IntroductionAlthough the developmental stages leading to committed B and T cells have been dissected in great detail, 1 the developmental intermediates downstream of the common lymphoid progenitor (CLP) leading to a committed natural killer (NK)-cell precursor remain poorly defined. The earliest committed NK-cell progenitor (NKP) identified to date is characterized through the expression of the IL-2 receptor  chain (CD122) and the lack of pan-NK-cell surface markers (NK1.1 and CD49b), but this population is likely to be heterogeneous because only a minority of cells give rise to NK cells in in vitro assays. 2,3 The transcription factor inhibitor of DNA binding 2 (ID2) is essential for NK-cell development. In its absence, fetal and adult NK-cell development is blocked at an early stage. [4][5][6][7] Id2 inhibits E proteins in NK cells and is itself regulated by E4BP4, which is also required for the differentiation of NKPs to immature NK cells. 8,9 Using a reporter line in which the green fluorescent protein (GFP) gene has been introduced into the Id2 locus, we report the identification of the earliest committed NK-cell precursor in adult BM. By analogy to B-cell development, we termed these pre-pro NK cells, and they express high levels of ID2, IL-7R␣ (CD127), and lack expression of most, but not all, NK-cell markers. Furthermore, we demonstrate that ID2 gfp expression can be used to isolate the committed NK-cell progenitors within the heterogeneous NKP population.
Methods
MiceWe generated ID2 gfp reporter mice by inserting an internal ribosome entry site-GFP cassette into the 3Ј untranslated region of the Id2 locus (G.T.B., S.C., and S.L.N. manuscript submitted April 2011). PU.1 gfp and Rag1 gfp mice have been described previously. 10,11 All mice were maintained under specific pathogen-free conditions at the Walter and Eliza Hall Institute according to institute guidelines.
Antibodies, cell analysis, and sortingLineage (lin) marker-positive BM cells were depleted and analyzed by flow cytometry as described in supplemental Methods (available on the Blood Web site; see the Supplemental Materials link at the top of the online article).
Cell cultureTo determine precursor frequencies, sorted progenitors cells were seeded in limiting dilution on OP9 or OP9-DL1 cells in the presence of 2% IL-7 supernatant and 5 ng/mL FMS-like tyrosine kinase 3 (Flt3) ligand for Band T-cell development, respectively, and on OP9 cells plus 0.5% IL-7 supernatant and 50 ng/mL human IL-15 (R&D Systems) for NK-cell differentiation as described. 12 After 7-10 days, wells were scored for the presence of colonies. Colonies were harvested for flow cytometric analysis, and precursor frequency was calculated according to Poisson statistics with the use of ELDA (extreme limiting dilution analysis) software Version 1.0 (http://bioinf.wehi.edu.au/software/elda/). 13
Results and discussionThe earliest committed NKP identified to date expresses CD122 but not the pan-NK cell-specific markers NK1.1 and CD49b. 2 This population is likely to be heterogene...