IL-15 serum levels sharply increase after HSCT and cyclophosphamide administration and correlate with NK cell dynamics. (A) IL-15 concentration (green triangles) measured in the sera of 7 patients before and after HSCT with PT-Cy is shown with the number of NK cells (red circles) and T cells (blue squares) in the PB of the same patients. Mean values with SEM are shown. (B) Correlation between IL-15 serum concentration at 3 days after HSCT (x-axis) and NK cell counts at day 15 after HSCT (y-axis). The red line denotes the best-fit line of the linear regression analysis. (C) Correlation between IL-15 serum concentration at 3 days after HSCT (x-axis) and the percentage of NK cells proliferating at the same time point, measured by Ki-67 intracellular staining (y-axis). The red line denotes the best-fit line of the linear regression analysis.