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Short Communication |
1 CNRS, Institut de Biologie de Lille (UMR8161), Institut Pasteur de Lille, 1 rue Calmette, BP447, 59021 Lille cedex, France
2 Free University of Brussels, Faculty of Medicine, Laboratory of Molecular Virology, Brussels, Belgium
3 INSERM-U412, ENS de Lyon, Lyon, France
4 Hôpital Erasme, Free University of Brussels, Brussels, Belgium
5 Laboratory of Virology, Centre Hospitalier Universitaire d'Amiens, Amiens, France
Correspondence
Ngoc Vu-Dac
ngoc.vu-dac{at}ibl.fr
Jean Dubuisson
jean.dubuisson{at}ibl.fr
| ABSTRACT |
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| MAIN TEXT |
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HCVpp containing subtype 1a envelope glycoproteins (strain H) and infectious JFH1 virus (HCVcc) were generated as described previously (Bartosch et al., 2003b
; Rouillé et al., 2006
; Wakita et al., 2005
). Infection experiments were performed on Huh-7 cells (Nakabayashi et al., 1982
). HCVpp and HCVcc were produced in lipoprotein-free medium (2 % fetal calf lipoprotein-depleted serum, LPDS) and neutralization experiments were also performed in medium supplemented with 2 % LPDS. Human HDL (density, 1.131.18 g ml1) fractions from fresh human plasma were isolated as described by Hatch (1968)
. A series of sera from patients infected chronically with various HCV genotypes was used.
To evaluate the impact of HDLs from HCV-positive sera on HCVpp neutralization, sera containing HDLs and antibodies were co-incubated with HCVpp and target cells. Classically, neutralization assays are performed by pre-incubating virus and sera from infected patients before contact with target cells. However, as HDLs potentially have an effect on virus entry by acting directly on the target cell, we considered that it would be more relevant to study the effect of HDLs on neutralization by co-incubating sera, HCVpp and target cells. In these conditions, we observed that only two of the 10 HCV-positive sera tested were able to neutralize HCVpp entry by >50 % (Fig. 1a
). In contrast, all of the sera displayed a neutralizing activity in a classical neutralization assay based on a pre-incubation of HCVpp and serum before contact with target cells (Fig. 1a
). These data show that, in most cases, HCV-positive sera are not able to neutralize HCVpp when antibodies and HDLs are incubated simultaneously with HCVpp and the cells.
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To further investigate the mechanism of HDL-mediated attenuation of neutralization, we knocked down SR-BI expression. HDLs are indeed a physiological ligand of SR-BI (Rhainds & Brissette, 2004
). RNA-interference experiments were done as described previously (Voisset et al., 2005
). SR-BI-specific small interfering RNA (siRNA) downregulated SR-BI expression to approximately 70 % compared with the negative-control siRNA (data not shown). As observed previously (Voisset et al., 2005
), when a specific siRNA against SR-BI was used, HCVpp infectivity was not affected in the absence of HDLs, whereas HDL-mediated enhancement of infectivity was reduced strongly (Fig. 2a
). This contrasts with another study showing that silencing of SR-BI expression reduces HCVpp infectivity (Lavillette et al., 2005b
). The discrepancy between these data may potentially be explained by differences in the levels of downregulation of expression of SR-BI in these experiments. Indeed, one cannot exclude the possibility that HCV requires only very small amounts of SR-BI to enter target cells. Interestingly, when SR-BI expression was downregulated, the efficiency of neutralization of sera was increased markedly and reached neutralization levels similar to those obtained with purified antibodies in the absence of HDLs (Fig. 2a
). To further investigate the role of SR-BI in HDL-mediated attenuation of neutralization, we also analysed the effect of BLT-4, a drug known to block SR-BI-mediated selective uptake of HDL lipids (Nieland et al., 2002
). This compound has also been shown to strongly reduce the HDL-mediated enhancement of HCVpp entry (Bartosch et al., 2005
; Voisset et al., 2005
). Interestingly, when the cells were treated by BLT-4, HCVpp were neutralized more efficiently by antibodies from HCV-positive patients, despite the presence of HDLs (Fig. 2b
). These data indicate that HDL-mediated attenuation of HCVpp neutralization is dependent on SR-BI and its selective lipid-uptake property. In addition, they also show that HDL-mediated facilitation of HCVpp entry and attenuation of neutralization are two related phenomena.
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The exact role of SR-BI in HCV entry is far from understood. SR-BI has been shown to modulate the plasma-membrane composition (Huang et al., 2003
; Peng et al., 2004
). The relationship between SR-BI and HCV infection may thus be based on the capacity of SR-BI to modulate the lipid composition of the plasma membrane to render the membrane permissive to HCV entry. In agreement with this hypothesis, the SR-BI-mediated selective uptake of lipids from HDLs has been shown to enhance HCVpp entry markedly (Bartosch et al., 2005
; Voisset et al., 2005
). It is thus tempting to speculate that SR-BI is involved at some stage of HCV entry by modulating the plasma-membrane composition.
In conclusion, our data demonstrate that HDLs reduce HCVpp neutralization by accelerating HCVpp entry, through the lipid-transfer property of SR-BI. The rapid kinetics of HDL facilitation of HCVpp entry may allow HCVpp to escape some neutralizing antibodies. While antibodies bind to HCVpp and neutralize their entry, HDLs accelerate the entry of HCVpp that are not yet neutralized.
| ACKNOWLEDGEMENTS |
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| REFERENCES |
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Received 9 February 2006;
accepted 24 April 2006.
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