Page last updated at 00:25 GMT, Friday, 22 January 2010

Cell 'surfing' aids virus spread

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Video shows the vaccinia virus spreading rapidly over a 16-hour period

Some viruses may be able to accelerate around the body by helping each other find uninfected cells to attack, scientists say.

The mechanism, caught on camera by experts at London's Imperial College, may explain the improbable speed with which some viruses spread.

The virus detects if a cell is occupied by another virus and simply "bounces" off in search of a free one.

One expert said it was a convincing and fascinating study.

The study in the journal Science says researchers used the vaccinia virus which was used to eradicate smallpox.

When a virus meets an unoccupied cell it burrows inside leaving a protein marker on the outside membrane.

It is these "occupied" signs that actually push other viruses away towards uninfected cells.

Mechanism

Viruses multiply by entering cells and hijacking the cell machinery to start copying themselves.


This fundamentally changes how we think about virus dissemination and similar strategies may very well be exploited by many viruses

Professor Geoffrey Smith, Imperial College London

When a large number of copies are made, the cell bursts and releases the new viruses to repeat the process on nearby cells.

However, scientists have always been puzzled that some viruses appear able to spread very rapidly, even though their rate of replication does not match that speed.

The Imperial College researchers, led by Professor Geoffrey Smith, think they may have found a neat trick used by some viruses to spread more efficiently.

If more than one virus enters the cell, it would be a waste, as only one can replicate once inside, and many viruses have ways to prevent this "superinfection".

'Surfing'

However, the vaccinia poxvirus does more - on entering the cell, it leaves two viral proteins on the cell surface.

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Snake-like "actins" bounce other viruses away from an infected cell

If another virus approached, these proteins would trigger it to push out snake-like projections called "actins" from its surface, actively bouncing it away to "surf" towards uninfected cells further afield.

Prof Smith said: "This effectively says to additional virus particles trying to infect the cell 'I'm infected already, there is no point coming here, you need to go elsewhere' - and remarkably the virus particles are physically repelled until they find an uninfected cell."

He said that other viruses might share this feature, and that the discovery might lead to strategies to slow the spread of viruses, limiting the ability of an infection to cause illness.

"This fundamentally changes how we think about virus dissemination and similar strategies may very well be exploited by many viruses," said Prof Smith.

Dr Stacey Efstathiou, a virologist from the University of Cambridge, said that other viruses, such as herpes viruses, also spread unusually quickly, and could well be employing a similar method.

He said: "The novelty here is the 'bounce', and this study is both very convincing, and fascinating.

"In theory, if you could block this mechanism of onward propulsion, you could end up severely attenuating the ability of the virus to spread."



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