Distinct mechanisms of antibody-mediated HCV neutralization revealed by nanobody-guided epitope mapping

Hepatitis C virus (HCV) remains a major global health challenge despite the availability of highly effective antiviral therapies, underscoring the need for a broadly protective vaccine. The envelope glycoprotein E2 is the principal target of neutralizing antibodies, yet the full repertoire of vulnerable epitopes and mechanisms of antibody-mediated neutralization remains incompletely understood. Here, we exploited the unique binding properties of camelid nanobodies to probe the antigenic landscape of HCV E2 beyond the immunodominant human antibody response. We isolated a diverse panel of E2-specific nanobodies, including broadly neutralizing antibodies with high-affinity cross-reactivity toward genetically diverse HCV isolates. By combining cross-neutralization assays, competition binding experiments, and high-resolution hydrogen-deuterium exchange mass spectrometry (HDX-MS), we identified three mechanistically distinct classes of neutralizing epitopes. While one class targets the canonical E2 neutralization face, a second class recognizes antigenic region 1 (AR1), independently validating and extending recent evidence that this region represents a functional site of viral vulnerability. These findings demonstrate that broadly neutralizing antibody responses extend beyond the canonical neutralization face and establish a broader framework for understanding HCV neutralization. More broadly, our study illustrates how alternative antibody repertoires can reveal functionally important antigenic surfaces that are underrepresented in conventional human antibody responses, providing new opportunities for the rational design of next-generation HCV vaccine immunogens.