Mechanisms of GII.4 norovirus persistence in human populations

Diagnosis and treatment of acute or persistent diarrhea

Studies of microbial pathogens and the toxins they produce are important for determining the mechanisms by which they cause disease and spread throughout a population. Some bacteria produce secretory enterotoxins (such as cholera toxin or the heat-labile or stable enterotoxins produced by Escherichia coli) that invade cells directly. Others invade cells or produce cytotoxins (such as those produced by Shigella, enteroinvasive E coli, or Clostridium difficile) that damage cells or trigger host responses that cause small or large bowel diseases (such as enteroaggregative or enteropathogenic E coli or Salmonella). Viruses (such as noroviruses and rotaviruses) and protozoa (such as Cryptosporidium, Giardia, or Entamoeba histolytica) disrupt cell functions and cause short- or long-term disease. Much epidemiologic data about these pathogens have been collected from community- and hospital-acquired settings, as well as from patients with traveler's or persistent diarrhea. These studies have led to practical approaches for prevention, diagnosis, and treatment.

Conservation of carbohydrate binding interfaces: evidence of human HBGA selection in norovirus evolution

Background

Human noroviruses are the major viral pathogens of epidemic acute gastroenteritis. These genetically diverse viruses comprise two major genogroups (GI and GII) and approximately 30 genotypes. Noroviruses recognize human histo-blood group antigens (HBGAs) in a diverse, strain-specific manner. Recently the crystal structures of the HBGA-binding interfaces of the GI Norwalk virus and the GII VA387 have been determined, which allows us to examine the genetic and structural relationships of the HBGA-binding interfaces of noroviruses with variable HBGA-binding patterns. Our hypothesis is that, if HBGAs are the viral receptors necessary for norovirus infection and spread, their binding interfaces should be under a selection pressure in the evolution of noroviruses.

Methods and Findings

Structural comparison of the HBGA-binding interfaces of the two noroviruses has revealed shared features but significant differences in the location, sequence composition, and HBGA-binding modes. On the other hand, the primary sequences of the HBGA-binding interfaces are highly conserved among strains within each genogroup. The roles of critical residues within the binding sites have been verified by site-directed mutagenesis followed by functional analysis of strains with variable HBGA-binding patterns.

Conclusions and Significance

Our data indicate that the human HBGAs are an important factor in norovirus evolution. Each of the two major genogroups represents an evolutionary lineage characterized by distinct genetic traits. Functional convergence of strains with the same HBGA targets subsequently resulted in acquisition of analogous HBGA binding interfaces in the two genogroups that share an overall structural similarity, despite their distinct locations and amino acid compositions. On the other hand, divergent evolution may have contributed to the observed overall differences between and within the two lineages. Thus, both divergent and convergent evolution, as well as the polymorphic human HBGAs, likely contribute to the diversity of noroviruses. The finding of genogroup-specific conservation of HBGA binding interfaces will facilitate the development of rational strategies to control and prevent norovirus-associated gastroenteritis.

Quasispecies dynamics and molecular evolution of human norovirus capsid P region during chronic infection

In this novel study, we have for the first time identified evolutionarily conserved capsid residues in an individual chronically infected with norovirus (GGII.3). From 2000 to 2003, a total of 147 P1-1 and P2 capsid sequences were sequenced and investigated for evolutionarily conserved and functionally important residues by the evolutionary trace (ET) algorithm. The ET algorithm revealed more absolutely conserved residues (ACR) in the P1-1 domain (47/53, 88 %) as compared with the P2 domain (86/133, 64 %). The capsid P1-1 and P2 domains evolved in time-dependent manner, with a distinct break point observed between autumn/winter of year 2000 (isolates P1, P3 and P5) and spring to autumn of year 2001 (isolates P11, P13 and P15), which presumably coincided with a change of clinical symptoms. Furthermore, the ET analysis revealed a similar receptor-binding pattern as reported for Norwalk and VA387 strains, with the CS-4 and CS-5 patch (Norwalk strain) including residues 329 and 377 and residues 306 and 310, respectively, all being ACR in all partitions. Most interesting was that residues 343, 344, 345, 374, 390 and 391 of the proposed receptor A and B trisaccharide binding site (VA387 strain) within the P2 domain remained ACR in all partitions, presumably because there was no selective advantage to alter the histo blood group antigens (HBGA) receptor binding specificity. In conclusion, this study provides novel insights to the evolutionary process of norovirus during chronic infection.

Herd immunity to GII.4 noroviruses is supported by outbreak patient sera

Noroviruses (NoVs) of genogroup II, cluster 4 (GII.4), are the most common cause of outbreaks of acute gastroenteritis worldwide. During the past 13 years, GII.4 NoVs caused four seasons of widespread activity globally, each associated with the emergence of a new strain. In this report, we characterized the most recent epidemic strain, GII.4-2006 Minerva, by comparing virus-like particle (VLP) antigenic relationships and histo-blood group antigen (HBGA) binding profiles with strains isolated earlier. We also investigated the seroprevalence and specificity of GII.4 antibody in the years prior to, during, and following the GII.4 pandemic of 1995 and 1996 using a large collection of acute- and convalescent-phase serum pairs (n = 298) collected from 34 outbreaks. In a surrogate neutralization assay, we measured the blockade of HBGA binding using a panel of GII.4 VLPs representing strains isolated in 1987, 1997, 2002, and 2006 and a GII.3 VLP representing a strain isolated in the mid-1990s. Serum titers required for 50% HBGA blockade were compared between populations. In general, blockade of GII.4 VLP-HBGA binding was greater with convalescent-phase outbreak sera collected near the time of origin of the VLP strain. Heterotypic genotypes did not contribute to herd immunity against GII.4 NoVs based on their inability to block GII.4 VLP binding to HBGA. However, previous exposure to GII.4 NoV followed by infection by GII.3 NoV appeared to evoke an immune response to GII.4 NoV. These results support the hypothesis that herd immunity is a driving force for GII.4 evolution in the U.S. population. The data also suggest that complex patterns of cross-protection may exist across NoV genotypes in humans.

Predominance and circulation of enteric viruses in the region of Greater Cairo, Egypt

The circulation of enteric viruses among the population of Cairo, Egypt, between March 2006 and February 2007 was studied. At least one virus was detected in 50% of fecal samples, 57.4% of which were positive for rotavirus, 26% for norovirus, 10.4% for adenovirus, and 1.7% for astrovirus. Over 10% of infections were mixed infections. Rotavirus typing showed that G1P[8] and G2P[4] were predominant but that the unusual G12P[4] and G12P[6] reassortants were also present. Among the noroviruses, half belonged to the predominant GGII.4 cluster. The phylogenetic analysis of the capsid gene suggested that GGII.4 strains from Cairo were similar to those circulating elsewhere. It also showed the emergence of new GGII.4 variants that were not associated with any previously known GGII.4 isolate. Further studies are required to assess the disease burden of enteric viruses in Egypt and the impact of atypical strains.

Alphavirus-adjuvanted norovirus-like particle vaccines: heterologous, humoral, and mucosal immune responses protect against murine norovirus challenge

The development of an effective norovirus vaccine likely requires the capacity to protect against infection with multiple norovirus strains. Advanced recombinant genetic systems and the recent discovery of a mouse-tropic norovirus strain (MNV) provide robust model systems for vaccine efficacy studies. We coadministered multivalent norovirus-like particle (VLP) vaccines with alphavirus adjuvant particles to mice and evaluated homotypic and heterotypic humoral and protective immunity to human and murine norovirus strains. Multivalent VLP vaccines induced robust receptor-blocking antibody responses to heterologous human strains not included in the vaccine composition. Inclusion of alphavirus adjuvants in the inoculum significantly augmented VLP-induced systemic and mucosal immunity compared to the responses induced by low-dose CpG DNA, validating the utility of such adjuvants with VLP antigens. Furthermore, multivalent vaccination, either including or excluding MNV VLP, resulted in significantly reduced viral loads following MNV challenge. Passive transfer of sera from mice monovalently vaccinated with MNV VLP to immunodeficient or immunocompetent mice protected against MNV infection; however, adoptive transfer of purified CD4(+) or CD8(+) cells did not influence viral loads in murine tissues. Together, these data suggest that humoral immunity induced by multivalent norovirus vaccines may protect against heterologous norovirus challenge.

Direct metagenomic detection of viral pathogens in nasal and fecal specimens using an unbiased high-throughput sequencing approach

With the severe acute respiratory syndrome epidemic of 2003 and renewed attention on avian influenza viral pandemics, new surveillance systems are needed for the earlier detection of emerging infectious diseases. We applied a “next-generation” parallel sequencing platform for viral detection in nasopharyngeal and fecal samples collected during seasonal influenza virus (Flu) infections and norovirus outbreaks from 2005 to 2007 in Osaka, Japan. Random RT-PCR was performed to amplify RNA extracted from 0.1–0.25 ml of nasopharyngeal aspirates (N = 3) and fecal specimens (N = 5), and more than 10 µg of cDNA was synthesized. Unbiased high-throughput sequencing of these 8 samples yielded 15,298–32,335 (average 24,738) reads in a single 7.5 h run. In nasopharyngeal samples, although whole genome analysis was not available because the majority (>90%) of reads were host genome–derived, 20–460 Flu-reads were detected, which was sufficient for subtype identification. In fecal samples, bacteria and host cells were removed by centrifugation, resulting in gain of 484–15,260 reads of norovirus sequence (78–98% of the whole genome was covered), except for one specimen that was under-detectable by RT-PCR. These results suggest that our unbiased high-throughput sequencing approach is useful for directly detecting pathogenic viruses without advance genetic information. Although its cost and technological availability make it unlikely that this system will very soon be the diagnostic standard worldwide, this system could be useful for the earlier discovery of novel emerging viruses and bioterrorism, which are difficult to detect with conventional procedures.

Continuous presence of noroviruses and sapoviruses in raw sewage reflects infections among inhabitants of Toyama, Japan (2006 to 2008)

Various genotypes of norovirus (NoV) (genogroup I genotype 1 [GI.1], -2, -4, -5, -8, -11, -12, and -14; GII.3, -4, -6, -7, -10, -13, -14, and -15), and sapovirus (SaV) (GI.1 and GI.2, GII.1, and GIV.1) were detected from raw sewage from April 2006 to March 2008, while limited numbers of genotypes of NoV (GI.8, GII.4, GII.6, and GII.13) and SaV (GII.3 and GIV.1) and of NoV (GII.4, GII.7, and GII.13) were detected from clinical cases and healthy children, respectively. During the winter 2006 to 2008, a large number of sporadic gastroenteritis outbreaks and many outbreaks caused by NoV GII.4 occurred among inhabitants in Toyama, Japan. The copy number of genomes of NoV GII detected from raw sewage changed in relation to the number of outbreaks. NoV strains of the same genotypes observed in both raw sewage and human specimens belonged to the same cluster by phylogenetic analysis and had almost identical nucleotide sequences among each genotype. These data suggest that NoVs and SaVs detected from raw sewage reflect the viruses circulating in the community, irrespective of symptoms, and that subclinical infections of NoV are common in Japan. Combined surveys of raw sewage with those of clinical cases help us to understand the relationship between infection of these viruses and gastroenteritis.

Immune mechanisms responsible for vaccination against and clearance of mucosal and lymphatic norovirus infection

Two cardinal manifestations of viral immunity are efficient clearance of acute infection and the capacity to vaccinate against secondary viral exposure. For noroviruses, the contributions of T cells to viral clearance and vaccination have not been elucidated. We report here that both CD4 and CD8 T cells are required for efficient clearance of primary murine norovirus (MNV) infection from the intestine and intestinal lymph nodes. Further, long-lasting protective immunity was generated by oral live virus vaccination. Systemic vaccination with the MNV capsid protein also effectively protected against mucosal challenge, while vaccination with the capsid protein of the distantly related human Lordsdale virus provided partial protection. Fully effective vaccination required a broad immune response including CD4 T cells, CD8 T cells, and B cells, but the importance of specific immune cell types varied between the intestine and intestinal lymph nodes. Perforin, but not interferon gamma, was required for clearance of MNV infection by adoptively transferred T lymphocytes from vaccinated hosts. These studies prove the feasibility of both mucosal and systemic vaccination against mucosal norovirus infection, demonstrate tissue specificity of norovirus immune cells, and indicate that efficient vaccination strategies should induce potent CD4 and CD8 T cell responses.

Human noroviruses are the most common cause of epidemic nonbacterial gastroenteritis in the world. Despite their importance as human pathogens, little is known about how the immune system controls and clears norovirus infection, and the potential and mechanisms of vaccination remain unclear. Here, we used norovirus infection of mice to show that vaccination can provide long-lasting immunity against mucosal norovirus challenge and to identify the types of immune cells that are important in vaccination against norovirus infection. Similarly, we identified the types of immune T cells that are important for clearance of acute infection. Efficient vaccination required all three major arms of adaptive immunity: CD4 T cells, CD8 T cell, and B cells. Importantly, protective vaccination against mucosal challenge was observed after either mucosal or systemic norovirus antigen exposure. The pore-forming molecule perforin was important for T cell-mediated control of norovirus infection. Our study has important implications for understanding adaptive immunity to norovirus infection, and may provide insight into the directions to take in developing a human norovirus vaccine.

Self-assembly of the recombinant capsid protein of a swine norovirus into virus-like particles and evaluation of monoclonal antibodies cross-reactive with a human strain from genogroup II

Noroviruses (NoVs) are responsible for the majority of gastroenteritis outbreaks in humans. Recently, NoV strains which are genetically closely related to human genogroup II (GII) NoVs have been detected in fecal specimens from swine. These findings have raised concern about the possible role of pigs as reservoirs for NoVs that could infect humans. To better understand the epidemiology of swine NoVs in both the swine and the human populations, rapid immunoassays are needed. In this study, baculovirus recombinants were generated to express the capsid gene of a swine NoV GII genotype 11 (GII.11) strain which self-assembled into virus-like particles (VLPs). Subsequently, the purified VLPs were used to evoke monoclonal antibodies (MAbs) in mice. A panel of eight promising MAbs was obtained and evaluated for their ability to bind to heterologous VLPs, denaturated antigens, and truncated capsid proteins. The MAbs could be classified into two groups: two MAbs that recognized linear epitopes located at the amino-terminal half (shell domain) of the swine NoV GII.11 VLPs and that cross-reacted with human GII.4 NoV VLPs. The other six MAbs bound to conformational epitopes and did not cross-react with the human GII.4 VLPs. To our knowledge, this is the first report on the characterization of MAbs against swine NoVs. The swine NoV VLPs and the MAbs described here may be further used for the design of diagnostic reagents that could help increase our knowledge of the prevalence of NoV infections in pigs and the possible role of pigs as reservoirs for NoVs.

Common variants of FUT2 are associated with plasma vitamin B12 levels

We identified a strong association (P = 5.36 x 10(-17)) between rs492602 in FUT2 and plasma vitamin B(12) levels in a genome-wide scan (n = 1,658) and an independent replication sample (n = 1,059) from the Nurses' Health Study. Women homozygous for the rs492602[G] allele had higher B(12) levels. This allele is in strong linkage disequilibrium with the FUT2 nonsecretor variant encoding W143X, suggesting a plausible mechanism for altered B(12) absorption and plasma levels.

Histo-blood group antigen assay for detecting noroviruses in water

We evaluated a novel, magnetic-bead-based histo-blood group antigen assay for the recovery of low numbers of norovirus particles. Using this assay, with Norwalk virus seeded in environmental waters as a model, we were able to recover 30 to 300 genomic copies of the virus.

Identification of monomorphic and divergent haplotypes in the 2006-2007 norovirus GII/4 epidemic population by genomewide tracing of evolutionary history

Our norovirus (NoV) surveillance group reported a >4-fold increase in NoV infection in Japan during the winter of 2006-2007 compared to the previous winter. Because the increase was not linked to changes in the surveillance system, we suspected the emergence of new NoV GII/4 epidemic variants. To obtain information on viral changes, we conducted full-length genomic analysis. Stool specimens from 55 acute gastroenteritis patients of various ages were collected at 11 sites in Japan between May 2006 and January 2007. Direct sequencing of long PCR products revealed 37 GII/4 genome sequences. Phylogenetic study of viral genome and partial sequences showed that the two new GII/4 variants in Europe, termed 2006a and 2006b, initially coexisted as minorities in early 2006 in Japan and that 2006b alone had dominated over the resident GII/4 variants during 2006. A combination of phylogenetic and entropy analyses revealed for the first time the unique amino acid substitutions in all eight proteins of the new epidemic strains. These data and computer-assisted structural study of the NoV capsid protein are compatible with a model of antigenic drift with tuning of the structure and functions of multiple proteins for the global outgrowth of new GII/4 variants. The availability of comprehensive information on genome sequences and unique protein changes of the recent global epidemic variants will allow studies of diagnostic assays, molecular epidemiology, molecular biology, and adaptive changes of NoV in nature.

Elucidation of strain-specific interaction of a GII-4 norovirus with HBGA receptors by site-directed mutagenesis study

Noroviruses interact with histo-blood group antigen (HBGA) receptors in a strain-specific manner probably detecting subtle structural differences in the carbohydrate receptors. The specific recognition of types A and B antigens by various norovirus strains is a typical example. The only difference between the types A and B antigens is the acetamide linked to the terminal galactose of the A but not to the B antigen. The crystal structure of the P dimer of a GII-4 norovirus (VA387) bound to types A and B trisaccharides has elucidated the A/B binding site on the capsid but did not explain the binding specificity of the two antigens. In this study, using site-directed mutagenesis, we have identified three residues on the VA387 capsid that are sterically close to the acetamide and are required for binding to A but not B antigens, indicating that the acetamide determines the binding specificity between the A and B antigens. Further mutational analysis showed that a nearby open cavity may also be involved in binding specificity to HBGAs. In addition, a systematic mutational analysis of residues in and around the binding interface has identified a group of amino acids that are required for binding but do not have direct contact with the carbohydrate antigens, implying that these residues may be involved in the structural integrity of the receptor binding interface. Taken together, our study provides new insights into the carbohydrate/capsid interactions which are a valuable complement to the atomic structures in understanding the virus/host interaction and in the future design of antiviral agents.

Analysis of integrated virological and epidemiological reports of norovirus outbreaks collected within the Foodborne Viruses in Europe network from 1 July 2001 to 30 June 2006

The Foodborne Viruses in Europe network has developed integrated epidemiological and virological outbreak reporting with aggregation and sharing of data through a joint database. We analyzed data from reported outbreaks of norovirus (NoV)-caused gastroenteritis from 13 European countries (July 2001 to July 2006) for trends in time and indications of different epidemiology of genotypes and variants. Of the 13 countries participating in this surveillance network, 11 were capable of collecting integrated epidemiological and virological surveillance data and 10 countries reported outbreaks throughout the entire period. Large differences in the numbers and rates of reported outbreaks per country were observed, reflecting the differences in the focus and coverage of national surveillance systems. GII.4 strains predominated throughout the 5-year surveillance period, but the proportion of outbreaks associated with GII.4 rose remarkably during years in which NoV activity was particularly high. Spring and summer peaks indicated the emergence of genetically distinct variants within GII.4 across Europe and were followed by increased NoV activity during the 2002-2003 and 2004-2005 winter seasons. GII.4 viruses predominated in health care settings and in person-to-person transmission. The consecutive emergence of new GII.4 variants is highly indicative of immune-driven selection. Their predominance in health care settings suggests properties that facilitate transmission in settings with a high concentration of people such as higher virus loads in excreta or a higher incidence of vomiting. Understanding the mechanisms driving the changes in epidemiology and clinical impact of these rapidly evolving RNA viruses is essential to design effective intervention and prevention measures.

The evolution of norovirus, the "gastric flu"

The authors discuss the implications of a new study that presents compelling data to show that norovirus evolution is driven by immune selection pressure.