Improved NK cell maturation and expansion ex vivo have been achieved with cytokines alone (IL-2, IL-12, IL-15, IL-18 and/or IL-21) or in combination with co-stimulatory antibodies/agonists, feeder cells generated from autologous mononuclear cells, EBV-transformed lymphoblastoid cell lines (EBV-LCLs) or tumor cells (K562)/feeder cells that have been genetically modified to express membrane-bound cytokines and co-stimulatory molecules such as 4-1BB, MICA, IL15 and IL21 [83,84,85,86,87]. Here, IL21 is linked to neoplasm.