Hi-Cy/Flu induces endogenous IL-15, which correlates with in vivo haploidentical NK-cell expansion. Plasma was collected from patients before and after the indicated preparative regimens. The day-0 sample was collected prior to haploidentical NK-cell infusion. A marked increase in IL-15 concentrations was detected in patients receiving Hi-Cy/Flu (•) compared with those receiving Flu alone (□). (A) The individual symbols represent patient samples obtained at the indicated time points. The plotted lines represent the mean plus or minus SEM of samples from each preparative regimen. Endogenous IL-15 was significantly higher with the Hi-Cy/Flu preparative regimen. A single pediatric patient (designated **) on oral etoposide prior to enrollment was the only patient who had high IL-15 levels prior to treatment. (B) At all postchemotherapy time points, an inverse correlation between the IL-15 concentration and the absolute lymphocyte count was detected among all patients receiving Flu alone or Hi-Cy/Flu.