In this study, we report survival of bioactive human neural progenitor cells in the subretinal space of normal nonhuman primates for more than one month without evident complication and with preservation of retinal function in the transplant area. The work shows that introduction of cells into an eye with close similarity to the human eye9,10
is feasible with a pars plana surgical approach that would be appropriate to the human clinical situation (with additional vitrectomy).
The cells, suspended in non-nutrient medium, were infused through a retinotomy and formed a continuous single sheet of cells in the subretinal space. The surgical method was low cost, clinically acceptable and scalable for the treatment of a significant number of patients. The procedure and continued presence of the grafts failed to elicit any deleterious manifestations in normal retinas, at any of the doses tested, when evaluated clinically, physiologically and histologically.
Surgeries were performed in healthy adult retinas since no validated models of retinal degeneration exist in nonhuman primates. This model provides information regarding dosage, location of cell engraftment and surgical technique that cannot be addressed in small rodent eyes. Retinas with advanced and widespread disease (e.g., in retinitis pigmentosa) may behave differently, for example with regard to subretinal bleb formation. However, the model is pertinent to early disease states, before marked degenerative changes have developed, and to macular degenerations such as AMD and Stargardt disease where juxtamacular delivery of cells to the surrounding more ‘normal’ retina might be employed.
Autologous RPE grafts introduced into the subretinal space of advanced patients with neovascular or “wet” AMD11, 12
appear to have little positive impact on vision and are associated with a high rate of significant complications including retinal detachment and hemorrhage affecting the graft. These disappointing findings are likely related to the complexity of performing this particular surgery as well as the advanced stage of the disease.13, 14
Indeed, creation of a retinal detachment by subretinal injection can lead to reduced visual function in the area of the detachment without subsequent recovery. Previous work has shown that introduction of growth factors such as brain-derived and glial-derived neurotrophic factors (BDNF and GDNF) can diminish this detachment effect.15, 16
Whether factors released by these neural progenitor cells may provide similar protection was not explored explicitly here but would be consistent with the preservation of mfERG responses in retinal areas where subretinal blebs had been elevated.
An issue of some importance is whether cells migrate from their site of engraftment. This question is critical to future use of cell therapy for AMD, given the need to introduce cells as close to the macula as possible without directly compromising macular vision. Indeed it would be advantageous if the cells could be delivered to an extra-macular location and then would migrate in the subretinal space under the fovea and throughout the retina. The degree of lateral migration or the retinal area covered by the cells could not be quantified in the present study, because it was not possible to define the border of the bleb in the post-mortem histology. Therefore, the potential area over which cells introduced into the bleb might have spread by active migration is unknown in the present study. Further work will address this issue.
The question of whether precautions are needed to prevent immune rejection, beyond the steroid application used here, also deserves more attention. Previous work in RCS rats has shown that even allogeneic grafts can undergo rejection and lead to loss of vision, while syngeneic grafts can survive when introduced under similar conditions.3, 17
In pigs, triple immune suppression with prednisone, cyclosporine and azathioprine was still not sufficient to sustain RPE graft survival.18
In the present study, cells survived up to 5 weeks even in the monkey treated postsurgically only with 5 days of topical steroids but not with systemic cyclosporine. Therefore it is possible that the donor cells used are less immunogenic than the RPE cells used in previous work. Two studies have suggested that human fetal RPE cells can survive in the primate subretinal space for significant periods.9, 19
It has also been suggested that the primate eye shows a stronger level of immune privilege than other species,20, 21
or that the pars plana approach avoids overt graft rejection, at least to 5 weeks, the longest time point studied here without cyclosporine.
Our study shows that human embryonic tissue-derived progenitor cells can survive transplantation into the subretinal space of non-human primates for at least five weeks. A more extensive and longer study is required to address issues (e.g. biodistribution, safety) that must be resolved before the application of cell-based therapies to the treatment of human retinal disorders including AMD.