The natural killer (NK) cell line YTS was purchased from American Type Culture Collection (ATCC, Manassas, VA, USA) and maintained in RPMI 1640 medium supplemented with 10% Fetal Bovine Serum (FBS, Takara Biotechnology Co., Ltd., Dalian, China), 1% nonessential amino acids (NEAA, Invitrogen), 1% sodium pyruvate (Sigma-Aldrich), 10 mM HEPES (PAA, Invitrogen), 2 mM L-glutamine (Biochrom, Berlin, Germany), and 1% penicillin-streptomycin (100 μg/ml; Invitrogen Life Technologies, Beijing, China) and 5% CO2 at 37°C.