Light Chain Dominant Quaternary Epitope Recognition and IgG3 Switching Drive Broad Dengue and Zika Virus Neutralization

Defining how antibodies achieve broad neutralization across antigenically related flaviviruses could inform rational vaccine design. Here, we describe 3A06, a human monoclonal antibody isolated from a dengue-infected donor that potently neutralizes dengue virus serotypes 1-3 (DENV1-3) and Zika virus (ZIKV). Binding studies show that 3A06 selectively recognizes envelope (E) protein dimers, but not monomeric E, EDIII, or viral lysates, indicating strict dependence on a quaternary epitope. Affinity measurements reveal stronger binding to DENV1, DENV3, and ZIKV E dimers, with weaker binding to DENV2 and DENV4. High-resolution structural analysis of 3A06 Fab bound to a chimeric ZIKV particle shows that the antibody engages a quaternary epitope spanning three E protomers at the dimer interface. Strikingly, structural and functional analyses demonstrate that epitope recognition is dominated by the antibody light chain, with somatic mutations driving neutralization potency. While the IgG1 form neutralizes DENV1-3 and ZIKV, subclass switching to IgG3 enables neutralization of all four dengue serotypes. Structural studies using intact IgG3 with DENV support the hypothesis that the enhanced breadth is mediated by extended reach and flexibility of the IgG3 Fc-hinge scaffold. These results reveal a previously unrecognized class of flavivirus broadly neutralizing antibodies and demonstrate how antibody light chain and subclass architecture can be engineered to overcome viral diversity. Our findings provide a structural blueprint for the rational design of universal flavivirus vaccines.