A neutralizing nanobody targeting a conserved lateral patch on HA1 confers protection against multiple H7 avian...

Human infections with H7 avian influenza viruses (AIVs) have been documented globally, involving multiple subtypes and geographic regions. However, effective therapeutics targeting H7 influenza viruses remain limited. Here, a panel of nanobodies targeting the HA1 domain of hemagglutinin (HA) was identified by yeast two-hybrid (Y2H) screening, and six candidates were subsequently validated to exhibit hemagglutination inhibition (HI) activity. Of these, a subset also displayed virus microneutralization (MN) activity, while all showed binding activity in ELISA assays. Among them, Nb74 exhibited inhibitory activity against four Chinese recombinant vaccine-matched strains (Rv1-Rv4), which were generated based on the HA sequences of the corresponding inactivated vaccine strains H7-Re1 to H7-Re4. The HI-IC₅₀ values were 0.23, 0.57, 3.65, and 43.75 µg/mL, respectively, and the MN-IC₅₀ values for Rv1-Rv3 were 0.02, 0.06, and 1.09 µg/mL. It also retained activity against diverse clinical isolates although HI potency varied among strains. In mouse challenge experiments, intratracheal administration of Nb74 conferred robust protection, achieving 100% and 80% survival against Rv1 and Rv2, respectively, when administered prophylactically (2 mg/kg) or therapeutically (4 mg/kg). Treated mice showed accelerated body weight recovery, reduced lung viral load, and alleviated pulmonary pathology. Mechanistic analyses indicated that Nb74 neutralizes virus by blocking viral attachment to the host. Furthermore, combined hydrogen-deuterium exchange mass spectrometry (HDX-MS) with escape mutant analysis mapped its epitope to a conserved lateral patch on the HA1 subunit, consistent with a conformational epitope. Overall, these results demonstrate the therapeutic promise of intratracheally delivered Nb74 and provide insights for H7 AIVs vaccine design.IMPORTANCEH7 avian influenza viruses (AIVs), including both low- and highly pathogenic strains, pose a persistent threat to poultry production and public health, while continuous antigenic evolution of the HA1 subunit undermines vaccine effectiveness. Broadly neutralizing agents against H7 AIVs are urgently needed for pandemic preparedness, yet nanobody-based therapeutics remain largely unexplored. In this study, we identified Nb74, a nanobody that inhibits hemagglutination and neutralizes multiple H7 strains. Importantly, intratracheal delivery of Nb74 conferred protective efficacy, supporting respiratory administration of nanobody therapeutics. Mechanistic analyses show that Nb74 blocks viral attachment by recognizing a conserved conformational epitope within a lateral patch of the HA1 subunit. These findings reveal a conserved and functionally vulnerable region on H7 hemagglutinin (HA) and support the development of nanobody-based antivirals and improved vaccine strategies against emerging H7 AIV threats.