Antigenic Characterization of a Novel Recombinant GII.P16-GII.4 Sydney Norovirus Strain With Minor Sequence Variation Leading to Antibody Escape

Sensitive Electrochemical and Thermal Detection of Human Noroviruses Using Molecularly Imprinted Polymer Nanoparticles Generated against a Viral Target

Norovirus (NoV) is the predominant cause of foodborne illness globally; current detection methods are typically expensive, have inadequate sensitivities, and utilize biological receptors with poor stability. Therefore, accurate, cost-effective, and highly stable detection methods are needed to screen for NoV in foods. We developed molecularly imprinted polymer nanoparticles (nanoMIPs) to detect NoV using a small target epitope (12 amino acids) with a solid-phase synthesis approach. The performance of three batches of nanoMIPs with varying monomer compositions (nanoMIP-1, -2, and -3) were compared both experimentally and computationally. Surface plasmon resonance examined nanoMIP binding affinity to norovirus virus-like particles (NoV-LPs), whereby nanoMIP-1 had the lowest KD value of 0.512 μM. This is significant, as traditional targets for generation of norovirus ligands previously reported were generated against drastically larger norovirus capsid segments that have limitations in ease of production. Further, an electrochemical sensor was developed by covalently attaching the nanoMIPs to glassy carbon electrodes. In agreement with our predictions from density functional theory simulations, electrochemical impedance spectroscopy showed a sensitive response toward NoV-LPs for nanoMIP batches tested; however, nanoMIP-1 was optimal, with an excellent detection limit of 3.4 pg/mL (1.9 × 105 particles/mL). Due to its exceptional performance, nanoMIP-1 was immobilized to screen-printed electrodes and utilized within a thermal sensor, where it exhibited a low detection limit of 6.5 pg/mL (3.7 × 105 particles/mL). Crucially, we demonstrated that nanoMIP-1 could detect NoV in real food samples (romaine lettuce) by using electrochemical and thermal sensors. Consequently, the study highlights the exceptional potential of nanoMIPs to replace traditional biological materials (e.g., antibodies) as sensitive, versatile, and highly stable receptors within NoV sensors.

Norovirus evolves as one or more distinct clonal populations in immunocompromised hosts

IMPORTANCE

Noroviruses are an important cause of chronic diarrhea in patients with compromised immune systems. Presently, there are no effective therapies to clear the virus, which can persist for years in the intestinal tract. The goal of our study was to develop a better understanding of the norovirus strains that are associated with these long-term infections. With the remarkable diversity of norovirus strains detected in the immunocompromised patient cohort we studied, it appears that most, if not all, noroviruses circulating in nature may have the capacity to establish a chronic infection when a person is unable to mount an effective immune response. Our work is the most comprehensive genetic data set generated to date in which near full-length genomes from noroviruses associated with chronic infection were analyzed by high-resolution next-generation sequencing. Analysis of this data set led to our discovery that certain patients in our cohort were shedding noroviruses that could be subdivided into distinct haplotypes or populations of viruses that were co-evolving independently. The ability to track haplotypes of noroviruses during chronic infection will allow us to fine-tune our understanding of how the virus adapts and maintains itself in the human host, and how selective pressures such as antiviral drugs can affect these distinct populations.

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

IMPORTANCE

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

Mapping human norovirus antigens during infection reveals the breadth of the humoral immune response

Human noroviruses (HuNoV) are the leading cause of acute gastroenteritis worldwide. The humoral immune response plays an important role in clearing HuNoV infections and elucidating the antigenic landscape of HuNoV during an infection can shed light on antibody targets to inform vaccine design. Here, we utilized Jun-Fos-assisted phage display of a HuNoV genogroup GI.1 genomic library and deep sequencing to simultaneously map the epitopes of serum antibodies of six individuals infected with GI.1 HuNoV. We found both unique and common epitopes that were widely distributed among both nonstructural proteins and the major capsid protein. Recurring epitope profiles suggest immunodominant antibody footprints among these individuals. Analysis of sera collected longitudinally from three individuals showed the presence of existing epitopes in the pre-infection sera, suggesting these individuals had prior HuNoV infections. Nevertheless, newly recognized epitopes surfaced seven days post-infection. These new epitope signals persisted by 180 days post-infection along with the pre-infection epitopes, suggesting a persistent production of antibodies recognizing epitopes from previous and new infections. Lastly, analysis of a GII.4 genotype genomic phage display library with sera of three persons infected with GII.4 virus revealed epitopes that overlapped with those identified in GI.1 affinity selections, suggesting the presence of GI.1/GII.4 cross-reactive antibodies. The results demonstrate that genomic phage display coupled with deep sequencing can characterize HuNoV antigenic landscapes from complex polyclonal human sera to reveal the timing and breadth of the human humoral immune response to infection.

Epidemiology and genetic diversity of norovirus GII genogroups among children in Hubei, China, 2017-2019

Norovirus (NoV) is an important cause of viral acute gastroenteritis (AGE). To gain insights into the epidemiological characteristics and genetic diversity of NoV among children in Hubei, 1216 stool samples from children (≤ 5 years) obtained under AGE surveillance from January 2017 to December 2019 were analyzed. The results showed that NoV was responsible for 14.64% of AGE cases, with the highest detection rate in children aged 7-12 months (19.76%). Statistically significant differences were found between male and female infection rates (χ2 ​= ​8.108, P ​= ​0.004). Genetic analysis of RdRp and VP1 sequences showed that NoV GII genotypes were GII.4 Sydney [P31] (34.35%), GII.3 [P12] (25.95%), GII.2 [P16] (22.90%), GII.4 Sydney [P16] (12.98%), GII.17 [P17] (2.29%), GII.6 [P7] and GII.3 [P16] (each at 0.76%). GII.17 [P17] variants were divided into the Kawasaki323-like lineage and the Kawasaki308-like lineage. A unique recombination event was detected between strains of GII.4 Sydney 2012 and GII.4 Sydney 2016. Significantly, all GII.P16 sequences associated with GII.4/GII.2 obtained in Hubei were correlated with novel GII.2 [P16] variants that re-emerged in Germany in 2016. Antigenic site analysis of complete VP1 sequences from all GII.4 variants from Hubei identified notable variable residues of antibody epitopes. Genotyping under continuous AGE surveillance and observation of the antigenic sites of VP1 are important monitoring strategies for emerging NoV strains.

Antigenic Site Immunodominance Redirection Following Repeat Variant Exposure

Human norovirus is a leading cause of acute gastroenteritis, driven by antigenic variants within the GII.4 genotype. Antibody responses to GII.4 vaccination in adults are shaped by immune memory. How children without extensive immune memory will respond to GII.4 vaccination has not been reported. Here, we characterized the GII.4 neutralizing antibody (nAb) landscape following natural infection using a surrogate assay and antigenic site chimera virus-like particles. We demonstrate that the nAb landscape changes with age and virus exposure. Among sites A, C, and G, nAbs from first infections are focused on sites A and C. As immunity develops with age/exposure, site A is supplemented with antibodies that bridge site A to sites C and G. Cross-site nAbs continue to develop into adulthood, accompanied by an increase in nAb to site G. Continued exposure to GII.4 2012 Sydney correlated with a shift to co-dominance of sites A and G. Furthermore, site G nAbs correlated with the broadening of nAb titer across antigenically divergent variants. These data describe fundamental steps in the development of immunity to GII.4 over a lifetime, and illustrate how the antigenicity of one pandemic variant could influence the pandemic potential of another variant through the redirection of immunodominant epitopes.

Epidemiological Impact of GII.17 Human Noroviruses Associated With Attachment to Enterocytes

For the last 30 years, molecular surveys have shown that human norovirus (HuNoV), predominantly the GII.4 genotype, is one of the main causative agents of gastroenteritis. However, epidemiological surveys have revealed the worldwide emergence of GII.17 HuNoVs. Genetic analysis confirmed that GII.17 strains are distributed into three variants (i.e., Kawasaki 308, Kawasaki 323, and CS-E1). Here, virus-like particles (VLPs) were baculovirus-expressed from these variants to study putative interactions with HBGA. Qualitative analysis of the HBGA binding profile of each variant showed that the most recent and predominant GII.17 variant, Kawasaki 308, possesses a larger binding spectrum. The retrospective study of GII.17 strains documented before the emergence of the dominant Kawasaki 308 variant showed that the emergence of a new GII.17 variant could be related to an increased binding capacity toward HBGA. The use of duodenal histological sections confirmed that recognition of enterocytes involved HBGA for the three GII.17 variants. Finally, we observed that the relative affinity of recent GII.17 VLPs for HBGA remains lower than that of the GII.4-2012 variant. These observations suggest a model whereby a combination of virological factors, such as polymerase fidelity and increased affinity for HBGA, and immunological factors was responsible for the incomplete and non-persistent replacement of GII.4 by new GII.17 variants.

Epidemiological and Genetic Characterization of Norovirus Outbreaks That Occurred in Catalonia, Spain, 2017–2019

Molecular characterization of human norovirus (HuNoV) genotypes enhances the understanding of viral features and illustrates distinctive evolutionary patterns. The aim of our study was to describe the prevalence of the genetic diversity and the epidemiology of the genotypes involved in HuNoV outbreaks in Catalonia (Spain) between 2017 and 2019. A total of 100 HuNoV outbreaks were notified with the predominance of GII (70%), followed by GI (27%) and mixed GI/GII (3%). Seasonality was observed for GII outbreaks only. The most prevalent genotypes identified were GII.4[P31] Sydney 2012, GII.4[P16] Sydney 2012 and GII.2[P16]. As compared to person-to-person (P/P) transmitted outbreaks, foodborne outbreaks showed significantly higher attack rates and lower duration. The average attack rate was higher in youth hostel/campgrounds compared to nursing homes. Only genotypes GI.4[P4], GII.2[P16], GII.4[P16], GII.4[P31] and GII.17[P17] were consistently detected every year, and only abundance of GII.2[P16] showed a negative trend over time. GII.4 Sydney 2012 outbreaks were significantly associated to nursing homes, while GII.2[P16] and GI.3[P3] were most frequently identified in youth hostel/campgrounds. The average attack rate was significantly higher when comparing GII.2[P16] vs. GI.4[P4], GII.2[P16] vs. GII.4[P31] Sydney 2012, and GII.6[P7] vs. GII.4[P31] Sydney 2012. No correlations were found between genotype and outbreak duration or age of affected individuals.

Molecular Epidemiology of Norovirus (NoV) Infection in Mie Prefecture: The Kinetics of Norovirus Antigenemia in Pediatric Patients

Few studies have shown the presence of norovirus (NoV) RNA in blood circulation but there is no data on norovirus antigenemia. We examined both antigenemia and RNAemia from the sera of children with NoV infections and studied whether norovirus antigenemia is correlated with the levels of norovirus-specific antibodies and clinical severity of gastroenteritis. Both stool and serum samples were collected from 63 children admitted to Mie National Hospital with acute NoV gastroenteritis. Norovirus antigen and RNA were detected in sera by ELISA and real-time RT-PCR, respectively. NoV antigenemia was found in 54.8% (34/62) and RNAemia in 14.3% (9/63) of sera samples. Antigenemia was more common in the younger age group (0–2 years) than in the older age groups, and most patients were male. There was no correlation between stool viral load and norovirus antigen (NoV-Ag) levels (r_s = −0.063; Cl −0.3150 to 0.1967; p = 0.6251). Higher levels of acute norovirus-specific IgG serum antibodies resulted in a lower antigenemia OD value (n = 61; r = −0.4258; CI −0.62 to −0.19; p = 0.0006). Norovirus antigenemia occurred more commonly in children under 2 years of age with NoV-associated acute gastroenteritis. The occurrence of antigenemia was not correlated with stool viral load or disease severity.

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.

Antigenic cartography reveals complexities of genetic determinants that lead to antigenic differences among pandemic GII.4 noroviruses

Noroviruses are the predominant cause of acute gastroenteritis, with a single genotype (GII.4) responsible for the majority of infections. This prevalence is characterized by the periodic emergence of new variants that present substitutions at antigenic sites of the major structural protein (VP1), facilitating escape from herd immunity. Notably, the contribution of intravariant mutations to changes in antigenic properties is unknown. We performed a comprehensive antigenic analysis on a virus-like particle panel representing major chronological GII.4 variants to investigate diversification at the inter- and intravariant level. Immunoassays, neutralization data, and cartography analyses showed antigenic similarities between phylogenetically related variants, with major switches to antigenic properties observed over the evolution of GII.4 variants. Genetic analysis indicated that multiple coevolving amino acid changes-primarily at antigenic sites-are associated with the antigenic diversification of GII.4 variants. These data highlight complexities of the genetic determinants and provide a framework for the antigenic characterization of emerging GII.4 noroviruses.

Next generation sequencing of near-full length genome of norovirus GII.4 from Botswana

Noroviruses are highly diverse, with genotype GII.4 causing most epidemics. This study aimed to investigate the evolutionary dynamics of norovirus genogroup GII strains among acutely infected children under 5 years in Botswana, between 2016 and 2018. Reverse transcriptase polymerase chain reaction (RT-PCR) was used to amplify the whole norovirus genome, followed by next-generation sequencing using Oxford Nanopore technology. Twelve samples were successfully analyzed, with 11 identified as norovirus GII.4 Sydney [P31] and one as GII.4 Sydney [P13]. This study generated the first near-full length norovirus sequences in Botswana (93-95% coverage). Our results show that the norovirus GII.4 strains circulating in Botswana are under evolution through recombination and antigenic drift. Recombination in the GII.4 Sydney [P31] and GII.4 Sydney [P13] strains occurred in the ORF1/ORF2 junction and within ORF1, respectively. This study provides the first description of the GII.4 Sydney [P13] recombinant. Amino acid variation in the immunogenic sites was analyzed. Mutations in epitope A correlate with the emergence of novel norovirus GII.4 strains with altered antigenicity. In this study, we identified 43 unique amino acid substitutions in the VP1 region, with six occurring in epitopes, A (G295N, and E368Q) and E (S40T, N412D, N412K and T413H). The shell subdomain of the GII.4 Sydney [P13] variant was closely related to norovirus GII.17. Lastly, we also observed several mutations in the T cell restricted epitopes of both strains. Our study has made a novel contribution to understanding the evolution of norovirus GII.4 in Botswana.

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.

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.

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.

Increasing Recombinant Strains Emerged in Norovirus Outbreaks in Jiangsu, China: 2015-2018

From January 2015 to December 2018, 213 norovirus outbreaks with 3,951 patients were reported in Jiangsu, China. Based on viral RdRp and VP1 genes, eight genotypes, GII.2[P16] (144, 67.6%), GII.3[P12] (21, 9.9%), GII.6[P7] (5, 2.3%), GII.14[P7] (4, 1.9%), GII.4 Sydney[P31] (3, 1.4%), GII.1[P33] (1, 0.5%), GII.2[P2] (3, 1.4%), and GII.17[P17] (16, 7.5%) were identified throughout the study period. These genotypes were further regrouped as GII.R (Recombinant) and GII.Non-R (Non-recombinant) strains. In this report we showed that GII.R strains were responsible for at least 178 (83.6%) of 213 norovirus-positive outbreaks with a peak in 2017 and 2018. Most norovirus outbreaks occurred in primary schools and 94 of 109 (86.2%) outbreaks in primary schools were caused by GII.R, while GII.Non-R and GII.NT (not typed) strains accounted for 6 (5.5%) and 9 (8.3%) norovirus outbreaks, respectively. The SimPlot analysis showed recombination breakpoints near the ORF1/2 junction for all six recombinant strains. The recombination breakpoints were detected at positions varying from nucleotides 5009 to 5111, localized in the ORF1 region for four strains (GII.2[P16], GII.3[P12], GII.6[P7], and GII.14[P7]) and in the ORF2 region for the other (GII.4 Sydney[P31] and GII.1[P33]). We identified four clusters, Cluster I through IV, in the GII.P7 RdRp gene by phylogenetic analysis and the GII.14[P7] variants reported here belonged to Cluster IV in the RdRp tree. The HBGA binding site of all known GII.14 strains remained conserved with several point mutations found in the predicted conformational epitopes. In conclusion, gastroenteritis outbreaks caused by noroviruses increased rapidly in the last years and these viruses were classified into eight genotypes. Emerging recombinant noroviral strains have become a major concern and challenge to public health.

Tracking the polyclonal neutralizing antibody response to a dengue virus serotype 1 type-specific epitope across two populations in Asia and the Americas

The four dengue virus serotypes (DENV1-4) cause major public health problems worldwide. Highly neutralizing type-specific human monoclonal antibodies (hmAbs) target conformation-dependent epitopes on the DENV envelope protein, including 1F4, a DENV1 type-specific hmAb. Using a recombinant DENV2 virus displaying the DENV1 1F4 epitope (rDENV2/1), we measured the proportion and kinetics of DENV1 neutralizing antibodies targeting the 1F4 epitope in individuals living in Asia and the Americas where different DENV1 genotypes were circulating. Samples from 20 individuals were analyzed 3 and 18 months post-primary DENV1 infection, alongside samples from 4 individuals collected annually for four years post-primary DENV1 infection, from two studies in Nicaragua. We also analyzed convalescent post-primary DENV1 plasma samples from Sri Lankan individuals. We found that neutralizing antibodies recognizing the 1F4 epitope vary in prevalence across both populations and were detected from 20 days to four years post-infection. Additionally, both populations displayed substantial variability, with a range of high to low proportions of DENV1 type-specific neutralizing antibodies recognizing the 1F4 epitope seen across individuals. Thus, the 1F4 epitope is a major but not exclusive target of type-specific neutralizing antibodies post-primary infection with different DENV1 genotypes in Asia and Latin America, and additional epitopes likely contribute to type-specific neutralization of DENV1.

Sera Antibody Repertoire Analyses Reveal Mechanisms of Broad and Pandemic Strain Neutralizing Responses after Human Norovirus Vaccination

Rapidly evolving RNA viruses, such as the GII.4 strain of human norovirus (HuNoV), and their vaccines elicit complex serological responses associated with previous exposure. Specific correlates of protection, moreover, remain poorly understood. Here, we report the GII.4-serological antibody repertoire—pre- and post-vaccination—and select several antibody clonotypes for epitope and structural analysis. The humoral response was dominated by GII.4-specific antibodies that blocked ancestral strains or by antibodies that bound to divergent genotypes and did not block viral-entry-ligand interactions. However, one antibody, A1431, showed broad blockade toward tested GII.4 strains and neutralized the pandemic GII.P16-GII.4 Sydney strain. Structural mapping revealed conserved epitopes, which were occluded on the virion or partially exposed, allowing for broad blockade with neutralizing activity. Overall, our results provide high-resolution molecular information on humoral immune responses after HuNoV vaccination and demonstrate that infection-derived and vaccine-elicited antibodies can exhibit broad blockade and neutralization against this prevalent human pathogen.

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Serum vaccine response is dominated by a small number of abundant antibody clonotypes

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Vaccine-boosted antibodies predominantly target conserved norovirus epitopes

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Identified cross-genogroup and strain-specific epitopes

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Discovered a pandemic-genotype neutralizing antibody recognizing a conserved epitope

Population Genomics of GII.4 Noroviruses Reveal Complex Diversification and New Antigenic Sites Involved in the Emergence of Pandemic Strains

Noroviruses are an important cause of viral gastroenteritis around the world. An obstacle delaying the development of norovirus vaccines is inadequate understanding of the role of norovirus diversity in immunity. Using a population genomics approach, we identified new residues on the viral capsid protein (VP1) from GII.4 noroviruses, the predominant genotype, that appear to be involved in the emergence and antigenic topology of GII.4 variants. Careful monitoring of the substitutions in those residues involved in the diversification and emergence of new viruses could help in the early detection of future novel variants with pandemic potential. Therefore, this novel information on the antigenic diversification could facilitate GII.4 norovirus vaccine design.

GII.4 noroviruses are a major cause of acute gastroenteritis. Their dominance has been partially explained by the continuous emergence of antigenically distinct variants. To gain insights into the mechanisms of viral emergence and population dynamics of GII.4 noroviruses, we performed large-scale genomics, structural, and mutational analyses of the viral capsid protein (VP1). GII.4 noroviruses exhibited a periodic replacement of predominant variants with accumulation of amino acid substitutions. Genomic analyses revealed (i) a large proportion (87%) of conserved residues; (ii) variable residues that map on the previously determined antigenic sites; and (iii) variable residues that map outside the antigenic sites. Residues in the third pattern category formed motifs on the surface of VP1, which suggested extensions of previously predicted and new uncharacterized antigenic sites. The role of two motifs (C and G) in the antigenic makeup of the GII.4 capsid protein was confirmed with monoclonal antibodies and carbohydrate blocking assays. Amino acid profiles from antigenic sites (A, C, D, E, and G) correlated with the circulation patterns of GII.4 variants, with three of them (A, C, and G) containing residues (352, 357, 368, and 378) linked with the diversifying selective pressure on the emergence of new GII.4 variants. Notably, the emergence of each variant was followed by stochastic diversification with minimal changes that did not progress toward the next variant. This report provides a methodological framework for antigenic characterization of viruses and expands our understanding of the dynamics of GII.4 noroviruses and could facilitate the design of cross-reactive vaccines.

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.

Emerging Novel GII.P16 Noroviruses Associated with Multiple Capsid Genotypes

Noroviruses evolve by antigenic drift and recombination, which occurs most frequently at the junction between the non-structural and structural protein coding genomic regions. In 2015, a novel GII.P16-GII.4 Sydney recombinant strain emerged, replacing the predominance of GII.Pe-GII.4 Sydney among US outbreaks. Distinct from GII.P16 polymerases detected since 2010, this novel GII.P16 was subsequently detected among GII.1, GII.2, GII.3, GII.10 and GII.12 viruses, prompting an investigation on the unique characteristics of these viruses. Norovirus positive samples (n = 1807) were dual-typed, of which a subset (n = 124) was sequenced to yield near-complete genomes. CaliciNet and National Outbreak Reporting System (NORS) records were matched to link outbreak characteristics and case outcomes to molecular data and GenBank was mined for contextualization. Recombination with the novel GII.P16 polymerase extended GII.4 Sydney predominance and increased the number of GII.2 outbreaks in the US. Introduction of the novel GII.P16 noroviruses occurred without unique amino acid changes in VP1, more severe case outcomes, or differences in affected population. However, unique changes were found among NS1/2, NS4 and VP2 proteins, which have immune antagonistic functions, and the RdRp. Multiple polymerase-capsid combinations were detected among GII viruses including 11 involving GII.P16. Molecular surveillance of protein sequences from norovirus genomes can inform the functional importance of amino acid changes in emerging recombinant viruses and aid in vaccine and antiviral formulation.

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.

GII.4 Human Norovirus: Surveying the Antigenic Landscape

Human norovirus is the leading cause of viral acute onset gastroenteritis disease burden, with 685 million infections reported annually. Vulnerable populations, such as children under the age of 5 years, the immunocompromised, and the elderly show a need for inducible immunity, as symptomatic dehydration and malnutrition can be lethal. Extensive antigenic diversity between genotypes and within the GII.4 genotype present major challenges for the development of a broadly protective vaccine. Efforts have been devoted to characterizing antibody-binding interactions with dynamic human norovirus viral-like particles, which recognize distinct antigenic sites on the capsid. Neutralizing antibody functions recognizing these sites have been validated in both surrogate (ligand blockade of binding) and in vitro virus propagation systems. In this review, we focus on GII.4 capsid protein epitopes as defined by monoclonal antibody binding. As additional antibody epitopes are defined, antigenic sites emerge on the human norovirus capsid, revealing the antigenic landscape of GII.4 viruses. These data may provide a road map for the design of candidate vaccine immunogens that induce cross-protective immunity and the development of therapeutic antibodies and drugs.

Changes in norovirus genotype diversity in gastroenteritis outbreaks in Alberta, Canada: 2012-2018

Background

The emergence of norovirus genotype GII.4 variants has been associated with gastroenteritis pandemics worldwide, prompting molecular surveillance for early detection of novel strains. In this study, we aimed to analyze the outbreak activity of norovirus and characterize the norovirus strains circulating in Alberta between July 2012 and February 2018.

Methods

Stool samples from gastroenteritis outbreaks in Alberta were tested for norovirus at the Provincial Laboratory for Public Health using a multiplex real time-RT PCR assay. The ORF1 and ORF2-genotypes of norovirus positive samples were assigned based on phylogenetic analyses of partial polymerase and capsid sequences, respectively.

Results

A total of 530 norovirus outbreaks were identified. During July 2012 and June 2017 there was a gradual decrease in the annual number of GII.4 outbreaks, however, outbreak numbers increased from June 2017–February 2018. Four novel strains emerged: GII.17 Kawasaki in July 2014–June 2015, GII.P16/GII.4 Sydney in July 2015–June 2016, GII.P16/GII.2 and GII.P4 New Orleans/GII.4 Sydney in July 2016–June 2017. GII.Pe/GII.4 Sydney was the single predominant strain responsible for the majority (over 50%) of all norovirus outbreaks up to June 2015. Between June 2017 and February 2018, GII.P16/GII.4 Sydney was the leading strain causing 63% of all norovirus outbreaks.

Conclusions

GII.4 stands as the predominant capsid genotype causing a large majority of the norovirus outbreaks in early 2018. An increase in genotype diversity was observed in the last years, characterized by a high circulation of non-GII.4 strains and GII.4 recombinants.

Electronic supplementary material

The online version of this article (10.1186/s12879-019-3792-y) contains supplementary material, which is available to authorized users.

Human Norovirus Epitope D Plasticity Allows Escape from Antibody Immunity without Loss of Capacity for Binding Cellular Ligands

Emergent strains of human norovirus seed pandemic waves of disease. These new strains have altered ligand binding and antigenicity characteristics. Study of viral variants isolated from immunosuppressed patients with long-term norovirus infection indicates that initial virus in vivo evolution occurs at the same antigenic sites as in pandemic strains. Here, cellular ligand binding and antigenicity of two cocirculating strains isolated from a patient with long-term norovirus infection were characterized. The isolated GII.4 viruses differed from previous strains and from each other at known blockade antibody epitopes. One strain had a unique sequence in epitope D, including loss of an insertion at residue 394, corresponding to a decreased relative affinity for carbohydrate ligands. Replacement of 394 with alanine or restoration of the contemporary strain epitope D consensus sequence STT improved ligand binding relative affinity. However, monoclonal antibody blockade of binding potency was only gained for the consensus sequence, not by the alanine insertion. In-depth study of unique changes in epitope D indicated that ligand binding, but not antibody blockade of ligand binding, is maintained despite sequence diversity, allowing escape from blockade antibodies without loss of capacity for binding cellular ligands.IMPORTANCE Human norovirus causes ∼20% of all acute gastroenteritis and ∼200,000 deaths per year, primarily in young children. Most epidemic and all pandemic waves of disease over the past 30 years have been caused by type GII.4 human norovirus strains. The capsid sequence of GII.4 strains is changing over time, resulting in viruses with altered ligand and antibody binding characteristics. The carbohydrate binding pocket of these strains does not vary over time. Here, utilizing unique viral sequences, we study how residues in GII.4 epitope D balance the dual roles of variable antibody binding site and cellular ligand binding stabilization domain, demonstrating that amino acid changes in epitope D can result in loss of antibody binding without ablating ligand binding. This flexibility in epitope D likely contributes to GII.4 strain persistence by both allowing escape from antibody-mediated herd immunity and maintenance of cellular ligand binding and infectivity.

Recombinant GII.P16/GII.4 Sydney 2012 Was the Dominant Norovirus Identified in Australia and New Zealand in 2017

For the past two decades, norovirus pandemic variants have emerged every 3–5 years, and dominate until they are replaced by alternate strains. However, this scenario changed in 2016 with the co-circulation of six prevalent viruses, three of which possessed the pandemic GII.4 Sydney 2012 capsid. An increased number of institutional gastroenteritis outbreaks were reported within the Oceania region in mid-2017. This study identified emerging noroviruses circulating in Australia and New Zealand in 2017 to assess the changing dynamics of the virus infection. RT-PCR-based methods, next generation sequencing, and phylogenetic analyses were used to genotype noroviruses from both clinical and wastewater samples. Antigenic changes were observed between the capsid of pandemic Sydney 2012 variant and the two new Sydney recombinant viruses. The combination of these antigenic changes and the acquisition of a new ORF1 through recombination could both facilitate their ongoing persistence in the population. Overall, an increased prevalence of GII.P16/GII.4 Sydney 2012 viruses was observed in 2017, replacing the GII.P16/GII.2 recombinant that dominated in the region at the end of 2016. This shift in strain dominance was also observed in wastewater samples, demonstrating the reliability of wastewater as a molecular surveillance tool.

Prevalence and genetic diversity of norovirus genogroup II in children less than 5 years of age with acute gastroenteritis in Tehran, Iran

Viral gastroenteritis is a major public health problem worldwide. In Iran, very limited studies have been performed with regard to the epidemiology of noroviruses. This study aimed to evaluate the prevalence and molecular epidemiology of GII noroviruses in hospitalized children less than 5 years of age with acute gastroenteritis (AGE). A total of 210 stool specimens were collected from Ali Asghar Children's Hospital and Bahrami Children's Hospital in Tehran, from June 2015 to June 2016. The samples were screened by real-time RT-PCR for genogroup II (GII). Positive samples were genotyped by semi-nested PCR followed by Sanger sequencing and phylogenetic analysis. Norovirus was identified in 36 (17.1%) of 210 specimens. Based on genetic analysis of RdRp and capsid sequences, the strains were clustered into eight RdRp-capsid genotypes: GII.P4-GII.4 Sydney_2012 (41.7%), GII.Pe-GII.4 Sydney_2012 (30.6%), GII.P21-GII.3 (13.9%), GII.P16-GII.4 Sydney_2012 (2.8%), GII.P16-GII.12 (2.8%), GII.P2-GII.4 Sydney_2012 (2.8%), GII.P7-GII.7 (2.8%) and GII.P2-GII.2 (2.8%). We determined several different co-circulating norovirus genotypes in children < 5 years of age with AGE in our hospital in Tehran, Iran. Continued molecular surveillance of noroviruses, including typing of both RdRp and capsid genes, is important for monitoring emerging strains in our continued efforts to reduce the overall burden of norovirus disease.