In vitro propagation of human hepatitis B virus in a rat hepatoma cell line.

A rat hepatoma cell line (Q7) of Morris hepatoma origin was transfected with a construct containing the tandem dimer genome of human hepatitis B virus (HBV) and the neomycin-resistant selection marker. The culture medium of several neomycin-resistant single-cell clones was found to accumulate high levels of secreted HBV surface antigen and core-related e antigen. HBV-specific replication intermediates, including relaxed circular and single-stranded DNA with a minus-strand polarity, could be found in both the intracellular fraction and the extracellular culture medium by the Southern blot procedure. One of these clones, designated Q7 HBV-21, was characterized in further detail. DNA polymerase activity was present in the virus particles produced by Q7 HBV-21 cells. Characteristic transcripts of HBV, including the 3.5-, 2.5-, and 2.1-kilobase mRNA as well as a core-gene-related transcript of 2.2 kilobases could be detected. Electron microscopic examination of the conditioned medium from Q7 HBV-21 cells identified 42-nm Dane-like particles as well as 22-nm subviral particles with a spherical or filamentous shape. This Q7 HBV-21 cell line has been maintained in the absence of neomycin for 1 year without losing the properties of HBV DNA replication and Dane-like particle production. Our results strongly suggest that the species barrier of HBV infection is at an early step of viral absorption onto or penetration into the target hepatocytes. This nonhuman system for HBV production in culture could be used to complement the human HepG2 system.