Inhibition of hepatitis B virus replication by the interferon-inducible MxA protein

Human MxA is an alpha/beta interferon-inducible intracytoplasmic protein th at mediates antiviral activity against several RNA viruses. We had previous ly shown that overexpression of the hepatitis B virus (HBV) capsid led to s elective downregulation of MxA gene expression, suggesting a mechanism by w hich the virus escapes from the host defense system (O. Rosmorduc, Ef. Sirm a, P. Soussan, E. Gordien, P. Lebon, M. Horisberger, C. Brechot and D. Krem sdorf, J Gen. Virol. 80:1253-1262, 1999), In the present study, we investig ated the antiviral activity of MxA protein against HBV. MxA-expressing HuH7 clones were established and transiently transfected with HBV, and viral re plication was then studied. Viral protein secretion was profoundly reduced in MxA-expressing clones by 80% for HBV surface antigen (HBsAg) and 70% for HBV e antigen (HBeAg). The levels of intracytoplasmic HBsAg and HBeAg were reduced by about 80 and 50% in the two MxA-positive clones tested. A nearl y complete disappearance of HEV DNA replicative intermediates was observed in MxA-expressing clones. Although the expression of total viral RNAs was n ot modified, two- to fourfold reductions in HBV cytoplasmic RNAs were found in MxA-expressing clones. This suggests the inhibition of HBV replication at a posttranscriptional level. Indeed, using the well-characterized posttr anscriptional regulation element (PRE) reporter system, we were able to dem onstrate a marked reduction (three- to eightfold) in the nucleocytoplasmic export of unspliced RNA in MxA-expressing clones. In addition, MxA protein did not interact with HBV nucleocapsid or interfere with HBV nucleocapsid f ormation. Our results show an antiviral effect of MxA protein on a DNA viru s for the first time, MxA protein acts, at least in part, by inhibiting the nucleocytoplasmic export of viral mRNA via the PRE sequence.