Characterization of a cross-reactive linear epitope in human genogroup I and bovine genogroup III norovirus capsid proteins

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.

First detection of bovine noroviruses and detection of bovine coronavirus in Australian dairy cattle

Background and objective

Noroviruses have been recognised as a significant cause of neonatal enteritis in calves in many countries, but there has been no investigation of their occurrence in Australian cattle. This study aimed to establish whether bovine noroviruses could be detected in faecal samples from Australian dairy cattle. It also sought to determine whether bovine coronaviruses, also associated with neonatal enteritis in calves, could be detected in the same faecal samples.

Methods

A selection of faecal samples that were negative for rotaviruses from dairy farms located in three geographically distinct regions of Victoria were pooled and tested by reverse transcription‐PCR for the presence of noroviruses (genogroup III), neboviruses and bovine coronaviruses.

Results and conclusion

Genetically distinct genogroup III noroviruses were detected in two sample pools from different geographic regions and bovine coronavirus was detected in a third pool of samples. This is the first report of bovine norovirus infection in Australian cattle and suggests that future work is required to determine the significance of these agents as a cause of bovine enteric disease in Australia.

Mapping and modeling of a strain-specific epitope in the Norwalk virus capsid inner shell

Noroviruses are diverse positive-strand RNA viruses associated with acute gastroenteritis. Cross-reactive epitopes have been mapped primarily to conserved sequences in the capsid VP1 Shell (S) domain, and strain-specific epitopes to the highly variable Protruding (P) domain. In this work, we investigated a strain-specific linear epitope defined by MAb NV10 that was raised against prototype (Genogroup I.1) strain Norwalk virus (NV). Using peptide scanning and mutagenesis, the epitope was mapped to amino acids 21-32 (LVPEVNASDPLA) of the NV S domain, and its specificity was verified by epitope transfer and reactivity with a recombinant MAb NV10 single-chain variable fragment (scFv). Comparative structural modeling of the NV10 strain-specific and the broadly cross-reactive TV20 epitopes identified two internal non-overlapping sites in the NV shell, corresponding to variable and conserved amino acid sequences among strains, respectively. The S domain, like the P domain, contains strain-specific epitopes that contribute to the antigenic diversity among the noroviruses.

Structural analysis of bovine norovirus protruding domain

We determined a structure of a bovine (genogroup III, GIII) norovirus capsid protruding (P) domain using X-ray crystallography. The bovine P domain was reminiscent of other norovirus genogroups (GI, GII, GIV, and GV), but closely matched the human GI P domain. We also identified a monoclonal antibody that was capable of binding the five different (GI-GV) P domains. Our data suggests that genetically diverse noroviruses still contain common epitopes.

Llama nanoantibodies with therapeutic potential against human norovirus diarrhea

Noroviruses are a major cause of acute gastroenteritis, but no vaccines or therapeutic drugs are available. Llama-derived single chain antibody fragments (also called VHH) are small, recombinant monoclonal antibodies of 15 kDa with several advantages over conventional antibodies. The aim of this study was to generate recombinant monoclonal VHH specific for the two major norovirus (NoV) genogroups (GI and GII) in order to investigate their potential as immunotherapy for the treatment of NoV diarrhea. To accomplish this objective, two llamas were immunized with either GI.1 (Norwalk-1968) or GII.4 (MD2004) VLPs. After immunization, peripheral blood lymphocytes were collected and used to generate two VHH libraries. Using phage display technology, 10 VHH clones specific for GI.1, and 8 specific for GII.4 were selected for further characterization. All VHH recognized conformational epitopes in the P domain of the immunizing VP1 capsid protein, with the exception of one GII.4 VHH that recognized a linear P domain epitope. The GI.1 VHHs were highly specific for the immunizing GI.1 genotype, with only one VHH cross-reacting with GI.3 genotype. The GII.4 VHHs reacted with the immunizing GII.4 strain and showed a varying reactivity profile among different GII genotypes. One VHH specific for GI.1 and three specific for GII.4 could block the binding of homologous VLPs to synthetic HBGA carbohydrates, saliva, and pig gastric mucin, and in addition, could inhibit the hemagglutination of red blood cells by homologous VLPs. The ability of Nov-specific VHHs to perform well in these surrogate neutralization assays supports their further development as immunotherapy for NoV treatment and immunoprophylaxis.

Newly isolated mAbs broaden the neutralizing epitope in murine norovirus

Here, we report the isolation and functional characterization of mAbs against two murine norovirus (MNV) strains, MNV-1 and WU20, which were isolated following oral infection of mice. The mAbs were screened for reactivity against the respective homologous and heterologous MNV strain by ELISA. Selected mAbs were of IgA, IgG1, IgG2a or IgG2b isotype and showed a range of Western blot reactivities from non-binding to strong binding, suggesting recognition of conformational and linear epitopes. Some of the anti-MNV-1 antibodies neutralized both MNV-1 and WU20 infections in culture and in mice, but none of the anti-WU20 mAbs neutralized either virus. The non-neutralizing anti-MNV-1 IgG2b antibody 5C4.10 was mapped to the S domain of the MNV-1 capsid, whilst the epitopes of the neutralizing anti-MNV-1 IgA antibodies 2D3.7 and 4F9.4 were mapped to the P domain. Generation of neutralization escape viruses showed that two mutations (V339I and D348E) in the C'D' loop of the MNV-1 P domain mediated escape from mAb 2D3.7 and 4F9.4 neutralization. These findings broaden the known neutralizing epitopes of MNV to the main surface-exposed loops of the P domain. In addition, the current panel of antibodies provides valuable reagents for studying norovirus biology and development of diagnostic tools.

Identification and characterization of antibody-binding epitopes on the norovirus GII.3 capsid

Genotype II.3 (GII.3) noroviruses are a major cause of sporadic gastroenteritis, particularly in children. The greater incidence of GII.3 noroviruses in the pediatric population compared to the adult demographic suggests development of herd immunity to this genotype, possibly as a consequence of limited evolution of immune epitopes. This study aimed to identify and characterize immune epitopes on the GII.3 capsid protein and to determine the level of immune cross-reactivity within the genotype. A panel of seven GII.3 virus-like particles (VLPs), representing norovirus strains isolated during 1975 to 2008, was tested by enzyme-linked immunosorbent assay (ELISA) for reactivity with human sera and a rabbit anti-GII.3 strain-specific polyclonal serum generated against the 2008 GII.3 VLP. Immunoprecipitation of protease-digested GII.3 VLPs and sequencing of bound peptides via mass spectrometry were used to locate epitopes on the capsid. Two epitopes were investigated further using Mimotopes technology. Serum binding studies demonstrated complete intragenotype GII.3 cross-reactivity using both human and rabbit serum. Six immunoreactive regions containing epitopes were located on the GII.3 capsid protein, two within each capsid domain. Epitopes in the S and P1 domains were highly conserved within GII.3 noroviruses. P2 domain epitopes were variable and contained evolutionarily important residues and histo-blood group antigen (HBGA) binding residues. In conclusion, anti-GII.3 antibody-binding epitopes are highly cross-reactive and mostly conserved within GII.3 strains. This may account for the limited GII.3 prevalence in adults and suggests that a GII.3 strain may be a valuable inclusion in a multivalent pediatric targeted VLP vaccine. Exploration of norovirus immune epitopes is vital for effective vaccine design. IMPORTANCE This study represents an important contribution to the understanding of norovirus immunology in a pediatric genotype. The high cross-reactivity and conservation of GII.3 epitopes suggest development of herd immunity against GII.3 and indicate that a GII.3 strain would be a valuable inclusion in a pediatric targeted multivalent vaccine. Immunological understanding of pediatric norovirus strains is important since norovirus vaccines will likely target high-risk groups such as the pediatric population.

Identification of a Broadly Cross-Reactive Epitope in the Inner Shell of the Norovirus Capsid

Noroviruses are major pathogens associated with acute gastroenteritis. They are diverse viruses, with at least six genogroups (GI-GVI) and multiple genotypes defined by differences in the major capsid protein, VP1. This diversity has challenged the development of broadly cross-reactive vaccines as well as efficient detection methods. Here, we report the characterization of a broadly cross-reactive monoclonal antibody (MAb) raised against the capsid protein of a GII.3 norovirus strain. The MAb reacted with VLPs and denatured VP1 protein from GI, GII, GIV and GV noroviruses, and mapped to a linear epitope located in the inner shell domain. An alignment of all available VP1 sequences showed that the putative epitope (residues 52–56) is highly conserved across the genus Norovirus. This broadly cross-reactive MAb thus constitutes a valuable reagent for the diagnosis and study of these diverse viruses.

Exposure to human and bovine noroviruses in a birth cohort in southern India from 2002 to 2006

Human and bovine norovirus virus-like particles were used to evaluate antibodies in Indian children at ages 6 and 36 months and their mothers. Antibodies to genogroup II viruses were acquired early and were more prevalent than antibodies to genogroup I. Low levels of IgG antibodies against bovine noroviruses indicate possible zoonotic transmission.

Bioinformatics analysis of the epitope regions for norovirus capsid protein

BACKGROUND

Norovirus is the major cause of nonbacterial epidemic gastroenteritis, being highly prevalent in both developing and developed countries. Despite of the available monoclonal antibodies (MAbs) for different sub-genogroups, a comprehensive epitope analysis based on various bioinformatics technology is highly desired for future potential antibody development in clinical diagonosis and treatment.

METHODS

A total of 18 full-length human norovirus capsid protein sequences were downloaded from GenBank. Protein modeling was performed with program Modeller 9.9. The modeled 3D structures of capsid protein of norovirus were submitted to the protein antigen spatial epitope prediction webserver (SEPPA) for predicting the possible spatial epitopes with the default threshold. The results were processed using the Biosoftware.

RESULTS

Compared with GI, we found that the GII genogroup had four deletions and two special insertions in the VP1 region. The predicted conformational epitope regions mainly concentrated on N-terminal (1~96), Middle Part (298~305, 355~375) and C-terminal (560~570). We find two common epitope regions on sequences for GI and GII genogroup, and also found an exclusive epitope region for GII genogroup.

CONCLUSIONS

The predicted conformational epitope regions of norovirus VP1 mainly concentrated on N-terminal, Middle Part and C-terminal. We find two common epitope regions on sequences for GI and GII genogroup, and also found an exclusive epitope region for GII genogroup. The overlapping with experimental epitopes indicates the important role of latest computational technologies. With the fast development of computational immunology tools, the bioinformatics pipeline will be more and more critical to vaccine design.

Multiple antigenic sites are involved in blocking the interaction of GII.4 norovirus capsid with ABH histo-blood group antigens

Noroviruses are major etiological agents of acute viral gastroenteritis. In 2002, a GII.4 variant (Farmington Hills cluster) spread so rapidly in the human population that it predominated worldwide and displaced previous GII.4 strains. We developed and characterized a panel of six monoclonal antibodies (MAbs) directed against the capsid protein of a Farmington Hills-like GII.4 norovirus strain that was associated with a large hospital outbreak in Maryland in 2004. The six MAbs reacted with high titers against homologous virus-like particles (VLPs) by enzyme-linked immunoassay but did not react with denatured capsid protein in immunoblots. The expression and self-assembly of newly developed genogroup I/II chimeric VLPs showed that five MAbs bound to the GII.4 protruding (P) domain of the capsid protein, while one recognized the GII.4 shell (S) domain. Cross-competition assays and mutational analyses showed evidence for at least three distinct antigenic sites in the P domain and one in the S domain. MAbs that mapped to the P domain but not the S domain were able to block the interaction of VLPs with ABH histo-blood group antigens (HBGA), suggesting that multiple antigenic sites of the P domain are involved in HBGA blocking. Further analysis showed that two MAbs mapped to regions of the capsid that had been associated with the emergence of new GII.4 variants. Taken together, our data map antibody and HBGA carbohydrate binding to proximal regions of the norovirus capsid, showing that evolutionary pressures on the norovirus capsid protein may affect both antigenic and carbohydrate recognition phenotypes.

Human antibody responses to bovine (Newbury-2) norovirus (GIII.2) and association to histo-blood group antigens

Serum antibodies to bovine norovirus have been found recently in about 22% of humans. Whether this prevalence reflects limited virulence properties of the virus or that inherited host factors provide protection against bovine norovirus infection in humans remains to be established. To investigate whether histo-blood group antigens correlate with the presence of bovine norovirus (GIII.2) antibody, plasma (n = 105) from Swedish blood donors, genotyped and phenotyped for secretor, Lewis and ABO, were tested and compared for the frequency of IgG antibody and antibody titer to Bo/Newbury2/76/UK. In total, 26.7% (28/105) of Swedish blood donors were antibody-positive. Two non-secretors (2/21, 9.5%) were antibody-positive compared with 26/84 (31%) secretors (P = 0.047). While no statistically significant correlation was found between the frequency of antibodies to bovine norovirus and different ABO blood groups, individuals with blood type B presented the highest frequency of antibodies (37.5%) compared with 0-30% among other blood groups. Individuals with Le(a-b+) had not only higher frequency of antibodies (31.3%) compared with Le(a+b-) (11%) (P = 0.068) but also higher antibody titer (P = 0.085) although this was not significant statistically. No detectable cross-reaction between bovine GIII.2 and human GII.3 NoV VLP was found with human and animal sera. The results of this study suggest that bovine norovirus infections occur in Sweden and that secretor status but not ABO blood groups is a possible risk factor for infection.

SYBR Green based real-time RT-PCR assay for detection and genotype prediction of bovine noroviruses and assessment of clinical significance in Norway

A novel SYBR Green based real-time RT-PCR assay for detection of genogroup III bovine noroviruses (BoNoV) was developed and the assay applied to 419 faecal samples from calves with and without diarrhoea. The samples were obtained from 190 Norwegian dairy and beef herds. BoNoV was detected in 49.6% of the samples from 61.1% of the herds indicating that BoNoV is ubiquitous in Norway. The overall prevalence was not significantly different in diarrhoea and non-diarrhoea samples. Analyses of polymerase gene sequences revealed both genotype III/1 and III/2 with genotype III/2 (Newbury2-like) being the most prevalent. Detected capsid sequences were restricted to Newbury2-like and the chimeric Bo/Thirsk10/00/UK strain. The RNA polymerase genotypes of the circulating BoNoVs in Norway were predicted by melting temperature analysis. Additional data from a challenge experiment suggest that a high proportion of young calves are shedding low levels of BoNoV for a prolonged time after recovering from the associated diarrhoea. The findings may explain some of the discrepancies in detection rates from previous studies and explain why some studies have failed to detect significant prevalence differences between calves with and without diarrhoea. It may also shed new light on some epidemiological aspects of norovirus infections.

The alphaGal epitope of the histo-blood group antigen family is a ligand for bovine norovirus Newbury2 expected to prevent cross-species transmission

Among Caliciviridae, the norovirus genus encompasses enteric viruses that infect humans as well as several animal species, causing gastroenteritis. Porcine strains are classified together with human strains within genogroup II, whilst bovine norovirus strains represent genogroup III. Various GI and GII human strains bind to carbohydrates of the histo-blood group family which may be shared among mammalian species. Genetic relatedness of human and animal strains as well as the presence of potentially shared ligands raises the possibility of norovirus cross-species transmission. In the present study, we identified a carbohydrate ligand for the prototype bovine norovirus strain Bo/Newbury2/76/UK (NB2). Attachment of virus-like particles (VLPs) of the NB2 strain to bovine gut tissue sections showed a complete match with the staining by reagents recognizing the Galalpha1,3 motif. Alpha-galactosidase treatment confirmed involvement of a terminal alpha-linked galactose. Specific binding of VLPs to the alphaGal epitope (Galalpha3Galbeta4GlcNAcbeta-R) was observed. The binding of Galalpha3GalalphaOMe to rNB2 VLPs was characterized at atomic resolution employing saturation transfer difference (STD) NMR experiments. Transfection of human cells with an alpha1,3galactosyltransferase cDNA allowed binding of NB2 VLPs, whilst inversely, attachment to porcine vascular endothelial cells was lost when the cells originated from an alpha1,3galactosyltransferase KO animal. The alphaGal epitope is expressed in all mammalian species with the exception of the Hominidaea family due to the inactivation of the alpha1,3galactosyltransferase gene (GGTA1). Accordingly, the NB2 carbohydrate ligand is absent from human tissues. Although expressed on porcine vascular endothelial cells, we observed that unlike in cows, it is not present on gut epithelial cells, suggesting that neither man nor pig could be infected by the NB2 bovine strain.

Identification and characterization of a native epitope common to norovirus strains GII/4, GII/7 and GII/8

Norovirus is an important cause of acute non-bacterial gastroenteritis in humans. The norovirus genus is comprised of at least five genogroups based on sequence differences. The norovirus genogroup II (GII/4) strain is recognized as the predominant genotype worldwide. We expressed a 60 kDa full-length recombinant capsid protein of norovirus GII/4 in Escherichia coli and generated three monoclonal antibodies (MAbs) against it. Western blotting indicated that all three MAbs had reactivity against the recombinant capsid protein and a 58 kDa native capsid protein of norovirus obtained from stool samples. MAb-capture ELISA showed that MAb detected segmental strains within GII antigens in clinical material. To identify the existent range of this epitope, epitope analyses were processed by expressing 12 amino acids of the GST-fusion peptides. The epitope analyses revealed that the MAb N2C3 recognized a continuous native epitope (55)WIRNNF(60) in the shell domain, which not only belongs to strain GII/4, but also to strains GII/7 and GII/8. This is a new native epitope to be reported for norovirus GII/4.

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

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

Immunomagnetic separation combined with real-time reverse transcriptase PCR assays for detection of norovirus in contaminated food

We developed an immunomagnetic separation (IMS) technique combined with real-time TaqMan reverse transcriptase PCR (RT-PCR), which allowed detection of norovirus at a level as low as 3 to 7 RT-PCR units from artificially contaminated strawberries. The inoculum recovery rate ranged from 14 to 30%. The data demonstrate that IMS combined with real-time RT-PCR will be useful as a rapid and sensitive method for detecting food-borne microbial contaminants.

Immunology of Norovirus Infection

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

Human noroviruses in swine and cattle

Detection of GII.4 norovirus sequences in animal fecal samples and retail meats demonstrates that noroviruses may be transmitted zoonotically.

Human noroviruses are the predominant cause of foodborne gastroenteritis worldwide. Strains of norovirus also exist that are uniquely associated with animals; their contribution to the incidence of human illness remains unclear. We tested animal fecal samples and identified GIII (bovine), GII.18 (swine), and GII.4 (human) norovirus sequences, demonstrating for the first time, to our knowledge, that GII.4-like strains can be present in livestock. In addition, we detected GII.4-like noroviral RNA from a retail meat sample. This finding highlights a possible route for indirect zoonotic transmission of noroviruses through the food chain.

Characterization of a broadly reactive monoclonal antibody against norovirus genogroups I and II: recognition of a novel conformational epitope

Norovirus, which belongs to the family Caliciviridae, is one of the major causes of nonbacterial acute gastroenteritis in the world. The main human noroviruses are of genogroup I (GI) and genogroup II (GII), which were subdivided further into at least 15 and 18 genotypes (GI/1 to GI/15 and GII/1 to GII/18), respectively. The development of immunological diagnosis for norovirus had been hindered by the antigen specificity of the polyclonal antibody. Therefore, several laboratories have produced broadly reactive monoclonal antibodies, which recognize the linear GI and GII cross-reactive epitopes or the conformational GI-specific epitope. In this study, we characterized the novel monoclonal antibody 14-1 (MAb14-1) for further development of the rapid immunochromatography test. Our results demonstrated that MAb14-1 could recognize 15 recombinant virus-like particles (GI/1, 4, 8, and 11 and GII/1 to 7 and 12 to 15) and showed weak affinity to the virus-like particle of GI/3. This recognition range is the broadest of the existing monoclonal antibodies. The epitope for MAb14-1 was identified by fragment, sequence, structural, and mutational analyses. Both terminal antigenic regions (amino acid positions 418 to 426 and 526 to 534) on the C-terminal P1 domain formed the conformational epitope and were in the proximity of the insertion region (positions 427 to 525). These regions contained six amino acids responsible for antigenicity that were conserved among genogroup(s), genus, and Caliciviridae. This epitope mapping explained the broad reactivity and different titers among GI and GII. To our knowledge, we are the first group to identify the GI and GII cross-reactive monoclonal antibody, which recognizes the novel conformational epitope. From these data, MAb14-1 could be used further to develop immunochromatography.