Differentiation of NK cells in BALB/c Rag2^−/−γcR^−/− mice. (A) NK-cell number in different organs from humanized mice that have received PBS, or human IL-15 + human IL-15Rα-Fc intraperitoneally weekly for 1 or 4 weeks (w), as determined by flow cytometry (n = 3 per group; mean ± SEM) (see supplemental Figure 2 for gating algorithm used to identify human NK cells), BM (bone marrow). (B) Representative staining for CD57 on gated human NK cells from the indicated organs of PBS- or IL-15–treated mice at the indicated time point (4 weeks). (C) Frequency of CD57 expression on gated human NK cells in the indicated organs of PBS- or IL-15–treated mice at 1 and 4 weeks (n = 3 per group; mean ± SEM), Thy (thymus), LN (lymph nodes). (D) Frequency of CD57 expression within subsets of KIR⁻ and KIR⁺ human NK cells from spleens of mice treated with IL-15 for 4 weeks (n = 3 KIR⁻, 9 KIR⁺; mean). (E) Representative staining for Ki67 and CD57 on gated human splenic NK cells from mice treated with IL-15 for 4 weeks. (F) The indicated subsets were sorted and transferred into irradiated BALB/c Rag2^−/−γcR^−/− mice. Shown are recovered NK cells at day +7 following 2 rounds of IL-15/IL-15Rα stimulation. One representative experiment of 2 is shown (n = 3 per group).