Retinal cell transplantation: prospects for the future

Keywords::

• photoreceptor
• retina
• stem cell
• transplantation

In a recent issue of The Lancet, Schwartz and colleagues have reported preliminary findings of the first trials of transplantation of retinal cells derived from embryonic stem cells [1]. Trials in the USA for Stargardt’s disease [101] and age-related macular degeneration (AMD) [102], and our own in the UK for Stargardt’s disease [103], mark an important stage in the development of cell therapies for blinding retinal diseases. Degenerations of the outer retina are responsible for the overwhelming majority of people with untreatable sight loss. AMD is the leading cause of blindness, and Stargardt’s disease is the most common cause of macular degeneration in young people. These conditions have a devastating impact on quality of life and present a huge economic cost to individuals, their families and society. Rapid progress in the development of retinal cell transplantation techniques during recent years has led to striking results that demonstrate the real potential for this technology to help address these blinding retinal degenerations.

The rationale for transplantation of healthy cells to replace diseased cells in the retina is clear. The hope is that, by repairing the retina using this approach, the rate of degeneration might be slowed to protect vision, or possibly even reversed to restore sight. The retina has a number of specific advantages as a recipient tissue for cell transplantation. Cells can be precisely delivered to the natural cleavage plane of the subretinal space under direct visualization using modern microsurgical techniques, and a relatively small number of donor cells may be sufficient to provide benefit for vision. The blood–retinal barrier is likely to help protect transplanted cells against systemic immune responses and immunological rejection. Functional integration of photoreceptor cells will require the formation of just a single synaptic connection with a bipolar cell. The retina is accessible to high-resolution optical imaging techniques with which to assess cell survival, and sophisticated psychophysical techniques can measure the impact of transplantation on retinal function. These advantages make the eye a valuable ‘model system’ for cell transplantation that has attracted the attention of investigators outside the field of ophthalmology.

A wide range of sources of donor cells has been described and evaluated for their potential in transplantation. One key question relates to the relative advantages of autologous versus allogeneic donor cells. Proof of principle for transplantation of autologous cells has been demonstrated in clinical trials of retinal translocation surgery, in which retinal pigment epithelium (RPE) from outside the macula is relocated to support foveal photoreceptors. Autologous cells are not subject to immunological rejection and improvement in outcome following surgery can be dramatic. However, the frequency of adverse events, particularly proliferative vitreoretinopathy, associated with the complex surgical manipulation involved is unacceptably high. Furthermore, the transplanted cells may be predisposed to degeneration by virtue of genetic risk factors or their own senescence. By contrast, allogeneic cells may be healthier and, although at risk of immune rejection, may benefit from the protection of the blood–retinal barrier.

The ability of pluripotent stem cells to differentiate into a wide range of cell types makes them a highly attractive source of donor cells for transplantation. Adult neural stem cells can be harvested from the subventricular zone of the adult brain, cultured in vitro as neurospheres and differentiated into neurones. However, the potential of adult neural stem cells appears to be restricted, and the number of cells that can be generated is limited. Embryonic stem cells isolated from the inner cell mass of the 5-day-old blastocyst, by contrast, are pluripotent and can replicate indefinitely to provide large numbers of cells for transplantation. Embryonic stem cells can be differentiated readily in vitro into cells that bear many of the hallmarks RPE cells. The cells transplanted in the current trials are fully differentiated human RPE cells derived from human embryonic stem cells (hESC-RPE cells).

Stargardt’s disease is typically inherited as an autosomal-recessive disorder caused by mutations in the gene encoding ABCA4, which is a photoreceptor-specific ATP-binding cassette transmembrane transporter for vitamin A intermediates that normally facilitates the removal of reactive retinal metabolites from photoreceptors excited by exposure to light. The associated accumulation of lipofuscin in RPE cells causes a secondary RPE dysfunction that compounds photoreceptor injury and degeneration. In AMD, oxidative stress and chronic inflammation are associated with the formation of drusen and progressive degeneration of RPE and photoreceptor cells. By replenishing the compromised RPE in these conditions with healthy transplanted cells, the progression of disease in these conditions might be slowed. Experimental data suggest that, when delivered to the subretinal space, these cells can form a monolayer and can protect against loss of visual function. The aim of the current clinical trials is to determine the safety of surgery to deliver these cells in humans, and to find out whether transplanted cells can survive in the subretinal space, with or without systemic immunosuppression. To minimize the impact of any adverse effects on sight, these first trials involve subjects who already have advanced retinal degeneration and sight loss, and for this reason any potential for them to benefit directly is limited. In their recent report, Schwartz and colleagues describe the preliminary results following subretinal administration of a suspension of 50,000 hESC-RPE cells to one eye of one subject with Stargardt’s disease and to another with AMD [1]. The early results suggest that the transplanted cells are well tolerated, with no evidence of inflammation, proliferation or deterioration in vision in the short term.

In the longer term, these trials will begin to address important questions about the survival of transplanted hESC-RPE cells in the human subretinal space, and the extent to which this is dependent on systemic immunosuppression. The requirement for immunosuppression may be limited to a short period perioperatively until the integrity of the blood–retinal barrier is restored. Should sustained immunosuppression be necessary to protect unmatched cells, the use of matched tissue could be considered. Ultimately, the risks of immunosuppression will need to be weighed carefully against any benefit to sight. The current trials will also help to address the question of whether a suspension of hESC-RPE cells can form a monolayer of cells with the ability to polarize appropriately, and which physiological roles of the many critical for photoreceptor function and survival might be recapitulated. An alternative to injection of a cell suspension is to deliver a preformed monolayer of RPE cells that has been cultured in vitro on a substrate. An artificial substrate might serve to perform the role of Bruch’s basement membrane, which is integrally involved in the pathogenesis of AMD. However, the surgical implantation of a sheet into the subretinal space will be technically more challenging than the injection of a cell suspension, and the presence of the substrate might adversely influence the diffusion of essential metabolites between the outer retina and the underlying choroidal circulation.

In AMD and Stargardt’s disease, degeneration of the RPE is often associated with concomitant degeneration of photoreceptor cells and, in many retinal degenerations, it is primarily the photoreceptors that are lost. Recent progress in the field of photoreceptor transplantation has demonstrated that postmitotic, terminally differentiated rod precursor cells delivered to the retina can survive, integrate and form functional synapses with bipolar cells [2], and are able to improve vision in mice [3] . Since the optimal developmental age of these donor cells in the human would be that of a second-trimester fetus, an alternative source of human photoreceptor precursor cells for transplantation is required. The recent finding that ESCs can spontaneously form stratified neural retinal tissue in 3D culture by an intrinsic self-organizing program [4] points to a potential source of photoreceptor cell sheets for transplantation [5]. The ability to induce pluripotent stem cells from a differentiated cell by the forced expression of specific genes (see work by Yamanaka) offers another potential source of autologous somatic cells that are likely to be at less risk of immune rejection [6].

Optimal timing of intervention with cell transplantation will depend on defining a window of opportunity during which the potential benefits can be expected to outweigh the associated risks. It is anticipated that this period is likely to begin after the onset of sight impairment and end prior to extensive cell death and associated gliosis. Given that the time course of retinal degenerations can be slowly progressive and variable, the identification of valid surrogate outcome measures will be valuable to help determine efficacy in early clinical trials. In conditions where degeneration of RPE cells and photoreceptors progress concurrently, the simultaneous transplantation of both cell types may be required. Photoreceptor cells might be transplanted together with sheets of RPE cells as an outer retinal graft in advanced retinal degeneration.

Significant progress in basic science and experimental ophthalmology has justifiably raised expectations about the possible impact of retinal cell transplantation. There are a number of challenges to be met before this potential can be realized, but the first clinical trials of ESC-derived RPE cells mark an important early step in the translation of these findings for the benefit of people with blinding retinal diseases.

Financial & competing interests disclosure

RR Ali and JWB Bainbridge are supported by the NIHR Biomedical Research Centre for Ophthalmology, Medical Research Council, Moorfields Eye Hospital Special Trustees, RP Fighting Blindness UK, The Newman Foundation and the Rosetrees Trust. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.