IL-15 treatments are associated with decreased numbers and decreased proliferations of leukemic cells in participants with ATL. Leukemic cell counts are shown for 7 participants with ATL before and after 2-week courses of IL-15. Six of 8 patients had reduced leukemic cell counts (A), and 7 of these 8 patients also showed a significant reduction in their rate of proliferation as measured by Ki67 expression (B). (C) Cell counts and proliferation rates were not correlated. ∗P < .01; NS, not significant.