IL15 and neoplasm: Artificial antigen presenting cells such as K562 with membrane-bound 4-1BBL/IL15Rα [16], IL-21 [17], or IL-15 [18] can be used for NK cell activation and expansion in culture and give rise to effector cells with potent anti-tumor capacity [16] that have been tested in phase 1 clinical trials [19, 20] with conflicting results.