Cross-Reactive Influenza-Specific Antibody-Dependent Cellular Cytotoxicity Antibodies in the Absence of Neutralizing Antibodies

A better understanding of immunity to influenza virus is needed to generate cross-protective vaccines. Engagement of Ab-dependent cellular cytotoxicity (ADCC) Abs by NK cells leads to killing of virus-infected cells and secretion of antiviral cytokines and chemokines. ADCC Abs may target more conserved influenza virus Ags compared with neutralizing Abs. There has been minimal interest in influenza-specific ADCC in recent decades. In this study, we developed novel assays to assess the specificity and function of influenza-specific ADCC Abs. We found that healthy influenza-seropositive young adults without detectable neutralizing Abs to the hemagglutinin of the 1968 H3N2 influenza strain (A/Aichi/2/1968) almost always had ADCC Abs that triggered NK cell activation and in vitro elimination of influenza-infected human blood and respiratory epithelial cells. Furthermore, we detected ADCC in the absence of neutralization to both the recent H1N1 pandemic strain (A/California/04/2009) as well as the avian H5N1 influenza hemagglutinin (A/Anhui/01/2005). We conclude that there is a remarkable degree of cross-reactivity of influenza-specific ADCC Abs in seropositive humans. Targeting cross-reactive influenza-specific ADCC epitopes by vaccination could lead to improved influenza vaccines.