Blimp1 is constitutively expressed in NK cells and is up-regulated during their maturation. (A-B) Blimp1^gfp/+ splenocytes were stained with antibodies as indicated and gated on either (A) NK1.1⁺CD49b⁺TCRβ⁻ NK cells (R1) or (B) TCRβ⁺CD8⁺CD44⁺CD122⁺ or TCRβ⁺CD4⁺CD62L⁻ effector/memory phenotype T cells. (C) NK cells from indicated organs of Blimp1^gfp/+ mice were identified as in (A) and analyzed for GFP and Mac-1 expression. Numbers in the plots show the proportion of NK cells in each quadrant. (D) Peripheral blood NK cells were isolated from healthy human donors and sorted into CD16⁺ and CD16⁻ subpopulations. After RNA isolation and cDNA synthesis, transcripts were measured for BLIMP1 and HPRT by semiquantitative PCR. (E-F) DX5⁺ NK cells isolated from Blimp1^gfp/+ (solid line) and wild-type mice (dotted line) were cultured in IL-15 for 5 days before being transferred into IL-15 plus cytokines as indicated. GFP expression was measured after 2 days (E). Wild-type NK cells were also subjected to Western blotting using antibodies as indicated (F). The results are representative of 3-6 independent experiments.