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Human retinal transplantation has followed many years of experimental research showing that transplanted retinal pigment epithelial (RPE) cells have the potential to rescue photoreceptors (PR).1–3 Histopathological studies have demonstrated integration of cultured cell suspensions in the subretinal space in animals, and blind rats showed regain of functions after RPE transplantation.4 However, it was also demonstrated from several groups that freshly harvested or cultured RPE suspensions may fail to survive or function on aged or damaged Bruch membrane (BM).5–7 An uneven distribution of RPE cells caused the formation of multilayers of RPE alternating with bare areas of basal lamina.8 Furthermore, in a hostile environment, the subfoveally delivered cells have less chance of survival. The inability to grow well on a defective or diseased basal lamina is considered a major problem as to why transplanted RPE cells fail to function for a prolonged period and cannot significantly restore vision in human eyes.9 Today, transplantation of a polarised RPE monolayer as a sheet seems to be more promising.
In human eyes with advanced age-related macular degeneration (AMD), a logical consequence is to translocate an autologous RPE BM choroid patch, taken from a more distant location of the same eye.10 11 While some cases have shown good functional results, and visual acuities up to 20/40 can be achieved in single cases with this technique, the overall visual gain reported is about one line, and complications such as proliferative vitreoretinopathy are observed up to 45%.12 13
Also, sheet transplants of fetal retina together with RPE are used in patients with retinitis pigmentosa and AMD from one group, so far with very limited visual gain.14 If retinal transplantation is to be performed more successfully and safely, we need alternative sources of cells in sufficient numbers. Scaffolds are needed to shelter these cells.
A carrier substrate, which at the same time would serve as an either temporary or permanent BM prosthesis, might dramatically improve the chance for permanent survival and function. Having such an arrangement would enable transplantation of an intact epithelial sheet that is simultaneously protected from hostile influences of aged and diseased BM. As the RPE is a polarised epithelial cell, the scaffold needs to facilitate nutrient flow, so that physiological processes such as vitamin A, glucose and fatty acid transport can be restored. In the clinical situation, the whole procedure should carry little surgical risk for the patient to allow intervention at much earlier stages of retinal degenerative disease.15
Many different basal laminas have already been tested in experiments, primarily those who are already available naturally such as Descemet membrane,16 lens capsule,17 aged BM,18 amniotic membrane19 20 and inner limiting membrane.21 While these scaffolds are biocompatible and can mimic natural mechanical properties they carry the disadvantage of limited availability, possible transmission of disease and an unclear absorbtion time. Technical problems include curling, stiffness and thickness.
Polymers, natural and synthetic have been widely investigated, too. Among natural occurring polymers are collagens. Crosslinked and non-crosslinked collagens collagen film and recently ultrathin collagen foils were delivered subretinally with a special designed instrument in animal experiments.22–24 Gelatin, a further processed collagen, and fibrinogen, which, if crosslinked in the presence of thrombin, forms a dense mesh network as well as cryoprecipitates, were examined by several groups as well.25–27 As these materials closely mimic the extracellular matrix of the tissue type involved and are biocompatible, their use is intriguing. But also here, concerns do exist about the consistency of a natural occurring product, its purity and transmission of disease and possible allergies to single components.28
The great advantage of synthetic polymers lies in the ability to be designed as either non-degradable or degradable in a desired time course.29
Among the biodegradable polymers, poly(alpha-hydroxy)ester was tested as a RPE substrate and retinal progenitor cells examined on poly(L-lactic-coglycolic)acid and poly(d,l-lactic-coglycolic acid) films, showing better survival and differentiation in comparison with cell suspensions.30 31
Another approach to improve cell survival is surface modification. The use of oxygen plasma treatment of non-porous films of biodegradable poly(hydroxybutyrate-co-hydroxy)valerate to change a hydrophobic surface into a hydrophilic one to optimise cell attachment was described by Tezcaner et al in 2003, and air plasma treatment on polyurethanes performed by Williams and Krishna in 2005.32 33
However, aside from surface modification, surface topography might also influence cell attachment and survival. Today, microfabrication technology allows the creation of 3-D tissue scaffolds to further improve interconnection of cells.34 Porous elastic scaffolds are created from poly(glycerol) sebacate and ultrathin nanowire scaffolds from poly(e-capro) lactone. As a supplement, a laminin coating is frequently used to enhance cell survival.35 36
In this issue (see page 569), Krishna and coworkers describe the use of an expanded polytetrafluoroethylene (ePTFE) as a possible substrate for RPE cell growth and transplantation.37 Surface modification with ammonia gas plasma treatment to create a hydrophilic surface was performed.
ARPE-19 cells, a non-immortalised human cell line, but also human RPE (hRPE) cells, to mimic more closely the human situation, were seeded on ePTFE and compared with controls (tissue culture polystrene). In order to create a more realistic situation if cells are transplanted in patients with degenerative retinal disease combined with choroidal atrophy, reduced serum levels were used for cell culture. Both cell types showed homogenous cell coverage of the ePTFE surface with increasing numbers over time, maintenance of the phenotype and phagocytosis of photoreceptor outer segments.
As cells need to be arrested in their differentiated state to be suitable for transplantation, these findings are important. In the given time frame, hRPE responded almost as well as ARPE-19 cells.
Interestingly, in their experiments, a biostable membrane was used in contrast to the biodegradable scaffolds used so far. The authors explain their choice with the need for a long-term support of the cell layer and the presence of a disease-free basement membrane. We do not know how long the ideal support of a cell layer should last in order to guarantee cell survival but also to provide foveal vision. Until now, many groups were convinced that 3–6 weeks would be an ideal time for an artificial scaffold to degrade. To answer this, in vivo experiments will be necessary.
Finally, for human studies, mechanical properties of scaffolds will also play an important role for retinal transplantation, as this will influence safe delivery and stability of a well-adjusted sheet in the foveal area. For example, temperature-dependent materials have demonstrated support of cellular growth.38 A cell- and nutrient-enriched gel would allow easy introduction via a small sclerotomy, precise delivery under the retina and definitely reduce surgical risk.
Although biotechnology does open numerous avenues for surface modification, and artificial scaffolds play an increasingly important role to control cell growth and improve RPE survival in experimental settings, the main questions about choosing the right cell type, its ability to phagocytose and provide visual function and the response of the transplant in a diseased environment will only be answered in in vivo experiments and human studies. Progress in the field of retinal transplantation does come in small steps, and Krishna and coworkers have provided another important step in this area.
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