David N. Leff Science Editor

When it comes to tuberculosis, guinea pigs are more nearly human than mice or rats. These chubby rodents, Cavia by name, are extremely vulnerable to infection, disease and death by Mycobacterium tuberculosi. Hence, guinea pigs make ideal guinea pigs for developing new vaccines against the pathogen.

More than ever in recent years, a truly new departure in anti- tuberculosis immunization is a vital imperative in the world today. M. tuberculosis currently infects 2 billion people worldwide DD one in three of the planet's population. It causes 8 million new cases a year of active tuberculosis, and 2.9 million deaths _ more than dealt by any other single infectious agent.

In the U.S. alone, there were 28,000 cases in a recent year, and 40 percent of AIDS fatalities are due to tuberculosis.

Eight different antibiotics are specific to M. tuberculosis, but the wily pathogen keeps evolving resistance to each new drug as it comes along.

As for immunization, the only half-way effective vaccine is BCG _ Bacille bili de Calmette-Gurin. Developed early this century in France, this avirulent strain of M. tuberculosis is widely used in the world, but not in the U.S. or Britain, because of its severe side-effects and occasional fatal backlash.

Adding microbial insult to the injury, the mycobacterium of tuberculosis doesn't fight fair. When it infects a human victim, the intracellular pathogen hides away, of all places, in the very cells of the immune system, the mononuclear phagocytes, that are programmed to eat up and spit out invading microbes.

Now, here come guinea pigs to the rescue, heralded by an article in the current Proceedings of the Academy of Sciences (PNAS), dated Feb. 28. Its title: "Protective immunity against tuberculosis induced by vaccination with major extracellular proteins of Mycobacterium tuberculosis."

Blowing Mycobacterium's Cover

Instead of tinkering with a weakened whole bacterium, such as BCG, researchers at the University of California (UCLA), are exploiting one of M. tuberculosis' guiltiest secrets: Holed up in a phagocyte, where it survives and thrives, the microbe releases up to 100 extracellular proteins, which turn out to be variously immunogenic.

Unlike whole-microbe BCG, which antibodies of the humoral immune system target, these subunit proteins activate T cells of the cell-mediated immune arm.

Vaccinologist Marcus Horwitz, first author of the PNAS paper, zeroed in on half a dozen of M. tuberculosis' most abundant extracellular proteins, and used them to immunize guinea pigs, by intradermal injection. Then, after confirming their sensitization to the presumed immunogens by skin testing, he and his co-authors sprayed their animals' nostrils with high levels of highly virulent aerosolized M. tuberculosis microbes. This aerosol dose delivered approximately 200 live bacilli to the lungs of each animal.

Most humans who contract pulmonary tuberculosis are infected by this same airway route DD inhaling the microbe.

A quick way to tell if such a challenged guinea pig has contracted tuberculosis is to watch its body weight for a sudden drop. Paradoxically, after a brief dip, some of the immunized, germ- inhaling animals actually gained grams over their baseline weight. One hundred percent of controls, which received dummy vaccine injections before the lethal aerosol, swiftly shed 11 to 25 percent of their total weight, and developed full-blown pulmonary tuberculosis within two or three months.

In one of two experiments, 83 percent of the controls died after challenge, versus 33 percent of immunized animals. In the second trial, half the controls succumbed, but all of the vaccinated animals survived.

However upbeat these numbers look, UCLA's subunit tuberculosis vaccine is not yet home free. "Although impressive," Horwitz wrote in PNAS, "protection was nevertheless incomplete. M. tuberculosis multiplied by several orders of magnitude even in immunized animals, although less so than in controls."

"Despite its drawbacks," he added, "BCG remains the standard against which new vaccines ultimately will be compared." Horwitz is in Madrid, Spain at a World Health Organization conference on tuberculosis vaccines.

His team is now undertaking the side-by-side comparison of its construct with BCG in guinea pigs, one of his co-authors, Barbara Dillon, told BioWorld Today. "This trial is just getting under way," she said, "using exactly the same protocol."

Another co-author, molecular biologist GYnther Harth, told BioWorld, that their goal of creating a recombinant vaccine "is more than a tough nut to crack. We have cloned more or less all of the extracellular protein molecules; we are trying to trim them to their structural genes, determine their DNA sequences, and express them as soluble molecules, which we are doing right now very intensely."

Reasons Why Subunit Does it Better

Horwitz gave four good reasons in PNAS why his team's subunit vaccine is "capable of inducing protective immunity and could have substantial advantages over BCG:"

First, consisting of "only a few selected molecules [e.g., proteins] rather than the thousands of molecules of different types [proteins, lipids . . . nucleic acids, etc.] that make up a whole bacterium, it is more likely to be safe."

Second, "the subunit vaccine can be constructed so as to eliminate irrelevant or even immunosuppressive components of the whole bacterium." Horwitz noted that "Mycobacteria contain a number of non-protein molecules shown to suppress T-cell proliferation."

Third, "In contrast to BCG, the subunit vaccine can be rigorously standardized, allowing . . . meaningful predictions regarding its efficacy and safety."

Finally, Horwitz observed, "That these proteins can induce protective immunity against tuberculosis in the high susceptible guinea pig gives reason for optimism that [they can do so] in humans, who have much greater innate resistance to the development of active tuberculosis." n

(c) 1997 American Health Consultants. All rights reserved.