The protein, called alpha-1 antitrypsin (AAT), seems to prevent the AIDS virus from infecting cells, the team at the University of Colorado Health Sciences Center reports.

"In the laboratory, we confirmed the surprising observation that the AIDS virus did not grow in blood," Dr. Leland Shapiro, an assistant professor of medicine who led the study, said in a statement.

"This suggested that there was at least one substance in the blood that blocked the virus. We believe we have identified one of the substances as AAT."

In a study to be published in the Federation of the American Societies of Experimental Biology (FASEB) Journal, Shapiro and colleagues said AAT works in several ways against the virus.

Looking at blood donated to the American Red Cross, Shapiro and colleagues saw the HIV virus did not proliferate when added to blood from healthy volunteers, but it did when added to blood from patients who had genetic defects that caused them to have low levels of AAT.

They also found, working in the laboratory, that AAT blocked the ability of the virus to infect previously healthy, uninfected cells.

And in cells infected with HIV, adding AAT to the lab dish virtually stopped the virus's ability to spread out of the cell.

Shapiro said it may be that the virus reproduces in areas naturally low in AAT. They think it might be possible to give extra AAT to HIV-infected patients.

"These findings are preliminary but promising," Shapiro said. "If clinical results are as promising as what we have seen in the laboratory, methods of increasing AAT or use of an AAT 'mimic' might be an effective form of AIDS therapy."

He said he would propose running tests in humans.

AAT is used to treat patients with cystic fibrosis and other genetic defects. It is being grown in the milk of genetically engineered animals by PPL Therapeutics, the Scotland-based company involved in cloning the first mammal, Dolly the sheep, in 1997.

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