A 'first loop' linear epitope accessible on native hepatitis B surface antigen that persists in the face of 'second loop' immune escape.

Murine monoclonal antibodies (mAbs) were raised following immunization with native mutant hepatitis B surface antigen (HBsAg) purified from human sera. A set of antibodies binding to a linear epitope carried between residues 121 and 129 of the s region was demonstrated. These antibodies were shown by cross-competition assays to bind to a single epitope whose antigenicity was influenced by the TTP motif lying between residues 125 and 127. This first loop epitope remained accessible on the surface of HBsAg in spite of major second loop mutations abrogating the normal a conformational epitopes. The mAb and its binding region in the first loop are important diagnostically and may represent an importance immunological target, one that is stable in the face of immunologically driven escape.