Project Summary: There have been four influenza outbreaks that caused global pandemics causing major threats to public health. Although influenza vaccines have been available for protection, current vaccines are strain-specific and require annual renewal, whereas the development of antiviral drugs encountered the problem of drug resistance. In addition to improving the prevention and treatment of flu, developing a rapid and precise method for detecting specific influenza variants could assist prophylactic measures, and glycan arrays have been an effective tool to meet this purpose. During influenza infection, the virus surface hemagglutinin first interacts with a sialoside receptor on the respiratory tract followed by neuraminidase-mediated cleavage of sialic acid from the receptor for viral entry. Determining the precise structure of the sialoside receptor and the detailed mechanism of infection will facilitate the development of better diagnostic methods. Unfortunately, current N-glycan samples occupy only a small percentage of the glycans required for the study of the influenza virus. The availability of asymmetrically branched N-glycans is instrumental to the development of next-generation diagnostic tools and glycan sequencing methods. The improved chemoenzymatic and modular strategies developed in this study will enable the previously infeasible synthesis of complex glycans commonly found in the respiratory tract but absent in the current arrays. These complex glycans will be used to create a comprehensive glycan array, thus advancing our understanding of the precise binding specificity of HA subtypes and the rapid detection of influenza variants. Monoclonal antibodies have been used for the treatment of life-threatening diseases, including viral infections. While the Fab region defines antibody specificity against viral targets, the Fc region recruits immune cells to fight infection. In particular, the function of a single N-glycan at N297 of Fc is to engage FcγRIIIA and FcγRIIA receptors, activating the antibody-depended cell-mediated cytotoxicity (ADCC) and the long-lasting vaccinal effect respectively for neutralization and clearance of infected cells. We have identified the α2,6-sialylated complex type biantennary glycan without core fucose (SCT) as an optimal modification for the broadly neutralizing anti-influenza antibody FI6 to exhibit the best ADCC and vaccinal effect. In this study, we will evaluate the 7F-SCT glycoform of FI6, which is stable toward sialidase degradation and is expected to have optimal effector functions yet can be synthesized enzymatically. We anticipate that the strategies and methods developed in this study will further transform the field of influenza research and glycoscience enabling access to multi-antennary N-glycans and precise arrays for the study of HA binding specificity and evaluation of homogenous antibodies against influenza infection.