On the science show This Week in Virology we receive many questions and comments, which are read every week. I also get many questions here on virology blog, which I tend to answer by email. However I think that everyone could benefit from these questions, so I’ve decided to post one here each week along with my answer.
This week’s question is from Joseph, who wrote:
I’m relatively new to virology or anything biology-related. Hell, I’m studying computer science as an undergrad at the moment; however, there’s something about virology that fascinates me – the simplistic fact that we can’t cure viruses, which are less complex than bacterium (in which we can treat, and they’ll eventually pack their bags and leave).
I’ll get to my question … since most, if not all, cells in the body replicate and reproduce and none of them merge, why do our cells let virions in? You would think after years of viral/immune system encounters, our bodies would have adapted to repelling these viruses off. I understand it’s probably much more complicated than that, but I would love to hear your answer. Does it have anything to do with virions’ size being so small?
This is a great question. In fact, I had a similar question on a midterm examination in my virology course. I phrased it this way: Could cells evolve to not have receptors for binding viruses?
I sent this answer to Joseph:
Viruses get into cells by binding to proteins on the cell surface – viruses have evolved to do this: they are safecrackers.
You would think that the cells would evolve to change these proteins – and you would be right. Over thousands of years, the cell proteins change, so the viruses can’t bind anymore.
But guess what? The viruses change right back so that they can bind to the cell protein once more.
Now you might ask: why doesn’t the cell get rid of that surface protein? The answer there is that they are needed for the cell, so they can’t be removed.
There seems to be one exception to the last statement: about 4-16% of people of Northern European descent don’t make one of the receptors for HIV. They are resistant to infection. But this doesn’t happen for most other viruses.
Joseph wrote back:
Hmm. I thought by definition virions weren’t living organisms, yet they “adapt” to bind to living cells. Sounds like those emotional virions just can’t deal with rejection – that and our cells just aren’t as smart as we need them to be. I’m not sure if you are a Trekkie; however, it reminds me of the Borg and The Enterprise’s encounter – The Enterprise adapting to The Borg’s every frequency of their phasers, bypassing their bruteforce.
That does make sense that our cells do need that protein surface for energy; however, I never thought it would actually be the surface itself. Interesting.
I did read about that somewhere – because of the Bubonic Plague causing some genetic mutation, if I’m not mistaken.
To which I responded:
Virus particles are not alive – but once they infect a living cell they can evolve.
Both cells and viruses are smart – they both have managed to be around for a long time. We have great immune systems; virus infected cells can evolve very quickly. It’s an arms race.
Correct, one idea is that the mutation conferring resistance to HIV was acquired in the Plague, but that’s hard to prove.
The mutation we are discussing is of course ccr5delta32, which confers resistance to infection with HIV-1 (the illustration shows the HIV-1 glycoprotein binding CD4 and ccr, a chemokine receptor). You can read more about ccr5delta32 here or listen to us discuss it on TWiV #278. We also talked about virus-receptor arms races on TWiV #242, and I wrote about it here.