Naturally Occurring Human Monoclonal Antibodies Neutralize both 1918 and 2009 Pandemic Influenza A (H1N1) Viruses

The 2009 pandemic influenza A (H1N1) virus exhibits hemagglutinin protein sequence homology with the 1918 pandemic influenza virus. We found that human monoclonal antibodies recognized the Sa antigenic site on the head domains of both 1918 and 2009 hemagglutinins, a site that is hypervariable due to immune selection. These antibodies exhibited high potency against the 2009 virus in vitro, and one exerted a marked therapeutic effect in vivo.