Cancers often bounce back after the initial assault of chemotherapy. In breast cancers this is thanks, at least in part, to the activities of a class of immune cell called macrophages, say Johanna Joyce and her colleagues at Memorial Sloan-Kettering Cancer Center in New York.

The scientists treated tumour-bearing mice with high-dose paclitaxel (Taxol), a common breast-cancer chemotherapeutic. They found a higher influx of macrophages into treated than untreated tumours. This, in turn, resulted in elevated levels of enzymes called cathepsin proteases, which are made by the macrophages and are known to facilitate several disease processes, including tumour growth. Mouse cancer cells cultured with macrophages and treated with Taxol had significantly lower death rates than Taxol-treated cell lines cultured alone. Treating the cells with a cathepsin inhibitor called JPM completely reversed this effect.

Giving mice both Taxol and JPM significantly improved Taxol's efficacy against both primary and metastatic tumours.

Genes Dev. 25, 2465–2479 (2011)