Nat. Immunol. 14, 290–297 (2013); published online 3 February 2013; corrected after print 21 March 2013

In the version of this article initially published, the number for the National Health and Medical Research Council grant to I.V. is incorrect in the Acknowledgements section. The correct number is 1021374. The error has been corrected in the HTML and PDF versions of the article.