Nat. Med. 16, 1400–1406 (2010); published online 21 November 2010; corrected after print 7 April 2011

In the version of this article initially published, the flow cytometry plot in Figure 4b corresponding to the condition HCMEC and TGF-β2 was incorrect. This plot has been replaced with the correct one in the HTML and PDF versions of the article.