Plaque assay and PRNT 50 assay. MDCK cells were grown in 6-well plates at a density of 8 × 10 5 /well. On the next day, cells were washed with PBS. 10-fold dilutions of virus were added in 500 µl DME and incubated at 37°C for 1 h, with mixing every 10 min. Cells were washed with PBS and overlayed with 199 media containing 0.5% agarose (Seakem), 1x antibiotics (100 U/ml penicillin and 100 mg/ml streptomycin), 0.2% BSA (Sigma-Aldrich), and 0.5 µg/ml TPCK-Trypsin (Sigma-Aldrich). Cells were incubated for 36-40 h and fixed with 2% PFA for 10 min. Agarose plugs were removed and cells were stained with 0.1% crystal violet in 25% EtOH for 1 min. After removal from the crystal violet solution, plates were dried and used to count plaques in each well. For PRNT 50 assay, MDCK cells were prepared as above. On the next day, mAbs were threefold-diluted (60-0.74 µg/ml). 100 PFU of virus in 250 µl DME were incubated with equal volume of diluted mAbs at 37°C for 1 h before the plaque assay. Plaques were counted and the final concentration of antibodies that reduced plaques to <50 PFU were scored as PRNT 50 .