Expression of membrane-bound Interleukin-15 sustains the growth and survival of CAR-NK cells

Natural killer (NK) cells display a short lifespan in vivo without cytokine support. In the present study, we investigated whether the expression of a membrane-bound interleukin-15 (IL-15) on chimeric antigen receptor (CAR)-NK cells could help to overcome the hurdle. The CAR-NK cells were generated through electroporation of a piggyBac transposase plasmid and a bicistronic vector containing a 2nd generation natural killer group 2 member D (NKG2D) CAR gene and a cDNA fragment encoding human IL-15 linked to the CD8 hinge and transmembrane domain (membrane-bound interleukin-15, mbIL15), followed by repeated stimulation with K562 artificial antigen-presenting cells. This protocol resulted in a robust expansion of the CAR-NK cells up to 590,000-fold and CAR expression over 90 %. mbIL15 substantially enhanced the in vitro survival of the CAR-NK cells when being cultured without IL-2 support or in the presence of low concentrations of IL-2. Importantly, mbIL15 enhanced the in vivo survival of the CAR-NK cells in normal mice. In mice with tumors, the in vivo expansion of mbIL15/NKG2D CAR-NK cells was observable. These CAR-NK cells were effective in lysing target human cancer cells in vitro and in reducing tumor burden in a mouse xenograft model generated with human cancer cells expressing the NKG2D ligands. Our work provides empirical evidence that the co-expression of mbIL15 is a potential approach to improve the therapeutic potential of CAR-NK cells.