Glaucoma is one of the leading causes of irreversible blindness worldwide, and elevated intraocular pressure remains the most important modifiable risk factor. Pressure depends on the resistance to aqueous humor outflow through the trabecular meshwork and Schlemm's canal. Aging raises glaucoma risk more than anything else, yet most older people do not develop higher pressure despite clear structural changes in the drainage system. This contradiction points to compensatory mechanisms that have so far been poorly understood.
The researchers performed single-cell RNA sequencing of anterior chamber angle tissues from young (6-week-old) and old (15-month-old) mice to see changes in gene expression. Then, using immunostaining of mouse whole-eye flat mounts and sections of human donor tissues, they compared the size of Schlemm's canal and the number of macrophages around it. They tested the functional role of macrophages by deleting the Vegfa gene specifically in these cells using a genetic system, then measured intraocular pressure and outflow facility. Additionally, they used mice with Tie2 haploinsufficiency as a model of genetic predisposition to glaucoma and administered gene therapy based on an adeno-associated virus carrying a TIE2 activator.
With age, in both mice and humans, macrophages expressing CX3CR1 and VEGFA accumulate around Schlemm's canal, while the canal itself narrows. In mice, canal area decreased from 1.299 to 1.118 ×10⁵ µm², and macrophage density rose from 8.914 to 13.07 cells per 10⁴ µm². In people over 50, canal length was shorter (526 versus 857 µm), and there were more macrophages (3.129 versus 1.606 per 100 µm). Deleting Vegfa from CX3CR1-positive macrophages led to increased intraocular pressure in 6- and 9-month-old mice and reduced outflow facility. Mice with Tie2 haploinsufficiency showed canal narrowing, macrophage accumulation, and elevated pressure already at a young age, which worsened with age. Gene therapy with a TIE2 activator in old mice improved outflow, lowered pressure, and reduced macrophage numbers, while in Tie2-deficient mice it preserved more retinal ganglion cells.
The work shows that immune cells do not merely react to damage but actively help maintain normal eye pressure during aging. This changes the view of glaucoma as a purely mechanical problem and brings an immune component into the picture. The discovery of the TIE2 signaling pathway as a regulator of drainage system resilience provides a concrete molecular target for therapy.
Further studies could clarify exactly how aging reduces the TIE2 signal and triggers macrophage accumulation. It may be possible to find ways to boost this signal without gene therapy, for example using small molecules. It is also important to test whether this approach works in other glaucoma models and in human tissues.
The results could influence the development of glaucoma drugs, especially for older patients. Gene therapy targeting Schlemm's canal could become a long-term alternative to daily eye drops.
Conduct experiments on other animal models of glaucoma and on human cell cultures. Determine the optimal dose and timing of gene therapy administration, and study long-term safety.
It remains unclear why compensatory mechanisms are sufficient in some people but glaucoma develops in others. The exact molecular cascade linking reduced TIE2 to macrophage accumulation is unknown. Also unresolved is how long these macrophages live and how they are renewed.
The area of Schlemm's canal in mice decreases with age by about 14%, comparable to the loss of aortic cross-sectional area in aging humans.
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