Binding activity of norovirus and sapovirus to histo-blood group antigens

Chimeric virus-like particles of human norovirus constructed by structure-guided epitope grafting elicit cross-reactive immunity against both GI.1 and GII.4 genotypes

IMPORTANCE

Human norovirus (HuNoV) is highly infectious and can result in severe illnesses in the elderly and children. So far, there is no effective antiviral drug to treat HuNoV infection, and thus, the development of HuNoV vaccines is urgent. However, NoV evolves rapidly, and currently, at least 10 genogroups with numerous genotypes have been found. The genetic diversity of NoV and the lack of cross-protection between different genotypes pose challenges to the development of broadly protective vaccines. In this study, guided by structural alignment between GI.1 and GII.4 HuNoV VP1 proteins, several chimeric-type virus-like particles (VLPs) were designed through surface-exposed loop grafting. Mouse immunization studies show that two of the designed chimeric VLPs induced cross-immunity against both GI.1 and GII.4 HuNoVs. To our knowledge, this is the first designed chimeric VLPs that can induce cross-immune activities across different genogroups of HuNoV, which provides valuable strategies for the development of cross-reactive HuNoV vaccines.

Human Sapovirus Replication in Human Intestinal Enteroids

Human sapoviruses (HuSaVs), like human noroviruses (HuNoV), belong to the Caliciviridae family and cause acute gastroenteritis in humans. Since their discovery in 1976, numerous attempts to grow HuSaVs in vitro were unsuccessful until 2020, when these viruses were reported to replicate in a duodenal cancer cell-derived line. Physiological cellular models allowing viral replication are essential to investigate HuSaV biology and replication mechanisms such as genetic susceptibility, restriction factors, and immune responses to infection. In this study, we demonstrate replication of two HuSaV strains in human intestinal enteroids (HIEs) known to support the replication of HuNoV and other human enteric viruses. HuSaVs replicated in differentiated HIEs originating from jejunum, duodenum and ileum, but not from the colon, and bile acids were required. Between 2h and 3 to 6 days postinfection, viral RNA levels increased up from 0.5 to 1.8 log10-fold. Importantly, HuSaVs were able to replicate in HIEs independent of their secretor status and histo-blood group antigen expression. The HIE model supports HuSaV replication and allows a better understanding of host-pathogen mechanisms such as cellular tropism and mechanisms of viral replication. IMPORTANCE Human sapoviruses (HuSaVs) are a frequent but overlooked cause of acute gastroenteritis, especially in children. Little is known about this pathogen, whose successful in vitro cultivation was reported only recently, in a cancer cell-derived line. Here, we assessed the replication of HuSaV in human intestinal enteroids (HIEs), which are nontransformed cultures originally derived from human intestinal stem cells that can be grown in vitro and are known to allow the replication of other enteric viruses. Successful infection of HIEs with two strains belonging to different genotypes of the virus allowed discovery that the tropism of these HuSaVs is restricted to the small intestine, does not occur in the colon, and replication requires bile acid but is independent of the expression of histo-blood group antigens. Thus, HIEs represent a physiologically relevant model to further investigate HuSaV biology and a suitable platform for the future development of vaccines and antivirals.

Association between breastfeeding, host genetic factors, and calicivirus gastroenteritis in a Nicaraguan birth cohort

BACKGROUND

Norovirus and sapovirus are important causes of childhood acute gastroenteritis (AGE). Breastfeeding prevents AGE generally; however, it is unknown if breastfeeding prevents AGE caused specifically by norovirus and sapovirus.

METHODS

We investigated the association between breastfeeding and norovirus or sapovirus AGE episodes in a birth cohort. Weekly data on breastfeeding and AGE episodes were captured during the first year of life. Stools were collected from children with AGE and tested by RT-qPCR for norovirus and sapovirus. Time-dependent Cox models estimated associations between weekly breastfeeding and time to first norovirus or sapovirus AGE.

FINDINGS

From June 2017 to July 2018, 444 newborns were enrolled in the study. In the first year of life, 69 and 34 children experienced a norovirus and a sapovirus episode, respectively. Exclusive breastfeeding lasted a median of 2 weeks, and any breastfeeding lasted a median of 43 weeks. Breastfeeding in the last week did not prevent norovirus (HR: 1.09, 95% CI: 0.62, 1.92) or sapovirus (HR: 1.00, 95% CI: 0.82, 1.21) AGE in a given week, adjusting for household sanitation, consumption of high-risk foods, and mother's and child's histo-blood group phenotypes. Maternal secretor-positive phenotype was protective against norovirus AGE, whereas child's secretor-positive phenotype was a risk factor for norovirus AGE.

INTERPRETATION

Exclusive breastfeeding in this population was short-lived, and no conclusions could be drawn about its potential to prevent norovirus or sapovirus AGE. Non-exclusive breastfeeding did not prevent norovirus or sapovirus AGE in the first year of life. However, maternal secretor-positive phenotype was associated with a reduced hazard of norovirus AGE.

Receptor profile and immunogenicity of the non-epidemic norovirus GII.8 variant

Noroviruses are causative agents of acute nonbacterial gastroenteritis epidemics worldwide. There are various genotypes, among which the non-epidemic genotype GII.8 can cause norovirus outbreaks. We previously demonstrated that the immunogenicity of GII.8 differed from that of epidemic variants. This study aimed to comprehensively compare the receptor profile and immunogenicity of the GII.8 variant with those of the epidemic variants. Using the baculovirus-insect cell expression system, we observed that recombinant capsid protein VP1 of the norovirus GII.8 GZ2017-L601 strain formed virus-like particles (VLPs) with a diameter of approximately 30 nm, as evidenced by transmission electron microscopy analysis. The GII.8 VLPs showed weak or moderate binding with all secretor histo-blood group antigens (HBGAs), but not with non-secretors, as evidenced by the HBGA-VLP binding test. The GII.8 VLP antiserum obtained from immunized BALB/c mice was tested for cross-reactivity with other norovirus genotypes (n = 28). The results showed that this antiserum demonstrated moderate cross-reactivity with GI.1, GII.3, and GII.15; however, no cross-reactivity with the epidemic variants of GII.2, GII.4, and GII.17 was observed. Additionally, the blocking-antibody activity of GII.8 antisera against GII.4 VLP-HBGAs and GII.17 VLP-HBGAs interactions and the cross-blocking of GII.8 VLP-HBGAs interactions by GI.1 and GII.4 antisera were evaluated using the HBGAs-VLP blocking test. However, no cross-blocking effect was observed. In summary, the characterization of norovirus GII.8 VLPs and derived antisera revealed that the GII.8 immunogenicity differed from that of epidemic variants.

Molecular Epidemiology of Sapovirus in Children Living in the Northwest Amazon Region

Sapovirus is an important etiological agent of acute gastroenteritis (AGE), mainly in children under 5 years old living in lower-income communities. Eighteen identified sapovirus genotypes have been observed to infect humans. The aim of this study was to identify sapovirus genotypes circulating in the Amazon region. Twenty-eight samples were successfully genotyped using partial sequencing of the capsid gene. The genotypes identified were GI.1 (n = 3), GI.2 (n = 7), GII.1 (n = 1), GII.2 (n = 1), GII.3 (n = 5), GII.5 (n = 1), and GIV.1 (n = 10). The GIV genotype was the most detected genotype (35.7%, 10/28). The phylogenetic analysis identified sapovirus genotypes that had no similarity with other strains reported from Brazil, indicating that these genotypes may have entered the Amazon region via intense tourism in the Amazon rainforest. No association between histo-blood group antigen expression and sapovirus infection was observed.

ELISA-Based Methods to Detect and Quantify Norovirus Virus-Like Particle Attachment to Histo-Blood Group Antigens

Histo-blood group antigen (HBGA) recognition by norovirus (NoV) has been studied using various techniques. Enzyme-linked immunosorbent assays (ELISAs) using virus-like particles (VLPs) have enabled us to visualize the last step of HBGAs-NoV binding with a total reaction time of approximately 8 h. Herein, we describe two ELISA-based methods to detect and quantify NoV VLP attachment to HBGAs: saliva-VLP binding assay and carbohydrate-VLP binding assay.

Risk Factors and Clinical Profile of Sapovirus-associated Acute Gastroenteritis in Early Childhood: A Nicaraguan Birth Cohort Study

BACKGROUND

Sapovirus is increasingly recognized as an important cause of acute gastroenteritis (AGE) in children. We identified risk factors and characterized the clinical profile of sapovirus AGE in a birth cohort in León, Nicaragua.

METHODS

We conducted a case-control study nested within a birth cohort (n = 444). Fieldworkers conducted weekly household AGE surveillance. AGE stools were tested for sapovirus by reverse transcriptase quantitative polymerase chain reaction. For each first sapovirus episode, we selected 2 healthy age-matched controls and estimated independent risk factors of sapovirus AGE using conditional logistic regression. We compared clinical characteristics of sapovirus AGE episodes with episodes associated with other etiologies and identified co-infections with other enteric pathogens.

RESULTS

From June 2017 to July 2019, we identified 63 first sapovirus AGE episodes and selected 126 controls. Having contact with an individual with AGE symptoms and vaginal delivery were independent risk factors for sapovirus AGE. All cases experienced diarrhea, lasting a median 6 days; 23% experienced vomiting. Compared with children with AGE due to another etiology, sapovirus AGE was similar in severity, with less reported fever. Most cases experienced co-infections and were more likely than controls to be infected with diarrheagenic Escherichia coli or astrovirus.

CONCLUSIONS

Sapovirus was a commonly identified AGE etiology in this Central American setting, and symptoms were similar to AGE associated with other etiologies. The association between vaginal delivery and sapovirus is a novel finding. Gut microbiome composition might mediate this relationship, or vaginal delivery might be a proxy for other risk factors. Further investigation into more specific biological mechanisms is warranted.

Comprehensive review of human sapoviruses

Sapoviruses cause acute gastroenteritis in humans and animals. They belong to the genus Sapovirus within the family Caliciviridae. They infect and cause disease in humans of all ages, in both sporadic cases and outbreaks. The clinical symptoms of sapovirus gastroenteritis are indistinguishable from those caused by noroviruses, so laboratory diagnosis is essential to identify the pathogen. Sapoviruses are highly diverse genetically and antigenically. Currently, reverse transcription-PCR (RT-PCR) assays are widely used for sapovirus detection from clinical specimens due to their high sensitivity and broad reactivity as well as the lack of sensitive assays for antigen detection or cell culture systems for the detection of infectious viruses. Sapoviruses were first discovered in 1976 by electron microscopy in diarrheic samples of humans. To date, sapoviruses have also been detected from several animals: pigs, mink, dogs, sea lions, and bats. In this review, we focus on genomic and antigenic features, molecular typing/classification, detection methods, and clinical and epidemiological profiles of human sapoviruses.

Removal properties of human enteric viruses in a pilot-scale membrane bioreactor (MBR) process

In order to evaluate removal properties of human enteric viruses from wastewater by a membrane bioreactor (MBR), influent, anoxic and oxic mixed liquor, and membrane effluent samples were collected in a pilot-scale anoxic-oxic MBR process for 16 months, and concentrations of enteroviruses, norovirus GII, and sapoviruses were determined by real-time PCR using murine norovirus as a process control. Mixed liquor samples were separated into liquid and solid phases by centrifugation, and viruses in the bulk solution and those associated with mixed liquor suspended solids (MLSS) were quantified. Enteroviruses, norovirus GII, and sapoviruses were detected in the influent throughout the sampling period (geometrical mean, 4.0, 3.1, and 4.4 log copies/mL, respectively). Enterovirus concentrations in the solid phase of mixed liquor were generally lower than those in the liquid phase, and the mean log reduction value between influent and anoxic mixed liquor was 0.40 log units. In contrast, norovirus GII and sapovirus concentrations in the solid phase were equal to or higher than those in the liquid phase, and higher log reduction values (1.3 and 1.1 log units, respectively) were observed between influent and anoxic mixed liquor. This suggested that enteroviruses were less associated with MLSS than norovirus GII and sapoviruses, resulting in lower enterovirus removal in the activated sludge process. Enteroviruses and norovirus GII were detected in the MBR effluent but sapoviruses were not in any effluent samples. When MLSS concentration was reduced to 50-60% of a normal operation level, passages of enteroviruses and norovirus GII through a PVDF microfiltration membrane were observed. Since rejection of viruses by the membrane was not related to trans-membrane pressure which was monitored as a parameter of membrane fouling, the results indicated that adsorption to MLSS plays an important role in virus removal by an MBR, and removal properties vary by viruses reflecting different adsorptive behavior to MLSS. Our observations suggested that sapoviruses are more associated with MLSS and removed more efficiently than enteroviruses and norovirus GII.

Both α2,3- and α2,6-linked sialic acids on O-linked glycoproteins act as functional receptors for porcine Sapovirus

Sapovirus, a member of the Caliciviridae family, is an important cause of acute gastroenteritis in humans and pigs. Currently, the porcine sapovirus (PSaV) Cowden strain remains the only cultivable member of the Sapovirus genus. While some caliciviruses are known to utilize carbohydrate receptors for entry and infection, a functional receptor for sapovirus is unknown. To characterize the functional receptor of the Cowden strain of PSaV, we undertook a comprehensive series of protein-ligand biochemical assays in mock and PSaV-infected cell culture and/or piglet intestinal tissue sections. PSaV revealed neither hemagglutination activity with red blood cells from any species nor binding activity to synthetic histo-blood group antigens, indicating that PSaV does not use histo-blood group antigens as receptors. Attachment and infection of PSaV were markedly blocked by sialic acid and Vibrio cholerae neuraminidase (NA), suggesting a role for α2,3-linked, α2,6-linked or α2,8-linked sialic acid in virus attachment. However, viral attachment and infection were only partially inhibited by treatment of cells with sialidase S (SS) or Maackia amurensis lectin (MAL), both specific for α2,3-linked sialic acid, or Sambucus nigra lectin (SNL), specific for α2,6-linked sialic acid. These results indicated that PSaV recognizes both α2,3- and α2,6-linked sialic acids for viral attachment and infection. Treatment of cells with proteases or with benzyl 4-O-β-D-galactopyranosyl-β-D-glucopyranoside (benzylGalNAc), which inhibits O-linked glycosylation, also reduced virus binding and infection, whereas inhibition of glycolipd synthesis or N-linked glycosylation had no such effect on virus binding or infection. These data suggest PSaV binds to cellular receptors that consist of α2,3- and α2,6-linked sialic acids on glycoproteins attached via O-linked glycosylation.

The role of red blood cells in enhancing or preventing HIV infection and other diseases

Aim. To highlight the apparently neglected role of erythrocyte antigens in the epidemiology of infectious diseases, especially HIV, with the prime objective of stimulating research in this area. Method. A literature search was performed on the PubMed for relevant papers from 1984 to 2013, the era covering active HIV research. This was achieved by using the phrases “erythrocyte blood groups HIV” (81 papers) or “red cell antigen, blood groups, and HIV” (60 papers). A manual Google Scholar search was done and supplemented by original papers referenced by various authors. However, the review was limited by the relative scarcity of papers on the subject, and only papers written in English were reviewed during the period October 2012 to September 2013. Results. Many communicable and noncommunicable diseases are associated with specific blood groups. Examples of these diseases are discussed in detail. HIV has been shown to bind to erythrocytes, and candidate erythrocyte-binding molecules and mechanisms are also discussed. Moreover, erythrocyte-HIV binding is associated with increased viral infectivity, thus, underscoring the need to study this phenomenon and its implications for HIV epidemiology. Conclusion. Erythrocyte antigens may be important in the pathogenesis and epidemiology of many diseases, including HIV.

Susceptibility of children to sapovirus infections, Nicaragua, 2005-2006

We describe the genetic diversity of sapovirus (SaV) in children in Nicaragua and investigate the role of host genetic factors and susceptibility to SaV infections. Our results indicate that neither ABO blood group, Lewis phenotype, nor secretor status affects susceptibility to SaV infection in Nicaragua.

Structural basis for the recognition of Lewis antigens by genogroup I norovirus

Noroviruses (NoVs) bind to histo-blood group antigens, namely, ABH antigens and Lewis antigens. We previously showed the NoVs GI/2, GI/3, GI/4, and GI/8 were able to strongly bind to Lewis a (Le(a)) antigen, which is expressed by individuals who are nonsecretors. In this study, to investigate how Lewis antigens interact with GI NoV virion protein 1 (VP1), we determined the crystal structures of the P domain of the VP1 protein from the Funabashi 258 (FUV258) strain (GI/2) in complexes with Le(a), Le(b), H type 1, or A type 1 antigens. The structures were compared with those of the NV/68 strain (GI/1), which does not bind to the Le(a) antigen. The four loop structures, loop P, loop S, loop A, and loop B, continuously deviated by more than 2 Å in length between the Cα atoms of the corresponding residues of the FUV258 and NV/68 P domains. The most pronounced differences between the two VP1 proteins were observed in the structures of loop P. In the FUV258 P domain, loop P protruded toward the next protomer, forming a Le(a) antigen-binding site. The Gln389 residue make a significant contribution to the binding of the Le(a) antigen through the stabilization of loop P as well as through direct interactions with the α4-fucosyl residue (α4Fuc) of the Le(a) antigen. Mutation of the Gln389 residue dramatically affected the degree of binding of the Lewis antigens. Collectively, these results suggest that loop P and the amino acid residue at position 389 affect Lewis antigen binding.

Structural analysis of histo-blood group antigen binding specificity in a norovirus GII.4 epidemic variant: implications for epochal evolution

Susceptibility to norovirus (NoV), a major pathogen of epidemic gastroenteritis, is associated with histo-blood group antigens (HBGAs), which are also cell attachment factors for this virus. GII.4 NoV strains are predominantly associated with worldwide NoV epidemics with a periodic emergence of new variants. The sequence variations in the surface-exposed P domain of the capsid protein resulting in differential HBGA binding patterns and antigenicity are suggested to drive GII.4 epochal evolution. To understand how temporal sequence variations affect the P domain structure and contribute to epochal evolution, we determined the P domain structure of a 2004 variant with ABH and secretor Lewis HBGAs and compared it with the previously determined structure of a 1996 variant. We show that temporal sequence variations do not affect the binding of monofucosyl ABH HBGAs but that they can modulate the binding strength of difucosyl Lewis HBGAs and thus could contribute to epochal evolution by the potentiated targeting of new variants to Lewis-positive, secretor-positive individuals. The temporal variations also result in significant differences in the electrostatic landscapes, likely reflecting antigenic variations. The proximity of some of these changes to the HBGA binding sites suggests the possibility of a coordinated interplay between antigenicity and HBGA binding in epochal evolution. From the observation that the regions involved in the formation of the HBGA binding sites can be conformationally flexible, we suggest a plausible mechanism for how norovirus disassociates from salivary mucin-linked HBGA before reassociating with HBGAs linked to intestinal epithelial cells during its passage through the gastrointestinal tract.

Crystal structures of GII.10 and GII.12 norovirus protruding domains in complex with histo-blood group antigens reveal details for a potential site of vulnerability

Noroviruses are the dominant cause of outbreaks of gastroenteritis worldwide, and interactions with human histo-blood group antigens (HBGAs) are thought to play a critical role in their entry mechanism. Structures of noroviruses from genogroups GI and GII in complex with HBGAs, however, reveal different modes of interaction. To gain insight into norovirus recognition of HBGAs, we determined crystal structures of norovirus protruding domains from two rarely detected GII genotypes, GII.10 and GII.12, alone and in complex with a panel of HBGAs, and analyzed structure-function implications related to conservation of the HBGA binding pocket. The GII.10- and GII.12-apo structures as well as the previously solved GII.4-apo structure resembled each other more closely than the GI.1-derived structure, and all three GII structures showed similar modes of HBGA recognition. The primary GII norovirus-HBGA interaction involved six hydrogen bonds between a terminal αfucose1-2 of the HBGAs and a dimeric capsid interface, which was composed of elements from two protruding subdomains. Norovirus interactions with other saccharide units of the HBGAs were variable and involved fewer hydrogen bonds. Sequence analysis revealed a site of GII norovirus sequence conservation to reside under the critical αfucose1-2 and to be one of the few patches of conserved residues on the outer virion-capsid surface. The site was smaller than that involved in full HBGA recognition, a consequence of variable recognition of peripheral saccharides. Despite this evasion tactic, the HBGA site of viral vulnerability may provide a viable target for small molecule- and antibody-mediated neutralization of GII norovirus.

Antiviral strategies to control calicivirus infections

Caliciviridae are human or non-human pathogenic viruses with a high diversity. Some members of the Caliciviridae, i.e. human pathogenic norovirus or rabbit hemorrhagic disease virus (RHDV), are worldwide emerging pathogens. The norovirus is the major cause of viral gastroenteritis worldwide, accounting for about 85% of the outbreaks in Europe between 1995 and 2000. In the United States, 25 million cases of infection are reported each year. Since its emergence in 1984 as an agent of fatal hemorrhagic diseases in rabbits, RHDV has killed millions of rabbits and has been dispersed to all of the inhabitable continents.

In view of their successful and apparently increasing emergence, the development of antiviral strategies to control infections due to these viral pathogens has now become an important issue in medicine and veterinary medicine. Antiviral strategies have to be based on an understanding of the epidemiology, transmission, clinical symptoms, viral replication and immunity to infection resulting from infection by these viruses. Here, we provide an overview of the mechanisms underlying calicivirus infection, focusing on the molecular aspects of replication in the host cell. Recent experimental data generated through an international collaboration on structural biology, virology and drug design within the European consortium VIZIER is also presented. Based on this analysis, we propose antiviral strategies that may significantly impact on the epidemiological characteristics of these highly successful viral pathogens.

Binding of Norovirus virus-like particles (VLPs) to human intestinal Caco-2 cells and the suppressive effect of pasteurized bovine colostrum on this VLP binding

Noroviruses (NoVs), which cannot be grown in cell culture, are a major infectious agent of gastroenteritis. An in vitro assay system was established for the evaluation of NoV binding to enterocytes using virus-like particles (VLPs) produced in a baculovirus system expressing a NoV VP1 capsid protein. After confirmation of the purity by MS analysis, VLPs were incubated with human intestinal Caco-2 cells. NoV VLPs were detected clearly by confocal laser microscopy only on a certain population of Caco-2 cells, and were semi-quantified by immunoblotting of cell lysates. Then the suppressive effect of pasteurized bovine colostrum was analyzed on the VLP binding to Caco-2 cells by immunoblotting. The colostrum reduced VLP binding in a dose-dependent manner, at about 50% suppression with 12.5 microg of the colostral proteins. Furthermore, the colostrum contained IgG antibodies reacting to VLPs, suggesting that cross-reactive antibodies in the bovine colostrums block human NoV binding to intestinal cells.

Insights into the enzyme-substrate interaction in the norovirus 3C-like protease

The Glu54 residue of the norovirus 3C-like protease was implicated in proteolysis as a third-member carboxylate of the catalytic triad. The E54L mutant protease cleaved the sequence (133)LSFE/AP between the 3B and 3C regions of norovirus polyprotein, but did not cleave the sequence (198)ATSE/GK between the 3A and 3B. The 3BC junction mutation (3B-L133A or 3B-F135S) hampered the cleavage by the E54L protease, whereas the 3AB junction mutation (3A-A198L, S200F) allowed the E54L protease to digest. These results indicate that the E54L mutant protease is a substrate-specificity mutant and requires large hydrophobic amino acid residues at both P4 and P2 positions of the substrate. It was notable that the 3A-S200F P2 position mutation caused tight interaction between the wild-type protease and the C-terminus of the 3A protein, hence a decreased release rate of the product from the enzyme. This tight binding was dependent on the hydrophobicity of amino acid residues introduced at position 200 of the 3A region and was affected by the mutation in the bII-cII loop of the protease or the mutation of position 198 of 3A corresponding to the P4 position of the substrate. These results suggest that the protease and the substrate sense each other in the process of the proteolysis, being supported by crystal structures.

Saturation mutagenesis reveals that GLU54 of norovirus 3C-like protease is not essential for the proteolytic activity

The norovirus 3C-like protease is a member of the chymotrypsin-like serine protease superfamily. Previous characterization of its crystal structure has implicated the Glu54-His30-Cys139 triad in the catalysis. In the present study, the Glu54 residue of the protease was subjected to site-saturation mutagenesis, with the result that nearly half of the mutants retained the significant proteolytic activity. It was suggested that a carboxylate at position 54 was not essential for the activity. The in vitro assays of the proteolysis revealed that most of Glu54 mutants retained relatively high proteolytic activity. When the Glu54 mutation was combined with the Ser mutation of the Cys139 residue, a nucleophile, only the Asp54 and Gln54 mutations showed proteolytic activity comparable to that of the Ser139 single mutant, suggesting that a hydrogen bond between Glu54 and His30 was critical in the Ser139 background. These results suggested that the mechanism of the proteolysis by the wild-type norovirus 3C-like protease was different from that of typical chymotrypsin-like serine proteases.

Noroviruses distinguish between type 1 and type 2 histo-blood group antigens for binding

Norovirus (NoV) is a causative agent of acute gastroenteritis. NoV binds to histo-blood group antigens (HBGAs), namely, ABH antigens and Lewis (Le) antigens, in which type 1 and type 2 carbohydrate core structures constitute antigenically distinct variants. Norwalk virus, the prototype strain of norovirus, binds to the gastroduodenal junction, and this binding is correlated with the presence of H type 1 antigen but not with that of H type 2 antigen (S. Marionneau, N. Ruvoen, B. Le Moullac-Vaidye, M. Clement, A. Cailleau-Thomas, G. Ruiz-Palacois, P. Huang, X. Jiang, and J. Le Pendu, Gastroenterology 122:1967-1977, 2002). It has been unknown whether NoV distinguishes between the type 1 and type 2 chains of A and B antigens. In this study, we synthesized A type 1, A type 2, B type 1, and B type 2 pentasaccharides in vitro and examined the function of the core structures in the binding between NoV virus-like particles (VLPs) and HBGAs. The attachment of five genogroup I (GI) VLPs from 5 genotypes and 11 GII VLPs from 8 genotypes, GI/1, GI/2, GI/3, GI/4, GI/8, GII/1, GII/3, GII/4, GII/5, GII/6, GII/7, GII/12, and GII/14, to ABH and Le HBGAs was analyzed by enzyme-linked immunosorbent assay-based binding assays and Biacore analyses. GI/1, GI/2, GI/3, GI/4, GI/8, and GII/4 VLPs were more efficiently bound to A type 2 than A type 1, and GI/8 and GII/4 VLPs were more efficiently bound to B type 2 than B type 1, indicating that NoV VLPs distinguish between type 1 and type 2 carbohydrates. The dissociation of GII/4 VLPs from B type 1 was slower than that from B type 2 in the Biacore experiments; moreover, the binding to B type 1 was stronger than that to B type 2 in the ELISA experiments. These results indicated that the type 1 carbohydrates bind more tightly to NoV VLPs than the type 2 carbohydrates. This property may afford NoV tissue specificity. GII/4 is known to be a global epidemic genotype and binds to more HBGAs than other genotypes. This characteristic may be linked with the worldwide transmission of GII/4 strains. GI/2, GI/3, GI/4, GI/8, GII/4, and GII/7 VLPs bound to Le(a) expressed by nonsecretors, suggesting that NoV can infect individuals regardless of secretor phenotype. Overall, our results indicated that HBGAs are important factors in determining tissue specificity and the risk of transmission.

[Interaction between noroviruses and human histo-blood group antigens]

Norovirus (NOV), a member of the family Caliciviridae, is a major cause of water and food-borne acute nonbacterial gastroenteritis, and forms many morphologically similar but antigenically diverse groups of viruses. The virus-like particles (VLPs) derived from the prototype strain of NoV, Norwalk virus (NV/68), bind to histo-blood group antigens (HBGAs). HBGAs are carbohydrates that contain structurally related saccharide moieties, and are found in saliva and mucosal secretions from intestinal epithelial cells of secretor individuals who have FUT2 gene encoding a fucosyltransferase. From volunteer challenge studies, there is strong evidence that the carbohydrate-binding is essential for the NV/68 infection. Non-secretors, who do not express FUT2 fucosyltransferase and consequently do not express H type 1 or Leb in the gut, were not infected after the challenge with NV/68. However, other NoV VLPs display different ABH and Lewis carbohydrate-binding profiles, and indeed epidemiological studies showed that some NoV strains could infect individuals with another ABH phenotypes. GII/4 is known to be global epidemic strain and bound more HBGAs when compared with other strains. The strength of the transmission of GII/4 strains may be linked with their wide recognition of HBGAs. It is obvious that HBGAs are important factors to determine the host specificity, although it is still unclear whether the HBGAs act as the primary receptor or enhance NoV infectivity. Further investigation is needed.

Human sapovirus in clams, Japan

Human sapovirus was detected in 4 of 57 clam packages by reverse transcription-PCR and sequence analysis. This represents the first finding of sapovirus contamination in food. Closely matching sequences have been detected in stool specimens from patients with gastroenteritis in Japan, which indicates a possible food-to-human transmission link.