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Reviewed by David J. Wilson, MD
Take-home message: Cell-based therapies are in their infancy but may be the future of salvaging vision in patients with retinal diseases.
Portland, OR-Cell-based therapy is a relatively new area of research in ophthalmology. Though interest is high, there are limited applications and the field is as yet “unsettled,” said David J. Wilson, MD.
A number of trials are ongoing and a variety of cell types are being explored for use in the eye. Most trials are focusing on replacing or augmenting the retinal pigment epithelium (RPE) in order to impact the diseases of interest, according to Dr. Wilson, chairman, Department of Ophthalmology, Casey Eye Institute, Oregon Health and Science University, Portland.
A number of cellular sources are available for therapies, including the most common: embryonal stem cells (ES), induced pleuripotent cells (iPS), bone marrow-derived stem cells, and mesenchymal stem cells.
Macular degeneration is the primary target in this therapeutic area, but other diseases that could possibly benefit from cell-based therapy are Stargardt disease, Best disease, and some of the advanced stages of other inherited retinal diseases. This is because the RPE is primarily or secondarily affected in these conditions as well, he explained.
Small animal models, notably the RCS rat and Rd1 mouse, are the basis for the excitement surrounding these cellular therapies. The results of work with those animal models showed that the photoreceptors could be salvaged when cell-based therapies were implemented, Dr. Wilson said, adding that because of those results, large animal trials were started.
Studies of RPE transplants performed in pigs and primates showed immune reactions to allografts. In humans, researchers are evaluating allogenic RPE cell suspensions in geographic atrophy and in Stargardt disease.
In addition, autologous RPE sheets and RPE on scaffolds are also being evaluated. These studies are limited because the fate of the cells remains unclear, although the treatments do appear to be safe.
Dr. Wilson indicated there is much work to be done to determine the best approach to cell based therapies.
Dr. Wilson described his group’s work in development and delivery of allographs and autografts and the associated problems and successes.
The ideal goal for cell-based therapies is to replace the RPE monolayer and establish a perfect relationship with the photoreceptors, Dr. Wilson said.
However, he noted, several preliminary steps must be undertaken to reach that goal, the first of which involves cellular preparation of ES and iPS cells.
For cells to be useful as RPE cells, they must be polygonal, pigmented, phagocytic, polarized, and have phenotypic markers typical of RPE, he explained. These ES cells are created by somatic cell nuclear transfer and iPS cells that are differentiated toward RPE lineage. This is performed using a technique developed by Kapil Bharti, PhD, and Sheldon S. Miller, PhD, from the National Eye Institute. Using both allogenic and autologous cell lines, the cells placed in the eye are able to be tracked because they were transduced with an AV vector which expresses green fluorescent protein (GFP).
A problem during cell delivery into the eye is that cell leakage into the vitreous can occur when a transretinal approach is used, possibly resulting in a proliferative vitreoretinopathy. In light of this, Dr. Wilson and colleagues developed a transcleral route of injection.
“First, we inject balanced saline solution under the retina using a tiny cannula and then inject the cells into the bleb from a transcleral approach using a larger cannula,” Dr. Wilson said. This prevents damage to the cells from the injection process.
Thus far, the investigators have performed experiments in a small number of primates. Specifically, they have injected seven ES-derived RPE allografts, four iPS-derived RPE allografts, and one iPS-derived RPE autograft.
After injecting the cells into the first eye, serology and ophthalmic imaging are performed weekly and compared with the baseline imaging. The fellow eye is injected from four to five weeks after the first and followed by weekly serology and ophthalmic imaging and necropsy three weeks after the second surgery.
The most important consideration is cell survival with avoidance of immune rejection or other causes of apoptosis. Clinical monitoring of the cells through photography or OCT is important to observe the fate of the cells.
“An important clinical marker is that the location of the cells can be seen on images under the retina. We can then look at the relationship of those cells to the overlying retina,” he said.
When viewing OCT images of allografts over time, the EZ band next to the injection site recovers but not overlying the cells. Dr. Wilson believes that this is a valuable clinical marker by which the survivability of grafts can be gauged.
Using immunohistochemical studies post necropsy, immune responses were apparent with T and B cells in the area of the injected cells in allografts. Activated microglial cells were present in both the allografts and autografts.
Dr. Wilson believes autografts may be necessary to avoid immune reactions. Although microglial activation occurred in the autographs, there was no immune response in the choroid- much different than with the allografts.
Dr. Wilson and colleagues have reached many interesting conclusions from these studies, but questions still remain.
“iPS and ES cells are suitable sources for RPE,” he said. “Many laboratories have produced cells with the features that are desirable for RPE cells. However, the question remains about whether these cells maintain their differentiation in the subretinal space.”
Regarding delivery, the caliber of the cannula and the delivery route are important considerations.
“The cannula caliber needs to be appropriate to eliminate sheer damage to the cells with injection,” he said. “Cells do not survive when injected using a small cannula; the tolerable cannula size will need to be determined for each cell type. In addition, transcleral delivery minimizes leakage of cells and allows use of a large cannula.”
Because cell survival is the most important factor with all cell-based therapies delivered into the subretinal space, allograft therapy may mandate use of immune suppression.
However, Dr. Wilson suggested that nonimmune factors may negatively affect cell survival, such as the cell characteristics, local environment, and transport media.
The original thought associated with this therapy was that if cells were injected into the subretinal space they would become integrated into the normal RPE cell layer, which was not the case.
“Without this happening, it is not as likely that we can treat patients with early-stage disease,” he said “The presence of native RPE cells may prevent healthy cells from surviving in that space.”
Dr. Wilson emphasized the need to address these problems in a large primate model in which a fovea is present.
David J. Wilson, MD
This article was adapted from Dr. Wilson’s delivery of the Zimmerman Lecture at the 2015 meeting of the American Academy of Ophthalmology. Dr. Wilson has no financial interest in any aspect of this report.