Demonstration of IL-15/sIL-15Rα heterodimers in normal and lymphodepleted mice. (A) Serum levels of IL-15/IL-15Rα heterodimer in normal mice () and on CTX treatment (□). Mice were bled at different time points (days 1-19) after CTX or placebo administration. Circulating IL-15/IL-15Rα was assessed by ELISA (eBioscience) and reported as mean ± SEM of 6 to 33 mice per time point. (B-C) Serum samples from 6 untreated (B) and 9 CTX-treated (day 3 after treatment; C) mice were preincubated with exogenous recombinant mouse IL-15Rα-Fc (100-1000 pg/mL, as indicated) to allow the in vitro formation of IL-15/sIL-15Rα complexes with any free IL-15 potentially present in the sample. Serum samples were next evaluated for the amount of sIL-15Rα–associated IL-15 by ELISA.