of rejection include local upregulation of major histocompatability complex and adhesion molecules, an influx of mononuclear cells into the cornea and anterior chamber, and local production of some inflammatory cytokines (3-7). The major target of corneal graft rejection is the corneal endothelium. Human (but not rodent) corneal endothelium is essentially amitotic (8), so that damage to the monolayer during graft rejection cannot be repaired.
Gene therapy has the potential to influence an allograft response through local expression of a modulatory gene product within transplanted donor tissue. The present inventors consider that the cornea may be uniquely amenable to such an approach because of its small size, which may allow modification of the whole tissue, and because of the ease with which a donor cornea may be manipulated in vitro and stored for a considerable period (for example, up to 28 days) prior to transplantation. The anatomical location and clarity of the cornea allow in vivo assessment of the entire graft in the post-operative period and loss of function is easy to detect. Furthermore, the cornea and anterior chamber are at least partially immunologically privileged sites (2), which may allow the use of otherwise immunogenic or pro-inflammatory vectors.