Retinal Prostheses and Transplantation Session
May 2nd, 2022; ARVO; Denver CO
Drs. Jacob Light, Abtin Shahlaee, Meera Sivalingam
Vitreoretinal Fellows
Wills Eye Hospital
One of the biggest challenges facing ophthalmic researchers and clinicians in end-stage retinal degenerations, whether in AMD or inherited dystrophies, is how to reverse outer-retinal loss and restore visual acuity. This exciting paper session focused on some of the latest work in this area, highlighting both technical innovations and early clinical trial data in both the fields of stem cell therapy and artificial electrode implantation.
Dr. Boris Stanzel kicked off the session with a description of a novel technique for preparing donor sites for injection of stem cells in a rabbit model. In both experimental and clinical stem cell injections, a subretinal bleb was created, using either transvitreal or suprachoroidal approaches. One challenge hinges upon successful creation of an RPE monolayer after injection. Injecting over intact RPE results in clumping and an irregular interface. In prior work, a mechanical RPE scraping technique has been described, however this often results in accumulation of hyperreflective material into the denuded area. Dr. Stanzel described using micropulse laser to selectively remove a large-area of RPE. He found that subretinal injection and bleb formation was more facile in the laser-treated regions than over controls with intact RPE. He also showed great images demonstrating that RPE monolayer formation was nicely facilitated by the laser ablation technique. Finding optimal laser energy titration is critical and remains a goal for further refinement.
Dr. Phuc Nguyen from the University of Michigan then presented his work on in vivo tracking of transplanted progenitor cells after photocoagulation. After stem cells are injected into the subretinal space, it is important to be able to track their location and distribution over time, specifically because therapeutic benefit is closely tied to the treatment reaching areas of disease. Broadly speaking, methods to track stem cells post implantation include MRI, PET, OCT, and an innovative technique known as photoacoustic microscopy (PAM). PAM relies on laser-induced light absorption of gold nanoparticles which can be attached to the injected stem cells. Heating these nanoparticles then generates an ultrasound signal which can be detected with a probe. Utilizing PAM images and a rabbit model, Dr. Nguyen demonstrated that injected RPE precursor cells were found to migrate to laser-injured sites within 5-7 days of injection. The signal coming from these PAM-imaged cells lasted for roughly 56 days. This is significantly longer than what is seen with fluorescently labeled cells, which were nearly undetectable after 14 days. While no direct toxicity was seen from the nanoparticles, the question still remains of whether they may generate significant intraocular inflammation.
Following, Dr. Budd Tucker from the University of Iowa presented his work on patient derived iPSCs for photoreceptor replacement. In order for patients to enjoy the potential benefits of retinal stem cell therapies, there must be efficient and reliable ways to produce the injectable cells. Dr. Tucker not only described methods to produce these autologous patient-derived cells, but showed it can be done in an efficient, automated, and less labor intensive way. The automated system can pick individual colonies in a precise fashion, cutting down on tedious human-dependent tasks. These colonies go on to form well-structured retinal organoids with cellular markers showing successful differentiation with good cone and rod localization. Finally, an automated incubator and a robotic arm to extract and manipulate the final products complete Dr. Tucker’s current prototype. The future of mass-produced retinal stem cells looks to be well on the way thanks to the important work being done by Dr. Tucker and others.
Sonali Nashine from UC Irvine then presented virtually her work on the CRISPR knockout of the Humanin trimeric CNTFR/gp130/WSX-1 receptor in AMD. Mitochondrial DNA damage has been implicated in AMD pathogenesis and the protective effects of Humanin G (HNG) mitochondrial derived peptide have been identified in RPE transmitochondrial cell lines. Dr. Nashine gave a virtual presentation on the effects of CRISPR knockout of the HNG receptor in AMD. Her team took cells derived from a patient with AMD and created a novel CRISPR-edited triple knock-out (KO) cell line which lacked the extracellular trimeric CNTFR/gp130/WSX-1 receptor. They exposed these cells (and wild type AMD cells) to exogenous HNG. The KO cells showed higher caspase staining than did the AMD wild type cells, indicating higher rates of apoptosis in the KO group. Furthermore, cell proliferation was also attenuated and cellular metabolic activity was also diminished in the KO group. In summary, the protective effects of exogenous HNG administration was diminished in the KOs, confirming that the knocked-out receptors are important components of the HNG-related cytoprotective mechanism.
Dr. Allen Ho from Wills Eye had the very exciting task of reporting remarkable results from the OpRegen Phase I/IIa clinical trial. This is an open-label, single-arm, multi-center trial in which allogeneic RPE cells were injected subretinally in 24 patients with poor (<20/200) and intermediate (20/64 to 20/200) vision. Both transvitreal and suprachoroidal approaches were used. Dr. Ho showed promising safety data, with adverse events mostly being related to the surgical procedure itself rather than the injected cells. Encouragingly, a trend in visual improvement was seen in treated eyes in all cohorts, though improvement was also seen in fellow untreated eyes, highlighting the limitations of a single-arm study with likely intervention bias. Structure/function correlation was demonstrated, as 5 patients in which the OpRegen cells were delivered to most or all of the area of geographic atrophy had increases in ETDRS scores well above the mean increases seen in the total cohort. While Dr. Ho admits this is anecdotal, it is very intriguing. We all look forward to seeing what’s on the horizon for this incredible clinical trial.
Dr. Penelope Allen from Australia then presented her work on suprachoroidal retinal prostheses. Electronic retinal prostheses, which replace or bypass degenerated photoreceptors with artificial electrodes, can be implanted in different locations: on the retinal surface (epiretinal), under the retina but above the RPE (subretinal), or between the choroid and sclera (suprachoroidal). Each of these methods carry with them potential pros and cons. In this excellent talk, Dr. Allen presented impressive long term data on two generations of suprachoroidal retinal prostheses. In 7 total implantation surgeries, no intraoperative complications were encountered. Instances of post-operative subretinal and suprachoroidal hemorrhage were mild and resolved. Impressively, Dr. Allen shared that after some initial post operative movement, devices overall remained stable over 8+ years of follow up and that the amount of retinal thinning seen over time was consistent with that seen in the natural history of the disease and unlikely to be from device toxicity. As such, the suprachoroidal approach clearly remains a viable option to implant artificial electrodes in retinal degeneration patients in a safe and biocompatible way.
Dr. Binig-Yi Wang from Stanford rounded off the session with a brilliant presentation on a 3D implantable electronic device aimed to restore vision in patients with central geographic atrophy. Flat electrode arrays have limited ability to maintain adequate retinal stimulation within safe power limits when the small pixels needed for high resolution vision are used. Increasing tissue integration of the device using a 3D honeycomb implant structure could theoretically overcome this challenge. Dr. Wang and the Stanford group implanted just such an array into rat models of outer-retinal degeneration. Using special stains, they found that cone bipolar cells, a critical part of the inner retinal circuitry, were well-preserved and integrated well into the honeycomb wells of the device. VEP testing, which shows electrical activity in the brain with visual stimulus, confirmed that the devices successfully stimulated retinal tissue and at excellent resolution. Dr. Wang’s 3D design is remarkable and shows definite promise in helping to achieve high-resolution vision with implanted electrode arrays.
