Upregulation of interferon (IFN)-stimulated genes (ISGs), including IFN-stimulated gene 15 (ISG15) and other members of the ISG15 pathway, in pre-treatment liver tissue of patients chronically infected with hepatitis C virus (HCV) is associated with subsequent treatment failure (pegylated IFN-a/ribavirin). This study assessed the effect of ISG15 on HCV production in vitro. The levels of ISG15 and of its conjugation to target proteins (ISGylation) were increased by plasmid transfection, but ISGylation was inhibited by small interfering RNA directed against the E1 activating enzyme, Ube1L, in Huh7.5 cells. Cells were infected with HCV FL-J6/JFH virus, and HCV RNA and viral titres were determined. Levels of both HCV RNA and virus increased when levels of ISG15 and ISGylation were increased, and decreased when ISGylation was inhibited. The effects of ISGylation on HCV were independent of upstream IFN signalling: IFN-a-induced ISG expression was not altered by Ube1L knockdown. Thus, although ISG15 has antiviral activity against most viruses, ISG15 promotes HCV production. HCV might exploit ISG15 as a host immune evasion mechanism, and this may in part explain how increased expression of ISGs, especially ISG15, correlates with subsequent IFN-based treatment failure. INTRODUCTIONHepatitis C virus (HCV) is adept at evading host antiviral mechanisms and is often resistant to the current standard of care -combination treatment with pegylated interferon (IFN)-a and ribavirin (PegIFN/Rib). This regimen eradicates the virus in only 50 % of cases. A number of mechanisms contribute to evasion and treatment resistance, including cleavage of the RIG-I adaptor protein IPS1/ MAVS/Cardif by the HCV NS3/NS4A protease and modulation of the host response by the HCV core protein Loo et al., 2006). However, none of these mechanisms has consistently been demonstrated to play a role in the clinical disease and thus cannot explain the ability of the virus to escape the host response in patients.Response to treatment can be predicted by levels of expression of IFN-stimulated genes (ISGs) in the liver prior to initiation of PegIFN/Rib treatment. Non-responders have increased expression of a number of ISGs (Chen et al., 2005;Feld et al., 2007; Asahina et al., 2008;Asselah et al., 2008;Sarasin-Filipowicz, et al., 2008). Three of these ISGs are components of the ISG15 ubiquitin-like pathway. ISG15 was the first ubiquitin-like protein to be described and, like its homologue ubiquitin, is conjugated to proteins in a tightly regulated process called ISGylation. The ISG15 E1 activating protein, Ube1L, coordinates with the E2 conjugating enzyme (UbcH8) and the E3 ligase (CEB1) to join the C terminus of ISG15 to a wide variety of proteins (Loeb & Haas, 1992). ISG15 can be removed from its target proteins by USP18, an ISG15 protease (Kim et al., 2008b). Thus, ISG15 inhibits virus production for many viruses, but may promote production of some.In this study, we examined the role of ISG15 and ISGylation in HCV production in vitro, using the FL-J6/JFH...

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