Researchers used stem cells from patients and 3D bioprinting to produce eye tissue that will advance understanding of the mechanisms behind blinding diseases. The research team from the National Eye Institute (NEI), part of the National Institutes of Health, printed a combination of cells that form the outer blood-retina barrier – eye tissue that supports the retina’s light-sensing photoreceptors. The technique provides a theoretically unlimited supply of patient-derived tissue to study degenerative retinal diseases such as age-related macular degeneration (AMD).
“We know that AMD starts in the outer blood-retina barrier,” said Kapil Bharti, Ph.D., who directs the NEI Section of Ocular and Stem Cell Translational Research. “However, the 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 outer blood-retina barrier consists of the retinal pigment epithelium (RPE), separated by Bruch’s membrane from the blood-rich choriocapillaris. Bruch’s membrane regulates the exchange of nutrients and waste between the choriocapillaris and the RPE. In AMD, lipoprotein deposits called drusen form outside Bruch’s membrane, preventing its function. Over time, the RPE breaks down, leading to photoreceptor degeneration and vision loss.
Bharti and colleagues combined three immature choroidal cell types in a hydrogel: pericytes and endothelial cells, which are key components of capillaries; and fibroblasts, which give tissue structure. The researchers then printed the gel onto a biodegradable scaffold. Within days, the cells began to mature into a dense capillary network.
On day nine, the researchers saw retinal pigment epithelial cells on the back of the scaffold. The printed tissue reached full maturity by day 42. Tissue analysis and genetic and functional tests showed that the printed tissue looked and behaved similarly to the natural outer blood-retina barrier. Under induced stress, printed tissues showed patterns of early AMD such as drusen deposits under the RPE and progression to late dry stage AMD, where tissue degradation was observed. Low oxygen-induced wet AMD-like appearance, with hyperproliferation of choroidal vessels that migrated into the sub-RPE zone. Anti-VEGF drugs, used to treat AMD, suppressed this vessel overgrowth and migration and restored tissue morphology.
“By printing cells, we facilitate the exchange of cellular signals necessary for normal outer blood-retinal barrier 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 proposed many years ago but was not proven until our model.”
Among the technical challenges 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 was heated. Good row consistency enabled a more precise system for quantifying tissue structures. They also optimized the cell mixing ratio between pericytes, endothelial cells and fibroblasts.
Co-author Marc Ferrer, Ph.D., director of the 3D Tissue Bioprinting Laboratory at NIH’s National Center for Advancing Translational Sciences, and his team provided expertise for the biofabrication of the outer blood-retina barrier tissue “in a well,” along with analytical measurements for to enable drug screening.
“Our collaborative efforts have resulted in highly relevant retinal tissue models of degenerative eye diseases,” said Ferrer. “Such tissue models have many potential uses in translational applications, including therapeutic development.”
Bharti and collaborators are using printed blood-retina barrier models to study AMD, and they are experimenting with adding more cell types to the printing process, such as immune cells, to better recapitulate native tissue.
Materials provided by NIH/National Eye Institute. Note: Content may be edited for style and length.