Structural basis for ultrapotent neutralization of human metapneumovirus

Human metapneumovirus (hMPV) is a leading cause of morbidity and hospitalization among children worldwide, however, no vaccines or therapeutics are currently available for hMPV disease prevention and treatment. The hMPV fusion (F) protein is the sole target of neutralizing antibodies. To map the immunodominant epitopes on the hMPV F protein, we isolated a panel of human monoclonal antibodies (mAbs), and the mAbs were assessed for binding avidity, neutralization potency, and epitope specificity. We found the majority of the mAbs target diverse epitopes on the hMPV F protein, and we discovered multiple mAb binding approaches for antigenic site III. The most potent mAb, MPV467, which had picomolar potency, was examined in prophylactic and therapeutic mouse challenge studies, and MPV467 limited virus replication in mouse lungs when administered 24 hrs before or 72 hrs after viral infection. We determined the structure of MPV467 in complex with the hMPV F protein using cryo-electron microscopy to a resolution of 3.3 [A], which revealed a complex novel prefusion-specific epitope overlapping antigenic sites II and V on a single protomer. Overall, our data reveal new insights into the immunodominant antigenic epitopes on the hMPV F protein, identify a new mAb therapy for hMPV F disease prevention and treatment, and provide the discovery of a unique pre-fusion-specific epitope on the hMPV F protein.