The saliva of a fly may save human lives--if researchers can transform it into a vaccine. A new study shows that sand flies, tiny insects that transmit a parasitic disease called leishmaniasis, also secrete a protein in their saliva that protects mice against the disease. The researchers believe a similar vaccine may one day protect humans.
If true, it would be one of the strangest vaccines ever. Almost all existing vaccines directly target a pathogen--whether it's a virus, a bacterium, or a parasite. Instead, this experimental vaccine goes for one of the carrier's proteins. "It's a very intriguing and promising approach," says epidemiologist Barbara Herwaldt of the Centers for Disease Control and Prevention in Atlanta. Herwaldt says the vaccine would also be a welcome new weapon in the battle against leishmaniasis, a disease that afflicts about 2 million people a year and can disfigure the face or kill.
All blood-sucking insects have small arsenals of chemicals in their saliva--including blood vessel dilators and anticlotting agents--that help them guzzle blood. In the late '80s, researchers discovered that these cocktails are vital to some parasites too: When injected without saliva, Leishmania, the parasite that causes leishmaniasis, couldn't cause an infection. What if the immune system could learn to block the action of saliva? A team led by José Ribeiro of the National Institute of Allergy and Infectious Diseases (NIAID) showed in 1998 that inoculation with sand fly saliva could protect mice from the disease.
Of course, practical and safety issues make vaccinating people with insect spit impossible. So in their new study, which appears in the 6 August issue of the Journal of Experimental Medicine, Ribeiro's team isolated the 12 major proteins in the saliva of the sand fly Phlebotomus papatasi. One protein, which they called SP-15, seemed best at protecting mice. When vaccinated with a DNA vaccine based on SP-15, all animals eliminated the parasites, while a control group remained infected and developed large skin ulcers.
Although the mechanism isn't nailed down, Ribeiro suspects that the vaccine triggers a localized immune reaction, called delayed hypersensitivity. Immune messenger molecules called cytokines and certain types of immune cells race to the bite site, making it inhospitable for the parasite.