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The Merits of Mutants
by Elizabeth Zubritsky
Bob Johnston has turned a mutant virus
against a deadly
infection, and millions of people may soon have one less reason to fear
the mosquito.
Bob Johnston didn't set out to make a vaccine. Twenty years ago, he
began studying the life cycles of viruses - basic research.
Johnston, professor of microbiology and immunology, wanted to learn
how viruses cause disease. He pictured fatal infections as a pathway, a
sequence of unknown steps - each an opportunity to block the virus. One
way to identify those steps was to study mutant viruses that had stopped
prematurely. By determining how their structures and RNA sequences had
changed, Johnston thought he could specify what the virus needed to continue.
He and his co-workers had begun by working with a mutant version of
Sindbis, a "model" virus that infects mice yet causes little
disease. Several years later, they could describe critical steps in the
virus' life cycle. That's when a colleague half-jokingly challenged
Johnston to work with a "real" virus - one that causes illness
in humans.
"I asked him, 'Just what real virus do you have in
mind?'" Johnston says. "He suggested the one he was working
on, Venezuelan equine encephalitis (VEE)."
VEE can reproduce inside mosquitoes, which periodically bring it out
of its secluded home in Central and South American forests to infect
livestock and people. One hundred thousand people were infected, and
hundreds, mainly children, died during an outbreak in Venezuela and
Columbia that ended about a year ago. Thousands of horses also died. In
1971, a similar outbreak reached southern Texas.
Early symptoms of VEE infection are flu-like: headache, chills,
fever, muscle pain, and nausea. Severe infections can lead to
encephalitis, an infection of the brain, causing convulsions, paralysis,
and death. Although some South American livestock are immunized against
VEE, the current vaccine is risky for humans, sometimes causing flu-like
symptoms.
Johnston decided to switch to VEE, once more studying crippled
mutants. The approach worked again, and this time success came with a
bonus - a new vaccine.
"When we did the work with a 'real' virus, we anticipated that
the experiments would hand us a vaccine," says Nancy Davis, a
research associate professor in Johnston's lab. The mutants had all the
right properties for a vaccine: They would grow inside a mouse and induce
an immune response, but they would not cause disease. And the animal was
protected from the naturally occurring virus, even a year after inoculation.
The final VEE vaccine has been tested successfully in monkeys. If all
goes well and if the FDA approves the vaccine, it should be ready for
human testing by the end of this year.
Meanwhile, Johnston is making good use of other VEE mutants. He
turned some into "expression vectors" - shuttles that carry
foreign genes. Genes from virtually any organism can be inserted to
produce or "express" a protein.
Johnston realized the expression vectors made from VEE could be
vaccines, too. If the inserted gene belongs to another infectious
organism, then the vector induces immunity to it and to VEE. Johnston,
Davis, and other lab members developed a version of
this "double-promoter" vaccine to protect mice against VEE
and influenza.
And they, and their collaborator, Dr. Jonathan F. Smith, took the
idea one step further, creating a second type of vector that immunizes
against the inserted gene but not VEE. This vector, called a
"replicon," does not carry the gene for the coat protein that
encapsulates VEE, making VEE virtually invisible to
the immune system and allowing the vector to be re-used for multiple
vaccines or booster shots. The lack of a coat protein gene also prevents
the virus from reproducing on its own, making it a very safe vaccine.
"The replicon is a 'suicide' particle," Johnston says.
"After the initial infection, the virus hits a dead end because it
can't reproduce without a helper. The virus can't spread."
A collaborative effort among Johnston, Davis, and the labs of Drs.
Jeff Frelinger and Ron Swanstrom combined the replicon with the
double-promoter vector to make a combination vaccine for SIV, a monkey
virus related to HIV. If these trials - conducted
by collaborator Phil Johnson - are successful, the next step will be to
test an HIV vaccine. Johnston says he can't predict how long it might
take to reach that stage.
In the meantime, he is putting together a company to explore other
uses for the expression vectors, such as vaccines for influenza or the
virus that causes croup. Later, the vectors might be adapted to boost a
person's immunity to cancer-causing viruses or to target specific cells
with gene therapy.
Turning these projects over to the company should let his lab focus
exclusively on basic research again. "There are a lot of
fundamental issues left," Johnston says. "How does the virus
spread in the body? How does it invade the brain? How does it grow in
neurons?"
Johnston's expression vectors might help him find the answers. The
lab now has a version that produces a green fluorescent protein,
normally found in jellyfish. This protein highlights the virus,
literally letting the researchers trace its path through a mouse's body.
Products like this are bringing Johnston back to basics. Or maybe he
never really left.
Johnston and Davis are members of the Department of Microbiology and
Immunology in the School of Medicine. Their work has been supported by
the North Carolina Biotechnology Center, the U.S. Army Medical Research
and Development Command, and the U.S. Public Health Service.
The Office of Technology Development (OTD) manages invention
licensing and helps form start-up companies based on UNC-CH inventions.
For more information, contact OTD at 919/966-3929 or Campus Box 4105,
308 Bynum Hall, UNC-CH, Chapel Hill, NC 27599-410
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