Identification of a variant A-specific neutralizing epitope on glycoprotein B (gB) of human herpesvirus-6 (HHV-6).

Based on genetic, antigenic, and growth properties, human herpesvirus-6 (HHV-6) can be classified into two groups, variant A (HHV-6A) and variant B (HHV-6B). We have mapped the HHV-6A-specific epitope on glycoprotein B (gB), which was recognized by a monoclonal antibody (MAb), 87-y-13, with a complement-independent neutralizing activity. Plasmids carrying various chimeric gB sequences formed between strains U1102 (HHV-6A) and HST (HHV-6B) and carrying sequences for a series of carboxy-terminal deletions of U1102 gB were constructed. By using the plasmids, in vitro transcription and subsequent in vitro translation were carried out. Immunoprecipitation assay of the translated products with MAb 87-y-13 revealed that MAb 87-y-13 was able to react only with in vitro translation products containing the sequence between amino acid residues 335 and 395 of U1102 gB. Amino acid sequence comparison between HHV-6A and HHV-6B in this region showed that amino acid residues 347, 387, and 393-395 were HHV-6A-specific. To determine which amino acid residue(s) was involved in recognition by MAb 87-y-13 as well as in the neutralizing activity, point mutations were introduced at those amino acid positions. Immunoprecipitation assay of the mutagenized gB with point mutations suggested that the neutralizing MAb-y-13 was involved in the recognition of the amino acid Asn at residue 347 of U1102 gB (HHV-6A). This site may play an important role in viral infection.