PECAM1 and leukemia: We first assessed the in vitro cytotoxicity of expanded NK cells by co-culturing them with CD31+ ECs derived from H9-WT (A02/03B35/44DRB1+/+), HLA-edited (A02/−B35/−D−/−), and B2M knockout (B2M−/−DRB1−/−) hESCs, as well as with K562, a human leukemia cell line that lacks the HLA complex.