Structural and Mechanistic Basis for Antibody Neutralization of the Measles Fusion Protein

Measles virus (MeV) is a highly contagious viral pathogen and remains a major global health threat. Resurgent infections, driven by insufficient vaccine coverage, waning herd immunity, and the vulnerability of immunocompromised individuals, highlight the urgent need for effective countermeasures. Monoclonal antibodies (mAbs) represent a promising strategy, both as antiviral agents and as probes of viral entry mechanisms. While most vaccine-elicited neutralizing antibodies target the hemagglutinin (H) protein, emerging evidence suggests that antibodies against the fusion (F) protein are also potent inhibitors. Still, there is insufficient information on the target sites and activities of antibodies against the F protein. Like other class I fusion proteins, MeV F exists in a metastable prefusion state that undergoes dramatic conformational changes during viral entry. Here, we selected four mAbs that recognize conformational patterns of F-prefusion and/or postfusion, characterized their epitopes, specificities, and antiviral activities. Structural analyses mapped antibody interactions onto pre- and postfusion F conformations, revealing that all three neutralizing mAbs are specific for the prefusion form, while the non-neutralizing mAb recognizes only the postfusion F. Biophysical and functional assays defined distinct mechanisms: neutralization occurs either by stabilizing the prefusion protein or by preventing the extended intermediate from completing fusion. We also describe a novel mechanism of neutralization in which an antibody prematurely triggers F activation but blocks the subsequent refolding required for viral entry. Together, these findings provide the first detailed mapping of neutralizing epitopes on the MeV F protein and establish a framework for the rational design of F-targeted intervention.