Eomes expression identifies the early bone marrow precursor to classical NK cells

Classical NK cells are the prototypical type 1 innate lymphocytes mounting cytolytic and IFNγ responses to intracellular pathogens and tumors. A similar but distinct population termed ILC1 was recently described, differing from NK cells in multiple though relatively subtle ways, including tissue residency vs recirculation, and levels of NK receptors. Whether the differences reflect distinct lineages or mere variations in program expression is not fully understood. Here, using transcription factor reporter mice and cell transfers of bone marrow precursors, we found that, upon transfer in vivo, Eomes-expressing, lineage-negative, NK receptor-negative cells acquired properties typically associated with classical NK cells including the capacity to prevent metatstatic disease. In contrast, as previously reported, PLZF-expressing cells mostly gave rise to ILC1 cells. These findings identify the Eomes-expressing NK precursor as the long-elusive bone marrow precursor to classical NK cells, and demonstrate that the NK and ILC1 lineages diverge early during development.