Hepatitis C Virus Entry and Neutralization
Section snippets
Tools to study hepatitis C virus entry
Currently, there are two methods for studying HCV entry. The first is based on the capacity of retroviruses lacking endogenous glycoproteins (gps) to incorporate foreign envelope proteins. The resulting pseudoparticles infect cells in a manner that is defined by the heterologous encoded gps. HCV encodes two gps, E1 and E2, and their detailed processing and biogenesis have been reviewed elsewhere [4]. The pseudotyping approach has allowed the characterization of viruses bearing a range of HCV
Elucidation of the receptors defining hepatitis C virus attachment and internalization
HCVpp entry into hepatoma cell lines and primary human hepatocytes is via pH-dependent clathrin-mediated endocytosis [17], [18]. Current evidence suggests that at least three host cell molecules are important for HCV entry in vitro: the tetraspanin CD81 [5], [9], [13], [19], scavenger receptor class B member I (SR-BI) [9], [20], [21], [22], and the tight junction (TJ) protein claudin-1 (CLDN1) [23]. Other factors, such as glycosaminoglycans (GAGs) [24], [25] and low-density lipoprotein (LDL)
Scavenger receptor class B member I
The first evidence for a role of SR-BI in HCV entry was the demonstration that sE2 bound to CD81-negative HepG2 cells by means of SR-BI [20]. SR-BI is the major receptor for high-density lipoprotein (HDL) and is involved in the trafficking of cholesterol into hepatocytes by means of selective uptake from cholesterol-rich lipoproteins and by the uptake of HDL particles into endosomes. The role of SR-BI in lipid physiology was recently reviewed [57]. Unlike CD81, which is expressed ubiquitously,
Tight junction claudin proteins
An additional host cell molecule reported to be important for HCV entry was recently identified as the TJ protein CLDN1 [78]. TJs are continuous intercellular contacts at the apical poles of lateral cell membranes that form a barrier to regulate the paracellular transit of solutes across a cell layer and to establish cell polarity [79]. The CLDN family of transmembrane proteins extends into the paracellular space, wherein they form homo- and heteroassociations with CLDNs on apposing cells [80].
Role of cell polarization in hepatitis C virus infection
Many tissues in the body contain polarized cells, and hepatocytes in the liver are known to be polarized, with TJs separating the apical (canalicular) and basolateral (sinusoidal) domains. Hepatic polarity is critical for normal liver function, with particular membrane domains performing specific tasks, such as biliary secretion from the canaliculi and serum protein secretion from the sinusoidal surface(s). To initiate infection, pathogens must breach the epithelial barrier to gain access to
Humoral immune response in hepatitis C virus infection
HCV-infected individuals often have detectable RNA levels as early as 1 week after infection; however, antibodies against the virus are not detected until much later. Chen and colleagues [94] noted the delayed appearance, low titer, and restricted isotype of antibodies to structural and nonstructural HCV proteins. The contribution of the humoral response to disease outcome remains unclear; in contrast, it has been well documented that a potent multi-specific and long-lasting cellular immune
In vitro systems for the measurement of neutralizing antibodies
Before the development of in vitro infection systems, the neutralization potential of HCV-specific antibodies was tested indirectly using “neutralization of binding” (NOB) assays, in which antibodies were screened for their ability to prevent the binding of sE2 to mammalian cells [116]. Baumert and colleagues [117] used a recombinant baculovirus system to express the HCV structural proteins that formed viral-like particles (VLPs) to study antibody reactivity and inhibition of VLP-cell
Detection of autologous and heterologous neutralizing antibodies
Dissecting the humoral immune response to HCV at an individual level is a challenging endeavor; readily available gp sequences may represent certain viral genotypes but differ considerably from a patient's autologous viral quasispecies. Most studies are limited to establishing the heterologous cross-reactivity of nAbs and do not measure their efficacy to neutralize autologous circulating strains in the subject under test. Several studies have demonstrated that chronically infected subjects
Epitope specificity of the neutralizing antibody response
nAbs can exert their effect(s) by binding directly to virus particles and blocking subsequent interaction(s) with receptors or by inhibiting post-entry events, such as viral uncoating and subsequent replication (reviewed by Burton [147]). The former may occur by inducing conformational changes in the viral envelope that disable infection or by steric hindrance, physically shielding important viral interaction sites. The modes of action of HCV-specific nAbs have yet to be defined; however,
Effect of lipoproteins on the sensitivity of virus to neutralizing antibodies
Recent in vitro studies highlight the role of the VLDL pathway in the assembly and release of HCVcc particles [69], [70], [71]. These data support earlier observations reporting on the density and sedimentation properties of human plasma-derived virus, concluding that the virus exists in association with lipoproteins or immunoglobulins [73], [157], [158], [159]. Thosmsen and colleagues [160] reported that HCV RNA was associated with β-lipoproteins in sera, a finding that was later confirmed by
Inducing protective immunity in vivo
The propensity of HCV to establish chronic infection, to reinfect previously exposed individuals [164], to transmit directly by cell-cell routes in vitro [165], and to evolve neutralization escape variants [166] makes the development of an HCV vaccine a major challenge. Vaccine development has been hampered by the lack of a convenient small animal model [154], [167]. The existence of natural immunity to infection in some humans [168] and in chimpanzees [142], [169], [170], [171] is encouraging,
Summary
The processes of HCV entry and antibody-mediated neutralization are intimately linked. The high frequency of nAbs that inhibit E2-CD81 interaction(s) suggests that this is a major target for the humoral immune response. The observation that HCV can transmit to naive cells by means of CD81-dependent and -independent routes in vitro awaits further investigation to assess the significance in vivo but may offer new strategies for HCV to escape nAbs. The identification of CLDNs in the entry process
Acknowledgments
The authors thank all their colleagues for stimulating discussions over the years. Research in the McKeating laboratory is currently supported by Public Health Service grants AI40034 and AI50798, the Medical Research Council, and The Wellcome Trust.
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