HPV vaccine-induced cross-neutralising antibodies target complex epitopes on the major capsid protein

Abstract

The current human papillomavirus (HPV) vaccines consist of major capsid protein (L1) virus-like particles (VLP) and target the two most prevalent oncogenic genotypes, HPV16 and HPV18. Prophylactic HPV vaccination is highly effective at preventing HPV16 and HPV18 infection and associated cervical disease, with type-specific neutralising antibodies thought to be the immune mediators of vaccine type protection. A degree of vaccine-induced cross-protection has also been demonstrated against genetically-related genotypes in the Alpha-7 (HPV18-like) and Alpha-9 (HPV16-like) species groups and although the underlying immune mechanism is uncertain, cross-protection is coincident with the detection of cross-neutralising antibodies. The aim of this thesis was to delineate the HPV L1 domains that are recognised by inter-genotype cross-neutralising antibodies. The formal analysis of the vaccine-induced A9 L1 antibody response demonstrated that cross-neutralising antibodies were a minor component of the total HPV16 antibody response and comprised antibody specificities which recognised single and multiple non-vaccine genotypes. The bioinformatic examination of A9 capsid amino acid sequences demonstrated that the L1L2 pseudovirions (PsV) used to measure cross-neutralising responses were generally representative of available contemporary sequences. The potential impact of amino acid variation within the L1 capsid protein was investigated for HPV31 and found differences in cross-neutralising antibody recognition of the L1 variants; however, this was of a low magnitude. L1 crystallographic homology models predicted structural changes in the loops between HPV16 and the non-vaccine A9 genotypes, informing the design and generation of chimeric PsV with inter-genotype loop swaps. These chimeric PsV demonstrated that cross-neutralising antibodies recognise DE and FG loop amino acid residues within close proximity to each other on the capsid surface. These data contribute to our understanding of the antigenicity of the L1 major capsid protein of HPV by identifying the L1 regions recognised by vaccine-induced cross-neutralising antibodies. Such specificities may play a critical role in vaccine-induced cross-protection.