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

Bivalent norovirus mRNA vaccine elicits cellular and humoral responses protecting human enteroids from GII.4 infection

Nucleoside-modified mRNA-LNP vaccines have revolutionized vaccine development against infectious pathogens due to their ability to elicit potent humoral and cellular immune responses. In this article, we present the results of the first norovirus vaccine candidate employing mRNA-LNP platform technology. The mRNA-LNP bivalent vaccine encoding the major capsid protein VP1 from GI.1 and GII.4 of human norovirus, generated high levels of neutralizing antibodies, robust cellular responses, and effectively protected human enteroids from infection by the most prevalent genotype (GII.4). These results serve as a proof of concept, demonstrating that a modified-nucleoside mRNA-LNP vaccine based on norovirus VP1 sequences can stimulate an immunogenic response in vivo and generates neutralizing antibodies capable of preventing viral infection in models of human gastrointestinal tract infection.

Nanocellulose-Based Passivated-Carbon Quantum Dots (P-CQDs) for Antimicrobial Applications: A Practical Review

Passivated-carbon quantum dots (P-CQDs) have been attracting great interest as an antimicrobial therapy tool due to their bright fluorescence, lack of toxicity, eco-friendly nature, simple synthetic schemes, and possession of photocatalytic functions comparable to those present in traditional nanometric semiconductors. Besides synthetic precursors, CQDs can be synthesized from a plethora of natural resources including microcrystalline cellulose (MCC) and nanocrystalline cellulose (NCC). Converting MCC into NCC is performed chemically via the top-down route, while synthesizing CODs from NCC can be performed via the bottom-up route. Due to the good surface charge status with the NCC precursor, we focused in this review on synthesizing CQDs from nanocelluloses (MCC and NCC) since they could become a potential source for fabricating carbon quantum dots that are affected by pyrolysis temperature. There are several P-CQDs synthesized with a wide spectrum of featured properties, namely functionalized carbon quantum dots (F-CQDs) and passivated carbon quantum dots (P-CQDs). There are two different important P-CQDs, namely 2,2'-ethylenedioxy-bis-ethylamine (EDA-CQDs) and 3-ethoxypropylamine (EPA-CQDs), that have achieved desirable results in the antiviral therapy field. Since NoV is the most common dangerous cause of nonbacterial, acute gastroenteritis outbreaks worldwide, this review deals with NoV in detail. The surficial charge status (SCS) of the P-CQDs plays an important role in their interactions with NoVs. The EDA-CQDs were found to be more effective than EPA-CQDs in inhibiting the NoV binding. This difference may be attributed to their SCS as well as the virus surface. EDA-CQDs with surficial terminal amino (-NH2) groups are positively charged at physiological pH (-NH3+), whereas EPA-CQDs with surficial terminal methyl groups (-CH3) are not charged. Since the NoV particles are negatively charged, they are attracted to the positively charged EDA-CQDs, resulting in enhancing the P-CQDs concentration around the virus particles. The carbon nanotubes (CNTs) were found to be comparable to the P-CQDs in the non-specific binding with NoV capsid proteins, through complementary charges, π-π stacking, and/or hydrophobic interactions.

A randomized, double-blind, placebo-controlled, dose-escalating phase I trial to evaluate safety and immunogenicity of a plant-produced, bivalent, recombinant norovirus-like particle vaccine

Noroviruses (NoV) are the leading cause of epidemic acute gastroenteritis in humans worldwide and a safe and effective vaccine is needed. Here, a phase I, double-blind, placebo-controlled clinical trial was performed in 60 healthy adults, 18 to 40 years old. Safety (primary objective) and immunogenicity (secondary and exploratory objectives) of a bivalent (GI.4 and GII.4), plant-produced, virus-like particle (VLP), NoV vaccine candidate formulation were investigated at two dose levels (50 µg + 50 µg and 150 µg + 150 µg) without adjuvant. Overall, 13 subjects (65.0%) in the 50 µg group, 16 subjects (80.0%) in the 150 µg group, and 14 subjects (70.0%) in the placebo group reported at least 1 solicited local or general symptom during the 7-day post-vaccination periods following each dose. Severe solicited adverse events (AEs) were rare (2 events in the 50 µg group). A total of 8 subjects (40.0%) in each group reported at least one unsolicited AE during the 28-day post-vaccination periods. Immunogenicity was assessed on days 1, 8, 29, 57, 183 and 365. All subjects were pre-exposed to norovirus as indicated by baseline levels of the different immunological parameters examined. Vaccine-specific humoral and cellular immune responses increased after the first dose but did not rise further after the second vaccination. Increased GI.4- and GII.4-specific IgG titers persisted until day 365. The vaccine elicited cross-reactive IgG antibodies against non-vaccine NoV VLPs, which was more pronounced for NoV strains of the same genotype as the GII.4 vaccine strain than for non-vaccine genotypes. Significant blocking anti-GI.4 and anti-GII.4 VLP titers were triggered in both dose groups. Lymphoproliferation assays revealed strong cell-mediated immune responses that persisted until day 365. In conclusion, both dose levels were safe and well-tolerated, and no higher incidence of AEs was observed in the higher dose group. The data show that a single dose of the vaccine formulated at 50 µg of each VLP is sufficient to reach a peak immune response after 8 to 28 days. The results of this Phase I study warrant further evaluation of the non-adjuvanted vaccine candidate.

Clinical trial registration

https://clinicaltrials.gov/ct2/show/record/NCT05508178, identifier (NCT05508178).

6-Valent Virus-Like Particle-Based Vaccine Induced Potent and Sustained Immunity Against Noroviruses in Mice

Norovirus is a major cause of acute gastroenteritis worldwide, and no vaccine is currently available. The genetic and antigenic diversity of Norovirus presents challenges for providing broad immune protection, which calls for a multivalent vaccine application. In this study, we investigated the possibility of developing a virus-like particle (VLP)-based 6-valent Norovirus vaccine candidate (Hexa-VLPs) that covers GI.1, GII.2, GII.3, GII.4, GII.6, and GII.17 genotypes. Hexa-VLPs (30 µg) adjuvanted with 500 µg of aluminum hydroxide (alum) were selected as the optimal immunization dose after a dose-escalation study. Potent and long-lasting blockade antibody responses were induced by 2-or 3-shot Hexa-VLPs, especially for the emerging GII.P16-GII.2 and GII.17 (Kawasaki 2014) genotypes. Hexa-VLPs plus alum elicited Th1/Th2 mixed yet Th2-skewed immune responses, characterized by an IgG1-biased subclass profile and significant IL-4⁺ T-cell activation. Notably, simultaneous immunization with a mixture of six VLPs revealed no immunological interference among the component antigens. These results demonstrate that Hexa-VLPs are promising broad-spectrum vaccines to provide immunoprotection against major GI/GII epidemic strains in the future.

Stochastic modeling of antibody binding predicts programmable migration on antigen patterns

Viruses and bacteria commonly exhibit spatial repetition of surface molecules that directly interface with the host immune system. However the complex interaction of patterned surfaces with immune molecules containing multiple binding domains is poorly understood. We developed a pipeline for constructing mechanistic models of antibody interactions with patterned antigen substrates. Our framework relies on immobilized DNA origami nanostructures decorated with precisely placed antigens. The results revealed that antigen spacing is a spatial control parameter that can be tuned to influence antibody residence time and migration speed. The model predicts that gradients in antigen spacing can drive persistent, directed antibody migration in the direction of more stable spacing. These results depict antibody-antigen interactions as a computational system wherein antigen geometry constrains and potentially directs antibody movement. We propose that this form of molecular programmability could be exploited during co-evolution of pathogens and immune systems or in the design of molecular machines.

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.

Development and Application of a Novel Rapid and Throughput Method for Broad-Spectrum Anti-Foodborne Norovirus Antibody Testing

Foodbone norovirus (NoV) is the leading cause of acute gastroenteritis worldwide. Candidate vaccines are being developed, however, no licensed vaccines are currently available for managing NoV infections. Screening for stimulated antibodies with broad-spectrum binding activities can be performed for the development of NoV polyvalent vaccines. In this study, we aimed to develop an indirect enzyme-linked immunosorbent assay (ELISA) for testing the broad spectrum of anti-NoV antibodies. Capsid P proteins from 28 representative NoV strains (GI.1–GI.9 and GII.1–GII.22 except GII.11, GII.18, and GII.19) were selected, prepared, and used as coating antigens on one microplate. Combined with incubation and the horseradish peroxidase chromogenic reaction, the entire process for testing the spectrum of unknown antibodies required 2 h for completion. The intra-assay and inter-assay coefficients of variation were less than 10%. The new method was successfully performed with monoclonal antibodies and polyclonal antibodies induced by multiple antigens. In conclusion, the indirect ELISA assay developed in this study had a good performance of reliability, convenience, and high-throughput screening for broad-spectrum antibodies.

Linear B-Cell Epitopes in Human Norovirus GII.4 Capsid Protein Elicit Blockade Antibodies

Human norovirus (HuNoV) is the leading cause of nonbacterial gastroenteritis worldwide with the GII.4 genotype accounting for over 80% of infections. The major capsid protein of GII.4 variants is evolving rapidly, resulting in new epidemic variants with altered antigenic potentials that must be considered for the development of an effective vaccine. In this study, we identify and characterize linear blockade B-cell epitopes in HuNoV GII.4. Five unique linear B-cell epitopes, namely P2A, P2B, P2C, P2D, and P2E, were predicted on the surface-exposed regions of the capsid protein. Evolving of the surface-exposed epitopes over time was found to correlate with the emergence of new GII.4 outbreak variants. Molecular dynamic simulation (MD) analysis and molecular docking revealed that amino acid substitutions in the putative epitopes P2B, P2C, and P2D could be associated with immune escape and the appearance of new GII.4 variants by affecting solvent accessibility and flexibility of the antigenic sites and histo-blood group antigens (HBAG) binding. Testing the synthetic peptides in wild-type mice, epitopes P2B (336-355), P2C (367-384), and P2D (390-400) were recognized as GII.4-specific linear blockade epitopes with the blocking rate of 68, 55 and 28%, respectively. Blocking rate was found to increase to 80% using the pooled serum of epitopes P2B and P2C. These data provide a strategy for expanding the broad blockade potential of vaccines for prevention of NoV infection.

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.

Simultaneous Immunization with Multivalent Norovirus VLPs Induces Better Protective Immune Responses to Norovirus Than Sequential Immunization

Human noroviruses (NoVs) are a genetically diverse, constantly evolving group of viruses. Here, we studied the effect of NoV pre-existing immunity on the success of NoV vaccinations with genetically close and distant genotypes. A sequential immunization as an alternative approach to multivalent NoV virus-like particles (VLPs) vaccine was investigated. Mice were immunized with NoV GI.3, GII.4-1999, GII.17, and GII.4 Sydney as monovalent VLPs or as a single tetravalent mixture combined with rotavirus VP6-protein. Sequentially immunized mice were primed with a trivalent vaccine candidate (GI.3 + GII.4-1999 + VP6) and boosted, first with GII.17 and then with GII.4 Sydney VLPs. NoV serum antibodies were analyzed. Similar NoV genotype-specific immune responses were induced with the monovalent and multivalent mixture immunizations, and no immunological interference was observed. Multivalent immunization with simultaneous mix was found to be superior to sequential immunization, as sequential boost induced strong blocking antibody response against the distant genotype (GII.17), but not against GII.4 Sydney, closely related to GII.4-1999, contained in the priming vaccine. Genetically close antigens may interfere with the immune response generation and thereby immune responses may be differently formed depending on the degree of NoV VLP genotype identity.

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

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.

In Depth Breadth Analyses of Human Blockade Responses to Norovirus and Response to Vaccination

To evaluate and understand the efficacy of vaccine candidates, supportive immunological measures are needed. Critical attributes for a norovirus vaccine are the strength and breadth of antibody responses against the many different genotypes. In the absence of suitable neutralization assays to test samples from vaccine clinical trials, blockade assays offer a method that can measure functional antibodies specific for many of the different norovirus strains. This paper describes development and optimization of blockade assays for an extended panel of 20 different norovirus strains that can provide robust and reliable data needed for vaccine assessment. The blockade assays were used to test a panel of human clinical samples taken before and after vaccination with the Takeda TAK-214 norovirus vaccine. Great variability was evident in the repertoire of blocking antibody responses prevaccination and postvaccination among individuals. Following vaccination with TAK-214, blocking antibody levels were enhanced across a wide spectrum of different genotypes. The results indicate that adults may have multiple exposures to norovirus and that the magnitude and breadth of the complex preexisting antibody response can be boosted and expanded by vaccination.

Human Norovirus: Experimental Models of Infection

Human noroviruses (HuNoVs) are a leading cause of acute gastroenteritis worldwide. HuNoV infections lead to substantial societal and economic burdens. There are currently no licensed vaccines or therapeutics for the prevention or treatment of HuNoVs. A lack of well-characterized in vitro and in vivo infection models has limited the development of HuNoV countermeasures. Experimental infection of human volunteers and the use of related viruses such as murine NoV have provided helpful insights into HuNoV biology and vaccine and therapeutic development. There remains a need for robust animal models and reverse genetic systems to further HuNoV research. This review summarizes available HuNoV animal models and reverse genetic systems, while providing insight into their usefulness for vaccine and therapeutic development.

Therapeutics and Immunoprophylaxis Against Noroviruses and Rotaviruses: The Past, Present, and Future

Background:

Noroviruses and rotaviruses are important viral etiologies of severe gastroenteritis. Noroviruses are the primary cause of nonbacterial diarrheal outbreaks in humans, whilst rotaviruses are a major cause of childhood diarrhea. Although both enteric pathogens substantially impact human health and economies, there are no approved drugs against noroviruses and rotaviruses so far. On the other hand, whilst the currently licensed rotavirus vaccines have been successfully implemented in over 100 countries, the most advanced norovirus vaccine has recently completed phase-I and II trials.

Methods:

We performed a structured search of bibliographic databases for peer-reviewed research litera-ture on advances in the fields of norovirus and rotavirus therapeutics and immunoprophylaxis.

Results:

Technological advances coupled with a proper understanding of viral morphology and replication over the past decade has facilitated pioneering research on therapeutics and immunoprophylaxis against noroviruses and rotaviruses, with promising outcomes in human clinical trials of some of the drugs and vaccines. This review focuses on the various developments in the fields of norovirus and rotavirus thera-peutics and immunoprophylaxis, such as potential antiviral drug molecules, passive immunotherapies (oral human immunoglobulins, egg yolk and bovine colostral antibodies, llama-derived nanobodies, and anti-bodies expressed in probiotics, plants, rice grains and insect larvae), immune system modulators, probiot-ics, phytochemicals and other biological substances such as bovine milk proteins, therapeutic nanoparti-cles, hydrogels and viscogens, conventional viral vaccines (live and inactivated whole virus vaccines), and genetically engineered viral vaccines (reassortant viral particles, virus-like particles (VLPs) and other sub-unit recombinant vaccines including multi-valent viral vaccines, edible plant vaccines, and encapsulated viral particles).

Conclusions:

This review provides important insights into the various approaches to therapeutics and im-munoprophylaxis against noroviruses and rotaviruses..

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.

Development of a Broadly Accessible Venezuelan Equine Encephalitis Virus Replicon Particle Vaccine Platform

Zoonotic viruses circulate as swarms in animal reservoirs and can emerge into human populations, causing epidemics that adversely affect public health. Portable, safe, and effective vaccine platforms are needed in the context of these outbreak and emergence situations. In this work, we report the generation and characterization of an alphavirus replicon vaccine platform based on a non-select agent, attenuated Venezuelan equine encephalitis (VEE) virus vaccine, strain 3526 (VRP 3526). Using both noroviruses and coronaviruses as model systems, we demonstrate the utility of the VRP 3526 platform in the generation of recombinant proteins, production of virus-like particles, and in vivo efficacy as a vaccine against emergent viruses. Importantly, packaging under biosafety level 2 (BSL2) conditions distinguishes VRP 3526 from previously reported alphavirus platforms and makes this approach accessible to the majority of laboratories around the world. In addition, improved outcomes in the vulnerable aged models as well as against heterologous challenge suggest improved efficacy compared to that of previously attenuated VRP approaches. Taking these results together, the VRP 3526 platform represents a safe and highly portable system that can be rapidly deployed under BSL2 conditions for generation of candidate vaccines against emerging microbial pathogens.

IMPORTANCE While VEE virus replicon particles provide a robust, established platform for antigen expression and vaccination, its utility has been limited by the requirement for high-containment-level facilities for production and packaging. In this work, we utilize an attenuated vaccine strain capable of use at lower biocontainment level but retaining the capacity of the wild-type replicon particle. Importantly, the new replicon platform provides equal protection for aged mice and following heterologous challenge, which distinguishes it from other attenuated replicon platforms. Together, the new system represents a highly portable, safe system for use in the context of disease emergence.

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.

Development of a Surrogate Neutralization Assay for Norovirus Vaccine Evaluation at the Cellular Level

Noroviruses (NoVs) are the main pathogens responsible for sporadic and epidemic nonbacterial gastroenteritis, causing an estimated 219,000 deaths annually worldwide. There is no commercially available vaccine for NoVs, due partly to the difficulty in establishing NoV cell culture models. The histo-blood group antigen (HBGA) blocking assay is used extensively to assess the protective potential of candidate vaccine-elicited antibodies, but there is still no widely used cellular evaluation model. In this study, we have established a cell line-based NoV vaccine evaluation model through the construction of human α1,2-fucosyltransferase 2-overexpressing 293T (293T-FUT2) cell lines. The 293T-FUT2 cells stably expressed H type 2 and Lewis y antigens. Virus-like particles (VLPs) of the NoV prototype strain genogroup I.1 (GI.1) and the predominant strains GII.4 and GII.17 could attach to the cell line efficiently in a dose-dependent manner. Importantly, antisera against these NoV VLPs could inhibit the attachment of the VLPs, where the inhibitory effects measured by the attachment inhibition assay correlated significantly with the antibody levels determined by the HBGA blocking assay. Collectively, our attachment inhibition assay could serve as a surrogate neutralization assay for the evaluation of NoV vaccines at the cellular level.

Norovirus Infection in Older Adults: Epidemiology, Risk Factors, and Opportunities for Prevention and Control

Norovirus is the leading cause of acute gastroenteritis. In older adults, it is responsible for an estimated 3.7 million illnesses; 320,000 outpatient visits; 69,000 emergency department visits; 39,000 hospitalizations; and 960 deaths annually in the United States. Older adults are particularly at risk for severe outcomes, including prolonged symptoms and death. Long-term care facilities and hospitals are the most common settings for norovirus outbreaks in developed countries. Diagnostic platforms are expanding. Several norovirus vaccines in clinical trials have the potential to reap benefits. This review summarizes current knowledge on norovirus infection in older adults.

Binding of rabbit hemorrhagic disease virus-like particles to host histo-blood group antigens is blocked by antisera from experimentally vaccinated rabbits

During infection host histo-blood group antigens (HBGAs) act as attachment factors that interact with rabbit hemorrhagic disease virus (RHDV) and participate in the infectious process. In the present study, baculovirus expressing recombinant RHDV capsid protein (VP60r) as a vaccine immunogen was used to test its antigenicity and immunogenicity via immunization experiments. Each group of rabbits immunized with VP60r was found to be fully protected against RHDV challenge. The duration of immunity of the vaccine following the inoculation of a single dose was determined to be at least 240 days. RHDV-specific humoral responses in antisera from inoculated rabbits were analyzed using VP60r virus-like particle (VLP)-based ELISA. Anti-VP60-specific antibody was produced by 7 days post-primary immunization. Following this stage, the levels of this antibody increased steadily, peaking at 90 days and maintaining a high level until 240 days. We developed a synthetic carbohydrate assay to detect blockage in attachment of RHDV VLPs to HBGAs by the rabbit antisera. On day 7 post-immunization, serum samples were demonstrated to block the binding of H type 2 to RHDV VLPs, with a blocking rate of almost 60%, a value that then increased steadily over time. From day 60 to day 240 post-immunization, serum samples completely blocked the binding of H type 2 to RHDV VLPs, with a blocking rate of almost 100%. This indicated that VP60-induced antibodies neutralize the interaction of RHDV with HBGAs.

Intranasal delivery of a bivalent norovirus vaccine formulated in an in situ gelling dry powder

The global health community is beginning to understand the burden of norovirus-associated disease, which has a significant impact in both developed and developing countries. Norovirus virus like particle (VLP)-based vaccines are currently under development and have been shown to elicit systemic and mucosal immune responses when delivered intranasally. In the present study, we describe the use of a dry powder formulation (GelVac™) with an in situ gelling polysaccharide (GelSite™) extracted from Aloe vera for nasal delivery of a bivalent vaccine formulation containing both GI and GII.4 norovirus VLPs. Dose-ranging studies were performed to identify the optimal antigen dosages based on systemic and mucosal immune responses in guinea pigs and determine any antigenic interference. A dose-dependent increase in systemic and mucosal immunogenicity against each of the VLPs were observed as well as a boosting effect for each VLP after the second dosing. A total antigen dose of ≥50 μg of each GI and GII.4 VLPs was determined to be the maximally immunogenic dose in guinea pigs. The immunogenicity results of this bivalent formulation, taken together with previous work on monovalent GelVac™ norovirus vaccine formulation, provides a basis for future development of this norovirus VLP vaccine.

Carbon Dots' Antiviral Functions Against Noroviruses

This study reported the first assessment of carbon dots’ (CDots) antiviral activity to human norovirus virus-like-particles (VLPs), GI.1 and GII.4 VLPs. CDots with different surface passivation molecules, 2,2′-(ethylenedioxy)bis(ethylamine) (EDA)-CDots and 3-ethoxypropylamine (EPA)-CDots, were synthesized and evaluated. The results indicated both EDA- and EPA- CDots were highly effective to inhibit both strains of VLPs’ bindings to histo-blood group antigens (HBGA) receptors on human cells at CDots concentration of 5 µg/mL, with EDA-CDots achieving 100% inhibition and EPA CDots achieving 85–99% inhibition. At low CDots concentration (2 µg/mL), positively charged EDA-CDots exhibited higher inhibitory effect (~82%) than non-charged EPA-CDots (~60%), suggesting the surface charge status of CDots played a role in the interactions between CDots and the negatively charged VLPs. Both types of CDots also exhibited inhibitory effect on VLP’s binding to their respective antibodies, but much less effective than those to HBGA binding. After CDots treatments, VLPs remained intact, and no degradation was observed on VLPs’ capsid proteins. Taken together, the observed antiviral effects of CDots on noroviruses were mainly through the effective inhibition of VLPs’ binding to HBGA receptors and moderate inhibition of VLPs’ binding to their antibodies, without affecting the integrity of viral capsid protein and the viral particle.

Type-specific and cross-reactive antibodies and T cell responses in norovirus VLP immunized mice are targeted both to conserved and variable domains of capsid VP1 protein

Norovirus (NoV)-specific antibodies, which block binding of the virus-like particles (VLPs) to the cell receptors are conformation dependent and directed towards the most exposed domain of the NoV capsid VP1 protein, the P2 domain. Limited data are available on the antibodies directed to other domains of the VP1, and even less on the NoV VP1-specific T cell epitopes. In here, BALB/c mice were immunized with six VLPs derived from NoV GII.4-1999, GII.4-2009 (New Orleans), GII.4-2012 (Sydney), GII.12, GI.1, and G1.3. Serum immunoglobulin G binding antibodies, histo-blood group antigen blocking antibodies and T cell responses using type-specific and heterologous NoV VLPs, P-dimers and 76 overlapping synthetic peptides, spanning the entire 539 amino acid sequence of GII.4 VP1, were determined. The results showed that at least half of the total antibody content is directed towards conserved S domain of the VP1. Only a small fraction (<1%) of the VP1 binding antibodies were blocking/neutralizing. With the use of matrix peptide pools and individual peptides, seven CD4⁺ and CD8⁺ T cell restricted epitopes were mapped, two located in S domain, four in P2 domain and one in P1 domain of NoV VP1. The epitopes were GII.4 strain-specific but also common GII.4 genotype-specific T cell epitopes were identified. More importantly, the results suggest a 9-amino acids long sequence (³¹⁸PAPLGTPDF³²⁶) in P2 domain of VP1 as a universal NoV genogroup II-specific CD8⁺ T cell epitope. Distribution of the T cell epitopes alongside the capsid VP1 indicates the need of the complete protein for high immunogenicity.

Vaccines against norovirus: state of the art trials in children and adults

Noroviruses (NoVs), a group of nonenveloped, single-stranded RNA viruses belonging to the Caliciviridae family, are the leading cause worldwide of acute infectious gastroenteritis. Serious and eventual fatal outcomes may be observed in at-risk populations such as the very young or older adults, especially in those with underlying diseases. NoVs are highly infectious, with a low number of virus particles causing infection, and they are highly resistant to environmental conditions. NoVs have multiple routes of transmission including faecal-oral, aerosolized vomitus, person to person and via contaminated surfaces or food and water. NoVs can cause frequent and dramatic outbreaks where people congregate in close quarters such as hospitals, long-term care facilities, cruise liners and military barracks and ships. Of the seven NoV genogroups, human disease is most frequently caused by genogroups I and II, although genogroup IV has also been associated with illness. The absence of reliable, high-yield cell culture systems or animal models has steered the development of vaccines towards nonreplicating recombinant capsid proteins including viruslike particles and the sub-virus-sized P particles. Takeda Vaccines is developing a candidate NoV vaccine formulation based on adjuvanted viruslike particles from the GI.1 genotype and a consensus GII.4 sequence derived from three natural GII.4 variants. Early clinical trial results show good tolerability and robust immune responses to both components. This approach is designed to induce broad protective immune responses in adults and children.

Intranasal Immunization with Influenza Virus-Like Particles Containing Membrane-Anchored Cholera Toxin B or Ricin Toxin B Enhances Adaptive Immune Responses and Protection against an Antigenically Distinct Virus

Vaccination is the most effective means to prevent influenza virus infection, although current approaches are associated with suboptimal efficacy. Here, we generated virus-like particles (VLPs) composed of the hemagglutinin (HA), neuraminidase (NA) and matrix protein (M1) of A/Changchun/01/2009 (H1N1) with or without either membrane-anchored cholera toxin B (CTB) or ricin toxin B (RTB) as molecular adjuvants. The intranasal immunization of mice with VLPs containing membrane-anchored CTB or RTB elicited stronger humoral and cellular immune responses when compared to mice immunized with VLPs alone. Administration of VLPs containing CTB or RTB significantly enhanced virus-specific systemic and mucosal antibody responses, hemagglutination inhibiting antibody titers, virus neutralizing antibody titers, and the frequency of virus-specific IFN-γ and IL-4 secreting splenocytes. VLPs with and without CTB or RTB conferred complete protection against lethal challenge with a mouse-adapted homologous virus. When challenged with an antigenically distinct H1N1 virus, all mice immunized with VLPs containing CTB or RTB survived whereas mice immunized with VLPs alone showed only partial protection (80% survival). Our results suggest that membrane-anchored CTB and RTB possess strong adjuvant properties when incorporated into an intranasally-delivered influenza VLP vaccine. Chimeric influenza VLPs containing CTB or RTB may represent promising vaccine candidates for improved immunological protection against homologous and antigenically distinct influenza viruses.

Assessment of Functional Norovirus Antibody Responses by Blocking Assay in Mice

Norovirus (NoV)-specific serum antibodies bind to NoV-derived virus-like particles (VLPs) and block the binding of VLPs to the host cell attachment factors/receptors, histo-blood group antigens (HBGAs). Blocking antibodies in human sera have been associated with a protection from NoV infection and disease. Studies of experimental NoV VLP-based vaccines measure blocking antibodies in animal sera instead of a traditional virus neutralization assay. This chapter describes the methodology for analyzing blocking antibodies from NoV GII.4 VLP-immunized mouse sera. Protocol for obtaining mouse NoV GII.4-specific immune sera is described, followed by the detailed protocol for blocking assay using synthetic HBGAs.

Preclinical dose-ranging studies of a novel dry powder norovirus vaccine formulation

Norovirus is the primary cause of viral gastroenteritis in humans with multiple genotypes currently circulating worldwide. The development of a successful norovirus vaccine is contingent on its ability to induce both systemic and mucosal antibody responses against a wide range of norovirus genotypes. Norovirus virus-like particles (VLPs) are known to elicit systemic and mucosal immune responses when delivered intranasally. Incorporation of these VLPs into an intranasal powder vaccine offers the advantage of simplicity and induction of neutralizing systemic and mucosal antibodies. Nasal immunization, which provides the advantage of ease of administration and a mucosal delivery mechanism, faces the real issue of limited nasal residence time due to mucociliary clearance. Herein, we describe a novel dry powder (GelVac™) formulation of GI or GII.4 norovirus VLPs, two dominant circulating genotypes, to identify the optimal antigen dosages based on systemic and mucosal immune responses in guinea pigs. Systemic and mucosal immunogenicity of each of the VLPs was observed in a dose-dependent manner. In addition, a boosting effect was observed after the second dosing of each VLP antigen. With the GelVac™ formulation, a total antigen dose of ≥ 15 μg was determined to be the maximally immunogenic dose for both GI and GII.4 norovirus VLPs based on evaluation for 56 days. Taken together, these results indicate that norovirus VLPs could be used as potential vaccine candidates without using an immunostimulatory adjuvant and provide a basis for the development of a GelVac™ bivalent GI/GII.4 norovirus VLP vaccine.

Tropical and travel-associated norovirus: current concepts

PURPOSE OF REVIEW

We highlight recent advances relevant to understanding norovirus infections in the tropics, both in populations living in developing settings and travelers to these regions.

RECENT FINDINGS

Because of the decrease in diarrheal disease associated with the global rollout of vaccines against rotavirus, norovirus is emerging as the predominant cause of diarrhea morbidity among children in the tropics, and evidence suggests that it contributes to adult disease in endemic populations and travelers. In addition to identifying potential target populations for preventive measures, we provide an update on norovirus vaccine development and concepts related to their implementation in low-income and middle-income countries.

SUMMARY

These current concepts related to norovirus-attributable disease burden, clinical significance, and economic impact can potentially be applied to tailoring efforts to prevent and mitigate the effects of this important enteropathogen.

Norovirus vaccines: Correlates of protection, challenges and limitations

Norovirus (NoV) is responsible for at least 50% of all gastroenteritis outbreaks worldwide. NoVs are classified into 6 different genogroups (GGI- GGVI) based on the viral capsid protein with NoV genogroup II genotype 4 (GII.4) being the predominant strain causing human diseases. Supportive therapy involving reversal of dehydration and electrolyte deficiency is the main treatment of NoV gastroenteritis. However, the worldwide increased recognition of NoV as an important agent of diarrheal gastroenteritis prompted researchers to focus on establishing preventive strategies conferring long-lasting immunity. This review describes the current status of animal and human vaccine models/studies targeting NoV and addresses the factors hampering the development of a broadly effective vaccine.

Vaccines for viral and bacterial pathogens causing acute gastroenteritis: Part I: Overview, vaccines for enteric viruses and Vibrio cholerae

Efforts to develop vaccines for prevention of acute diarrhea have been going on for more than 40 y with partial success. The myriad of pathogens, more than 20, that have been identified as a cause of acute diarrhea throughout the years pose a significant challenge for selecting and further developing the most relevant vaccine candidates. Based on pathogen distribution as identified in epidemiological studies performed mostly in low-resource countries, rotavirus, Cryptosporidium, Shigella, diarrheogenic E. coli and V. cholerae are predominant, and thus the main targets for vaccine development and implementation. Vaccination against norovirus is most relevant in middle/high-income countries and possibly in resource-deprived countries, pending a more precise characterization of disease impact. Only a few licensed vaccines are currently available, of which rotavirus vaccines have been the most outstanding in demonstrating a significant impact in a short time period. This is a comprehensive review, divided into 2 articles, of nearly 50 vaccine candidates against the most relevant viral and bacterial pathogens that cause acute gastroenteritis. In order to facilitate reading, sections for each pathogen are organized as follows: i) a discussion of the main epidemiological and pathogenic features; and ii) a discussion of vaccines based on their stage of development, moving from current licensed vaccines to vaccines in advanced stage of development (in phase IIb or III trials) to vaccines in early stages of clinical development (in phase I/II) or preclinical development in animal models. In this first article we discuss rotavirus, norovirus and Vibrio cholerae. In the following article we will discuss Shigella, Salmonella (non-typhoidal), diarrheogenic E. coli (enterotoxigenic and enterohemorragic), and Campylobacter jejuni.

Molecular epidemiology of norovirus in children and the elderly in Atlanta, Georgia, United States

Noroviruses are an important cause of gastroenteritis, which can be severe at the extremes of ages. Data documenting the endemic burden of norovirus among children and elderly adults are lacking. Stool specimens submitted for clinical testing were collected from elderly (≥ 65 years) adults and children (<18 years) with acute vomiting and/or diarrhea seeking care at several metropolitan Atlanta adult and pediatric hospitals from January 2013-June 2013. Specimens were tested for norovirus with real-time RT-PCR and sequenced if norovirus was detected. Corresponding clinical and demographic data were abstracted from retrospective chart review. Norovirus was detected in 11% (11/104) of elderly specimens and 11% (67/628) of pediatric, with GII.4 Sydney_2012 detected in 64% (7/11) of elderly norovirus-positive and 11% (8/67) of pediatric specimens, P < 0.001. In comparison to hospitalized children, hospitalized elderly with norovirus were more commonly admitted to the intensive care unit (ICU) (36% vs. 7%, P = 0.02). Norovirus in the elderly can be associated with severe illness requiring ICU admissions. The pediatric group demonstrated greater variability in genotype distribution. Ongoing surveillance of norovirus genotypes is crucial for norovirus vaccine development in understanding circulating and emerging genotypes.

Infection of Murine Macrophages by Salmonella enterica Serovar Heidelberg Blocks Murine Norovirus Infectivity and Virus-induced Apoptosis

Gastroenteritis caused by bacterial and viral pathogens constitutes a major public health threat in the United States accounting for 35% of hospitalizations. In particular, Salmonella enterica and noroviruses cause the majority of gastroenteritis infections, with emergence of sporadic outbreaks and incidence of increased infections. Although mechanisms underlying infections by these pathogens have been individually studied, little is known about the mechanisms regulating co-infection by these pathogens. In this study, we utilized RAW 264.7 murine macrophage cells to investigate the mechanisms governing co-infection with S. enterica serovar Heidelberg and murine norovirus (MNV). We demonstrate that infection of RAW 264.7 cells with S. enterica reduces the replication of MNV, in part by blocking virus entry early in the virus life cycle, and inducing antiviral cytokines later in the infection cycle. In particular, bacterial infection prior to, or during MNV infection affected virus entry, whereas MNV entry remained unaltered when the virus infection preceded bacterial invasion. This block in virus entry resulted in reduced virus replication, with the highest impact on replication observed during conditions of co-infection. In contrast, bacterial replication showed a threefold increase in MNV-infected cells, despite the presence of antibiotic in the medium. Most importantly, we present evidence that the infection of MNV-infected macrophages by S. enterica blocked MNV-induced apoptosis, despite allowing efficient virus replication. This apoptosis blockade was evidenced by reduction in DNA fragmentation and absence of poly-ADP ribose polymerase (PARP), caspase 3 and caspase 9 cleavage events. Our study suggests a novel mechanism of pathogenesis whereby initial co-infection with these pathogens could result in prolonged infection by either of these pathogens or both together.

Biological and immunological characterization of norovirus major capsid proteins from three different genotypes

Noroviruses (NoVs) are the leading cause of non-bacterial acute gastroenteritis worldwide. Due to a lack of cell culture system and animal model, our understanding of NoVs has been lagging behind. In this study, NoV major capsid proteins (VP1) from three different genotypes (GI.2, GII.3 and GII.4) were expressed by using recombinant baculovirus expression system and which led to successful assembly of virus-like particles (VLPs). The receptor binding patterns of three kinds of VLPs were characterized by using synthetic and salivary HBGA-VLP binding assay. Cross-reactivity and cross-blocking activity of rabbit hyperimmune sera against these VLPs were determined by ELISA/Western blot analysis and saliva-VLP binding blockade assay, respectively. Expression of the major capsid proteins from three genotypes all led to smaller VLPs in dominance when sf9 cells were cultured in suspension, which was in consistence with our previous report. These smaller VLPs were used for in vitro synthetic and salivary HBGA-VLP binding and binding blockade assays. VLPs from GII.3 strain exhibited no binding to all synthetic HBGAs and saliva samples tested while VLPs from GI.2 and GII.4 strain showed similar binding pattern and bound to all salivary HBGAs tested. Rabbit anti-GII.3 VLPs hyperimmune serum didn't block the binding of GI.2 and GII.4 VLPs to salivary HBGAs while rabbit anti-GI.2 VLP hyperimmune serum blocked the binding of GII.4 VLPs to salivary HBGAs but not vice versa. Our results provide further evidence indirectly in support of presence of other factors involved in receptor binding other than HBGAs for NoVs, and demonstrate poor cross-blocking activities of antibodies against VLPs within or across genogroups.

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.

Noroviruses: epidemiology, immunity and prospects for prevention

In recent years, noroviruses have become recognized as an important cause of both sporadic and epidemic acute gastroenteritis (AGE), largely due to the improved availability of broadly reactive real-time RT-PCR (TaqMan-based RT-PCR) assays. While there is substantial diversity among noroviruses, one specific genotype, GII.4, is the most common etiology in sporadic and epidemic AGE. Outbreaks of norovirus AGE most commonly occur in healthcare facilities and restaurants and result in significant morbidity and mortality and substantial healthcare costs. Norovirus vaccine development is progressing, and Phase I and II human trials have shown proof-of-principle that norovirus vaccines can reduce illness and infection.

Progress toward norovirus vaccines: considerations for further development and implementation in potential target populations

Human norovirus infection causes significant medical and financial costs in the USA and abroad. Some populations, including young children, the elderly, and the immunocompromised, are at heightened risk of infection with this virus and subsequent complications, while others, such as healthcare workers and food handlers are at increased risk of transmitting it, and some are at risk of both. Human noroviruses are heterogeneous with new strains emerging periodically. In addition to viral diversity, incompletely understood characteristics, such as virus-host cell binding and duration of immunity after infection add to the challenges of creating a norovirus vaccine. Although much progress has been made in recent years, many questions remain to be answered. In this review, we discuss the important areas and relevant literature in considering human norovirus vaccine development and potential targets for implementation.

Immunogenicity of Newcastle disease virus vectors expressing Norwalk virus capsid protein in the presence or absence of VP2 protein

Noroviruses are the most common cause of acute gastroenteritis in humans. Development of an effective vaccine is required for reducing their outbreaks. In order to develop a GI norovirus vaccine, Newcastle disease virus vectors, rLaSota and modified rBC, were used to express VP1 protein of Norwalk virus. Co-expression of VP1 and VP2 proteins by Newcastle disease virus vectors resulted in enhanced expression of Norwalk virus VP1 protein and self-assembly of VP1 protein into virus-like particles. Furthermore, the Norwalk virus-specific IgG response induced in mice by Newcastle disease virus vectors was similar to that induced by baculovirus-expressed virus-like particles in mice. However, the modified rBC vector in the presence of VP2 protein induced significantly higher levels of cellular and mucosal immune responses than those induced by baculovirus-expressed VLPs. These results indicate that Newcastle disease virus has great potential for developing a live Norwalk virus vaccine by inducing humoral, cellular and mucosal immune responses in humans.

Serological Correlates of Protection against a GII.4 Norovirus

Noroviruses are the leading cause of acute gastroenteritis worldwide, and norovirus vaccine prevention strategies are under evaluation. The immunogenicity of two doses of bivalent genogroup 1 genotype 1 (GI.1)/GII.4 (50 μg of virus-like particles [VLPs] of each strain adjuvanted with aluminum hydroxide and 3-O-desacyl-4'monophosphoryl lipid A [MPL]) norovirus vaccine administered to healthy adults in a phase 1/2 double-blind placebo-controlled trial was determined using virus-specific serum total antibody enzyme-linked immunosorbent assay (ELISA), IgG, IgA, and histoblood group antigen (HBGA)-blocking assays. Trial participants subsequently received an oral live virus challenge with a GII.4 strain, and the vaccine efficacy results were reported previously (D. I. Bernstein et al., J Infect Dis 211:870-878, 2014, doi:10.1093/infdis/jiu497). This report assesses the impact of prechallenge serum antibody levels on infection and illness outcomes. Serum antibody responses were observed in vaccine recipients by all antibody assays, with first-dose seroresponse frequencies ranging from 88 to 100% for the GI.1 antigen and from 69 to 84% for the GII.4 antigen. There was little increase in antibody levels after the second vaccine dose. Among the subjects receiving the placebo, higher prechallenge serum anti-GII.4 HBGA-blocking and IgA antibody levels, but not IgG or total antibody levels, were associated with a lower frequency of virus infection and associated illness. Notably, some placebo subjects without measurable serum antibody levels prechallenge did not become infected after norovirus challenge. In vaccinees, anti-GII.4 HBGA-blocking antibody levels of >1:500 were associated with a lower frequency of moderate-to-severe vomiting or diarrheal illness. In this study, prechallenge serum HBGA antibody titers correlated with protection in subjects receiving the placebo; however, other factors may impact the likelihood of infection and illness after virus exposure. (This study is registered at ClinicalTrials.gov under registration number NCT1609257.).

Application of salivary antibody immunoassays for the detection of incident infections with Norwalk virus in a group of volunteers

Norovirus infection is the most common cause of acute gastroenteritis in developed countries. Developing an assay based on a non-invasive biomarker for detecting incident norovirus infections could improve disease surveillance and epidemiological investigations. This project involved analysis of IgA and IgG norovirus-specific antibody responses in saliva samples from a Norwalk virus (Genogroup I, genotype 1 norovirus) challenge study involving infected and symptomatic, and non-infected asymptomatic individuals. Saliva was collected at the challenge, and two weeks and 40 days post-challenge. Samples were analyzed using the Luminex fluorometric and Meso Scale Discovery (MSD) electrochemiluminescence immunoassays. Recombinant P domains of Norwalk virus capsid protein, as well as similar recombinant proteins of two genogroup II noroviruses (VA387 and VA207) were used as antigens. Immunoconversions were defined as >4-fold increase in antibody responses to the norovirus antigens. Various sample pre-treatment options, buffers, saliva dilution ratios, and data adjustment approaches to control for sample-to-sample variability in saliva composition were compared using the Luminex assay. The results suggest that adjusting responses to the norovirus antigens for responses to the protein purification tag, glutathione-S-transferase (GST), significantly improved the odds of producing a correct immunoconversion test result. IgG-based tests were more accurate compared to IgA-based tests. At optimal conditions, both Luminex and MSD assays for Norwalk-specific IgG antibodies correctly identified all infected and non-infected individuals. There was no evidence of cross-reactivity of anti-Norwalk virus antibodies with genogroup II noroviruses. These results suggest that salivary antibody responses can be used for the detection of incident infections with Norwalk virus in prospective surveys.

Norovirus in healthcare settings

Purpose of review

To provide an overview of the burden of norovirus disease in healthcare settings and the factors responsible for outbreaks in these institutions; to assess progress on interventions aimed at reducing the burden of norovirus disease.

Recent findings

Norovirus outbreaks in healthcare settings are driven by confluence of viral diversity, the built environment, and host factors. Some of these characteristics may be modifiable and the target of successful interventions.

Summary

Most norovirus outbreaks in hospital and residential care institutions are associated with a particular genotype, known as GII.4. The persistence of norovirus is associated with strain diversity, which is driven by immune evasion and viral adaptation to interaction with a variety of human histo-blood group antigens. The healthcare environment presents serious challenges for control, both because of the physical structure of the built space and the high levels of contact among patient populations who may have compromised hygiene. Increased vulnerability among the populations in healthcare institutions is likely to be multifactorial and may include the following: nutritional status, immunodeficiency or senescence, chronic inflammation, and microbiome alterations. Current control measures are based on general infection control principles, and treatment is mainly supportive and nonspecific. Vaccines and antiviral agents are being developed with promising results, but none are currently available.