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Scientists from the National Eye Institute (NEI), part of the National Institutes of Health, report they have used patient stem cells and 3D bioprinting to produce eye tissue that will advance understanding of the mechanisms of blinding diseases.
Their findings “Bioprinted 3D outer retina barrier uncovers RPE-dependent choroidal phenotype in advanced macular degeneration,” were published in Nature Methods.
“Age-related macular degeneration (AMD), a leading cause of blindness, initiates in the outer-blood-retina-barrier (oBRB) formed by the retinal pigment epithelium (RPE), Bruch’s membrane, and choriocapillaris,” wrote the researchers. “The mechanisms of AMD initiation and progression remain poorly understood owing to the lack of physiologically relevant human oBRB models. To this end, we engineered a native-like three-dimensional (3D) oBRB tissue (3D-oBRB) by bioprinting endothelial cells, pericytes, and fibroblasts on the basal side of a biodegradable scaffold and establishing an RPE monolayer on top.”
“We know that AMD starts in the outer blood-retina barrier,” explained Kapil Bharti, PhD, who heads the NEI Section on Ocular and Stem Cell Translational Research. “However, mechanisms of AMD initiation and progression to advanced dry and wet stages remain poorly understood due to the lack of physiologically relevant human models.”
The scientists combined three immature choroidal cell types in a hydrogel: pericytes and endothelial cells, and fibroblasts. The scientists then printed the gel on a biodegradable scaffold. Within days, the cells began to mature into a dense capillary network.
On day nine, the scientists seeded retinal pigment epithelial cells on the flip side of the scaffold. The printed tissue reached full maturity on day 42. Tissue analyses and genetic and functional testing showed that the printed tissue looked and behaved similarly to the native oBRB.
“By printing cells, we’re facilitating the exchange of cellular cues that are necessary for normal oBRB anatomy,” said Bharti. “For example, the presence of RPE cells induces gene expression changes in fibroblasts that contribute to the formation of Bruch’s membrane—something that was suggested many years ago but wasn’t proven until our model.”
Among the technical challenges that Bharti’s team addressed were generating a suitable biodegradable scaffold and achieving a consistent printing pattern through the development of a temperature-sensitive hydrogel that achieved distinct rows when cold but dissolved when the gel warmed. Good row consistency enabled a more precise system of quantifying tissue structures. They also optimized the cell mixture ratio of pericytes, endothelial cells, and fibroblasts.
“Our collaborative efforts have resulted in very relevant retina tissue models of degenerative eye diseases,” Ferrer said. “Such tissue models have many potential uses in translational applications, including therapeutics development.”
Bharti and collaborators are using printed blood-retina barrier models to study AMD, and they are experimenting with adding additional cell types to the printing process, such as immune cells, to better recapitulate native tissue.