Immune response to synthetic peptide analogues of hepatitis B surface antigen specific for the a determinant.

Recovery from a natural infection with hepatitis B virus or vaccination with purified envelope protein leads to production of antibodies against the hepatitis B surface antigen (HBsAg). Such physiologic response in man is generally directed against the a determinant of HBsAg common to all serotypes of the virus. To define the immunochemical specificity of this determinant, the secondary structure of HBsAg was derived from its sequence of 226 amino acids. Hydrophilic stretches expected to contain the antigenic determinants were located between residues 32 and 76 and between residues 110 and 156. Loss of the antigenic activity after chemical modification of lysine residues of HBsAg indicated their critical importance in antigenicity. Because all lysines are located between residues 121 and 160, we selected this region for localization of HbsAg determinants. Solid-phase synthesis was used to prepare seven peptide analogues of HBsAg (PsAs): 122-137, 128-134, 139-147, 139-158, 140-158, 145-158, and 150-158. For experimental immunization of rabbits the synthetic peptides were coupled to keyhole limpet hemocyanin. We studied the antigenicity of each peptide analogue by serologic neutralization of human antibodies specific for the a determinant of HBsAg. Analogues 139-147, 139-158, and 140-158 showed antigenicity as well as function of anti-HBsAg. The rabbit antibodies were inhibited with each of the three peptide analogues and all serotypes of natural HbSag, having only the a determinant in common. These results indicate that the nonapeptide sequence 139-147 represents the total or an essential part of the a determinant of HBsAg.