The A/PR/8/34 (H1N1) and A/Milan/UHSR1/2009 (H1N1) viruses were also tested in plaque reduction assay as previously described [26] . Briefly, neutralizing assays were carried out in 6 wells plates using MDCK cells (56105 cells/well). Two dilutions, 1-0.1 mg/ml, of IgG PN-SIA49 were preincubated for 1 hour at 37uC with 100 TCID50 of virus. Following this incubation, 1 ml of each virus-antibody mix was added on MDCK monolayer and the plate was incubated 1 hour at 37uC in 5% CO2. Then, the medium was removed and the monolayer washed twice with PBS. Two ml of MEM-agarose 0.8% supplemented with penicillin (50 mg/ml) (Gibco Invitrogen, Carlsbad, CA, USA), streptomycin (100 mg/ml) (Gibco Invitrogen, Carlsbad, CA, USA), L-glutamine (2 mM) (Gibco Invitrogen, Carlsbad, CA, USA) and trypsin (2 mg/ml) (Roche Applied Sciences) were added to each well and the plates were incubated 48 hours at 37uC in 5% CO2. After this incubation, the agarose medium was removed from each well and 1 ml of 70% methanol-crystal violet 1% (w/v) was added to each well at room temperature. Finally, the wells were washed with tap water and dried. An infection control without antibody was added as well as a negative control with anti-HCV/E2 e137 mAb. The neutralization was determined counting the PFU reduction in presence of antibodies compared to the infection control.