By David N. Leff
It will take the U.S. armed forces seven years to vaccinate its 2.4 million personnel against the bioweaponized anthrax bacterium. Defense Secretary William Cohen made that projection in late 1997, when he directed the Pentagon to launch its six-shot, 18-month immunization program.
"The major constituent of the military's vaccine against anthrax," observed microbial immunologist Michael Starnbach, at Harvard Medical School in Boston, is a bacterial molecule called 'Protective Antigen' (PA). He explained how the virulent anthrax organism fatally intoxicates people. "In its worst, most deadly form, the spores of this bacterium are inhaled into the lungs. There they germinate into growing vegetative organisms. What they secrete is a three-part toxin.
"PA, the first of these molecules," he continued, "is not toxic by itself. It simply sticks to cells. Then one of the two toxic proteins that this organism secretes - Edema Factor (EF) or Lethal Factor (LF) - binds to the PA that's already adhering to a cell surface. Then the whole complex is internalized into the cell, where it undergoes a conformational change that pushes the toxin proteins into the cytoplasm of the host cell."
Starnbach is co-senior author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated July 5, 2000, which embodies a new and improved approach to immunization against AIDS. Its title: "Genetically modified anthrax lethal toxin safely delivers whole HIV protein antigens into the cytosol to induce T cell immunity."
The article's lead author is virologist Yichen Lu, at Harvard School of Public Health. "To define the difference between our AIDS vaccine and all the others," Lu told BioWorld Today, "I can point out two things:
"No. One: This is the first such vaccine designed exclusively to stimulate T-cell responses.
"No. Two: In this category of vaccines, targeted for T-cell response, our approach is unique in that it's not using either a live or attenuated virus vector or a bacterial vector or naked-DNA vaccination. These are the only approaches where people had luck with the cellular immune response.
"Our approach," Lu pointed out, "is based on a recombinant protein antigen - the entire Gag fragment of HIV's p24 core protein - using the biological pathway of the anthrax Lethal Factor toxin. So that's why it's unique."
Domesticating Anthrax 'Lethal Factor'
Starnbach enlarged on this T-cell singularity: "Natural anthrax toxin," he told BioWorld Today, "delivers its toxic cargo into the cytoplasm of host cells. What we've done is modify the Lethal Factor, one of these toxic molecules that gets pushed in. We made them into fusion proteins that have no toxic activity. Instead, we replaced that activity with known antigenic proteins that stimulate CD8-positive killer T cells - particularly intracellular viral pathogens.
"These CD8 killer T cells," Starnbach pointed out, are also known as cytotoxic T lymphocytes, or CTLs. They blow up the cell that they're recognizing. And if it does that with a virally infected cell, it deprives the virus of its life cycle. So the idea of these vaccines that stimulate CD8 T cells is to mimic an antigen that those T cells can recognize, so if the person encounters the real pathogen, he has an expanded set of these T cells that can more quickly and effectively respond.
"And that's really what our toxin-delivery system does," he went on. "It's a mechanism of injecting this into someone, in a way that delivers this vaccine not only to the bloodstream, but into the cells of that person, so it can be recognized by this particular class of T cells."
The co-authors' new AIDS vaccine, Starnbach recounted, "is really a natural follow-through to studies that we had been doing with mice. In this preclinical in vivo model, we tested our detoxified version of anthrax toxin that can be used to deliver heterologous T-cell antigens into the cytoplasm of host cells in vivo.
"We exposed the mice to both a bacterial and a viral pathogen, and the vaccine showed protection against either of those pathogens. (See BioWorld Today, Oct. 29, 1996, p. 1.)
"And then as a preliminary to answering the question: Will this work in people? Using HIV as a model, we built a toxin fusion, which was different from previous toxin fusions in that this one carried the entire p-24 Gag fragment.
"It really would be the scale-up of what we had developed in mice," Starnbach went on, "where we took the PA delivery molecule and the LF-n fusion protein, and injected them together in the same syringe into an animal, and showed that those animals are protected against the pathogen. So that would be our initial approach in people as well."
Academia, Industry, Military Plan Clinical Trial
John McNeil is chief of HIV development at Walter Reed Army Institute of Research in Rockville, Md. "We have a Cooperative Research and Development Agreement with Avant Immunotherapeutics in Needham, Mass.," he told BioWorld Today. "Yichen Lu came to us about a year ago demonstrating this delivery platform with a few experimental HIV vectors, for which he had constructed fusion proteins. The system of this recombinant anthrax protein, carrying an HIV gene, then combined with the protective antigen, is called Therapore - Avant's proprietary trade name.
"Our interest," McNeil explained, "was solely to develop this as a novel T-cell vaccine to deliver HIV immunogens, and hopefully to induce more effective CTL responses. This was to be a joint project where we in our GMP facility at Walter Reed were to develop the Lethal Factor-n p-24 component of the vaccine, and the company was developing the PA component.
"At a successful pre-IND meeting with the FDA on June 28th," he summed up, "We described our clinical plan and toxicity data and provided them with the Phase I trial we propose to do here in our vaccine clinic at Walter Reed."
Starnbach added, "We are also interested in seeing whether we can use this approach as proof of principle for an antitumor therapeutic or vaccine. We would apply the information to cellular immunology, and see whether or not it has clinical efficacy. We've proven it effective as a mouse vaccine and a mouse therapeutic. It's now got to be proven in the clinic," he concluded. "That's where the system will live or die."