Cross-Neutralizing mAb to Influenza Viruses Figure 2. Sequence conservation in hemagglutinin groups and subtypes. Boxes indicate mutated residues which decrease PN-SIA28 binding to mutated HAs. Circles on the top indicate PN-SIA28 percent binding to each HA alanine mutants compared to binding to wild-type HA: red 25% binding, yellow 50–75% binding. Sequence numbering is based on H1N1 A/PR/8/34 coding region (GenBank accession number ABO21709). Neutralizing activity of PN-SIA28 against each strain is highlighted by a green ‘+’ or a red ‘2’ on the left, indicating neutralizing activity and no neutralizing activity, respectively. *Recombinant HA from H1N1 A/South Carolina/1/1918 pandemic strain was previously shown to be bound by PN- SIA28 [19,20]; analogously, recombinant HA from H5N1 A/Cygnus Olor/Italy/742/2005 was recognized by PN-SIA28 (data not shown). # H1N1 A/New Caledonia/20/1999 was previously shown to be neutralized by PN-SIA28 as Fab fragment [19,20]. doi:10.1371/journal.pone.0028001.g002 robust neutralizing activity (IC50 = 0.4–3.7 mg/ml) against all tested strains belonging to group 1, thus demonstrating that its epitope is broadly shared among group 1 viruses, including the highly divergent H9 subtype. More importantly, PN-SIA28 showed a potent neutralizing activity also against group 2 viruses demonstrating that its epitope is also present in the highly PLoS ONE | www.plosone.org 4 December 2011 | Volume 6 | Issue 12 | e28001