Optimal genetic feeder cell-expanded and engineered NK cell products are composed of CD56^bright and CD56^dim NK cells

Natural killer (NK) cells represent a promising source for off-the-shelf cellular therapies. Typically, NK cells are expanded with cytokines and K562 feeder cells expressing 4-1BBL and membrane-bound IL-21, which induce a uniform and activated CD56^superbright phenotype. However, the developmental origin of these cells remains unclear, arising either from CD56^dim NK cells through phenotypic plasticity or from functional maturation of the CD56^bright subset. In this study, we dissected the respective contributions of CD56^bright and CD56^dim NK cells to engineered NK cell products. NK cell populations were sorted, and their activation, expansion, phenotype, and functionality were assessed. We found that CD56^bright NK cells were initially critical for proliferation and feeder clearance, but after restimulation, they exhibited comparable cytotoxicity and expansion to CD56^dim NK cells. This was further highlighted by the ability of both subsets to mediate CAR- or TCR-driven cytotoxicity. Eventually, sorted and expanded NK cells acquire the same activated phenotype, but can be distinguished by CD16 expression, predominantly maintained by CD56^dim cells, resulting in superior antibody-dependent cellular cytotoxicity. Together these findings indicate that preselection of a single subset is redundant, and that bulk isolation enables optimal NK cell engineering by leveraging the complementary strengths of CD56^bright and CD56^dim NK cells.