Stem Cell Reports Article Surgical Transplantation of Human RPE Stem Cell-Derived RPE Monolayers into Non-Human Primates with Immunosuppression Zengping Liu, 1,2,3,10 Bhav Harshad Parikh, 1,2,10 Queenie Shu Woon Tan, 1 Daniel Soo Lin Wong, 2 Kok Haur Ong, 1 Weimiao Yu, 1 Ivan Seah, 4 Graham E. Holder, 2,4,7 Walter Hunziker, 1,8 Gavin S.W. Tan, 3,9 Veluchamy Amutha Barathi, 2,3,9 Gopal Lingam, 2,3,4 Boris V. Stanzel, 2,5, * Timothy A. Blenkinsop, 6, * and Xinyi Su 1,2,3,4, * 1 Institute of Molecular and Cell Biology (IMCB), Agency for Science, Technology and Research (A*STAR), Singapore, Singapore 2 Department of Ophthalmology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore 3 Singapore Eye Research Institute (SERI), Singapore, Singapore 4 Department of Ophthalmology, National University Hospital, Singapore, Singapore 5 Macula Center Saar, Eye Clinic Sulzbach, Knappschaft Hospital Saar, Sulzbach, Saar, Germany 6 Department of Cellular, Developmental and Regenerative Biology, Icahn School of Medicine at Mount Sinai, New York, NY, USA 7 UCL Institute of Ophthalmology, London, UK 8 Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore 9 Academic Clinical Program in Ophthalmology, Duke-NUS Medical School, Singapore, Singapore 10 These authors contributed equally *Correspondence: boris.stanzel@kksaar.de (B.V.S.), timothy.blenkinsop@mssm.edu (T.A.B.), xysu@imcb.a-star.edu.sg (X.S.) https://doi.org/10.1016/j.stemcr.2020.12.007 SUMMARY Recent trials of retinal pigment epithelium (RPE) transplantation for the treatment of disorders such as age-related macular degeneration have been promising. However, limitations of existing strategies include the uncertain survival of RPE cells delivered by cell suspension and the inherent risk of uncontrolled cell proliferation in the vitreous cavity. Human RPE stem cell-derived RPE (hRPESC-RPE) transplan- tation can rescue vision in a rat model of retinal dystrophy and survive in the rabbit retina for at least 1 month. The present study placed hRPESC-RPE monolayers under the macula of a non-human primate model for 3 months. The transplant was able to recover in vivo and maintained healthy photoreceptors. Importantly, there was no evidence that subretinally transplanted monolayers underwent an epithelial-mesenchymal transition. Neither gliosis in adjacent retina nor epiretinal membranes were observed. These findings suggest that hRPESC-RPE monolayers are safe and may be a useful source for RPE cell replacement therapy. INTRODUCTION The retinal pigment epithelium (RPE) is a single-layer epithelium present under the neurosensory retina and is essential for vision. Estimates suggest there are more than 200 million individuals with vision loss due to RPE-related diseases (Wong et al., 2014), reflecting the multiple roles that the RPE has in the maintenance of normal visual trans- duction. Those roles include provision of the outer blood- retina barrier, management of fluid transport, regulation of cytokine release, processing of reactive oxygen species, recy- cling of phototransduction components, and regulation of the subretinal space ionic balance (Bharti et al., 2011; Bo- nilha et al., 2006; Sparrow et al., 2010; Strauss, 2005). The clinical trials of RPE transplantation as a ‘‘cell replace- ment therapy’’ for vision-threatening complications of age- related macular degeneration (AMD) showed some prelimi- nary signs of success (da Cruz et al., 2018; Kashani et al., 2018; Mandai et al., 2017; Singh et al., 2020). Cell replace- ment involves the provision of healthy RPE cells to replace those that are dysfunctional, with the goal of restoring phys- iological function to the retina. In the case of advanced geographic atrophy (GA) in AMD, wherein both photorecep- tors and RPE are lost, RPE replacement aims to prevent further atrophy and, thereby, additional visual loss. More- over, if dormant photoreceptor nuclei are present within the atrophic area (Bird et al., 2014; Kashani et al., 2018), limited recovery of visual function might also be possible, dependent upon on the extent of damage prior to transplant. Previous successes with autologous RPE replacements, such as peripheral RPE/choroid patch and macular translo- cation, laid the foundation for stem cell-based RPE therapy (Stanga et al., 2002; van Zeeburg et al., 2012). Notwith- standing guarded visual outcomes, those early approaches had limited uptake due to the complexity of the surgical procedures, resulting in the search for alternative stem cell-based RPE sources for clinical applications. Recent clin- ical trials on eye disease have demonstrated promising re- sults using human embryonic stem cell (hESC)-RPE and human induced pluripotent stem cell (iPSC)-RPE (da Cruz et al., 2018; Mandai et al., 2017; Schwartz et al., 2012; Schwartz et al., 2015). Human RPE stem cell-derived RPE (hRPESC-RPE) is an additional, potentially unlimited cell source of human leukocyte antigen (HLA) matching and an unlimited donor source with some qualities favorable to translation, including stability, ubiquity, and cost. A Stem Cell Reports j Vol. 16 j 237–251 j February 9, 2021 j ª 2021 The Authors. 237 This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).