Ancient viruses buried in our genomes help to fight off new ones

Ancient viral DNA could be helping our immune system fight off infections from modern viruses and other pathogens.

A new study in Science suggests that ancient viruses that integrated into our genomes millions of years ago, called endogenous retroviruses (ERVs), could have been harnessed during the evolution of our immune systems.

'Many viruses originally entered our genomes as part of the process of viral replication. The evolutionary process turned the tables to our benefit,' said study co-author Dr Nels Elde, assistant professor of human genetics at the University of Utah in Salt Lake City.

ERVs are 'viral fossils' – DNA sequences that integrated into our genome in our evolutionary past and have lost the ability to produce new viral particles and be infectious. They comprise 5-8 percent of the human genome and for a long time were thought to have no function – or even to have a negative effect.

The research team analysed the human genome looking for ERVs. They saw that many of these were found close to genes that are important for the innate immune response, which is the body's first line of defence against pathogens. The researchers wondered if they might play a role in triggering the activation of the immune system and therefore be important in fighting off infections – including viruses, bacteria and other pathogens.

'These were the first signs to us that some of these elements may be truly involved in switching on immunity genes,' said Dr Cédric Feschotte, the study's other co-author, also at the University of Utah.

In order to confirm the role of the ERVs, the researchers grew human cells in the laboratory and used the CRISPR/Cas9 genome-editing technique to remove an ERV sequence that was found close to an immunity gene. When they tried to trigger the innate immune response in these cells, the immunity gene did not turn on.

The researchers also found that the immune response was only partially activated when they infected the cells with a virus similar to smallpox. The researchers then re-introduced the ERV sequence into the cells and confirmed that the normal immune response was restored.

'We show that some of these endogenous viruses have shaped our biology,' said Dr Feschotte. 'Within mammalian genomes are reservoirs of viral DNA that have fuelled innovation of the innate immune system.'

Scientists will next investigate whether endogenous retroviruses play a similar role in the immune systems of other animals. 'Do they contribute on a major scale or are there just a few small cases? The question is whether it opens new vistas,' Dr Jonathan Stoye, a retrovirologist at the Francis Crick Institute in London, told Wired.