Class A Scavenger Receptor 1 (MSR1) Restricts Hepatitis C Virus Replication by Mediating Toll-like Receptor 3 Recognition of Viral RNAs Produced in Neighboring Cells

Abstract

Persistent infections with hepatitis C virus (HCV) may result in life-threatening liver disease, including cirrhosis and cancer, and impose an important burden on human health. Understanding how the virus is capable of achieving persistence in the majority of those infected is thus an important goal. Although HCV has evolved multiple mechanisms to disrupt and block cellular signaling pathways involved in the induction of interferon (IFN) responses, IFN-stimulated gene (ISG) expression is typically prominent in the HCV-infected liver. Here, we show that Toll-like receptor 3 (TLR3) expressed within uninfected hepatocytes is capable of sensing infection in adjacent cells, initiating a local antiviral response that partially restricts HCV replication. We demonstrate that this is dependent upon the expression of class A scavenger receptor type 1 (MSR1). MSR1 binds extracellular dsRNA, mediating its endocytosis and transport toward the endosome where it is engaged by TLR3, thereby triggering IFN responses in both infected and uninfected cells. RNAi-mediated knockdown of MSR1 expression blocks TLR3 sensing of HCV in infected hepatocyte cultures, leading to increased cellular permissiveness to virus infection. Exogenous expression of Myc-MSR1 restores TLR3 signaling in MSR1-depleted cells with subsequent induction of an antiviral state. A series of conserved basic residues within the carboxy-terminus of the collagen superfamily domain of MSR1 are required for binding and transport of dsRNA, and likely facilitate acidification-dependent release of dsRNA at the site of TLR3 expression in the endosome. Our findings reveal MSR1 to be a critical component of a TLR3-mediated pattern recognition receptor response that exerts an antiviral state in both infected and uninfected hepatocytes, thereby limiting the impact of HCV proteins that disrupt IFN signaling in infected cells and restricting the spread of HCV within the liver.

SIDT2 Transports Extracellular dsRNA into the Cytoplasm for Innate Immune Recognition

“…However, HSV-1 is well-known for its extensive ability to suppress immunity in cells that it infects (Melchjorsen et al, 2009), so SIDT2-mediated bystander activation – whereby uninfected host cells take up, detect, and respond to extracellular HSV-1 dsRNA released from infected cells – may represent an important counter strategy for host immunity. Along similar lines, recent in vitro experiments have demonstrated a role for dsRNA-based activation of bystander cells in limiting hepatitis C virus (HCV) infection (Dansako et al, 2013), but our results provide evidence that this activation is important in vivo and highlight a physiological role for extracellular RNA at a time when this is a matter of considerable interest and debate (Leslie, 2013). …”

“…To circumvent this inhibition, infected cells can transfer viral dsRNA extracellularly to uninfected bystander cells, thereby promoting IFN-I production and limiting viral replication in vitro (Dansako et al, 2013). How such transfer occurs is yet to be determined, and the importance of this transfer to immunity in vivo – while postulated – remains unclear.…”

“…What is clear is that once dsRNA is present within the extracellular milieu it can bind various cell surface receptors, such as MSR, Raftlin and CD14 (Dansako et al, 2013; DeWitte-Orr et al, 2010; Lee et al, 2006; Watanabe et al, 2011), and then be internalised via clathrin-dependent endocytosis (Itoh et al, 2008). Once endocytosed, dsRNA can be recognised by Toll-Like Receptor 3 (TLR3) within the acidic environment of the lysosome (Dansako et al, 2013; Lee et al, 2006; Schulz et al, 2005; Watanabe et al, 2011), leading to subsequent signalling via the adaptor protein, TRIF (Yamamoto et al, 2003). dsRNA that has been internalised from the extracellular environment also activates the RLR sensing pathway within the cytoplasm, and it is the cytoplasmic pathway that is responsible for the vast majority of the IFN-I response upon in vivo exposure to extracellular dsRNA (in the form of poly(I:C) (Gitlin et al, 2006).…”

Activation and Evasion of Antiviral Innate Immunity by Hepatitis C Virus

“…59 Additional HCV PAMPS for TLR3 could come following uptake of HCV RNA from the extracellular milieu (possibly from dying infected cells) by scavenger receptors or through autophagic processes. 62; 63 This uptake could lead to viral PAMP presentation to TLR3 in the endosome in uninfected cells setting up an antiviral state even in uninfected hepatocytes. HCV sensing by TLR7 occurs in both plasmacytoid dendritic cells (pDCs) and Kupffer cells, leading to production of IFN or activation of the inflammasome (Fig.…”

Scavenger Receptor C Mediates Phagocytosis of White Spot Syndrome Virus and Restricts Virus Proliferation in Shrimp

“…The most studied SRs are SRA and B. SRA recognizes Gram-positive bacteria via binding to Lipoteichoic acid (LTA) [9]. Studies also showed that SRA could restrict hepatitis C virus replication by interacting with TLR3 in human hepatocytes [10]. CD36, which is the first cloned class B SR, is a sensor for LTA and diacylated lipopeptide, as well as a co-receptor for TLR2 in responses to microbial diacylglycerides [11].…”