A drug that activates the immune system against a type of skin cancer does so by promoting new immune responses to cancer, rather than amplifying pre-existing ones.

The melanoma drug ipilimumab boosts the activity of immune-system cells called T cells, which recognize a variety of molecules on tumour cells, marking them for destruction. Pia Kvistborg and Ton Schumacher of the Netherlands Cancer Institute in Amsterdam and their team compared T cells from melanoma patients before and after treatment and found that the drug broadened the range of melanoma targets recognized by T cells. But melanoma-specific T cells that were already present before treatment did not expand their ranks, suggesting that the drug stimulates new responses.

Boosting the range of T cells could be a useful strategy for other cancer immunotherapies, the authors say.

Sci. Transl. Med. 6, 254ra128 (2014)