A new front in the war against cancer has been opened by scientists tapping the potential of "natural killer" cells.
Researchers in Australia have identified a mechanism that causes natural killer (NK) immune cells to show mercy to cancer.
Switching it off had a dramatic effect on mice with normally lethal skin, prostate and breast cancers.
NK action against the tumours was stepped up, preventing deadly metastasis, the spread of cancer to vital organs in the body.
In the case of breast cancer, tumour growth in the mammary glands was significantly reduced.
Lead scientist Dr Nicholas Huntington, from the Walter and Eliza Hall Institute of Medical Research in Parkville, Victoria, said: "This is about learning how to activate the NK cells of the individual patient and boost their immune system to tackle the disease.
"We are hopeful our research will lead to new immunotherapies that supercharge the body's natural killer cell, and maintain it in a highly active state to more efficiently and specifically fight cancer."
Natural killer cells are specialised white blood cells that act as the immune system's assassins.
Like murderous agents working for a totalitarian state, their job is to locate and eradicate "deviant" cells in the body that may pose a threat.
Some of their targets are infected cells, while others show signs of becoming cancerous.
Once a deviant cell is identified, the NK cell releases a chemical called perforin that blasts holes in its outer membrane. Other molecules fired through the holes cause the cell to fall apart or self-destruct.
However, the immune system also possesses a complex system of so-called "checkpoints" that in certain situations tone down its responses to prevent accidental damage to healthy tissue.
Dr Huntington's team discovered a particular checkpoint pathway that had the effect of taming NK cells.
An inhibitor protein made inside the cells limited their ability to respond to an activating signal that issues the command to kill cancer.
By silencing the protein's gene, the researchers were able to ramp up the ability of NK cells to protect mice against melanoma skin cancer, prostate cancer and breast cancer.
After 14 days, melanoma mice without the genetic modification had extensive tumour growth in their lungs. In contrast, metastatic growth was "largely absent" from mice with boosted NK cells.
Mice with prostate cancer responded in a similar way, said the scientists, writing in the journal Nature Immunology.
A detailed study of mice with breast cancer showed that despite some microscopic spread to the lungs, there was no sign of the "large metastases" that would normally be observed. Tumour growth in the mammary glands was also suppressed.
