By David N. Leff
What is the U.S. Navy doing waging war against a landlocked enemy — Plasmodium falciparum?
This highly pathogenic protozoan parasite is the grim reaper of millions of lives in the poverty-ridden villages of Africa and East Asia.
Capt. Stephen Hoffman, a tropical-disease specialist who directs the malaria vaccine program of the Naval Medical Research Institute, in Bethesda, Md., explained:
"I don't have the exact numbers," Hoffman told BioWorld Today, "but it's estimated that tens of millions of travellers from North America, Europe, [and Japan] . . . travel every year to countries where malaria is transmitted. They would all need the vaccine.
"So we're talking about 10, 20, 30 million travellers being immunized per year. If one adds to that," he added, "military personnel, State Department personnel, Peace Corps personnel and so on, one begins to develop a rather substantial demand for an effective anti-malaria vaccine."
But that's not all. "At the same time, Hoffman continued, "there are people in the developing world who are in the middle class and live in non-endemic areas. Not everybody lives in a village, with malaria being transmitted. [Many are] city dwellers who visit their parents, families and so on. They're just as non-immune as any of us and in many cases quite well off, so they will need it too." (See BioWorld Today, Oct. 13, 1994, p. 1.)
Hoffman is senior author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated June 23, 1998, which reports the latest salvo in his two-decades-long engagement to arm human immune defenses against the malaria parasite foe. Its title is "Boosting with recombinant vaccinia increases immunogenicity and protective efficacy of malaria DNA vaccine."
The immunogenic target in the cross-hairs of this naval task force is the full-length major sporozoite surface protein of Plasmodium. Sporozoites are the form of the parasite that malarial mosquitoes squirt from their saliva into the bite they inflict in order to suck mammalian blood.
Once deposited in the circulation, this microbe heads straight for its victim's liver. There it sojourns for 5.5 days, causing no symptoms, but multiplying prodigiously. These progeny then break out of the liver cells and infect red blood cells. That's when the severe chills, high fever and malaise of full-blown malaria begins.
First Clinical Trial Looking Good
Just a year ago, Hoffman and his crew had reached a point in their preclinical in vivo vaccine development in mice to undertake a Phase I (toxicity and dosage) vaccine trial in human subjects.
"We initiated it last July," he recounted, "here at the Naval Medical Research Institute, with healthy adult volunteers recruited from the Maryland community. The results are in review at Science now," he revealed, "but I can tell you that the vaccine was safe, well-tolerated and induced good cellular immune responses."
He expects a Phase II clinical trial — challenging volunteers with live sporozoites and measuring the vaccine's efficacy — to begin "in about ten months."
Hoffman's report in PNAS described malarial challenge and efficacy in mice of the Naval Institute's three-pronged vaccine:
The animals received a rodent-specific version of the parasite Plasmodium yoelii, rather than the human-directed P. falciparum, tested in the just-ended Phase I trial. Hoffman's team inoculated them with combinations of three molecules.
"They included that sporozoite surface protein one way or another," Hoffman recounted. "Whether the DNA or the recombinant vaccinia or a synthetic protein, in a form called multiple antigenic peptide. It contained amino acids from the sequence of this sporozoite protein.
"These were given in different orders of time," he continued, "priming with one, boosting with the other — DNA-DNA, DNA-vaccinia, vaccinia-vaccinia, vaccinia-DNA, etcetera — for a total of nine different combinations.
The two top-performing inocula turned out to be:
* A naked-DNA construct encoding P. yoelii's major sporozoite surface protein and code-named "D."
* A second antigen delivery system, designated "V," which wrapped the protein in a recombinant vaccinia virus engineered by co-author John Tine, of Virogenetics Corp., in Troy, N.Y.
New Vaccine Technology Makes Great Strides
"Our experimental method was very straightforward," Hoffman narrated. "As the initial, or primary, immunization dose, we injected mice in their tail veins with either the vaccinia expressing the sporozoite protein (V) or with D, for the DNA plasmid. Then, six weeks later, we gave some of the groups of mice a second, or booster, shot as D and other groups as V.
"So that way, we ended up with four trial cohorts — VV, DD, DV or VD."
Two weeks later, the co-authors challenged the mice with live sporozoites. Seven to 14 days after that, they could detect the parasites in the bloodstream of their murine subjects.
"The only two of our nine variant vaccinations," Hoffman recalled, "that gave protection against malaria in those mice were DD (both primary and booster shots of DNA) or DV (DNA followed by vaccinia). But DV," he pointed out, "gave significantly better protection than DD, with 69 percent resistant to the malarial challenge. This was understandable because it elicited better antibody and T cell responses than did DD, with 44 percent."
Seven additional experimental combinations yielded results at 14 days post-challenge ranging from 27 percent down to zero — same as all control mice.
"Our work with this brand-new DNA vaccine technology," Hoffman observed, "is propelling us forward in ways that were unthinkable in the past because of what it provides in simplicity and lower cost of making new types of vaccines.
"And that," he went on, "will allow us to capitalize on the Plasmodium falciparum genome-sequencing project, which we're doing in parallel with the vaccine development. With its 14 chromosomes," he said, "P. falciparum's genome is much larger than the other microorganisms that have been published to date."
Hoffman and his collaborators have just finished sequencing the first of those 14 chromosomes. *