Extended Heterosubtypic Neutralization and Preclinical Model In Vivo Protection from Clade 2.3.4.4b H5 Influenza Virus Infection by Broadly Neutralizing Antibodies

<b>Background/Objective</b>: The influenza virus remains one of the most prevalent respiratory pathogens, posing significant global health and economic challenges. According to the World Health Organization, the seasonal influenza virus infects up to 1 billion people and causes up to 650,000 deaths, annually. Despite influenza vaccination is the most effective available preventive strategy, its reliance on strain predictions and yearly updates limits its effectiveness. The virus' ability to cause both epidemics and pandemics, driven by zoonotic transmissions, underscores its continuous threat. The ongoing H5N1 avian influenza outbreak is the perfect example, renewing concerns due to its ability to infect over 70 mammalian species and sporadically transmit to humans. This study aims to evaluate the protective potential of two human monoclonal antibodies against diverse and recent influenza virus strains. <b>Method</b>: PN-SIA28 and PN-SIA49 monoclonal antibodies were previously isolated from an individual undergoing seasonal influenza vaccination and with no known recent influenza virus exposure. Their breadth of recognition, neutralization, and conferred in vivo protection were assessed against multiple influenza viruses, including pre-pandemic strains. Structural analyses were performed to characterize antibody-antigen interactions for epitope identification. <b>Results</b>: Both antibodies recognize a broad range of strains and neutralize pre-pandemic avian influenza viruses, including the currently circulating H5N1 clade. Moreover, a structural analysis revealed that PN-SIA49 binds a conserved HA stem region, overlapping with epitopes recognized by other broadly neutralizing antibodies. <b>Conclusions</b>: These findings underscore the potential of broadly neutralizing antibodies as a basis for universal influenza countermeasures against both seasonal and pandemic threats. Additionally, they provide guidance for the design of targeted vaccine strategies to steer immune responses toward broadly protective epitopes.