Background: Highly pathogenic avian H5 influenza viruses have spread and diversified genetically and antigenically into multiple clades and subclades. Most isolates of currently circulating H5 viruses are in clade 2.3.2.1 or 2.3.4.4.

Methods: Panels of murine monoclonal antibodies (mAbs) were generated to the influenza hemagglutinin (HA) of H5 viruses from the clade 2.3.2.1 H5N1 vaccine virus A/duck/Bangladesh/19097/2013 and the clade 2.3.4.4 H5N8 vaccine virus A/gyrfalcon/Washington/41088-6/2014. Antibodies were selected and characterized for binding, neutralization, epitope recognition, cross-reactivity with other H5 viruses, and the ability to provide protection in passive transfer experiments.

Results: All mAbs bound homologous HA in an ELISA format; mAbs 5C2 and 6H6 were broadly binding for other H5 HAs. Potently neutralizing mAbs were identified in each panel, and all neutralizing mAbs provided protection in passive transfer experiments in mice challenged with a homologous clade influenza virus. Cross-reacting mAb 5C2 neutralized a wide variety of clade 2.3.2.1 viruses, as well as H5 viruses from other clades, and also provided protection against heterologous H5 clade influenza virus challenge. Epitope analysis indicated that the majority of mAbs recognized epitopes in the globular head of the HA. The mAb 5C2 appeared to recognize an epitope below the globular head but above the stalk region of HA.

Conclusions: The results suggested that these H5 mAbs would be useful for virus and vaccine characterization. The results confirmed the functional cross-reactivity of mAb 5C2, which appears to bind a novel epitope, and suggest the therapeutic potential for H5 infections in humans with further development.