A novel pre-fusion conformation-specific neutralizing epitope on the respiratory syncytial virus fusion protein.

Jarrod J Mousa,Nurgun Kose,Pranathi Matta,Pavlo Gilchuk,James E Crowe

doi:10.1038/nmicrobiol.2016.271

Abstract

Respiratory syncytial virus (RSV) remains a major human pathogen, infecting the majority of infants before age two and causing re-infection throughout life. Despite decades of RSV research, there is no licensed RSV vaccine. Most candidate vaccines studied to date have incorporated the RSV fusion (F) surface glycoprotein, because the sequence of F is highly conserved among strains of RSV. To better define the human B cell response to RSV F, we isolated from a single donor 13 new neutralizing human monoclonal antibodies (mAbs) that recognize the RSV F protein in the pre-fusion conformation. Epitope binning studies showed that the majority of neutralizing mAbs targeted a new antigenic site on the globular head domain of F, designated here antigenic site VIII, which occupies an intermediate position between the previously defined major antigenic sites II and site Ø. Antibodies to site VIII competed for binding with antibodies to both of those adjacent neutralizing sites. The new mAbs exhibited unusual breadth for pre-fusion F-specific antibodies, cross-reacting with F proteins from both RSV subgroups A and B viruses. We solved the X-ray crystal structure of one site VIII mAb, hRSV90, in complex with pre-fusion RSV F protein. The structure revealed a large footprint of interaction for hRSV90 on RSV F, in which the heavy chain and light chain both have specific interactions mediating binding to site VIII, the heavy chain overlaps with site Ø, and the light chain interacts partially with site II.

Highlights

Respiratory syncytial virus (RSV) expresses three surface proteins: attachment (G), small hydrophobic (SH) and fusion (F) proteins
We recently described the isolation and characterization of several new human monoclonal antibodies (mAbs) targeting antigenic sites I and II, which were identified by screening for binding to the RSV strain A2 F protein in the post-fusion conformation[14]
All mAbs except hRSV130 were of the immunoglobulin G1 (IgG1) subclass, and the majority of light chains were of the kappa subtype (Table 1)

Summary

Introduction

RSV expresses three surface proteins: attachment (G), small hydrophobic (SH) and fusion (F) proteins. We recently described the isolation and characterization of several new human mAbs targeting antigenic sites I and II, which were identified by screening for binding to the RSV strain A2 F protein in the post-fusion conformation[14]. Recent experiments suggested a dominant role for epitopes in the pre-fusion conformation of RSV F in the induction of serum neutralizing antibodies, a major role for antigenic site Ø in immunogenicity[15]. We determined the half-maximal effective concentration for binding (EC50) values by enzyme-linked immunosorbent assay to test the F-protein strain specificity and preference for pre-fusion versus post-fusion F conformations. Cross-reactive pre-fusion conformation-specific mAbs have been reported only in one case, that of the quaternary-epitope dependent mAb AM14 HRSV90 and hRSV20 shared nearly identical gene segment usage, the two mAbs are probably not clonal siblings due to a heavy chain complementarity determining region 3 (HCDR3) insertion in hRSV20

Methods

Conclusion