Cellular and humoral immunity following Snow Mountain virus challenge

Biological and immunological characterization of major capsid protein VP1 from distinct GII.2 norovirus clusters

Human noroviruses (HuNoVs) are a leading cause of acute viral gastroenteritis worldwide. Infectious outbreaks due to recombinant NoV genotype called GII.P16-GII.2 have been frequently reported since 2016. In this study, we expressed the major capsid protein VP1 from three GII.2 NoV strains using the recombinant baculovirus expression system. The assembly, histo-blood group antigen (HBGA)-binding patterns, and cross-blocking abilities of VP1 proteins were investigated. All the three NoV VP1 proteins successfully assembled into virus-like particles (VLPs). The HBGA-binding assay demonstrated a temporal binding pattern. The latest isolate bound to saliva samples of all blood types. Sequence alignment suggested that the observed gain in HBGA-binding ability was attributed to a limited number of amino acid mutations. Using chimeric VP1 proteins, we demonstrated that synergistic effects resulted in enhanced binding ability. Bile salts increased GII.2 VLP avidity for HBGAs except GII.2-2011/M1. In vitro blockade assay of salivary HBGA-VLP binding demonstrated the presence of cross-blocking effects among different strains. This study provides insight into the evolutionary binding characteristics and cross-blocking effects of GII.2 NoVs to facilitate the development of measures to control this type of viruses.

Molecular Epidemiology of Human Norovirus Variants from Outbreaks in Zhejiang Province, China, during 2021

Background

Noroviruses are the most frequent cause of epidemic acute viral gastroenteritis in China.

Objectives

The aim of this study was to determine the molecular epidemiological characteristics of norovirus outbreaks and the molecular genetic features of norovirus in Zhejiang Province during 2021.

Methods

First, the local Centers for Disease Control and Prevention in the outbreak area conducted on-site epidemiologic investigations and collected samples from ill patients for initial testing. The general epidemiologic characteristics of the demographic information are presented through descriptive analysis. Positive samples were sent to the Microbiology Laboratory of Zhejiang Provincial Center for Disease Control and Prevention for further verification. The presence of norovirus genogroups I (GI) and II (GII), along with sapovirus, was detected. Subsequently, the specimens positive for norovirus were sequenced for genotyping purposes. Furthermore, the whole genomes of positive samples were sequenced, enabling the characterization of both nucleotide and amino acid differences within the virus. Finally, phylogenetic trees were constructed to further analyze and understand the genetic relationships among the detected viruses.

Result

227 norovirus outbreaks were reported in Zhejiang Province, China, during 2021. Schools were the main setting while January was the peak month for outbreaks. A total of 17 diverse genotypes of norovirus were identified in 2021, and GII.P16-GII.2 was the most frequent genotype (30.19%). Seven genomes (five GI.P4-GI.5 and two GII.P16-GII.2) were obtained. Although GI.P4-GI.5 is considered to be a rare genotype of norovirus, the prevalence might have been underestimated. Capsid microvariation of GII.2 displayed histo-blood group antigen binding patterns compared to the GII.2 prototype, although VP1 sequences were considered to have a minimal impact on antigenicity.

Conclusion

This study revealed the diversity of norovirus strains' genotypes circulating in Zhejiang Province in 2021. Continued molecular surveillance of noroviruses should be strengthened in our further efforts to the development of vaccines.

Noroviruses: Evolutionary Dynamics, Epidemiology, Pathogenesis, and Vaccine Advances-A Comprehensive Review

Noroviruses constitute a significant aetiology of sporadic and epidemic gastroenteritis in human hosts worldwide, especially among young children, the elderly, and immunocompromised patients. The low infectious dose of the virus, protracted shedding in faeces, and the ability to persist in the environment promote viral transmission in different socioeconomic settings. Considering the substantial disease burden across healthcare and community settings and the difficulty in controlling the disease, we review aspects related to current knowledge about norovirus biology, mechanisms driving the evolutionary trends, epidemiology and molecular diversity, pathogenic mechanism, and immunity to viral infection. Additionally, we discuss the reservoir hosts, intra-inter host dynamics, and potential eco-evolutionary significance. Finally, we review norovirus vaccines in the development pipeline and further discuss the various host and pathogen factors that may complicate vaccine development.

Blockade Antibody Responses in Human Subjects Challenged with a New Snow Mountain Virus Inoculum

Background

Noroviruses (NoVs) are a leading cause of non-bacterial gastroenteritis in young children and adults worldwide. Snow Mountain Virus (SMV) is the prototype of NoV GII genotype 2 (GII.2) that has been developed as a viral model for human challenge studies, an important tool for studying pathogenesis and immune response of NoV infections and for evaluating NoV vaccine candidates. Previous studies have identified blockade antibodies that block the binding of NoV virus-like particles (VLPs) to histo-blood group antigens (HBGAs) as a surrogate for neutralization in human Norwalk virus and GII.4 infections but little is known about SMV blockade antibodies.

Methods

In this secondary data analysis study, blockade antibodies were characterized in pre-challenge and post-challenge serum samples from human subjects challenged with a new SMV inoculum. The correlation between blockade antibody geometric mean antibody titers (GMTs) and SMV-specific serum IgG/IgA GMTs were examined after stratifying the subjects by infection status. A linear mixed model was applied to test the association between HBGA blockade antibody concentrations and post-challenge days accounting for covariates and random effects.

Results

Laboratory results from 33 SMV inoculated individuals were analyzed and 75.7% (25/33) participants became infected. Serum SMV-specific blockade antibodies, IgA, and IgG were all significantly different between infected and uninfected individuals beginning day 15 post-challenge. Within infected individuals, a significant correlation was observed between both IgG and IgA and blockade antibody concentration as early as day 6 post-challenge. Analysis of blockade antibody using the linear mixed model showed that infected individuals, when compared to uninfected individuals, had a statistically significant increase in blockade antibody concentrations across the post-challenge days. Among the post-challenge days, blockade antibody concentrations on days 15, 30, and 45 were significantly higher than those observed pre-challenge. The intraclass correlation coefficient (ICC) analysis indicated that the variability of blockade antibody titers is more observed between individuals rather than within subjects.

Conclusions

These results indicate that HBGA-blockade antibody GMTs are generated after SMV challenge and the blockade antibodies were still detectable at day 45 post-challenge. These data indicate that the second-generation of SMV inoculum is highly effective.

Blockade Antibody Responses in Human Subjects Challenged with a New Snow Mountain Virus Inoculum

Background

Noroviruses (NoVs) are a leading cause of non-bacterial gastroenteritis in young children and adults worldwide. Snow Mountain Virus (SMV) is the prototype of NoV GII genotype 2 (GII.2) that has been developed as a viral model for human challenge models, an important tool for studying pathogenesis and immune response of NoV infections and for evaluating NoV vaccine candidates. Previous studies have identified blockade antibodies that block the binding of NoV virus-like particles (VLPs) to histo-blood group antigens (HBGAs) as a surrogate for neutralization in human Norwalk virus and GII.4 infections but little is known about SMV blockade antibodies.

Methods

In this secondary data analysis study, blockade antibodies were characterized in pre-challenge and post-challenge serum samples from human subjects challenged with a new SMV inoculum. The correlation between blockade antibody geometric mean antibody titers (GMTs) and SMV-specific serum IgG/IgA GMTs were examined after stratifying the subjects by infection status. A linear mixed model was applied to test the association between HBGA blockade antibody concentrations and post-challenge days accounting for covariates and random effects.

Results

Laboratory results from 33 SMV inoculated individuals were analyzed and 75.7% (25/33) participants became infected. Serum SMV-specific blockade antibodies, IgA, and IgG were all significantly different between infected and uninfected individuals beginning day 15 post-challenge. Within infected individuals, a significant correlation was observed between both IgG and IgA and blockade antibody concentration as early as day 6 post-challenge. Analysis of blockade antibody using the linear mixed model showed that infected individuals, when compared to uninfected individuals, had a statistically significant increase in blockade antibody concentrations across the post-challenge days. Among the post-challenge days, blockade antibody concentrations on days 15, 30, and 45 were significantly higher than those observed pre-challenge. The intraclass correlation coefficient (ICC) analysis indicated that the variability of blockade antibody titers is more observed between individuals rather than observations within subjects.

Conclusions

These results indicate that HBGA-blockade antibody GMTs are generated after SMV challenge and the blockade antibodies were still detectable at day 45 post-challenge. These data indicate that the second generation of SMV inoculum is highly effective.

Dose-Response of a Norovirus GII.2 Controlled Human Challenge Model Inoculum

BACKGROUND

Genogroup II noroviruses are the most common cause of acute infectious gastroenteritis. We evaluated the use of a new GII.2 inoculum in a human challenge.

METHODS

Forty-four healthy adults (36 secretor-positive and 8 secretor-negative for histo-blood group antigens) were challenged with ascending doses of a new safety-tested Snow Mountain virus (SMV) GII.2 norovirus inoculum (1.2 × 104 to 1.2 × 107 genome equivalent copies [GEC]; n = 38) or placebo (n = 6). Illness was defined as diarrhea and/or vomiting postchallenge in subjects with evidence of infection (defined as GII.2 norovirus RNA detection in stool and/or anti-SMV immunoglobulin G [IgG] seroconversion).

RESULTS

The highest dose was associated with SMV infection in 90%, and illness in 70% of subjects with 10 of 12 secretor-positive (83%) and 4 of 8 secretor-negative (50%) becoming ill. There was no association between prechallenge anti-SMV serum IgG concentration, carbohydrate-binding blockade antibody, or salivary immunoglobulin A and infection. The median infectious dose (ID50) was 5.1 × 105 GEC.

CONCLUSIONS

High rates of infection and illness were observed in both secretor-positive and secretor-negative subjects in this challenge study. However, a high dose will be required to achieve the target of 75% illness to make this an efficient model for evaluating potential norovirus vaccines and therapeutics.

CLINICAL TRIALS REGISTRATION

NCT02473224.

Association of histo-blood group antigens and predisposition to gastrointestinal diseases

Infectious gastroenteritis is a common illness afflicting people worldwide. The two most common etiological agents of viral gastroenteritis, rotavirus and norovirus are known to recognize histo-blood group antigens (HBGAs) as attachment receptors. ABO, Lewis, and secretor HBGAs are distributed abundantly on mucosal epithelia, red blood cell membranes, and also secreted in biological fluids, such as saliva, intestinal content, milk, and blood. HBGAs are fucosylated glycans that have been implicated in the attachment of some enteric pathogens such as bacteria, parasites, and viruses. Single nucleotide polymorphisms in the genes encoding ABO (H), fucosyltransferase gene FUT2 (Secretor/Se), FUT3 (Lewis/Le) have been associated with changes in enzyme expression and HBGAs production. The highly polymorphic HBGAs among different populations and races influence genotype-specific susceptibility or resistance to enteric pathogens and its epidemiology, and vaccination seroconversion. Therefore, there is an urgent need to conduct population-based investigations to understand predisposition to enteric infections and gastrointestinal diseases. This review focuses on the relationship between HBGAs and predisposition to common human gastrointestinal illnesses caused by viral, bacterial, and parasitic agents.

Current assumptions for quantitative microbial risk assessment (QMRA) of Norovirus contamination of drinking water catchments due to recreational activities: an update

Contamination of drinking water from Norovirus (NoV) and other waterborne viruses is a major public health concern globally. Increasingly, quantitative microbial risk assessment (QMRA) is being used to assess the various risks from waterborne pathogens and evaluate control strategies. As urban populations grow and expand, there is increasing demand for recreational activities in drinking water catchments. QMRA relies on context-specific data to map out the pathways by which viruses can enter water and be transferred to drinking water consumers and identify risk factors and appropriate controls. This review examines the current evidence base and assumptions for QMRA analysis of NoV and other waterborne viral pathogens and recommends numerical values based on the most recent evidence to better understand the health risks associated with recreators in Australian drinking water sources; these are broadly applicable to all drinking water sources where recreational access is allowed. Key issues include the lack of an agreed upon data and dose-response models for human infectious NoV genotypes, faecal shedding by bathers, the extent of NoV infectivity and aggregation, resistance (secretor status) to NoV and the extent of secondary transmission.

Controlled Human Infection Models To Accelerate Vaccine Development

The timelines for developing vaccines against infectious diseases are lengthy, and often vaccines that reach the stage of large phase 3 field trials fail to provide the desired level of protective efficacy. The application of controlled human challenge models of infection and disease at the appropriate stages of development could accelerate development of candidate vaccines and, in fact, has done so successfully in some limited cases. Human challenge models could potentially be used to gather critical information on pathogenesis, inform strain selection for vaccines, explore cross-protective immunity, identify immune correlates of protection and mechanisms of protection induced by infection or evoked by candidate vaccines, guide decisions on appropriate trial endpoints, and evaluate vaccine efficacy. We prepared this report to motivate fellow scientists to exploit the potential capacity of controlled human challenge experiments to advance vaccine development. In this review, we considered available challenge models for 17 infectious diseases in the context of the public health importance of each disease, the diversity and pathogenesis of the causative organisms, the vaccine candidates under development, and each model's capacity to evaluate them and identify correlates of protective immunity. Our broad assessment indicated that human challenge models have not yet reached their full potential to support the development of vaccines against infectious diseases. On the basis of our review, however, we believe that describing an ideal challenge model is possible, as is further developing existing and future challenge models.

Norovirus Infection in Young Nicaraguan Children Induces Durable and Genotype-Specific Antibody Immunity

There are significant challenges to the development of a pediatric norovirus vaccine, mainly due to the antigenic diversity among strains infecting young children. Characterizing human norovirus serotypes and understanding norovirus immunity in naïve children would provide key information for designing rational vaccine platforms. In this study, 26 Nicaraguan children experiencing their first norovirus acute gastroenteritis (AGE) episode during the first 18 months of life were investigated. We used a surrogate neutralization assay that measured antibodies blocking the binding of 13 different norovirus virus-like particles (VLPs) to histo-blood group antigens (HBGAs) in pre- and post-infection sera. To assess for asymptomatic norovirus infections, stools from asymptomatic children were collected monthly, screened for norovirus by RT-qPCR and genotyped by sequencing. Seroconversion of an HBGA-blocking antibody matched the infecting genotype in 25 (96%) of the 26 children. A subset of 13 (50%) and 4 (15%) of the 26 children experienced monotypic GII and GI seroconversion, respectively, strongly suggesting a type-specific response in naïve children, and 9 (35%) showed multitypic seroconversion. The most frequent pairing in multitypic seroconversion (8/12) were GII.4 Sydney and GII.12 noroviruses, both co-circulating at the time. Blocking antibody titers to these two genotypes did not correlate with each other, suggesting multiple exposure rather than cross-reactivity between genotypes. In addition, GII titers remained consistent for at least 19 months post-infection, demonstrating durable immunity. In conclusion, the first natural norovirus gastroenteritis episodes in these young children were dominated by a limited number of genotypes and induced responses of antibodies blocking binding of norovirus VLPs in a genotype-specific manner, suggesting that an effective pediatric norovirus vaccine likely needs to be multivalent and include globally dominant genotypes. The duration of protection from natural infections provides optimism for pediatric norovirus vaccines administered early in life.

Profiling of humoral immune responses to norovirus in children across Europe

Norovirus is a leading cause of epidemic acute gastroenteritis. More than 30 genotypes circulate in humans, some are common, and others are only sporadically detected. Here, we investigated whether serology can be used to determine which genotypes infect children. We established a multiplex protein microarray with structural and non-structural norovirus antigens that allowed simultaneous antibody testing against 30 human GI and GII genotypes. Antibody responses of sera obtained from 287 children aged < 1 month to 5.5 years were profiled. Most specific IgG and IgA responses were directed against the GII.2, GII.3, GII.4, and GII.6 capsid genotypes. While we detected antibody responses against rare genotypes, we found no evidence for wide circulation. We also detected genotype-specific antibodies against the non-structural proteins p48 and p22 in sera of older children. In this study, we show the age-dependent antibody responses to a broad range of norovirus capsid and polymerase genotypes, which will aid in the development of vaccines.

Understanding the relationship between norovirus diversity and immunity

Human noroviruses are the most common viral cause of acute gastroenteritis worldwide. Currently, there are no approved vaccines or specific therapeutics to treat the disease. Some obstacles delaying the development of a norovirus vaccine are: (i) the extreme diversity presented by noroviruses; (ii) our incomplete understanding of immunity to noroviruses; and (iii) the lack of a robust cell culture system or animal model for human noroviruses. Recent advances in in vitro cultivation of norovirus, novel approaches applied to viral genomics and immunity, and completion of vaccine trials and birth cohort studies have provided new information toward a better understanding of norovirus immunity. Here, we will discuss the complex relationship between norovirus diversity and correlates of protection for human noroviruses, and how this information could be used to guide the development of cross-protective vaccines.

Norovirus Vaccines: Current Clinical Development and Challenges

Noroviruses are the major viral pathogens causing epidemic and endemic acute gastroenteritis with significant morbidity and mortality. While vaccines against norovirus diseases have been shown to be of high significance, the development of a broadly effective norovirus vaccine remains difficult, owing to the wide genetic and antigenic diversity of noroviruses with multiple co-circulated variants of various genotypes. In addition, the absence of a robust cell culture system, an efficient animal model, and reliable immune markers of norovirus protection for vaccine evaluation further hinders the developmental process. Among the vaccine candidates that are currently under clinical studies, recombinant VP1-based virus-like particles (VLPs) that mimic major antigenic features of noroviruses are the common ones, with proven safety, immunogenicity, and protective efficacy, supporting a high success likelihood of a useful norovirus vaccine. This short article reviews the recent progress in norovirus vaccine development, focusing on those from recent clinical studies, as well as summarizes the barriers that are being encountered in this developmental process and discusses issues of future perspective.

Serological Humoral Immunity Following Natural Infection of Children with High Burden Gastrointestinal Viruses

Acute gastroenteritis (AGE) is a major cause of morbidity and mortality worldwide, resulting in an estimated 440,571 deaths of children under age 5 annually. Rotavirus, norovirus, and sapovirus are leading causes of childhood AGE. A successful rotavirus vaccine has reduced rotavirus hospitalizations by more than 50%. Using rotavirus as a guide, elucidating the determinants, breath, and duration of serological antibody immunity to AGE viruses, as well as host genetic factors that define susceptibility is essential for informing development of future vaccines and improving current vaccine candidates. Here, we summarize the current knowledge of disease burden and serological antibody immunity following natural infection to inform further vaccine development for these three high-burden viruses.

Zoonotic and reverse zoonotic transmission of viruses between humans and pigs

Humans and pigs share a close contact relationship, similar biological traits, and one of the highest estimated number of viruses compared to other mammalian species. The contribution and directionality of viral exchange between humans and pigs remain unclear for some of these viruses, but their transmission routes are important to characterize in order to prevent outbreaks of disease in both host species. This review collects and assesses the evidence to determine the likely transmission route of 27 viruses between humans and pigs.

Broadly cross-reactive human antibodies that inhibit genogroup I and II noroviruses

The rational development of norovirus vaccine candidates requires a deep understanding of the antigenic diversity and mechanisms of neutralization of the virus. Here, we isolate and characterize a panel of broadly cross-reactive naturally occurring human monoclonal IgMs, IgAs and IgGs reactive with human norovirus (HuNoV) genogroup I or II (GI or GII). We note three binding patterns and identify monoclonal antibodies (mAbs) that neutralize at least one GI or GII HuNoV strain when using a histo-blood group antigen (HBGA) blocking assay. The HBGA blocking assay and a virus neutralization assay using human intestinal enteroids reveal that the GII-specific mAb NORO-320, mediates HBGA blocking and neutralization of multiple GII genotypes. The Fab form of NORO-320 neutralizes GII.4 infection more potently than the mAb, however, does not block HBGA binding. The crystal structure of NORO-320 Fab in complex with GII.4 P-domain shows that the antibody recognizes a highly conserved region in the P-domain distant from the HBGA binding site. Dynamic light scattering analysis of GII.4 virus-like particles with mAb NORO-320 shows severe aggregation, suggesting neutralization is by steric hindrance caused by multivalent cross-linking. Aggregation was not observed with the Fab form of NORO-320, suggesting that this clone also has additional inhibitory features.

Simvastatin Reduces Protection and Intestinal T Cell Responses Induced by a Norovirus P Particle Vaccine in Gnotobiotic Pigs

Noroviruses (NoVs) are a leading cause of acute gastroenteritis worldwide. P particles are a potential vaccine candidate against NoV. Simvastatin is a cholesterol-reducing drug that is known to increase NoV infectivity. In this study, we examined simvastatin’s effects on P particle-induced protective efficacy and T-cell immunogenicity using the gnotobiotic pig model of human NoV infection and diarrhea. Pigs were intranasally inoculated with three doses (100 µg/dose) of GII.4/VA387-derived P particles together with monophosphoryl lipid A and chitosan adjuvants. Simvastatin-fed pigs received 8 mg/day orally for 11 days prior to challenge. A subset of pigs was orally challenged with 10 ID₅₀ of a NoV GII.4/2006b variant at post-inoculation day (PID) 28 and monitored for 7 days post-challenge. Intestinal and systemic T cell responses were determined pre- and postchallenge. Simvastatin abolished the P particle’s protection and significantly increased diarrhea severity after NoV infection. Simvastatin decreased proliferation of virus-specific and non-specific CD8 T cells in duodenum and virus-specific CD4 and CD8 T cells in spleen and significantly reduced numbers of intestinal mononuclear cells in vaccinated pigs. Furthermore, simvastatin significantly decreased numbers of duodenal CD4+IFN-γ+, CD8+IFN-γ+ and regulatory T cells and total duodenal activated CD4+ and CD8+ T cells in vaccinated pigs pre-challenge at PID 28. Following challenge, simvastatin prevented the IFN-γ+ T cell response in spleen of vaccinated pigs. These results indicate that simvastatin abolished P particle vaccine-induced partial protection through, at least in part, impairing T cell immunity. The findings have specific implications for the development of preventive and therapeutic strategies against NoV gastroenteritis, especially for the elderly population who takes statin-type drugs.

Characterizing Norovirus Transmission from Outbreak Data, United States

Norovirus is the leading cause of acute gastroenteritis outbreaks in the United States. We estimated the basic (R₀) and effective (R_e) reproduction numbers for 7,094 norovirus outbreaks reported to the National Outbreak Reporting System (NORS) during 2009–2017 and used regression models to assess whether transmission varied by outbreak setting. The median R₀ was 2.75 (interquartile range [IQR] 2.38–3.65), and median R_e was 1.29 (IQR 1.12–1.74). Long-term care and assisted living facilities had an R₀ of 3.35 (95% CI 3.26–3.45), but R₀ did not differ substantially for outbreaks in other settings, except for outbreaks in schools, colleges, and universities, which had an R₀ of 2.92 (95% CI 2.82–3.03). Seasonally, R₀ was lowest (3.11 [95% CI 2.97–3.25]) in summer and peaked in fall and winter. Overall, we saw little variability in transmission across different outbreaks settings in the United States.

Less severe clinical symptoms of Norwalk virus 8fIIb inoculum compared to its precursor 8fIIa from human challenge studies

Noroviruses (NoV) are a leading cause of epidemic gastroenteritis. Human challenge studies have been used to examine the infectivity, pathogenicity, and host immune response to NoV as well as vaccine efficacy. The goal of this study was to conduct a meta-analysis of data from five previously completed human challenge trials and compare the response to the secondary NV inoculum (8fIIb) to its precursor (8fIIa). We investigated a total of 158 subjects: 76 subjects were experimentally challenged with NV inoculum 8fIIa, and 82 subjects were challenged with 8fIIb. We compared demographic characteristics, infection, illness, mean severity score, blood types, and duration of viral shedding between the two groups of subjects. There were no statistically significant differences in overall infection and illness rates between subjects inoculated with 8fIIa and 8fIIb. However, individuals challenged with 8fIIa had significantly higher severity scores (5.05 vs. 3.22, p = .008) compared with those challenged with 8fIIb. We also observed that infection with 8fIIb was associated with significantly longer duration of viral shedding compared with 8fIIa (11.0 days vs. 5.0 days, p = .0005). These results have serious implications for the development of new NoV inocula for human challenge studies to test candidate vaccine efficacy-where illness severity and duration of viral shedding are important outcomes.

Comparison of a one-step real-time RT-PCR and a nested real-time RT-PCR for a genogroup II norovirus reveals differences in sensitivity depending upon assay design and visualization

Human norovirus (NoV) is the leading cause of acute viral gastroenteritis and a major source of foodborne illness. Detection of NoV in food and environmental samples is typically performed using molecular techniques, including real-time reverse transcription polymerase chain reaction (RT-PCR) and less frequently, nested real-time PCR. In this study, we conducted a controlled comparison of two published NoV detection assays: a broadly reactive one-step real-time RT-PCR and a two-step nested real-time PCR assay. A 20% human fecal suspension containing a genogroup II human NoV was serially diluted, genome extracted, and subjected to amplification using the two assays compared via PCR Units. Additional amplicon confirmation was performed by dot blot hybridization using digoxigenin (DIG)-labeled oligonucleotide probes. Both assays displayed similar amplification standard curves/amplification efficiencies; however, the nested assay consistently detected one log10 lower virus. Dot blot hybridization improved the detection limit of the nested real-time PCR by one log10 NoV genome copies but impaired the detection limit of the one-step real-time RT-PCR by one log10 NoV genome copies. These results illustrate the complexities in designing and interpreting molecular techniques having a sufficient detection limit to detect low levels of viruses that might be anticipated in contaminated food and environmental samples.

Humoral and Mucosal Immune Responses to Human Norovirus in the Elderly

BACKGROUND

Most information on mucosal and systemic immune response to norovirus infection is derived from human challenge studies, birth cohort studies, or vaccine trials in healthy adults. However, few data are available on immune responses to norovirus in the elderly.

METHODS

To study the mucosal and systemic immune response against norovirus, 43 long-term care facilities were enrolled prospectively in 2010-2014. Baseline saliva samples from 17 facilities, cases and controls up to day 84 from 10 outbreaks, as well as acute and convalescent sera were collected.

RESULTS

Norovirus-specific immunoglobulin A (IgA) levels in baseline saliva samples were low and increased in both symptomatic patients and asymptomatic shedders at day 5 after onset during outbreaks. Receiver operating characteristics analysis correctly assigned prior norovirus infection in 23 (92%) of 25 participants. Cases and asymptomatic shedders showed seroconversion for IgG (80%), IgA (78%), and blockade antibodies (87%). Salivary IgA levels strongly correlated with increased convalescent serum IgA titers and blockade antibodies.

CONCLUSIONS

Salivary IgA levels strongly correlated with serum IgA titers and blockade antibodies and remained elevated 3 months after a norovirus outbreak. A single salivary sample collected on day 14 could be used to identify recent infection in a suspected outbreak or to monitor population salivary IgA.

Norovirus-Specific CD8+ T Cell Responses in Human Blood and Tissues

BACKGROUND & AIMS

Noroviruses (NoVs) are the leading cause of acute gastroenteritis worldwide and are associated with significant morbidity and mortality. Moreover, an asymptomatic carrier state can persist following acute infection, promoting NoV spread and evolution. Thus, defining immune correlates of NoV protection and persistence is needed to guide the development of future vaccines and limit viral spread. Whereas antibody responses following NoV infection or vaccination have been studied extensively, cellular immunity has received less attention. Data from the mouse NoV model suggest that T cells are critical for preventing persistence and achieving viral clearance, but little is known about NoV-specific T-cell immunity in humans, particularly at mucosal sites.

METHODS

We screened peripheral blood mononuclear cells from 3 volunteers with an overlapping NoV peptide library. We then used HLA-peptide tetramers to track virus-specific CD8⁺ T cells in peripheral, lymphoid, and intestinal tissues. Tetramer⁺ cells were further characterized using markers for cellular trafficking, exhaustion, cytotoxicity, and proliferation.

RESULTS

We defined 7 HLA-restricted immunodominant class I epitopes that were highly conserved across pandemic strains from genogroup II.4. NoV-specific CD8⁺ T cells with central, effector, or tissue-resident memory phenotypes were present at all sites and were especially abundant in the intestinal lamina propria. The properties and differentiation states of tetramer⁺ cells varied across donors and epitopes.

CONCLUSIONS

Our findings are an important step toward defining the breadth, distribution, and properties of human NoV T-cell immunity. Moreover, the molecular tools we have developed can be used to evaluate future vaccines and engineer novel cellular therapeutics.

FUT2, Secretor Status and FUT3 Polymorphisms of Children with Acute Diarrhea Infected with Rotavirus and Norovirus in Brazil

Host susceptibility according to human histo-blood group antigens (HBGAs) is widely known for norovirus infection, but is less described for rotavirus. Due to the variable HBGA polymorphism among populations, we aimed to evaluate the association between HBGA phenotypes (ABH, Lewis and secretor status) and susceptibility to rotavirus and norovirus symptomatic infection, and the polymorphisms of FUT2 and FUT3, of children from southeastern Brazil. Paired fecal-buccal specimens from 272 children with acute diarrhea were used to determine rotavirus/norovirus genotypes and HBGAs phenotypes/genotypes, respectively. Altogether, 100 (36.8%) children were infected with rotavirus and norovirus. The rotavirus P[8] genotype predominates (85.7%). Most of the noroviruses (93.8%) belonged to genogroup II (GII). GII.4 Sydney represented 76% (35/46) amongst five other genotypes. Rotavirus and noroviruses infected predominantly children with secretor status (97% and 98.5%, respectively). However, fewer rotavirus-infected children were Lewis-negative (8.6%) than the norovirus-infected ones (18.5%). FUT3 single nucleotide polymorphisms (SNP) occurred mostly at the T59G > G508A > T202C > C314T positions. Our results reinforce the current knowledge that secretors are more susceptible to infection by both rotavirus and norovirus than non-secretors. The high rate for Lewis negative (17.1%) and the combination of SNPs, beyond the secretor status, may reflect the highly mixed population in Brazil.

Bile Facilitates Human Norovirus Interactions with Diverse Histoblood Group Antigens, Compensating for Capsid Microvariation Observed in 2016-2017 GII.2 Strains

Human norovirus (HuNoV) is the leading cause of global infectious acute gastroenteritis, causing ~20% of reported diarrheal episodes. Typically, GII.4 strains cause 50-70% of yearly outbreaks, and pandemic waves of disease approximately every 2-7 years due to rapid evolution. Importantly, GII.4 dominance is occasionally challenged by the sudden emergence of other GII strains, most recently by GII.2 strains which peaked in 2016-2017, dramatically increasing from 1% to 20% of total HuNoV outbreaks. To determine if viral capsid evolution may account for the sudden rise in GII.2 outbreaks, Virus Like Particles (VLPs) of two 2016-2017 GII.2 strains were compared by antigenic and histo blood group antigen (HBGA) binding profiles to the prototypic 1976 GII.2 Snow Mountain Virus (SMV) strain. Despite >50 years of GII.2 strain persistence in human populations, limited sequence diversity and antigenic differences were identified between strains. However, capsid microvariation did affect HBGA binding patterns, with contemporary strains demonstrating decreased avidity for type A saliva. Furthermore, bile salts increased GII.2 VLP avidity for HBGAs, but did not alter antigenicity. These data indicate that large changes in antigenicity or receptor binding are unlikely to explain GII.2 emergence, in contrast to the pandemic GII.4 strains, and indicate that host factors such as waning or remodeling of serum or mucosal immunity likely contributed to the surge in GII.2 prevalence.

Norovirus in health care and implications for the immunocompromised host

PURPOSE OF REVIEW

The majority of norovirus outbreaks in the United States occur in healthcare facilities. With the growing population of immunocompromised hosts who are in frequent contact with healthcare facilities, norovirus is not only a threat to hospitals and nursing homes but also to these individuals. This review summarizes the impact of norovirus infection on healthcare facilities and immunocompromised hosts.

RECENT FINDINGS

The natural history of norovirus infection in immunocompromised individuals remains poorly understood. Although host immune responses play a critical role in reducing duration of viral shedding and viral load in norovirus-infected individuals, why some immunocompromised patients spontaneously recover while others develop a chronic and protracted course of illness remains unclear. Norovirus outbreaks occur in healthcare facilities because the virus is highly contagious, resistant to disinfection and efficiently transmitted. The use of real-time metagenomic next-generation sequencing and phylogenetic analyses has provided valuable information on transmission patterns in complex hospital-associated norovirus outbreaks. The development of human intestinal enteroid cultures enables the determination of effectiveness of disinfectants against human noroviruses, circumventing the validity questions with surrogate virus models due to differences in susceptibility to inactivation and disinfectants.

SUMMARY

Metagenomics next-generation sequencing can enhance our understanding of norovirus transmission and lead to more timely mitigation strategies to curb norovirus outbreaks in healthcare facilities. With new in-vitro cultivation methods for human noroviruses, candidate vaccines and effective antivirals could be available in the near future.

Noroviruses are highly infectious but there is strong variation in host susceptibility and virus pathogenicity

Noroviruses are a major public health concern: their high infectivity and environmental persistence have been documented in several studies. Genetic sequencing shows that noroviruses are highly variable, and exhibit rapid evolution. A few human challenge studies have been performed with norovirus, leading to estimates of their infectivity. However, such incidental estimates do not provide insight into the biological variation of the virus and the interaction with its human host. To study the variation in infectivity and pathogenicity of norovirus, multiple challenge studies must be analysed jointly, to compare their differences and describe how virus infectivity and host susceptibility vary. Since challenge studies can only provide a small sample of the diversity in the natural norovirus population, outbreaks should be exploited as an additional source of information. The present study shows how challenge studies and 'natural experiments' can be combined in a multilevel dose response framework. Infectivity and pathogenicity are analysed by secretor status as a host factor, and genogroup as a pathogen factor. Infectivity, characterized as the estimated mean infection risk when exposed to 1 genomic copy (qPCR unit)is 0.28 for GI norovirus, and 0.076 for GII virus, both in Se⁺ subjects. The corresponding risks of acute enteric illness are somewhat lower, about 0.2 (GI) and 0.035 (GII), in outbreaks. Se^- subjects are protected, with substantially lower risks of infection (0.00007 and 0.015 at a dose of 1 GC of GI and GII virus, respectively). The present study shows there is considerable variability in risk of infection and especially risk of acute symptoms following infection with norovirus. These challenge and outbreak data consistently indicate high infectivity among secretor positives and protection in secretor negatives.

Interaction between norovirus and Histo-Blood Group Antigens: A key to understanding virus transmission and inactivation through treatments?

Human noroviruses (HuNoVs) are a main cause of acute gastroenteritis worldwide. They are frequently involved in foodborne and waterborne outbreaks. Environmental transmission of the virus depends on two main factors: the ability of viral particles to remain infectious and their adhesion capacity onto different surfaces. Until recently, adhesion of viral particles to food matrices was mainly investigated by considering non-specific interactions (e.g. electrostatic, hydrophobic) and there was only limited information about infectious HuNoVs because of the absence of a reliable in vitro HuNoV cultivation system. Many HuNoV strains have now been described as having specific binding interactions with human Histo-Blood Group Antigens (HBGAs) and non-HBGA ligands found in food and the environment. Relevant approaches to the in vitro replication of HuNoVs were also proposed recently. On the basis of the available literature data, this review discusses the opportunities to use this new knowledge to obtain a better understanding of HuNoV transmission to human populations and better evaluate the hazard posed by HuNoVs in foodstuffs and the environment.

Human Norovirus Proteins: Implications in the Replicative Cycle, Pathogenesis, and the Host Immune Response

Human noroviruses (HuNoVs) are the cause of more than 95% of epidemic non-bacterial gastroenteritis worldwide, with some lethal cases. These viral agents affect people of all ages. However, young children and older adults are the highest-risk groups, being affected with the greatest rate of hospitalizations and morbidity cases. HuNoV structural proteins, especially VP1, have been studied extensively. In contrast, the functions of the non-structural proteins of the virus have been undescribed in depth. Studies on HuNoV non-structural proteins have mostly been made by expressing them individually in in vitro cultures, providing insights of their functions and the role that they play in HuNoV replication and pathogenesis. This review examines exhaustively the functions of both HuNoV structural and non-structural proteins and their possible role within the viral replicative cycle and the pathogenesis of the virus. It also highlights recent findings regarding the host's innate and adaptive immune responses against HuNoV, which are of great relevance for diagnostics and vaccine development so as to prevent infections caused by these fastidious viruses.

Recent advances in human norovirus research and implications for candidate vaccines

INTRODUCTION

Noroviruses are a leading cause of acute gastroenteritis worldwide. An estimated 21 million illnesses in the United States and upwards of 684 million illnesses worldwide are attributed to norovirus infection. There are no licensed vaccines to prevent norovirus, but several candidates are in development.

AREAS COVERED

We review recent advances in molecular epidemiology of noroviruses, immunology, and in-vitro cultivation of noroviruses using human intestinal enteroids. We also provide an update on the status of norovirus vaccine candidates.

EXPERT OPINION

Molecular epidemiological studies confirm the tremendous genetic diversity of noroviruses, the continuous emergence of new recombinant strains, and the predominance of GII.4 viruses worldwide. Duration of immunity, extent of cross protection between different genotypes, and differences in strain distribution for young children compared with adults remain key knowledge gaps. Recent discoveries regarding which epitopes are targeted by neutralizing antibodies using the novel in vitro culture of human noroviruses in human intestinal enteroids are enhancing our understanding of mechanisms of protection and providing guidance for vaccine development. A future norovirus vaccine has the potential to substantially reduce the burden of illnesses due to this ubiquitous virus.

Vero Cells as a Mammalian Cell Substrate for Human Norovirus

Human norovirus (HuNoV) is a principal cause of acute gastroenteritis worldwide, particularly in developing countries. Its global prevalence is underscored by more serious morbidity and some mortality in the young (<5 years) and the elderly. To date, there are no licensed vaccines or approved therapeutics for HuNoV, mostly because there are limited cell culture systems and small animal models available. Recently described cell culture systems are not ideal substrates for HuNoV vaccine development because they are not clonal or only support a single strain. In this study, we show Vero cell-based replication of two pandemic GII.4 HuNoV strains and one GII.3 strain and confirm exosome-mediated HuNoV infection in Vero cells. Lastly, we show that trypsin addition to virus cultures or disruption of Vero cell host genes can modestly increase HuNoV replication. These data provide support for Vero cells as a cell culture model for HuNoV.

Virus-Host Interactions Between Nonsecretors and Human Norovirus

BACKGROUND & AIMS

Human norovirus infection is the leading cause of acute gastroenteritis. Genetic polymorphisms, mediated by the FUT2 gene (secretor enzyme), define strain susceptibility. Secretors express a diverse set of fucosylated histoblood group antigen carbohydrates (HBGA) on mucosal cells; nonsecretors (FUT2-/-) express a limited array of HBGAs. Thus, nonsecretors have less diverse norovirus strain infections, including resistance to the epidemiologically dominant GII.4 strains. Because future human norovirus vaccines will comprise GII.4 antigen and because secretor phenotype impacts GII.4 infection and immunity, nonsecretors may mimic young children immunologically in response to GII.4 vaccination, providing a needed model to study cross-protection in the context of limited pre-exposure.

METHODS

By using specimens collected from the first characterized nonsecretor cohort naturally infected with GII.2 human norovirus, we evaluated the breadth of serologic immunity by surrogate neutralization assays, and cellular activation and cytokine production by flow cytometry.

RESULTS

GII.2 infection resulted in broad antibody and cellular immunity activation that persisted for at least 30 days for T cells, monocytes, and dendritic cells, and for 180 days for blocking antibody. Multiple cellular lineages expressing interferon-γ and tumor necrosis factor-α dominated the response. Both T-cell and B-cell responses were cross-reactive with other GII strains, but not GI strains. To promote entry mechanisms, inclusion of bile acids was essential for GII.2 binding to nonsecretor HBGAs.

CONCLUSIONS

These data support development of within-genogroup, cross-reactive antibody and T-cell immunity, key outcomes that may provide the foundation for eliciting broad immune responses after GII.4 vaccination in individuals with limited GII.4 immunity, including young children.

Snow Mountain Virus recovery by synthetic human histo-blood group antigens is heavily influenced by matrix effects

Noroviruses are known to bind to histo-blood group antigens (HBGAs) and the specific binding patterns depend on the virus genotype. However, the development of point-of-care diagnostic assays based on this binding has been challenging due to low assay sensitivity. This study utilized a well-defined stool collection from a GII.2 Snow Mountain Virus (SMV) human challenge study to investigate virus recovery from stool and emesis samples using HBGA-coated beads. SMV was recovered from H type III-coated beads for 13 stool specimens out of 27 SMV-positive specimens tested. After adjusting for non-specific binding to PEG-coated beads, the mean percent recovery by H type III-coated beads was 308.11% +/− 861.61. Recovery by H type III ligands was subject-specific and weakly correlated with stool consistency. Input virus titer was not correlated with SMV recovery. The results suggest that the generally low virus recovery we observed may be due to bead saturation or hindrance by existing glycans in the matrix that precluded the virus from being captured by the synthetic glycans. These results indicate a strong role for subject-specific and matrix effects in HBGA binding by SMV. Further investigation of the nature of this interference is needed to facilitate development of high sensitivity diagnostic assays.

Production and Clinical Evaluation of Norwalk GI.1 Virus Lot 001-09NV in Norovirus Vaccine Development

BACKGROUND

Human noroviruses (HuNoV) are the leading cause of gastroenteritis. No vaccine is currently available to prevent norovirus illness or infection. Safe, infectious challenge strains are needed to assess vaccine efficacy in the controlled human infection model (CHIM).

METHODS

A stock of HuNoV strain Norwalk virus ([NV] GI.1) was prepared. Healthy, genetically susceptible adults were inoculated with NV Lot 001-09NV and monitored for infection, gastroenteritis symptoms, and immune responses.

RESULTS

Lot 001-09NV induced gastroenteritis in 9 (56%) and infection in 11 (69%) of 16 genetically susceptible subjects. All infected subjects developed strong immune responses to GI.1 with a 30-fold (geometric mean titer) increase in blocking titers (BT50) and a 161-fold increase in GI.1-specific immunoglobulin (Ig)G titers when compared with baseline. GI.1-specific cellular responses in peripheral blood were observed 9 days postchallenge with an average of 3253 IgA and 1227 IgG antibody-secreting cells per million peripheral blood mononuclear cells.

CONCLUSIONS

GI.1 Lot 001-09NV appears to be similar in virulence to previous passages of NV strain 8fIIa. The safety profile, attack rate, and duration of illness make GI.1 Lot 001-09NV a useful challenge strain for future vaccine studies aimed at establishing immune correlates.

ABO blood group-associated susceptibility to norovirus infection: A systematic review and meta-analysis

BACKGROUND

Norovirus is responsible for the viral gastroenteritis burden worldwide. Histo-blood type antigens (HBGAs) are the only well-known factor regarding their effect on the pathogenesis of norovirus. Here, we performed the study to further investigate the association of the ABO blood group with norovirus susceptibility.

METHODS

All relevant studies were retrieved from PubMed, Embase, Web of Science, and Cochrane Library databases and the associations of ABO blood groups with norovirus were assessed. The pooled odds ratios (ORs) and 95% confidence interval (CI) were calculated from extracted data. I² statistics, sensitivity analysis, and publication bias were used to confirm the findings. Subgroup analyses were performed for genotypes, publication years, development degree of the countries, and age if heterogeneity was recorded.

RESULTS

Seventeen articles covering 2304 participants were included. The overall analysis of the studies showed similar ORs of norovirus infection among individuals with blood type A (OR = 0.90, 95%CI = 0.71-1.14, P = 0.37) and blood type B (OR = 0.85, 95%CI = 0.66-1.12, P = 0.25) as compared to those controls. An increased odds of norovirus infection was found among individuals with blood type O (OR = 1.28, 95%CI = 1.03-1.59, P = 0.03), while the individuals with blood type AB (OR = 0.91, 95%CI = 0.60-1.39, P = 0.67) showed no correlation with norovirus infection. For blood type B and blood type AB, the results of subgroup analyses mirrored the observations above.

CONCLUSIONS

The meta-analysis suggested that the blood type A, B and AB might not affect susceptibility to norovirus infection. However, blood type O appeared to be more susceptible to norovirus infection.

Norovirus Seroprevalence among Adults in the United States: Analysis of NHANES Serum Specimens from 1999-2000 and 2003-2004

Norovirus is the most common cause of epidemic and endemic acute gastroenteritis. However, national estimates of the infection burden are challenging. This study used a nationally representative serum bank to estimate the seroprevalence to five norovirus genotypes including three GII variants: GI.1 Norwalk, GI.4, GII.3, GII.4 US95/96, GII.4 Farmington Hills, GII.4 New Orleans, and GIV.1 in the USA population (aged 16 to 49 years). Changes in seroprevalence to the three norovirus GII.4 variants between 1999 and 2000, as well as 2003 and 2004, were measured to examine the role of population immunity in the emergence of pandemic GII.4 noroviruses. The overall population-adjusted seroprevalence to any norovirus was 90.0% (1999 to 2000) and 95.9% (2003 to 2004). Seroprevalence was highest to GI.1 Norwalk, GII.3, and the three GII.4 noroviruses. Seroprevalence to GII.4 Farmington Hills increased significantly between the 1999 and 2000, as well as the 2003 and 2004, study cycles, consistent with the emergence of this pandemic strain. Seroprevalence to GII.4 New Orleans also increased over time, but to a lesser degree. Antibodies against the GIV.1 norovirus were consistently detected (population-adjusted seroprevalence 19.1% to 25.9%), with rates increasing with age. This study confirms the high burden of norovirus infection in US adults, with most adults having multiple norovirus infections over their lifetime.

Minimally Invasive Saliva Testing to Monitor Norovirus Infection in Community Settings

Background

Norovirus is a leading cause of acute gastroenteritis worldwide. Routine norovirus diagnosis requires stool collection. The goal of this study was to develop and validate a noninvasive method to diagnose norovirus to complement stool diagnostics and to facilitate studies on transmission.

Methods

A multiplex immunoassay to measure salivary immunoglobulin G (IgG) responses to 5 common norovirus genotypes (GI.1, GII.2, GII.4, GII.6, and GII.17) was developed. The assay was validated using acute and convalescent saliva samples collected from Peruvian children <5 years of age with polymerase chain reaction (PCR)–diagnosed norovirus infections (n = 175) and controls (n = 32). The assay sensitivity and specificity were calculated to determine infection status based on fold rise of salivary norovirus genotype-specific IgG using norovirus genotype from stool as reference.

Results

The salivary assay detected recent norovirus infections and correctly assigned the infecting genotype. Sensitivity was 71% and specificity was 96% across the evaluated genotypes compared to PCR-diagnosed norovirus infection.

Conclusions

This saliva-based assay will be a useful tool to monitor norovirus transmission in high-risk settings such as daycare centers or hospitals. Cross-reactivity is limited between the tested genotypes, which represent the most commonly circulating genotypes.

The Coxsackievirus and Adenovirus Receptor, a Required Host Factor for Recovirus Infection, Is a Putative Enteric Calicivirus Receptor

Human norovirus (HuNoV) is a leading cause of acute gastroenteritis in both developed and developing countries. Studies of HuNoV host cell interactions are limited by the lack of a simple, robust cell culture system. Due to their diverse HuNoV-like biological features, including histo-blood group antigen (HBGA) binding, rhesus enteric caliciviruses (ReCVs) are viable surrogate models for HuNoVs. In addition, several ReCV strains can be propagated to high titers in standard nonhuman primate cell lines while causing lytic infection and cell death. To identify the ReCV entry receptor, we performed CRISPR/Cas9 library screening in Vero cells, which identified the coxsackievirus and adenovirus receptor (CAR) as a candidate ReCV entry receptor. We showed that short interfering RNA, anti-human CAR (hCAR) monoclonal antibody RmcB treatment, and recombinant hCAR ectodomain blocked ReCV replication in LLC-MK2 cells. CRISPR/Cas9-targeted knockout of CAR in LLC-MK2 and Vero cells made these cell lines resistant to ReCV infection, and susceptibility to infection could be restored by transient expression of CAR. CHO cells do not express CAR or HBGAs and are resistant to ReCV infection. Recombinant CHO cells stably expressing hCAR or the type B HBGA alone did not support ReCV infection. However, CHO cells expressing both hCAR and the type B HBGA were susceptible to ReCV infection. In summary, we have demonstrated that CAR is required for ReCV infection and most likely is a functional ReCV receptor, but HBGAs are also necessary for infection.IMPORTANCE Because of the lack of a simple and robust human norovirus (HuNoV) cell culture system surrogate, caliciviruses still represent valuable research tools for norovirus research. Due to their remarkable biological similarities to HuNoVs, including the utilization of HBGAs as putative attachment receptors, we used rhesus enteric caliciviruses (ReCVs) to study enteric calicivirus host cell interactions. Using CRISPR/Cas9 library screening and functional assays, we identified and validated the coxsackievirus and adenovirus receptor (CAR) as a functional proteinaceous receptor for ReCVs. Our work demonstrated that CAR and HBGAs both are necessary to convert a nonsusceptible cell line to being susceptible to ReCV infection. Follow-up studies to evaluate the involvement of CAR in HuNoV infections are ongoing.

Human noroviruses: recent advances in a 50-year history

PURPOSE OF REVIEW

Noroviruses are a major cause of gastroenteritis. This review summarizes new information on noroviruses that may lead to the development of improved measures for limiting their human health impact.

RECENT FINDINGS

GII.4 strains remain the most common human noroviruses causing disease, although GII.2 and GII.17 strains have recently emerged as dominant strains in some populations. Histo-blood group antigen (HBGA) expression on the gut mucosa drives susceptibility to different norovirus strains. Antibodies that block virus binding to these glycans correlate with protection from infection and illness. Immunocompromised patients are significantly impacted by norovirus infection, and the increasing availability of molecular diagnostics has improved infection recognition. Human noroviruses can be propagated in human intestinal enteroid cultures containing enterocytes that are a significant primary target for initiating infection. Strain-specific requirements for replication exist with bile being essential for some strains. Several vaccine candidates are progressing through preclinical and clinical development and studies of potential antiviral interventions are underway.

SUMMARY

Norovirus epidemiology is complex and requires continued surveillance to track the emergence of new strains and recombinants, especially with the continued progress in vaccine development. Humans are the best model to study disease pathogenesis and prevention. New in-vitro cultivation methods should lead to better approaches for understanding virus-host interactions and ultimately to improved strategies for mitigation of human norovirus-associated disease.

Rotavirus VP6 as an Adjuvant for Bivalent Norovirus Vaccine Produced in Nicotiana benthamiana

Rotaviruses (RVs) and noroviruses (NoVs) are major causes of childhood acute gastroenteritis. During development of a combination vaccine based on NoV virus-like particles (VLP) and RV VP6 produced in baculovirus expression system in insect cells, a dual role of VP6 as a vaccine antigen and an adjuvant for NoV-specific immune responses was discovered. Here the VP6 adjuvant effect on bivalent GI.4 and GII.4-2006a NoV VLPs produced in Nicotiana benthamiana was investigated. BALB/c mice were immunized intradermally with suboptimal (0.3 µg) dose of each NoV VLP alone or combined with 10 µg of VP6, or equal doses of NoV VLPs and VP6 (1 µg/antigen). NoV-specific serum IgG antibodies and their blocking activity were analyzed using vaccine-homologous and heterologous NoV VLPs. Immunization with 0.3 µg NoV VLPs alone was insufficient to induce NoV-specific immune responses, but with co-administration of 10 µg of VP6, antibodies against vaccine-derived and heterologous NoV genotypes were generated. Furthermore, corresponding adjuvant effect of VP6 was observed with 1 µg dose. Efficient uptake and presentation of VP6 by dendritic cells was demonstrated in vitro. These results show that adjuvant effect of VP6 on bivalent NoV VLP vaccine is independent of the cell source used for vaccine production.

The Antigenic Topology of Norovirus as Defined by B and T Cell Epitope Mapping: Implications for Universal Vaccines and Therapeutics

Human norovirus (HuNoV) is the leading cause of acute nonbacterial gastroenteritis. Vaccine design has been confounded by the antigenic diversity of these viruses and a limited understanding of protective immunity. We reviewed 77 articles published since 1988 describing the isolation, function, and mapping of 307 unique monoclonal antibodies directed against B cell epitopes of human and murine noroviruses representing diverse Genogroups (G). Of these antibodies, 91, 153, 21, and 42 were reported as GI-specific, GII-specific, MNV GV-specific, and G cross-reactive, respectively. Our goal was to reconstruct the antigenic topology of noroviruses in relationship to mapped epitopes with potential for therapeutic use or inclusion in universal vaccines. Furthermore, we reviewed seven published studies of norovirus T cell epitopes that identified 18 unique peptide sequences with CD4- or CD8-stimulating activity. Both the protruding (P) and shell (S) domains of the major capsid protein VP1 contained B and T cell epitopes, with the majority of neutralizing and HBGA-blocking B cell epitopes mapping in or proximal to the surface-exposed P2 region of the P domain. The majority of broadly reactive B and T cell epitopes mapped to the S and P1 arm of the P domain. Taken together, this atlas of mapped B and T cell epitopes offers insight into the promises and challenges of designing universal vaccines and immunotherapy for the noroviruses.

Genetic Susceptibility to Human Norovirus Infection: An Update

Noroviruses are the most common etiological agent of acute gastroenteritis worldwide. Despite their high infectivity, a subpopulation of individuals is resistant to infection and disease. This susceptibility is norovirus genotype-dependent and is largely mediated by the presence or absence of human histo-blood group antigens (HBGAs) on gut epithelial surfaces. The synthesis of these HBGAs is mediated by fucosyl- and glycosyltransferases under the genetic control of the FUT2 (secretor), FUT3 (Lewis) and ABO(H) genes. The so-called non-secretors, having an inactivated FUT2 enzyme, do not express blood group antigens and are resistant to several norovirus genotypes, including the predominant GII.4. Significant genotypic and phenotypic diversity of HBGA expression exists between different human populations. Here, we review previous in vivo studies on genetic susceptibility to norovirus infection. These are discussed in relation to population susceptibility, vaccines, norovirus epidemiology and the impact on public health.

Development of T cell immunity to norovirus and rotavirus in children under five years of age

Most of the research effort to understand protective immunity against norovirus (NoV) has focused on humoral immunity, whereas immunity against another major pediatric enteric virus, rotavirus (RV), has been studied more thoroughly. The aim of this study was to investigate development of cell-mediated immunity to NoV in early childhood. Immune responses to NoV GI.3 and GII.4 virus-like particles and RV VP6 were determined in longitudinal blood samples of 10 healthy children from three months to four years of age. Serum IgG antibodies were measured using enzyme-linked immunosorbent assay and production of interferon-gamma by peripheral blood T cells was analyzed by enzyme-linked immunospot assay. NoV-specific T cells were detected in eight of 10 children by the age of four, with some individual variation. T cell responses to NoV GII.4 were higher than those to GI.3, but these responses were generally lower than responses to RV VP6. In contrast to NoV-specific antibodies, T cell responses were transient in nature. No correlation between cell-mediated and antibody responses was observed. NoV exposure induces vigorous T cell responses in children under five years of age, similar to RV. A role of T cells in protection from NoV infection in early childhood warrants further investigation.

Identification of a First Human Norovirus CD8+ T Cell Epitope Restricted to HLA-A*0201 Allele

Norovirus (NoV) causes a substantial global burden of acute gastroenteritis in all age groups and the development of NoV vaccine is a high priority. There are still gaps in understanding of protective NoV-specific immunity. Antibody mediated immune responses have been widely studied, but in contrast, the research on NoV-specific human T cell-mediated immunity is very limited. We have recently reported NoV capsid VP1-specific 18-mer peptide (¹³⁴SPSQVTMFPHIIVDVRQL¹⁵¹) to induce strong CD8⁺ T cell immune responses in healthy adult donors. This work extends to identify the precise NoV T cell epitope and the restricting human leucocyte antigen (HLA). Pentamer technology was used to detect HLA-A^*0201-restricted T cell-mediated responses to 10-mer peptide ¹³⁹TMFPHIIVDV¹⁴⁸ of four healthy adult blood donors. Immunogenicity of the 10-mer epitope was confirmed by ELISPOT IFN-γ and intracellular cytokine staining (ICS) on flow cytometry. A population of CD3⁺CD8⁺ T lymphocytes binding to HLA-A^*0201/TMFPHIIVDV pentamers was identified in two HLA-A^*0201-positive donors. Recognition of the 10-mer epitope by T cells resulted in a strong IFN-γ secretion as shown by ELISPOT assay. In addition, ICS confirmed that high proportion (31 and 59%) of the TMFPHIIVDV epitope-responsive CD3⁺CD8⁺ T cells in the two donors had multifunctional phenotype, simultaneously producing IFN-γ, IL-2 and TNF-α cytokines. In the present study novel human NoV HLA-A^*0201-restricted minimal 10-mer epitope ¹³⁹TMFPHIIVDV¹⁴⁸ in the capsid VP1 was identified. The HLA-peptide pentamer staining of T cells from healthy donor PBMCs and cytokine responses in ex-vivo ELISPOT and ICS assays suggest that this epitope is recognized during NoV infection and activates memory phenotype of the epitope-specific multifunctional CD8⁺ T cells. The importance of this epitope in protection from NoV infection remains to be determined.

Enteric Infections Are Common in Patients with Flares of Inflammatory Bowel Disease

OBJECTIVES

Few studies have examined the role of non-Clostridium difficile enteric infections in flares of inflammatory bowel disease (IBD). Our objective was to investigate enteric infection detected by multiplex PCR stool testing in patients with IBD.

METHODS

We performed a cross-sectional analysis of 9403 patients who underwent 13,231 stool tests with a gastrointestinal pathogen PCR panel during a diarrheal illness from March 2015 to May 2017. Our primary outcome was the presence of an infection. Secondary outcomes included endoscopic and histologic predictors of infection, and IBD outcomes following testing.

RESULTS

A total of 277 patients with Crohn's disease (CD), 300 patients with ulcerative colitis (UC), and 8826 patients without IBD underwent 454, 503, and 12,275 tests, respectively. Compared to patients without IBD, patients with IBD were less likely to test positive (CD 18.1%, UC 16.1%, no IBD 26.6%, p < 0.001). Compared to patients without IBD, CD had a higher prevalence of norovirus (p = 0.05) and Campylobacter (p = 0.043), whereas UC had a lower prevalence of norovirus (p = 0.001) and a higher prevalence of Campylobacter (p = 0.013), Plesiomonas (p = 0.049), and Escherichia coli species (p < 0.001). Of 77 patients who underwent endoscopy, there were no major endoscopic or histologic predictors of a positive test. Patients who tested negative were more likely to have IBD therapy escalated (p = 0.004). Enteric infection did not impact IBD outcomes following testing (log-rank 0.224).

CONCLUSIONS

Non-Clostridium difficile enteric infections were identified in 17% of symptomatic patients with IBD. Endoscopic and histologic findings may not differentiate flare from infection. Norovirus and E.coli may play an important role in flare of IBD.

Norovirus-specific mucosal antibodies correlate to systemic antibodies and block norovirus virus-like particles binding to histo-blood group antigens

The best acknowledged correlate of protection from norovirus (NoV) infection is the ability of serum antibodies to block binding of NoV virus-like particles (VLPs) to histo-blood group antigens (HBGAs). We investigated mucosal NoV-specific antibody levels in adult volunteers and used saliva from a single donor to determine whether purified saliva antibodies confer blocking. NoV-specific IgG and IgA levels in saliva and plasma samples were measured against four NoV genotype VLPs. NoV-specific IgG and IgA titers in saliva and plasma samples correlated significantly. Antibodies were detected against all VLPs with the highest level of antibodies directed against ancestral GII.4 99 genotype. Affinity chromatography purified salivary IgA and IgG blocked binding of GII.4 99 VLPs to HBGAs. Saliva sampling is a non-invasive alternative to blood drawing and an excellent biological fluid to study NoV-specific immune responses. Mucosal anti-NoV antibodies block binding of NoV VLPs to HBGAs, and may therefore be protective.

Human norovirus GII.4(MI001) P dimer binds fucosylated and sialylated carbohydrates

Human noroviruses (HuNoV), members of the family Caliciviridae, are the major cause of acute viral gastroenteritis worldwide. Successful infection is linked to the ability of the protruding (P) domain of the viral capsid to bind histo-blood group antigens (HBGA). Binding to gangliosides plays a major role for many nonhuman calici- and noroviruses. Increasing evidence points to a broader role of sialylated carbohydrates such as gangliosides in norovirus infection. Here, we compare HBGA and ganglioside binding of a GII.4 HuNoV variant (MI001), previously shown to be infectious in a HuNoV mouse model. Saturation transfer difference nuclear magnetic resonance spectroscopy, native mass spectrometry (MS) and surface plasmon resonance spectroscopy were used to characterize binding epitopes, affinities, stoichiometry and dynamics, focusing on 3'-sialyllactose, the GM3 ganglioside saccharide and B antigen. Binding was observed for 3'-sialyllactose and various HBGAs following a multistep binding process. Intrinsic affinities (Kd) of fucose, 3'-sialyllactose and B antigen were determined for the individual binding steps. Stronger affinities were observed for B antigen over 3'-sialyllactose and fucose, which bound in the mM range. Binding stoichiometry was analyzed by native MS showing the presence of four B antigens or two 3'-sialyllactose in the complex. Epitope mapping of 3'-sialyllactose revealed direct interaction of α2,3-linked sialic acid with the P domain. The ability of HuNoV to engage multiple carbohydrates emphasizes the multivalent nature of norovirus glycan-specificity. Our findings reveal direct binding of a GII.4 HuNoV P dimer to α2,3-linked sialic acid and support a broader role of ganglioside binding in norovirus infection.

Norovirus: The Burden of the Unknown

Human noroviruses (HNoVs) are primarily transmitted by the fecal-oral route, either by person-to-person contact, or by ingestion of contaminated food or water as well as by aerosolization. Moreover, HNoVs significantly contribute to foodborne diseases being the causative agent of one-fifth of acute gastroenteritis worldwide. As a consequence of globalization, transnational outbreaks of foodborne infections are reported with increasing frequency. Therefore, in this review, state-of-the-art information regarding molecular procedures for human norovirus detection in food as well common food processing technologies have been summarized. Besides, the purpose of this chapter is to consolidate basic information on various aspects of HNoVs and to summarize food processing technologies that can potentially be applied in the food industry.

Functionality and avidity of norovirus-specific antibodies and T cells induced by GII.4 virus-like particles alone or co-administered with different genotypes

Norovirus (NoV) is the main cause of acute gastroenteritis worldwide across all age groups. Current NoV vaccine candidates are based on non-infectious highly immunogenic virus-like particles (VLPs) produced in cell cultures in vitro. As NoVs infecting human population are highly divergent, it is proposed that the vaccine should contain at least two different NoV genotypes, potentially affecting the immunogenicity of each other. We investigated the immunogenicity of NoV GII.4 VLPs administered by intramuscular (IM) or intradermal (ID) injections to BALB/c mice either alone or co-delivered with genogroup I (GI) and other genogroup GII VLPs. Serum NoV-specific IgG binding antibody titers and antibody functionality in terms of avidity and blocking potential were assessed. Furthermore, the specificity and functional avidity of CD4⁺ and CD8⁺ T cell responses were analyzed using synthetic peptides previously identified to contain NoV VP1 P2 domain-specific H-2^d epitopes. The results showed that IM and ID immunization induced comparable GII.4-specific antibodies and T cell responses. Similar magnitude and functionality of antibodies and interferon-gamma producing T cells were developed using monovalent GII.4 VLPs or different genotype combinations. For the first time, degranulation assay using multicolor flow cytometry showed that NoV GII.4-specific CD8⁺ T cells had cytotoxic T lymphocyte phenotype. To conclude, our results demonstrate that there is no immunological interference even if up to five different NoV VLP genotypes were co-administered at the same time. Furthermore, no inhibition of NoV-specific antibody functionality or the magnitude, specificity and affinity of T cell responses was observed in any of the immunized animals, observations relevant for the development of a multivalent NoV VLP vaccine.