The rapid progression of breast cancer has been linked to immune cells called macrophages. Now researchers have identified a protein that these 'tumour-associated' macrophages (TAMs) secrete that drives metastasis.

Erwei Song at Sun-Yat-Sen University in Guangzhou, China, Xuebiao Yao at the University of Science and Technology of China in Hefei and their colleagues found that the most abundant cytokine — a cell-signalling protein — produced by TAMs isolated from patients with invasive breast cancer is CCL18. Injection of this molecule into tumour-bearing mice boosted metastasis. Meanwhile, human breast cancer cells cultured with TAMs displayed a six- to ninefold increase in invasiveness. Adding an anti-CCL18 antibody tempered this response.

Cancer Cell 19, 541–555 (2011)