IL-29 Is the Dominant Type III Interferon Produced by Hepatocytes During Acute Hepatitis C Virus Infection

HCV poses a significant health problem, with more than 170 million chronically infected people worldwide.1 Treatment is based on interferon (IFN)-α in combination with ribavirin and direct antiviral agents,2 but the role of IFN-α and other IFNs in the spontaneous outcome of infection is unclear. Type I IFNs (13 IFN-α proteins plus IFN-β, IFN-e, IFN-κ, and IFN-ϖ) form the frontline of innate host defenses by inducing an antiviral state in infected and neighboring cells, and by modulating adaptive immune responses directly and by the induction of IFN-stimulated genes ISGs.3 Whereas ISGs are strongly induced during HCV infection,4,5 neither the ISG-inducing cyto-kines nor their cellular sources have been defined. HCV has been shown to interfere with Toll-like receptor (TLR)3- and retinoic-acid–inducible gene (RIG) I–mediated induction of IFN-β and with Janus kinase/signal transducer and activator of transcription (JAK-STAT) signaling downstream of the IFN-α/β receptor,6 thus reducing IFN-α/β production to levels that are undetectable in HCV-infected patients. In vitro studies suggest that plasmacytoid dendritic cells (pDCs) may be the source of the ISG-inducing type I IFNs,7 but the role of pDCs has not been studied in the HCV-infected liver. In this context, type III IFNs have become of interest. This family is composed of interleuking (IL)-29, IL-28A, and IL-28B, and induced in response to several viral pathogens.8 Although signaling by the JAK-STAT pathway is shared with type I IFNs and similar sets of ISGs are induced,9 receptors for type III IFNs are distinct from those for type I IFNs10 and are expressed in a cell-type–specific manner.11 In the liver, type III IFN receptors are expressed at significant levels as a functional full-length form,10,11 suggesting intact type III IFN signaling as part of the intrahepatic innate immune response. Furthermore, single nucleotide polymorphisms (SNPs) near and within IL28B are strong predictive markers for spontaneous, treatment-induced HCV clearance,12–15 suggesting that variations in type III IFN expression or function affect the outcome of HCV infection. In this context, Langhans et al. reported that IL-29 serum levels do not differ between patients with acute HCV infection and healthy controls, but that they are lower in chronically infected patients.16 Whereas recombinant type III IFNs are known to suppress HCV replication in vitro,17–19 their expression level in the liver has never been studied prospectively during acute HCV infection. Thus, the relative antiviral effect of endogenously produced type I and type III IFNs is not known. Chimpanzees constitute the sole animal model for HCV infection and recapitulate many features of the human immune responses to HCV. To examine the role of IFNs in acute HCV infection, we studied the expression of type I (IFN-β and IFN-α2) and III (IL28-A/B and IL-29) IFNs in relation to ISGs and viremia using serial liver biopsies and plasma samples throughout the first 6 months of HCV infection. To further evaluate the HCV-induced IFN profile, we infected primary human hepatocytes (PHHs) with HCV in the presence and absence of pDCs, identified the induced IFNs and the requirements for their induction, and studied their antiviral potency relative to their expression level.