Abstract

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.