Identification of neutralizing conformational epitopes on the human papillomavirus type 31 major capsid protein and functional implications

Abstract

The aim of this study was to characterize the conformational neutralizing epitopes of the major capsid protein of human papillomavirus type 31. Analysis of the epitopes was performed by competitive epitope mapping using 15 anti-HPV31 and by reactivity analysis using a HPV31 mutant with an insertion of a seven-amino acid motif within the FG loop of the capsid protein. Fine mapping of neutralizing conformational epitopes on HPV L1 was analyzed by a new approach using a system displaying a combinatorial library of constrained peptides exposed on E. coli flagella. The findings demonstrate that the HPV31 FG loop is dense in neutralizing epitopes and suggest that HPV31 MAbs bind to overlapping but distinct epitopes on the central part of the FG loop, in agreement with the exposure of the FG loop on the surface of HPV VLPs, and thus confirming that neutralizing antibodies are mainly located on the tip of capsomeres. In addition, we identified a crossreacting and partially crossneutralizing conformational epitope on the relatively well conserved N-terminal part of the FG loop. Moreover, our findings support the hypothesis that there is no correlation between neutralization and the ability of MAbs to inhibit VLP binding to heparan sulfate, and confirm that the blocking of virus attachment to the extracellular matrix is an important mechanism of neutralization.

Naturally Occurring Capsid Protein Variants of Human Papillomavirus Genotype 31 Represent a Single L1 Serotype

“…These data corroborate previous data from a bacterial cell surface display model which demonstrated that these three MAbs recognize overlapping, yet distinct, FG loop epitopes (36). Residues 267 and 274 are in close proximity to the Lys279 and Lys362 residues or near the Lys54, Asn57, Lys60, and Lys367 residues.…”

“…The FG loop of HPV31 has been shown to be an important antigenic domain targeted by both type-specific and cross-reactive L1 MAbs (36,37). In the present study, we generated additional HPV31 L1 and L2 sequences and synthesized representative antigens in order to investigate the potential impact of this variation.…”

Naturally Occurring Major and Minor Capsid Protein Variants of Human Papillomavirus 45 (HPV45): Differential Recognition by Cross-Neutralizing Antibodies Generated by HPV Vaccines

“…The L1 protein multimerizes to form the nonenveloped icosahedral viral capsid (comprising 72 L1 pentameric capsomers) that mediates attachment to host cells (11), while the L2 protein is essential for viral infectivity (12). Structural alterations of the external surface topography of L1 can be conferred by minor sequence differences between genotypes (13), supporting observations that almost all neutralizing monoclonal antibodies (MAbs) that target these external surfaces are type specific (14)(15)(16)(17). Nevertheless, functional antibody cross-reactivity is a common feature of sera from recipients of the Cervarix (bivalent) and Gardasil (quadrivalent) vaccines (18)(19)(20)(21)(22) and may be responsible for conferring HPV vaccine-induced cross-protection (23).…”

The DE and FG loops of the HPV major capsid protein contribute to the epitopes of vaccine-induced cross-neutralising antibodies

“…The FG loop contains a Lys 278 (HPV16 numbering) which mediates primary host attachment9, an interaction which is inhibited in vivo by vaccine induced L1 antibodies53 providing a possible mechanistic reason behind the antigenic targeting of the FG loop by both type-specific MAbs and natural infection antibodies1316455154. The early region of the FG loop is known to harbour residues involved in the epitope footprints of type-specific, neutralising HPV31 MAbs16 whilst the late region contains the majority of residues involved in the epitope footprints of type-specific, neutralising HPV16 MAbs14.…”

“…The epitope of one of these MAbs, H16.V5, included loops from two neighbouring L1 monomers with the majority of contact residues predicted to be in the DE and FG loops with a minor number of contact residues located in the EF and HI loops15. In comparison, the epitope footprints recognised by four HPV31 MAbs appear to be restricted to amino acid residues within the FG loop16.…”