CP-690,550 inhibited the 6-dayex vivospontaneous proliferation of PBMCs from patients with ATL and with HAM/TSP. The PBMCs from a patient with ATL (A) or a patient with HAM/TSP (B) were cultured ex vivo for 6 days with and without CP-690,550 or with 10 μg/mL of antibody when a single antibody directed against the cytokine IL-2, IL-9, or IL-15 was used or with their combination (5 μg/mL each). ³H-thymidine was added during the last 6 hours of the cultures. Cells were then harvested and analyzed for ³H-thymidine incorporation. The percentage of inhibition of proliferation with CP-690,550 addition was determined for 12 patients with ATL (C) or 9 patients with HAM/TSP (D). The value was calculated as: % inhibition = (cpm of proliferation without CP − cpm of proliferation with CP)/(cpm of proliferation without CP) × 100. Black bars indicate the mean inhibition percentage.