HIV-fighting protein in human saliva.

Researchers have long known that HIV, the AIDS-causing virus, rarely gets transmitted among humans via saliva. They've also demonstrated that something in saliva prevents HIV from infecting white blood cells, its usual target. Yet this protective substance eluded them.

Now, investigators from the National Institute of Dental Research (NIDR) in Bethesda, Md., have bagged a protein they believe is responsible for saliva's anti-HIV properties. Tessie B. McNeely and Sharon M. Wahl of NIDR call it secretory leukocyte protease inhibitor, or SLPI (pronounced "slippy").

The researchers tested a series of proteins found in saliva. In naturally occurring concentrations, only SLPI seemed to protect white blood cells from HIV infection.

Next, the team showed that SLPI works by interacting with white cells, not HIV. However, this guardian protein doesn't seem to dock with the CD4 receptor, the portal through which HIV gains entry to a white cell.

"The next step is to identify the SLPI receptor and determine the role it plays in HIV entry into host cells," McNeely says. The team presented its data Jan. 30 at the Second National Conference on Human Retroviruses and Related Infections, sponsored by the American Society for Microbiology and held in Washington, D.C.

Can SLPI protect people against HIV transmitted by exposure to blood? That's one possibility the researchers plan to explore.