MDCK and Human embryonic kidney 293T cells were respectively maintained in MEM and DMEM (Invitrogene, USA) supplemented with 10% heat-inactivated fetal bovine serum (FBS) and antibiotics (100 U penicillin G/mL and 100 ug streptomycin/mL). Influenza virus strains A/New Caledonia/ 20/1999 (H1N1) and A/Hong Kong/486/97 (H5N1) used in these experiments were prepared in MDCK cells and virus titers were determined by TCID 50 . All experiments with H5N1 virus were performed in BSL-3 laboratory.