ST. LOUIS - Scientists at Washington University have made a key discovery in a disease that is the leading cause of blindness in people older than age 65.

The research also might have implications for vision loss associated with diabetes and premature birth.

The new study shows that an immune-system protein controls blood-vessel growth in the eye. Abnormal blood-vessel growth is an important step in the blinding disease called age-related macular degeneration. The protein could be a target for treatments to prevent the disease.

Results of the new study, led by Rajendra Apte at Washington University, appear online in the August issue of the journal PLoS Medicine, a publication of the Public Library of Science.

Age-related macular degeneration affects the central part of the retina, called the macula. It is the part of the eye responsible for the central field of vision. The area around the macula provides peripheral vision. The disease has two stages, "dry" and "wet."

People with macular degeneration first go through the dry stage of the disease. It affects about 7 million people in the United States. In that phase, yellow deposits of pigment, called drusen, form under the macula. The clumps grow slowly and lead to gradual loss of vision.

As the disease progresses, abnormal blood vessels "sprout like little weeds" under the retina in the areas where the pigment clumps are found, Apte said. The blood vessels leak and damage the retina, leading to sudden and dramatic loss of vision. About 1 million people in the United States are blind from the disease, and 100,000 to 200,000 people each year are diagnosed with it.

Researchers have had clues that the immune system might be involved in regulating the growth of blood vessels in the eye, said Peter Gehlbach, a retina specialist at the Wilmer Ophthalmological Institute at Johns Hopkins University School of Medicine in Baltimore.

The Washington University team used a laser to create a defect in the eyes of mice. Blood vessels then could grow into the area, mimicking macular degeneration. The researchers measured blood-vessel growth in mice lacking certain immune-system proteins.

Mice lacking an anti-inflammatory protein called interleukin-10 (IL-10) had fewer blood vessels invading the retina and more immune cells called macrophages. Injecting more IL-10 into the eyes increased blood-vessel growth, but injecting macrophages reduced their formation. If researchers can learn to control IL-10, they might be able to develop drugs that could prevent dry degeneration from converting to the blinding, wet form, researchers said.