Project Summary Vaccines are powerful tools against microbial pathogens; however, it has not been possible to elicit antibodies that neutralize each pathogen nor to generate sufficient neutralizing antibodies in each person. Along with neutralizing antibodies, vaccines and infection often generate non-neutralizing antibodies that can provide protection through cell-dependent and cell-independent mechanisms. Mechanisms that require the Fc portion of IgG antibodies include antibody-dependent cell-mediated cytotoxicity and phagocytosis (ADCC and ADCP, respectively) and complement-dependent cytotoxicity (CDC). Unfortunately, protection from non-neutralizing antibodies often proves inferior compared to neutralizing antibodies, especially against viral pathogens, limiting the efficacy of vaccines as well as antibodies treating bacterial pathogens that likewise do not neutralize. Additionally, even many broadly neutralizing antibodies in vitro require Fc functions for efficacy in vivo. Currently, the complete protective potential of the entire antibody repertoire is not realized because factors affecting Fc- mediated immune responses are not fully understood, and thus cannot be fully tuned to the host’s advantage. We expect that defining general principles of Fc-mediated immune responses will promote the design of efficacious vaccines and antibody treatments for many pathogens. In this proposal, we will investigate multiple factors that modulate the Fc-mediated responses to antibodies, to increase the potency of protection resulting from antibodies requiring Fc functions for influenza virus or antibodies against Streptococcus pneumoniae. Multiple groups have defined a set of factors that affect the Fc-mediated immune response. These include location of the epitope, antibody glycosylation, antibody oligomerization, and properties of the responding immune cell, including glycosylation of the Fcγ receptors (FcγRs). We will evaluate the hypothesis that the success of Fc-mediat