Influenza Virus A ( H 1 N 1 ) 2009 Antibody-Dependent Cellular Cytotoxicity in Young Children Prior to the H 1 N 1 Pandemic

Pre-existing immunity played a significant role in protection during the latest influenza A virus H1N1 pandemic, especially in older age groups. Structural similarities were found between A(H1N1) 2009 and older H1N1 virus strains to which humans had already been exposed. Broadly cross-reactive antibodies capable of neutralizing the A(H1N1) 2009 virus have been implicated in this immune protection in adults. We investigated the serological profile of a group of young children aged 9 years (n=55), from whom paired blood samples were available, just prior to the pandemic wave (March 2009) and shortly thereafter (March 2010). On the basis of A(H1N1) 2009 seroconversion, 27 of the 55 children (49 %) were confirmed to be infected between these two time points. Within the non-infected group of 28 children (51 %), high levels of seasonal antibodies to H1 and H3 HA1 antigens were detected prior to pandemic exposure, reflecting past infection with H1N1 and H3N2, both of which had circulated in The Netherlands prior to the pandemic. In some children, this reactivity coincided with specific antibody reactivity against A(H1N1)2009. While these antibodies were not able to neutralize the A(H1N1) 2009 virus, they were able to mediate antibody-dependent cellular cytotoxicity (ADCC) in vitro upon interaction with the A(H1N1) 2009 virus. This finding suggests that cross-reactive antibodies could contribute to immune protection in children via ADCC.