Preparation and characterization of a Llama VHH-hFc chimeric antibody recognizing conserved neutralization epitope of H5N1 hemagglutinin with high affinity.

Highly pathogenic avian influenza (HPAI) H5N1 virus poses a continuing global public health threat due to its outbreaks in poultry farms and zoonotic transmission from birds to humans. In the quest of effective therapeutics against H5N1 infection, antibodies with broad neutralizing activity have attracted significant attention. In this study, we employed a phage display technique to select and identify VHH antibodies with specific neutralizing activity against H5N1 hemagglutinin (HA) from an immune llama-derived antibody library. Subsequently, we prepared fusions of VHH10 antibody with human Fc fragment. The chimeric antibody VHH10-hFc was characterized and evaluated for its specificity, binding affinity, serum persistence and antigen recognition epitope. Following purification from 293 F cell cultures, VHH10-hFc chimeric antibody retained its specificity to H5N1 HA. Its antigen-binding affinity was enhanced by up to 130-fold, and its serum persistence was extended by up to 170-fold compared to VHH10. The VHH10-hFc chimeric antibody demonstrated high affinity, excellent thermal stability, and broad reactivity against H5N1 HA in clades 0, 1, 2, and 4. Through epitope mapping, we identified a conformational epitope consisting amino acid residues at positions Q187, K189, L190, Y191, N193, T215, S217 and N220 located on the top region of HA, which was specific and conserved epitopes among H5N1 strains. Consequently, VHH10-hFc, with great specificity, high affinity, prolonged serum persistence and good thermal stability, recognizes a conserved neutralization epitope on the globular head of H5N1 HA, indicating great potential in therapeutic strategies against H5N1 infection.