A mole, as every espionage buff knows, is a secret agent who perpetrates his traitorous machinations from deep within the organization he is supposed to serve.

Such a treacherous operator is Mycobacterium species, the family of pathogens that cause tuberculosis (TB), leprosy and M. avium, an opportunistic infection in AIDS patients, once strictly for the birds.

Of these three diseases, by far the most dangerous and widespread is the work of M. tuberculosis, which infects an estimated one-third of Earth's 6 billion people. Of this number, 8 million come down each year with active pulmonary TB, and 3 million a year die of it.

Mycobacteria answer to the description of moles, because they invade, and lie low in, macrophages. These are cells of the immune system expressly employed to engulf, dismember and spit out infectious microorganisms and other alien invaders. (See BioWorld Today, April 16, 1997, p. 4.)

Now it turns out that Mycobacteria practice an additional form of treachery in the body: They highjack a protein of the immune system's complement cascade to ease their entry into macrophages. The irony of this just-unmasked mole-like subversion is that the complement system's main mission in the body is to detect pathogens and kill them off.

Today's Science, dated Aug. 22, 1997, blows this latest Mycobacterial cover in an article aptly titled: "A macrophage invasion mechanism of pathogenic Mycobacteria." Its lead author is cellular microbiologist Jeffrey Schorey, in the Division of Infectious Diseases at Washington University, of St. Louis.

The unsuspected molecule that Mycobacteria "turned" carries the code name C2a. This identifies it as the second protein in the body's 20-protein complement system. Complement is the hyper-intricate ballet of immune-system proteins, receptors and enzymes that puts the finger on intruding substances, marks them for termination and carries out the execution.

C2a enters the Mycobacterial picture bound to another complement component, C4b, both perched on the bacterial cell surface. "The C4b/C2a complex is very unstable," Schorey told BioWorld Today. "It only lasts for a couple of minutes, then dissociates. C4b stays on the microbe's surface, while C2a is released into the bloodstream."

He continued: "As far as anyone has ever shown, C2a has no function; it's just released into the serum, and there degraded. As we have now reported in Science, although the host's immune system says of C2a, 'We don't need you any- more,' Mycobacterium quickly butts in, 'Fine! I'll take you.'"

The pathogen then somehow activates this orphan C2a protein, Schorey explained, "so its active site is able to recognize and cleave C3." This third complement component is like a spy's secret cutout--the mutually trusted go-between the Mycobacterium and its safe house in the macrophage.

Macrophages: From Scavenger To Sanctuary

The skeleton key that admits the C3-coated pathogen to its hideaway consists of complement receptors to that C3b protein on the germ's surface.

Just how Mycobacteria can con or coerce macrophages into rolling over instead of doing their job of repelling boarders, Schorey said, "We don't really know. We do know that the Mycobacteria manipulates the phagocyte's intracellular environment. Its first maneuver is to enter a phagosome, which is a vesicle inside the macrophage."

Normally, this entry-stage sac would promptly fuse with a neighboring lysosome, a bag full of enzymes by which macrophages break down their prey. "They're just digestive," Schorey observed, "like your dishwasher."

But the wily Mycobacterium has other designs. "What it has learned to do, or adapted to do," Schorey said, "is to block that progression from phagosome to lysosome. They simply remain in the phagosome indefinitely. One ploy it deploys to achieve this stand-off," he observed, "is keeping its acid-alkaline ratio at 6.2 or 6.3, which is more basic than the lysosome's degradative enzymes can put up with."

Chemical countermeasures against TB are fetching up against counter-countermeasures by which M. tuberculosis is blunting the efficacy of once-potent antibiotics (See BioWorld Today, June 18, 1996, p. 1.)

"One of the mechanisms that's believed to be very important for Mycobacterial survival in the phagosomes," Schorey observed, "is its very thick cell wall. It's very impermeable and resistant to whatever the macrophage throws at it. What antibiotics do," he continued, "is inhibit biosynthesis of the bacterial cell wall, or increase its permeability--essentially, just make the bug less virulent."

Therapeutic Thought Experiments

He suggested that in the light of the C2a discovery, "the therapeutic scenario most likely to have an effect would be to make antibiotics that target the particular Mycobacterial cell-wall surface that binds C2a. You can't really target the C2a itself," he pointed out, "because the host needs it as part of the complement system. So you go after whatever it is the Mycobacterium has that's binding the C2a."

He went on: "One way you can do this is through mounting an immune response quickly to that component, such that having antibodies bound to it, it's more targeted for degradation. The other approach is to see if you can develop drugs that block that C2a synthesis. If you can develop antibiotics that target this particular part of the cell wall surface, as well as other components on it, you might have more of an effect on the ability of the Mycobacterium not only to survive, but to get inside the macrophages."

But Schorey qualified these therapeutic scenarios as "premature--just a lot of hand-waving. The next important step," he said, "is to find the Mycobacterial molecule that interacts with C2a."

Meanwhile, he and his co-authors are gearing up to move from the in vitro experiments by which they isolated C2a to in vivo testing in knockout mice and guinea pigs deficient in selected complement components and challenged with M. avium.

This species of Mycobacterium is ubiquitous in the planet's soil and water, Schorey pointed out, and until onset of the HIV epidemic it infected birds primarily. "In humans," he said, "M. avium's major route of infection is through the gut."

While specific for macrophages like M. tuberculosis, it differs from that pathogen, he said, "in that AIDS patients have to have a very low CD4 count before they are susceptible to M. avium." He added: "Although its usual route is through the stomach, if someone with avium coughs on you, you can get it in your lungs." *