Use of recombinant hepatitis delta antigen in diagnostic assays for HDV antibody

The gene encoding the hepatitis delta virus (HDV) structural antigen (HDAg) was inserted into a Rous sarcoma virus expression vector and the recombinant plasmid used to direct the synthesis of recombinant HDAg (rHDAg) in a continuous hepatoma cell line. A competitive radioimmunoassay for serum antibody to HDV using rHDAg was developed and was found to be equally suitable for diagnostic purposes to a radioimmunoassay using infected liver-derived HDAg. Similarly, rHDAg was shown to be serologically equivalent to liver-derived HDAg within the limit of the blocking titrations performed. The rHDAg-positive cell line was also used in an indirect immunofluorescence assay to detect anti-HD. Similar titres of anti-HD were detected by both radioimmunoassay and immunofluorescence and identical samples were positive for anti-HD by either assay. In a sample of prison inmates with high prevalence of both HBV and HDV, anti-HD was confined almost exclusively to those with persistent HBV infection and not to those in whom HBV infection had cleared. The availability of rHDAg will permit wider development of diagnostic anti-HD assays, and the use of two such assays is presented in this study.