Bats are a natural host of over 100 diseases, many of which are fatal to humans and other animals. But despite carrying diseases such as Ebola, Hendra virus, and MERS, bats do not get sick or show signs of disease.
In a study published this week in Proceedings of the National Academy of Sciences (PNAS), researchers have thoroughly examined the immune system of the Australian black flying fox (Pteropus alecto).
The international research team led by CSIRO found that while in most species, including humans, an immune response is switched ‘on’ following infection, bats’ immune systems are switched on all the time.
“Whenever our body encounters a foreign organism, like bacteria or a virus, a complicated set of immune responses are set in motion – one of which is the defence mechanism known as innate immunity,” says leading bat immunologist at CSIRO’s Australian Animal Health Laboratory, Dr Michelle Baker.
The researchers focused on the innate immune systems of bats, particularly the role of interferons.
Interferons work like hormones of the immune system, and help protect the body from viral infections by ‘interfering’ with viral replication. They play an important role in the innate immune system – the first line of defence against viruses – and also activate other immune cells, leading to a targeted immune response.
While humans have 14 interferon genes, the bat genome – sequenced in 2013 – revealed only four genes.
“That was really unexpected. We thought we’d find a large family of these genes, considering bats are so good at controlling viruses, and [that] this is a really important family in antiviral immunity,” Baker says.
But thanks to their highly efficient interferons, bats are protected from succumbing to the diseases they carry, such as Ebola – a trait that likely developed through evolution.
“They’re just such gregarious mammals, passing viruses to each other constantly,” says Baker. “They’ve had to co-evolve to come to some equilibrium between the host response and the virus, so that they can both coexist.”
“Bats have found a way of dealing with a whole range of pathogens using the same mechanism, which clearly humans don’t have,” says Director of the Centre for Animal Biotechnology at Melbourne University, Professor Jean-Pierre Scheerlinck, who was not involved in the research. “We have to deal with each pathogen individually, and that’s why we respond so differently to each one, and have different clinical outcomes.”
While this research won’t lead directly to a targeted therapy or new treatment just yet, it helps us better understand interferon genes and the innate immune response.
In other mammals, high levels of interferon are damaging, as it’s toxic to cells and tissues. In humans, high levels of interferons are associated with fever, muscle pains, dizziness and fatigue – they are responsible for these ‘flu-like symptoms’ we often experience at the start of a viral infection.
Understanding how bats can withstand constant interferons and avoid the toxic, inflammatory side effects could lead to new therapies.
“People should be learning a lot more from bats, I think. They’re just amazing,” says Baker, admitting she is “a bit biased”.
“Because we’re encroaching on the habitats of these animals more and more, if we can learn what they’re doing, it might help us predict spill-over of viruses, and we can try and develop some new therapeutics based on what they’re doing.”