Emergent strains of human norovirus seed pandemic waves of disease. These new strains have altered
ligand binding and antigenicity characteristics. Study of viral variants isolated from immunosuppressed patients with long-term
norovirus infection indicates that initial virus in vivo evolution occurs at the same antigenic sites as in pandemic strains. Here, cellular
ligand binding and antigenicity of two cocirculating strains isolated from a patient with long-term
norovirus infection were characterized. The isolated GII.4 viruses differed from previous strains and from each other at known blockade antibody
epitopes. One strain had a unique sequence in
epitope D, including loss of an insertion at residue 394, corresponding to a decreased relative affinity for
carbohydrate ligands. Replacement of 394 with
alanine or restoration of the contemporary strain
epitope D consensus sequence STT improved
ligand binding relative affinity. However,
monoclonal antibody blockade of binding potency was only gained for the consensus sequence, not by the
alanine insertion. In-depth study of unique changes in
epitope D indicated that
ligand binding, but not antibody blockade of
ligand binding, is maintained despite sequence diversity, allowing escape from blockade
antibodies without loss of capacity for binding cellular
ligands.IMPORTANCE Human norovirus causes ∼20% of all acute
gastroenteritis and ∼200,000 deaths per year, primarily in young children. Most epidemic and all pandemic waves of disease over the past 30 years have been caused by type GII.4 human norovirus strains. The capsid sequence of GII.4 strains is changing over time, resulting in viruses with altered
ligand and antibody binding characteristics. The
carbohydrate binding pocket of these strains does not vary over time. Here, utilizing unique viral sequences, we study how residues in GII.4
epitope D balance the dual roles of variable antibody binding site and cellular
ligand binding stabilization domain, demonstrating that
amino acid changes in
epitope D can result in loss of antibody binding without ablating
ligand binding. This flexibility in
epitope D likely contributes to GII.4 strain persistence by both allowing escape from antibody-mediated herd immunity and maintenance of cellular
ligand binding and infectivity.