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Credit: JJ Harrison

Disrupting the Stealth Mode of a Contagious Cancer

You cannot catch cancers. For all their terrifying and destructive properties, tumours aren’t contagious. Unlike, say, a viral infection, you cannot transmit cancer to someone else by coughing in their face, or having sex with them, or sharing the same contaminated water. *

But while this is true for humans, it’s not for Tasmanian devils. These scruffy black-and-white Australian mammals are being slowly wiped out by a contagious cancer, which can jump from the face of one devil to another as they squabble and bite each other over food. This parasite started off as a Schwann cell – a type of cell that wraps around neurons outside of the brain. In a single unfortunate devil, this cell turned into a tumour. And while almost all tumours go down with their hosts, this one somehow evolved the ability to jump into another. It became an independent, free-living parasite.

This infectious cancer, known as devil facial tumour disease (DFTD), invariably kills the devils it infects. It causes such large disfiguring tumours that its hosts can no longer eat properly. First documented in 1996, it has since spread to the majority of the devils and threatens to wipe them out entirely within a few decades. Caught off-guard, a small team of scientists is now racing to find ways of saving the devils and to understand DFTD. (Carl Zimmer has a great round-up of their work at the New York Times.)

How could a single cancer spread so effectively among the Tasmanian devils? By right, the immune system of any new host should be able to recognise and kill these invading cells. The people working on the devils had always assumed that their narrow genetic diversity had inadvertently given the tumour a free pass. Ages ago, the devil population crashed, sending them through a ‘genetic bottleneck’. That is, all the living devils are meant to be extremely close relatives. Their immune systems won’t kill DFTD cells because they can’t tell the difference between these invaders and their own tissues.

But that explanation is wrong. The devils actually have plenty of genetic diversity, and their immune systems will reject skin grafts from a different individual. So how does the tumour sneak past?

Hannah Siddle from the University of Cambridge has the answer. Like us, a devil’s cells display proteins called MHC on their surface, which help the immune system to recognise threats like tumours or virus-infected cells. Siddle expected to see that DFTD displays very similar MHC molecules to those of the various devils. She actually found that they don’t display MHC at all.

This isn’t because the genes for these protein have been broken through mutations. Instead, MHC molecules need an entourage of other proteins that escort it from factories within the cell to their posts on the surface. And DFTD cells don’t make any of these escorts. They have a cloaking device that renders them invisible to the immune system.

But Siddle found that they can be persuaded to. When she treated the cancerous cells with an immune chemical called interferon-gamma, they started displaying MHC. That’s big news, and the team is now working to develop a vaccine using this discovery. The idea is to expose healthy devils to DFTD cells that have been treated to  expose their hidden MHC molecules. This would train the immune system to recognise these threats and mount a vigorous defence should the devils ever become infected. It’s an early step, but a hopeful one.

Reference: Siddle et al, 2013. PNAS.

* You can transmit certain viruses and bacteria that can cause cancer, such as HPV, but that’s not the same as saying that the tumours themselves are contagious.