1C,D and Table 1). The apparent Kd (Kdapp) corresponding to the half-saturating
concentrations for binding to Huh7.5.1 cells ranged from 0.5 to 7.4 nM, demonstrating that these antibodies recognize SR-BI with high affinity (Table 1). It is noteworthy that there seems to be a correlation between the antibody affinity and inhibitory capacity, with the low affinity antibodies unable to block HCV infection. We next aimed to characterize the viral entry steps targeted by these anti–SR-BI mAbs. We first assessed their ability to interfere with viral binding. To reflect the complex interaction between HCV and hSR-BI during viral binding, we studied the effect of anti–SR-BI mAbs on HCVcc binding to Huh7.5.1 selleck chemicals cells at 4°C. Incubation of Huh7.5.1 cells with anti–SR-BI mAbs before and during HCVcc binding did not inhibit virus particle binding (Fig. 2A). Similar results were obtained using sE2 as a surrogate model for HCV (Supporting Results and Supporting Fig. 1). These data suggest that, in contrast to described anti–SR-BI mAbs,20 these novel anti–SR-BI mAbs do not inhibit HCV binding but interfere with HCV entry during postbinding steps. Next, to characterize potential postbinding steps targeted by these anti–SR-BI mAbs, we assessed HCVcc entry kinetics into Huh7.5.1 cells in the presence of anti–SR-BI mAbs inhibiting HCV infection (QQ-4A3-A1, QQ-2A10-A5, QQ-4G9-A6, and NK-8H5-E3) added at different time RXDX-106 clinical trial points during or after viral binding (Fig. 2B). This assay was
performed side-by-side with an anti-CD81 mAb inhibiting HCV postbinding15, 18, 29 and proteinase K36 to remove HCV from the cell surface. HCVcc binding to Huh7.5.1 cells was performed for 1 hour at 4°C in the presence or absence of compounds. Subsequently, unbound virus was washed
away, cells were shifted to 37°C to allow HCVcc entry, and compounds were added every 20 minutes for up to 120 minutes after viral binding. These Thymidylate synthase kinetic experiments indicate that anti–SR-BI mAbs inhibited HCVcc infection when added immediately after viral binding as well as 20-30 minutes after initiation of viral entry (Fig. 2C), demonstrating that QQ-4A3-A1, QQ-2A10-A5, QQ-4G9-A6, and NK-8H5-E3 indeed target postbinding steps of the HCV entry process. This time frame is comparable to the kinetics of resistance of internalized virus to proteinase K (Fig. 2C), indicating that these postbinding steps precede completion of virus internalization. Taken together, these data indicate that a postbinding function of SR-BI is essential for initiation of HCV infection. In contrast to previous anti–SR-BI mAbs inhibiting HCV binding20 as well as polyclonal anti–SR-BI antibodies and small molecules interfering with both viral binding and postbinding,15, 17, 23 these antibodies are the first molecules exclusively targeting the postbinding function of SR-BI and thus represent a unique tool to more thoroughly assess the relevance of this function for HCV infection. HCV disseminates via direct cell-to-cell transmission.