Antibody directed to IL9Rα inhibited the spontaneous proliferation of ATL PBMCs ex vivo. (A) Six-day spontaneous proliferation of ATL PBMCs with and without antibody to IL9Rα in the ex vivo culture. A nonspecific antibody, UPC10, was used as a control. The monoclonal antibody to IL9Rα or UPC10 was added to the 96-well plates at day 0. ³H thymidine was added to the culture during the last 6 hours of culture. Cells were then harvested and analyzed for the ³H thymidine incorporation. (B) CD4⁺CD25⁺ T cells from ATL patients proliferated in ex vivo culture. FACS analysis of CD4/CD25 expression was performed at different time points (day 0, day 3, and day 6). (C) CFSE staining of CD3⁺ lymphocytes to monitor their cell division. CFSE was labeled at day 0, and the cells were then put in culture without any stimulation. FACS analysis of CFSE-positive or CD25 CFSE double-positive cells was done at day 6. Unlabeled cells were used as controls. (D) CFSE staining of monocytes (CD14-expressing cells) to monitor cell division.