While United Nations arms inspectors continue to probe Saddam Hussein's well-hidden ability to wage germ warfare, such a system is killing upwards of 2 million people a year, most of them children, and disabling 300 million men and women around the world.
Just as Iraq's bio-weaponeers fitted out aircraft to deliver pathogen-tipped missiles, the malaria system relies on a specialized airborne fleet of Anopheles gambiae mosquitoes ferrying a parasitic missile, Plasmodium falciparum, to bomb its human targets with delayed-action lethal infection.
"P. falciparum is the only malarial parasite, of four human-infecting species, that causes death in its victims," observed molecular parasitologist Mats Wahlgren, at the Karolinska Institute, in Stockholm. "Its lethality arises," he explained, "from its ability to bind to the endothelial cells lining our blood vessel walls and inner organs."
When a mosquito plunges its hollow nose-cone into the skin of a human victim to suck up a meal of blood, the insect's saliva deposits a contingent of P. falciparum parasites into the itchy bite. These invaders, known as sporozoites, make straight for the liver, where they proliferate prodigiously. Their innumerable progeny, the merozoites, then set off in the bloodstream, where they attack the body's oxygen-carrying erythrocytes — the red blood cells.
Parasite Has Strategy For Evading Immune System
"When a merozoite touches a red cell," Wahlgren related, "it invades it, in what is known as the ring stage of its life cycle. These then develop into trophozoites."
He and his colleagues have found that "at this very early stage after invasion, the parasite's genome is activated in some way and transcribes many of its family of 50 to 100 var — for 'variable' — genes, which doesn't make sense, actually."
It's almost as if, Wahlgren suggested, P. falciparum "knew" it was locked in a life-and-death duel with the human immune defenses.
"One would think," he observed, "that maybe this multiplicity of genes could have to do with generating antigenic variation and variability. The parasite has a need to express a large number of surface antigens and keep changing its coat, because it lives and thrives in the bloodstream, actually inside the immune system.
"Of course, it must keep a balance," he went on. "If the parasite expressed too many variants of the var gene family, it would be recognized too quickly. So it doesn't switch so fast to the other variants. First, it imprints a specific gene, the one that predominates. Then, each time it replicates inside infected red cells, it slowly switches antigens on two percent of its surface. So that a small part of the whole parasite population changes all the time, although the major part of it remains the same."
These antigens are high-molecular-weight polypeptides — 200 to 350 kiloDaltons — and the parasite also expresses them on the surface of infected red cells. "There," Wahlgren pointed out, "they are also the target for immune attack on the infected red cells."
Wahlgren is senior author of a paper in today's issue of Nature, dated July 23, 1998, titled "Developmental selection of var gene expression in Plasmodium falciparum."
"What we did," he recounted, "was to look at the gene expression in single parasites. One parasite is the size of a red cell — something like six or seven microns in diameter. By microscopy, we could examine at it, check the one we wanted, and spit it out from the pipette. Then, after fiddling around with it a bit, we put that single cell into the tube. By amplifying large parts of its genome, we could see if it activated the genes, or not."
The results, as he reported in Nature, "show that there are many genes that are activated — which is completely against the genetic dogma of one gene, one protein.
"We found many genes," he recounted, "switched on in the ring, in the trophozoite, early stages of the parasite, and then all of them went silent, except for one. One gene only came up as a protein on the surface of the red cell. An unknown mechanism silenced its other genes."
Wahlgren continued: "Although we showed that the parasite expresses multiple genes in the ring, and then only one transcript in the trophozoite, a key outstanding issue was to show that the protein on its surface was the only one. So," he recounted, "we examined blood serum from people in Liberia, because inhabitants of that African country have long, heavy exposure to the malarial parasite.
"So if there were more proteins than one on their surface, the serum should recognize all kinds of variants. Through the antibody in these patients' serum, they recognize all kinds of things. If that antigen was there they would see it. But they saw only one single protein on the surface. That's one of the important findings in our paper."
Wahlgren compared this Liberian serum to the immune system's humoral B cell. "The B cell," he pointed out, "makes one antibody on the surface, like one gene, one protein. We used this as an instrument to show that, because it should react with something else, if there was something else also expressed on the surface, it would be recognized by these sera."
Possible Payoff: A Malarial Vaccine With A Difference
Hence, he allowed, that single protein suggested itself as a possible candidate for a malarial vaccine.
"We are working right now," he revealed, "to develop that into a vaccine. My vision, my thoughts, have been on that for a long time. And now that we have the means to do it, we will try to pursue that line."
A clinical trial of such a vaccine in human subjects, Wahlgren said, "is not so very close by; it will be at least five years from now. Testing in animal models must come first, but our problem is that there aren't so many human models for P. falciparum. Only New World primates are good. I already have collaborators who have access to these animal models," he said, "so I hope to test it in vivo pretty soon — within two years maybe."
Malariologist Louis Miller, who directs the Laboratory of Parasitic Diseases at NIAID, the National Institute of Allergy and Infectious Diseases, in Bethesda, Md., knows Wahlgren's work well.
"I feel very positive about his paper in Nature," Miller told BioWorld Today. "It's fascinating!" *