Study shows how smallpox virus tricks healthy cells
By Michael Kahn
LONDON (Reuters) - A virus closely related to smallpox disguises itself as a piece of a broken cell to trick its way into cells, Swiss researchers said on Thursday in a discovery that could lead to better drugs and vaccines.
The vaccinia virus tricks scavenging immune system cells into devouring it, and can invade the body from there, said Ari Helenius, who led the study published in the journal Science.
Vaccinia is used to make smallpox vaccines and as a research model for the more dangerous variola virus that causes smallpox, said Helenius, a biochemist at the Swiss Federal Institute of Technology.
"The virus is really fooling the cell," he said in a telephone interview. "It is deposited outside the city wall and for the cell to get invaded by the virus what must happen is for the Trojans to bring the virus in."
There are other pox viruses such as monkeypox spread by animals but researchers are focused on smallpox because of fears it could be used as a biological weapon.
A 2003 outbreak of monkeypox in the United States that sickened a family exposed to an infected prairie dog highlights the threat from other strains as well, Helenius said.
Viruses have devised many ways of entering cells, Helenius said. Most do so by binding to a cell and turning on a chemical signal that causes the cell to absorb the virus.
Pox viruses, however, are 10 times the size of most other viruses and far more complex. This means they have to find another way in to healthy cells, Helenius said.
The virus achieves this by mimicking a piece of a broken cell to trick immune system cells called macrophages into picking them up and disposing of them as part of the normal process when cells die, the study found.
"What we are finding is the virus is disguised as a piece of garbage with the same 'eat me' signal," Helenius said. "Instead of getting degraded the virus escapes."
The finding provides targets for drugs that would seek to block the healthy cells from bringing in the virus rather than targeting the virus itself, Helenius added.
"Drugs are clearly what we are after," he said. "You can either target the virus or the trojans involved."
Smallpox was eradicated in 1979 but many experts fear that samples of the virus, which when it was common killed 30 percent of its victims, were developed into biological weapons in countries such as Iraq and of the former Soviet Union.
Because of this the U.S. government began stockpiling smallpox vaccine after the September 11 attacks. On Wednesday, British company Acambis said it had won a 10-year, $425 million contract with the U.S. government to supply smallpox vaccines.
Smallpox is so dangerous because as an airborne disease it spreads easily and can kill within weeks, Helenius added.
"The main reason there is so much work on this virus is the threat of bioterrorism," he said. "Smallpox is one of the worst plagues humans have ever had."
(Reporting by Michael Kahn; Editing by Maggie Fox)