By William J. Cromie

Gazette Staff

Deadly bacteria are being engineered into vaccines expected to treat and protect humans against a variety of diseases.

Medical School scientists have combined bits of anthrax toxin, used in germ warfare, with proteins that stimulate the body to fight everything from food poisoning to AIDS and cancer.

"Almost all the frightening bacteria that people know of -- anthrax, diphtheria, cholera, botulism, and tetanus -- make toxins that have the extraordinary ability to cross the membrane that surrounds and protects our cells," notes John Collier, Presley Professor of Microbiology and Molecular Genetics.

"We're trying to engineer these toxins to eliminate their poisonous character and replace it with molecules that shore up the body's natural defense system," adds Michael Starnbach, assistant professor of microbiology and molecular genetics. In other words, the researchers want to utilize the toxins' unique transport system to deliver beneficial, rather than harmful, freight into human cells.

Working with postdoc Jimmy Ballard, Collier and Starnbach separated the transport and poison parts of anthrax toxin and commandeered the former to deliver pieces of protein from a bacterium that causes food poisoning in humans. They injected the combination into mice. Other mice got injections of salt water.

Later, they exposed all the mice to the whole bacterium, Listeria monocytogenes. Mice who received the salt water got sick; those who got the experimental vaccine did not.

Without the anthrax transport, the load of helpful protein would have motored right past the cells. With it, the cargo of Listeria protein reached the inside of cells. Once there, it was processed and fragments of the proteins worked their way out to the cell surface. As part of their protective immune system, mice and humans boast white blood cells that circulate through the body checking the outside of cells for abnormal proteins, signatures of invading bacteria or viruses. When such invaders are found, the vigilant scouts turn into killer T-cells that destroy any cells with the abnormal markings.

When challenged later with the whole bacteria, killer cells recognized them as abnormal interlopers and destroyed them. This is precisely how vaccines work, so the researchers successfully vaccinated the mice against human food poisoning.

A Trick to Treat Cells

Ballard, Collier, and Starnbach see the experiments with food-poisoned mice as a model for a system that could be used against a variety of severe viral infections and, possibly, against cancer tumors.

Future vaccines are likely to contain more than one type of cargo being pulled into cells by the Toxin Freight Lines. The first experiments got rid of most but not all Listeria bacteria in the mice; it will probably take an improved vaccine with more than one protein to do a more effective job in humans.

"We're now building additional proteins on the carrier molecule to stimulate a stronger protective response against bacteria," Starnbach notes. "We're also attempting to design vaccines against viruses."

One project uses diphtheria toxin as a delivery system. Another involves immunizing mice against a virus that causes meningitis in humans. Other possible targets include HIV, papillomavirus, and cytomegalovirus. HIV (human immunodeficiency virus) causes AIDS; papillomavirus produces genital warts associated with cancer of the cervix; cytomegalovirus is responsible for an eye inflammation that can lead to blindness.

Viruses invade cells, then take over the cell's resources to reproduce more of themselves. As this happens, they make abnormal bits of protein that work their way to the cell surface. That's the signal for killer T-cells, stimulated by a vaccine, to recognize and attack the invaders.

Another possible application, already under pursuit, would be to custom-tailor vaccines against cancer tumors. Tumor cells produce abnormal proteins as they grow and fragments of these proteins find their way to the cell surface. Researchers have identified such markers for breast, bowel, lung, and bladder cancer.

"As we learn more about these markers, we should be able to build compounds that will stimulate T-cells to attack them, or to prevent the tumor in the first place," Starnbach speculates. "Our lab has started building vaccines for such tumors in mice. This is complicated because each tumor has specific proteins. We'll probably never be able to make a vaccine for all types of cancer cells. However, we may be able to concoct a 'cocktail' that protects against selected tumors and disease-causing organisms like viruses and bacteria."

While working with diphtheria, Collier first discovered that toxins carry "free passes" that let them into cells. "I realized, along with others, that this ability might be used to transport beneficial proteins of various kinds into cells," he says.

Collier did experiments to show that the anthrax system is very efficient at this, and he showed how the carrier and poisonous parts of a toxin can be separated from each other.

"This project is very exciting," he says. "It would be marvelous if this works in people as well as it does in mice."

"It's kind of a fun system to work with," Starnbach adds. "We've had a good time learning how dreaded diseases can be used for applications that nature never intended."