Canonical Antibodies Adopt Distinct Binding Modes to Recognize Viral Glycan Shields.

Viral entry glycoproteins are often shielded from immune recognition by dense N-linked glycans that limit antibody access to protein epitopes. While glycan-reactive antibodies with unusual architectures have been described, how canonical Y-shaped antibodies engage these glycan-rich surfaces remains poorly defined. Here, we characterize two human antibodies, VRC35 and VRC36, isolated from an HIV-1-infected donor, that recognize diverse glycosylated viral glycoproteins. Cryo-electron microscopy structural analyses of these antibodies in complex with viral entry glycoproteins, including HIV-1 envelope, influenza hemagglutinin, SARS-CoV-2 spike, and the Lassa virus glycoprotein complex, reveal adaptive Fab stoichiometries ranging from single-Fab binding to dimeric and higher-order assemblies are mediated by intra- and inter-IgG interactions that depend on local glycan organization. Dense glycan clustering on HIV-1 and influenza glycoproteins supports multivalent Fab assemblies and correlates with neutralization activity, whereas sparse glycan environments on SARS-CoV-2 and Lassa virus favor weak or heterogeneous engagement without neutralization. Structural and mutational analyses further demonstrate that homotypic Fab-Fab interactions stabilize multivalent engagement and contribute to neutralizing activity. Together, these findings define a structural framework in which viral glycan organization constrains antibody valency and engagement, while somatic hypermutation contributes to the acquisition of homotypic Fab-Fab interactions that facilitate multivalent recognition of viral glycan shields.