THE INTERSUBUNIT REGION OF THE INFLUENZA-VIRUS HEMAGGLUTININ IS RECOGNIZED BY ANTIBODIES DURING INFECTION

The influenza virus haemagglutinin has an important role in the infect ious cycle of the virus and carries multiple B and T cell epitopes. It is synthesized as a single polypeptide chain but viral infectivity de pends 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. Syn thetic peptides comprising the intact and cleaved forms of the intersu bunit segment were used to examine the specificity of virus- or peptid e-induced antibodies. The immune response elicited by viral infection resulted in the appearance of antibodies capable of neutralizing the v irus without interfering with its binding to the receptor. A monoclona l antibody (MoAb) of such functional properties was shown to recognize the intact intersubunit region both in the uncleaved haemagglutinin m olecule and in a 25-mer synthetic peptide comprising the intact inters ubunit region. Specificity and functional studies revealed the conform ation-dependent recognition of the C-terminal segment of the haemagglu tinin 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 haemagglu tinin was demonstrated to abolish antibody recognition of the infectiv e virus suggesting an escape mechanism mediated by the functional dest ruction of this highly conserved region. The synthetic peptide corresp onding to the intact intersubunit region is characterized by an ordere d structure and is able to elicit an antibody response in BALB/c mice while itp subfragments are nonimmunogenic. Furthermore, this peptide e licited a protective immune response demonstrated by in vivo experimen ts.