David N. LeffScience Editor

When it comes to tuberculosis, guinea pigs are more nearly humanthan mice or rats. These chubby rodents, Cavia by name, areextremely vulnerable to infection, disease and death byMycobacterium tuberculosi. Hence, guinea pigs make ideal guineapigs 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 inthree of the planet's population. It causes 8 million new cases a yearof active tuberculosis, and 2.9 million deaths _ more than dealt byany other single infectious agent.

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

Eight different antibiotics are specific to M. tuberculosis, but the wilypathogen keeps evolving resistance to each new drug as it comesalong.

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

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

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

Blowing Mycobacterium's Cover

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

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

Vaccinologist Marcus Horwitz, first author of the PNAS paper,zeroed in on half a dozen of M. tuberculosis' most abundantextracellular proteins, and used them to immunize guinea pigs, byintradermal injection. Then, after confirming their sensitization to thepresumed immunogens by skin testing, he and his co-authors sprayedtheir animals' nostrils with high levels of highly virulent aerosolizedM. tuberculosis microbes. This aerosol dose delivered approximately200 live bacilli to the lungs of each animal.

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

A quick way to tell if such a challenged guinea pig has contractedtuberculosis 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 vaccineinjections before the lethal aerosol, swiftly shed 11 to 25 percent oftheir total weight, and developed full-blown pulmonary tuberculosiswithin two or three months.

In one of two experiments, 83 percent of the controls died afterchallenge, versus 33 percent of immunized animals. In the secondtrial, half the controls succumbed, but all of the vaccinated animalssurvived.

However upbeat these numbers look, UCLA's subunit tuberculosisvaccine is not yet home free. "Although impressive," Horwitz wrotein PNAS, "protection was nevertheless incomplete. M. tuberculosismultiplied by several orders of magnitude even in immunizedanimals, although less so than in controls."

"Despite its drawbacks," he added, "BCG remains the standardagainst which new vaccines ultimately will be compared." Horwitz isin Madrid, Spain at a World Health Organization conference ontuberculosis vaccines.

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

Another co-author, molecular biologist GYnther Harth, toldBioWorld, that their goal of creating a recombinant vaccine "is morethan a tough nut to crack. We have cloned more or less all of theextracellular protein molecules; we are trying to trim them to theirstructural genes, determine their DNA sequences, and express themas 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 subunitvaccine is "capable of inducing protective immunity and could havesubstantial 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 ismore likely to be safe."

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

Third, "In contrast to BCG, the subunit vaccine can be rigorouslystandardized, allowing . . . meaningful predictions regarding itsefficacy and safety."

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

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