Correction to: Nature Medicine https://doi.org/10.1038/s41591-019-0737-3, published online 20 January 2020.

In the version of this article initially published, a value in the final sentence of the legend to Extended Data Fig. 8 (spots/40,000 total input PBMC) was incorrect (twice). The correct value is ‘spots/400,000 total input PBMC’ (for both). The error has been corrected in the HTML and PDF versions of the article.