Nat. Immunol. 16, 1114–1123 (2015); published online 20 October 2015; corrected after print 13 November 2015

In the version of this article initially published, the vertical axes of Figure 4 were labeled incorrectly as “(per 1 × 105 T cells).” The correct label is “(per 1 × 106 T cells).” These errors have been corrected for the PDF and HTML versions of this article.