Age-related macular degeneration (AMD) is the leading cause of blindness in people over 50 y of age in at least three continents. Choroidal neovascularization (CNV) is the process by which abnormal blood vessels develop underneath the retina. CNV develops in 10% of patients with AMD but accounts for up to 90% of the blindness from AMD.
Although the precise etiology of CNV in AMD remains unknown, the macrophage component of the inflammatory response, which has been shown to promote tumor growth and support atherosclerotic plaque formation, is thought to stimulate aberrant angiogenesis in blinding eye diseases. The current theory is that macrophage infiltration promotes the development of neovascularization in CNV.
Methods and Findings
We examined the role of macrophages in a mouse model of CNV. IL-10−/− mice, which have increased inflammation in response to diverse stimuli, have significantly reduced CNV with increased macrophage infiltrates compared to wild type. Prevention of macrophage entry into the eye promoted neovascularization while direct injection of macrophages significantly inhibited CNV. Inhibition by macrophages was mediated by the TNF family death molecule Fas ligand (CD95-ligand).
Immune vascular interactions can be highly complex. Normal macrophage function is critical in controlling pathologic neovascularization in the eye. IL-10 regulates macrophage activity in the eye and is an attractive therapeutic target in order to suppress or inhibit CNV in AMD that can otherwise lead to blindness.
Apte and colleagues examined the role of macrophages in a mouse model of choroidal neovascularization, and showed that normal macrophage function is critical in controlling pathologic neovascularization in the eye.
The most common cause of poor eyesight in later life in the developed world is known as age-related macular degeneration (AMD). The macula is the central part of the retina (the film-like membrane at the back of the eye), which is the most sensitive and important for sharp central vision. There are two types of advanced AMD: so-called wet, or neovascular, AMD (neovascular means “new vessel”) and dry, or geographic atrophy, AMD (atrophy means “to waste away”). Wet AMD occurs when abnormal, fragile new blood vessels grow under the macula behind the retina. These blood vessels often leak blood and fluid, which lift the macula. Dry AMD occurs as the light-sensitive cells in the macula (the rods and cones) break down. To study this disease further, researchers use animal models. One such animal model is made by using a laser to damage the back of the eye in a mouse, which causes the formation of new vessels. Various treatments can then be tested to see if they have any effect on the damage.
Why Was This Study Done?
One theory about AMD is that the immune system may be involved in determining how severe the damage at the back of the eye is, and how much new vessel formation occurs. The researchers wanted to look at the effect of the immune system on AMD, in particular, the effect of one type of cell called a macrophage, and a substance, IL-10, that is secreted from bone marrow cells and that affects how these macrophages work.
What Did the Researchers Do and Find?
The researchers used a mouse strain in which IL-10 was absent, induced damage in the eyes that mimicked AMD, and then looked at what role macrophages had in the eye abnormalities. They found that in the eyes of mice that lacked IL-10, there was reduced new vessel formation and increased numbers of macrophages compared to mice that had normal amounts of IL-10. Also, preventing macrophages from getting into the eyes of such mice by injecting IL-10 into the eyes made the new vessel formation worse, while direct injection of macrophages made it better.
What Do These Findings Mean?
Although animal models cannot completely replicate disease in humans, they can give us an idea of how diseases might come about and suggest possible treatment strategies. It is possible that inhibiting the effect of IL-10, or other strategies that make macrophages more efficient in the eye, may be a useful treatment for AMD. In a related Perspective (DOI: 10.1371/journal.pmed.0030364), Susan Lightman and Virginia Calder discuss the findings further, including suggesting new experiments that will need to be done as a next step.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0030310.
MedlinePlus encyclopedia entry on macular degeneration
National Institutes of Health Senior Health page of information on AMD
National Eye Institute AMD fact page