The role of antibody-dependent phagocytosis (ADP) and the control of viral infections

© 2017 Dr. Fernanda Elizabeth Ana-Sosa-BatizViral infectious diseases are major killers worldwide. Most current vaccine strategies against viral infections rely in eliciting neutralising antibodies, that are effective as long the virus does not change. However, both HIV and influenza have a high rate of replication error leading to continuous change and evasion to neutralising antibodies. Fortunately, antibodies have other function besides neutralisation, such antibody-dependent phagocytosis (ADP) and antibody dependent-cellular cytotoxicity (ADCC). These functions are known as Fc-mediated functions and consist in antibodies that bind with their Fab region to available antigens and engage Fc-receptor on innate immune cells with their Fc region. This engagement triggers a response in the innate immune cell. Recent studies indicate that ADCC and ADP may play a role in protection against HIV and influenza virus. To evaluate the role of ADP in clearing viral infections it is necessary to utilise robust methods to detect and measure ADP in vitro. In the field of HIV, an established ADP assay measures the uptake of IgG-opsonised HIV Env-coated fluorescent microspheres by a monocytic cell line. To advance the standard ADP assay we added an internalisation sensor to measure truly phagocytosed opsonised-antigen-coated beads. The development of our assay is covered in the context of HIV-specific ADP in Chapter II of this thesis. An advantage of this ADP assay is that can be adapted to measure ADP-mediating antibodies against any other viral antigen. The standardisation of the assay in the context of influenza-specific ADP is described in Chapter III. Using the modified ADP assay we investigated whether ADP-mediating antibodies mature during HIV infection (Chapter II). We compared ADP activity in purified plasma IgG from HIV-infected individuals during early primary infection and early chronic infection. We found that even though ADP-mediating antibodies were present during early primary infection there was a significant increase during early chronic infection in the absence of antiretroviral therapy in all individuals. We then studied if RV144 vaccination elicited ADP-mediating antibodies. We did not detect a significant increase in ADP-mediating antibodies in RV144 vaccinees compared to placebo recipients. The role ADP plays as a mechanism of protection against human influenza virus infection is currently unclear. To better understand how to elicit or boost anti-influenza ADP responses it is necessary to generate basic knowledge regarding the prevalence of these antibodies in the general population (Chapter III). We evaluated ADP activity in plasma IgG from influenza exposed healthy individuals and intravenous immunoglobulin (IVIG). We found that most individuals develop ADP-mediating antibodies against circulating influenza viruses and some individuals develop highly cross-reactive antibody responses that recognise non-circulating strains. We also investigated whether natural influenza infected humans and experimentally influenza infected macaques developed ADP-mediating antibodies. We found that influenza infection in humans and macaques elicits strain-specific and cross-reactive ADP-mediating antibodies. We also developed an in vitro functional assay to measure Ab-mediated clearance of live influenza virus. Using this assay, we found that uptake mediated by Abs of pandemic 2009 H1N1 virus and seasonal H1N1 does not lead to productive infection in THP-1 cells. 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 strains. Vaccine effectiveness of TIV varies year to year, depending on how well antigenically matched the strains in the vaccine are compared to circulating strains. 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 assays. We investigated the level of ADP-mediating Abs induced by 2008 TIV in healthy Australian adults aged over and under 60 years (Chapter IV). We found that young and older individuals mounted similar ADP responses against HA contained in the 2008 TIV, despite older adults having diminished HAI responses. This thesis highlights the potential role of ADP-mediating antibodies in HIV and influenza virus infections. Overall, understanding the prevalence and specificity of Abs with Fc-functions, such ADP, elicited either by infection or vaccination to HIV and influenza virus should contribute to the future rational design of more effective vaccines