FIG 3.

Neutralization of virus attached to cell surfaces. A/Puerto Rico/8/1934(H1N1) or A/California/7/2009(H1N1) virus was preadsorbed to MDCK cells at 4°C to avoid virus internalization. The attached viruses were then incubated with titrated amounts of HA stem-reactive (3.1, 1.12, and FI6) or hemagglutination-inhibiting (30D1 and H36-4) MAbs, and residual infectivity was detected. As a control, viruses were mock incubated in cell-free medium before titrated amounts of the MAbs were added. The HIV-1 gp120-specific MAb b12 (αHIV) was used as a nonneutralizing control. Gray areas highlight the differences between the neutralization curves, and numbers indicate the fold difference in the corresponding IC₅₀s. Representatives of at least 2 independent, consistent experiments (performed in duplicate) are shown.