Cellular and humoral immunity following Snow Mountain virus challenge

QCM-D studies of human norovirus VLPs binding to glycosphingolipids in supported lipid bilayers reveal strain-specific characteristics

Susceptibility to norovirus infection has been linked to secretor status. Norovirus virus-like particles (VLPs; 0- 20 microg/mL) from the Norwalk (GI.1) and Dijon (GII.4) strains were assayed for binding to H type 1 and Lewis a pentaglycosylceramides, incorporated in laterally fluid supported lipid bilayers. Binding kinetics was monitored in real time in 40 microL stationary reaction chambers, using quartz crystal microbalance with dissipation (QCM-D) monitoring. Both strains displayed binding only to H type 1 and not to Lewis a glycosphingolipids, typical for epithelial cells of susceptible and resistant individuals, respectively. This binding specificity was confirmed by VLPs binding to the two glycosphingolipids chromatographed on TLC-plates. Experiments using bilayers with mixtures of H type 1 and Lewis a, with the total glycosphingolipid concentration constant at 10 wt%, showed that binding was only dependent on H type 1 concentrations and identical to experiments without additional Lewis a. Both strains showed a threshold concentration of H type 1 below which no binding was observable. The threshold was one order of magnitude higher for the Dijon strain (2 wt% versus 0.25 wt%) demonstrating that the interaction with a significantly larger number of glycosphingolipids was needed for the binding of the Dijon strain. The difference in threshold glycosphingolipid concentrations for the two strains suggests a lower affinity for the glycosphingolipid for the Dijon compared to the Norwalk strain. We propose that VLPs initially bind only a few glycosphingolipids but the binding is subsequently strengthened by lateral diffusion of additional glycosphingolipids moving into the interaction area.

Genetic basis of host resistance to norovirus infection

Noroviruses have emerged as a major cause of acute gastroenteritis in humans of all ages and are responsible for 200,000 deaths every year, mainly in developing countries. Despite high infectivity and lack of long-term immunity, authentic and volunteer studies have shown existence of inherited protective factors. Recent studies have shown that secretor status controlled by the α1,2-fucosyltransferase gene located on chromosome 19 determines susceptibility to most, if not all, norovirus infections, with individuals homozygous for the G428A nonsense mutation (nonsecretors) representing 20% of the highly protected European population.

Model systems for the study of human norovirus Biology

The relative contribution of norovirus to disease burden on society has only recently been established and they are now established as a major cause of gastroenteritis in the developed world. However, despite the medical relevance of these viruses, an efficient in vitro cell culture system for human noroviruses has yet to be developed. As a result, much of our knowledge on the basic mechanisms of norovirus biology has come from studies using other members of the Caliciviridae family of small positive stranded RNA viruses. Here we aim to summarise the recent advances in the field, highlighting how model systems have played a key role in increasing our knowledge of this prevalent pathogen.

The G428A nonsense mutation in FUT2 provides strong but not absolute protection against symptomatic GII.4 Norovirus infection

In November 2004, 116 individuals in an elderly nursing home in El Grao de Castellón, Spain were symptomatically infected with genogroup II.4 (GII.4) norovirus. The global attack rate was 54.2%. Genotyping of 34 symptomatic individuals regarding the FUT2 gene revealed that one patient was, surprisingly, a non-secretor, hence indicating secretor-independent infection. Lewis genotyping revealed that Lewis-positive and negative individuals were susceptible to symptomatic norovirus infection indicating that Lewis status did not predict susceptibility. Saliva based ELISA assays were used to determine binding of the outbreak virus to saliva samples. Saliva from a secretor-negative individual bound the authentic outbreak GII.4 Valencia/2004/Es virus, but did not in contrast to secretor-positive saliva bind VLP of other strains including the GII.4 Dijon strain. Amino acid comparison of antigenic A and B sites located on the external loops of the P2 domain revealed distinct differences between the Valencia/2004/Es and Dijon strains. All three aa in each antigenic site as well as 10/11 recently identified evolutionary hot spots, were unique in the Valencia/2004/Es strain compared to the Dijon strain. To the best of our knowledge, this is the first example of symptomatic GII.4 norovirus infection of a Le(a+b-) individual homozygous for the G428A nonsense mutation in FUT2. Taken together, our study provides new insights into the host genetic susceptibility to norovirus infections and evolution of the globally dominating GII.4 viruses.

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.

Kinetics of transmission, infectivity, and genome stability of two novel mouse norovirus isolates in breeding mice

Murine noroviruses are a recently discovered group of viruses found within mouse research colonies in many animal facilities worldwide. In this study, we used 2 novel mouse norovirus (MNV) wildtype isolates to examine the kinetics of transmission and tissue distribution in breeding units of NOD.CB17-Prkdc(scid)/J and backcrossed NOD.CB17-Prkdc(scid)/J x NOD/ShiLtJ (N1) mice. Viral shedding in feces and dissemination to tissues of infected offspring mice were monitored by RT-PCR over a 6-wk period postpartum. Histologic sections of tissues from mice exposed to MNV were examined for lesions and their sera monitored for the presence of antibodies to MNV. Viruses shed in feces of parental and offspring mice were compared for sequence homology of the Orf2 gene. Studies showed that the wildtype viruses MNV5 and MNV6 behaved differently in terms of the kinetics of transmission and distribution to tissues of offspring mice. For MNV5, virus transmission from parents to offspring was not seen before 3 wk after birth, and neither isolate was transmitted between cages of infected and control mice. Susceptibility to infection was statistically different between the 2 mouse strains used in the study. Both immunodeficient NOD.CB17-Prkdc(scid)/J mice and NOD. CB17-Prkdc(scid)/J x NOD/ShiLtJ offspring capable of mounting an immune response shed virus in their feces throughout the 6-wk study period, but no gross or histologic lesions were present in infected tissues. Progeny viruses isolated from the feces of infected offspring showed numerous mutations in the Orf2 gene for MNV5 but not MNV6. These results confirm previous studies demonstrating that the biology of MNV in mice varies substantially with each virus isolate and mouse strain infected.

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.

Long-term inactivation study of three enteroviruses in artificial surface and groundwaters, using PCR and cell culture

Since the transmission of pathogenic viruses via water is indistinguishable from the transmission via other routes and since the levels in drinking water, although significant for health, may be too low for detection, quantitative viral risk assessment is a useful tool for assessing disease risk due to consumption of drinking water. Quantitative viral risk assessment requires information concerning the ability of viruses detected in drinking water to infect their host. To obtain insight into the infectivity of viruses in relation to the presence of virus genomes, inactivation of three different enteroviruses in artificial ground and surface waters under different controlled pH, temperature, and salt conditions was studied by using both PCR and cell culture over time. In salt-peptone medium, the estimated ratio of RNA genomes to infectious poliovirus 1 in freshly prepared suspensions was about 10(0). At 4 degrees C this ratio was 10(3) after 600 days, and at 22 degrees C it was 10(4) after 200 days. For poliovirus 1 and 2 the RNA/infectious virus ratio was higher in artificial groundwater than in artificial surface water, but this was not the case for coxsackievirus B4. When molecular detection is used for virus enumeration, it is important that the fraction of infectious virus (based on all virus genomes detected) decays with time, especially at temperatures near 22 degrees C.

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.

Outbreak studies of a GII-3 and a GII-4 norovirus revealed an association between HBGA phenotypes and viral infection

Noroviruses are the major viral pathogen of epidemic acute gastroenteritis. Two outbreaks of norovirus gastroenteritis that occurred in China in 2003 and 2006, caused by a GII-4 and a GII-3 strain, respectively, were studied to investigate potential association between viral infection and histo-blood types of hosts. The histo-blood group antigen (HBGA) phenotypes of 146 subjects (16 from the GII-3 and 130 from the GII-4 outbreaks) were determined in a saliva-based EIA. Our results showed that the secretor status of individuals was strongly associated with infection in the two outbreaks (P=0.0007, OR=0.044, 95% CI, 0.003-0.765); none of the nonsecretor in either outbreak developed symptomatic infection. The infection rate of individuals with the ABH and Lewis blood types varied between the two outbreaks. In the GII-4 outbreak, association of ABH blood types with noroviral infection (P=0.001, Chi-square=16.13) has been observed, in which the type A individuals had an increased risk of infection [61% in the symptomatic group (n=41) vs. 30% in the asymptomatic group (n=89), P=0.0001], while the type O individuals showed a decreased infection rate (17% vs. 48% in the two groups, P=0.0048). In the GII-3 outbreak, however, individuals with the H antigen only appeared to have a higher rate of infection (33% vs. 14%, P=0.059). Our study provided further evidence in the association between noroviral infection and the HBGA types of hosts. While the nonsecretor phenotype appears naturally resistant to these two strains, additional determinants on the HBGAs also may play roles in host range of the two strains.

Antibody is critical for the clearance of murine norovirus infection

Human noroviruses cause more than 90% of epidemic nonbacterial gastroenteritis. However, the role of B cells and antibody in the immune response to noroviruses is unclear. Previous studies have demonstrated that human norovirus specific antibody levels increase upon infection, but they may not be protective against infection. In this report, we used murine norovirus (MNV), an enteric norovirus, as a model to determine the importance of norovirus specific B cells and immune antibody in clearance of norovirus infection. We show here that mice genetically deficient in B cells failed to clear primary MNV infection as effectively as wild-type mice. In addition, adoptively transferred immune splenocytes derived from B-cell-deficient mice or antibody production-deficient mice were unable to efficiently clear persistent MNV infection in RAG1(-/-) mice. Further, adoptive transfer of either polyclonal anti-MNV serum or neutralizing anti-MNV monoclonal antibodies was sufficient to reduce the level of MNV infection both systemically and in the intestine. Together, these data demonstrate that antibody plays an important role in the clearance of MNV and that immunoglobulin G anti-norovirus antibody can play an important role in clearing mucosal infection.

Chronic norovirus and adenovirus infection in a solid organ transplant recipient

A 10-month-old boy developed chronic diarrhea 2 months after a combined liver, pancreas, and small bowel transplant. Norovirus and adenovirus were detected in multiple stool specimens during a 114-day period. Enteric viral infectious should be considered in solid organ transplant recipients with chronic diarrhea.

Genetic relatedness of noroviruses identified in sporadic gastroenteritis in children and gastroenteritis outbreaks in northern Alberta

We compared the proportion and genotype distribution of norovirus (NoV) identified in sporadic acute gastroenteritis in children younger than 7 years old with the NoV strains found in outbreaks from January 2003 through April 2004 in northern Alberta, Canada. Eight genogroup I (GI) and 133 GII NoV cases were detected in 1,166 cases of acute sporadic childhood gastroenteritis with a monthly detection rates varying from 6.0% to 20.4% and no sporadic gastroenteritis case in October 2003. Seventy-eight outbreaks (65%) tested positive for NoV during the study period with an obvious winter predominance and no NoV outbreaks in August, September, and October 2003. Three GI and 51 GII strains from the sporadic childhood gastroenteritis cases and seven GI and 37 GII strains from gastroenteritis outbreaks were sequenced and analyzed. Strains belonging to the GII.4 cluster predominated in outbreaks (68%) while the strains in sporadic childhood gastroenteritis demonstrated significant heterogeneity with the majority belonging to the GII.3 cluster (36%). Further analysis of NoV strains from 34 sporadic childhood gastroenteritis cases and 38 gastroenteritis outbreaks in chronologically and geographically related groups failed to demonstrate clear link between strains circulating in the setting of sporadic childhood gastroenteritis and those found in outbreaks.

Mechanisms of GII.4 norovirus persistence in human populations

BACKGROUND

Noroviruses are the leading cause of viral acute gastroenteritis in humans, noted for causing epidemic outbreaks in communities, the military, cruise ships, hospitals, and assisted living communities. The evolutionary mechanisms governing the persistence and emergence of new norovirus strains in human populations are unknown. Primarily organized by sequence homology into two major human genogroups defined by multiple genoclusters, the majority of norovirus outbreaks are caused by viruses from the GII.4 genocluster, which was first recognized as the major epidemic strain in the mid-1990s. Previous studies by our laboratory and others indicate that some noroviruses readily infect individuals who carry a gene encoding a functional alpha-1,2-fucosyltransferase (FUT2) and are designated "secretor-positive" to indicate that they express ABH histo-blood group antigens (HBGAs), a highly heterogeneous group of related carbohydrates on mucosal surfaces. Individuals with defects in the FUT2 gene are termed secretor-negative, do not express the appropriate HBGA necessary for docking, and are resistant to Norwalk infection. These data argue that FUT2 and other genes encoding enzymes that regulate processing of the HBGA carbohydrates function as susceptibility alleles. However, secretor-negative individuals can be infected with other norovirus strains, and reinfection with the GII.4 strains is common in human populations. In this article, we analyze molecular mechanisms governing GII.4 epidemiology, susceptibility, and persistence in human populations.

METHODS AND FINDINGS

Phylogenetic analyses of the GII.4 capsid sequences suggested an epochal evolution over the last 20 y with periods of stasis followed by rapid evolution of novel epidemic strains. The epidemic strains show a linear relationship in time, whereby serial replacements emerge from the previous cluster. Five major evolutionary clusters were identified, and representative ORF2 capsid genes for each cluster were expressed as virus-like particles (VLPs). Using salivary and carbohydrate-binding assays, we showed that GII.4 VLP-carbohydrate ligand binding patterns have changed over time and include carbohydrates regulated by the human FUT2 and FUT3 pathways, suggesting that strain sensitivity to human susceptibility alleles will vary. Variation in surface-exposed residues and in residues that surround the fucose ligand interaction domain suggests that antigenic drift may promote GII.4 persistence in human populations. Evidence supporting antigenic drift was obtained by measuring the antigenic relatedness of GII.4 VLPs using murine and human sera and demonstrating strain-specific serologic and carbohydrate-binding blockade responses. These data suggest that the GII.4 noroviruses persist by altering their HBGA carbohydrate-binding targets over time, which not only allows for escape from highly penetrant host susceptibility alleles, but simultaneously allows for immune-driven selection in the receptor-binding region to facilitate escape from protective herd immunity.

CONCLUSIONS

Our data suggest that the surface-exposed carbohydrate ligand binding domain in the norovirus capsid is under heavy immune selection and likely evolves by antigenic drift in the face of human herd immunity. Variation in the capsid carbohydrate-binding domain is tolerated because of the large repertoire of similar, yet distinct HBGA carbohydrate receptors available on mucosal surfaces that could interface with the remodeled architecture of the capsid ligand-binding pocket. The continuing evolution of new replacement strains suggests that, as with influenza viruses, vaccines could be targeted that protect against norovirus infections, and that continued epidemiologic surveillance and reformulations of norovirus vaccines will be essential in the control of future outbreaks.

Immunology of Norovirus Infection

Noroviruses are the leading cause of epidemic non-bacterial gastroenteritis worldwide. Despite their discovery over three decades ago, little is known about the host immune response to norovirus infection. The purpose of this chapter is to review the field of norovirus immunology and discuss the contributions of outbreak investigations, human and animal challenge studies and population-based studies. This chapter will survey both humoral and cellular immunity as well as recent advances in norovirus vaccine development.

Intranasal administration of a recombinant adenovirus expressing the norovirus capsid protein stimulates specific humoral, mucosal, and cellular immune responses in mice

Norovirus (NV) is a major cause of acute, epidemic nonbacterial gastroenteritis in individuals of all ages. The immunological mechanism of NV infection and the approaches used to prevent infection remain to be elucidated. In this study, the specific immune responses of BALB/c mice were assessed following intranasal immunization with a recombinant adenovirus vector expressing the genogroup II4 (GGII/4) norovirus capsid protein. Analysis of IgM, IgG, and IgA antibodies specific for the recombinant virus-like particles (VLPs) of NV demonstrated that a high level of humoral immunity developed following immunization. Mucosal immune responses were also detectable in stool, intestinal homogenates, lung homogenates, and lung lavage samples. Specific cellular immune responses were observed in NV VLPs-restimulated splenocytes by ELISPOT and Th1/Th2 cytokine cytometric array (CBA). Serum IgG subclass analysis showed that a balanced Th1- and Th2-like cellular immune response was induced in BALB/c mice following immunization with recombinant adenovirus. These findings demonstrate that the intranasal immunization of a recombinant adenovirus expressing the NV capsid protein is an efficient strategy to stimulate systemic, mucosal, and cellular Th1/Th2 immune responses in mice, and could serve as a novel approach for designing NV vaccines.

Pathogenesis and immune responses in gnotobiotic calves after infection with the genogroup II.4-HS66 strain of human norovirus

We previously characterized the pathogenesis of two host-specific bovine enteric caliciviruses (BEC), the GIII.2 norovirus (NoV) strain CV186-OH and the phylogenetically unassigned NB strain, in gnotobiotic (Gn) calves. In this study we evaluated the Gn calf as an alternative animal model to study the pathogenesis and host immune responses to the human norovirus (HuNoV) strain GII.4-HS66. The HuNoV HS66 strain caused diarrhea (five/five calves) and intestinal lesions (one/two calves tested) in the proximal small intestine (duodenum and jejunum) of Gn calves, with lesions similar to, but less severe than, those described for the Newbury agent 2 (NA-2) and NB BEC. Viral capsid antigen was also detected in the jejunum of the proximal small intestine of one of two calves tested by immunohistochemistry. All inoculated calves shed virus in feces (five/five calves), and one/five had viremia. Antibodies and cytokine (proinflammatory, tumor necrosis factor alpha [TNF-alpha]; Th1, interleukin-12 [IL-12] and gamma interferon [IFN-gamma]; Th2, IL-4; Th2/T-regulatory, IL-10) profiles were determined in serum, feces, and intestinal contents (IC) of the HuNoV-HS66-inoculated calves (n = 5) and controls (n = 4) by enzyme-linked immunosorbent assay in the acute (postinoculation day 3 [PID 3]) and convalescent (PID 28) stages of infection. The HuNoV-HS66-specific antibody and cytokine-secreting cells (CSCs) were quantitated by ELISPOT in mononuclear cells of local and systemic tissues at PID 28. Sixty-seven percent of the HuNoV-HS66-inoculated calves seroconverted, and 100% coproconverted with immunoglobulin A (IgA) and/or IgG antibodies to HuNoV-HS66, at low titers. The highest numbers of antibody-secreting cells (ASC), both IgA and IgG, were detected locally in intestine, but systemic IgA and IgG ASC responses also occurred in the HuNoV-HS66-inoculated calves. In serum, HuNoV-HS66 induced higher peaks of TNF-alpha and IFN-gamma at PIDs 2, 7, and 10; of IL-4 and IL-10 at PID 4; and of IL-12 at PIDs 7 and 10, compared to controls. In feces, cytokines increased earlier (PID 1) than in serum and TNF-alpha and IL-10 were elevated acutely in the IC of the HS66-inoculated calves. Compared to controls, at PID 28 higher numbers of IFN-gamma and TNF-alpha CSCs were detected in mesenteric lymph nodes (MLN) or spleen and Th2 (IL-4) CSCs were elevated in intestine; IL-10 CSCs were highest in spleen. Our study provides new data confirming HuNoV-HS66 replication and enteropathogenicity in Gn calves and reveals important and comprehensive aspects of the host's local (intestine and MLN) and systemic (spleen and blood) immune responses to HuNoV-HS66.

A human norovirus-like particle vaccine adjuvanted with ISCOM or mLT induces cytokine and antibody responses and protection to the homologous GII.4 human norovirus in a gnotobiotic pig disease model

We inoculated gnotobiotic pigs orally/intranasally with human norovirus GII.4 HS66 strain virus-like particles (VLP) and immunostimulating complexes (ISCOM) or mutant E. coli LT toxin (mLT, R192G) as mucosal adjuvants, then assessed intestinal and systemic antibody and cytokine responses and homologous protection. Both vaccines induced high rates of seroconversion (100%) and coproconversion (75-100%). The VLP+mLT vaccine induced Th1/Th2 serum cytokines and cytokine secreting cells, whereas the VLP+ISCOM vaccine induced Th2 biased responses with significantly elevated IgM, IgA and IgG antibody-secreting cells in intestine. Nevertheless, both vaccines induced increased protection rates against viral shedding and diarrhea (75-100%) compared to controls; however, only 57% of controls shed virus.

Norovirus–host interaction: implications for disease control and prevention

Noroviruses (NVs) are a major cause of acute gastroenteritis epidemics in both developing and developed countries and affect people of all ages. Three main human histo-blood group antigens (HBGAs) – the ABO, Lewis and secretor families – are involved in NV recognition and eight strain-specific receptor-binding patterns in two major binding groups have been described. The receptor-binding interface is located at the outermost surface of the P domain of the viral capsid. Each interface contains two major binding sites and each site interacts with a carbohydrate side-chain of the HBGAs via multiple hydrogen bonds. Soluble HBGAs in human milk are able to block binding of NV to HBGA receptors, suggesting a potential decoy receptor for the protection of infants from NV infection. Phylogenetic analysis has revealed limited genetic relatedness among NVs with similar receptor-binding patterns. This review summarises and discusses recent advances and highlights implications for future studies in the control and prevention of NV gastroenteritis.

Cytokine and antibody responses in gnotobiotic pigs after infection with human norovirus genogroup II.4 (HS66 strain)

A human norovirus genogroup II.4 strain HS66 (HuNoV-HS66) infects and causes mild diarrhea in gnotobiotic (Gn) pigs (S. Cheetham, M. Souza, T. Meulia, S. Grimes, M. G. Han, and L. J. Saif, J. Virol. 80:10372-10381, 2006). In this study we evaluated systemic and intestinal humoral and cellular immune responses to HuNoV-HS66 in orally inoculated pigs. Antibodies and type I interferon (IFN-I or IFN-alpha), proinflammatory interleukin-6 (IL-6), Th1 (IL-12 and IFN-gamma), Th2 (IL-4), and Th2/regulatory T ([T(reg)] IL-10) cytokine profiles in serum and intestinal contents (IC) of the HuNoV-HS66-inoculated pigs and controls were assessed by enzyme-linked immunosorbent assay at selected postinoculation days (0 to 28). Using an enzyme-linked immunospot assay, we evaluated immunoglobulin M (IgM), IgA, and IgG antibody-secreting cells (ASC) and cytokine-secreting cells (CSC) in intestine, spleen, and blood. In the HuNoV-inoculated pigs, antibody titers in serum and IC were generally low, and 65% seroconverted. Pigs with higher diarrhea scores were more likely to seroconvert and developed higher intestinal IgA and IgG antibody titers. The numbers of IgA and IgG ASC were higher systemically than in the gut. In serum, HuNoV induced persistently higher Th1 (low transient IFN-gamma and high IL-12) than the other cytokines, but also low Th2 (IL-4) and Th2/T(reg) (IL-10) levels; low, transient proinflammatory (IL-6) cytokines; and, notably, a delayed IFN-alpha response. In contrast, intestinal innate (IFN-alpha early and late) and Th1 (IL-12 late) cytokines were significantly elevated postinfection. HuNoV-HS66 also elicited higher numbers of Th1 (IL-12 and IFN-gamma) CSC than Th2 (IL-4) and proinflammatory (IL-6) CSC, with the latter responses low in blood and intestine, reflecting low intestinal inflammation in the absence of gut lesions. These data provide insights into the kinetics of cytokine secretion in serum and IC of HuNoV-inoculated Gn pigs and new information on intestinal humoral and cellular immune responses to HuNoV that are difficult to assess in human volunteers.

Asymptomatic norovirus infection in Mexican children

Sixty-three children in periurban Mexico City were examined for the occurrence of asymptomatic norovirus (NoV) infection from June to August 1998. NoV was detected in 48 of 161 stool specimens (29.8%), with 31 children (49.2%) having at least one positive stool. Asymptomatic NoV infection occurred commonly during summertime in a Mexican pediatric population.

Complete genomic characterization and antigenic relatedness of genogroup III, genotype 2 bovine noroviruses

Bovine enteric noroviruses form a genogroup, III, distinct from the 2 human norovirus genogroups, I and II. Two genogroup III genotypes were suggested by partial genomic analyses. In the present study, analysis of the full-length genome sequence of Bo/Newbury2/76/UK and the more contemporary Newbury2-like virus, Bo/Dumfries/1994/UK, showed that both were 7311 nucleotides in length and had three open reading frames (ORFs), amino acids motifs typical of noroviruses, and 95% or greater amino acid identities to each other in all regions of their genome. Apart from the ORF1 NTPase region, their ORF1 regions had less than 90% identity to the genogroup III genotype 1 Bo/Jena/80/DE virus, confirming two genogroup III genotypes. A close antigenic relationship was demonstrated by ELISA between the genotype 2 viruses, which will allow their serological diagnosis.

Noroviruses everywhere: has something changed?

PURPOSE OF REVIEW

Noroviruses causing gastroenteritis in humans have increasingly been described in both the scientific literature and the lay press. This review summarizes new information about where and why these viruses are detected as well as new developments to prevent or treat these infections.

RECENT FINDINGS

Noroviruses are highly infectious and quite stable. Noroviruses infect all age groups, with particularly severe disease occurring in young children, the elderly, and persons with chronic diseases. Noroviruses also cause persistent infections in immunosuppressed patients. Host susceptibility factors have been identified, including histo-blood group antigens that likely function as initial receptors for some noroviruses. The patterns of susceptibility are complex, however, and not completely understood for all virus strains. X-ray crystallographic information on the virus capsid and other viral-encoded proteins provides targets for structure-based drug development.

SUMMARY

The availability of new methods of detecting noroviruses has resulted in increased detection of these pathogens. Advances in understanding of virus replication and structure should lead to development of strategies to prevent and treat these infections. Such efforts may be challenged by the emergence of new virus strains that appear through mechanisms that remain to be understood.

Mendelian resistance to human norovirus infections

Noroviruses have emerged as a major cause of acute gastroenteritis in humans of all ages. Despite high infectivity of the virus and lack of long-term immunity, volunteer and authentic studies has suggested the existence of inherited protective factors. Recent studies have shown that histo-blood group antigens (HBGAs) and in particular secretor status controlled by the α1,2fucosyltransferase FUT2 gene determine susceptibility to norovirus infections, with nonsecretors (FUT2−/−), representing 20% of Europeans, being highly resistant to symptomatic infections with major strains of norovirus. Moreover, the capsid protein from distinct strains shows different HBGA specificities, suggesting a host–pathogen co-evolution driven by carbohydrate–protein interactions.

Fecal cytokines and markers of intestinal inflammation in international travelers with diarrhea due to Noroviruses

The intestinal immune and inflammatory responses of Norovirus (NoV) are poorly defined. The objective of this study was to investigate fecal cytokine and lactoferrin profiles in response to NoV gastroenteritis in travelers. Both fecal cytokines and fecal lactoferrin were measured for NoV-associated diarrhea (N = 7), mixed infection of NoV and enterotoxigenic E. coli (ETEC)-associated diarrhea (N = 10) and in pathogen-negative diarrhea cases (N = 19). Both IL-2 and IFN-gamma were significantly increased in NoV-associated diarrhea specimens, suggesting a predominant Th1 immune response to NoV infection in the gut. When a mixed infection of NoV and ETEC occurred, a combined Th1/Th2 response was observed suggesting a dual immune response secondary to infection by both pathogens. Intestinal inflammation associated with increased fecal lactoferrin, important in bacterial enteric infection, was not found in NoV-associated gastroenteritis.

The epidemiologic and clinical importance of norovirus infection

Noroviruses are a major cause of sporadic cases and epidemic outbreaks of gastroenteritis. The development of molecular diagnostic assays has led to an increased recognition of the significance of these viruses as causes of gastroenteritis in all age groups. This article reviews the epidemiology, clinical manifestations and pathogenesis of norovirus infection, and it describes the diagnostic and therapeutic approaches to this disease.

Multivalent norovirus vaccines induce strong mucosal and systemic blocking antibodies against multiple strains

Noroviruses are important agents of human gastroenteritis characterized by extensive sequence variation in the major capsid structural protein that likely encodes critical antigenic determinants of protective immunity. The lack of an infection model has limited detailed characterizations of viral antigenic relationships and identification of the essential components for protective immunity. This information would contribute to efficacious vaccine design against a broad array of norovirus strains. To understand the extent of heterotypic norovirus antibody specificity to inter- and intra-genogroup strains and its applicability to vaccine design, we collected sera from humans infected with different norovirus strains and from mice inoculated with alphavirus vectors expressing strain-specific recombinant norovirus-like particles (VLPs). We used VLPs that were assembled from Norwalk virus (NV), Hawaii virus (HV), Snow Mountain virus (SM) and Lordsdale virus (LV) as antigens to define and compare heterotypic antibody responses in humans and mice. We also examined if these heterotypic antibodies could block specific binding of ABH histo-blood group antigens, putative receptors for norovirus binding and entry, to norovirus VLPs. Furthermore, we examined the effect of multivalent inocula on the specificity, titer, and ligand blockade properties of systemic and mucosal norovirus-specific antibodies in mice. Our studies suggest that infection with one of several different genogroup I (GI) strains in humans induces heterotypic antibodies that block NV binding to ABH antigens, although comparable findings were not evident following infection with genogroup (GII) strains. Additionally, inoculating mice with vaccine cocktails encoding multiple norovirus VLPs enhances heterotypic and ligand attachment-blocking antibody responses against the LV strain not included in the cocktail. These data suggest that multivalent vaccination may provide better protection from a broader range of noroviruses than monovalent vaccination.

The p domain of norovirus capsid protein forms a subviral particle that binds to histo-blood group antigen receptors

Norovirus is the most important cause of nonbacterial acute gastroenteritis. We have shown previously that the isolated P domain containing the hinge forms a dimer and binds to histo-blood group antigen (HBGA) receptors with a low affinity (M. Tan, R. S. Hegde, and X. Jiang, J. Virol. 78:6233-6242, 2004). Here, we reported that the P domain of VA387 without the hinge forms a small particle with a significantly increased receptor binding affinity. An end-linked oligopeptide containing one or more cysteines promoted P-particle formation by forming intermolecular disulfide bridges. The binding sensitivity of the P particle to HBGAs was enhanced >700-fold compared to the P dimer, which was comparable to that of virus-like particles. The binding specificity of the P particle was further confirmed by strong binding to the Caco-2 cells, a human colon carcinoma cell line. This binding enhancement was observed in the P particles of both norovirus GI and GII strains. The P particle is estimated to contain 12 P dimers, in which the P2 subdomain builds up the outer layer, while the P1 subdomain forms the internal core. Taken together, our data indicate that the P domain is involved not only in dimerization but also in polymerization of the protein during the capsid assembling. The enhanced receptor binding of the P particle reflects the intrinsic feature of the viral capsid. The easy production of the P particle and its strong binding to HBGAs suggest that the P particle is useful in studying pathogenesis and morphogenesis of norovirus and candidates for antiviral or vaccine development.

Norovirus capsid protein expressed in yeast forms virus-like particles and stimulates systemic and mucosal immunity in mice following an oral administration of raw yeast extracts

Norovirus (NV) gastroenteritis is a widespread disease affecting people of all ages worldwide. A simple, safe, and easily deliverable vaccine may be the key for the control and prevention of NV gastroenteritis. In this study, we demonstrated that a NV recombinant capsid protein (strain VA387, genogroup II.4) expressed in yeast (Pichia pastoris) spontaneously formed virus-like particles (VLPs) like those expressed in other in vitro systems. Oral administration of raw material from the yeast cell lysates containing 0.1 mg of VLPs without an adjuvant resulted in systemic and mucosal immune responses in mice. Significantly higher and earlier responses were observed in mice receiving a higher dose (1 mg per dose) of the antigen. Both the serum and fecal antibodies blocked VA387 VLP binding to its histo-blood group antigen receptors. The animals did not reveal any side effect following the administration of the yeast lysates. Our results indicated that yeast is a simple, effective alternative for NV VLP production. The mice immunization study also indicated that the oral administration of raw yeast extracts without an adjuvant is a safe and simple way which is worth to be studied for vaccine delivery in humans.

Development of a microsphere-based serologic multiplexed fluorescent immunoassay and a reverse transcriptase PCR assay to detect murine norovirus 1 infection in mice

Murine norovirus 1 (MNV-1) is a newly recognized pathogen of mice that causes lethal infection in mice deficient in components of the innate immune response but not in wild-type 129 mice. In this study, in vitro-propagated MNV-1 was used as antigen to develop a multiplexed fluorescent immunoassay (MFI) to detect antibodies to MNV-1 in infected mice. The MNV-1 MFI was 100% specific and 100% sensitive in detecting anti-MNV-1 antibody in sera from experimentally infected mice. Testing of a large number of mouse serum samples (n = 12,639) submitted from contemporary laboratory mouse colonies in the United States and Canada revealed that 22.1% of these sera contained antibodies to MNV-1, indicating infection with MNV-1 is widespread in research mice. In addition, a reverse transcriptase PCR primer pair with a sensitivity of 25 virus copies was developed and used to demonstrate that MNV-1 RNA could be detected in the spleen, mesenteric lymph node, and jejunum from some experimentally infected mice 5 weeks postinoculation. These diagnostic assays provide the necessary tools to define the MNV-1 infection status of research mice and to aid in the establishment of laboratory mouse colonies free of MNV-1 infection.

Norwalk virus infection associates with secretor status genotyped from sera

ABO histo-blood group type and secretor status are two genetically determined factors that contribute to resistance and susceptibility to Norwalk virus (NV). Archived serum samples but not saliva samples are available from NV and many other norovirus challenge studies and outbreaks. A person's ABO phenotype is easily determined from their archived sera, but the individual's secretor phenotype cannot easily be ascertained without saliva. We now report that a person's secretor genotype can also be determined from the archived serum samples. Of the 51 volunteers who participated in a NV challenge study, all eight non-secretors were resistant to NV infection, all of the 42 NV-infected volunteers were secretor positive, and a single uninfected secretor was histo-blood group type B. In agreement with a previous report, secretor status was most predictive of risk of NV infection. The methods described in this report should rapidly improve our knowledge of the associations between carbohydrate antigen expression and susceptibility to different strains of the non-cultivatable noroviruses by enabling retrospective studies from previously collected volunteer challenge and outbreak sera.

Immune responses to bovine norovirus-like particles with various adjuvants and analysis of protection in gnotobiotic calves

We evaluated serum and fecal antibody responses to bovine norovirus (BoNoV) virus-like particles (VLP) and protection against virulent BoNoV in gnotobiotic calves. Calves were vaccinated with two or three doses of VLPs (250 microg/dose) coadministered with oil, mutant E. coli heat-labile toxin (R192G) (mLT) or immunostimulating complexes (ISCOM). Fecal IgA antibody was detected only in calves vaccinated intranasally with VLP+mLT and after challenge, partial protection with delayed, shortened diarrhea (1-2 days) was observed only in these calves. Diarrhea presented from 2 to 6 days in calves vaccinated with VLP with oil or ISCOM and 8 to 9 days in controls. Virus shedding was detected post-challenge by ELISA in all vaccinated calves. All calves recovered from BoNoV infection had high titers of fecal IgA antibodies and were completed protection against challenge. We conclude that only BoNoV VLP+mLT given intranasally stimulated both serum and fecal IgA antibodies and partial protection.

Norovirus and its histo-blood group antigen receptors: an answer to a historical puzzle

Recent findings demonstrate that human histo-blood group antigens (HBGAs) serve as receptors for norovirus infection. The recognition of human HBGAs by noroviruses is a typical protein-carbohydrate interaction, in which the protruding domain of the viral capsid protein forms an interface with the oligosaccharide side-chains of the antigens, with a wide diversity among different strains. The human HBGA system is also highly polymorphic and is controlled by multiple gene families with silent alleles. The presence of such diversified molecules on the cell surfaces indicates a possible host defense mechanism against the changing external environment. As mild pathogens that replicate possibly only in the intestinal tract, noroviruses have developed unique strategies to overcome the host defense system. This has been shown by their genetic and structural variations, which explains why norovirus-associated diseases are so common and widespread in every population worldwide.