Virus yield reduction assay

The extracellular virus yield reduction assay was performed as described previously in 24-well plates containing confluent Calu-3 cells grown on membrane supports (6.5-mm Transwell, Corning Inc., Corning, NY, USA) [12]. The concentrations of MAA or SNA lectins ranged from 1 to 1000 μg/ml and they were added to the apical compartment of Calu-3 cells for 2 h. After pretreatment, the cells were overlaid with 2× lectin-containing medium (100 μl/well), infected with influenza virus at a multiplicity of infection (MOI) of 0.1 PFU/cell, and incubated for 48 h at 37°C. Virus yields were determined as the number of PFU/ml in MDCK cells. The drug concentration that caused a 50% decrease in the PFU titer in comparison to control wells without lectin was defined as EC₅₀. The results of two independent experiments were averaged.