Scientists Develop Novel Therapeutic Model to Treat Incurable Eye Diseases

In a study published as a Reviewed Preprint in eLife, researchers have effectively transplanted human microglia cells into the mouse retina. This innovative model holds promise for testing novel treatments targeting incurable eye diseases.

The editors have deemed this research significant, showcasing robust data that underscores the potential of microglial replacement therapy in addressing retinal and central nervous system ailments.

Microglia, the intrinsic immune cells of the central nervous system, including the retina, serve crucial functions in normal nerve and synapse development. However, their role can take a detrimental turn, contributing to the onset of brain and ocular conditions such as age-related macular degeneration (AMD), glaucoma, diabetic retinopathy, and uveitis.

While microglia maintain retinal function by attentively monitoring environmental changes under normal circumstances, they can swiftly respond to injuries under pathological conditions. Unfortunately, they can also undergo inappropriate activation, proliferation, and migration into nearby tissues.

"Our understanding of microglia function comes predominantly from rodent studies due to the difficulty of sourcing human tissue and isolating the microglia from these tissues. But there are genetic and functional differences between microglia in mice and humans, so these studies may not accurately represent many human conditions," says lead author Wenxin Ma, a Ph.D. Biologist at the Retinal Neurophysiology Section, National Eye Institute, National Institutes of Health, US.

"To address this concern, researchers have been growing human microglia from human stem cells. We wanted to take this a step further and see if we could transplant human microglia into the mouse retina, to serve as a platform for screening therapeutic drugs as well as explore the potential of microglia transplantation as a therapy itself," adds senior author Wai Wong, Vice President of Retinal Disease, Janssen Research and Development LLC, California, U.S.

To validate their method, researchers cultivated microglia using human induced pluripotent stem cells (hiPSCs). They underwent a series of assessments to ensure that the generated microglia functioned as typical immune cells before introducing them into the retinas of mice.

Differentiation and characterization of human iPSC-derived microglia cell. (A) Human iPSC were cultured in a 6-well plate. Scale bar = 200µm. (B) Embryoid bodies formation in AggreWell™800 plate at day eight in culture medium mTeSR1 plus BMP4, VEGF, and SCF. Scale bar = 200µm. (C) Myeloid precursor cluster at the 1-month culture of embryoid bodies under TheraPEAK™ X-vivo™-15 Serum-free Hematopoietic Cell Medium with M-CSF and IL3. Scale bar = 50µm. (D) Microglia cells maturation culture for two weeks with DMEM/F12 plus nonessential amino acids, glutamine, IL34, CSF1, TGFb2, and CX3CL1. Scale bar = 50µm. (E) Immunocytochemistry staining with Iba1 and human CD34, CX3CR1, P2ry12, CD11b, CD68. Scale bar = 100µm. (F) The cell counts and collaboration analysis of CD34 and Iba1 positive cells. (G) Percentage analysis of myeloid cell marker CX3CR1, CD11b, activation marker CD68, and microglia cell signature marker P2ry12. Credit: (2023). DOI: 10.7554/eLife.90695.1

Initially, they used a drug to eliminate the existing microglial cell population and subsequently transplanted the newly labeled cells to take their place. At the four and eight-month marks post-transplantation, the human microglial cells had effectively migrated into the retina, exhibiting proper distribution and responding appropriately to chemical signals within the eye. Remarkably, the introduction of hiPSC-microglia had no adverse impact on the neighboring retinal cells.

Having established the seamless integration of transplanted hiPSC-microglia with the existing mouse microglia population, the researchers tested whether these transplanted cells displayed a standard immune response to retinal cell injury. They observed that the behavior of the transplanted cells mirrored that of resident mouse microglia—migrating to the subretinal space near damaged cells. Although this led to a reduced number of human microglia in their original location, the transplanted cells rapidly replenished themselves and ceased multiplying upon reaching their initial count.

An essential function of microglia is phagocytosis, where they eliminate unhealthy cells. The transplanted microglia not only moved into damaged areas but also effectively removed impaired light-receiving cells. This implies that these transplanted cells replicated the normal functions of native microglia within the eye.

However, a limitation of the study pertains to the uncertainty surrounding the maturity of microglia generated from hiPSCs and their comparability to fully mature human-derived microglia cells. According to feedback from the eLife public review, further investigation is required to confirm their maturity, potentially involving a comparative analysis with publicly available gene expression data for primary human microglia.

"Understanding microglial cell function is essential for investigating disease mechanisms and identifying accurate targets for treating degenerative retinal diseases," explains senior author Wei Li, a Senior Investigator in the Retinal Neurophysiology Section, National Eye Institute, National Institutes of Health. "Our model provides a new way of studying the functions of human microglia within disease mechanisms and a platform for evaluating the potential therapeutic effects of microglia cell transplants from diverse patient backgrounds."

Reference

Wenxin Ma et al, Human iPSC-derived Microglia Cells Integrated into Mouse Retina and Recapitulated Features of Endogenous Microglia Cells, eLife (2023). DOI: 10.7554/eLife.90695.1