Introduction & Aims : IFN-l4 is a newly identified type III IFN and the dinucleotide polymorphism of IFN-l4 has been a genetic marker strongly associated with response to IFN-α based therapy in hepatitis C virus (HCV)-infected patients. The IFNL4-TT genotype does not produce IFN-l4 due to a premature stop codon, while the IFNL4-DG genotype is a frameshift variant that produces functional IFN-l4. It is reported that HCV-infected patients with the IFNL4-TT genotype show favorable response to IFN-α based therapy whereas patients with the IFNL4-DG show poor response to IFN-α based therapy. However, the underlying mechanism of these genotype-phenotype correlation is not fully investigated. In our previous study, we demonstrated that prolonged exposure to IFNs results in unresponsiveness to IFN-α by ISG15 upregulation and USP18 stabilization. In this study, we investigated the induction of IFN-l4 by HCV infection and the biological effects of IFN-l4 in relation to IFN-α responsiveness.
Methods : Huh7 cells were transfected with IFNL4 encoding plasmid or treated with recombinant human IFN-l4. Primary human hepatocytes (PHHs) with IFNL4-DG allele were infected with JFH1 HCVcc at high titer to examine the effects of IFN-l4 on IFN-α responsiveness
Results : HCV infection induced IFN-l4 at mRNA and protein level in PHHs with IFNL4-DG allele. HCV-infected PHHs showed attenuated response to exogenous IFN-α. IFN-l4 treatment or IFNL4 encoding plasmid transfection caused sustained upregulation of ISG15 and USP18 in Huh7 cells, resulting in IFN-α unresponsiveness. The ISG15/USP18 mediated IFN-α unresponsiveness was confirmed by transfection of siRNA targeting ISG15 or USP18. Direct-acting antivirals (DAAs) treatment to HCV-infected PHHs decreased the induction of IFN-λs including IFN-λ4 and restored the IFN-α responsiveness.
Conclusion : Our data demonstrate that IFN-l4 induced by HCV infection confers unresponsiveness to IFN-α by upregulation of ISG15 and USP18. HCV-infected hepatocytes restore IFN-α responsiveness after DAA treatment.