Trivalent combination vaccine induces broad heterologous immune responses to norovirus and rotavirus in mice

Production of Norovirus VLPs of the Nine Representative Genotypes Widely Distributed in Japan using the Silkworm-Baculovirus Expression Vector System

Norovirus (NoV) is one of the major causes of acute viral gastroenteritis in humans. Genetic variation is abundant, and prevalent genotypes vary from year to year and region to region. Since NoVs are difficult to amplify in cultured cells, genome RNA-free virus-like particles (VLPs) that mimic the capsid structure of the virus are promising vaccine candidates for the prevention of NoVs infection, and the development of multivalent VLP vaccines is required to prevent NoV infection in a wide range of genotypes. In this study, we attempted to produce NoV VLPs of the top nine genotypes that have a history of epidemics in Japan using the silkworm-baculovirus expression vector system (silkworm-BEVS), which has a proven track record in the mass production of recombinant proteins. In silkworm pupae infected with recombinant baculoviruses constructed to express VP1s, the major protein that forms VLP, the NoV VP1 protein was expressed in large amounts. Most genotypes of VP1 accumulated in the cytoplasm as soluble proteins, but solubility was reduced for that of two genotypes. VP1s of five genotypes could be purified in large quantities (>0.9mg per pupa) by a two-step purification process, and gel filtration chromatography analysis confirmed the formation of VLPs. This study demonstrates the utility of silkworm-BEVS in producing NoV VLPs of multiple genotypes and provides the basis for the development of a multivalent vaccine against genetically diverse NoV infections.

Unravelling aggregation propensity of rotavirus A VP6 expressed as E. coli inclusion bodies through in silico prediction

The inner capsid protein of rotavirus, VP6, emerges as a promising candidate for next-generation vaccines against rotaviruses owing to its abundance in virion particles and high conservation. However, the formation of inclusion bodies during prokaryotic VP6 expression poses a significant hurdle to rotavirus research and applications. Here, we employed experimental and computational approaches to investigate inclusion body formation and aggregation-prone regions (APRs). Heterologous recombinant VP6 expression in Escherichia coli BL21(DE3) cells resulted in inclusion body formation, confirmed by transmission electron microscopy revealing amorphous aggregates. Thioflavin T assay demonstrated incubation temperature-dependent aggregation of VP6 inclusion bodies. Computational predictions of APRs in rotavirus A VP6 protein were performed using sequence-based tools (TANGO, AGGRESCAN, Zyggregator, Waltz, FoldAmyloid, ANuPP, Camsol intrinsic) and structure-based tools (SolubiS, CamSol structurally corrected, Aggrescan3D). A total of 24 consensus APRs were identified, with 21 of them being surface-exposed in VP6. All identified APRs display a predominance of hydrophobic amino acids, ranging from 33 to 100%. Computational identification of these APRs corroborates our experimental observation of VP6 inclusion body or aggregate formation. Characterization of VP6's aggregation propensity facilitates understanding of its behaviour during prokaryotic expression and opens avenues for protein engineering of soluble variants, advancing research on rotavirus VP6 in pathology, therapy, and diagnostics.

A quadrivalent norovirus vaccine based on a chimpanzee adenovirus vector induces potent immunity in mice

Norovirus (NoV) infection is a major cause of gastroenteritis worldwide. The virus poses great challenges in developing vaccines with broad immune protection due to its genetic and antigenic diversity. To date, there are no approved NoV vaccines for clinical use. Here, we aimed to develop a broad-acting quadrivalent NoV vaccine based on a chimpanzee adenovirus vector, AdC68, carrying the major capsid protein (VP1) of noroviral GI and GII genotypes. Compared to intramuscular (i.m.), intranasal (i.n.), or other prime-boost immunization regimens (i.m. ​+ ​i.m., i.m. ​+ ​i.n., i.n. ​+ ​i.m.), AdC68-GI.1-GII.3 (E1)-GII.4-GII.17 (E3), administered via i.n. ​+ ​i.n. induced higher titers of serum IgG antibodies and higher IgA antibodies in bronchoalveolar lavage fluid (BALF) and saliva against the four homologous VP1s in mice. It also significantly stimulated the production of blocking antibodies against the four genotypes. In response to re-stimulation with virus-like particles (VLP)-GI.1, VLP-GII.3, VLP-GII.4, and VLP-GII.17, the quadrivalent vaccine administered according to the i.n. ​+ ​i.n. regimen effectively triggered specific cell-mediated immune responses, primarily characterized by IFN-γ secretion. Furthermore, the preparation of this novel quadrivalent NoV vaccine requires only a single recombinant adenovirus to provide broad preventive immunity against the major GI/GII epidemic strains, making it a promising vaccine candidate for further development.

A narrative review of norovirus epidemiology, biology, and challenges to vaccine development

Norovirus is a leading cause of acute gastroenteritis (AGE) globally. AGE resulting from norovirus causes significant morbidity and mortality in countries of all income levels, particularly among young children and older adults. Prevention of norovirus AGE represents a unique challenge as the virus is genetically diverse with multiple genogroups and genotypes cocirculating globally and causing disease in humans. Variants of the GII.4 genotype are typically the most common genotype, and other genotypes cause varying amounts of disease year-to-year, with GII.2, GII.3, and GII.6 most prevalent in recent years. Noroviruses are primarily transmitted via the fecal-oral route and only a very small number of virions are required for infection, which makes outbreaks of norovirus extremely difficult to control when they occur. Settings like long-term care facilities, daycares, and hospitals are at high risk of outbreaks and can have very high attack rates resulting in substantial costs and disease burden. Severe cases of norovirus AGE are most common in vulnerable patient populations, such as infants, the elderly, and immunocompromised individuals, with available treatments limited to rehydration therapies and supportive care. To date, there are no FDA-approved norovirus vaccines; however, several candidates are currently in development. Given the substantial human and economic burden associated with norovirus AGE, a vaccine to prevent morbidity and mortality and protect vulnerable populations could have a significant impact on global public health.

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

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

Combatting infectious diarrhea: innovations in treatment and vaccination strategies

INTRODUCTION

The escalating prevalence of infectious diseases is an important cause of concern in society. Particularly in several developing countries, infectious diarrhea poses a major problem, with a high fatality rate, especially among young children. The condition is divided into four classes, namely, acute diarrhea, invasive diarrhea, acute bloody diarrhea, and chronic diarrhea. Various pathogenic agents, such as bacteria, viruses, protozoans, and helminths, contribute to the onset of this condition.

AREAS COVERED

The review discusses the scenario of infectious diarrhea, the prevalent types, as well as approaches to management including preventive, therapeutic, and vaccination strategies. The vaccination techniques are extensively discussed including the available vaccines, their advantages as well as limitations.

EXPERT OPINION

There are several approaches available to develop new-improved vaccines. In addition, route of immunization is important and aerosols/nasal sprays, oral route, skin patches, powders, and liquid jets to minimize needles can be used. Plant-based vaccines, such as rice, might save packing and refrigeration costs by being long-lasting, non-refrigerable, and immunogenic. Future research should utilize predetermined PCR testing intervals and symptom monitoring to identify persistent pathogens after therapy and symptom remission.

Co-administration of Oral Cholera Vaccine With Oral Polio Vaccine Among Bangladeshi Young Children: A Randomized Controlled Open Label Trial to Assess Interference

BACKGROUND

Cholera remains a public health threat for low- and middle-income countries, particularly in Asia and Africa. Shanchol™, an inactivated oral cholera vaccine (OCV) is currently in use globally. OCV and oral poliovirus vaccines (OPV) could be administered concomitantly, but the immunogenicity and safety of coadministration among children aged 1-3 years is unknown.

METHODS

We undertook an open-label, randomized, controlled, inequality trial in Dhaka city, Bangladesh. Healthy children aged 1-3 years were randomly assigned to 1 of 3 groups: bivalent OPV (bOPV)-alone, OCV-alone, or combined bOPV + OCV and received vaccines on the day of enrollment and 28 days later. Blood samples were collected on the day of enrollment, day 28, and day 56. Serum poliovirus neutralizing antibodies and vibriocidal antibodies against Vibrio cholerae O1 were assessed using microneutralization assays.

RESULTS

A total of 579 children aged 1‒3 years were recruited, 193 children per group. More than 90% of the children completed visits at day 56. Few adverse events following immunization were recorded and were equivalent among study arms. On day 28, 60% (90% confidence interval: 53%-67%) and 54% (46%-61%) of participants with co-administration of bOPV + OCV responded to polioviruses type 1 and 3, respectively, compared to 55% (47%-62%) and 46% (38%-53%) in the bOPV-only group. Additionally, >50% of participants showed a ≥4-fold increase in vibriocidal antibody titer responses on day 28, comparable to the responses observed in OCV-only arm.

CONCLUSIONS

Co-administration of bOPV and OCV is safe and effective in children aged 1-3 years and can be cost-beneficial.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov (NCT03581734).

Established and new rotavirus vaccines: a comprehensive review for healthcare professionals

Robust scientific evidence related to two rotavirus (RV) vaccines available worldwide demonstrates their significant impact on RV disease burden. Improving RV vaccination coverage may result in better RV disease control. To make RV vaccination accessible to all eligible children worldwide and improve vaccine effectiveness in high-mortality settings, research into new RV vaccines continues. Although current and in-development RV vaccines differ in vaccine design, their common goal is the reduction of RV disease risk in children <5 years old for whom disease burden is the most significant. Given the range of RV vaccines available, informed decision-making is essential regarding the choice of vaccine for immunization. This review aims to describe the landscape of current and new RV vaccines, providing context for the assessment of their similarities and differences. As data for new vaccines are limited, future investigations will be required to evaluate their performance/added value in a real-world setting.

Rotavirus VP6: involvement in immunogenicity, adjuvant activity, and use as a vector for heterologous peptides, drug delivery, and production of nano-biomaterials

The first-generation, live attenuated rotavirus (RV) vaccines, such as RotaTeq and Rotarix, were successful in reducing the number of RV-induced acute gastroenteritis (AGE) and child deaths globally. However, the low efficacy of these first-generation oral vaccines, coupled with safety concerns, required development of improved RV vaccines. The highly conserved structural protein VP6 is highly immunogenic, and it can generate self-assembled nano-sized structures, including tubes and spheres (virus-like particles; VLPs). Amongst the RV proteins, only VP6 shows these features. Interestingly, VP6-assembled structures, in addition to being highly immunogenic, have several other useful characteristics that could allow them to be used as adjuvants, immunological carriers, and drug-delivery vehicles as well as acting a scaffold for production of valuable nano-biomaterials. This review provides an overview of the self-assembled nano-sized structures of VP6-tubes/VLPs and their various functions.

Rotavirus vaccines: progress and new developments

INTRODUCTION

Rotavirus is the primary cause of severe acute gastroenteritis among children under the age of five globally, leading to 128,500 to 215,000 vaccine-preventable deaths annually. There are six licensed oral, live-attenuated rotavirus vaccines: four vaccines pre-qualified for global use by WHO, and two country-specific vaccines. Expansion of rotavirus vaccines into national immunization programs worldwide has led to a 59% decrease in rotavirus hospitalizations and 36% decrease in diarrhea deaths due to rotavirus in vaccine-introducing countries.

AREAS COVERED

This review describes the current rotavirus vaccines in use, global coverage, vaccine efficacy from clinical trials, and vaccine effectiveness and impact from post-licensure evaluations. Vaccine safety, particularly as it relates to the risk of intussusception, is also summarized. Additionally, an overview of candidate vaccines in the pipeline is provided.

EXPERT OPINION

Considerable evidence over the past decade has demonstrated high effectiveness (80-90%) of rotavirus vaccines at preventing severe rotavirus disease in high-income countries, although the effectiveness has been lower (40-70%) in low-to-middle-income countries. Surveillance and research should continue to explore modifiable factors that influence vaccine effectiveness, strengthen data to better evaluate newer rotavirus vaccines, and aid in the development of future vaccines that can overcome the limitations of current vaccines.

Understanding the relationship between norovirus diversity and immunity

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

Effect of rotavirus genetic diversity on vaccine impact

Group A rotaviruses (RVAs) are the leading cause of gastroenteritis, causing 0.2 million deaths and several million hospitalisations globally each year. Four rotavirus vaccines (Rotarix^TM , RotaTeq^TM , Rotavac^® and ROTASIIL^® ) have been pre-qualified by the World Health Organization (WHO), but the two newly pre-qualified vaccines (Rotavac^® and ROTASIIL^® ) are currently only in use in Palestine and India, respectively. In 2009, WHO strongly proposed that rotavirus vaccines be included in the routine vaccination schedule of all countries around the world. By the end of 2019, a total of 108 countries had administered rotavirus vaccines, and 10 countries have currently been approved by Gavi for the introduction of rotavirus vaccine in the near future. With 39% of global coverage, rotavirus vaccines have had a substantial effect on diarrhoeal morbidity and mortality in different geographical areas, although efficacy appears to be higher in high income settings. Due to the segmented RNA genome, the pattern of RVA genotypes in the human population is evolving through interspecies transmission and/or reassortment events for which the vaccine might be less effective in the future. However, despite the relative increase in some particular genotypes after rotavirus vaccine use, the overall efficacy of rotavirus mass vaccination worldwide has not been affected. Some of the challenges to improve the effect of current rotavirus vaccines can be solved in the future by new rotavirus vaccines and by vaccines currently in progress.

Inactivation of Foodborne Viruses by UV Light: A Review

Viruses on some foods can be inactivated by exposure to ultraviolet (UV) light. This green technology has little impact on product quality and, thus, could be used to increase food safety. While its bactericidal effect has been studied extensively, little is known about the viricidal effect of UV on foods. The mechanism of viral inactivation by UV results mainly from an alteration of the genetic material (DNA or RNA) within the viral capsid and, to a lesser extent, by modifying major and minor viral proteins of the capsid. In this review, we examine the potential of UV treatment as a means of inactivating viruses on food processing surfaces and different foods. The most common foodborne viruses and their laboratory surrogates; further explanation on the inactivation mechanism and its efficacy in water, liquid foods, meat products, fruits, and vegetables; and the prospects for the commercial application of this technology are discussed. Lastly, we describe UV’s limitations and legislation surrounding its use. Based on our review of the literature, viral inactivation in water seems to be particularly effective. While consistent inactivation through turbid liquid food or the entire surface of irregular food matrices is more challenging, some treatments on different food matrices seem promising.

Norovirus: Facts and Reflections from Past, Present, and Future

Human Norovirus is currently the main viral cause of acute gastroenteritis (AGEs) in most countries worldwide. Nearly 50 years after the discovery of the "Norwalk virus" by Kapikian and colleagues, the scientific and medical community continue to generate new knowledge on the full biological and disease spectrum of Norovirus infection. Nevertheless, several areas remain incompletely understood due to the serious constraints to effectively replicate and propagate the virus. Here, we present a narrated historic perspective and summarize our current knowledge, including insights and reflections on current points of interest for a broad medical community, including clinical and molecular epidemiology, viral-host-microbiota interactions, antivirals, and vaccine prototypes. We also include a reflection on the present and future impacts of the COVID-19 pandemic on Norovirus infection and disease.

Parenteral, non-live rotavirus vaccine: recent history and future perspective

Since the widespread introduction of oral and live attenuated rotavirus vaccines around the world in 2009, the impacts of disease burden and the effects of disease reduction in developing countries have been proven. However, in low and middle-income countries, the vaccine efficacy is somewhat lower than in developed countries due to differences in nutritional conditions, microbial environments of individuals, and other factors. In addition, as oral, live vaccines have been found to be associated with rare but serious side effects, the development of a next-generation vaccine with safety, improved effectiveness, and ease of storage is currently underway. New vaccine strain developed by the Centers for Disease Control and Prevention in the United States are undergoing preclinical testing of efficacy, antigen dose, and administration route in the form of a heat-treated inactive vaccine, and a recombinant protein-based trivalent subunit vaccine developed by the Program for Appropriate Technology in Health is undergoing clinical trial in phase III. Several research groups are also developing non-replicating protein-based rotavirus vaccines using virus-like particles and nanoparticles. This review provides a brief overview of the development status and technology of parenteral, non-live rotavirus vaccines worldwide.

Norovirus Protease Structure and Antivirals Development

Human norovirus (HuNoV) infection is a global health and economic burden. Currently, there are no licensed HuNoV vaccines or antiviral drugs available. The protease encoded by the HuNoV genome plays a critical role in virus replication by cleaving the polyprotein and is an excellent target for developing small-molecule inhibitors. The current strategy for developing HuNoV protease inhibitors is by targeting the enzyme’s active site and designing inhibitors that bind to the substrate-binding pockets located near the active site. However, subtle differential conformational flexibility in response to the different substrates in the polyprotein and structural differences in the active site and substrate-binding pockets across different genogroups, hamper the development of effective broad-spectrum inhibitors. A comparative analysis of the available HuNoV protease structures may provide valuable insight for identifying novel strategies for the design and development of such inhibitors. The goal of this review is to provide such analysis together with an overview of the current status of the design and development of HuNoV protease inhibitors.

Epidemiology and evolution of Norovirus in China

Norovirus (NoV) has been recognized as a leading cause of gastroenteritis worldwide. This review estimates the prevalence and genotype distribution of NoV in China to provide a sound reference for vaccine development. Studies were searched up to October 2020 from CNKI database and inclusion criteria were study duration of at least one calendar year and population size of >100. The mean overall NoV prevalence in individuals with sporadic diarrhea/gastroenteritis was 16.68% (20796/124649, 95% CI 16.63-16.72), and the detection rate of NoV was the highest among children. Non-GII.4 strains have replaced GII.4 as the predominant caused multiple outbreaks since 2014. Especially the recombinant GII.P16-GII.2 increased sharply, and virologic data show that the polymerase GII.P16 rather than VP1 triggers pandemic. Due to genetic diversity and rapid evolution, predominant genotypes might change unexpectedly, which has become major obstacle for the development of effective NoV vaccines.

Antigenic Diversity of Human Norovirus Capsid Proteins Based on the Cross-Reactivities of Their Antisera

Human norovirus (HuNoV), which is the major causative agent of acute gastroenteritis, has broad antigenic diversity; thus, the development of a broad-spectrum vaccine is challenging. To establish the relationship between viral genetic diversity and antigenic diversity, capsid P proteins and antisera of seven GI and 16 GII HuNoV genotypes were analyzed. Enzyme-linked immunosorbent assays showed that HuNoV antisera strongly reacted with the homologous capsid P proteins (with titers > 5 × 10⁴). However, 17 (73.9%) antisera had weak or no cross-reactivity with heterologous genotypes. Interestingly, the GII.5 antiserum cross-reacted with seven (30.4%) capsid P proteins (including pandemic genotypes GII.4 and GII.17), indicating its potential use for HuNoV vaccine development. Moreover, GI.2 and GI.6 antigens reacted widely with heterologous antisera (n ≥ 5). Sequence alignment and phylogenetic analyses of the P proteins revealed conserved regions, which may be responsible for the immune crossover reactivity observed. These findings may be helpful in identifying broad-spectrum epitopes with clinical value for the development of a future vaccine.

Advances in Human Norovirus Vaccine Research

Human norovirus (HuNoV) is the leading cause of acute gastroenteritis (AGE) worldwide, which is highly stable and contagious, with a few virus particles being sufficient to establish infection. Although the World Health Organization in 2016 stated that it should be an absolute priority to develop a HuNoV vaccine, unfortunately, there is currently no licensed HuNoV vaccine available. The major barrier to the development of an effective HuNoV vaccine is the lack of a robust and reproducible in vitro cultivation system. To develop a HuNoV vaccine, HuNoV immunogen alone or in combination with other viral immunogens have been designed to assess whether they can simultaneously induce protective immune responses against different viruses. Additionally, monovalent and multivalent vaccines from different HuNoV genotypes, including GI and GII HuNoV virus-like particles (VLPs), have been assessed in order to induce broad protection. Although there are several HuNoV vaccine candidates based on VLPs that are being tested in clinical trials, the challenges to develop effective HuNoV vaccines remain largely unresolved. In this review, we summarize the advances of the HuNoV cultivation system and HuNoV vaccine research and discuss current challenges and future perspectives in HuNoV vaccine development.

Vaccines against gastroenteritis, current progress and challenges

Enteric viral and bacterial infections continue to be a leading cause of mortality and morbidity in young children in low-income and middle-income countries, the elderly, and immunocompromised individuals. Vaccines are considered an effective and practical preventive approach against the predominantly fecal-to-oral transmitted gastroenteritis particularly in the resource-limited countries or regions where implementation of sanitation systems and supply of safe drinking water are not quickly achievable. While vaccines are available for a few enteric pathogens including rotavirus and cholera, there are no vaccines licensed for many other enteric viral and bacterial pathogens. Challenges in enteric vaccine development include immunological heterogeneity among pathogen strains or isolates, a lack of animal challenge models to evaluate vaccine candidacy, undefined host immune correlates to protection, and a low protective efficacy among young children in endemic regions. In this article, we briefly updated the progress and challenges in vaccines and vaccine development for the leading enteric viral and bacterial pathogens including rotavirus, human calicivirus, Shigella, enterotoxigenic Escherichia coli (ETEC), cholera, nontyphoidal Salmonella, and Campylobacter, and introduced a novel epitope- and structure-based vaccinology platform known as MEFA (multiepitope fusion antigen) and the application of MEFA for developing broadly protective multivalent vaccines against heterogenous pathogens.

Current and new rotavirus vaccines

As of 2019, four rotavirus vaccines have been prequalified by the WHO for use worldwide. This review highlights current knowledge regarding rotavirus vaccines available, and provides a brief summary of the rotavirus vaccine pipeline.

Rotavirus Inner Capsid VP6 Acts as an Adjuvant in Formulations with Particulate Antigens Only

Novel adjuvants present a concern for adverse effects, generating a need for alternatives. Rotavirus inner capsid VP6 protein could be considered a potential candidate, due to its ability to self-assemble into highly immunogenic nanospheres and nanotubes. These nanostructures exhibit immunostimulatory properties, which resemble those of traditional adjuvants, promoting the uptake and immunogenicity of the co-administered antigens. We have previously elucidated an adjuvant effect of VP6 on co-delivered norovirus and coxsackievirus B1 virus-like particles, increasing humoral and cellular responses and sparing the dose of co-delivered antigens. This study explored an immunostimulatory effect of VP6 nanospheres on smaller antigens, P particles formed by protruding domain of a norovirus capsid protein and a short peptide, extracellular matrix protein (M2e) of influenza A virus. VP6 exhibited a notable improving impact on immune responses induced by P particles in immunized mice, including systemic and mucosal antibody and T cell responses. The adjuvant effect of VP6 nanospheres was comparable to the effect of alum, except for induction of superior mucosal and T cell responses when P particles were co-administered with VP6. However, unlike alum, VP6 did not influence M2e-specific immune responses, suggesting that the adjuvant effect of VP6 is dependent on the particulate nature of the co-administered antigen.

Norovirus Vaccine: Priorities for Future Research and Development

Soon after its identification, norovirus (NoV) has been indicated as one of the most common causes of outbreaks of acute gastroenteritis (AGE) and sporadic acute diarrhea episodes in subjects of any age. In 2016 the World Health Organization stated that the development of a NoV vaccine should be considered an absolute priority. Unfortunately, the development of an effective NoV vaccine has proven extremely difficult, and only in recent years, some preparations have been tested in humans in advanced clinical trials. In this paper, reasons that justify efforts to develop a NoV vaccine, difficulties encountered during NoV vaccine development, and NoV vaccine candidates will be discussed. In recent years, identification of some NoV antigens that alone or in combination with other viral antigens can induce a potentially protective immune response has led to the development of a large series of preparations that seem capable of coping with the problems related to NoV infection. Epidemiological and immunological studies have shown that multivalent vaccines, including both GI and GII NoV, are the only solution to induce sufficiently broad protection. However, even if the road to formulation of an effective and safe NoV vaccine seems to be definitively traced, many problems still need to be solved before the total burden of NoV infections can be adequately controlled. Whether currently available vaccines are able to protect against all the heterologous NoV strains and the variants of the most common serotypes that frequently emerge and cause outbreaks must be defined. Moreover, as performed clinical trials have mainly enrolled adults, it is mandatory to know whether vaccines are effective in all age groups, including younger children. Finally, we must know the immune response of immunocompromised patients and the duration of protection induced by NoV vaccines. Only when all these problems have been solved will it be possible to establish an effective immunization schedule against NoV infection and calculate whether systematic vaccination can be cost effective.

Rotavirus VP6 Adjuvant Effect on Norovirus GII.4 Virus-Like Particle Uptake and Presentation by Bone Marrow-Derived Dendritic Cells In Vitro and In Vivo

We have previously shown that rotavirus (RV) inner capsid protein VP6 has an adjuvant effect on norovirus (NoV) virus-like particle- (VLP-) induced immune responses and studied the adjuvant mechanism in immortalized cell lines used as antigen-presenting cells (APCs). Here, we investigated the uptake and presentation of RV VP6 and NoV GII.4 VLPs by primary bone marrow-derived dendritic cells (BMDCs). The adjuvant effect of VP6 on GII.4 VLP presentation and NoV-specific immune response induction by BMDC in vivo was also studied. Intracellular staining demonstrated that BMDCs internalized both antigens, but VP6 more efficiently than NoV VLPs. Both antigens were processed and presented to antigen-primed T cells, which responded by robust interferon γ secretion. When GII.4 VLPs and VP6 were mixed in the same pulsing reaction, a subpopulation of the cells had uptaken both antigens. Furthermore, VP6 copulsing increased GII.4 VLP uptake by 37% and activated BMDCs to secrete 2-5-fold increased levels of interleukin 6 and tumor necrosis factor α compared to VLP pulsing alone. When in vitro-pulsed BMDCs were transferred to syngeneic BALB/c mice, VP6 improved NoV-specific antibody responses. The results of this study support the earlier findings of VP6 adjuvant effect in vitro and in vivo.

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.

Human noroviruses: recent advances in a 50-year history

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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

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

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

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

Immunological Cross-Reactivity of an Ancestral and the Most Recent Pandemic Norovirus GII.4 Variant

Norovirus (NoV) genotype GII.4 is responsible for the majority of NoV infections causing pandemics every few years. A NoV virus-like particle (VLP)-based vaccine should optimally cover the high antigenic variation within the GII.4 genotype. We compared the immune responses generated by VLPs of the ancestral GII.4 1999 strain (GII.4 1995/96 US variant) and the most recent GII.4 Sydney 2012 pandemic strains in mice. No significant differences were observed in the type-specific responses but GII.4 1999 VLPs were more potent in inducing high-avidity antibodies with better cross-reactivity. GII.4 1999 immune sera blocked binding of GII.4 2006 and GII.4 2012 VLPs to the putative receptors in a surrogate neutralization assay, whereas GII.4 2012 immune sera only had low blocking activity against GII.4 2006 VLPs. Amino acid substitution in the NERK motif (amino acids 310, 316, 484, and 493, respectively), altering the access to conserved blocking epitope F, moderately improved the cross-blocking responses against mutated GII.4 2012 VLPs (D310N). NoV GII.4 1999 VLPs, uptaken and processed by antigen-presenting cells, induced stronger interferon gamma (IFN-γ) production from mice splenocytes than GII.4 2012 VLPs. These results support the use of GII.4 1999 VLPs as a major component of a NoV vaccine.

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..

Progress on norovirus vaccine research: public health considerations and future directions

INTRODUCTION

Noroviruses are the leading cause of foodborne illness worldwide, account for approximately one-fifth of acute gastroenteritis (AGE) cases globally, and cause a substantial economic burden. Candidate norovirus vaccines are in development, but there is currently no licensed vaccine.

AREAS COVERED

Noroviruses cause approximately 684 million cases and 212,000 deaths per year across all age groups, though burden estimates vary by study and region. Challenges to vaccine research include substantial and rapidly evolving genetic diversity, short-term and homotypic immunity to infection, and the absence of a single, well-established correlate of protection. Nonetheless, several norovirus vaccine candidates are currently in development, utilizing virus-like particles (VLPs), P particles, and recombinant adenoviruses. Of these, a bivalent GI.1/GII.4 VLP-based intramuscular vaccine (Phase IIb) and GI.1 oral vaccine (Phase I) are in clinical trials.

EXPERT COMMENTARY

A norovirus vaccine should target high-risk populations, including the young and the elderly, and protect them against the most common circulating norovirus strains. A norovirus vaccine would be a powerful tool in the prevention and control of norovirus while lessening the burden of AGE worldwide. However, more robust burden and cost estimates are needed to justify investments in and guide norovirus vaccine development.

Highly sensitive ELISA for the serological detection of murine rotavirus EDIM based on its major immunogen VP6

Precise health monitoring of laboratory animals is a critical factor for surveillance and accuracy of animal experiments. Rotavirus epizootic diarrhea of infant mice (EDIM) leads to infections in mice that can influence animal studies, e.g., by altering the intestinal physiology. Thus, the aim of this study was establishing a highly sensitive and specific ELISA for the serological detection of EDIM infections in rodents. First, virus proteins were separated by SDS-PAGE and immunogenic proteins were visualized by immunoblotting and identified after in-gel digestion by tandem mass spectrometry. Subsequently, the major immunogen VP6 (virus protein 6) was expressed in Escherichia coli in high yields, purified by affinity chromatography, and used to establish an indirect ELISA. The diagnostic sensitivity and specificity were both above 99 % and the selectivity better than 98.7 % for animals infected by other pathogens listed by the Federation of Laboratory Animal Science Associations. Importantly, the Strep-rVP6-His-ELISA was more sensitive than a commercial virus-based ELISA and is a time- and cost-efficient complement to EDIM-specific immune-fluorescence assays. In conclusion, the assay can improve health monitoring by reducing the risk of missed EDIM infections in animal housing facilities, thereby improving animal welfare, reliability of animal studies, and protection of precious mice breeds.

Parenterally Administered Norovirus GII.4 Virus-Like Particle Vaccine Formulated with Aluminum Hydroxide or Monophosphoryl Lipid A Adjuvants Induces Systemic but Not Mucosal Immune Responses in Mice

Norovirus (NoV) is a main cause of acute gastroenteritis across all ages worldwide. NoV vaccine candidates currently in clinical trials are based on noninfectious highly immunogenic virus-like particles (VLPs) delivered intramuscularly (IM). Since NoV is an enteric pathogen, it is likely that mucosal immunity has a significant role in protection from infection in the intestine. Due to the fact that IM delivery of NoV VLPs does not generate mucosal immunity, we investigated whether NoV genotype GII.4 VLPs coadministered with aluminum hydroxide (Al(OH)₃) or monophosphoryl lipid A (MPLA) would induce mucosal antibodies in mice. Systemic as well as mucosal IgG and IgA antibodies in serum and intestinal and nasal secretions were measured. As expected, strong serum IgG, IgG1, and IgG2a antibodies as well as a dose sparing effect were induced by both Al(OH)₃ and MPLA, but no mucosal IgA antibodies were detected. In contrast, IN immunization with GII.4 VLPs without an adjuvant induced systemic as well as mucosal IgA antibody response. These results indicate that mucosal delivery of NoV VLPs is needed for induction of mucosal responses.

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.

Rotavirus capsid VP6 protein acts as an adjuvant in vivo for norovirus virus-like particles in a combination vaccine

Rotavirus (RV) and norovirus (NoV) are the 2 leading causes of acute viral gastroenteritis worldwide. We have developed a non-live NoV and RV vaccine candidate consisting of NoV virus-like particles (VLPs) and recombinant polymeric RV VP6 protein produced in baculovirus-insect cell expression system. Both components have been shown to induce strong potentially protective immune responses. As VP6 nanotubes are highly immunogenic, we investigated here a possible adjuvant effect of these structures on NoV-specific immune responses in vivo. BALB/c mice were immunized intramuscularly with a suboptimal dose (0.3 μg) of GII.4 or GI.3 VLPs either alone or in a combination with 10 μg dose of VP6 and induction of NoV-specific antibodies in sera of experimental animals were measured. Blocking assay using human saliva or synthetic histo-blood group antigens was employed to test NoV blocking antibodies. Suboptimal doses of the VLPs alone did not induce substantial anti-NoV antibodies. When co-administered with the VP6, considerable titers of not only type-specific but also cross-reactive IgG antibodies against NoV VLP genotypes not included in the vaccine composition were induced. Most importantly, NoV-specific blocking antibodies, a surrogate for neutralizing antibodies, were generated. Our results show that RV VP6 protein has an in vivo adjuvant effect on NoV-specific antibody responses and support the use of VP6 protein as a part of the NoV-RV combination vaccine, especially when addition of external adjuvants is not desirable.

Live baculovirus acts as a strong B and T cell adjuvant for monomeric and oligomeric protein antigens

Recombinant proteins produced by baculovirus (BV) expression systems contain residual BV after crude purification. We studied adjuvant effect of BV on antibody and T cell responses against two model antigens, monomeric ovalbumin (OVA) protein and oligomeric norovirus (NoV) virus-like particles (VLPs). BALB/c mice were immunized intradermally with OVA alone or OVA formulated with live or inactivated BV, and VLP formulations comprised of chromatographically purified NoV GII.4 VLPs alone or mixed with BV, or of crude purified VLPs containing BV impurities from expression system. Live BV improved immunogenicity of NoV VLPs, sparing VLP dose up to 10-fold. Moreover, soluble OVA protein induced IgG2a antibodies and T cell response only when co-administered with live BV. BV adjuvant effect was completely abrogated by removal or inactivation of BV. These findings support the usage of crude purified proteins containing residual BV as vaccine antigens.

Molecular analysis of norovirus in specimens from children enrolled in a 1982-1986 study in Belém, Brazil: A community-based longitudinal study

Fecal specimens were collected during a longitudinal, community-based study in the city of Belém, North Brazil, that was conducted over 3 years (October 1982 to March 1986), in which 20 children were included from birth to 3 years of age. A total of 229 fecal samples were screened by real time RT-PCR targeting the junction region (ORF 1/2) of the norovirus (NoV) genome. NoV-positive samples were subjected to PCR and sequencing of the viral polymerase (ORF1) and viral protein 1 (VP1) genes (ORF2). The junction region was also sequenced to assess for recombination when ORF1 and ORF2 genotyping results were dissimilar. Samples classified as GII.P4/GII.4 were further characterized by sequencing the P2 subdomain of the viral capsid to determine possible alterations. An overall positivity of 16.1% (37/229) was observed, including GI (16.2%-6/37) and GII (83.8%-31/37) genogroups. Cases of NoV reinfection in at least 2-month intervals were observed, and 12 children developed at least one case of asymptomatic NoV infection. In total, 48.6% (18/37) NoV-positive samples were subjected to nucleotide sequencing analysis targeting the following polymerase genes: GI.P3 (n = 1), GII.Pa (n = 1), GII.Pc (n = 1), GII.P4 (n = 5), GII.P6 (n = 5), GII.P7 (n = 3), GII.P12 (n = 1), and GII.P22 (n = 1). For the VP1 gene, characterization was performed in 14 (77.8%) samples: GI.3 (n = 1), GII.2 (n = 1), GII.4 (n = 4), GII.6 (n = 4), GII.7 (n = 1), GII.12 (n = 1), GII.14 (n = 1), and GII.23 (n = 1). Recombination events were confirmed in three cases (GII.P12/GII.2, GII.P7/GII.14, and GII.Pa/GII.12), and four samples genotyped as GII.P4/GII.4 were analyzed to identify variants. None had contemporary counterparts. Three children developed consecutive NoV infections by different genotypes. The present report documents the importance of NoV as a cause of childhood infection during a longitudinal study conducted more than 30 years ago.

Prospects and Challenges in the Development of a Norovirus Vaccine

PURPOSE

Norovirus is the leading cause of acute epidemic gastroenteritis among children under the age of 5 years and adults in the United States and in adults worldwide, accounting for an estimated 20% of episodes of acute gastroenteritis across all ages. No effective vaccine is presently available. This article provides an overview of the current state of norovirus vaccine development, emphasizing barriers and challenges in the development of an effective vaccine, correlates of protection used to assess vaccine efficacy, and the results of clinical trials of the major candidate vaccines.

METHODS

We performed an unstructured literature review of published articles listed in PubMed in the field of norovirus vaccine development, with an emphasis on studies in humans.

FINDINGS

Two candidate vaccines have reached clinical trials, and a number of other candidates are in the preclinical stages of development. Multivalent vaccination may be effective in inducing broadly neutralizing antibodies protective against challenge with novel and heterologous norovirus strains. Most identified correlates of protection have not been validated in large-scale challenge studies, nor have the degrees to which these correlates covary been assessed.

IMPLICATIONS

Immune correlates of protection against norovirus infection need to be further developed to facilitate additional studies of the tolerability and efficacy of candidate norovirus vaccines in humans.

Norovirus vaccines under development

Noroviruses (NoVs) are one of the leading causes of acute gastroenteritis, including both outbreaks and endemic infections. The development of preventive strategies, including vaccines, for the most susceptible groups (children <5years of age, the elderly and individuals suffering crowding, such as military personnel and travelers) is desirable. However, NoV vaccine development has faced many difficulties, including genetic/antigenic diversity, limited knowledge on NoV immunology and viral cycle, lack of a permissive cell line for cultivation and lack of a widely available and successful animal model. Vaccine candidates rely on inoculation of virus-like particles (VLPs) formed by the main capsid protein VP1, subviral particles made from the protruding domain of VP1 (P-particles) or viral vectors with a NoV capsid gene insert produced by bioengineering technologies. Polivalent vaccines including multiple NoV genotypes and/or other viruses acquired by the enteric route have been developed. A VLP vaccine candidate has reached phase II clinical trials and several others are in pre-clinical stages of development. In this article we discuss the main challenges facing the development of a NoV vaccine and the current status of prevailing candidates.

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.

Rotavirus capsid VP6 tubular and spherical nanostructures act as local adjuvants when co-delivered with norovirus VLPs

A subunit protein vaccine candidate based on norovirus (NoV) virus‐like particles (VLPs) and rotavirus (RV) VP6 protein against acute childhood gastroenteritis has been proposed recently. RV VP6 forms different oligomeric nanostructures, including tubes and spheres when expressed in vitro, which are highly immunogenic in different animal models. We have shown recently that recombinant VP6 nanotubes have an adjuvant effect on immunogenicity of NoV VLPs in mice. In this study, we investigated if the adjuvant effect is dependent upon a VP6 dose or different VP6 structural assemblies. In addition, local and systemic adjuvant effects as well as requirements for antigen co‐delivery and co‐localization were studied. The magnitude and functionality of NoV GII.4‐specific antibodies and T cell responses were tested in mice immunized with GII.4 VLPs alone or different combinations of VLPs and VP6. A VP6 dose‐dependent adjuvant effect on GII.4‐specific antibody responses was observed. The adjuvant effect was found to be strictly dependent upon co‐administration of NoV GII.4 VLPs and VP6 at the same anatomic site and at the same time. However, the adjuvant effect was not dependent on the types of oligomers used, as both nanotubes and nanospheres exerted adjuvant effect on GII.4‐specific antibody generation and, for the first time, T cell immunity. These findings elucidate the mechanisms of VP6 adjuvant effect in vivo and support its use as an adjuvant in a combination NoV and RV vaccine.

Rotavirus Recombinant VP6 Nanotubes Act as an Immunomodulator and Delivery Vehicle for Norovirus Virus-Like Particles

We have recently shown that tubular form of rotavirus (RV) recombinant VP6 protein has an in vivo adjuvant effect on the immunogenicity of norovirus (NoV) virus-like particle (VLP) vaccine candidate. In here, we investigated in vitro effect of VP6 on antigen presenting cell (APC) activation and maturation and whether VP6 facilitates NoV VLP uptake by these APCs. Mouse macrophage cell line RAW 264.7 and dendritic cell line JAWSII were used as model APCs. Internalization of VP6, cell surface expression of CD40, CD80, CD86, and major histocompatibility class II molecules, and cytokine and chemokine production were analyzed. VP6 nanotubes were efficiently internalized by APCs. VP6 upregulated the expression of cell surface activation and maturation molecules and induced secretion of several proinflammatory cytokines and chemokines. The mechanism of VP6 action was shown to be partially dependent on lipid raft-mediated endocytic pathway as shown by methyl-β-cyclodextrin inhibition on tumor necrosis factor α secretion. These findings add to the understanding of mechanism by which VP6 exerts its immunostimulatory and immunomodulatory actions and further support its use as a part of nonlive RV-NoV combination vaccine.

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.