Screening a monoclonal antibody with a fusion-phage display library shows a discontinuity in a linear epitope within PreS1 of hepatitis B virus.

The epitope recognized by the monoclonal antibody MA18/7, specific for the PreS1-domain of the hepatitis B virus surface antigen, has been defined precisely by means of a library of fusion-phage carrying random hexapeptides on the tip of filamentous phage fd particles. Phage, isolated after only one round of affinity selection, displayed hexapeptides showing strong conservation of the PreS1 primary sequence in the region 19-23 with three noncontiguous residues, DP (20 and 21) and F (23) appearing in phage that bound the antibody. The importance of these core residues was supported by comparing the antibody binding of individual phage in solution, which provided relative dissociation constants for these interactions. Replacement of F (23) by Y was the only substitution observed in the three core residues, and resulted in somewhat weaker binding. Synthetic tetra- and hexapeptides containing these key residues inhibited the reaction between the phage and the antibody.