Block That Plasmodium!

A DNA vaccine stops malaria from spreading in mice. Instead of conferring immunity, the new vaccine attacks the Plasmodium parasite inside its mosquito host. Experts say the findings, described in the April issue of Infection and Immunity, could lead to a new approach for curbing the spread of the disease in people.

A human vaccine against malaria has faltered in the face of the sophisticated life cycle of Plasmodium falciparum, the one-celled parasite responsible for the most severe form of the disease. The parasite shuttles between two hosts--humans and female Anopheles mosquitoes--all the while proceeding from one developmental stage to the next and constantly changing its appearance. Antibodies in human blood against certain parasite proteins can mess up Plasmodium development in the mosquito gut, but manufacturing these delicate antigens for use in vaccines is tricky.

Nirbhay Kumar, a molecular biologist at the Johns Hopkins School of Public Health in Baltimore, wondered whether injecting the genes for the Plasmodium antigens would work as a vaccine. His team injected mice with the Plasmodium Pfs25 gene, which is active only when the parasite resides inside a mosquito. When they mixed serum from immunized mice with cultures of the parasite and fed the concoction to mosquitoes, they found 75% fewer infected insects compared to controls. What's more, the remaining 25% carried about 97% fewer Plasmodium oocysts in their midguts. A vaccine such as this in the real world would work like a time bomb--a mosquito would pick up the antibodies only after infecting an individual with malaria. Although this type of vaccination "does not offer immediate protection on an individual level," Kumar says, "if done on a population level, the transmission of malaria might be curbed in the long run."

Malaria DNA vaccine pioneer Stephen Hoffman of the Naval Medical Research Institute in Bethesda, Maryland, is excited about the study. "It's the first report of a DNA vaccine that can elicit a transmission blocking response," he says, adding that it would be a valuable component of a vaccine cocktail designed to confer immunity and block transmission.

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