An immune system balancing act performed by scientists could yield new cancer treatments, experts hope.
Researchers succeeded in harnessing the immune system to attack tumours without harming healthy cells.
Under normal conditions, regulatory T-cells - part of the immune system - help prevent extreme and dangerous auto-immune reactions.
But this can stop the immune system mounting a strong enough attack on cancer.
Scientists discovered that blocking a particular enzyme can safely reduce the effect of the regulatory cells, known as Tregs.
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By suppressing the enzyme with a drug, they were able to lower Treg function in mice with cancer and at the same time limit tumour growth.
"This preclinical study demonstrates proof of principle that using a drug to regulate the function of a special immunosuppressive subset of so-called T-regulatory (Treg) cells safely controls tumour growth," said US study leader Dr Wayne Hancock, from The Children's Hospital of Philadelphia. "It really moves the field along towards a potentially major, new cancer immunotherapy."
The findings are reported in the journal Nature Medicine.