Viruses 'can spread faster than previously thought'
22 January 2010
Viruses are thought to spread in the body by entering a cell and replicating before being released to infect new cells. The speed at which a virus can spread would be limited by how quickly it can reproduce in each cell.
Now, researchers from Imperial College London have found that vaccinia virus - the vaccine used to eradicate smallpox - spreads in a different and much faster way.
The scientists observed the virus producing two proteins on a cell's surface soon after it had infected the cell. When other virus particles reach the infected cell, these proteins cause the host cell to push out tentacle-like projections that drive the virus particles away and towards other cells that they can infect. The particles thus bounce from one cell surface to another until they land on an uninfected cell.
"The cell expressing the two virus proteins effectively says to other virus particles, 'I'm infected already, there is no point coming here.' And, remarkably, the virus particles are physically repelled until they find an uninfected cell. Thus the virus can spread quickly to distant uninfected cells without needing to replicate in each cell on the way," said Professor Geoffrey L Smith, a Wellcome Trust Principal Research Fellow at Imperial College London and lead author on the study.
Using live video microscopy, the researchers found that vaccinia could spread four times quicker than previously thought possible. They were also able to slow the spread of the virus by preventing it from making the proteins needed for the projections at the early stage of infection.
Video: Vaccinia virus infection spreading across cells. Recorded by phase microscopy over 16 hours. Note virus replication takes 5-6 hours per cell, but the infection spreads across each cell in only 1.2 hours. Running time: 6 s.
Video: Green virus particles moving on the tip of red actin tails. Note that a virus-tipped red actin tail produced by this cell induces the formation of another actin tail after re-contacting the cell surface (white box). Running time: 4 s.
The researchers believe that other viruses use similar mechanisms. For example, herpes simplex virus (HSV-1) - which causes cold sores - spreads at a faster rate than should be possible given its rate of replication. Targeting this mechanism could lead to a new source of antiviral drugs.
The study was funded by the Medical Research Council.
Image: An electron micrograph of Vaccinia virions; a virion is a single complete and infectious virus particle. Credit: CDC
Reference
Doceul V et al. Repulsion of superinfecting virions: a mechanism for rapid virus spread. Science 2010 [Epub ahead of print].
