MHC class II contributed to the proliferation induced by the interaction between purified leukemic cells and monocytes. (A) Purified leukemic cells and monocytes were cultured in the same chamber or a different chamber of the Transwell (0.4-μm) for 6 days. ³H-thymidine was added to the culture during the last 6 hours of the culture. (B) Proliferation of purified leukemic cells cultured with or without 10 ng/mL recombinant human IL-1-α, IL-1b, IL-6, IL-12, IL-2, and IL-9. (C) Six-day spontaneous proliferation of purified leukemic cells, monocytes, and mixtures of purified leukemic cells and monocytes (leukemic cells: monocytes = 1:1). ³H-thymidine was added to the culture during the last 6 hours of culture. Anti-MHC class II or control antibody UPC10 were added to the culture at day 0. (D) Six-day spontaneous proliferation of smoldering/chronic ATL PBMCs with or without anti-MHC class II antibody and control antibody UPC10. ³H-thymidine was added to the cultures during the last 6 hours of culture. Anti-MHC class II or control antibody UPC10 were added to the cultures at day 0. The data are representative of data from 4 different patients.