Antiviral Immunity – a gift from viruses?

Anyone who’s ever taken an evolutionary biology class has probably heard the famous Dobzhansky quote, “Nothing in Biology Makes Sense Except in the Light of Evolution”. It’s actually the title of an essay he wrote promoting evolution by natural selection as the only way to make sense of both the unity and the diversity of life.

Many years ago, discovering this quote as a graduate student simultaneously studying virology and evolutionary biology, I wondered how viruses fit in to evolutionary theory. At the time, I was particularly curious about when the first viruses originated, and if they could have come before the first cells. I ended up assembling my thoughts on this subject into a scholarly paper, a requirement for my non-thesis masters degree.

Since then, more has been published on the subject of virus-host evolution. This includes the paper which inspired this post, “Viral Ancestors of Antiviral Systems”, published in Viruses this month. In this review article, Luis Villarreal of UC Irvine argues that early antiviral defenses may have been provided by viruses.

Why would a virus want to improve host defenses against itself? Since a virus depends upon its host, it is desirable for a host to protect itself well enough to survive viral infection without completely eliminating virus. Since viruses evolve a lot faster than their hosts – up to a million times faster – they could easily kill the host if that was their goal. But once the host dies, the virus dies with it. So in the light of evolution, or rather in the light of avoiding extinction, it makes sense for a virus to do what it takes to keep its host alive.

Now the question is: What can a virus do to help keep its host alive? Villarreal suggests several possible mechanisms, including the following:

• Horizontal transfer of restriction-modification sets from phage to bacteria.
• Expression of siRNA from LTR regions of endogenous retroviruses and transposons in eukaryotes.
• Viral reprogramming of cells for participation in adaptive immunity.

The mechanisms presented here are merely attempts to make sense of various host-virus interactions, and are yet to be proven. However, they present interesting possibilities to explore.

Citations:

Villarreal, L. (2011). Viral Ancestors of Antiviral Systems Viruses, 3 (10), 1933-1958 DOI: 10.3390/v3101933