Blockade of Axl/Gas6 pathway inhibits IL-15–induced in vitro differentiation of human CD34⁺ HPCs into NK cells. (A) Representative flow cytometry plot showing differentiation of NK cells (CD3⁻CD56⁺) from blood CD34⁺ HPCs. Value in parentheses indicates a percentage of CD3⁻CD56⁺ NK cells (upper left quadrant) detected among the total cells after a 3-week culture under the experimental conditions indicated. (B) Top graph shows a percentage of differentiated CD3⁻CD56⁺ NK cells among the total cells after culture for 3 weeks (triplicate wells). Bottom graph shows absolute numbers of NK cells (calculated by multiplying total number of cells counted in each well times the percentage of NK cells noted by flow cytometric analysis). Results for panel B are summarized from at least 3 experiments; error bar represents SD. *P < .03; **P < .001. (C) CD34⁺ cells isolated from human bone marrow were cultured with IL-15 in the presence of irrelevant Fc or Axl-Fc for 3 weeks, and the resultant percentage of CD3⁻CD56⁺ NK cells among the total cells is shown. The results show the mean plus or minus SD and are from 3 separate donors, each in triplicate wells. *P < .02.