Replacing retinal pigment epithelial cells

Retinal pigment epithelial (RPE) cells have a number of important jobs, including looking after the adjacent retina. If these cells stop working properly due to damage or disease, then certain parts of the retina die. As the retina is the component of the eye responsible for detecting light, this leads to the onset of blindness. RPE cells can be damaged in a variety of diseases such as: age-related macular degeneration (AMD), retinitis pigmentosa and Lebers congenital aneurosis.

One way to treat these diseases would be to replace the damaged RPE cells with transplanted healthy cells. Unfortunately, it is not possible to take healthy RPE cells from donors so it is necessary to find another source of cells for transplantation. Scientists have recently produced new RPE cells from both embryonic stem cells and iPS cells in the lab. The safety of embryonic stem cell-derived RPE cells has been tested in phase I/II clinical trials for patients with Stargardts macular dystrophy, and for thse affected by AMD by a stem cell biotech company called Advanced Cell Technologies. Theresults of the trial, published in 2014, demonstrated safety and showed engraftment of the transplanted RPE cells. However, some participants experienced adverse side effects from the immunosuppression and the transplantation procedure itself. Interestingly, despite not being an endpoint of this trial, several patients also reported an improvement in vision.

A second Phase I/II trial exploringthe use of RPEs derived from human embryonic stem cells for people with wet AMDis currently underway in the United Kingdom. The first patient received their transplant in September 2015. This work, led by Prof Pete Coffey, is ongoing and is being carried out at Moorfields Eye Hospital as part of the London Project to Cure Blindness.

Finally, Japanese researcher, Dr Masayo Takahashi is leading a clinical trial in Japan which transplants RPE cells made from iPS cells into patients with wet AMD. The trial was put on hold for several months due to regulatory changes in Japan and concerns about mutations in an iPS cell product to be used in the trial. The trial has recommenced June 2016 and many await the results.

There areseveral other phase I or I/II clinical trials using pluripotent stem cells world-wideinvolving small numbers of participants. These trials are examining primarily the safety, but in some cases also the effectiveness, of the use of RPEs developed from pluripotent stem cells in dry and wet AMD and Stargardts macular degeneration.

Replacement of damaged RPE cells will only be effective in patients who still have at least part of a working retina, and therefore some level of vision (i.e. at early stages of the disease). This is because the RPE cells are not themselves responsible for seeing, but are actually responsible for supporting the seeing retina. Sight is lost in these types of diseases when the retina begins to degenerate because the RPE cells are not doing their job properly. So the RPE cells need to be replaced in time for them to support a retina that is still working. It is hoped that transplantation of new RPE cells will then permanently halt further loss of vision, and in some cases may even improve vision to some degree.

Replacing retinal pigment epithelial cells:Techniques for growing cells for therapies are being researched and tested in early clinical safety trials.

Replacing retinal cells

In many of the cases where vision is lost, we often find that the problem lies with malfunctioning retinal circuitry. Different disorders occur when particular, specialized cells in the circuit either stop working properly or die off. Despite the retina being more complicated than other components of the eye, it is hoped that if a source of new retinal cells can be found, we may be able to replace the damaged or dying cells to repair the retina. In addition, this approach may also help to repair damage caused to the optic nerve.

Again, scientists have turned to stem cell technology to provide the source of replacement cells. Several studies have now reported that both embryonic stem cells and iPS cells can be turned into different types of retinal cells in the lab. Within the eye, a type of cell called the Mller cell, which is found in the retina, is known to act as a stem cell in some species, such as the zebra fish. It has been suggested that this cell may also be able to act as a stem cell in humans, in which case it may provide another source of retinal cells for repair of the retina.

Unlike RPE cell transplantation, direct repair of the retina may allow patients who have already lost their vision to have it restored to some degree. This gives hope for patients with disorders like late-stage age-related macular degeneration, where the light-sensitive photoreceptor cells in the retina have already been lost. This type of research may also provide new treatments for people who suffer from retinal diseases like retinitis pigmentosa and glaucoma. However, despite encouraging evidence, such research is very much in its infancy. There are currently no patient clinical trials planned using this type of approach, as significant further research is still required first.

Replacing the nerve cells of the retina:Current research aims to understand how to produce retinal nerve cells that could be used in future therapies.

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The eye and stem cells: the path to treating blindness