Nat. Immunol. 16, 276–285 (2015); published online 19 January 2015; corrected after print 8 April 2015

In the version of this article initially published, the Acknowledgments section was incomplete. The correct text should begin “We thank P. O'Brien, M. Mochizuki and N. Takeno for assistance with tissue collection....” The error has been corrected in the HTML and PDF versions of the article.