A novel hepatitis B virus variant S 129 (Gln-->Leu): lack of correlation between antigenicity and immunogenicity

A point mutation has been detected in the "a" determinant of hepatitis B surface antigen (HBsAg) in an infant immunised with hepatitis B vaccine after exposure to hepatitis B virus (HBV). This A-to-T point mutation at nucleotide 540 resulted in a glutamine-to-leucine substitution at amino acid residue 129 (129L). The S gene fragment (nucleotide 58-1072) of this isolate was cloned and used to substitute the wild-type S gene in a plasmid (p3.8II), containing 1.2 copy of full-length HBV genome with expression and replication efficiency. This plasmid p3.8II-129L was used to transfect HepG2 cells. HBsAg expressed by p3.8II-129L showed higher binding efficiency compared with the original plasmid containing the wild-type clone. A panel of 24 anti-HBs monoclonal antibodies (MAbs) was used to characterise the binding efficiency of HBsAg expressed by p3.8II-129L. Eighteen showed higher binding to the antigen, whereas HBsAg expressed by p3.8II-145R gave a consistently lower absorbance or were negative. Surprisingly, when the immunogenicity of plasmid constructs was used for DNA immunisation in Balb/c mice, the anti-HBs response induced by p3.8II-129L was significantly lower than that of the wild-type p3.8II. The lack of correlation between the antigenicity profile (binding of expressed HBsAg to anti-HBs in vitro), and the immunogenicity (induction of anti-HBs by plasmid DNA in vivo) of HBsAg with leucine substitution at position 129 indicates that biological characteristics other than the binding efficiency of HBsAg to anti-HBs could occur in HBsAg variants. These different aspects of the biological characteristics of S mutants merit further investigation.