Helen J. Harris

Hepatitis C virus cell‐cell transmission in hepatoma cells in the presence of neutralizing antibodies

Hepatitis C virus (HCV) infection of Huh‐7.5 hepatoma cells results in focal areas of infection where transmission is potentiated by cell‐cell contact. To define route(s) of transmission, HCV was allowed to infect hepatoma cells in the presence or absence of antibodies that neutralize cell‐free virus infectivity. Neutralizing antibodies (nAbs) reduced cell‐free virus infectivity by >95% and had minimal effect(s) on the frequency of infected cells in the culture. To assess whether cell‐cell transfer of viral infectivity occurs, HCV‐infected cells were cocultured with fluorescently labeled naïve cells in the presence or absence of nAbs. Enumeration by flow cytometry demonstrated cell‐cell transfer of infectivity in the presence or absence of nAbs and immunoglobulins from HCV+ patients. The host cell molecule CD81 and the tight junction protein Claudin 1 (CLDN1) are critical factors defining HCV entry. Soluble CD81 and anti‐CD81 abrogated cell‐free infection of Huh‐7.5 and partially inhibited cell‐cell transfer of infection. CD81‐negative HepG2 hepatoma cells were resistant to cell‐free virus infection but became infected after coculturing with JFH‐infected cells in the presence of nAb, confirming that CD81‐independent routes of cell‐cell transmission exist. Further experiments with 293T and 293T‐CLDN1 targets suggested that cell‐cell transmission is dependent on CLDN1 expression. Conclusion: These data suggest that HCV can transmit in vitro by at least two routes, cell‐free virus infection and direct transfer between cells, with the latter offering a novel route for evading nAbs. (HEPATOLOGY 2007.)

CD81 and Claudin 1 Coreceptor Association: Role in Hepatitis C Virus Entry

ABSTRACT Hepatitis C virus (HCV) is an enveloped positive-stranded RNA hepatotropic virus. HCV pseudoparticles infect liver-derived cells, supporting a model in which liver-specific molecules define HCV internalization. Three host cell molecules have been reported to be important entry factors or receptors for HCV internalization: scavenger receptor BI, the tetraspanin CD81, and the tight junction protein claudin-1 (CLDN1). None of the receptors are uniquely expressed within the liver, leading us to hypothesize that their organization within hepatocytes may explain receptor activity. Since CD81 and CLDN1 act as coreceptors during late stages in the entry process, we investigated their association in a variety of cell lines and human liver tissue. Imaging techniques that take advantage of fluorescence resonance energy transfer (FRET) to study protein-protein interactions have been developed. Aequorea coerulescens green fluorescent protein- and Discosoma sp. red-monomer fluorescent protein-tagged forms of CD81 and CLDN1 colocalized, and FRET occurred between the tagged coreceptors at comparable frequencies in permissive and nonpermissive cells, consistent with the formation of coreceptor complexes. FRET occurred between antibodies specific for CD81 and CLDN1 bound to human liver tissue, suggesting the presence of coreceptor complexes in liver tissue. HCV infection and treatment of Huh-7.5 cells with recombinant HCV E1-E2 glycoproteins and anti-CD81 monoclonal antibody modulated homotypic (CD81-CD81) and heterotypic (CD81-CLDN1) coreceptor protein association(s) at specific cellular locations, suggesting distinct roles in the viral entry process.

Claudin Association with CD81 Defines Hepatitis C Virus Entry

Viruses initiate infection by attaching to molecules or receptors at the cell surface. Hepatitis C virus (HCV) enters cells via a multistep process involving tetraspanin CD81, scavenger receptor class B member I, and the tight junction proteins Claudin-1 and Occludin. CD81 and scavenger receptor class B member I interact with HCV-encoded glycoproteins, suggesting an initial role in mediating virus attachment. In contrast, there are minimal data supporting Claudin-1 association with HCV particles, raising questions as to its role in the virus internalization process. In the present study we demonstrate a relationship between receptor active Claudins and their association and organization with CD81 at the plasma membrane by fluorescence resonance energy transfer and stoichiometric imaging methodologies. Mutation of residues 32 and 48 in the Claudin-1 first extracellular loop ablates CD81 association and HCV receptor activity. Furthermore, mutation of the same residues in the receptor-inactive Claudin-7 molecule enabled CD81 complex formation and virus entry, demonstrating an essential role for Claudin-CD81 complexes in HCV infection. Importantly, Claudin-1 associated with CD81 at the basolateral membrane of polarized HepG2 cells, whereas tight junction-associated pools of Claudin-1 demonstrated a minimal association with CD81. In summary, we demonstrate an essential role for Claudin-CD81 complexes in HCV infection and their localization at the basolateral surface of polarized hepatoma cells, consistent with virus entry into the liver via the sinusoidal blood and association with basal expressed forms of the receptors.

Inhibition of hepatitis C virus infection by anti-claudin-1 antibodies is mediated by neutralization of E2–CD81–Claudin-1 associations†

The tight junction protein claudin‐1 (CLDN1) has been shown to be essential for hepatitis C virus (HCV) entry—the first step of viral infection. Due to the lack of neutralizing anti‐CLDN1 antibodies, the role of CLDN1 in the viral entry process is poorly understood. In this study, we produced antibodies directed against the human CLDN1 extracellular loops by genetic immunization and used these antibodies to investigate the mechanistic role of CLDN1 for HCV entry in an infectious HCV cell culture system and human hepatocytes. Antibodies specific for cell surface–expressed CLDN1 specifically inhibit HCV infection in a dose‐dependent manner. Antibodies specific for CLDN1, scavenger receptor B1, and CD81 show an additive neutralizing capacity compared with either agent used alone. Kinetic studies with anti‐CLDN1 and anti‐CD81 antibodies demonstrate that HCV interactions with both entry factors occur at a similar time in the internalization process. Anti‐CLDN1 antibodies inhibit the binding of envelope glycoprotein E2 to HCV permissive cell lines in the absence of detectable CLDN1‐E2 interaction. Using fluorescent‐labeled entry factors and fluorescence resonance energy transfer methodology, we demonstrate that anti‐CLDN1 antibodies inhibit CD81‐CLDN1 association. In contrast, CLDN1‐CLDN1 and CD81‐CD81 associations were not modulated. Taken together, our results demonstrate that antibodies targeting CLDN1 neutralize HCV infectivity by reducing E2 association with the cell surface and disrupting CD81‐CLDN1 interactions. Conclusion: These results further define the function of CLDN1 in the HCV entry process and highlight new antiviral strategies targeting E2‐CD81‐CLDN1 interactions. (HEPATOLOGY 2010.)

Polarization Restricts Hepatitis C Virus Entry into HepG2 Hepatoma Cells

ABSTRACT The primary reservoir for hepatitis C virus (HCV) replication is believed to be hepatocytes, which are highly polarized with tight junctions (TJ) separating their basolateral and apical domains. HepG2 cells develop polarity over time, resulting in the formation and remodeling of bile canalicular (BC) structures. HepG2 cells expressing CD81 provide a model system to study the effects of hepatic polarity on HCV infection. We found an inverse association between HepG2-CD81 polarization and HCV pseudoparticle entry. As HepG2 cells polarize, discrete pools of claudin-1 (CLDN1) at the TJ and basal/lateral membranes develop, consistent with the pattern of receptor staining observed in liver tissue. The TJ and nonjunctional pools of CLDN1 show an altered association with CD81 and localization in response to the PKA antagonist Rp-8-Br-cyclic AMPs (cAMPs). Rp-8-Br-cAMPs reduced CLDN1 expression at the basal membrane and inhibited HCV infection, supporting a model where the nonjunctional pools of CLDN1 have a role in HCV entry. Treatment of HepG2 cells with proinflammatory cytokines, tumor necrosis factor alpha and gamma interferon, perturbed TJ integrity but had minimal effect(s) on cellular polarity and HCV infection, suggesting that TJ integrity does not limit HCV entry into polarized HepG2 cells. In contrast, activation of PKC with phorbol ester reduced TJ integrity, ablated HepG2 polarity, and stimulated HCV entry. Overall, these data show that complex hepatocyte-like polarity alters CLDN1 localization and limits HCV entry, suggesting that agents which disrupt hepatocyte polarity may promote HCV infection and transmission within the liver.

Hepatitis C Virus Receptor Expression in Normal and Diseased Liver Tissue

The principal site of hepatitis C virus (HCV) replication is the liver. HCV pseudoparticles infect human liver derived cell lines and this suggests that liver‐specific receptors contribute to defining HCV hepatotropism. At least three host cell molecules have been reported to be important for HCV entry: the tetraspanin CD81, scavenger receptor class B member I (SR‐BI), and the tight junction (TJ) protein Claudin 1 (CLDN1). Hepatocytes in liver tissue coexpress CD81, SR‐BI, and CLDN1, consistent with their ability to support HCV entry. CLDN1 localized at the apical‐canalicular TJ region and at basolateral‐sinusoidal hepatocyte surfaces in normal tissue and colocalized with CD81 at both sites. In contrast, CLDN1 appeared to colocalize with SR‐BI at the basolateral‐sinusoidal surface. CLDN1 expression was increased on basolateral hepatocyte membranes in HCV‐infected and other chronically inflamed liver tissue compared with normal liver. In contrast, CLDN4 hepatocellular staining was comparable in normal and diseased liver tissue. Conclusion: HCV infection of Huh‐7.5 hepatoma cells in vitro significantly increased CLDN1 expression levels, consistent with a direct modulation of CLDN1 by virus infection. In HCV infected livers, immunohistochemical studies revealed focal patterns of CLDN1 staining, suggesting localized areas of increased CLDN1 expression in vivo which may potentiate local viral spread within the liver. (HEPATOLOGY 2007.)

Effect of Cell Polarization on Hepatitis C Virus Entry

ABSTRACT The primary reservoir for hepatitis C virus (HCV) replication in vivo is believed to be hepatocytes within the liver. Three host cell molecules have been reported to be important entry factors for receptors for HCV: the tetraspanin CD81, scavenger receptor BI (SR-BI), and the tight-junction (TJ) protein claudin 1 (CLDN1). The recent discovery of a TJ protein as a critical coreceptor highlighted the importance of studying the effect(s) of TJ formation and cell polarization on HCV entry. The colorectal adenocarcinoma Caco-2 cell line forms polarized monolayers containing functional TJs and was found to express the CD81, SR-BI, and CLDN1 proteins. Viral receptor expression levels increased upon polarization, and CLDN1 relocalized from the apical pole of the lateral cell membrane to the lateral cell-cell junction and basolateral domains. In contrast, expression and localization of the TJ proteins ZO-1 and occludin 1 were unchanged upon polarization. HCV infected polarized and nonpolarized Caco-2 cells to comparable levels, and entry was neutralized by anti-E2 monoclonal antibodies, demonstrating glycoprotein-dependent entry. HCV pseudoparticle infection and recombinant HCV E1E2 glycoprotein interaction with polarized Caco-2 cells occurred predominantly at the apical surface. Disruption of TJs significantly increased HCV entry. These data support a model where TJs provide a physical barrier for viral access to receptors expressed on lateral and basolateral cellular domains.

Protein Kinase A-Dependent Step(s) in Hepatitis C Virus Entry and Infectivity

ABSTRACT Viruses exploit signaling pathways to their advantage during multiple stages of their life cycle. We demonstrate a role for protein kinase A (PKA) in the hepatitis C virus (HCV) life cycle. The inhibition of PKA with H89, cyclic AMP (cAMP) antagonists, or the protein kinase inhibitor peptide reduced HCV entry into Huh-7.5 hepatoma cells. Bioluminescence resonance energy transfer methodology allowed us to investigate the PKA isoform specificity of the cAMP antagonists in Huh-7.5 cells, suggesting a role for PKA type II in HCV internalization. Since viral entry is dependent on the host cell expression of CD81, scavenger receptor BI, and claudin-1 (CLDN1), we studied the role of PKA in regulating viral receptor localization by confocal imaging and fluorescence resonance energy transfer (FRET) analysis. Inhibiting PKA activity in Huh-7.5 cells induced a reorganization of CLDN1 from the plasma membrane to an intracellular vesicular location(s) and disrupted FRET between CLDN1 and CD81, demonstrating the importance of CLDN1 expression at the plasma membrane for viral receptor activity. Inhibiting PKA activity in Huh-7.5 cells reduced the infectivity of extracellular virus without modulating the level of cell-free HCV RNA, suggesting that particle secretion was not affected but that specific infectivity was reduced. Viral particles released from H89-treated cells displayed the same range of buoyant densities as did those from control cells, suggesting that viral protein association with lipoproteins is not regulated by PKA. HCV infection of Huh-7.5 cells increased cAMP levels and phosphorylated PKA substrates, supporting a model where infection activates PKA in a cAMP-dependent manner to promote virus release and transmission.

Hepatitis C Virus Induces CD81 and Claudin-1 Endocytosis

ABSTRACT Hepatitis C virus (HCV) leads to progressive liver disease and hepatocellular carcinoma. Current treatments are only partially effective, and new therapies targeting viral and host pathways are required. Virus entry into a host cell provides a conserved target for therapeutic intervention. Tetraspanin CD81, scavenger receptor class B member I, and the tight-junction proteins claudin-1 and occludin have been identified as essential entry receptors. Limited information is available on the role of receptor trafficking in HCV entry. We demonstrate here that anti-CD81 antibodies inhibit HCV infection at late times after virus internalization, suggesting a role for intracellular CD81 in HCV infection. Several tetraspanins have been reported to internalize via motifs in their C-terminal cytoplasmic domains; however, CD81 lacks such motifs, leading several laboratories to suggest a limited role for CD81 endocytosis in HCV entry. We demonstrate CD81 internalization via a clathrin- and dynamin-dependent process, independent of its cytoplasmic domain, suggesting a role for associated partner proteins in regulating CD81 trafficking. Live cell imaging demonstrates CD81 and claudin-1 coendocytosis and fusion with Rab5 expressing endosomes, supporting a role for this receptor complex in HCV internalization. Receptor-specific antibodies and HCV particles increase CD81 and claudin-1 endocytosis, supporting a model wherein HCV stimulates receptor trafficking to promote particle internalization.

Clearance of persistent hepatitis C virus infection in humanized mice using a claudin-1-targeting monoclonal antibody

Hepatitis C virus (HCV) infection is a leading cause of liver cirrhosis and cancer. Cell entry of HCV and other pathogens is mediated by tight junction (TJ) proteins, but successful therapeutic targeting of TJ proteins has not been reported yet. Using a human liver–chimeric mouse model, we show that a monoclonal antibody specific for the TJ protein claudin-1 (ref. 7) eliminates chronic HCV infection without detectable toxicity. This antibody inhibits HCV entry, cell-cell transmission and virus-induced signaling events. Antibody treatment reduces the number of HCV-infected hepatocytes in vivo, highlighting the need for de novo infection by means of host entry factors to maintain chronic infection. In summary, we demonstrate that an antibody targeting a virus receptor can cure chronic viral infection and uncover TJ proteins as targets for antiviral therapy.