Koalas help solve a viral puzzle

A team of Australian and US researchers has discovered a new type of immunity, mounted by the genome, that koalas are using to fight off an AIDS-like illness endemic in parts of Australia.

KoRV-A is a virus linked to Koala Immune Deficiency Syndrome (KIDS), which leaves our furry friends vulnerable to the blood cancer leukaemia and infections such as Chlamydia, which causes blindness, pneumonia and infertility.

But KoRV-A isn’t just any type of virus. It’s a retrovirus, which means when it invades the body it hacks into the cell’s genetic machinery to make DNA in its own likeness, altering the cell’s genome in the process.

That is an impressive bit of chicanery. But KoRV-A has plenty more where that came from.

Not only does it infect the koala’s workaday somatic cells – the ones that aren’t for reproduction – it also gets into the cells that are for reproduction, the so-called “germline” cells of the testes and ovaries.

Which sets KoRV-A up for something rather special, and worrying.

In a feat of what you may want to call viral immortality, it gets passed down from one koala generation to the next as a fully ordained member of the cuddly marsupial’s genome.

Surprisingly, this is not unusual – around 8% of the human genome is made up of fragments of retroviruses that infected us down the ages.

“What we are seeing with koalas is something that every organism on the planet has gone through,” says Zhiping Weng from the University of Massachusetts in the US, a co-author on the study which appears in the journal Cell.

“Animals get infected by retroviruses that enter the germline cells. These viruses multiply and insert themselves into the germline genome, altering host genome organisation and function, and the process continues until the invader is tamed by the host,” she says. 

Thanks to the current study, we are now much closer to knowing precisely how that viral invader is tamed.

The genome, it turns out, can mount its very own type of immune response to the virus, independent of the infection-fighting cells – such as lymphocytes and antibodies – that we typically link with immunity.

The genetic material in the germ cell recognises strands of RNA that are peculiar to the virus. It can then slice and dice them into smaller sections called “sense piRNAs” which, in turn, sabotage the virus’s replication.

“What we’ve uncovered, we believe, is an ‘innate’ genome immune system that can tell a virus from one of your genes,” says co-author William Theurkauf.

“We think this is getting at how your genome says, ‘This is something we want; this is a gene.’ And, ‘That is something we don’t want; that’s a virus’,” he says.

The hope is that a better understanding of “genomic immunity” could help combat the koala retrovirus. To that end, Weng and Theurkauf are planning more research on just how the viral RNA is detected and then chopped up.

Which means the iconic Aussie tree dweller may need to step up again.

“We think we can sort that out by looking at koalas,” says Theurkauf.