"Mine, all mine!" That's what the vaccinia virus seems to be saying after it invades a cell and prevents its companions from following suit. But the behavior is hardly selfish. Researchers have found that the strategy actually helps the virus spread, as it speeds the targeting of uninfected cells. This previously unknown mechanism may lead to new ways to combat viral infections.
Vaccinia doesn't cause diseases in humans, but it has helped us fight them. Thanks to its resemblance to the smallpox virus, researchers were able to use vaccinia as a vaccine for the disease, eventually leading to its eradication in the late 1970s. Since then, scientists have continued to study vaccinia as a platform to develop vaccines against other diseases.
Despite all of this study, researchers have missed a curious feature of the virus's biology, says virologist Geoffrey Smith of Imperial College London. While looking carefully at the areas of cell destruction induced by the viruses--called plaques--he and colleagues noticed that vaccinia spread four times faster than its replication cycle would allow. "No one seemed to have done the arithmetic," Smith says.
So Smith and colleagues dug a bit deeper. They discovered that vaccinia produces two proteins right after it infects a cell. The proteins work together to form a complex on the cell's surface that prevents other vaccinia virus particles from entering (a process called superinfection). When other vaccinia viruses come knocking, long projections of another protein, actin, shoot out from the cell membrane, causing the virus to bounce off. "The novelty here, with vaccinia, is that it's not just preventing the virus superinfecting, but physically it's repelling the virus away," says Smith, whose team reports its findings today in Science.
Many other viruses, such as herpes simplex virus, appear to employ a similar infection strategy, says Smith. Assuming they utilize the same protein complex as vaccinia, he says, researchers may be able to fight these infections by blocking the interaction of the two proteins.
Cell biologist Michael Way of the London Research Institute agrees that other viruses may also use the vaccinia infection strategy. "I think it may be more general than people realize."