By David N. Leff

Science Editor

During World War II, the U.S. Army sprang a dozen hardened convicts from prison, and sent them to fight the Nazis behind enemy lines. That, at least, is the plot of an old movie, "The Dirty Dozen," inspired by true military history.

Those cinematic criminals can in a way compare to the human immune system in its never-ending war against the serial-killer pathogen Mycobacterium tuberculosis. It's not as if the immune defenses are all that bad; but against TB they are relatively weak, and don't always shoot straight at their antigenic targets. This is notably the case in the BCG vaccine.

"BCG is sometimes referred to as an antigen," observed bacteriologist Douglas Lowrie, "but it's a whole, live, attenuated mycobacterium, so it has many protein antigens in it." (See BioWorld Today, June 1, 1999, p. 1.)

Somewhat as a military establishment fields land, sea and air forces, the immune system launches two main arms against its antigenic foes - antibody responders and cellular T-cell responders. One division of the T-cell army, T helper-2 cells, promotes production of antibodies. T helper-1 cells mobilize cell-killing forces. One of the weaknesses of BCG vaccine, Lowrie pointed out, is that "antibodies seem to have no protective role against TB." That is, the BCG vaccine is fighting with one of its arms tied behind its back.

M. tuberculosis has formidable arms of its own, with which it seems right now to be winning the war against the human population. One-third of those 6 billion people are infected with the pathogen, which kills 3 million of them a year.

Besides thumbing its nose at the human immune system, the microorganism deploys three so-far-unbeatable strategies and tactics:

1. High infectivity: A mere cough or sneeze, let alone a kiss, can carry the bug from an infected individual to its next victim.

2. Drug resistance: Almost as fast as a new antibiotic appears on the market, the bacterium trumps it with resistance factors that render that chemotherapeutic agent helpless.

3. Persistence: Despite occasional setbacks - by an antibacterial drug that still works - M. tuberculosis holes up in human cells for ages, only to re-emerge as virulent as ever.

This was the triple threat that confronted Lowrie, a senior scientist in the British Medical Research Council's London-based division of mycobacterial diseases. He is the senior author of a progress report in today's issue of Nature, dated July 15, 1999, and titled: "Therapy of tuberculosis in mice by DNA vaccination."

Approach More Like Rifle Than Shotgun

Unlike the BCG vaccine's hodgepodge of protein antigens, Lowrie's new DNA arsenal contains only one antigenic dart board per vaccine. Another "surprising" difference: Instead of merely preventing the infection, his DNA vaccines have proven effective in dramatically reducing the bacterium's body count, and reversing the infection - so far in mice and guinea pigs.

"Our DNA vaccine," Lowrie explained, "causes the immune systems of mice to switch from an ineffective type-2 helper immune response to a more effective type-1 helper, which stimulates the cytolytic T cells to start killing the bacteria. The vaccine package consisted of an E. coli plasmid vector backbone, into which we introduced a gene promoter that drives a length of DNA containing the gene sequence for a selected mycobacterial antigen."

The first two such targets they selected for preclinical testing were the mycobacterium's heat-shock and the secretory antigens. "Initially," Lowrie recalled, "heat-shock was serendipity in a way. It was the first piece of mycobacterial DNA that had been cloned, and it turned up in our laboratory because we knew it encoded a prominent antigen of both the antibody and cellular responses."

He observed that after British scientists had sequenced the entire 4.40-megabase sequence of M. tuberculosis last year, "there was an upsurge of interest in its secretory antigen. Its function is obtaining phosphate for the bacteria to grow."

Lowrie recounted how he and his co-authors had tested these DNA vaccine versions in mice and guinea pigs:

"In the first batch of animals, we gave the mice a large challenge dose of tuberculosis bacteria, and allowed the heavy infection to develop for about five weeks. Next, we gave them standard chemotherapeutic drugs, and the number of bacteria went down to very low levels. Those mice remained healthy for a long time. But when we then injected them with corticosteroids - which suppress immune responses - the persisting bacteria that were there in invisible form started to multiply, and reached large numbers again. If we had left the animals alive, they would have died of TB."

"The second batch of mice also received chemotherapy, but this was followed immediately by DNA vaccine, injected into their leg muscles. Then we waited and gave corticosteroids again. This time, in a proportion of the mice, we couldn't get the bacteria back at all. The mice remained, seemingly, completely cured of TB."

Lowrie continued: "We then killed the animals at intervals, and counted the bacteria in their lungs and spleen. We found that the number of microorganisms decreased in the animals that had received the vaccine, compared to placebo or artificial control vaccines." Among those controls was BCG, deemed more of a placebo than a vaccine."

Since reporting those preclinical trials in Nature, Lowrie related, "We've now done a little bit of work with a different kind of infection, intranasal aerosol vaccine delivery, which more closely resembles the natural transmission of TB. In guinea pigs, we allowed the aerosol infection to develop for five weeks, before we gave the vaccine. The vaccinated animals remain healthy; the not-vaccinated became sick. So that's as far as we've gone."

He sees as the group's next steps "consolidating the position in animals. We need to do more experiments in mice, guinea pigs and rabbits. Provided it still looks good - in other words, we're still getting beneficial effects and not toxicity - one might think of putting some of this DNA into people. My guess is that one such clinical trial cohort would be in HIV, and the other in multidrug-resistance TB, where patients are dying, and may be running out of things to try. It might be reasonable to try a DNA vaccine in that context. But," Lowrie concluded, "that's a year or two away, I think."