Grove, J., Nielsen, S., Zhong, J., Bassendine, M. F., Drummer, H. E., Balfe, P. and McKeating, J. A.
J. Virol. 82:12020-29, 2008
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With 170 million infected individuals worldwide, Hepatitis C Virus (HCV) poses a global health problem. The primary site of virus replication is the liver, leading to progressive disease often culminating in life threatening conditions such as hepatocellular carcinoma. HCV has a propensity to persist, with 70-80% of infected individuals failing to clear the virus. Precisely how the virus achieves this is unclear, current evidence suggests it has adopted numerous strategies to evade host immune responses. Intriguingly, HCV associates with host lipoproteins to form lipo-viro-particles (LVPs). These structures are predicted to exhibit the characteristics of both lipoproteins and virus particles, suggesting that HCV exploits hepatic physiology to adopt a "wolf in sheep's clothing" strategy to aid viral transmission and persistence. Recent evidence suggests that HCV entry is dependent on at least three cellular entry factors: a tetraspanin, CD81; a lipoprotein receptor, Scavenger Receptor B-I (SR-BI) and the tight junction protein Claudin-1. How these molecules coordinate HCV entry and the role(s) of particle associated lipoproteins in this process is unknown.
It is becoming increasingly clear that HCV entry involves a complex interplay between 1) the virus encoded structural proteins; 2) lipoprotein components (apoproteins) and 3) host cellular receptors. This interdependence suggests that HCV is exquisitely adapted to its host environment, making it difficult to study any one aspect in isolation. Indeed, this conclusion is supported by our recent observation that a single amino acid change (G451R) within the second viral envelope glycoprotein (E2) significantly altered particle characteristics. This adaptive mutation modulated SR-BI/CD81 co-receptor dependence, disrupted the relationship between infectivity and particle-associated lipoproteins and increased the sensitivity of virus to neutralisation by glycoprotein-specific antibodies. These studies highlight the resistance of HCV LVPs to neutralising antibodies, suggesting that particle association with lipoproteins may help HCV evade the humoral immune response.
HCV infection is dependent on at least three co-receptors, CD81, scavenger receptor BI (SR-BI) and claudin-1. The mechanism of how these molecules coordinate HCV entry is unknown. In this study we demonstrate that a cell culture adapted JFH-1 mutant, with an amino acid change in E2 at position 451 (G451R), has a reduced dependency on SR-BI. This altered receptor dependency is accompanied by an increased sensitivity to neutralization by soluble CD81 and enhanced binding of recombinant E2 to cell surface expressed and soluble CD81. Fractionation of HCV by density gradient centrifugation allows the analysis of particle-lipoprotein associations. The cell culture adapted mutation alters the relationship between particle density and infectivity, with the peak infectivity occurring at higher density than the parental virus. No association was observed between particle density and SR-BI or CD81 co-receptor dependence. JFH-1 G451R is highly sensitive to neutralization by gp-specific antibodies, suggesting increased epitope exposure at the virion surface. Finally, an association was observed between JFH-1 particle density and sensitivity to neutralizing antibodies (nAbs), suggesting that lipoprotein association reduces the sensitivity of particles to nAbs. In summary, mutation of E2 at position 451 alters the relationship between particle density and infectivity, disrupts co-receptor dependence and increases virion sensitivity to receptor mimics and nAbs. Our data suggest that a balanced interplay between HCV particles, lipoprotein components and viral receptors allows the evasion of host immune responses.
JFH-1 G451R is insensitive to neutralization by anti-SR-BI serum.
JFH-1 G451R is 50 fold more sensitive to antibody mediated neutralization.
Low density, lipoprotein like, JFH-1 wt particles are less sensitive to antibody mediated neutralisation.