Date Published: May 21, 2018
Publisher: Public Library of Science
Author(s): Yisha Liang, Xuezhi Cao, Qiang Ding, Yanan Zhao, Zhenliang He, Jin Zhong, Stacy M. Horner.
Toll-like receptor 3 (TLR3) senses dsRNA intermediates produced during RNA virus replication to activate innate immune signaling pathways through adaptor protein TRIF. Many viruses have evolved strategies to block TLR3-mediated interferon signaling via targeting TRIF. Here we studied how hepatitis C virus (HCV) antagonizes the TLR3-mediated interferon signaling. We found that HCV-encoded NS4B protein inhibited TLR3-mediated interferon signaling by down-regulating TRIF protein level. Mechanism studies indicated that the downregulation of TRIF by NS4B was dependent on caspase8. NS4B transfection or HCV infection can activate caspase8 to promote TRIF degradation, leading to suppression of TLR3-mediated interferon signaling. Knockout of caspase8 can prevent TRIF degradation triggered by NS4B, thereby enhancing the TLR3-mediated interferon signaling activation in response to HCV infection. In conclusion, our work revealed a new mechanism for HCV to evade innate immune response by blocking the TLR3-mediated interferon signaling via NS4B-induced TRIF degradation.
Hepatitis C virus (HCV) is an enveloped, single-stranded RNA virus belonging to the Flaviviridae family. HCV has a 9.6-kb RNA genome and encodes a large polyprotein of over 3000 amino acids which is cleaved into structural proteins (core, E1 and E2) and nonstructural proteins (p7, NS2, NS3, NS4A, NS4B, NS5A and NS5B). HCV infects approximately 170 million people worldwide, about 80% of whom develop into persistent infection. Persistent HCV infection leads to severe liver diseases, such as liver cirrhosis and hepatocellular carcinoma . No vaccine is available for preventing HCV infection. Interferon (IFN) plus ribavirin, the traditional therapy to treat chronic hepatitis C, is not always effective and has strong side effect. Recently developed direct-acting antiviral agents (DAA), including NS3 protease inhibitors, NS5A inhibitors and NS5B nucleotide inhibitors have greatly improved curing efficiency. However, the impact of these highly effective DAAs on global control of HCV infection remains to be seen in the long run as drug-resistant mutations, severe liver disease progression in DAA-cured patients and other newly emerging problems arise . Therefore, HCV infection is still a big threat to human public health.
To establish chronic infection, HCV has evolved multiple strategies to counteract IFN signaling. It has been demonstrated that HCV-encoded NS3/4A serine protease cleaves MAVS and TRIF to shut down the IFN signaling mediated by RLRs and TLR3 [24, 28]. Growing evidence showed that HCV employs additional strategies to disrupt the RLR- and TLR3-mediated IFN signaling. We and others previously reported that HCV NS4B can block RLR-mediated interferon signaling by targeting STING [15–17]. In this study, we provided several lines of evidence to demonstrate that NS4B can also disrupt the TLR3-mediated signaling by targeting the adaptor protein TRIF for its degradation. First, NS4B transfection blocks IFN signaling activated by extracellular poly(I:C), known to be recognized by TLR3 (Fig 1). Second, the disruption of TLR3-mediated signaling by NS4B was associated with the reduction of TRIF at the protein level (Fig 3A–3E), but not at the mRNA level (S1A Fig). Third, the NS4B-triggered TRIF protein degradation and blockade of TLR3-mediated IFN signaling were recapitulated in HCV infected cells (Figs 3F, 5A and 7), suggesting that interference of the TLR3-mediated IFN signaling by NS4B indeed takes place in the context of HCV infection.