Jallow's two-year-old son had died of malaria the day before. Yet there he was, back at work fighting the disease, checking on some of the 372 adults who had been vaccinated in the first large-scale trial of a novel approach to vaccine design. Losing children to malaria is an everyday event in The Gambia. Parents mourn; life goes on.

A new generation of potential malaria vaccines, such as the one developed by Hill, comes too late to save Jallow's little boy. But the HHMI international research scholar believes that the "prime-boost" malaria vaccine he and colleagues at Oxford University in England developed might save many others. He is one of several HHMI international research scholars working to develop vaccines and drugs against malaria.

Caused by a parasite spread by the bite of an infected mosquito, malaria kills more than 1 million people a year, most of them children in sub-Saharan Africa. Only tuberculosis and aids take a greater toll. Half a billion people are infected by malaria annually, so most do survive, though many of them still suffer years later from the anemia and developmental disorders caused by severe malaria infection.

There are drugs that prevent malaria temporarily—long enough to be useful to travelers but not for residents of endemic regions. Such drugs include doxycycline, mefloquine and Malarone, but they are too costly for those in poor countries and can have serious side effects.

Quinine sulfate, chloroquine, Malarone and mefloquine can be used to treat some strains of malaria, such as the one caused by Plasmodium vivax, which broke out unexpectedly in northern Virginia in 2002, but Plasmodium falciparum, the most lethal malaria parasite, has developed resistance to most of these drugs. Even where antimalarial drugs are still effective, they are prohibitively expensive for malaria-prone developing nations. One treatment can cost more than $5, and children in Africa—where the per capita expenditure for all medicines averages less than $5 per year—often contract four or five malaria infections a year, says Monica Parise, a researcher with the Centers for Disease Control and Prevention's Malaria Epidemiology Branch.

So some scientists are focusing on preventive measures, including vaccines, insecticides, mosquito netting and public education, while others work to develop new treatments to replace drugs to which P. falciparum has become resistant.

"Malaria is a complex problem that must be addressed in many different ways," says Thomas E. Wellems, acting chief of the Laboratory of Malaria and Vector Research at the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. "The more approaches we use to learn the secrets of the parasites, the better it will be for the future of malaria control."

A VACCINE IN FIVE YEARS?
Hill hopes to fill a major gap: the lack of effective vaccines. "We have some drugs that work, but we don't have vaccines," he says.

Researchers around the world have been trying to create malaria vaccines for decades. They've come up with a few promising candidates, but none have proved effective enough. P. falciparum is a formidable opponent. It has a complex life cycle, changing character and virulence as it moves from an infected female Anopheles gambiae mosquito to the liver of the human host, and from there into the bloodstream and back into another mosquito. P. falciparum also is clever at mutating to evade a vaccine's antibodies. "It's almost as if the antigen taunts the immune system," Hill explains, "saying 'take a good look at me, because the next time you see me I'm going to be entirely different.'"

Until recently, malaria vaccines were all based on antibodies, proteins produced by B cells in the immune system to neutralize the antigens or proteins that cause disease. Hill's vaccine is different. It is designed to rev up the immune system's production of T cells that target and kill infected cells. It is called "prime-boost" because immunization is a two-step process: First, a fragment of P. falciparum DNA "primes" the immune system to recognize the malaria antigen; then a virus—modified so that it can't replicate and cause illness (called modified vaccinia virus Ankara, or MVA)—boosts production of T cells. These in turn attack and destroy the malaria parasite at an early stage, in the liver, before it can burst free to flood the bloodstream and produce the high fevers and other life-threatening effects of malaria.

"At first, we thought this approach wouldn't work," Hill recalls. In studies in mice, the DNA alone provided no protection, and the MVA by itself protected less than 20 percent of the time. MVA followed by DNA did not work very well either, but—"amazingly," says Hill—DNA followed by MVA produced 100 percent protection. "We all remember that day," he remarks.

Only three years after the groundbreaking mouse studies, Hill and colleagues moved into human safety studies, or phase I trials. At Oxford, the group recruited volunteers who were willing to be vaccinated and then bitten by mosquitoes infected with a treatable form of malaria. The results looked promising, so his team moved on to phase I trials in The Gambia, collaborating with the United Kingdom's Medical Research Council unit based there.

In 2002, the first large-scale trial began. Working with tribal chiefs and village leaders, Hill and co-investigators Vasee Moorthy and Kalifa Bojang found 372 adult villagers from rural parts of the country who volunteered to be vaccinated. The trial was randomized and blinded, so half of the volunteers received a placebo—in this case rabies vaccine. Rabies vaccine is much needed in The Gambia but is prohibitively expensive. Even if the placebo hadn't offered benefits, Hill thinks his group would have had no trouble getting volunteers. "Malaria is such a devastating part of their lives that both the people and the government of The Gambia are eager to assess potential new malaria vaccines," he explains. "Happily, neither the test vaccine nor the rabies placebo caused any significant adverse side effects."

If the adult trials are a success, the next step would be a phase II trial testing the vaccine's efficacy in 500 to 1,000 children. Here too, Hill doesn't anticipate any difficulty finding parents willing to have their children immunized with the experimental vaccine. "If you've got something that works in adults, you're going to get lots of volunteers," he says.

The final stage, Hill continues, would be a phase III clinical trial in more than 1,000 infants, which would take another two years. All told, he estimates the earliest any vaccine could become generally available is around 2008. But even getting there that fast would require more resources, he says.

DIFFERENT PATHS, SAME GOAL
Regina Rabinovich, a pediatrician and epidemiologist who headed the $50 million Malaria Vaccine Initiative at the Program for Appropriate Technology in Health in Rockville, Maryland, funded by the Bill and Melinda Gates Foundation, thinks Hill's work is important. "He has been uniquely able to move his concepts into successful clinical trials," says Rabinovich, who recently moved to the Gates Foundation as director of its infectious diseases program. "Whether or not the prime-boost approach proves itself, we're really interested in his viral vectors, the modified viruses used to boost T-cell production."

Hill is 1 of 12 HHMI international research scholars working on different aspects of malaria. Louis Schofield of the Walter and Eliza Hall Institute of Medical Research in Melbourne, Australia, is developing a vaccine based on glycosylphosphatidylinositol (GPI), a toxin responsible for the intense fevers of malaria. In August 2002, Schofield and colleagues Peter H. Seeberger and Michael C. Hewitt at the Massachusetts Institute of Technology reported success in tests of an anti-GPI vaccine in mice.

"We are a long way from clinical trials with a vaccine," says Schofield. "The vaccine has to be put through preclinical testing in monkeys." Daniel E. Goldberg, an HHMI investigator at Washington University in St. Louis, whose own research focuses on the biochemistry of malaria infection in humans, calls Schofield's published findings on the GPI toxin "elegant and important." Goldberg explains: "He actually synthesized this complex carbohydrate molecule to show that the activity seen was due to the GPI, then he demonstrated the central role of the GPI toxin in P. falciparum pathogenesis, suggesting that this molecule would be a good vaccine target." Schofield's approach is also controversial, says the Gates Foundation's Rabinovich, because essentially it treats the fever, which is itself deadly, but does not attack the underlying cause.

Meanwhile, Chetan Chitnis, an HHMI international research scholar at the International Centre for Genetic Engineering and Biotechnology in New Delhi, India, is doing extremely promising malaria research, according to Rabinovich. Using Plasmodium knowlesi—a malaria parasite that infects monkeys—as a model, Chitnis is studying the invasion pathways that P. knowlesi uses to infect red blood cells. P. falciparum and P. vivax—malaria parasites that infect humans—use similar invasion pathways. Understanding the molecular interactions that mediate these pathways has enabled Chitnis to develop a vaccine for P. vivax that blocks invasion of red blood cells. He expects to begin phase I clinical trials by the end of 2003.

"His fundamental scientific rationale is sound, and he has shown the ability to take a protein made on the benchtop and translate it into a real product of sufficient quality to test in humans," says Rabinovich. "To say that is not easy is an understatement."

Alan Cowman, an HHMI international research scholar also at Australia's Walter and Eliza Hall Institute of Medical Research, studied a population with uncommon resistance to malaria and identified an important pathway—used by P. falciparum to infect red blood cells—that might be exploited to prevent or slow infection (see Melanesians Provide Malaria Clues).

The work of another Australian research scholar, Magdalena Plebanski, at the Austin Research Institute in Melbourne, focuses on dendritic cells—key players in the immune response because they activate pathogen-fighting T cells. Plebanski has shown that blood-stage malaria parasites impair the ability of dendritic cells to mature, thereby suppressing some of the T cells that could protect against the disease. However, when she injected mice with immature dendritic cells that had been altered by interaction with parasite-infected whole blood cells, the animals developed 80-100 percent immunity after one injection. Four months later—a long time in mice—protective immunity had not diminished, and the procedure did not make the mice sick.

Plebanski has developed a method to target dendritic cells in vivo with a novel carrier-adjuvant—a substance added to a vaccine to take it to the right place in the body and to boost the immune response—that elicits similarly high and long-lasting levels of protection against lethal blood-stage malaria. "Some vaccines are carriers, and others are adjuvants. Good vaccines aim to be both," she explains.

Although the United States has been and continues to be a major contributor to malaria research, the enormity of the problem and the importance of the solution mandate an international effort, says Jill Conley, director of HHMI's international program. "History has shown that no single country has the answer," she observes.

A PORTFOLIO OF APPROACHES
Prevent it. Treat it. Protect people from mosquito bites. Kill the mosquitoes. Transform them into harmless nuisances through genetic engineering. There are myriad potential approaches to combating malaria, and a great many are being tried.

Washington University's Goldberg believes that's a good thing. He also endorses the private-public partnerships that have been forming to take on malaria. "They're the only way to go. Industry will not take the lead because malaria is not a financially rewarding venture for them," he explains, "and public funding is not adequate by itself to take things forward."

In addition to the Gates-funded Malaria Vaccine Initiative, there is the Multilateral Initiative on Malaria, whose players include the Wellcome Trust, the National Institutes of Health and HHMI; Roll Back Malaria, a global partnership of the World Health Organization, the United Nations and the World Bank; and the Medicines for Malaria Venture, a private-public partnership underwritten by the Gates Foundation, ExxonMobil Corporation, the Global Forum for Health Research, the International Federation of Pharmaceutical Manufacturers Associations and others. In November 2002, pharmaceutical company GlaxoSmithKline awarded $1.5 million in community development grants to combat malaria in seven African nations.

Meanwhile, two international consortia of scientists published the completed genomes of P. falciparum (Nature, October 3, 2002) and Anopheles gambiae (Science, October 4, 2002). Many scientists hailed the work as a breakthrough in the war on malaria, predicting that data mined from the parasite, mosquito and human genomes will yield new and effective drugs, insecticides and vaccines. One new antimalarial, fosmidomycin, already has been developed in Germany by using P. falciparum genome data, and preliminary results of animal tests have been promising.

Hill still pins his hopes on prime-boost vaccines. "If the genome helps," he says, "that's really bad news!" Come again? "That means we'd have to wait another 25 years for an effective vaccine," Hill explains. "With anything that comes out of the genome, we'd be starting at square one, with unfamiliar, untested genes and antigens. There are several candidate vaccines in trials right now that can be made at reasonable cost and that could be useful well before then, if the resources are provided for vaccine development. A useful malaria vaccine is needed by this generation's children, not their grandchildren."

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Photos: Graham Trott, Vasee Morthy

Reprinted from the HHMI Bulletin,
March 2003, pages 14–19.
©2003 Howard Hughes Medical Institute

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