A protective human antibody reveals a quaternary epitope on the Epstein-Barr virus fusion apparatus.

Epstein-Barr virus (EBV) entry into B lymphocytes and epithelial cells requires the conserved fusion machinery gHgL and the tropism-switching glycoprotein gp42. While the individual roles of these proteins have been extensively studied, the functional contribution and antibody accessibility of the assembled gHgL-gp42 complex remain unclear. Here, we describe a human monoclonal antibody, 4G12, that selectively recognizes this complex by binding a quaternary epitope spanning the gHgL DII domain and the gp42 N-terminal interface. 4G12 potently neutralizes EBV infection in both B cells and epithelial cells by blocking viral binding and membrane fusion. A cryogenic electron microscopy (cryo-EM) structure reveals a vulnerable site within the fusion apparatus and defines the molecular basis of inhibition. Importantly, 4G12 confers robust protection against EBV challenge in humanized mice. These findings identify a neutralizing epitope on the gHgL-gp42 fusion complex and establish 4G12 as a mechanistic probe and candidate for EBV prophylaxis.