Nat. Immunol. 17, 976–984 (2016); published online 4 July 2016; corrected after print 20 July 2016

In the version of this article initially published, some of the statistical comparisons in Figures 3f, 3l, 3m, 4a, 4d, 4e, 5g, 6b, 6c and 7d were presented incorrectly in the plots. Also, 'in vitro' was not fully in italics in the abstract, and the author list for reference 36 was incorrect. The errors have been corrected in the HTML and PDF versions of the article.