Expression and localization of the small and large delta antigens during the replication cycle of hepatitis D virus

To study the expression and localization of delta antigen during the replication cycle of hepatitis D virus, we cotransfected HuH‐7 hepatoma cells with a hepatitis B virus expression plasmid and plasmids expressing the small or large delta antigen or the entire HDV genome. The transfected cells and culture medium were analyzed on double immunofluorescence staining for delta antigen and HBsAg, on Western blotting and on Northern‐blot hybridization from 4 hr to 9 days after transfection. In cells transfected with the expression plasmid carrying the small delta antigen open reading frame, only the small delta antigen was expressed; it was localized in the nucleolus and was not released into the medium during the culture period. In cells transfected with the large delta antigen expression plasmid, the large delta antigen expressed was localized in the nucleolus at the initial stage; this was followed by relocalization in nucleoplasm. Finally, large delta antigen was released in HBsAg‐enveloped particles within 1 day of transfection. In cells cotransfected with hepatitis B virus and hepatitis D virus expression plasmids, the small delta antigen was expressed 4 hr after transfection, whereas the large form was expressed 3 days after transfection. Expression of the large delta antigen coincided with the localization changes from nucleolar to mixed stage and, finally, to nucleoplasm and release of the hepatitis D virus particles. The large delta antigen appears to play a key role in relocalization of the delta antigen and packaging of the hepatitis D virus virions. However, early expression of the large delta antigen suppressed hepatitis D virus replication, resulting in the release of defective particles containing an excess amount of the large delta antigen devoid of hepatitis D virus RNA. (H EPATOLOGY 1992;16:1120–1127.)