Viruses use 'Hive Intelligence' to spread Infection

A tactic familiar from insect behaviour seems to give viruses the edge in the eternal battle between them and their host – and the remarkable proof can be seen in a video.

The video catches viruses only a few hundred nanometres in size in the act of hopping over cells that are already infected. This allows them to concentrate their energies on previously uninfected cells, accelerating the spread of infection fivefold.

Geoffrey Smith and his team of virologists at Imperial College London were curious about the vaccinia virus, and set up a video microscope to watch how the virus spreads through cells.

Vaccinia and Smallpox
Vaccinia was used in the vaccine that rid the world of smallpox some 35 years ago. It doesn't cause disease in humans or any other animal, and its origin is unknown.

Spreading Infection
The traditional idea of how viruses spread goes like this. A virus first enters a cell and hijacks its machinery to make its own viral proteins and replicate. Thousands of replicated viruses then spread to neighbouring cells to wreak havoc.

When Smith watched the vaccinia virus infecting monkey liver cells, he thought that it was spreading far too quickly. "It takes 5 to 6 hours for the virus to replicate, but it was spreading from cell to cell within 1 or 2 hours," he says.

Spread of Vaccinia
Vaccinia is known to spread from cell to cell in a characteristic way. After attaching to the cell membrane of its target, it releases a protein that enters the cell, where it communicates with actin – a protein that helps maintain the cell's structure.

The actin responds by growing longer, and then attaches itself to the virus, still sitting on the surface of the cell, as a so-called "actin tail". This tail helps the virus take off from the cell and find the next victim.

Marking the Virus
Smith's team labelled the virus with green fluorescent protein, and labelled some – but not all – cells with a red marker that tagged the actin. They found, to their amazement, that a virus leaving a cell would travel to another cell and merely bounce off it if it already contained the virus.

Virus Changes
The researchers could tell that a single virus had travelled over more than one cell because some viruses which left a cell with an uncoloured actin tail picked up a red actin tail from another cell. "This means that the viruses can change their actin tails as they bounce along the surfaces of cells," says Smith. "This allows the virus to reach distant cells really quickly."

Smith reckons that two viral proteins which are presented on the surface of the infected cell effectively tell the virus not to bother reinfecting that cell. When he looked at virus strains lacking each of these proteins, the virus spread at the slower rate that would expected without the "bouncing infection" mechanism.

"It's as if the proteins are telling the virus: 'Hey guys, there's no point in coming in here'," says Smith. "If you think about it, it makes sense – it's very Darwinian."

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