Adsorption and penetration of hepatitis B virus in a nonpermissive cell line

A cell monolayer radioimmunoassay was established to detect hepatitis B virus (HBV)-binding activity. HBV was shown to bind to HepG2 and HuH7 cells, but not to LTK- and HeLa cells. The binding activity was inhibited by peptide 21-47 from the large hepatitis B surface antigen (L-HBsAg) region, but not by other peptides from the L- and middle-HBsAg. A monoclonal antibody to L-HBsAg (MA18/7) and a polyclonal antibody to HBsAg (anti-HBs) also inhibited the binding. However, only the Fab fragment of MA18/7 showed blocking activity, suggesting that these two antibodies may neutralize virus by different mechanisms. Excess HBV was able to saturate the binding activity of HepG2 cells. The virus was also shown to be internalized; virus DNA was detected in the cytoplasmic fraction 1-2 hr postadsorption and was associated with the nuclear fraction 2 hr later. Furthermore, immunoprecipitation experiments showed that the virus DNA remained encapsidated 48 hr after internalization. Nuclear fractionation experiments showed that the encapsidated HBV DNA remained associated with the nuclear membrane. Trypsinization of intact nuclei resulted in disassociation of the nucleocapsid from the nuclear membranes, suggesting that the nuclear membrane presents a barrier to intact HBV virions or nucleocapsids. This may explain why HepG2 cells are refractile to infection although permissive for HBV replication after transfection of viral DNA.