Secret of smallpox's success may lead to bioterror cure
By Arizona State University
May 22, 2005, 14:33
In disease, as in war, offensive strategies can become weaknesses, if the defenders see the enemy coming and compensate for its weapons.
By manipulating what is perhaps the most devastating trick in cellular weaponry of pox viruses like smallpox, Arizona State University virologist and Biodesign Institute researcher Bertram Jacobs believes that he can turn the biochemical machinery of the pox viruses against themselves – and protect the public against catastrophic bioterror attacks.
And if he is correct, Jacobs may not only be able to create a vaccine that can cure smallpox infections in their early stages, but he may also have a powerful tool for fighting a host of other viral pathogens, including HIV.
Jacobs has received a $1 million grant from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH), to develop an effective post-exposure vaccine for smallpox. The research is one of ten projects funded under Project Bioshield, which gives federal agencies new tools to accelerate research on medical countermeasures to safeguard Americans against chemical, biological, radiological or nuclear attack.
The idea for the vaccine comes from Jacobs and his team's discovery of a gene that gives pox viruses uncanny ability to camouflage themselves from mammalian immune systems. The ability to stay hidden allows the smallpox virus, in particular, time to grow and multiply to the point of causing devastating disease before the immune system detects it and attempts to mount a counter-attack.
Jacobs notes that there have been clues to this property of the virus present in historical results from using vaccinia, or cowpox, to immunize against smallpox. In most people, exposure to vaccinia generally only causes a mild infection in humans before the immune system responds and eliminates the virus, also leaving immunity to smallpox and other pox viruses in its wake.
"And there has been anecdotal evidence that if you get exposed to smallpox and get vaccinated within 4 days of exposure, the vaccine will protect you," Jacobs said. The immune system is apparently better able to detect the vaccinia virus than it is smallpox, and thus vaccinia alerts the immune system in time to counteract the smallpox infection.
Jacobs' idea for a post-exposure vaccine is to strengthen the immune response to the vaccinia virus by completely eliminating the viral gene that allows pox viruses to hide. The more visible the vaccinia virus is to the immune system, the stronger the immune response should be and the better the body should be able to fight off a smallpox infection.
Over the past 15 years, Jacobs has developed a variety of mutant vaccinia viruses that lack the camouflage gene, and have proved harmless to mice that otherwise get sick from vaccinia. Currently, he is testing these strains for their ability to protect mice that have already been exposed to a particularly lethal strain of vaccinia, or to mouse pox, another pox virus.
"We want to look at all of our mutants and see which ones work best in this post-exposure prophylaxis," explained Jacobs. Though results are preliminary, several strains look promising.
Jacobs' experimental results verify that the de-cloaked viruses not only do not cause disease, but improve disease resistance when they are present. When a mouse has been exposed to a lethal vaccinia virus and then is vaccinated with a very low dose of the mutant virus – only 100 virus particles – the mouse gets sick but eventually recovers. The more mutant virus the mice are given, the less sick they become.
"If you give them a million particles, they don't even get sick -- the animals stay healthy," notes Jacobs.
"The body reacts and it fights off both the mutant, and also the normal virus that normally would hide itself," said Jacobs. "The more you put in, the more likely it is for the immune system to say 'Hey, wait a minute! There is something going on here! I need to start fighting a virus infection!' and so it fights the mutant virus and a normal pathogenic virus as well."
The gene that Jacobs has isolated and eliminated from the mutants is a special adaptation of pox viruses that appears to help the viruses hide double-stranded RNA, a form of RNA that only viruses produce and that animal immune systems have learned to recognize.
"It's what we call a 'pathogen associated molecular pattern' – a biomarker that is only present in your body when you are infected with a virus and that the body knows to look for and respond to," said Jacobs. "The body responds in a lot of different ways, but the main way is it starts producing an immune response – double-stranded RNA is very powerful in eliciting this.
"What this gene in pox viruses does is make a protein that binds up all the double-stranded RNA like a little sponge, and this is how the viruses hide infection from your cells," he explained. "Take the gene out and now we have a vaccinia virus that can no longer hide itself – it can't camouflage its signature anymore from the cells in its body."
Once the virus calls attention to itself, the immune system begins a complex immune response, including developing antigens that specifically target the virus. Because the virus's double-stranded RNA signal is very clear, the immune response tends to be very strong when the quantity of the uncloaked virus is high.
According to Jacobs, the uncloaked vaccinia virus has the potential to be useful for developing novel post-exposure vaccines for other dangerous viruses as well, making available a who new arsenal of weapons against disease. Antigen-eliciting genes from hard-to-treat viruses such as HIV could potentially be added to the mutant vaccinia virus, which would draw the immune system's attention to the target virus's proteins, thus creating a strong immune response to the target virus. Though perhaps controversial, such a method could ultimately provide effective protection from some of humankind's most challenging viral enemies.
Jacob's current attempt to develop a post-exposure smallpox vaccine is also not with its challenges. Because of the extreme dangerousness of the smallpox virus, the effectiveness of the vaccine cannot actually be tested through clinical trials on humans. Instead, the vaccine is tested on mice and other animal models, using viruses closely related to smallpox that are lethal to those animals.
"We're testing with vaccinia and mousepox in lab mice, and in dormice with monkeypox. These are good model systems – if we can protect against mousepox in mice, then we have a high level of certainty that the vaccine will be able to protect against smallpox in humans," Jacobs said.
"Since you can't test in people, the FDA's animal efficacy rule applies. If you can show efficacy in several animal models, then you can presume efficacy in humans, if it is for a disease that you can't ethically expose people to in trial. The vaccine will go through safety studies in humans, though."
Because of the importance of the research for national security, Jacob's work is on a fast-track, with the studies projected to yield a usable vaccine virus within eighteen months. At that point, the virus will go to a commercial manufacturer, who will be responsible for developing and then producing a complete nasal-delivery vaccine. If all is successful, a large number of doses will then be produced and put into storage for possible use in case of emergency, in accordance with the procedures of Project Bioshield.
Though the final product may take a while to be produced and, one hopes, may never have to be used, Jacobs has a feeling now of years of research coming to fruition.
"This is the result of work that we have been doing for 15 years now and I think we are really very close to producing some really important findings, especially now that we are engaged in these fast-track projects," Jacobs said. "It's really exciting."
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