Previously, viruses were thought to spread by entering a cell, replicating there, and then being released to infect new cells, so that the rate of spread of a virus would be limited by how quickly it could replicate in each cell. However, a virus called vaccinia spreads in a different and much faster way, according to a new study. Using live video microscopy, the scientists discovered that the poxvirus vaccinia was spreading four times more quickly than thought possible, based on the rate at which it replicates.
Videos of virus-infected cells revealed that the virus spreads by surfing from cell to cell, using a mechanism that allows it to bounce past cells that are already infected and reach uninfected cells as quickly as possible. Early after vaccinia infects a cell, it expresses two viral proteins on the cell surface, which marks the cell as infected. When further virus particles reach the infected cell, these proteins cause the host cell to push out snake-like projections called "actin tails," which drive the virus particles away towards other cells that they can infect. The particles thus bounce from one cell surface to another until they land on an uninfected cell.
Lead study author Professor Geoffrey L. Smith explains: "Shortly after infection vaccinia expresses two virus proteins on the cell surface that mark the cell as infected. 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. Thus the virus can spread quickly to distant uninfected cells without needing to replicate in each cell on the way.”
In the study, the researchers prevented the virus from making the proteins needed to make the actin tails in the early stages of infecting a cell and showed that this slowed the spread of the virus. The researchers believe that other viruses also employ rapid spreading mechanisms. For instance, herpes simplex virus spreads at a faster rate than should be possible given its replication rate. The discovery may enable scientists to create new antiviral drugs that target this spreading mechanism.
MEDICA.de; Source: Imperial College London