Neutralizing antibodies derived from the B cells of 1918 influenza pandemic survivors

Investigation of the human antibody response to influenza virus infection has been largely limited to serology, with relatively little analysis at the molecular level. The 1918 H1N1 influenza virus pandemic was the most severe of the modern era1. Recent work has recovered the gene sequences of this unusual strain2, so that the 1918 pandemic virus could be reconstituted to display its unique virulence phenotypes3,4. However, little is known about adaptive immunity to this virus. We took advantage of the 1918 virus sequencing and the resultant production of recombinant 1918 hemagglutinin (HA) protein antigen to characterize at the clonal level neutralizing antibodies induced by natural exposure of survivors to the 1918 pandemic virus. In our study, each of 32 individuals tested that were born in or before 1915 exhibited seroreactivity with 1918 virus, nearly 90 years after the pandemic. Seven of 8 donor samples tested had circulating B cells that secreted antibodies that bound 1918 HA. We isolated B cells from subjects and generated five monoclonal antibodies that exhibited potent neutralizing activity against 1918 virus from three separate donors. These antibodies also cross-reacted with the genetically similar HA of a 1930 swine H1N1 influenza strain, but not with HAs of more contemporary human influenza viruses. The antibody genes exhibited an unusually high degree of somatic mutation. The antibodies bound to the 1918 HA protein with high affinity, exhibited exceptional virus neutralizing potency, and protected mice from lethal infection. Isolation of viruses that escaped inhibition suggested that the antibodies recognize classical antigenic sites on the HA surface. Thus, these studies reveal that survivors of the 1918 influenza pandemic possess highly functional, virus-neutralizing antibodies to this uniquely virulent virus, and that humans can sustain circulating B memory cells to viruses for many decades after exposure - well into the tenth decade of life.