Structural basis of host ligand specificity change of GII porcine noroviruses from their closely related GII human noroviruses

Genetic characterization of rarely reported GII.3[P25] norovirus strain detected in patients with acute gastroenteritis in Huzhou, China, 2021

•The first genome of GII.3 [P25] strain isolated in China was determined by NGS. •About 19 unique amino acid substitutions were identified in the VP1 region of GII.3 [P25]. •Antigenic variation may have contributed to the re-emerge of GII.3 [P25] strains.

Structural Insight into Terminal Galactose Recognition by Two Non-HBGA Binding GI.3 Noroviruses

Human noroviruses (huNoVs) cause epidemic acute gastroenteritis using histo-blood group antigens (HBGAs) as host receptors or attachment factors to initiate an infection. While most huNoVs have been shown to bind HBGAs, some known clinical isolates, such as GI.3 DSV and VA115, do not recognize any HBGAs and thus the molecular mechanism behind their infections remains elusive. In this study, we provided both phenotypic and structural evidence to show that huNoV DSV and VA115 recognize a group of glycans with terminal galactoses as ligands. First, through glycan array we found that both DSV and VA115 protruding (P) domain proteins bound two oligosaccharides that share common terminal galactoses. Then, by determination of the crystal structures of DSV/VA115 P proteins in complex with Galα1-3Galβ1-4Glc and/or NA2 N-Glycan, respectively, we showed that the terminal galactose is the main saccharide recognized by the two viral proteins. Our data demonstrated that GI huNoVs can interact with non-HBGA glycans through their conserved galactose binding site, shedding light on the mechanism of huNoV adaptation through recognizing new glycan receptors to facilitate their widespread nature in human population. These findings are also of significance in strategy development for huNoV control and prevention, as well as development of antiviral drugs.

IMPORTANCE Human noroviruses (huNoVs) are the most important viral pathogens causing epidemic acute gastroenteritis worldwide. Previous studies indicated that histo-blood group antigens (HBGAs) are critical host-susceptibility factors affecting huNoV host susceptibility, host range, and probably prevalence. However, certain huNoVs, such as GI.3 DSV and VA115, do not recognize any HBGAs. This implies that other unknown host factors might exist and the molecular mechanism underlying their host receptor recognition or attachment remains elusive. In this study, we found that purified capsid protruding domain proteins from two GI.3 huNoVs specifically bind two glycans that contain a common terminal galactose. We solved the crystal structures of the complexes at atomic resolution and validated the vital amino acids involved in glycan recognition. Our findings elucidate the mechanism of GI.3 huNoV-non-HBGA glycan interaction, which explains why GI.3 virus strains could not bind human HBGAs, paving a way to the prevention and treatment of huNoV-associated diseases.

Identification of a novel norovirus species in fox

A novel Norovirus (NoV) was identified by viral metagenomic analysis in fox fecal samples from the Xinjiang Uygur Autonomous Region of China. The virus exhibited typical genomic characteristics of NoVs. It was closely related to the canine NoV GVII strains with 86.0-86.2% and 91.9% amino acid identities in the capsid protein VP1 and RNA-dependent RNA polymerase (RdRp), respectively. The fox NoV clustered phylogenetically with the two canine NoV GVII strains, and it was distant from other NoVs. According to the new classification criteria of NoVs, the new fox NoV belongs to the same genotype as GVII, similar to canine GVII NoVs. Moreover, key amino acid residues in the Histo-blood group antigen (HBGA) binding sites and the HBGA binding pattern of the fox NoV differed significantly from those of human and canine GVII NoVs. This study identified a new GVII norovirus from wild foxes in China. These findings enrich our understanding of the diversity of NoVs and provide further evidence regarding the genetic heterogeneity of NoVs in carnivores.

Identification of Human Norovirus GII.3 Blockade Antibody Epitopes

Human noroviruses are a common pathogen causing acute gastroenteritis worldwide. Among all norovirus genotypes, GII.3 is particularly prevalent in the pediatric population. Here we report the identification of two distinct blockade antibody epitopes on the GII.3 capsid. We generated a panel of monoclonal antibodies (mAbs) from mice immunized with virus-like particle (VLP) of a GII.3 cluster 3 strain. Two of these mAbs, namely 8C7 and 8D1, specifically bound the parental GII.3 VLP but not VLPs of GII.4, GII.17, or GI.1. In addition, 8C7 and 8D1 efficiently blocked GII.3 VLP binding with its ligand, histo-blood group antigens (HBGA). These data demonstrate that 8C7 and 8D1 are GII.3-specific blockade antibodies. By using a series of chimeric VLPs, we mapped the epitopes of 8C7 and 8D1 to residues 385-400 and 401-420 of the VP1 capsid protein, respectively. These two blockade antibody epitopes are highly conserved among GII.3 cluster 3 strains. Structural modeling shows that the 8C7 epitope partially overlaps with the HBGA binding site (HBS) while the 8D1 epitope is spatially adjacent to HBS. These findings may enhance our understanding of the immunology and evolution of GII.3 noroviruses.

Human Noroviruses Attach to Intestinal Tissues of a Broad Range of Animal Species

Human noroviruses are the most common nonbacterial cause of gastroenteritis outbreaks, with new variants and genotypes frequently emerging. The origin of these new viruses is unknown; however, animals have been proposed as a potential source, as human noroviruses have been detected in animal species. Here, we investigated the potential of animals to serve as a reservoir of human noroviruses by testing norovirus attachment to formalin-fixed intestinal tissues of a range of potential reservoir animals. We set up a novel method to study norovirus binding using fluorescein isothiocyanate (FITC)-labeled virus-like particles (VLPs). In humans, noroviruses interact with histo-blood group antigens (HBGAs), carbohydrates that are expressed, among others, on the epithelial lining of the gastrointestinal tract. In animals, this interaction is not well understood. To test if virus binding depends on HBGAs, we characterized the HBGA phenotype in animal tissues by immunohistochemistry. With the exception of the black-headed gull and the straw-colored fruitbat, we observed the attachment of several human norovirus genotypes to the intestinal epithelium of all tested animal species. However, we did not find an association between the expression of a specific HBGA phenotype and virus-like particle (VLP) attachment. We show that selected human noroviruses can attach to small-intestinal tissues across species, supporting the hypothesis that human noroviruses can reside in an animal reservoir. However, whether this attachment can subsequently lead to infection needs to be further assessed.IMPORTANCE Noroviruses are a major cause of acute gastroenteritis in humans. New norovirus variants and recombinants (re)emerge regularly in the human population. From animal experiments and surveillance studies, it has become clear that at least seven animal models are susceptible to infection with human strains and that domesticated and wild animals shed human noroviruses in their feces. As virus attachment is an important first step for infection, we used a novel method utilizing FITC-labeled VLPs to test for norovirus attachment to intestinal tissues of potential animal hosts. We further characterized these tissues with regard to their HBGA expression, a well-studied norovirus susceptibility factor in humans. We found attachment of several human strains to a variety of animal species independent of their HBGA phenotype. This supports the hypothesis that human strains could reside in an animal reservoir.