Speaking of expansion. From this: ''We apply a human B-lymphoblastoid cell-line 721.221 (hereinafter, 221)-based artificial feeder cell system with membrane-bound interleukin 21 (mIL 21) to propagate NK and CAR-NK cells. The expansion capability, purity, and cytotoxicity of NK cells expanded with 221-mIL-21 feeder cells are superior to that of conventional K562-mIL-21 feeder cells. RNA sequencing (RNA-seq) data show that 221-mIL-21 feeder cell-expanded NK cells display a less differentiated, non-exhausted, limited fratricidal, memory-like phenotype correlated with enriched metabolic pathways, which explains underlying mechanisms.'' https://www.cell.com/molecular-therapy-family/methods/fulltext/S2329-0501(20)30138-8