( ren each year. The disease can result ...)Malaria vaccine prompts victims' immune system to eliminate parasite from mosquitoes

ren each year. The disease can result in severe headache, high fever, chills, and vomiting. Malaria is caused by a single celled parasite, Plasmodium. In all, four species of Plasmodium cause malaria in people, with Plasmodium falciparum causing the most severe form. The malarial parasite spends part of its life cycle in humans, and part in mosquitoes. The parasite is injected into an individual by the bite of an infected mosquito. Numerous experimental vaccines have been tried against the form of the parasite that resides in humans, but have been unsuccessful or produced limited immunity. The Plasmodium cells escape the human immune system by hiding in liver and blood cells, making them difficult to target with a vaccine. During the human phase of the infection, these cells, for the most part, exist in an asexual form.

Some of the Plasmodium cells, however, transform into gametocytes—the sexual forms of the parasite that are equivalent to sperm and eggs. Fertilization takes place in the mosquito gut, after which the parasite imbeds itself in the gut lining. There, it passes through discrete stages, before migrating to the insect's salivary glands, where it is passed on to the next host through a mosquito bite.

The protein Pfs25 (Plasmodium falciparum surface protein 25) is found only on the surface of the ookinette, a stage of the parasite living in the mosquito gut, and does not appear on any other stage of the parasite. When injected into human volunteers, Pfs25 fails to generate a sufficient level of antibodies to target the parasite.

In their article, the researchers described several strategies for using conjugate technology to make an effective vaccine based on Pfs25. These consisted of chemically linking numerous Pfs25 molecules to each other and to other proteins: Pseudomonas aeruginosa exotoxin A, a protein from a species of bacteria that infects people with weakened immune systems, and ovalbumin, a protein found
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Source:NIH/National Institute of Child Health and Human Development

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