The interferon stimulated gene 15 functions as a proviral factor for HCV and is enhanced in the liver of patients with genotype 1

Aims: The outcome of HCV standard combination therapy has previously been associated with a strong hepatic up-regulation of a subset of interferon stimulated genes (ISGs), one of them is ISG15. The aim of this study was to further elucidate the functional role of ISG15 in HCV infection.

Methods: ISG15 gene expression was suppressed in human (con1) and murine (MH1) hepatoma cells harbouring the HCV con1 replicon I377/NS3–3´ using specific siRNAs. Effects on HCV replication were determined by quantitative rtPCR and Western blot analysis. In addition, ISG15 expression was analyzed in liver samples of HCV patients prior to therapy.

Results: ISG15 gene silencing led to suppression of HCV replication to levels comparable to IC50 concentrations of IFNs. A synergistic antiviral effect was observed when cells were treated with a combination of ISG15 siRNAs and IFN-α or ribavirin which resulted in complete suppression of NS5a expression on the protein level. The increased IFN response was associated with enhanced and prolonged expression of selected ISGs. Long-term treatment with ISG15 siRNA resulted in sustained suppression of HCV replication for more than 3 months without evidence for the selection of resistant mutations. Genotype 1 infected HCV patients showed higher ISG15 levels in the liver than patients with genotypes 2 and 3 which also correlated with a low viral drop in HCV viremia 36h after the beginning of therapy.

Conclusions: In conclusion, our data indicate that ISG15 plays an important role in the replication cycle of HCV. Therefore, therapies that are directed at the suppression of ISG15 may provide a promising strategy to overcome non-response to standard combination treatment in the future.

HCV - Hepatitis C Virus - ISG15 - Interferon - Interferon stimulated gene 15 - Therapy - subgenomic Replicon