Rapid generation of purified human RPE from pluripotent stem cells using 2D cultures and lipoprotein uptake-based sorting

Abstract

Background: Despite increasing demand, current protocols for human pluripotent stem cell (hPSC)-derived retinal pigment epithelium (RPE) remain time, labor, and cost intensive. Additionally, absence of robust methods for selective RPE purification and removal of non-RPE cell impurities prevents upscaling of clinical quality RPE production. We aimed to address these challenges by developing a simplified hPSC-derived RPE production and purification system that yields high-quality RPE monolayers within 90 days. Methods: Human pluripotent stem cells were differentiated into RPE using an innovative time and cost-effective protocol relying entirely on 2D cultures and minimal use of cytokines. Once RPE identity was obtained, cells were transferred onto permeable membranes to acquire mature RPE morphology. RPE differentiation was verified by electron microscopy, polarized VEGF expression, establishment of high transepithelial electrical resistance and photoreceptor phagocytosis assay. After 4 weeks on permeable membranes, RPE cell cultures were incubated with Dil-AcLDL (DiI-conjugated acetylated low-density lipoproteins) and subjected to fluorescence-activated cell sorting (FACS) for purification and subculture. Results: Using our 2D cytokine scarce protocol, hPSC-derived functional RPE cells can be obtained within 2 months. Nevertheless, at this stage, most samples contain a percentage of non-RPE/early RPE progenitor cells that make them unsuitable for clinical application. We demonstrate that functional RPE cells express high levels of lipoprotein receptors and that this correlates with their ability to uptake lipoproteins. Combining photoreceptor uptake assay with lipoprotein uptake assay further confirms that only functional RPE cells uptake AcLDL. Incubation of mixed RPE/non-RPE cell cultures with fluorophore conjugated AcLDL and subsequent FACS-based isolation of labeled cells allows selective purification of mature functional RPE. When subcultured, DiI-AcLDL-labeled cells rapidly form pure homogenous high-quality RPE monolayers. Conclusions: Pure functional RPE monolayers can be derived from hPSC within 90 days using simplified 2D cultures in conjunction with our RPE PLUS protocol (RPE Purification by Lipoprotein Uptake-based Sorting). The simplicity of this protocol makes it scalable, and the rapidity of production and purification allows for high-quality RPE to be produced in a short span of time making them ideally suited for downstream clinical and in vitro applications. © 2020 The Author(s).