The Spread of a Virus
Chris - Now, we're turning to some of the tiniest organisms that are known to exist and those are viruses that have been very well studied over the last 50 years, so it's something of a surprise that no one had noticed that some viruses seem to be able to spread far faster than they ought to be able to. And now, Cambridge virologist Geoff Smith and his team have discovered why.
Geoff - We study pox viruses and the virus that we've used most in all that is called vaccinia virus and that was the virus that was used to eradicate smallpox from the whole world. One of the things that we started to do a few years ago was to look at how the virus spreads and how it seems to do so, so very quickly. And so, we just made some very simple measurements of the rate at which the virus would spread from one cell to another, across a lawn of cells that were just growing in the lab. So what we found was, it was spreading across each cell in just over one hour and when we got that measurement, we really stopped and scratched our heads because it didn't make any sense because we knew that the virus takes at least 5 hours to replicate in each cell. So before you can make any new virus particles after you've infected a cell, you have to wait 5 to 6 hours before the very first particles are made.
Chris - But in your experiments, it appeared that the virus was spreading faster than it was being made.
Geoff - That's right. The numbers didn't add up. The virus was spreading about 4 or 5-fold faster than could be predicted from what was known about its replication rate.
Chris - So how did you take that forward? Because that's obviously a really fundamental and important finding? It could either be totally wrong or there's something very ingenious going on.
Geoff - Well up to the time we did our study, it was thought that the viruses were only using the host cell transport machinery to accelerate the rate at which newly formed virus particles were being released from the cell in which they had been made.
Chris - So you make the virus in the cell then you use the cell's machinery to expel that virus onto an adjacent uninfected cell and that's why you get the spread?
Geoff - Yeah, there are two parts to the virus hijacking the cell biology to accelerate spread. The first is, that it uses a network of tubules called microtubules inside the cell which are the cell's normal transport machinery for moving cargo around inside the cell, and the virus hooks up to this transport system and exploits it to get the newly formed virus particles out to the cell surface rapidly. So that's the first part, and then once the virus gets to the cell surface, it then will induce the formation of a protein in the cell called actin which is important for giving the cell its structure and it induces the polymerization of actin so that these growing actin filaments will physically propel the virus away from the cell's surface towards uninfected cells that it can subsequently infect.
Chris - The virus comes out from the cell on almost like a stalk towards adjacent cells?
Geoff - That's right and then you can see these by videos. If you label up the virus in a way for instance by fusing it to a fluorescent protein, you can follow the virus moving around inside cells and between cells by live video microscopy. They make very good pictures.
Chris - I'm sure they do. So that much was known and that doesn't actually explain how you could marry up the observation you made, the viruses are spreading much more quickly than they should with actually what was going on then.
Geoff - That's correct because the ability of the virus to exploit actin to push the virus particle away from the cell in which it had been made only happens very late in infection, about 5 or 6 hours or later. So that is too late to explain the very rapid spread from cell to cell, so there had to be another mechanism.
By looking more deeply into what the virus was doing, we found that once the virus had left the original cell in which it had replicated, and it had then entered the adjacent cell, shortly after that cell was infected, the virus expressed two proteins which we knew formed a complex and that complex is transported to the cell's surface which essentially marked that cell as being infected already. And the consequence of that was that when additional viral particles trying to re-infect or superinfect that same cell came into contact with that complex on the cell's surface, that induced the formation of additional actin tails which repulsed the superinfecting virions and pushed them away towards uninfected cells.
Chris - Gosh! So it's almost like a virus comes in, lands on a cell that has already been infected but isn't yet making virus particles, can detect that the cell is infected and then bounces off in the direction of other potentially infectable, but so far uninfected, cells.
Geoff - That's exactly right and it's almost as if the virus infection is marking the newly infected cell as infeicted and saying to the additional viral particles, "Go away! There's no point comng here. I'm infected already. You need to go elsewhere." And in fact, this is a common theme in virology. Many viruses have mechanisms by which they restrict the re-infection of an already infected cell. But what was different in this study was that the virus didn't just stop the virus particles re-entering the same cell. They physically repelled them. So as you say, they bounced, or surfed, across the surface of the infected cell until they would come to a cell that was uninfected which there was no restriction upon them infecting.
Chris - Given that this gives the virus an advantage and you now know how it does it, disabling it could therefore be quite a good target to treat these viruses with drugs.
Geoff - Indeed so. We know for instance that if we remove the genes which encode the proteins that enable this rapid spreading mechanism to take place, number one, the virus spreads much more slowly in cultured cells and it is also a virulent. It can no longer induce disease. So that one could build attenuated vaccines perhaps in this way - that's one angle. A second one would be to design molecules which would interfere with this spreading mechanism. And in fact, you could take the parts of the proteins which mediate this mechanism themselves. And if we were to produce a large amount of that protein in soluble form, that would compete with the receptor bound on the cell's surface and so, prevent the rapid spread that the cell bound protein would otherwise induce.