RMA-S/HLA-E cells56 (link) were maintained in complete medium (RPMI-1640 containing glutamine and supplemented with 10 % (vol/vol) fetal bovine serum (FBS), 20 μM β-mercaptoethanol and 100 U ml−1 penicillin–streptomycin; all Thermo Fisher) in the presence of 400 μg ml−1 hygromycin B (InvivoGen).