Characterisation of a human monoclonal antibody targeting a conserved epitope at the base of the HA head of influenza A(H3N2) virus

Abstract

Background Numerous broadly reactive human monoclonal antibodies (mAbs) against the haemagglutinin (HA) of influenza A viruses have recognised conserved epitopes across HA subtypes or within subtypes. Most heterosubtypic mAbs target the HA stem, the receptor-binding site (RBS), or the trimeric interface. Although at least three H3-specific mAbs recognise epitopes outside these regions, the overall landscape of conserved H3-specific epitopes remains incompletely understood. This study aimed to identify and characterise conserved epitopes on H3-HA to inform the development of vaccines resilient to antigenic change. Methods We screened a panel of previously reported H3-HA-reactive human mAbs to identify mAbs recognising conserved epitopes. The candidate clone, 034-10040 4F02 (4F02), was evaluated for neutralising, haemagglutination inhibiting, and HA-mediated fusion-inhibitory, Fc receptor-mediated effector functions in vitro, and for protective efficacy in a lethal mouse challenge model. Cryo-electron microscopy was used to define the structural basis of 4F02 binding. Human sera were screened for antibodies targeting similar epitopes. Findings Clone 4F02 recognised the HA of human influenza A (H3N2) viruses that circulated across multiple decades. It neutralised multiple H3N2 viruses, exhibited weak haemagglutination inhibition, blocked HA-mediated fusion activity, activated Fc receptor-mediated signalling, and protected mice against lethal challenge. Cryo-electron microscopy revealed that 4F02 targets the base of the HA head at a head-stem interface spanning antigenic sites C, D, and E. Antibodies targeting similar epitopes were detected, albeit at a low level, in human sera. Interpretation Characterisation of the 4F02 epitope reveals a previously underappreciated site of vulnerability at the H3-HA head-stem interface. This finding expands our understanding of conserved epitopes and provides a target for the development of influenza vaccines resilient to antigenic change. Funding This work was supported by the Japan Agency for Medical Research and Development, JSPS KAKENHI, the National Institutes of Allergy and Infectious Diseases.