Immune Boost Wards Off Malaria

Scientists have found that a jolt from a versatile immune-system chemical, interleukin-12, protects monkeys from malaria. The findings, reported in the January issue of Nature Medicine, suggest that the chemical could be the basis for a prophylactic shot for travelers to malaria-ridden countries.

A team led by Stephen L. Hoffman, director of the malaria program at the Naval Medical Research Institute Annex in Rockville, Maryland, injected one to two doses of interleukin-12--a chemical produced by macrophages that helps regulate the immune system--into 14 rhesus monkeys, then exposed the animals to the malaria parasite. All monkeys receiving the double dose remained uninfected up to 100 days later. Interleukin-12 "produces the most substantial protection we have ever seen in a primate model," Hoffman says. Hoffman's team believes that interleukin-12 exerts its protective effects during the liver stage of the malaria parasite's life cycle. After the parasite enters the bloodstream via a mosquito bite, it heads to the liver, where it replicates. Hoffman's team had found in earlier experiments that interleukin-12, when injected into a mouse, causes two types of immune cells--T-cells and natural killer cells--to produce higher than normal levels of another chemical, interferon-gamma. That boost in interferon-gamma triggers malaria-infected liver cells--but not adjacent liver cells--to produce nitric oxide, which kills the parasite. Says Hoffman, "It's quite an interesting cascade of events."

Hoffman argues that travelers to malarial regions, who now take other prophylactic drugs that fail to provide 100% protection, might get better short-term protection from an interleukin-12 shot. "This research tells us that we can prevent malaria by a totally immunological intervention," he says. But he and others are waiting for safety results from a trial of interleukin-12 as an anti-cancer drug before they test its effectiveness as a malaria prophylactic in people.