RPE transplantation and its role in retinal disease

Elucidating the phenomenon of HESC-derived RPE: anatomy of cell genesis, expansion and retinal transplantation

Healthy Retinal Pigment Epithelium (RPE) cells are required for proper visual function and the phenomenon of RPE derivation from Human Embryonic Stem Cells (HESC) holds great potential for the treatment of retinal diseases. However, little is known about formation, expansion and expression profile of RPE-like cells derived from HESC (HESC-RPE). By studying the genesis of pigmented foci we identified OTX1/2-positive cell types as potential HESC-RPE precursors. When pigmented foci were excised from culture, HESC-RPE expanded to form extensive monolayers, with pigmented cells at the leading edge assuming a precursor role: de-pigmenting, proliferating, expressing keratin 8 and subsequently re-differentiating. As they expanded and differentiated in vitro, HESC-RPE expressed markers of both developing and mature RPE cells which included OTX1/2, Pax6, PMEL17 and at low levels, RPE65. In vitro, without signals from a developing retinal environment, HESC-RPE could produce regular, polarised monolayers with developmentally important apical and basal features. Following transplantation of HESC-RPE into the degenerating retinal environment of Royal College of Surgeons (RCS) dystrophic rats, the cells survived in the subretinal space, where they maintained low levels of RPE65 expression and remained out of the cell cycle. The HESC-RPE cells responded to the in vivo environment by downregulating Pax6, while maintaining expression of other markers. The presence of rhodopsin-positive material within grafted HESC-RPE indicates that in the future, homogenous transplants of this cell type may be capable of supporting visual function following retinal dystrophy.

A novel rabbit model for studying RPE transplantation

PURPOSE

The goal of this project was to develop a model of retinal pigment epithelium (RPE) transplantation that permits extensive and reliable analysis of the transplants.

METHODS

Cultures of newborn rabbit RPE were evaluated by morphology, electrophysiology, and the expression of zonula occludens-1, cytokeratin, and the melanocyte marker S-100. Cells labeled with 5,6-carboxyfluorescein diacetate succinimidyl ester (CFDA-SE) were transplanted into the subretinal space of rabbits with a 30-gauge needle without making a conjunctival flap or sclerotomy. The transplants were examined by fundus photography, confocal scanning laser ophthalmoscopy (cSLO), optical coherence tomography (OCT), and angiography. At 2 months, the retina was examined histochemically.

RESULTS

A 1-minute incubation at 37 degrees C with 20 muM CFDA-SE did not affect morphology or the expression of marker proteins. In coculture, the labeled cells integrated into monolayers that developed a normal transepithelial electrical resistance of 400 to 450 Omega . cm(-2). Dye was not transferred from labeled to nonlabeled RPE cells. Transplanted RPE was detectable for at least 2 months. Angiography demonstrated an intact blood-retinal barrier. The normal morphology of the retina and lack of debris in the subretinal space suggested that the transplanted RPE was functional.

CONCLUSIONS

Primary cultures of newborn rabbit RPE were highly differentiated, even when labeled with CFDA-SE. Labeled cells were observed long-term in vitro and in vivo. This model can be used to examine how culture and transplantation protocols affect the reformation of a functional RPE monolayer. The similar size of rabbit and human eyes will facilitate the translation of these protocols to the bedside.