Conformational Flexibility in the CD81-Binding Site of the Hepatitis C Virus Glycoprotein E2.

Luisa J Ströh,Kumar Nagarathinam,Thomas Krey

doi:10.3389/fimmu.2018.01396

Abstract

Numerous antibodies have been described that potently neutralize a broad range of hepatitis C virus (HCV) isolates and the majority of these antibodies target the binding site for the cellular receptor CD81 within the major HCV glycoprotein E2. A detailed understanding of the major antigenic determinants is crucial for the design of an efficient vaccine that elicits high levels of such antibodies. In the past 6 years, structural studies have shed additional light on the way the host’s humoral immune system recognizes neutralization epitopes within the HCV glycoproteins. One of the most striking findings from these studies is that the same segments of the E2 polypeptide chain induce antibodies targeting distinct antigen conformations. This was demonstrated by several crystal structures of identical polypeptide segments bound to different antibodies, highlighting an unanticipated intrinsic structural flexibility that allows binding of antibodies with distinct paratope shapes following an “induced-fit” mechanism. This unprecedented flexibility extends to the entire binding site for the cellular receptor CD81, underlining the importance of dynamic analyses to understand (1) the interplay between HCV and the humoral immune system and (2) the relevance of this structural flexibility for virus entry. This review summarizes the current understanding how neutralizing antibodies target structurally flexible epitopes. We focus on differences and common features of the reported structures and discuss the implications of the observed structural flexibility for the viral replication cycle, the full scope of the interplay between the virus and the host immune system and—most importantly—informed vaccine design.

Highlights

71 million people worldwide are chronically infected with hepatitis C virus (HCV), which is one of the major causes of liver cirrhosis, liver failure, and hepatocellular carcinoma [1]
The conformational flexibility within HCV E2 extends to the entire composite CD81-binding site, which overlaps most of the conserved neutralization epitopes present in E2
This finding raises the question how such a conformational flexibility emerges during virus evolution? Which functional importance does this flexibility have—or in other words—which selective advantage does this flexibility provide for the virus?

Summary

INTRODUCTION

71 million people worldwide are chronically infected with hepatitis C virus (HCV), which is one of the major causes of liver cirrhosis, liver failure, and hepatocellular carcinoma [1]. Amino acid substitution frequently results in protein misfolding and thereby in false contact residues in case of conformation-sensitive epitopes—as illustrated for the bnAb AR3C, where the crystal structure revealed different contact residues than expected from previous alanine scanning [34] This pitfall is often alleviated by the use of non-competing conformational Abs to probe overall protein conformation and cross-competition analysis using a panel of well-characterized nAbs. In vitro studies of antibody escape can provide or confirm information about key epitopes [35,36,37,38,39,40,41,42,43]. E1 is less immunogenic but two regions targeted by nAbs have been identified: residues 192–202 (in the prototype H77 sequence), which are recognized by the weakly nAb H-111 [48] and residues 313–324, which interact with the cross-reactive nAbs IGH-526 and IGH-505 [49, 50]

STRUCTURE AND CONFORMATIONAL FLEXIBILITY

CONCLUSION AND FUTURE PERSPECTIVES