ABSTRACT
ABSTRACT Since 2013, H7N9 avian influenza viruses (AIVs) have caused more than 1500 human deaths and millions of poultry culling. Despite large-scale poultry vaccination, H7N9 AIVs continue to circulate among poultry in China and pose a threat to human health. Previously, we isolated and generated four monoclonal antibodies (mAbs) derived from humans naturally infected with H7N9 AIV. Here, we investigated the haemagglutinin (HA) epitopes of H7N9 AIV targeted by these mAbs (L3A-44, K9B-122, L4A-14 and L4B-18) using immune escape studies. Our results revealed four key antigenic epitopes at HA amino acid positions 125, 133, 149, and 217. The mutant H7N9 viruses representing escape mutations containing Alanine to Threonine at residue 125 (A125T), Glycine to Glutamic acid at residue 133 (G133E), Asparagine to Aspartic acid at residue 149 (N149D), or Leucine to Glutamine at residue 217 (L217Q) showed reduced or completely abolished cross-reactivity with the mAbs, as measured by hemagglutination inhibition (HI) assay. We further assessed the potential risk of these mutants to humans should they emerge following mAb treatment by measuring the impact of these HA mutations on virus fitness and evasion of host adaptive immunity. Here we showed that the L4A-14 mAb had broad neutralizing capability, and its escape mutant N149D had reduced viral stability and human receptor binding and could be neutralized by both post-infection and antigen-induced sera. Therefore, L4A-14 mAb could be a therapeutic candidate for H7N9 AIV infection in humans and warrants further investigation for therapeutic application. IMPORTANCE Avian Influenza virus (AIV) H7N9 continues to circulate and evolve in birds, posing a credible threat to humans. Antiviral drugs have been proven useful for the treatment of severe influenza infections in humans, however, concerns have been raised as antiviral resistant mutants have emerged. Monoclonal antibodies (mAbs) have been studied for both prophylactic and therapeutic applications in infectious disease control and have demonstrated great potential. For example, mAb treatment has significantly reduced the risk of people developing severe disease with SARS-COV 2 infection. In addition to the protection efficiency, we should also consider the potential risk of the escape mutants generated by mAb treatment to public health by assessing their viral fitness and potential to compromise host adaptive immunity. Considering these parameters, we assessed four human mAbs derived from humans naturally infected with H7N9 AIV and showed that the mAb L4A-14 displayed potential as a therapeutic candidate.