Hepatitis C Virus Entry into Hepatocytes and Engagement of Innate Immune Defenses
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Hepatitis C virus (HCV) infection is a major cause of liver disease and a global health problem with inadequate treatment options. An improved understanding of how HCV exploits and subverts host factors to establish infection should yield potential targets for therapy. This study uses a recently developed cell culture model of HCV infection to examine HCV entry and engagement of innate immune defenses. HCV associates with host apolipoproteins and enters hepatocytes through complex processes involving some combination of CD81, claudin-I, occludin, and scavenger receptor BI. Here I show that HCV forms a complex with very low density lipoprotein (VLDL) within infected hepatocytes and uses this association to support infection through the low density lipoprotein receptor (LDL-R). Blocking experiments demonstrate that beta-VLDL and apolipoprotein E (apoE) can compete with HCV for entry. Knockdown of the LDL-R by treatment with 25-hydroxycholesterol or siRNA ablated ligand uptake and reduced HCV infection of cells, whereas infection was rescued upon cell ectopic LDL-R expression. Analyses of gradient-fractionated HCV demonstrate that apoE is associated with HCV virions exhibiting peak infectivity and dependence upon the LDL-R for cell entry. These results define the LDL-R as a cooperative HCV co-receptor that supports viral entry and infectivity through interaction with apoE ligand present in an infectious HCV/lipoprotein complex comprising the virion. Furthermore, upon entry HCV induces an initial transient activation of interferon regulatory factor-3 (IRF3) which is dependent on retinoic acid inducible gene I (RIG-I) and interferon-beta promoter stimulator-1 (IPS-1). This activation produces an antiviral activity which inhibits HCV entry and replication. HCV NS3/4A protease activity blocks this activation within 48 hours. At later time points post infection HCV activates NF-kappaB in a RIG-I independent manner leading to inflammatory cytokine production. These studies identify 3 potential targets for future HCV therapy: 1) alteration of HCV-lipoprotein interaction to disrupt entry, 2) blockade of NS3/4A protease activity to restore innate antiviral response, and 3) modulation of HCV induced NF-kappaB signaling to downregulate chronic inflammation.