FL enhances IL-15–mediated development of CD56⁺ CD3⁻ NK cells from CD34⁺ HPCs in vitro. (A) Flow cytometric analysis of CD56 and CD3 expression on MNCs generated in 21-day cultures of CD34⁺ HPCs under the indicated conditions (right column). Nonreactive MsIg isotype control MoAb were used to determine background fluorescence (left column). Results displayed are representative of 10 separate experiments. (B) Fold increase in the absolute number of MNCs after 21 days of culture of CD34⁺ HPCs in the indicated conditions. (C) The absolute number of CD56⁺CD3⁻ cells after a 21-day culture of CD34⁺ HPCs in the indicated conditions. These values were calculated by multiplying the absolute number of viable MNCs by the percentage of cells that were CD56⁺CD3⁻ by flow cytometric analysis. Results shown in (B) and (C) represent the mean ± SEM of five separate experiments. The asterisk indicates a value of P ≤ .025 compared with IL-15 alone.