HEK 293T and MDCK cells were maintained in complete Dulbecco's Modified Eagle's Medium (DMEM; Gibco) containing 10% (v/v) Fetal Calf Serum (FCS; Bodinco B. V.), 100 U/ml Penicillin and 100 mg/ml Streptomycin. Influenza A/WSN/33 (H1N1) (IAV-WSN) was grown on MDCK cells. Briefly ,70% confluent MDCK cells were infected with IAV-WSN at a multiplicity of infection (MOI) of 0.02 50% tissue culture infectious dose (TCID50) per cell. Supernatant was harvested after 48 h of incubation at 37uC and cell debris was removed by centrifugation at 2,000 rpm for 10 min. Virus was stored at 280uC and TCID50 on MDCK cells was determined.