Thursday afternoon, at a media briefing in the Pentagon, Capt. Stephen Hoffman disclosed the results of what he described as "the first published report on the response of normal humans to DNA vaccines."
That publication figures in today's issue of Science, dated Oct. 16, 1998. Its title is, "Induction of antigen-specific cytotoxic T lymphocytes [CTLs] in humans by a malaria DNA vaccine." The paper's senior author, Hoffman is a tropical-disease specialist who directs the malaria vaccine program at the U.S. Naval Medical Center, in Bethesda, Md.
Among the Science article's 15 co-authors, three are scientists from Vical Inc., a biotechnology company based in San Diego. In collaboration with the institute, Vical designed and developed the naked-DNA malaria-vaccine vector that has just passed its first test in humans.
The safety and dose-ranging study enlisted 20 military and civilian healthy volunteers, recruited from the Maryland community. All of them were "malaria-naïve" — having never been infected by the parasite. They were randomized into five cohorts, who received three injections of the vaccine at four-week intervals, in the deltoid muscle of their upper arms. The shots ranged from 20 to 100 to 500 to 2,500 micrograms of plasmid DNA.
The endpoint of the study was to assess their immune response to malarial antigens generated by the naked-DNA vaccine. As Hoffman informed the media conference, "The majority of the 20 subjects immunized with a potential malaria DNA vaccine developed CTL [cytotoxic lymphocyte 'killer'] responses, and those responses were dose-related."
Parasitologist William Rogers, a scientist on Hoffman's staff, described the DNA vaccine plasmid to BioWorld Today:
"It consists of an expression cassette into which the full-length sequence of the circumsporozoite protein gene from the Plasmodium falciparum parasite was inserted. It is under the control of a cytomegalovirus enhancer promoter, and has a leader sequence from human tissue plasminogen activator."
In P. falciparum's life cycle, sporozoites are the stage that beelines for the liver after a mosquito bite has injected it into the victim's bloodstream.
"By far the majority of the DNA vaccine that gets into a subject's cells enters the muscle cells at the site of injection, where the gene is expressed. But muscle cells," Rogers said, "are not what we call 'professional antigen-presenting cells.' Their job in life is not to present foreign antigens to the body's immune system. They are not believed a priori to be good cells to stimulate the immune response."
He added: "Cells that are specialized to present antigen have a variety of co-stimulatory molecules that provide an additional kick to the T cells. Basically, they tell the immune system: 'Wake up! This is important, ready or not!'"
Rogers noted that "conventional vaccines must overcome a vastly formidable obstacle in the variablity of individuals in the human population to specific antigen recognition and response.
Multi-Genes Are Better Than One
"DNA vaccination," he suggested, "may make it possible to get around that drawback, because it is particularly easy to make cocktails of multiple foreign genes. In all the other traditional vaccines, whether you're using a killed organism, an attenuated organism, even a recombinant protein, you are injecting into the body the material against which you want the body to mount an immune response.
"In the case of a DNA vaccine," Rogers said, "you're injecting the DNA and then asking cells of the body to actually produce the foreign material, which is going to be the target of the immune response. In our present vaccine, the killer T cells are designed to move into action, and destroy the parasite within the liver cell. The idea is that this vaccine is targeted to kill the parasite before it can get out of the hepatocyte into the bloodstream."
The just-reported anti-malarial DNA, Rogers noted, is a single-gene vaccine. "Because this is one of the first DNA-vaccine trials in healthy humans, the FDA asked us to begin these safety studies with a single-gene construct. That in fact is the reason we didn't challenge the volunteers with malarial infection, because we suspected that only a fraction, perhaps less than half, would have been protected, owing to this phenomenon of variable population response."
Next Time, Hold Still For Mosquito Bites
"So our next step," he said, "will be a five-gene vaccine, and in that trial we will do a challenge, with infective parasites, and see if there is indeed protection." Such a challenge, he added, simply involves exposing the volunteers to malarial-mosquito bites. "Having achieved our goals for the [single-gene] study," Hoffman told the Pentagon press briefing audience, "we are now working to develop a [five-gene] vaccine to provide protective efficacy."
Vical's president and CEO, Alain Schreiber, noted, "Our latest achievement represents the proof of principle of safety and potential efficacy of DNA vaccines in man, so we can focus now on their further development against specific disease targets." The company is working to develop and test the multi-gene DNA vaccine under a cooperative agreement with Hoffman's group, supported by up to $2.7 million in funding from the Office of Naval Research.
The impending multi-gene challenge study will test "five completely different full-length genes," Rogers said, "each of which will contain multiple T-cell-targeting epitopes. One will be the same sporozoite single-protein that is in the present vaccine. The other four genes will encode proteins that are expressed in either the sporozoite stage of the parasite or the infected liver stage."
For that next-step trial, Rogers said, Hoffman and his team "plan to begin the six-months course of immunization in late spring or early summer of 1999. Then giving some time for the challenge and putting the results together, we would expect an answer from that study perhaps in February of 2000."
He added, "The five-gene vaccine is designed to prevent the parasites from getting out of the liver. Then, the longest long-term plan after that will be a 15-gene vaccine, including additional genes expressed in the blood-stage form of the parasite, and designed to reduce the severity of disease in endemic malarial areas." (See BioWorld Today, June 30, 1998, p. 1.) *