Detection of IL-15 and heterodimeric IL-15/IL-15Rα complexes in human sera. (A) Total IL-15 (filled bars) and sIL-15Rα–associated IL-15 (open bars) in sera of 10 lymphodepleted melanoma patients. Total IL-15 was evaluated using the commercially available IL-15 ELISA (Quantiglo Q1500B). sIL-15Rα–associated IL-15 was detected by the heterodimeric IL-15/IL-15Rα ELISA using capture anti–IL-15 antibody and detection anti–IL-15Rα antibody, as described in “IL-15 and IL-15/IL-15Rα ELISA.” Data are mean ± SEM of 2 to 4 independent measurements. *Value below the limit of detection of the assay. (B) Correlation of total IL-15 and sIL-15Rα–associated IL-15 levels in the sera of lymphodepleted patients. Graph represents mean ± SEM from 2 to 4 independent measurements for each sample. Data were fitted to a linear regression curve using Prism 4.0a software package (correlation coefficient r² = 0.92; slope = 0.92).