The promise of stem cells for agerelated macular degeneration and other retinal degenerative diseases

Transplanted cells can secrete numerous molecules that may exert a beneficial effect on the host retina and/or choroid even if they do not cure the underlying disease. Ideally, with a single transplant operation, many different pathways can be modified, which may reduce the chance of 'escape' associated with typical pharmacotherapy as well as the need for repeated drug administration. In addition, transplanted cells can replace dead cells (e.g. photoreceptors). Because of their pluripotency and unlimited proliferative capacity, stem cells seem to be a logical choice for starting material because they can be produced en masse safely and they can be induced to differentiate into ocular cells with potential for repla-cement and rescue therapy. Although preclinical studies demonstrate the feasibility of using embryonic stem cells and induced pluripotent stem cells for treating degenerative retinal diseases associated with abnormalities in the retinal pigment epithelium and/or photoreceptors, some issues may limit the use of stem cells in clinical practice. These issues include: immunogenicity of the cells, stability of cell phenotype (both inherent and environment-induced), the propensity to form tumors in situ, the abnormal micro-environment that can accompany degenerative dis-ease and the synaptic rewiring that accompanies retinal degeneration. In the case of non-exudative age-related macular degeneration, cell transplants might prevent progression of geographic atrophy (through replacement of dysfunctional or dead RPE) and might even bring about some visual improvement in selected cases (through rescue of photoreceptors that are dying but not dead). Cell-based therapy may one day be sight-restoring for patients who are blind due to retinal degeneration of various etiologies. RPE transplantation is an attractive starting point for this sort of therapy as these cells can integrate with the host retina easily.