Reversal of end-stage retinal degeneration and restoration of visual function by photoreceptor transplantation

Abstract

One strategy to restore vision in retinitis pigmentosa and agerelated macular degeneration is cell replacement. Typically, patients lose vision when the outer retinal photoreceptor layer is lost, and so the therapeutic goal would be to restore vision at this stage of disease. It is not currently known if a degenerate retina lacking the outer nuclear layer of photoreceptor cells would allow the survival, maturation, and reconnection of replacement photoreceptors, as prior studies used hosts with a preexisting outer nuclear layer at the time of treatment. Here, using a murine model of severe human retinitis pigmentosa at a stage when no host rod cells remain, we show that transplanted rod precursors can reform an anatomically distinct and appropriately polarized outer nuclear layer. A trilaminar organization was returned to rd1 hosts that had only two retinal layers before treatment. The newly introduced precursors were able to resume their developmental program in the degenerate host niche to become mature rods with light-sensitive outer segments, reconnecting with host neurons downstream. Visual function, assayed in the same animals before and after transplantation, was restored in animals with zero rod function at baseline. These observations suggest that a cell therapy approach may reconstitute a light-sensitive cell layer de novo and hence repair a structurally damaged visual circuit. Rather than placing discrete photoreceptors among preexisting host outer retinal cells, total photoreceptor layer reconstruction may provide a clinically relevant model to investigate cell-based strategies for retinal repair.stem cell | blindness | retinal surgery | visual cortex | neural regeneration

Bimodal in vivo imaging provides early assessment of stem-cell-based photoreceptor engraftment

“…Here we show, for the first time, that graft architecture can be highly variable at the time of transplantation, which could represent a considerable source of treatment variation among recipients and across the fundus. In this regard, bimodal imaging allows for early detection of a central problem inherent to cellsuspension therapeutics: undesired graft aggregation resulting in ischemia (reviewed by Robey et al 26 ) and/or spatial inhibition of functional connections, such as the highly limited contact observed between bolus PPC-grafts and host RPE 9 -a common architecture seen here and in other studies 7,9,27 ). Conversely, bimodal imaging can also detect extensive graft dispersion, which can leave insufficient density of cells for functional benefit, as small quantities of integrating PPCs are unlikely to rescue vision.…”

“…This will avoid prolonging experiments in animals where engrafted cells are incorrectly placed, under immune attack, or have died early in the experimental protocol. Although conventional in vivo imaging (rodent ophthalmoscopy) allows for some initial confirmation of subretinal bleb formation, 9,24,25 implying transplantation to the correct retinal layer, it is ineffective at determining subretinal graft presence past the day of transplantation. This is because ophthalmoscopy views the retina en face, making blebs of smaller profile harder to identify; moreover, aside from host erythrocytes (in cases where subretinal hemorrhaging occurs), graft and host cells cannot be differentiated through the neural retinal.…”

“…Expressed fluorescence, on the other hand, can be used for unambiguous tracking purposes in serial and end point analysis. 5,9 Utilizing this with bimodal imaging in future studies would allow for high-fidelity, in vivo imaging of graft survival. The lack of subretinal graft survival we observed at day 15 posttransplant is most likely due to acute rejection, though anoikis and ischemia probably also have a role in diminished graft survival.…”

“…In future preclinical trials, this feature could help streamline immunosuppression regimes and enable more rapid feedback of various approaches for increasing graft survival-to date, one of the most challenging barriers to overcome in the development of a photoreceptor replacement therapy, as 490% of even the most promising syngeneic-PPCs die within 2 weeks posttransplant. 9 The importance of graft architecture…”

“…Remarkably, a handful of studies have shown some restoration of vision in photoreceptor precursor cell (PPC) recipients, [6][7][8][9] though graft integration and survival remains too low to be clinically relevant. Short-term assessment in serial studies aimed at addressing these barriers currently relies on imaging, such as ophthalmoscopy and end point histology.…”