Bats provide a refuge for some of the most lethal viruses known, including Ebola, Marburg, Nipah and SARS. Now we may know why the animals tolerate these lethal viruses – and it’s because flying is such hard work.
Peng Zhou of the Wuhan Institution of Virology in China and his colleagues studied the immune systems of bats and flightless mammals.
They focused on free-floating DNA within cells. This can happen as the result of a viral infection, as the viruses hijack the cells’ DNA replication apparatus to copy their own genetic material. But it can also happen during strenuous exercise, which creates chemicals called free radicals that build up in cells and damage the DNA, releasing fragments of it.
Most mammals don’t have to perform hugely strenuous exercise, so their own DNA rarely leaks out into their cells. As a result, if their immune system detects any free DNA, it interprets it as an emerging viral threat and begins fighting back. The trigger for action is a sensor molecule called STING, which swamps the viral infection with antiviral substances called interferons.
However, bats fly and this is extremely strenuous, so their DNA often does leak out. This could lead the bat’s immune system to mistakenly attack the animal’s own tissues. To avoid this, bats appear to have evolved milder reactions to viral infections, allowing the bats and the viruses to tolerate each other.
Zhou’s team mimicked infections in the white blood cells of mice and of Chinese rufous horseshoe bats (Rhinolophus sinicus): the species that harboured the SARS virus, which killed almost 600 and infected approaching 7700 during the 2003 outbreak. The mouse cells produced at least 10 times more interferon.
They compared the gene for STING in 30 bat species and 10 flightless mammal species, including humans. In all the bats, STING had lost the amino acid serine at one site, but in all the other mammals STING had kept it. The presence or absence of serine at that site dictated how the cells responded to fake viral infections. By losing the serine, bats tolerate viruses that other mammals would fight off.
“Wild bats may carry viruses for a long time at a low level, less like control and more like coexistence,” says Zhou. “A milder response to viral infection is not always a bad thing.”
“Because some of these viruses may potentially lead to new global pandemics, it’s essential we begin to learn how the bats remain well and unaffected,” says John Mackenzie at Curtin University in Perth, Western Australia.