Nat. Immunol. 16, 534–543 (2015); published online 6 April 2015; corrected after print 21 May 2015

In the version of this article initially published, the bottom left plot in Figure 3c, the top right plot in Figure 4a, and the third and bottom plots in the left column of Figure 6a were duplicates of other plots in the same figure. Those plots have been replaced and the errors have been corrected in the HTML and PDF versions of the article.