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An Elusive Parasite
Plowe, who became an HHMI investigator in June 2008, likes to show off his lab’s new DNA extraction robot that “has vastly increased our throughput,” supporting the shift in his lab’s focus from drug resistance to gene resistance.
That transition is crucial to developing next-generation tools to fight malaria. Ever since British researcher Ronald Ross identified the malaria parasite in mosquitoes in 1897, thousands of scientists around the world have searched for effective treatments and vaccines. But that resilient organism—Plasmodium falciparum causes the deadliest form of human malaria—keeps developing resistance to drugs and presenting hurdles to vaccine makers.
At the heart of the problem is the parasite’s complexity, with its life-cycle stages and uncanny ability to fool the body’s immune system. Delivered by a mosquito’s bite, the parasite’s thread-like sporozoite stage travels through the human bloodstream and settles in the liver. There it morphs and multiplies into tear-shaped merozoites that reinvade the circulatory system and burst red blood cells, releasing toxins that sicken or kill the human host. Also in the bloodstream are the parasite’s reproductive gametocytes, which biting mosquitoes suck from the human host into their salivary glands to begin a new cycle of infection.
“It has a very plastic genome,” says Plowe. “Every 48 hours, the parasite multiplies roughly 10-fold,” speeding the genetic recombination that strengthens resistance.
“I don’t think we have the tools to [eradicate malaria] yet. This will require sustained funding and decades of research.”
Vaccine developers target the parasite at one stage or the other, using specific parasite proteins to incite a human immune response. The challenge, Plowe explains, is to target the right combination of proteins, which can vary based on genetic code. “Part of the difficulty is the speed of mutations of the parasite and the other part is not knowing the basis of immunity in the human populations,” he says.
With support from the NIH, Plowe developed a vaccine test site in the same village where Djimdé did his groundbreaking study of chloroquine resistance. Plowe and his colleagues in Mali are testing vaccines developed by scientists at the Walter Reed Army Institute of Research, combined with an adjuvant (immune system booster) from vaccine manufacturer GlaxoSmithKline Biologicals.
Among the most promising of those candidates is the AMA-1 blood-stage vaccine. Plowe’s group reported in the February 2010 issue of PLoS One that the vaccine candidate produced a 100-fold increase in antibodies against the AMA-1 malaria protein in 75 children in a Phase I study at the Mali test site.
Following 100 kids over time in Mali, his group sequenced the gene for AMA-1 in more than 1,300 infections. “By looking at which changes in the AMA-1 sequence in consecutive infections were more likely to make a child sick,” Plowe says, “we could pinpoint the genetic differences that seem to be important for clinical immunity.”
The new approach is aimed at a persistent problem with malaria vaccines, he says—the fact that “all the vaccines in the clinical pipeline were designed with no knowledge of whether the variant that was picked to put in the vaccine was the most common or the least common in nature.” Plowe suspects that genetic differences that matter most in natural immunity will turn out to be the same ones that drive vaccine-induced immunity. (See sidebar, “Potent Buzz.”)
In one previous trial, a malaria vaccine candidate was found ineffective, most likely because it used a protein variant that was rare in the Kenyan community where the vaccine was tested. In Mali, Plowe’s team has identified more than 200 variants of the AMA-1 protein in a single village, but the group is using genomic analysis to narrow down the number of variations the immune system must recognize to prevent illness.
The studies have indicated that “we might be able to reduce the number of variants we would need to address [in a vaccine] from more than 200 to just 10 or so—maybe even fewer,” Plowe says. A vaccine based on just one of those variants was tested first, with the Phase I trial among children reported in the recent PLoS One paper and the first efficacy (Phase II) trial results expected to be reported in a paper later this year. While the results of the Phase II trial aren’t yet public, Plowe offers a hint, saying “We haven’t hit a home run, but I think we’ve finally made it to first base.
“The big question is: Can you get overall efficacy based on a single variant when there are over 200 variants in a village?” he adds. “Or, if you don’t get broad overall protection, do you at least get the information that you need to go back and develop a more broadly protective vaccine?”