IL15 and leukemia: To expand ex vivo NK cells (over 1000-fold expansion), peripheral blood mononuclear cells of healthy donors or patients are co-cultured in the presence of irradiated K562 leukemia cells that have been modified to express membrane-bound IL-15 and 4-1BB ligands in the presence of IL-2 and IL-15 cytokines in the culture media [43].