Antibody-dependent phagocytosis (ADP) responses following trivalent inactivated influenza vaccination of younger and older adults

Globally the most commonly utilised immunisation against influenza is the trivalent inactivated influenza vaccine (TIV) derived from an A/H1N1, an A/H3N2 and a B type influenza virus. Vaccine effectiveness of TIV varies year to year, depending on how well antigenically matched the strains in the vaccine are compared to circulating strains [1,2]. Moreover, vaccine effectiveness can vary within certain subpopulations such as HIV-positive, young children and the elderly. Decreased vaccine effectiveness in the elderly is associated with impaired Ab production, as measured by standard hemagglutination inhibition (HAI) assays. We investigated the level of Antibody Dependent Phagocytosis (ADP)-mediating Abs induced by the 2008-TIV in healthy Australian adults aged over and under 60 years to determine if this immune function was also reduced in the elderly. We utilised an ADP assay that measures the uptake of IgG-opsonised HA-coated fluorescent microspheres by a monocytic cell line. We also measured HA-specific Abs that are close enough to bind to dimeric FcγRIIa ectodomains in an ELISA-based assay. Furthermore, we compared the extent of cross-reactive recognition of diverse influenza strains by ADP-mediating Abs found in pre- and post-vaccination sera in both of these groups. We found that young adults and older adults mounted similar ADP activity against HAs contained in the 2008-TIV, despite older adults have diminished HI responses. The level of cross-reactive antibodies against other HAs was limited in both groups. We conclude that seasonal influenza vaccination elicits limited cross-reactive ADP to HA in both young and older adults. New influenza vaccination strategies that elicit cross-reactive and polyfunctional antibodies are needed.