New front in fight against malaria

AN IMPORTANT step in the life cycle of the Plasmodium parasite - the bug that causes malaria - is its invasion of the salivary glands of the female Anopheles mosquito. Once inside the salivary gland, the parasite can then be launched into a human host when the insect takes a blood meal.

Now, researchers at the Johns Hopkins School of Public Health have identified a protein in the salivary glands of the female Anopheles gambiae mosquito - the primary malaria carrier in Africa - that appears to help Plasmodium recognize and gain entrance to that mosquito's salivary gland. What is more, the researchers have discovered they can greatly reduce the numbers of parasites in mosquito salivary glands if they neutralize this protein with an antibody they have raised. The study appears in the December 2000 issue of the Proceedings of the National Academy of Science, USA.

Senior author Nirbhay Kumar, PhD, professor, Molecular Microbiology and Immunology, the Johns Hopkins School of Public Health, said, "Although the results of this study are preliminary, they do show that salivary gland proteins from A. gambiae can be blocked by monoclonal antibodies, thus demonstrating that these proteins offer alternate targets for blocking A. gambiae's ability to transmit malaria."

Despite a longstanding struggle to control mosquito populations, malaria exacts a heavy burden on human health, causing 300-500 million infections and 1.5-2.7 million deaths each year. As mosquitos become more and more resistant to insecticides each year, and as health care funds remain inadequate in the developing world, new control strategies are sorely needed.

The scientists first raised a number of antibodies to particular salivary gland proteins in female A. gambiae mosquitos. They found two antibodies in particular, 2A3 and C26, that reacted strongly with certain proteins making up the insect's salivary glands. 2A3 was found to link up with a 100-kiloDalton (kDa) protein, whereas C26 pounced on a 29- kDa protein. (A Dalton is a unit of molecular mass, equivalent to one- twelfth the mass of the most abundant isotope of carbon, carbon 12.) Both of these proteins seemed promising targets because they are expressed only in the female mosquito and are both situated in the very parts of the female salivary gland to which Plasmodium gravitates. And one of the two, the 100-kDa protein, is only expressed after the insect takes a blood meal.

Next, the researchers infected A. gambiae females with Plasmodium and, nine days later, fed each of these mosquitos either one of the two antibodies created by the researchers or a third control antibody. Four or five days later, the salivary glands from each insect were removed and the parasites inside counted.

The salivary glands from mosquitos fed the antibody blocking the 100-kDa protein had 73 percent fewer Plasmodium parasites compared to the glands of those insects that received either the antibody against the 29- kDa protein or a control antibody.The authors emphasize that the antibodies used in the present study were "monoclonal" - that is, they were synthesized in test tubes to be exact replicas of a single epitope-specific B lymphocyte immune cell. But the researchers suspect that the 100-kDa protein is not the only protein that lets Plasmodium enter the salivary gland, and so they are planning further studies using "polyclonal" antibodies, those raised in the bloodstream of actual animals.

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