Correction to: Nature https://www.nature.com/articles/nature17947 Published online 5 May 2016

In the version of the article initially published, due to an error during figure preparation, the flow cytometry plot shown in the top-right panel of Extended Data Fig. 6j was an inadvertent duplicate of the bottom-left plot. Due to the age of the article, the figure cannot be replaced directly. The Supplementary information accompanying this amendment serves to update the article.

Supplementary information is available in the online version of this amendment.