Mice reveal key to locking up cancers

22 November 2007

Today's cancer treatments aim to kill tumours. But now scientists have discovered that mice keep cancer at bay using their own immune system. Could the same trick work for humans? Antenna investigates...

This research was published in the journal Nature on 18 November 2007.

Could mice teach us how to live with cancer?

Image: iStockphoto/Brandon Laufenberg

A third of us develop cancer at some point in our lives. Currently, the treatment strategy is clear: find the cells quickly and hit them hard with powerful treatments such as radiation and chemotherapy.
But now scientists have shown that mice can use their own immune system to keep cancers locked up and out of trouble for long periods of time. And doctors hope the findings could help them control nasty tumours in people.

The cancer was held in check by the mice's immune systems - a collection of cells like these that fight infection and diseases.

Image: iStockphoto/Andrei Tchernov

Scientists have long suspected that cancer can lie dormant for years. Sometimes, patients who seem to recover from cancer find it reappears decades later for no apparent reason. But until now, doctors had no idea what had been keeping the disease hidden away, or why it resurfaced.

In one case, doctors found that a woman had inactive cancer cells in her kidneys for 16 years. They only found out when her kidneys were transplanted to other patients, who both went on to develop active cancer.

80% of the mice didn't seem to develop cancer straight away - but were they hiding a secret?

mage: Ikayama

To find out more about these hidden tumours, the researchers tested a cancer-causing chemical on mice. Some of the animals developed active cancer straight away, but the scientists were more interested in looking at the ones that didn't.
Robert Schreiber led the research at Washington University School of Medicine: 'Most of the mice didn't seem to have cancer - but we wanted to know whether they had hidden tumours that were kept under control somehow.
'When we disabled the mice's immune systems, around half developed active cancer within a few weeks. This suggests that these animals had had cancer all along - but that their immune systems had been keeping it under control,' says Robert.

Professor Robert Schreiber and his team found hidden cancers in half of the mice.

Image: Washington University School of Medicine

So what keeps cancer quiet?
Robert explains: 'The cancer and immune system are locked in a long-term fight - the tumour cells are trying to grow, and the immune cells are keeping them in check. So it may seem like the cancer has disappeared, but really there's a lot happening to keep it under control.'

In these two slides, immune cells are shown in blue, and cancer cells in pink. When the immune system is knocked out (in the slide on the right) the cancer cells quickly take over.

Image: Washington University School of Medicine

There are many reasons that older people are more susceptible to cancer - but a weakening immune system could also contribute.

This could help explain why some people develop cancer in old age. Our immune systems weaken as we grow older, and the researchers suggest that this could be letting cancer cells gain the edge.

So could this be the beginning for new ways to treat cancer?
The researchers are hopeful, but cautious. Robert explains: 'These studies are in mice, and only look at one kind of cancer, so we'll need to make sure that they also apply to humans and for other types of cancer too.'

The Washington team and other scientists will continue to test whether all cancers act in this way.

Image: Washington University School of Medicine

'But we now have the potential to harness a patient's immune system to control cancers for a long time. If we succeed, cancer might be treated like diabetes - a serious illness, but one that you could control with drugs for the rest of your life,' says Robert.

Cancer expert Karol Sikora is impressed: 'We've long suspected that cancer and the immune system are linked, but until now we've only been able to take snapshots of what's going on. It's like we were trying to understand a complex ballet dance by looking at still photos.
'Research like this helps us finally get a sense of the movement - understanding which cells are involved and how they interact. That will really help us develop new ways of treating the disease.'

Karol Sikora is Professor of Cancer Medicine at Hammersmith Hospital.

Image: Cancer Partners UK

But the scientists say their work has also exposed something more sinister. Robert Schreiber explains: 'At the moment, we test a lot of chemicals on mice to make sure they won't cause cancer in humans.
'But these tests only look for active cancer - they wouldn't spot these hidden cancers being kept in check by the immune system. I think we need to test chemicals on mice with damaged immune systems to see the real effects.'

Further tests are needed to make sure chemicals aren't causing hidden cancers that remain undetected until the immune system becomes weak later in life.