By David N. Leff
Fairly early in our nearly ended Second Millennium, from 1346 AD to 1351 AD, Europe lost one-quarter of its population to a flea called Xenopsylla cheopsis. A bite from this blood-hungry parasite injected its victim with a bacterium known as Yersinia pestis, pathogen of Plague.
Unlike malaria, which requires the bite of an infected mosquito to deliver its pathogen, a mere cough or sneeze can waft the Yersinia pestis from an infected person to its next victim. Nowadays, if untreated, it kills 60 percent of its targets, but prompt antibiotic treatment cuts that body count down below 5 percent.
Better known as the rat flea, that plague-infecting bug lives on rodents - not just rats, but mice, squirrels, prairie dogs and their ilk. "In the southwestern U.S. particularly," observed infectious diseases specialist James Bliska, "and in other parts of the world, there are infected rodent populations." He estimates at "half a dozen to 10" the annual number of U.S. plague cases. "These are usually people associated with, say, trapping animals," Bliska told BioWorld Today, "occasionally army recruits out on maneuvers, crawling around on the ground in the desert. A flea bites one; that's the normal way to acquire the disease." Bliska is a research scientist at the State University of New York in Stony Brook.
Postgraduate biochemist Kim Orth, at the University of Michigan, Ann Arbor, picks up the Yersinia pestis rap sheet: "You have a bacterium that goes into the blood and targets the mammalian cells, especially the immune system's macrophages. When the bacterium makes contact with the macrophage, it injects what acts like a syringe into the mammalian cell. That so-called type III protein secretion system opens a channel between the bacterium and the macrophage. And it squirts in six different molecules called Yops - Yersinia outer proteins, which actually works on the inside of the mammalian cell. All six of these Yops are essential for complete virulence by the Yersinia pestis.
"And all of these effector molecules," Orth went on, "are going to be signaling essential machinery within the cell. So each one will give us a key as to what mechanism is essential for a cell to survive. And if you don't have this machinery working, you can't work as an organism. We work as an organism so we have to have cells communicate to each other. If we break that telecommunication cable link, we no longer can have our cells talking to each other, and we die."
Meet YopJ - Yersinia's Hit Molecule
Orth is lead author of a paper in the issue of Science dated Sept. 17, 1999, titled: "Inhibition of the mitogen-activated protein kinase kinase superfamily by a Yersinia effector."
One particular effector, named YopJ, she told BioWorld Today, "was targeting these signaling pathways in cells - pathways found in every cell in your body. Yersinia pestis selectively targets the macrophage, thereby shutting down the immune system before it gets off the ground. And it does it in a lethal way, because YopJ also facilitates cell death - apoptosis."
Two crucial molecular pathways, her Science paper reports, are critical to this mechanism. "In the immune system you receive cytokine signals," Orth pointed out. "In this case it's the MAP kinase pathway and the NFkB pathway. The MAP kinase pathway is involved in cell growth and cell death. It also sends our inflammatory signals, which induce cytokine responses. As for the NFkB pathway," she went on, "it's definitely involved in inducing inflammatory responses and controlling whether a cell lives or dies. If NFkB is not turned on you can't make any protective molecules, so the macrophage won't die."
What's new in the Science report, Orth indicated, "is that you have an effector molecule that can recognize not only the MAP kinase pathway but the NFkB pathway as well. And it shows that both, which in the past were thought to be dissimilar in many respects, are actually more similar than we first thought. So if you shut down the NFkB pathway, which can't make molecules to survive a death stimulus, then the default pathway is death. But what if you could use that same site on all these molecules, in conjunction with the death stimulus, if you wanted, say, to kill tumor cells? You could selectively inactivate all those pathways, just like Yersinia does. You could do it in a tumor, to control cell growth."
Orth said, "If you want to shut down the immune system, we know where to target on that molecule. You'd use it to modulate the signaling pathway, say, to treat autoimmune disease."
From Lethal Plague Strategy To Vaccines In Plants
Biological chemist Jack Dixon, at the University of Michigan, is senior author of the Science article. "This Yersinia protein, YopJ," he told BioWorld Today, "is not only found in bacteria that are basically pathogenic in humans and rodents, but a similar molecule, showing sequence identity to YopJ, is also found in bacteria that have pathogenic effects on plants. There are suggestions that many pathogenic organisms use this type III secretory mechanism - this ability to physically inject proteins into mammalian cells - as a strategy to knock out the immune system.
"So I think one can think about using this potential strategy in creative ways to produce vaccines, for example for Third World countries. One might envision the possibility that some day we might actually be able to introduce, via bacteria, selected genes into plants, which would become part of the ecosystem. And as a result of that, they might in fact generate antigenic responses in people, and provide a really new strategy for vaccine development.
"So I think," Dixon concluded, "one of the key factors is understanding the mechanism by which YopJ carries out this inhibition. Then we can selectively modulate it in a bunch of ways - in cancer, inflammatory responses, etc."
As for whether the university is in touch with any pharmaceutical or biotech companies with a view to actually developing this long-range approach, Dixon allowed, "We are just starting that. We've just written up the patent disclosure, and such contacts are the next thing that's on the page."