PM21 particles were generated and NK cells were expanded from peripheral blood mononuclear cells (PBMCs) as described previously.23 24 (link) Briefly, PBMCs were depleted of T-cells (EasySep CD3 positive selection kit; STEMCELL Technologies) and then cultured for up to 25 days with 100 U/mL interleukin-2 (IL-2; PeproTech) and 200 µg/mL of PM21 particles in SCGM media (CellGenix) supplemented with 10% non-HI FBS. PM21 particles were derived from CSTX-002 (K562-nmIL21-41BBL) cells, provided by Kiadis Pharma and maintained in RPMI media supplemented with 10% FBS.