During the past decade, there has been significant development in therapies for exudative age-related macular degeneration (AMD), but currently there is no effective treatment for the dry or atrophic forms of AMD. It has been reported that between 60% and 80% of patients with AMD have the dry or atrophic form of the disease.1

In both exudative and nonexudative AMD, important alterations have been detected in the retinal pigment epithelium (RPE) cells and Bruch's membrane.2 The RPE is a monolayer that plays several important roles, including transporting nutrients from the choroid to the photoreceptors, eliminating their waste, and forming the bloodretinal barrier that controls the transport of substances into the retina.

RPE cells have important roles in for the proper functioning of the photoreceptors. They are responsible for the disc shedding of the photoreceptors, and it has been demonstrated that they secrete different trophic (growth) and antiangiogenic factors important for the correct functioning of the retina.3 The alterations that occur in AMD cause malfunctioning of the RPE cells, which then fail to maintain the photoreceptors. In general, age-related RPE cell damage includes altered gene expression and protein synthesis, free radical oxidation, and accumulation of residual bodies, consisting of lipofuscin, drusen, and advanced glycation endproducts (AGEs). Accumulations of drusen (Figure 1), basal deposits, and AGEs cause damage to Bruch's membrane, which provides supports to RPE cells (Figure 2) to maintain their polarity and normal functionality.

Recently some therapeutic alternatives for atrophic AMD are under investigation, including encapsulated cell technology (ECT; Neurotech USA, Inc., Lincoln, RI) and other forms of cell therapy, described below.

ENCAPSULATED CELL TECHNOLOGY

ECT deserves detailed comment because it can be considered both a new cell therapy strategy and an alternative to the use of cells directly in contact with eye tissues. ECT is a technology that allows sustained delivery of therapeutic factors into the eye. The first ECT product is an intraocular polymer implant containing human RPE cells genetically modified to secrete ciliary neurotrophic factor (CNTF). Cells are encapsulated in a semipermeable hollow fiber membrane (Figure 3). The hollow fiber membrane is designed to promote long-term survival of the implanted cells. The cells continuously produce the therapeutic protein, which diffuses out of the implant into the vitreous cavity. Originally designed for treating Stargardt's disease, ECT is currently being tested for dry forms of AMD in a pilot study being conducted by Neurotech.

CELL THERAPY FOR DRY FORMS OF AMD
Cell therapies are currently considered the best hope for treatment of dry forms of AMD. Alterations to the retina, at least in the initial stages of AMD, are limited to the external layers, and in theory can be recovered by using cell therapy approaches. If the defective RPE cells can be replaced and the new RPE cells connect appropriately with the still-functional part of the host retina, a degenerating RPE might be repaired, resulting in the prevention of continuing degeneration of the remaining photoreceptors, restoring functional activity of the blood-retinal barrier and finally restoring eyesight. Investigators are trying to achieve this aim by applying different approaches.

REJUVENATION OF RPE CELLS
Autologous adult RPE cells can be expanded ex vivo before transplantation. Although ex vivo expansion may offer the potential to partially reverse the influence of aging, this method has a high chance of transdifferentiation of cells. Best results are obtained when the RPE cells are in contact with extracellular matrix; otherwise there is a dedifferentiation process that prevents the transplant from being effective. The aging RPE loses its ability to synthesize correct basal laminar proteins and also fails to attach onto the basal lamina of Bruch's membrane because it does not express the necessary integrins for attachment. Such integrin expression can be induced by intermittent culture of adult RPE cells, and some rejuvenation of these adult RPE cells can be achieved by providing physiologic signals from highly complex matrix on a culture substrate. Different types of extracellular matrices from natural or synthetic origin have been analyzed in the past decade. There are a number of biocompatible matrices that can be used as supporting matrices for RPE cells, but recent studies have shown that unfavorable diffusion characteristics across an extracellular matrix can lead to neural retinal atrophy despite ideal RPE differentiation.4

CELL TRANSPLANTATION
Adult RPE cell transplantation appears to be an easy way to restore the subretinal anatomy and the critical interactions between RPE and photoreceptors. Several human trials have been performed in the past few years after many animal experimental studies (Figure 4). Unfortunately, initial results in humans are not conclusive, and visual recovery has been limited. To avoid rejection, autologous RPE cells harvested from the midperiphery of the fundus are being transplanted as a cell suspension or a patch of RPE and choroid under the macular area. But autologous RPE transplants have the disadvantage of carrying the same genetic information that has led to AMD.4

Because harvesting fresh adult RPE cells is surgically complicated in patients, autologous iris pigmented epithelial cells (IPEs) have been used as an alternative cell type for transplantation in AMD because IPE and RPE have been shown to possess a number of key common characteristics. IPE cells have the ability to transdifferentiate into RPE-like phenotypes, but they are incapable of expressing crucial enzymes of the retinoid visual cycle.5 Therefore, their potential use as a cell-based delivery system for various neuroprotective biomolecules, such as pigment epithelium-derived facto or brain-derived neurotrophic factor, has been explored.

Progenitors or stem cells present tremendous promise in a variety of ocular diseases, and transplantation of stem cells may be a viable treatment for AMD. Reports are emerging about cells derived from different extraand intraocular sources to obtain RPE-like phenotypes. The most important cells for RPE replacement include bone marrow-derived cells, neural stem cells, and embryonic stem cells.

FETAL/UMBILICAL CORD-DERIVED CELLS
Retinas derived from human fetuses contain neuroblastic cells that have the potential to develop into all retinal cell types. A pilot study conducted in 10 patients (four with AMD) who received subretinal implants of neural retinal progenitor sheets with RPE demonstrated some improvement of vision and the safety of the procedure, with no clinical evidence of rejection at 1 year of followup. 6 The use of fetal tissues, however, remains controversial. Additionally, embryonic cells have yet to demonstrate the ability to differentiate into retinal phenotypes when transplanted into the adult retina.7

The subretinal transplantation of bone marrow mesenchymal stem cells or human umbilical cord bloodderived cells is also under investigation for potential use to differentiate into retinal neurons,8 or for prolonging photoreceptor survival in animal models.9 Bone marrow seems to be a good source of stem cells because it continuously produces uncommitted cells, which are capable of differentiating neuronal cells expressing retinal markers and to the cells with RPE-like properties.

RETINAL ADULT STEM CELLS
The recent discovery of retinal stem cell niches that survive into adulthood (in the region of the pars plana and ciliary margin),10 which have the ability to differentiate into neural retinal and RPE cells, may yield a new source of cells that can be differentiated and transplanted into patients. Retinal stem cells harvested from the pigmented and nonpigmented epithelium of the ciliary body would be a good source of RPE. Some work has been performed on stimulating the differentiation of neural progenitors into specific cell lines. When they are cultured in the presence of growth factors, these cells proliferate to form neurospheres. Using adherent monolayer culture system, these cells could be readily expanded maintaining a progenitor phenotype.11 Microarray analyses have demonstrated different gene expression (RPE markers or photoreceptors) when stem cells are cultivated on different extracellular matrices and neighboring cells.12,13 These findings suggest the possibility of harvesting these cells from a patient, differentiating them, and expanding them to replace the affected area of the macula using the concept of tissue engineering.11

Funding for this work was provided by Regenerative Medicine and Cell Therapy Network Center of Castilla y Leon.

José-Carlos Pastor Jimeno, MD, PhD, is Director of the Instituto Universitario de Oftalmobiología Aplicada at the Universidad de Valladolid in Spain. He has no financial interests to disclose. Dr. Pastor can be reached at +34 983 423274; or via e-mail at pastor@ioba.med.uva.es.

Girish Kumar Srivastava, PhD, is an investigator contracted by the Regenerative Medicine and Cell Therapy Network Center of Castilla y Leon, Spain. He is involved in developing cell therapy research in the Instituto Universitario de Oftalmobiología Aplicada at the Universidad de Valladolid in Spain. He has no financial interests to disclose. Dr. Srivastava can be reached at +34 983 184753; or via e-mail at girish@ioba.med.uva.es.

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