Characterization of hepatitis C virus (HCV) and HCV E2 interactions with CD81 and the low-density lipoprotein receptor

Hepatitis C virus (HCV) or HCV-low-density lipoprotein (LDL) complexes inte ract with the LDL receptor (LDLr) and the HCV envelope glycoprotein E2 inte racts with CD81 in vitro. However, E2 interactions with LDLr and HCV intera ctions with CD81 have not been clearly described. Using sucrose gradient-pu rified low-density particles (1.03 to 1.07 g/cm(3)), intermediate-density p articles (1.12 to 1.18 g/cm(3)), recombinant E2 protein, or control protein s, we assessed binding to MOLT-4 cells, foreskin fibroblasts, or LDLr-defic ient foreskin fibroblasts at 4 degrees C by flow cytometry and confocal mic roscopy, Viral entry was determined by measuring the coentry of alpha-sarci n, a protein synthesis inhibitor. We found that low-density HCV particles, but not intermediate-density HCV or controls bound to MOLT-4 cells and fibr oblasts expressing the LDLr, Binding correlated with the extent of cellular LDLr expression and was inhibited by LDL but not by soluble CD81. In contr ast, E2 binding was independent of LDLr expression and was inhibited by hum an soluble CD81 but not mouse soluble CD81 or LDL. Based on confocal micros copy, we found that low-density HCV particles and LDL colocalized on the ce ll surface. The addition of low-density HCV but not intermediate-density HC V particles to MOLT-4 cells allowed coentry of alpha-sarcin, indicating vir al entry. The amount of viral entry also correlated with LDLr expression an d was independent of the CD81 expression. Using a solid-phase immunoassay, recombinant E2 protein did not interact with LDL. Our data indicate that E2 binds CD81; however, virus particles utilize LDLr for binding and entry. T he specific mechanism by which HCV particles interact with LDL or the LDLr remains unclear.