Background Non-response to combination therapy by patients with hepatitis C virus (HCV) has previously been associated with a strong hepatic upregulation of interferon stimulated genes (ISGs) including ISG15. Therefore, the aim of this study was to further elucidate the functional role of this molecule.

Methods ISG15 expression was suppressed by siRNAs or enhanced by over-expression in genomic and subgenomic human or murine HCV replicon systems. In addition, ISG15 expression was analysed in liver samples of patients with HCV prior to antiviral therapy and correlated with clinical and virological parameters.

Results Short- or long-term knockdown of ISG15 expression suppressed HCV replication comparable to IFNs without evidence for the induction of resistant mutations. Triple therapy consisting of ISG15 knockdown, interferon α (IFNα) and ribavirin led to complete suppression of the HCV NS5A protein, corresponding to 99% suppression of HCV-RNA compared to 75% suppression by IFNα and ribavirin only. Combination treatment of ISG15 knockdown and IFN was associated with enhanced and prolonged expression of selected ISGs. Consistent with these in vitro data, high hepatic ISG15 levels correlated with the unfavourable HCV genotype 1, a high hepatic HCV load and a low antiviral response to IFN during the initial phase of treatment.

Conclusions ISG15 plays an important role in the HCV replication cycle. Therefore, therapies based on the suppression of ISG15 may provide a promising strategy to overcome non-response to standard combination treatment in the future. Furthermore, analysis of ISG15 prior to therapy may be useful to predict short-term and long-term outcome and thus tailor antiviral therapy with pegIFN and ribavirin.