Quantifying the number of cancer-fighting immune cells that a tumour contains could offer a way to predict whether it will spread through the body.

Cancer is often deadly when it spreads, but anticipating primary-tumour spread (or metastasis) is difficult. Jérôme Galon at the French National Institute of Health and Medical Research in Paris and his team analysed tumours from more than 800 people with colorectal cancer, comparing people whose tumours had metastasized with those whose had not. The primary tumours from both groups had similar patterns of mutations in cancer genes, but tumours that had spread had fewer cell-killing T cells. The invasive edges of the metastasized tumours also had a lower density of lymphatic vessels, which transport immune cells.

The authors conclude that these changes contribute to metastasis, and that immunotherapies that boost T-cell responses could block the spread of cancer in people with early-stage disease.

Science Transl. Med. 8, 327ra26 (2016)