ASHINGTON, April 7 (Reuters) — A single dose of an antibody that cleans up brain-clogging proteins improves memory in mice and could be a step toward an Alzheimer's disease vaccine for people, researchers reported today.

The experiment, reported in the journal Nature Neuroscience, also sheds new light on the causes of Alzheimer's, which affects four million Americans.

A team at Lilly Research Laboratories in Indianapolis, owned by Eli Lilly; Washington University in St. Louis; and Université Louis Pasteur in Strasbourg, France, has been reporting steady progress in mice with an antibody called m266. In the mice, which are genetically engineered to develop a syndrome similar to Alzheimer's, the antibody homes in on the beta amyloid peptide, which forms the brain-clogging "plaques" seen in the disease.

The antibody attaches to the peptide and causes it to be flushed from the brain into the blood. Other studies have shown that mice do better on a memory test after an injection of the antibody.

Steven Paul of Lilly, who helped lead the research, said he and his colleagues had assumed that the antibodies were clearing plaque from the mice's brains. It appears instead that the antibodies were affecting the peptide before it could form plaque. This suggests that beta amyloid causes problems even before clogs form.

Dr. Paul said the information was a long way from being useful to human patients.