The cells were maintained in RPMI-1640 complete medium in a humidified 5% CO2 atmosphere at 37 °C. RPMI-1640 complete medium was supplemented with 10% foetal calf serum (FCS), 2 mM l-glutamine, 50 μM β-mercaptoethanol, penicillin (100 U/ml) and streptomycin (100 μg/ml)