Isn鈥檛 the immune system wonderful? Among its many talents, it can keep cancer in check for years without you ever knowing anything about it.
That鈥檚 the implication of new research showing that animals can survive for years with tumours that are tiny and harmless because the dormant growths remain in equilibrium with the immune system.
A person will only develop full-blown cancer if the microtumour changes 鈥� perhaps through a mutation that alters its outward appearance 鈥� so that it falls off the immune system鈥檚 radar. Or the immune system may slacken its surveillance and lose track of the tumour because the person is physically or mentally stressed, or fighting off an infection.
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If researchers can discover which molecules keep the tumours in check, or find ways of increasing the immune system鈥檚 vigilance, it could open up new ways to fight cancer, or even stop the disease developing in the first place.
Robert Schreiber at Washington University School of Medicine in St Louis, Missouri, and his colleagues injected mice with a carcinogenic chemical called methylcholanthrene to stimulate the growth of tumours called sarcomas. While most of the tumours remained tiny and harmless, the researchers found that they could upset the equilibrium by knocking out important components of the immune system with artificial antibodies.
When they knocked out white blood cells called CD4 and CD8 cells, or the signalling molecule gamma interferon, the tumours began to grow larger (Nature, ).
Schreiber says that this could happen naturally if someone鈥檚 immune system breaks down, perhaps through ageing, prolonged stress, or if someone takes immunosuppressive drugs to stop their immune system attacking a transplanted organ. 鈥淭he immune system can keep tumours corralled in a dormant state that doesn鈥檛 become clinically apparent or harmful,鈥� says Schreiber. 鈥淢any of us may be walking round with tumours in this equilibrium state.鈥�
鈥淭he immune system can keep tumours corralled in a dormant state that doesn鈥檛 become apparent or harmful鈥�
It might be possible to stop cancers developing in the first place if researchers can identify the factors keeping the disease in equilibrium with the immune system. 鈥淢aybe we can convert cancer from a deadly disease to a chronic, controllable state,鈥� Schreiber says.
Judah Folkman of the Children鈥檚 Hospital in Boston suspects the crucial factors might be signalling molecules that enable tumours to grow blood vessels. 鈥淓xpansion of tumour mass beyond 2 millimetres requires continuous recruitment of new blood vessels,鈥� he says.
Additional clues have come from research showing a new mechanism by which immune cells lose track of potentially dangerous cells and tissues, including cancers. Leonie Taams and her colleagues at King鈥檚 College London showed that they could switch off the rallying call made by macrophages, white blood cells that alert the immune system to the presence of invaders and abnormal cells.
They did this by exposing the macrophages to immune cells called regulatory T cells, which are known to 鈥渟witch off鈥� macrophages once an infection is under control or a wound repaired. But Taams found that the regulatory T cells could make the macrophages give a false 鈥渁ll clear鈥� in certain situations (, ).
Cancer 鈥� Learn more about one of the world鈥檚 biggest killers in our comprehensive special report.
