Emergence of Novel Human Norovirus GII.17 Strains Correlates With Changes in Blockade Antibody Epitopes

Abstract

Background: Human norovirus is a significant public health burden, with >30 genotypes causing endemic levels of disease and strains from the GII.4 genotype causing serial pandemics as the virus evolves new ligand binding and antigenicity features. During 2014-2015, genotype GII.17 cluster IIIb strains emerged as the leading cause of norovirus infection in select global locations. Comparison of capsid sequences indicates that GII.17 is evolving at previously defined GII.4 antibody epitopes.

Methods: Antigenicity of virus-like particles (VLPs) representative of clusters I, II, and IIIb GII.17 strains were compared by a surrogate neutralization assay based on antibody blockade of ligand binding.

Results: Sera from mice immunized with a single GII.17 VLP identified antigenic shifts between each cluster of GII.17 strains. Ligand binding of GII.17 cluster IIIb VLP was blocked only by antisera from mice immunized with cluster IIIb VLPs. Exchange of residues 393-396 from GII.17.2015 into GII.17.1978 ablated ligand binding and altered antigenicity, defining an important varying epitope in GII.17.

Conclusions: The capsid sequence changes in GII.17 strains result in loss of blockade antibody binding, indicating that viral evolution, specifically at residues 393-396, may have contributed to the emergence of cluster IIIb strains and the persistence of GII.17 in human populations.

Keywords: GII.17; Norovirus; antibody neutralization; blockade antibody; viral evolution.

Figure 1.

GII.17 capsid P2 domain residue changes over time. A, Structural model of GII.17.2002 P domain dimer (shades of gray) with GII.4 surface-exposed blockade epitope A (yellow), D (blue), and E (magenta) highlighted. B–D, Residue changes between 1978 and 2002 (red; B), between 2002 (cluster I) and 2005 (cluster II; cyan; C), and between 2005 (cluster II) and 2015 (cluster IIIb; orange; D).

Figure 2.

Surface-exposed charge changes over time. The electrostatic potential (blue, positive; red, negative) of the top surface of the P2 subdomain is shown for GII.17.1978 (A), GII.17.2002 (B), GII.17.2005 (C), and GII.17.2015 (D). Surface-exposed charge changes in 2015 likely drove antigenic changes, particularly a 4 aspartic acid motif at positions 393–396 (circled).

Figure 3.

Adults have similar blockade antibody (Ab) titers to a time-ordered panel of GII.17 strains. Serum collected for donation from 16 adults (colored markers) living in the United States was evaluated for blockade Ab titer to GII.4.2012 and time-ordered GII.17 virus-like particles from clusters I (1978), II (2005), and IIIb (2015). Bars denote geometric mean titers, and whiskers denote 95% confidence intervals. EC₅₀, half-maximum effective concentration. Dashed line equals the limit of detection.

Figure 4.

GII.17.2015 is antigenically distinct from earlier GII.17 strains at blockade antibody (Ab) epitopes. Balb/c mice were immunized intramuscularly with 0.2 µg of a single GII.17 virus-like particle (VLP) adjuvanted with monophosphoryl lipid A and alhydrogel and sera were evaluated for blockade Ab titer to VLPs from clusters I (1978), II (2005), and IIIb (2015). Bars denote geometric mean titers, and whiskers denote 95% confidence intervals. Dashed line equals the limit of detection. *Significantly different from homotypic serum (by analysis of variance). EC₅₀, half-maximum inhibitory concentration.

Figure 5.

Exchange of residues 393–396 from GII.17.2015 to GII.17.1978 decreases antibody reactivity and virus-like particle (VLP) binding to ligand while maintaining particle structure. A–C, Chimeric VLPs maintain particle integrity, as visualized by electron microscopy (A) but reduced reactivity to polyclonal sera from hyperimmunized guinea pig or rabbit (B) and monoclonal antibodies to conserved genogroup II epitopes (C). D, Ligand binding to both pig gastric mucin (PGM) and human type B saliva also was reduced as compared to parental VLPs. Dashed line equals the limit of detection. *Significantly different from the chimeric VLP (by analysis of variance). EC₅₀, half-maximum effective concentration; IgG, immunoglobulin G.