Harris, E. D.; McGovern, M.; Pernikoff, S.; Ikeda, R.; Kipnis, L.; Hannon, W.; Sobolik, E. B.; Gray, M.; Greninger, A. L.; He, S.; Chin, C.-N.; Fu, T.-M.; Pancera, M.; Boonyaratanakornkit, J.

Human metapneumovirus (HMPV) is a major cause of respiratory infections, particularly among vulnerable populations, yet effective therapeutics remain unavailable. Monoclonal antibodies (mAbs) offer a promising approach for both treatment and prevention. Here, we describe the discovery and characterization of 4F11, a highly potent and broadly neutralizing mAb with demonstrated in vitro and in vivo efficacy against HMPV. Using cryo-electron microscopy, we defined a unique mechanism of binding HMPV employed by 4F11, which distinguishes it from previously characterized RSV and HMPV mAbs. 4F11 targets an epitope located at the apex of the prefusion F protein (site O) with a 1:1 stoichiometry, distinct from the 3:1 stoichiometry observed with other HMPV site O antibodies. Unlike other site O antibodies, which penetrate the glycan shield between Asn57 and Asn172, 4F11 binds vertically and directly interacts with the Asn172 glycan, representing a unique glycan-dependent mode of recognition. In vitro, 4F11 displayed high potency and broad neutralization across diverse HMPV strains. It also showed a low propensity for resistance development, with only a single escape mutation (K179E) identified, a mutation not found in any published HMPV sequence to date. Viruses rescued with the K179E escape mutation had significantly decreased fitness in vitro compared to wild-type virus. In a hamster challenge model, 4F11 significantly reduced viral loads in both the lungs and nasal turbinates. These findings highlight 4F11 as a promising candidate for therapeutic development, particularly for immunocompromised individuals and other high-risk groups.