Robust mucosal-homing antibody-secreting B cell responses induced by intramuscular administration of adjuvanted bivalent human norovirus-like particle vaccine

Unraveling the interplay between norovirus infection, gut microbiota, and novel antiviral approaches: a comprehensive review

Norovirus infection is a leading cause of acute gastroenteritis worldwide and can also cause harmful chronic infections in individuals with weakened immune systems. The role of the gut microbiota in the interactions between the host and noroviruses has been extensively studied. While most past studies were conducted in vitro or focused on murine noroviruses, recent research has expanded to human noroviruses using in vivo or ex vivo human intestinal enteroids culture studies. The gut microbiota has been observed to have both promoting and inhibiting effects on human noroviruses. Understanding the interaction between noroviruses and the gut microbiota or probiotics is crucial for studying the pathogenesis of norovirus infection and its potential implications, including probiotics and vaccines for infection control. Recently, several clinical trials of probiotics and norovirus vaccines have also been published. Therefore, in this review, we discuss the current understanding and recent updates on the interactions between noroviruses and gut microbiota, including the impact of norovirus on the microbiota profile, pro-viral and antiviral effects of microbiota on norovirus infection, the use of probiotics for treating norovirus infections, and human norovirus vaccine development.

Immune responses in healthy adults elicited by a bivalent norovirus vaccine candidate composed of GI.4 and GII.4 VLPs without adjuvant

The development of an efficacious vaccine against norovirus is of paramount importance given its potential to reduce the global burden of norovirus-associated morbidity and mortality. Here, we report a detailed immunological analysis of a phase I, double-blind, placebo-controlled clinical trial performed on 60 healthy adults, ages 18 to 40. Total serum immunoglobulin and serum IgA against vaccine strains and cross-reactive serum IgG against non-vaccine strains were measured by enzyme immunoassays, whereas cell-mediated immune responses were quantified using intracellular cytokine staining by flow cytometry. A significant increase in humoral and cellular responses, e.g., IgA and CD4⁺ polypositive T cells, was triggered by the GI.4 Chiba 407 (1987) and GII.4 Aomori 2 (2006) VLP-based norovirus vaccine candidate rNV-2v, which is formulated without adjuvant. No booster effect was observed after the second administration in the pre-exposed adult study population. Furthermore, a cross-reactive immune response was elicited, as shown by IgG titers against GI.3 (2002), GII.2 OC08154 (2008), GII.4 (1999), GII.4 Sydney (2012), GII.4 Washington (2018), GII.6 Maryland (2018), and GII.17 Kawasaki 308 (2015). Due to viral infection via mucosal gut tissue and the high variety of potentially relevant norovirus strains, a focus should be on IgA and cross-protective humoral and cell-mediated responses in the development of a broadly protective, multi-valent norovirus vaccine.

Clinical trial registration

https://clinicaltrials.gov, identifier NCT05508178. EudraCT number: 2019-003226-25.

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.

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.

Preclinical Characterization of Immunogenicity and Efficacy against Diarrhea from MecVax, a Multivalent Enterotoxigenic E. coli Vaccine Candidate

There are no vaccines licensed for enterotoxigenic Escherichia coli (ETEC), a leading cause of diarrhea for children in developing countries and international travelers. Virulence heterogeneity among strains and difficulties identifying safe antigens for protective antibodies against STa, a potent but poorly immunogenic heat-stable toxin which plays a key role in ETEC diarrhea, are challenges in ETEC vaccine development. To overcome these challenges, we applied a toxoid fusion strategy and a novel epitope- and structure-based multiepitope fusion antigen (MEFA) vaccinology platform to construct two chimeric multivalent proteins, toxoid fusion 3xSTa_N12S-mnLT_R192G/L211A and adhesin CFA/I/II/IV MEFA, and demonstrated that the proteins induced protective antibodies against STa and heat-labile toxin (LT) produced by all ETEC strains or the seven most important ETEC adhesins (CFA/I and CS1 to CS6) expressed by the ETEC strains causing 60 to 70% of diarrheal cases and moderate to severe cases. Combining two proteins, we prepared a protein-based multivalent ETEC vaccine, MecVax. MecVax was broadly immunogenic; mice and pigs intramuscularly immunized with MecVax developed no apparent adverse effects but had robust antibody responses to the target toxins and adhesins. Importantly, MecVax-induced antibodies were broadly protective, demonstrated by significant adherence inhibition against E. coli bacteria producing any of the seven adhesins and neutralization of STa and cholera toxin (CT) enterotoxicity. Moreover, MecVax protected against watery diarrhea and provided over 70% and 90% protection against any diarrhea from an STa-positive or an LT-positive ETEC strain in a pig challenge model. These results indicated that MecVax induces broadly protective antibodies and prevents diarrhea preclinically, signifying that MecVax is potentially an effective injectable vaccine for ETEC. IMPORTANCE Enterotoxigenic Escherichia coli (ETEC) bacteria are a top cause of children's diarrhea and travelers' diarrhea and are responsible for over 220 million diarrheal cases and more than 100,000 deaths annually. A safe and effective ETEC vaccine can significantly improve public health, particularly in developing countries. Data from this preclinical study showed that MecVax induces broadly protective antiadhesin and antitoxin antibodies, becoming the first ETEC vaccine candidate to induce protective antibodies inhibiting adherence of the seven most important ETEC adhesins and neutralizing the enterotoxicity of not only LT but also STa toxin. More importantly, MecVax is shown to protect against clinical diarrhea from STa-positive or LT-positive ETEC infection in a pig challenge model, recording protection from antibodies induced by the protein-based, injectable, subunit vaccine MecVax against ETEC diarrhea and perhaps the possibility of intramuscularly administered protein vaccines for protection against intestinal mucosal infection.

Norovirus in Cancer Patients: A Review

Norovirus (NoV) is the leading cause of viral-related diarrhea in cancer patients, in whom it can be chronic, contributing to decreased quality of life, interruption of cancer care, malnutrition, and altered mucosal barrier function. Immunosuppressed cancer patients shed NoV for longer periods of time than immunocompetent hosts, favoring quasispecies development and emergence of novel NoV variants. While nucleic acid amplification tests (NAATs) for NoV diagnosis have revolutionized our understanding of NoV burden of disease, not all NAATs provide information on viral load or infecting genotype. There is currently no effective antiviral or vaccine for chronic NoV infections. Screening for inhibitors of NoV replication in intestinal organoid culture models and creation of NoV-specific adoptive T cells are promising new strategies to develop treatments for chronic NoV in immunosuppressed patients. Herein we summarize data on the epidemiology, clinical manifestations, diagnostic challenges, and treatment of NoV infection in patients with cancer.

Paediatrics: how to manage viral gastroenteritis

Background

Viral gastroenteritis is the most common diarrhoeal disorder seen in general practice and emergency departments. This article aims to provide a narrative updated review on the evaluation and management of viral gastroenteritis in children.

Methods

A PubMed search was performed with Clinical Queries using the key term 'viral gastroenteritis'. The search strategy included clinical trials, meta-analyses, randomized controlled trials, observational studies and reviews. The search was restricted to the English literature and the paediatric population.

Results

Acute viral gastroenteritis is usually self-limiting. However, it can lead to dehydration and electrolyte imbalance if not properly treated. Adequate fluids containing physiological concentrations of glucose and electrolytes should be provided to compensate for gastrointestinal losses and cover maintenance needs. Oral rehydration therapy is as effective as intravenous (IV) fluid therapy for rehydration for children with mild-to-moderate dehydration. Measurements of serum electrolytes, creatinine and glucose are usually not necessary and should only be considered in a subset of children with severe dehydration who require hospitalization and IV therapy. Judicious use of ondansetron can increase the success rate of oral rehydration therapy and minimize the need for IV therapy and hospitalization.

Conclusion

Acute viral gastroenteritis is associated with substantial morbidity in developed countries and significant mortality in developing countries. Physicians should educate caregivers on proper personal hygiene and handwashing to prevent faecal to oral transmission of the pathogen as well as the importance of rotavirus vaccine in the prevention of rotavirus gastroenteritis. Several norovirus vaccines are currently undergoing clinical trials with promising results. It is hoped that development of an effective norovirus vaccine will further reduce the incidence of viral gastroenteritis.

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.

Preclinical optimization of an enterotoxigenic Escherichia coli adjuvanted subunit vaccine using response surface design of experiments

Enterotoxigenic E. coli (ETEC) is a leading cause of moderate-to-severe diarrhoea. ETEC colonizes the intestine through fimbrial tip adhesin colonization factors and produces heat-stable and/or heat-labile (LT) toxins, stimulating fluid and electrolyte release leading to watery diarrhoea. We reported that a vaccine containing recombinant colonization factor antigen (CfaEB) targeting fimbrial tip adhesin of the colonization factor antigen I (CFA/I) and an attenuated LT toxoid (dmLT) elicited mucosal and systemic immune responses against both targets. Additionally, the toll-like receptor 4 ligand second-generation lipid adjuvant (TLR4-SLA) induced a potent mucosal response, dependent on adjuvant formulation. However, a combination of vaccine components at their respective individual optimal doses may not achieve the optimal immune profile. We studied a subunit ETEC vaccine prototype in mice using a response surface design of experiments (DoE), consisting of 64 vaccine dose-combinations of CfaEB, dmLT and SLA in four formulations (aqueous, aluminium oxyhydroxide, squalene-in-water stable nanoemulsion [SE] or liposomes containing the saponin Quillaja saponaria-21 [LSQ]). Nine readouts focusing on antibody functionality and plasma cell response were selected to profile the immune response of parenterally administered ETEC vaccine prototype. The data were integrated in a model to identify the optimal dosage of each vaccine component and best formulation. Compared to maximal doses used in mouse models (10 µg CfaEB, 1 µg dmLT and 5 µg SLA), a reduction in the vaccine components up to 37%, 60% and 88% for CfaEB, dmLT and SLA, respectively, maintained or even maximized immune responses, with SE and LSQ the best formulations. The DoE approach can help determine the best vaccine composition with a limited number of experiments and may accelerate development of multi-antigen/component ETEC vaccines.

Evaluation of the 50% Infectious Dose of Human Norovirus Cin-2 in Gnotobiotic Pigs: A Comparison of Classical and Contemporary Methods for Endpoint Estimation

Human noroviruses (HuNoVs) are the leading causative agents of epidemic and sporadic acute gastroenteritis that affect people of all ages worldwide. However, very few dose–response studies have been carried out to determine the median infectious dose of HuNoVs. In this study, we evaluated the median infectious dose (ID₅₀) and diarrhea dose (DD₅₀) of the GII.4/2003 variant of HuNoV (Cin-2) in the gnotobiotic pig model of HuNoV infection and disease. Using various mathematical approaches (Reed–Muench, Dragstedt–Behrens, Spearman–Karber, logistic regression, and exponential and approximate beta-Poisson dose–response models), we estimated the ID₅₀ and DD₅₀ to be between 2400–3400 RNA copies, and 21,000–38,000 RNA copies, respectively. Contemporary dose–response models offer greater flexibility and accuracy in estimating ID₅₀. In contrast to classical methods of endpoint estimation, dose–response modelling allows seamless analyses of data that may include inconsistent dilution factors between doses or numbers of subjects per dose group, or small numbers of subjects. Although this investigation is consistent with state-of-the-art ID₅₀ determinations and offers an advancement in clinical data analysis, it is important to underscore that such analyses remain confounded by pathogen aggregation. Regardless, challenging virus strain ID₅₀ determination is crucial for identifying the true infectiousness of HuNoVs and for the accurate evaluation of protective efficacies in pre-clinical studies of therapeutics, vaccines and other prophylactics using this reliable animal model.

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.

IgA Responses Following Recurrent Influenza Virus Vaccination

Influenza is a highly contagious viral respiratory disease that affects millions of people worldwide each year. Annual vaccination is recommended by the World Health Organization to reduce influenza severity and limit transmission through elicitation of antibodies targeting mainly the hemagglutinin glycoprotein of the influenza virus. Antibodies elicited by current seasonal influenza vaccines are predominantly strain-specific. However, continuous antigenic drift by circulating influenza viruses facilitates escape from pre-existing antibodies requiring frequent reformulation of the seasonal influenza vaccine. Traditionally, immunological responses to influenza vaccination have been largely focused on IgG antibodies, with almost complete disregard of other isotypes. In this report, young adults (18–34 years old) and elderly (65–85 years old) subjects were administered the split inactivated influenza vaccine for 3 consecutive seasons and their serological IgA and IgG responses were profiled. Moreover, correlation analysis showed a positive relationship between vaccine-induced IgA antibody titers and traditional immunological endpoints, exposing vaccine-induced IgA antibodies as an important novel immune correlate during influenza vaccination.

Immune-Focusing Properties of Virus-like Particles Improve Protective IgA Responses

Virus-like particles (VLPs) provide a well-established vaccine platform; however, the immunogenic properties acquired by VLP structure remain poorly understood. In this study, we showed that systemic vaccination with norovirus VLP recalls human IgA responses at higher magnitudes than IgG responses under a humanized mouse model that was established by introducing human PBMCs in severely immunodeficient mice. The recall responses elicited by VLP vaccines depended on VLP structure and the disruption of VLP attenuated recall responses, with a more profound reduction being observed in IgA responses. The IgA-focusing property was also conserved in a murine norovirus-primed model under which murine IgA responses were recalled in a manner dependent on VLP structure. Importantly, the VLP-driven IgA response preferentially targeted virus-neutralizing epitopes located in the receptor-binding domain. Consequently, VLP-driven IgA responses were qualitatively superior to IgG responses in terms of the virus-neutralizing activity in vitro. Furthermore, the IgA in mucosa obtained remarkable protective function toward orally administrated virus in vivo. Thus, our results indicate the immune-focusing properties of the VLP vaccine that improve the quality/quantity of mucosal IgA responses, a finding with important implications for developing mucosal vaccines.

Spec-seq unveils transcriptional subpopulations of antibody-secreting cells following influenza vaccination

Vaccines are among the most effective public health tools for combating certain infectious diseases such as influenza. The role of the humoral immune system in vaccine-induced protection is widely appreciated; however, our understanding of how antibody specificities relate to B cell function remains limited due to the complexity of polyclonal antibody responses. To address this, we developed the Spec-seq framework, which allows for simultaneous monoclonal antibody (mAb) characterization and transcriptional profiling from the same single cell. Here, we present the first application of the Spec-seq framework, which we applied to human plasmablasts after influenza vaccination in order to characterize transcriptional differences governed by B cell receptor (BCR) isotype and vaccine reactivity. Our analysis did not find evidence of long-term transcriptional specialization between plasmablasts of different isotypes. However, we did find enhanced transcriptional similarity between clonally related B cells, as well as distinct transcriptional signatures ascribed by BCR vaccine recognition. These data suggest IgG and IgA vaccine-positive plasmablasts are largely similar, whereas IgA vaccine-negative cells appear to be transcriptionally distinct from conventional, terminally differentiated, antigen-induced peripheral blood plasmablasts.

Challenges in developing mucosal vaccines and antibodies against infectious diarrhea in children

Infectious diarrhea in children can be life-threatening and imposes a large economic burden on healthcare systems, therefore more effective prophylactic and therapeutic drugs are needed urgently. Because most of the pathogens responsible for childhood diarrhea infect the gastrointestinal mucosa, providing protective immunity at the mucosal surface is an ideal way to control pathogen invasion and toxic activity. Mucosal (e.g. oral, nasal) vaccines are superior to systemic (subcutaneous or intramuscular) vaccination for conferring both mucosal and systemic pathogen-specific immune responses. Therefore, great efforts has been focused on the development of cost-effective mucosal vaccines for the past 50 years. Recent progress in plant genetic engineering has revolutionized the production of inexpensive and safe recombinant vaccine antigens. For example, rice plant biotechnology has facilitated the development of a cold-chain-free rice-based oral subunit vaccine against Vibrio cholerae. Furthermore, this technology has led to the creation of a rice-based oral antibody for prophylaxis and treatment of rotavirus gastroenteritis. This review summarizes current perspectives regarding the mucosal immune system and the development of mucosal vaccines and therapeutic antibodies, particularly rice-based products, and discusses future prospects regarding mucosal vaccines for children.

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.

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.

B-Cell Responses to Intramuscular Administration of a Bivalent Virus-Like Particle Human Norovirus Vaccine

Human noroviruses (HuNoVs) are a leading cause of acute gastroenteritis worldwide. A virus-like particle (VLP) candidate vaccine induces the production of serum histo-blood group antigen (HBGA)-blocking antibodies, the first identified correlate of protection from HuNoV gastroenteritis. Recently, virus-specific IgG memory B cells were identified to be another potential correlate of protection against HuNoV gastroenteritis. We assessed B-cell responses following intramuscular administration of a bivalent (genogroup I, genotype 1 [GI.1]/genogroup II, genotype 4 [GII.4]) VLP vaccine using protocols identical to those used to evaluate cellular immunity following experimental GI.1 HuNoV infection. The kinetics and magnitude of cellular immunity to G1.1 infection were compared to those after VLP vaccination. Intramuscular immunization with the bivalent VLP vaccine induced the production of antibody-secreting cells (ASCs) and memory B cells. ASC responses peaked at day 7 after the first dose of vaccine and returned to nearly baseline levels by day 28. Minimal increases in ASCs were seen after a second vaccine dose at day 28. Antigen-specific IgG memory B cells persisted at day 180 postvaccination for both GI.1 and GII.4 VLPs. The overall trends in B-cell responses to vaccination were similar to the trends in the responses to infection, where there was a greater bias of an ASC response toward IgA and a memory B-cell response to IgG. The magnitude of the ASC and memory B-cell responses to the GI.1 VLP component of the vaccine was also comparable to that of the responses following GI.1 infection. The production of IgG memory B cells and persistence at day 180 is a key finding and underscores the need for future studies to determine if IgG memory B cells are a correlate of protection following vaccination. (This study has been registered at ClinicalTrials.gov under registration no. NCT01168401.)

The assessment of Hantaan virus-specific antibody responses after the immunization program for hemorrhagic fever with renal syndrome in northwest China

Xianyang city is one of the main hemorrhagic fever with renal syndrome (HFRS) epidemic areas in northwest China. Although the HFRS immunity program has been provided in this city, HFRS is still occurred every year. In order to implement the vaccination program effectively and to control HFRS, the analysis of antibody responses specific to Hantaan virus (HTNV) in individuals after vaccination is essential. In this study, a total of 100 subjects were divided into 5 groups: unvaccinated, 1, 3, 29 and 33 months after boost vaccination. The levels and the positive rates of HTNV-NP-specific IgM and IgG antibodies as well as HTNV neutralizing antibodies were significantly increased in the serum of the vaccinated individuals. The positive rates and levels of HTNV-NP-specific IgG and HTNV neutralizing antibody reached their highest values at 3 months respectively and could be sustained up to 33 months after vaccination. Moreover, the titres of HTNV-NP-specific IgM or IgG antibody and the titres of HTNV neutralizing antibody at 1 month after vaccination have a positive correlation. The level of HTNV-NP-specific IgG antibody was much higher than that of HTNV-NP-specific IgM antibody or HTNV neutralizing antibody. In addition, the strongest responses of antibody-secreting cells were observed at 3 months after vaccination, which was consistent with the serum results. Therefore, the HFRS immunization program is effective to induce humoral immunity in the population of northwest China.

Mucosal Antibodies Induced by Intranasal but Not Intramuscular Immunization Block Norovirus GII.4 Virus-Like Particle Receptor Binding

Noroviruses (NoVs) account for the majority of diagnosed cases of viral acute gastroenteritis worldwide. Virus-like particle (VLP)-based vaccines against NoV are currently under development. Serum antibodies that block the binding of NoV VLPs to histo-blood group antigens, the putative receptors for NoV, correlate with protection against NoV infection. The role of functional mucosal antibodies in protection is largely unknown, even though the intestinal mucosa is the entry port for NoV. Balb/c mice were immunized intramuscularly (IM) or intranasally (IN) with NoV GII.4 VLPs, and systemic and mucosal blocking antibody responses were studied. IN immunization elicited NoV-specific serum and mucosal IgG and IgA antibodies, whereas IM immunized animals completely lacked IgA. Both immunization routes induced similar blocking activity in serum but only IN route generated blocking antibodies in mucosa. The level of IgA in the mucosal (nasal) lavages strongly correlated (r = 0.841) with the blocking activity, suggesting that IgA, but not IgG, is the major NoV blocking antibody on mucosal surfaces. The results indicate that only mucosal immunization route induces the development of functional anti-NoV IgA on mucosal surface.

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.

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.

Norovirus: new developments and implications for travelers' diarrhea

Noroviruses are the leading cause of acute gastroenteritis in the United States and are responsible for at least 50 % of acute gastroenteritis outbreaks occurring worldwide each year. In addition, noroviruses have caused outbreaks on cruise ships, in nursing homes and hospitals, and in deployed military personnel, but its role in the etiology of travelers’ diarrhea is not well defined. The aim of this review is to describe the role of noroviruses in travelers’ diarrhea in terms of epidemiology, current diagnostics, treatment and vaccine development efforts. Studies have shown prevalence rates of noroviruses in travelers’ diarrhea cases ranging from 10–65 %. It is likely that norovirus prevalence rates are highly underestimated in travelers’ diarrhea due to rapid onset, short duration of the illness, limited availability of laboratory facilities, and the fact that most clinical laboratories lack the diagnostic capability to detect noroviruses in stool. Further, additional studies are needed to accurately determine the true prevalence rates of norovirus as an etiologic agent of diarrhea among travelers to different regions around the world. With the rapid progress in the development of a norovirus vaccine, travelers could serve as an ideal population for future norovirus clinical trials.

Production of Brazilian human norovirus VLPs and comparison of purification methods

Noroviruses (NVs) are responsible for most cases of human nonbacterial gastroenteritis worldwide. Some parameters for the purification of NV virus-like particles (VLPs) such as ease of production and yield were studied for future development of vaccines and diagnostic tools. In this study, VLPs were produced by the expression of the VP1 and VP2 gene cassette of the Brazilian NV isolate, and two purification methods were compared: cesium chloride (CsCl) gradient centrifugation and ion-exchange chromatography (IEC). IEC produced more and purer VLPs of NV compared to CsCl gradient centrifugation.

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.

Serum Immunoglobulin A Cross-Strain Blockade of Human Noroviruses

Background. Human noroviruses are the leading cause of acute viral gastroenteritis, justifying vaccine development despite a limited understanding of strain immunity. After genogroup I (GI).1 norovirus infection and immunization, blockade antibody titers to multiple virus-like particles (VLPs) increase, suggesting that GI cross-protection may occur.

Methods. Immunoglobulin (Ig)A was purified from sera collected from GI.1-infected participants, and potential neutralization activity was measured using a surrogate neutralization assay based on antibody blockade of ligand binding. Human and mouse monoclonal antibodies (mAbs) were produced to multiple GI VLPs to characterize GI epitopes.

Results. Immunoglobulin A purified from day 14 post-GI.1 challenge sera blocked binding of GI.1, GI.3, and GI.4 to carbohydrate ligands. In some subjects, purified IgA preferentially blocked binding of other GI VLPs compared with GI.1, supporting observations that the immune response to GI.1 infection may be influenced by pre-exposure history. For other subjects, IgA equivalently blocked multiple GI VLPs. Only strain-specific mAbs recognized blockade epitopes, whereas strain cross-reactive mAbs recognized nonblockade epitopes.

Conclusions. These studies are the first to describe a functional role for serum IgA in norovirus immunity and the first to characterize human monoclonal antibodies to GI strains, expanding our understanding of norovirus immunobiology.

Oral delivery of wafers made from HBsAg-expressing maize germ induces long-term immunological systemic and mucosal responses

BACKGROUND

The hepatitis B surface antigen (HBsAg) has been administered over the last 20 years as a parenteral vaccine against the hepatitis B virus (HBV). Despite high seroconversion rates, chronic infection rates are still high worldwide. Orally delivered vaccines provide a practical alternative to injected vaccines, potentially helping poorly responding populations and providing a viable alternative for populations in remote locations. Anamnestic responses are vital to establishing the efficacy of a given vaccine and have been assessed in this study using a plant-based oral delivery platform expressing HBsAg.

METHODS

Long-term immunological memory was assessed in mice injected with a primary dose of Recombivax and boosted with orally-delivered HBsAg wafers, control wafers, or parenterally-delivered commercial vaccine (Recombivax).

RESULTS

Mice boosted with HBsAg orally-administered wafers displayed sharp increases in mucosal IgA titers in fecal material and steep increases in serum IgA, whereas mice boosted with Recombivax showed no detectable levels of IgA in either fecal or serum samples following four boosting treatments. Long-term memory in the orally-treated mice was evidenced by sustained fecal IgA, and serum IgA, IgG, and mIU/mL over one year, while Recombivax-treated mice displayed sustained serum IgG and mIU/mL. Furthermore, sharp increases in these same antibodies were induced after re-boosting at 47 and 50 weeks post-primary injection.

CONCLUSIONS

Orally-delivered vaccines can provide long-term immune responses mucosally and systemically. For sexually-transmitted diseases that can be acquired at mucosal surfaces, such as HBV, an oral delivery platform may provide added protection over a conventional parenterally administered vaccine.

Next generation vaccines and vectors: Designing downstream processes for recombinant protein-based virus-like particles

In recent years, the development of novel recombinant virus-like particles (VLPs) has been generating new perspectives for the prevention of untreated and arising infectious diseases. However, cost-reduction and acceleration of manufacturing processes for VLP-based vaccines or vectors are key challenges for the global health system. In particular, the design of rapid and cost-efficient purification processes is a critical bottleneck. In this review, we describe and evaluate new concepts, development strategies and unit operations for the downstream processing of VLPs. A special focus is placed on purity requirements and current trends, as well as chances and limitations of novel technologies. The discussed methods and case studies demonstrate the advances and remaining challenges in both rational process development and purification tools for large biomolecules. The potential of a new era of VLP-based products is highlighted by the progress of various VLPs in clinical phases.