The intersubunit region of the influenza virus haemagglutinin is recognized by antibodies during infection.

The influenza virus haemagglutinin has an important role in the infectious cycle of the virus and carries multiple B and T cell epitopes. It is synthesized as a single polypeptide chain but viral infectivity depends on its post-translational enzymatic cleavage. The cleavage site of a trypsin-like enzyme responsible for this modification is found in the most conserved intersubunit region of the molecule. In this study the role of this region in antibody recognition was investigated. Synthetic peptides comprising the intact and cleaved forms of the intersubunit segment were used to examine the specificity of virus- or peptide-induced antibodies. The immune response elicited by viral infection resulted in the appearance of antibodies capable of neutralizing the virus without interfering with its binding to the receptor. A monoclonal antibody (MoAb) of such functional properties was shown to recognize the intact intersubunit region both in the uncleaved haemagglutinin molecule and in a 25-mer synthetic peptide comprising the intact intersubunit region. Specificity and functional studies revealed the conformation-dependent recognition of the C-terminal segment of the haemagglutinin 1 subunit by this MoAb. The binding of the antibody was shown to inhibit the trypsin-mediated cleavage of the haemagglutinin molecule and the membrane fusion event. The enzymatic cleavage of the haemagglutinin was demonstrated to abolish antibody recognition of the infective virus suggesting an escape mechanism mediated by the functional destruction of this highly conserved region. The synthetic peptide corresponding to the intact intersubunit region is characterized by an ordered structure and is able to elicit an antibody response in BALB/c mice while its subfragments are nonimmunogenic. Furthermore, this peptide elicited a protective immune response demonstrated by in vivo experiments.