IL-9Rα–expressing monocytes were required for the spontaneous proliferation of ATL PBMCs through a contact-dependent manner. (A) IL-9Rα was expressed on the monocyte population of ATL PBMCs, which also expressed CD14. IL-9Rα was not expressed on CD3 (T), CD20 (B), or CD16 (NK) positive cells. PBMCs from normal donors were used as a control. (B) Six-day spontaneous proliferation of purified T cells, monocytes, and mixture of purified T cells and monocytes (T cells: monocytes = 1:1). ³H thymidine was added to the culture during the last 6 hours of culture. Anti–IL-9Rα or control antibody UPC10 was added to the culture at T₀. Cells were then harvested and analyzed for their ³H thymidine incorporation. (C) Separated T cells and monocytes were cultured in the same chamber or different chambers of the transwell (0.4 μm) for 6 days. ³H thymidine was added to the culture during the last 6 hours of culture.