CXCR4 surface level and responsiveness to CXCL12 decrease during NK-cell maturation. (A) BM cells were stained for NK1.1, CD3, CD27, CD11b, and CXCR4 or isotype control. Expression of CXCR4 (black line) or isotype (gray histogram) by gated NK-cell subsets is shown, as indicated. Numbers above histograms indicate mean fluorescence intensity (MFI) of CXCR4 staining minus MFI of isotype control. Data show representative results of 3 independent experiments. (B) Transwell assay of the migration of NK-cell subsets assessing movement toward different concentrations of CXCL12, as indicated. The migration index is calculated as the ratio between the number of cells migrating in the chemokine and in the control (without chemokine) condition. Data are the mean ± SD of 3 independent experiments with duplicates.