IL-9 was secreted into the supernatants of ATL PBMCs and HTLV-I–infected cell lines. (A) NK-92 cell line assay of 6-day culture supernatants of PBMCs from smoldering and chronic ATL patients were performed. Monoclonal antibodies to IL-2 or IL-9 or to both were added to the assay to detect the presence of IL-2 or/and IL-9 in the 6-day culture supernatants. The normal control was typical of that observed from 10 normal donors. (B) IL-9 ELISA was performed to define the IL-9 levels in the 6-day culture supernatants of PBMCs from 11 patients with ATL. The growth medium and the supernatants from normal donor PBMCs culture were used as a control (n = 10, patient vs control, P < .001). (C) An IL-9 ELISA was performed to determine the IL-9 levels in the culture supernatants of HTLV-I–infected cell lines Hut102, MT-1, MT-2, and MJ as well as HTLV-I–negative cell lines Jurkat, CEM, and Hut78.