IL-26 inhibits hepatitis C virus replication in hepatocytes

Highlights

• IL-26 is a new weapon in the antiviral arsenal, functioning independently of the immune system.

• IL-26 penetrates HCV-infected cells and interacts with viral double-stranded RNA replication intermediates.

• IL-26 interferes with viral RNA-dependent RNA polymerase activity, preventing the de novo synthesis of viral genomic single-stranded RNA.

Background & Aims

Interleukin-26 (IL-26) is a proinflammatory cytokine that has properties atypical for a cytokine, such as direct antibacterial activity and DNA-binding capacity. We previously observed an accumulation of IL-26 in fibrotic and inflammatory lesions in the livers of patients with chronic HCV infection and showed that infiltrating CD3⁺ lymphocytes were the principal source of IL-26. Surprisingly, IL-26 was also detected in the cytoplasm of hepatocytes from HCV-infected patients, even though these cells do not produce IL-26, even when infected with HCV. Based on this observation and possible interactions between IL-26 and nucleic acids, we investigated the possibility that IL-26 controlled HCV infection independently of the immune system.

Methods

We evaluated the ability of IL-26 to interfere with HCV replication in hepatocytes and investigated the mechanisms by which IL-26 exerts its antiviral activity.

Results

We showed that IL-26 penetrated HCV-infected hepatocytes, where it interacted directly with HCV double-stranded RNA replication intermediates, thereby inhibiting viral replication. IL-26 interfered with viral RNA-dependent RNA polymerase activity, preventing the de novo synthesis of viral genomic single-stranded RNA.

Conclusions

These findings reveal a new role for IL-26 in direct protection against HCV infection, independently of the immune system, and increase our understanding of the antiviral defense mechanisms controlling HCV infection. Future studies should evaluate the possible use of IL-26 for treating other chronic disorders caused by RNA viruses, for which few treatments are currently available, or emerging RNA viruses.

Lay summary

This study sheds new light on the body’s arsenal for controlling hepatitis C virus (HCV) infection and identifies interleukin-26 (IL-26) as an antiviral molecule capable of blocking HCV replication. IL-26, which has unique biochemical and structural characteristics, penetrates infected hepatocytes and interacts directly with viral RNA, thereby blocking viral replication. IL-26 is, therefore, a new player in antiviral defenses, operating independently of the immune system. It is of considerable potential interest for treating HCV infection and other chronic disorders caused by RNA viruses for which few treatments are currently available, and for combating emerging RNA viruses.

Introduction

Interleukin-26 (IL-26) has been identified as a molecule overexpressed in human T cells transformed with saimiri herpesvirus,¹ and has been assigned to the IL-10 subfamily of cytokines.[1], [2], [3] However, IL-26 has unconventional physicochemical characteristics not found in other members of this family. It has an isoelectric point of 10.4, amphipathic properties and a predicted in-plane membrane (IPM)-anchoring motif reported to be involved in protein binding to cell membranes.^4,5 IL-26 resembles cationic cell-penetrating peptides designed for DNA delivery, accounting for its unusual functional properties. Indeed, we and others have reported that IL-26 binds extracellular DNA and shuttles DNA within myeloid cells⁵ and plasmacytoid dendritic cells,⁴ leading to the induction of type I interferons (IFNs) and inflammatory cytokines. This activation involves DNA sensors and occurs in the absence of the heterodimeric receptor IL-10R2/IL-20R1, which was initially identified as a receptor for IL-26.⁶

IL26 mRNA is expressed by activated T helper (Th)1 and Th17 lymphocytes,^7,8 natural killer (NK) cells⁸ and some non-immune cells (such as fibroblasts and smooth muscle cells) present in chronically inflamed tissues.^5,9 High serum IL-26 concentrations and IL-26 expression at sites of inflammation have been reported in patients with chronic inflammatory diseases, such as Crohn's disease,^7,8 rheumatoid arthritis⁹ and vasculitis.^5,10 IL-26 has thus emerged as a link between cell death and persistent inflammation, and has been implicated in the pathophysiology of chronic inflammatory disorders.

We suspected that, in addition to this harmful role, IL-26 might have protective physiological functions, particularly in antiviral protection. This hypothesis is supported by previous studies. IL-26 modulates the infection of epithelial cells with vesicular stomatitis virus (VSV), human cytomegalovirus (HCMV), and herpes simplex virus type 1 (HSV-1).¹¹ We found that treatment-naive patients with chronic HCV infection had high serum IL-26 concentrations, which were correlated with the degree of liver inflammation.¹² We also found that IL-26 increased the cytotoxic activity of NK cells against HCV-infected cells. We observed an accumulation of IL-26 protein in the cytoplasm of hepatocytes from patients with chronic HCV infection, in the absence of IL26 mRNA detection.¹² As hepatocytes are the primary site of HCV replication, we suspected that the IL-26 accumulating in hepatocytes might have a direct antiviral effect. We report here that IL-26 inhibits HCV infection by interacting with the viral RNA and blocking its replication in hepatocytes. This result sheds new light on the antiviral defense mechanisms involved in controlling HCV infection and identifies IL-26 as a novel antiviral molecule.