Influenza virus-like particle can accommodate multiple subtypes of hemagglutinin and protect from multiple influenza types and subtypes

Chimeric H5 influenza virus-like particle vaccine elicits broader cross-clade antibody responses in chickens than in ducks

Eurasian-lineage highly pathogenic avian influenza (HPAI) H5 viruses have spread throughout Asia, the Middle East, Europe, Africa, and most recently, North and South America. These viruses are independently evolving into genetically and antigenically divergent clades, and broad-spectrum vaccines protecting against these divergent clades are needed. In this study, we developed a chimeric virus-like particle (VLP) vaccine co-expressing hemagglutinins from two clades (clades 1 and 2.3.2.1) of HPAI H5 viruses and performed comparative cross-clade hemagglutination inhibition (HI) analysis in chickens and ducks. The chimeric VLP immunization induced a significantly broader spectrum of antibodies against various clades of HPAI H5 viruses than monovalent VLPs both in chickens and ducks. While the chimeric VLP led to broadened antibody responses in both species, significantly lower levels of HI antibodies were elicited in ducks than in chickens. Moreover, boost immunization failed to increase antibody responses in ducks regardless of the VLPs used, in contrast to chickens that showed significantly enhanced antibody responses upon boost immunization. These results suggest (1) the potential application of the chimeric VLP technology in poultry to help control HPAI H5 viruses by offering broader antibody responses against antigenically different strains and (2) possible obstacles in generating high levels of antibody responses against HPAI H5 viruses in ducks via vaccination, implying the need for advanced vaccination strategies for ducks.

A Single Vaccination of Chimeric Bivalent Virus-Like Particle Vaccine Confers Protection Against H9N2 and H3N2 Avian Influenza in Commercial Broilers and Allows a Strategy of Differentiating Infected from Vaccinated Animals

H9N2 and H3N2 are the two most important subtypes of low pathogenic avian influenza viruses (LPAIV) because of their ongoing threat to the global poultry industry and public health. Although commercially available inactivated H9N2 vaccines are widely used in the affected countries, endemic H9N2 avian influenza remains uncontrolled. In addition, there is no available avian H3N2 vaccine. Influenza virus-like particles (VLPs) are one of the most promising vaccine alternatives to traditional egg-based vaccines. In this study, to increase the immunogenic content of VLPs to reduce production costs, we developed chimeric bivalent VLPs (cbVLPs) co-displaying hemagglutinin (HA) and neuraminidase (NA) of H9N2 and H3N2 viruses with the Gag protein of bovine immunodeficiency virus (BIV) as the inner core using the Bac-to-Bac baculovirus expression system. The results showed that a single immunization of chickens with 40μg/0.3mL cbVLPs elicited an effective immune response and provided complete protection against H9N2 and H3N2 viruses. More importantly, cbVLPs with accompanying serological assays can successfully accomplish the strategy of differentiating infected animals from vaccinated animals (DIVA), making virus surveillance easier. Therefore, this cbVLP vaccine candidate would be a promising alternative to conventional vaccines, showing great potential for commercial development.

Nanotechnology against COVID-19: Immunization, diagnostic and therapeutic studies

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in early 2020 soon led to the global pandemic of Coronavirus Disease 2019 (COVID-19). Since then, the clinical and scientific communities have been closely collaborating to develop effective strategies for controlling the ongoing pandemic. The game-changing fields of recent years, nanotechnology and nanomedicine have the potential to not only design new approaches, but also to improve existing methods for the fight against COVID-19. Nanomaterials can be used in the development of highly efficient, reusable personal protective equipment, and antiviral nano-coatings in public settings could prevent the spread of SARS-CoV-2. Smart nanocarriers have accelerated the design of several therapeutic, prophylactic, or immune-mediated approaches against COVID-19. Some nanovaccines have even entered Phase IΙ/IIΙ clinical trials. Several rapid and cost-effective COVID-19 diagnostic techniques have also been devised based on nanobiosensors, lab-on-a-chip systems, or nanopore technology. Here, we provide an overview of the emerging role of nanotechnology in the prevention, diagnosis, and treatment of COVID-19.

Engineered Nanoparticle Applications for Recombinant Influenza Vaccines

Influenza viruses cause seasonal epidemics and represent a pandemic risk. With current vaccine methods struggling to protect populations against emerging strains, there is a demand for a next-generation flu vaccine capable of providing broad protection. Recombinant biotechnology, combined with nanomedicine techniques, could address this demand by increasing immunogenicity and directing immune responses toward conserved antigenic targets on the virus. Various nanoparticle candidates have been tested for use in vaccines, including virus-like particles, protein and carbohydrate nanoconstructs, antigen-carrying lipid particles, and synthetic and inorganic particles modified for antigen presentation. These methods have yielded some promising results, including protection in animal models against antigenically distinct influenza strains, production of antibodies with broad reactivity, and activation of potent T cell responses. Based on the evidence of current research, it is feasible that the next generation of influenza vaccines will combine recombinant antigens with nanoparticle carriers.

Influenza-Host Interplay and Strategies for Universal Vaccine Development

Influenza is an annual epidemic and an occasional pandemic caused by pathogens that are responsible for infectious respiratory disease. Humans are highly susceptible to the infection mediated by influenza A viruses (IAV). The entry of the virus is mediated by the influenza virus hemagglutinin (HA) glycoprotein that binds to the cellular sialic acid receptors and facilitates the fusion of the viral membrane with the endosomal membrane. During IAV infection, virus-derived pathogen-associated molecular patterns (PAMPs) are recognized by host intracellular specific sensors including toll-like receptors (TLRs), C-type lectin receptors, retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) either on the cell surface or intracellularly in endosomes. Herein, we comprehensively review the current knowledge available on the entry of the influenza virus into host cells and the molecular details of the influenza virus–host interface. We also highlight certain strategies for the development of universal influenza vaccines.

Interleukin 12-containing influenza virus-like-particle vaccine elevate its protective activity against heterotypic influenza virus infection

To produce monovalent and bivalent influenza vaccines composed of virus-like particles (VLPs) containing hemagglutinin (HA), we generated four recombinant Baculoviruses derived from Bombyx mori nuclear polyhedrosis virus (BmNPV) and Autographa california nuclear polyhedrosis virus (AcNPV). Monovalent Fukushima (A/tufted duck/Fukushima/16/2011 [H5N1]) (FkH5) and Anhui (A/Anhui/1/2013 [H7N9]) (AnH7) VLP influenza vaccines were produced in silkworm pupae infected with FkH5-BmNPV or AnH7-BmNPV. To produce a bivalent FkH5 and AnH7 vaccine, the pupae were simultaneously inoculated with FkH5-BmNPV and AnH7-BmNPV. Then, interleukin (IL)-containing bivalent vaccines were produced by Eri silkworm pupae following triple infection with FkH5-AcNPV, AnH7-AcNPV, and IL-12-AcNPV. Fluorescent antibody tests in Sf9 cells triple-infected with FkH5-AcNPV, AnH7-AcNPV, and IL-12-AcNPV showed coexpression of FkH5, AnH7, and IL-12 antigens, suggesting the presence of VLPs containing all three antigens. We then performed competitive hemagglutination inhibition (CHI) tests to calculate the VLP vaccine constituents. Inoculation with two recombinant viruses led to the production of bivalent vaccines containing very similar amounts of the H5 and H7 antigens, suggesting that our dual infection system can be used to produce bivalent VLP vaccines. Immunisation of mice with our developed monovalent and bivalent VLP vaccines induced the production of HI antibody, which protected against a sublethal dose of influenza virus. These IL-12-containing vaccines tended to display increased protection against hetero-subtype influenza viruses.

Influenza Virus Like Particles (VLPs): Opportunities for H7N9 Vaccine Development

In the midst of the ongoing COVID-19 coronavirus pandemic, influenza virus remains a major threat to public health due to its potential to cause epidemics and pandemics with significant human mortality. Cases of H7N9 human infections emerged in eastern China in 2013 and immediately raised pandemic concerns as historically, pandemics were caused by the introduction of new subtypes into immunologically naïve human populations. Highly pathogenic H7N9 cases with severe disease were reported recently, indicating the continuing public health threat and the need for a prophylactic vaccine. Here we review the development of recombinant influenza virus-like particles (VLPs) as vaccines against H7N9 virus. Several approaches to vaccine development are reviewed including the expression of VLPs in mammalian, plant and insect cell expression systems. Although considerable progress has been achieved, including demonstration of safety and immunogenicity of H7N9 VLPs in the human clinical trials, the remaining challenges need to be addressed. These challenges include improvements to the manufacturing processes, as well as enhancements to immunogenicity in order to elicit protective immunity to multiple variants and subtypes of influenza virus.

Genetic engineering strategies for construction of multivalent chimeric VLPs vaccines

Introduction: Over the past two decades, virus-like particles (VLPs) have been developed as a new generation of vaccines against viral infections. Based on VLPs, chimeric VLPs (chi-VLPs) have been generated through genetic modifications or chemical couplings. For construction of multivalent chi-VLPs vaccines, multiple genetic engineering strategies are continuously being developed. Thus, it is important to provide a summary as reference for researchers in this field.Areas covered: The representative studies on the genetic engineered multivalent chi-VLPs are summarized and mainly focused on chimeric capsid VLPs and chimeric enveloped VLPs. The advantages and limitations of each strategy are also discussed at last, as well as opinions on platform choice and future directions of eVLPs vaccines.Expert opinion: The design of multivalent chi-VLPs vaccines needs to meet the following specifications: 1) the incorporated antigens are suggested to display on the exposed surface of chi-VLPs and do not have excessive adverse effects on the stability of chi-VLPs; 2) the chi-VLPs should elicit protective antibodies against the incorporated antigen as well as the source virus of VLPs. However, there is no requirement of retaining the antigenicity of VLPs when using VLPs solely as carriers for antigens display or drug delivery.

Complexes Formed via Bioconjugation of Genetically Modified TMV Particles with Conserved Influenza Antigen: Synthesis and Characterization

Recently we obtained complexes between genetically modified Tobacco Mosaic Virus (TMV) particles and proteins carrying conserved influenza antigen such as M2e epitope. Viral vector TMV-N-lys based on TMV-U1 genome was constructed by insertion of chemically active lysine into the exposed N-terminal part of the coat protein. Nicotiana benthamiana plants were agroinjected and TMV-N-lys virions were purified from non-inoculated leaves. Preparation was analyzed by SDS-PAGE/Coomassie staining; main protein with electrophoretic mobility of 21 kDa was detected. Electron microscopy confirmed the stability of modified particles. Chemical conjugation of TMV-N-lys virions and target influenza antigen M2e expressed in E. coli was performed using 5 mM 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide and 1 mM N-hydroxysuccinimide. The efficiency of chemical conjugation was confirmed by Western blotting. For additional characterization we used conventional electron microscopy. The diameter of the complexes did not differ significantly from the initial TMV-N-lys virions, but complexes formed highly organized and extensive network with dense "grains" on the surface. Dynamic light scattering demonstrated that the single peaks, reflecting the complexes TMV-N-lys/DHFR-M2e were significantly shifted relative to the control TMV-N-lys virions. The indirect enzyme-linked immunosorbent assay with TMV- and DHFR-M2e-specific antibodies showed that the complexes retain stability during overnight adsorption. Thus, the results allow using these complexes for immunization of animals with the subsequent preparation of a candidate universal vaccine against the influenza virus.

Development and fit-for-purpose verification of an LC-MS method for quantitation of hemagglutinin and neuraminidase proteins in influenza virus-like particle vaccine candidates

Recombinant influenza Virus-Like Particle (VLP) vaccines are promising vaccine candidates to prevent influenza, contain two major viral antigenic glycoproteins, Hemagglutinin (HA) and Neuraminidase (NA), on the surface of recombinant VLPs. Accurate quantitation of the mass of these antigenic proteins is important to ensure the product quality and proper dosing. Currently, Single Radial Immunodiffusion (SRID) is a recognized assay for determination of the HA immuno-reactive concentration (potency) in vaccine products, based on immuno-reactivity of HA with strain-specific antisera. The SRID assay, however, requires availability of strain-specific and properly calibrated reagents, which can be time-consuming to generate and calibrate. In addition, the assay is not suitable for quantitation of low abundant proteins, such as NA. In order to accelerate the overall production cycle, we have developed and optimized a high-resolution (HR) LC-MS method for absolute quantitation of both HA and NA protein concentrations in influenza VLP vaccine candidates. In this work, we present the method development, optimization and verification of its suitability for the intended purpose, as a prerequisite for its potential application in Quality Control, by assessing specificity, precision and accuracy, detection characteristics, and dynamic linear range. The method can be also used for other HA/NA containing preparations including in-process samples, purified proteins, whole virus preparations, nano-particle and egg-based vaccine preparations, or for calibration of SRID reference antigens.

Synthetic biology for bioengineering virus-like particle vaccines

Vaccination is the most effective method of disease prevention and control. Many viruses and bacteria that once caused catastrophic pandemics (e.g., smallpox, poliomyelitis, measles, and diphtheria) are either eradicated or effectively controlled through routine vaccination programs. Nonetheless, vaccine manufacturing remains incredibly challenging. Viruses exhibiting high antigenic diversity and high mutation rates cannot be fairly contested using traditional vaccine production methods and complexities surrounding the manufacturing processes, which impose significant limitations. Virus‐like particles (VLPs) are recombinantly produced viral structures that exhibit immunoprotective traits of native viruses but are noninfectious. Several VLPs that compositionally match a given natural virus have been developed and licensed as vaccines. Expansively, a plethora of studies now confirms that VLPs can be designed to safely present heterologous antigens from a variety of pathogens unrelated to the chosen carrier VLPs. Owing to this design versatility, VLPs offer technological opportunities to modernize vaccine supply and disease response through rational bioengineering. These opportunities are greatly enhanced with the application of synthetic biology, the redesign and construction of novel biological entities. This review outlines how synthetic biology is currently applied to engineer VLP functions and manufacturing process. Current and developing technologies for the identification of novel target‐specific antigens and their usefulness for rational engineering of VLP functions (e.g., presentation of structurally diverse antigens, enhanced antigen immunogenicity, and improved vaccine stability) are described. When applied to manufacturing processes, synthetic biology approaches can also overcome specific challenges in VLP vaccine production. Finally, we address several challenges and benefits associated with the translation of VLP vaccine development into the industry.

Biological characteristics and immunological properties in Muscovy ducks of H5N6 virus-like particles composed of HA-TM/HA-TMH3 and M1

Highly pathogenic avian influenza viruses (HPAIVs), including H5N6 strains, pose threats to the health of humans and poultry. Waterfowl play a crucial role as a reservoir of HPAIVs. Since current influenza vaccines induce poor antibody titres in waterfowl, there is an urgent need to develop an efficient vaccine against H5N6 infection. In this study, we constructed two H5N6 virus-like particles (VLPs) composed of matrix-1 (M1) and haemagglutinin of wildtype (HA-TM) or haemagglutinin with transmembrane domain replacement (HA-TM_H3) (designated as H5N6 VLPs-TM and H5N6 VLPs-TM_H3). Biological characteristics of the composed H5N6 VLPs were compared including localization, expression, contents of HA trimers, thermal stability, morphology and immunogenicity in Muscovy ducks. Our results indicate that the H5N6 VLPs-TM_H3 contained more HA trimers and presented better thermal stability. Moreover, Muscovy ducks immunized with H5N6 VLPs-TM_H3 produced higher titres of HI antibody and IFN-γ compared with those immunized with the same dose of H5N6 VLP-TM, thus providing a promising approach for the development of influenza virus vaccines for waterfowl. RESEARCH HIGHLIGHTS H5N6 VLPs-TM_H3 had more HA trimers and resisted higher temperature than H5N6 VLPs-TM H5N6 VLPs-TM_H3 induced higher titre of HI than H5N6 VLPs-TM in Muscovy ducks.

How far have we reached in development of effective influenza vaccine?

Despite of ongoing research programs and numerous clinical trials, seasonal influenza epidemics remain a major concern globally. Vaccination remains the most effective method to prevent influenza infection. However, current flu vaccines have several limitations, including limited vaccine capacity, long production times, inconsistence efficacy in certain populations, and lack of a "universal" solution. Different next-generation approaches such as cell line-based culture, reverse genetics, and virus expression technology are currently under development to address the aforementioned challenges in conventional vaccine manufacture pipeline. Such approaches hope for safe and scalable production, induce broad-spectrum immunity, create premade libraries of vaccine strains, and target nonvariable regions of antigenic proteins for "universal" vaccination. Here, we discuss the process and challenges of the current influenza vaccine platform as well as new approaches that are being investigated. These developments indicate that an exciting future lies ahead in the influenza vaccine field.

The Multifaceted Zoonotic Risk of H9N2 Avian Influenza

Poultry-adapted H9N2 avian influenza viruses (AIVs) are commonly found in many countries in Asia, the Middle East, Africa, and Europe, and although classified as low pathogenic viruses, they are an economically important disease. Besides the importance of the disease in the poultry industry, some H9N2 AIVs are also known to be zoonotic. The disease in humans appears to cause primarily a mild upper respiratory disease, and doesn't cause or only rarely causes the severe pneumonia often seen with other zoonotic AIVs like H5N1 or H7N9. Serologic studies in humans, particularly in occupationally exposed workers, show a large number of people with antibodies to H9N2, suggesting infection is commonly occurring. Of the four defined H9N2 poultry lineages, only two lineages, the G1 and the Y280 lineages, are associated with human infections. Almost all of the viruses from humans have a leucine at position 226 (H3 numbering) of the hemagglutinin associated with a higher affinity of binding with α2,6 sialic acid, the host cell receptor most commonly found on glycoproteins in the human upper respiratory tract. For unknown reasons there has also been a shift in recent years of poultry viruses in the G1 and Y280 lineages to also having leucine instead of glutamine, the amino acid found in most avian viruses, at position 226. The G1 and Y280 poultry lineages because of their known ability to infect humans, the high prevalence of the virus in poultry in endemic countries, the lack of antibody in most humans, and the shift of poultry viruses to more human-like receptor binding makes these viruses a human pandemic threat. Increased efforts for control of the virus, including through effective vaccine use in poultry, is warranted for both poultry and public health goals.

Virus-like particle vaccines: immunology and formulation for clinical translation

INTRODUCTION

Virus-like particle (VLP) vaccines face significant challenges in their translation from laboratory models, to routine clinical administration. While some VLP vaccines thrive and are readily adopted into the vaccination schedule, others are restrained by regulatory obstacles, proprietary limitations, or finding their niche amongst the crowded vaccine market. Often the necessity to supplant an existing vaccination regimen possesses an immediate obstacle for the development of a VLP vaccine, despite any preclinical advantages identified over the competition. Novelty, adaptability and formulation compatibility may prove invaluable in helping place VLP vaccines at the forefront of vaccination technology.

AREAS COVERED

The purpose of this review is to outline the diversity of VLP vaccines, VLP-specific immune responses, and to explore how modern formulation and delivery techniques can enhance the clinical relevance and overall success of VLP vaccines.

EXPERT COMMENTARY

The role of formation science, with an emphasis on the diversity of immune responses induced by VLP, is underrepresented amongst clinical trials for VLP vaccines. Harnessing such diversity, particularly through the use of combinations of select excipients and adjuvants, will be paramount in the development of VLP vaccines.

Overexpression of a virus-like particle influenza vaccine in Eri silkworm pupae, using Autographa californica nuclear polyhedrosis virus and host-range expansion

Ecological investigations of silkworms have revealed that Eri silkworms (Samia cynthia ricini) possess useful morphological and ecological characteristics for virus-like particle (VLP) production, namely non-seasonal breeding, longer lengths, and heavier weights than Bombyx mori silkworms. Furthermore, when vector DNA from Bombyx mori nuclear polyhedrosis virus (BmNPV), which is unable to replicate in Sf9 cells from Eri silkworms, was replaced with the Autographa californica nuclear polyhedrosis virus (AcNPV) vector, three improved AcNPV influenza virus recombinants capable of replication in Sf9 cells were obtained. Although VLP antigens produced previously in silkworms were not evaluated individually, the present recombinant Fukushima (FkH5) and Anhui (AnH7) VLP antigens were detected in tissue fluids and fat bodies of Eri silkworms. Here, we aimed to determine the function of the AcNPV vector and P143 gene by expressing recombinants in Sf9 cells and eri silkworm pupae. The FkH5 recombinant produced high yields of haemagglutinin (HA)-positive VLPs, showing a mean HA titre of 1.2 million. Similarly, high production of H7 HA VLPs was observed in the fat bodies of eri silkworm pupae. Antigenic analysis and electron microscopy examination of Eri-silkworm-produced H5 HA VLPs showed characteristic antigenicity and morphology similar to those of the influenza virus. Although FkH5 recombinants possessing the AcNPV vector did not replicate in Bm-N cells, the introduction of the helicase p143 gene from BmNPV resulted in their production in Bm-N and Sf9 cells.

Multi-clade H5N1 virus-like particles: Immunogenicity and protection against H5N1 virus and effects of beta-propiolactone

During the past decade, H5N1 highly pathogenic avian influenza (HPAI) viruses have diversified genetically and antigenically, suggesting the need for multiple H5N1 vaccines. However, preparation of multiple vaccines from live H5N1 HPAI viruses is difficult and economically not feasible representing a challenge for pandemic preparedness. Here we evaluated a novel multi-clade recombinant H5N1 virus-like particle (VLP) design, in which H5 hemagglutinins (HA) and N1 neuraminidase (NA) derived from four distinct clades of H5N1 virus were co-localized within the VLP structure. The multi-clade H5N1 VLPs were prepared by using a recombinant baculovirus expression system and evaluated for functional hemagglutination and neuraminidase enzyme activities, particle size and morphology, as well as for the presence of baculovirus in the purified VLP preparations. To remove residual baculovirus, VLP preparations were treated with beta-propiolactone (BPL). Immunogenicity and efficacy of multi-clade H5N1 VLPs were determined in an experimental ferret H5N1 HPAI challenge model, to ascertain the effect of BPL on immunogenicity and protective efficacy against lethal challenge. Although treatment with BPL reduced immunogenicity of VLPs, all vaccinated ferrets were protected from lethal challenge with influenza A/VietNam/1203/2004 (H5N1) HPAI virus, indicating that multi-clade VLP preparations treated with BPL represent a potential approach for pandemic preparedness vaccines.

EDiP: the Epitope Dilution Phenomenon. Lessons learnt from a malaria vaccine antigen and its applicability to polymorphic antigens

INTRODUCTION

Polymorphism in vaccine antigens poses major challenges to vaccinologists. The Plasmodium falciparum Apical Membrane Antigen 1 (AMA1) poses such a challenge. We found that immunization with a mixture of three variants yielded functional antibody levels to all variants comparable to levels induced by monovalent immunization. The mechanism behind the observed broadening was shown to be an increase in the fraction of cross-reactive antibodies, most likely because strain-specific epitopes are present at lower frequency relative to conserved epitopes. Areas covered: We hereby introduce the Epitope Dilution Phenomenon (EDiP) as a practical strategy for the induction of broad, cross-variant antibody responses against polymorphic antigens and discuss the utility and applicability of this phenomenon for the development of vaccines against polymorphic antigens of pathogens like Influenza, HIV, Dengue and Plasmodium. Expert commentary: EDiP can be used to broaden antibody responses by immunizing with a mixture of at least 3 antigenic variants, where the variants included can differ, yet yield broadened responses.

Antibody Immunity Induced by H7N9 Avian Influenza Vaccines: Evaluation Criteria, Affecting Factors, and Implications for Rational Vaccine Design

Severe H7N9 avian influenza virus (AIV) infections in humans have public health authorities around the world on high alert for the potential development of a human influenza pandemic. Currently, the newly-emerged highly pathogenic avian influenza A (H7N9) virus poses a dual challenge for public health and poultry industry. Numerous H7N9 vaccine candidates have been generated using various platforms. Immunization trials in animals and humans showed that H7N9 vaccines are apparently poorly immunogenic because they induced low hemagglutination inhibition and virus neutralizing antibody titers. However, H7N9 vaccines elicit comparable levels of total hemagglutinin (HA)-reactive IgG antibody as the seasonal influenza vaccines, suggesting H7N9 vaccines are as immunogenic as their seasonal counterparts. A large fraction of overall IgG antibody is non-neutralizing antibody and they target unrecognized epitopes outside of the traditional antigenic sites in HA. Further, the Treg epitope identified in H7 HA may at least partially contribute to regulation of antibody immunity. Here, we review the latest advances for the development of H7N9 vaccines and discuss the influence of serological criteria on evaluation of immunogenicity of H7N9 vaccines. Next, we discuss factors affecting antibody immunity induced by H7N9 vaccines, including the change in antigenic epitopes in HA and the presence of the Treg epitope. Last, we present our perspectives for the unique features of antibody immunity of H7N9 vaccines and propose some future directions to improve or modify antibody response induced by H7N9 vaccines. This perspective would provide critical implications for rational design of H7N9 vaccines for human and veterinary use.

Vaccine approaches conferring cross-protection against influenza viruses

INTRODUCTION

Annual vaccination is one of the most efficient and cost-effective strategies to prevent and control influenza epidemics. Most of the currently available influenza vaccines are strong inducers of antibody responses against viral surface proteins, hemagglutinin (HA) and neuraminidase (NA), but are poor inducers of cell-mediated immune responses against conserved internal proteins. Moreover, due to the high variability of viral surface proteins because of antigenic drift or antigenic shift, many of the currently licensed vaccines confer little or no protection against drift or shift variants. Areas covered: Next generation influenza vaccines that can induce humoral immune responses to receptor-binding epitopes as well as broadly neutralizing conserved epitopes, and cell-mediated immune responses against highly conserved internal proteins would be effective against variant viruses as well as a novel pandemic influenza until circulating strain-specific vaccines become available. Here we discuss vaccine approaches that have the potential to provide broad spectrum protection against influenza viruses. Expert commentary: Based on current progress in defining cross-protective influenza immunity, it seems that the development of a universal influenza vaccine is feasible. It would revolutionize the strategy for influenza pandemic preparedness, and significantly impact the shelf-life and protection efficacy of seasonal influenza vaccines.

Increasing the breadth and potency of response to the seasonal influenza virus vaccine by immune complex immunization

The main barrier to reduction of morbidity caused by influenza is the absence of a vaccine that elicits broad protection against different virus strains. Studies in preclinical models of influenza virus infections have shown that antibodies alone are sufficient to provide broad protection against divergent virus strains in vivo. Here, we address the challenge of identifying an immunogen that can elicit potent, broadly protective, antiinfluenza antibodies by demonstrating that immune complexes composed of sialylated antihemagglutinin antibodies and seasonal inactivated flu vaccine (TIV) can elicit broadly protective antihemagglutinin antibodies. Further, we found that an Fc-modified, bispecific monoclonal antibody against conserved epitopes of the hemagglutinin can be combined with TIV to elicit broad protection, thus setting the stage for a universal influenza virus vaccine.

A clinical approach to the threat of emerging influenza viruses in the Asia-Pacific region

Seasonal influenza epidemics and periodic pandemics are important causes of morbidity and mortality. Patients with chronic co‐morbid illness, those at the extremes of age and pregnant women are at higher risks of complications requiring hospitalization, whereas young adults and obese individuals were also at increased risk during the A(H1N1) pandemic in 2009. Avian influenza A(H5N1) and A(H7N9) viruses have continued to circulate widely in some poultry populations and infect humans sporadically since 1997 and 2013, respectively. The recent upsurge in human cases of A(H7N9) infections in Mainland China is of great concern. Sporadic human cases of avian A(H5N6), A(H10N8) and A(H6N1) have also emerged in recent years while there are also widespread poultry outbreaks due to A(H5N8) in many countries. Observational studies have shown that treatment with a neuraminidase inhibitor (NAI) for adults hospitalized with severe influenza is associated with lower mortality and better clinical outcomes, especially when administered early in the course of illness. Whether higher than standard doses of NAI would provide greater antiviral effects in such patients will require further investigation. High‐dose systemic corticosteroids were associated with worse outcomes in patients with severe influenza. There is an urgent need for developing more effective antiviral therapies for treatment of influenza infections.

Neuraminidase-based recombinant virus-like particles protect against lethal avian influenza A(H5N1) virus infection in ferrets

Avian influenza A (H5N1) viruses represent a growing threat for an influenza pandemic. The presence of widespread avian influenza virus infections further emphasizes the need for vaccine strategies for control of pre-pandemic H5N1 and other avian influenza subtypes. Influenza neuraminidase (NA) vaccines represent a potential strategy for improving vaccines against avian influenza H5N1 viruses. To evaluate a strategy for NA vaccination, we generated a recombinant influenza virus-like particle (VLP) vaccine comprised of the NA protein of A/Indonesia/05/2005 (H5N1) virus. Ferrets vaccinated with influenza N1 NA VLPs elicited high-titer serum NA-inhibition (NI) antibody titers and were protected from lethal challenge with A/Indonesia/05/2005 virus. Moreover, N1-immune ferrets shed less infectious virus than similarly challenged control animals. In contrast, ferrets administered control N2 NA VLPs were not protected against H5N1 virus challenge. These results provide support for continued development of NA-based vaccines against influenza H5N1 viruses.

Strategies to induce broadly protective antibody responses to viral glycoproteins

INTRODUCTION

Currently, several universal/broadly protective influenza virus vaccine candidates are under development. Many of these vaccines are based on strategies to induce protective antibody responses against the surface glycoproteins of antigenically and genetically diverse influenza viruses. These strategies might also be applicable to surface glycoproteins of a broad range of other important viral pathogens. Areas covered: Common strategies include sequential vaccination with divergent antigens, multivalent approaches, vaccination with glycan-modified antigens, vaccination with minimal antigens and vaccination with antigens that have centralized/optimized sequences. Here we review these strategies and the underlying concepts. Furthermore, challenges, feasibility and applicability to other viral pathogens are discussed. Expert commentary: Several broadly protective/universal influenza virus vaccine strategies will be tested in humans in the coming years. If successful in terms of safety and immunological readouts, they will move forward into efficacy trials. In the meantime, successful vaccine strategies might also be applied to other antigenically diverse viruses of concern.

Combining stable insect cell lines with baculovirus-mediated expression for multi-HA influenza VLP production

Safer and broadly protective vaccines are needed to cope with the continuous evolution of circulating influenza virus strains and promising approaches based on the expression of multiple hemagglutinins (HA) in a virus-like particle (VLP) have been proposed. However, expression of multiple genes in the same vector can lead to its instability due to tandem repetition of similar sequences. By combining stable with transient expression systems we can rationally distribute the number of genes to be expressed per platform and thus mitigate this risk. In this work, we developed a modular system comprising stable and baculovirus-mediated expression in insect cells for production of multi-HA influenza enveloped VLPs. First, a stable insect High Five cell population expressing two different HA proteins from subtype H3 was established. Infection of this cell population with a baculovirus vector encoding three other HA proteins from H3 subtype proved to be as competitive as traditional co-infection approaches in producing a pentavalent H3 VLP. Aiming at increasing HA expression, the stable insect cell population was infected at increasingly higher cell concentrations (CCI). However, cultures infected at CCI of 3×10⁶cells/mL showed lower HA titers per cell in comparison to standard CCI of 2×10⁶cells/mL, a phenomenon named "cell density effect". To lessen the negative impact of this phenomenon, a tailor-made refeed strategy was designed based on the exhaustion of key nutrients during cell growth. Noteworthy, cultures supplemented and infected at a CCI of 4×10⁶cells/mL showed comparable HA titers per cell to those of CCI of 2×10⁶cells/mL, thus leading to an increase of up to 4-fold in HA titers per mL. Scalability of the modular strategy herein proposed was successfully demonstrated in 2L stirred tank bioreactors with comparable HA protein levels observed between bioreactor and shake flasks cultures. Overall, this work demonstrates the suitability of combining stable with baculovirus-mediated expression in insect cells as an efficient platform for production of multi-HA influenza VLPs, surpassing the drawbacks of traditional co-infection strategies and/or the use of larger, unstable vectors.

Quantitative analysis of the yield of avian H7 influenza virus haemagglutinin protein produced in silkworm pupae with the use of the codon-optimized DNA: A possible oral vaccine

In this study, we aimed to quantitatively compare the increased production of three H7 influenza virus-like particle (VLP) haemagglutinin (HA) with the use of a codon-optimized single HA gene in silkworm pupae. Recombinant baculovirus (Korea H7-BmNPV) could produce 0.40 million HA units per pupa, corresponding to 1832μg protein. The yield of the HA produced in larva was estimated to be approximately 0.31 million HA units per larva, and there were no significant differences between the three HA proteins. We could establish efficient recovery system of HA production in larvae and pupae with the use of three cycles sonication methods. Next, we compared yields of HA proteins from three different H7 and two H5 recombinant baculoviruses based on the amount of mRNA synthesized in BmN cells, suggesting that mRNA synthesis may be also a useful indicator for the production of HA. Based on HA titres from four recombinants, the yield of HA had a great influence on the codon-optimized effect and the characteristics of the viral HA gene. The recombinant containing codon optimized HA DNA of A/tufted duck/Fukushima/16/2011 (H5N1) did produce more than one million HA units, although another recombinant including of the wild H5N1 strain failed to show HA activity. Electron microscopy revealed the presence of large VLP and small HA particle in the heavy and light fractions. The purified VLPs reacted with the authentic anti-H7 antibodies and the antibodies prepared after immunization with the VLP H7 antigen. Also H5 and H7VLPs could produce HI antibody in chickens and mice with oral immunization. The antibodies elicited with oral immunization were confirmed in fluorescent antibody analysis and western blotting in Korea H5-BmNPV and H7HA-BmNPV recombinant infected BmN cells. Taken together, these findings provided important insights into future oral vaccine development.

Reverse Genetics Approaches for the Development of Influenza Vaccines

Influenza viruses cause annual seasonal epidemics and occasional pandemics of human respiratory disease. Influenza virus infections represent a serious public health and economic problem, which are most effectively prevented through vaccination. However, influenza viruses undergo continual antigenic variation, which requires either the annual reformulation of seasonal influenza vaccines or the rapid generation of vaccines against potential pandemic virus strains. The segmented nature of influenza virus allows for the reassortment between two or more viruses within a co-infected cell, and this characteristic has also been harnessed in the laboratory to generate reassortant viruses for their use as either inactivated or live-attenuated influenza vaccines. With the implementation of plasmid-based reverse genetics techniques, it is now possible to engineer recombinant influenza viruses entirely from full-length complementary DNA copies of the viral genome by transfection of susceptible cells. These reverse genetics systems have provided investigators with novel and powerful approaches to answer important questions about the biology of influenza viruses, including the function of viral proteins, their interaction with cellular host factors and the mechanisms of influenza virus transmission and pathogenesis. In addition, reverse genetics techniques have allowed the generation of recombinant influenza viruses, providing a powerful technology to develop both inactivated and live-attenuated influenza vaccines. In this review, we will summarize the current knowledge of state-of-the-art, plasmid-based, influenza reverse genetics approaches and their implementation to provide rapid, convenient, safe and more effective influenza inactivated or live-attenuated vaccines.

Virus-like particles displaying H5, H7, H9 hemagglutinins and N1 neuraminidase elicit protective immunity to heterologous avian influenza viruses in chickens

Avian influenza (AI) viruses circulating in wild birds pose a serious threat to public health. Human and veterinary vaccines against AI subtypes are needed. Here we prepared triple-subtype VLPs that co-localized H5, H7 and H9 antigens derived from H5N1, H7N3 and H9N2 viruses. VLPs also contained influenza N1 neuraminidase and retroviral gag protein. The H5/H7/H9/N1/gag VLPs were prepared using baculovirus expression. Biochemical, functional and antigenic characteristics were determined including hemagglutination and neuraminidase enzyme activities. VLPs were further evaluated in a chicken AI challenge model for safety, immunogenicity and protective efficacy against heterologous AI viruses including H5N2, H7N3 and H9N2 subtypes. All vaccinated birds survived challenges with H5N2 and H7N3 highly pathogenic AI (HPAI) viruses, while all controls died. Immune response was also detectable after challenge with low pathogenicity AI (LPAI) H9N2 virus suggesting that H5/H7/H9/N1/gag VLPs represent a promising approach for the development of broadly protective AI vaccine.

Mono- and quadri-subtype virus-like particles (VLPs) containing H10 subtype elicit protective immunity to H10 influenza in a ferret challenge model

Avian-origin influenza represents a global public health concern. In 2013, the H10N8 virus caused documented human infections for the first time. Currently, there is no approved vaccine against H10 influenza. Recombinant virus-like particles (VLPs) represent a promising vaccine approach. In this study, we evaluated H10 VLPs containing hemagglutinin from H10N8 virus as an experimental vaccine in a ferret challenge model. In addition, we evaluated quadri-subtype VLPs co-localizing H5, H7, H9 and H10 subtypes. Both vaccines elicited serum antibody that reacted with the homologous H10 derived from H10N8 virus and cross-reacted with the heterologous H10N1 virus. Quadri-subtype vaccine also elicited serum antibody to the homologous H5, H7, and H9 antigens and cross-reacted with multiple clades of H5N1 virus. After heterologous challenge with the H10N1 virus, all vaccinated ferrets showed significantly reduced titers of replicating virus in the respiratory tract indicating protective effect of vaccination with either H10 VLPs or with quadri-subtype VLPs.

Multi-subtype influenza virus-like particles incorporated with flagellin and granulocyte-macrophage colony-stimulating factor for vaccine design

Virus-like particle (VLP) technology is an attractive platform for seasonal and pandemic influenza vaccine development. We previously showed that influenza VLPs can be modified using M2 fusion with molecular adjuvants such as Salmonella typhimurium flagellin (FliC) to enhance VLP immunogenicity. For this study, three types of chimeric VLPs were incorporated with FliC, granulocyte-macrophage colony-stimulating factor (GM-CSF), or both GM-CSF and FliC (GM-CSF/FliC) to enhance anti-influenza immunogenicity. Our results indicate that immunizations with the chimeric FliC VLPs and GM-CSF/FliC H5N1 VLPs elicited more potent and broadly neutralizing antibodies and neuraminidase-inhibiting antibodies in sera, and induced higher numbers of hemagglutinin-specific antibody-secreting cells and germinal center B cell subsets in splenoctyes. Immunization with the chimeric GM-CSF H5N1 VLPs induced stronger Th1 and Th2 cellular responses. The chimeric GM-CSF/FliC H5N1 VLP constructs were further obtained to include H7 or H1H7 bi- or tri-subtype. It is our hope that these findings provide useful information for developing multi-subtype influenza vaccines.

Hemagglutinin amino acids related to receptor specificity could affect the protection efficacy of H5N1 and H7N9 avian influenza virus vaccines in mice

The continuous and sporadic human transmission of highly pathogenic avian H5N1 and H7N9 influenza viruses illustrates the urgent need for efficacious vaccines. However, all tested vaccines for the H5N1 and H7N9 viruses appear to be poorly immunogenic in mammals. In this study, a series of vaccines was produced using reverse genetic techniques that possess HA and NA genes from the H5N1 virus in the genetic background of the high-yield strain A/PR/8/34 (H1N1). Meanwhile, a group of H7N9 VLP vaccines that contain HA from H7N9 and NA and M1 from A/PR/8/34 (H1N1) was also produced. The HA amino acids of both the H5N1 and H7N9 vaccines differed at residues 226 and 228, both of which are critical for receptor specificity for an avian or mammalian host. Mice received two doses (3μg of HA each) of each vaccine and were challenged with lethal doses of wild type H5N1 or H7N9 viruses. The results showed that a recombinant H5N1 vaccine in which the HA amino acid G228 (avian specificity) was converted to S228 (mammalian specificity) resulted in higher HI titers, a lower viral titer in the lungs, and 100% protection in mice. However, a H7N9 VLP vaccine that contains L226 (mammalian specificity) and G228 (avian specificity) in HA showed better immunogenicity and protection efficacy in mice than VLP containing HA with either L226+S228 or Q226+S228. This observation indicated that specific HA residues could enhance a vaccine's protection efficacy and HA glycoproteins with both avian-type and human-type receptor specificities may produce better pandemic influenza vaccines for humans.

Vaccination with virus-like particles containing H5 antigens from three H5N1 clades protects chickens from H5N1 and H5N8 influenza viruses

Highly pathogenic avian influenza (HPAI) viruses, especially H5N1 strains, represent a public health threat and cause widespread morbidity and mortality in domestic poultry. Recombinant virus-like particles (VLPs) represent a promising novel vaccine approach to control avian influenza including HPAI strains. Influenza VLPs contain viral hemagglutinin (HA), which can be expressed in cell culture within highly immunogenic VLPs that morphologically and antigenically resemble influenza virions, except VLPs are non-infectious. Here we describe a recombinant VLP containing HA proteins derived from three distinct clades of H5N1 viruses as an experimental, broadly protective H5 avian influenza vaccine. A baculovirus vector was configured to co-express the H5 genes from recent H5N1 HPAI isolates A/chicken/Germany/2014 (clade 2.3.4.4), A/chicken/West Java/Subang/29/2007 (clade 2.1.3) and A/chicken/Egypt/121/2012 (clade 2.2.1). Co-expression of these genes in Sf9 cells along with influenza neuraminidase (NA) and retrovirus gag genes resulted in production of triple-clade H555 VLPs that exhibited hemagglutination activity and morphologically resembled influenza virions. Vaccination of chickens with these VLPs resulted in induction of serum antibody responses and efficient protection against experimental challenges with three different viruses including the recent U.S. H5N8 HPAI isolate. We conclude that these novel triple-clade VLPs represent a feasible strategy for simultaneously evoking protective antibodies against multiple variants of H5 influenza virus.

Production of Virus-Like Particles for Vaccination

The ability to make a large variety of virus-like particles (VLPs) has been successfully achieved in the baculovirus expression vector system (BEVS)/insect cell system. The production and scale-up of these particles, which are mostly sought as vaccine candidates, are currently being addressed. Furthermore, these VLPs are being investigated as delivery agents for use as therapeutics. The use of host insect cells allows mass production of VLPs in a proven scalable system.

Preparation of quadri-subtype influenza virus-like particles using bovine immunodeficiency virus gag protein

Influenza VLPs comprised of hemagglutinin (HA), neuraminidase (NA), and matrix (M1) proteins have been previously used for immunological and virological studies. Here we demonstrated that influenza VLPs can be made in Sf9 cells by using the bovine immunodeficiency virus gag (Bgag) protein in place of M1. We showed that Bgag can be used to prepare VLPs for several influenza subtypes including H1N1 and H10N8. Furthermore, by using Bgag, we prepared quadri-subtype VLPs, which co-expressed within the VLP the four HA subtypes derived from avian-origin H5N1, H7N9, H9N2 and H10N8 viruses. VLPs showed hemagglutination and neuraminidase activities and reacted with specific antisera. The content and co-localization of each HA subtype within the quadri-subtype VLP were evaluated. Electron microscopy showed that Bgag-based VLPs resembled influenza virions with the diameter of 150-200nm. This is the first report of quadri-subtype design for influenza VLP and the use of Bgag for influenza VLP preparation.

Evaluation of Trichodysplasia Spinulosa-Associated Polyomavirus Capsid Protein as a New Carrier for Construction of Chimeric Virus-Like Particles Harboring Foreign Epitopes

Recombinant virus-like particles (VLPs) represent a promising tool for protein engineering. Recently, trichodysplasia spinulosa-associated polyomavirus (TSPyV) viral protein 1 (VP1) was efficiently produced in yeast expression system and shown to self-assemble to VLPs. In the current study, TSPyV VP1 protein was exploited as a carrier for construction of chimeric VLPs harboring selected B and T cell-specific epitopes and evaluated in comparison to hamster polyomavirus VP1 protein. Chimeric VLPs with inserted either hepatitis B virus preS1 epitope DPAFR or a universal T cell-specific epitope AKFVAAWTLKAAA were produced in yeast Saccharomyces cerevisiae. Target epitopes were incorporated either at the HI or BC loop of the VP1 protein. The insertion sites were selected based on molecular models of TSPyV VP1 protein. The surface exposure of the insert positions was confirmed using a collection of monoclonal antibodies raised against the intact TSPyV VP1 protein. All generated chimeric proteins were capable to self-assemble to VLPs, which induced a strong immune response in mice. The chimeric VLPs also activated dendritic cells and T cells as demonstrated by analysis of cell surface markers and cytokine production profiles in spleen cell cultures. In conclusion, TSPyV VP1 protein represents a new potential carrier for construction of chimeric VLPs harboring target epitopes.

An Intranasal Virus-Like Particle Vaccine Broadly Protects Mice from Multiple Subtypes of Influenza A Virus

Influenza virus infections are a global public health problem, with a significant impact of morbidity and mortality from both annual epidemics and pandemics. The current strategy for preventing annual influenza is to develop a new vaccine each year against specific circulating virus strains. Because these vaccines are unlikely to protect against an antigenically divergent strain or a new pandemic virus with a novel hemagglutinin (HA) subtype, there is a critical need for vaccines that protect against all influenza A viruses, a so-called "universal" vaccine. Here we show that mice were broadly protected against challenge with a wide variety of lethal influenza A virus infections (94% aggregate survival following vaccination) with a virus-like particle (VLP) vaccine cocktail. The vaccine consisted of a mixture of VLPs individually displaying H1, H3, H5, or H7 HAs, and vaccinated mice showed significant protection following challenge with influenza viruses expressing 1918 H1, 1957 H2, and avian H5, H6, H7, H10, and H11 hemagglutinin subtypes. These experiments suggest a promising and practical strategy for developing a broadly protective "universal" influenza vaccine.

IMPORTANCE

The rapid and unpredictable nature of influenza A virus evolution requires new vaccines to be produced annually to match circulating strains. Human infections with influenza viruses derived from animals can cause outbreaks that may be associated with high mortality, and such strains may also adapt to humans to cause a future pandemic. Thus, there is a large public health need to create broadly protective, or "universal," influenza vaccines that could prevent disease from a wide variety of human and animal influenza A viruses. In this study, a noninfectious virus-like particle (VLP) vaccine was shown to offer significant protection against a variety of influenza A viruses in mice, suggesting a practical strategy to develop a universal influenza vaccine.

Recombinant Hemagglutinin and Virus-Like Particle Vaccines for H7N9 Influenza Virus

Cases of H7N9 human infection were caused by a novel, avian-origin H7N9 influenza A virus that emerged in eastern China in 2013. Clusters of human disease were identified in many cities in China, with mortality rates approaching 30%. Pandemic concerns were raised, as historically, influenza pandemics were caused by introduction of novel influenza A viruses into immunologically naïve human population. Currently, there are no approved human vaccines for H7N9 viruses. Recombinant protein vaccine approaches have advantages in safety and manufacturing. In this review, we focused on evaluation of the expression of recombinant hemagglutinin (rHA) proteins as candidate vaccines for H7N9 influenza, with the emphasis on the role of oligomeric and particulate structures in immunogenicity and protection. Challenges in preparation of broadly protective influenza vaccines are discussed, and examples of broadly protective vaccines are presented including rHA stem epitope vaccines, as well as recently introduced experimental multi-HA VLP vaccines.

The potential of nanoparticles for the immunization against viral infections

Vaccination has a great impact on the prevention and control of infectious diseases. However, there are still many infectious diseases for which an effective vaccine is missing. Thirty years after the discovery of the AIDS-pathogen (human immunodeficiency virus, HIV) and intensive research, there is still no protective immunity against the HIV infection. Over the past decade, nanoparticulate systems such as virus-like particles, liposomes, polymers and inorganic nanoparticles have received attention as potential delivery vehicles which can be loaded or functionalized with active biomolecules (antigens and adjuvants). Here we compare the properties of different nanoparticulate systems and assess their potential for the development of new vaccines against a range of viral infections.

Critical assessment of influenza VLP production in Sf9 and HEK293 expression systems

Background

Each year, influenza is responsible for hundreds of thousand cases of illness and deaths worldwide. Due to the virus’ fast mutation rate, the World Health Organization (WHO) is constantly on alert to rapidly respond to emerging pandemic strains. Although anti-viral therapies exist, the most proficient way to stop the spread of disease is through vaccination. The majority of influenza vaccines on the market are produced in embryonic hen’s eggs and are composed of purified viral antigens from inactivated whole virus. This manufacturing system, however, is limited in its production capacity. Cell culture produced vaccines have been proposed for their potential to overcome the problems associated with egg-based production. Virus-like particles (VLPs) of influenza virus are promising candidate vaccines under consideration by both academic and industry researchers.

Methods

In this study, VLPs were produced in HEK293 suspension cells using the Bacmam transduction system and Sf9 cells using the baculovirus infection system. The proposed systems were assessed for their ability to produce influenza VLPs composed of Hemagglutinin (HA), Neuraminidase (NA) and Matrix Protein (M1) and compared through the lens of bioprocessing by highlighting baseline production yields and bioactivity. VLPs from both systems were characterized using available influenza quantification techniques, such as single radial immunodiffusion assay (SRID), HA assay, western blot and negative staining transmission electron microscopy (NSTEM) to quantify total particles.

Results

For the HEK293 production system, VLPs were found to be associated with the cell pellet in addition to those released in the supernatant. Sf9 cells produced 35 times more VLPs than HEK293 cells. Sf9-VLPs had higher total HA activity and were generally more homogeneous in morphology and size. However, Sf9 VLP samples contained 20 times more baculovirus than VLPs, whereas 293 VLPs were produced along with vesicles.

Conclusions

This study highlights key production hurdles that must be overcome in both expression platforms, namely the presence of contaminants and the ensuing quantification challenges, and brings up the question of what truly constitutes an influenza VLP candidate vaccine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-015-0152-x) contains supplementary material, which is available to authorized users.

Ferret models of viral pathogenesis

Emerging and well-known viral diseases remain one the most important global public health threats. A better understanding of their pathogenesis and mechanisms of transmission requires animal models that accurately reproduce these aspects of the disease. Here we review the role of ferrets as an animal model for the pathogenesis of different respiratory viruses with an emphasis on influenza and paramyxoviruses. We will describe the anatomic and physiologic characteristics that contribute to the natural susceptibility of ferrets to these viruses, and provide an overview of the approaches available to analyze their immune responses. Recent insights gained using this model will be highlighted, including the development of new prophylactic and therapeutic approaches. To provide decision criteria for the use of this animal model, its strengths and limitations will be discussed.

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Ferrets as models for respiratory virus pathogenesis.

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Ferrets as models for vaccine and drug efficacy assessment.

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Immunological tools for ferrets.

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Housing and handling of ferrets.

Recombinant virus-like particles elicit protective immunity against avian influenza A(H7N9) virus infection in ferrets

In March 2013, diagnosis of the first reported case of human infection with a novel avian-origin influenza A(H7N9) virus occurred in eastern China. Most human cases have resulted in severe respiratory illness and, in some instances, death. Currently there are no licensed vaccines against H7N9 virus, which continues to cause sporadic human infections. Recombinant virus-like particles (VLPs) have been previously shown to be safe and effective vaccines for influenza. In this study, we evaluated the immunogenicity and protective efficacy of a H7N9 VLP vaccine in the ferret challenge model. Purified recombinant H7N9 VLPs morphologically resembled influenza virions and elicited high-titer serum hemagglutination inhibition (HI) and neutralizing antibodies specific for A/Anhui/1/2013 (H7N9) virus. H7N9 VLP-immunized ferrets subsequently challenged with homologous virus displayed reductions in fever, weight loss, and virus shedding compared to these parameters in unimmunized control ferrets. H7N9 VLP was also effective in protecting against lung and tracheal infection. The addition of either ISCOMATRIX or Matrix-M1 adjuvant improved immunogenicity and protection of the VLP vaccine against H7N9 virus. These results provide support for the development of a safe and effective human VLP vaccine with potent adjuvants against avian influenza H7N9 virus with pandemic potential.

Chimeric rabies virus-like particles containing membrane-anchored GM-CSF enhances the immune response against rabies virus

Rabies remains an important public health threat in most developing countries. To develop a more effective and safe vaccine against rabies, we have constructed a chimeric rabies virus-like particle (VLP), which containing glycoprotein (G) and matrix protein (M) of rabies virus (RABV) Evelyn-Rokitnicki-Abelseth (ERA) strain, and membrane-anchored granulocyte-macrophage colony-stimulating factor (GM-CSF), and it was named of EVLP-G. The immunogenicity and protective efficacy of EVLP-G against RABV were evaluated by intramuscular administration in a mouse model. The EVLP-G was successfully produced in insect cells by coinfection with three recombinant baculoviruses expressing G, M, and GM-CSF, respectively. The membrane-anchored GM-CSF possesses a strong adjuvant activity. More B cells and dendritic cells (DCs) were recruited and/or activated in inguinal lymph nodes in mice immunized with EVLP-G. EVLP-G was found to induce a significantly increased RABV-specific virus-neutralizing antibody and elicit a larger and broader antibody subclass responses compared with the standard rabies VLP (sRVLP, consisting of G and M). The EVLP-G also elicited significantly more IFN-γ- or IL-4-secreting CD4+ and CD8+ T cells than the sRVLP. Moreover, the immune responses induced by EVLP-G protect all vaccinated mice from lethal challenge with RABV. These results suggest that EVLP-G has the potential to be developed as a novel vaccine candidate for the prevention and control of animal rabies.

Improved virus purification processes for vaccines and gene therapy

The downstream processing of virus particles for vaccination or gene therapy is becoming a critical bottleneck as upstream titers keep improving. Moreover, the growing pressure to develop cost-efficient processes has brought forward new downstream trains. This review aims at analyzing the state-of-the-art in viral downstream purification processes, encompassing the classical unit operations and their recent developments. Emphasis is given to novel strategies for process intensification, such as continuous or semi-continuous systems based on multicolumn technology, opening up process efficiency. Process understanding in the light of the pharmaceutical quality by design (QbD) initiative is also discussed. Finally, an outlook of the upcoming breakthrough technologies is presented.

A viable recombinant rhabdovirus lacking its glycoprotein gene and expressing influenza virus hemagglutinin and neuraminidase is a potent influenza vaccine

The emergence of novel influenza viruses that cause devastating human disease is an ongoing threat and serves as an impetus for the continued development of novel approaches to influenza vaccines. Influenza vaccine development has traditionally focused on producing humoral and/or cell-mediated immunity, often against the viral surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). Here, we describe a new vaccine candidate that utilizes a replication-defective vesicular stomatitis virus (VSV) vector backbone that lacks the native G surface glycoprotein gene (VSVΔG). The expression of the H5 HA of an H5N1 highly pathogenic avian influenza virus (HPAIV), A/Vietnam/1203/04 (VN1203), and the NA of the mouse-adapted H1N1 influenza virus A/Puerto Rico/8/34 (PR8) in the VSVΔG vector restored the ability of the recombinant virus to replicate in cell culture, without the requirement for the addition of trypsin. We show here that this recombinant virus vaccine candidate was nonpathogenic in mice when given by either the intramuscular or intranasal route of immunization and that the in vivo replication of VSVΔG-H5N1 is profoundly attenuated. This recombinant virus also provided protection against lethal H5N1 infection after a single dose. This novel approach to vaccination against HPAIVs may be widely applicable to other emerging strains of influenza virus.

IMPORTANCE

Preparation for a potentially catastrophic influenza pandemic requires novel influenza vaccines that are safe, can be produced and administered quickly, and are effective, both soon after administration and for a long duration. We have created a new influenza vaccine that utilizes an attenuated vesicular stomatitis virus (VSV) vector, to deliver and express influenza virus proteins against which vaccinated animals develop potent antibody responses. The influenza virus hemagglutinin and neuraminidase proteins, expressed on the surface of VSV particles, allowed this vaccine to grow in cell culture and induced a potent antibody response in mice that was effective against infection with a lethal influenza virus. The mice showed no adverse reactions to the vaccine, and they were protected against an otherwise lethal influenza infection after only 14 days postvaccination and after as many as 140 days postvaccination. The ability to rapidly produce this safe and effective vaccine in cell culture is additionally advantageous.

Chimeric influenza-virus-like particles containing the porcine reproductive and respiratory syndrome virus GP5 protein and the influenza virus HA and M1 proteins

Both porcine reproductive and respiratory syndrome and swine influenza are acute, highly contagious swine diseases. These diseases pose severe threats for the swine industry and cause heavy economic losses worldwide. In this study, we have developed a chimeric virus-like particle (VLP) vaccine candidate for porcine reproductive and respiratory syndrome virus (PRRSV) and H3N2 influenza virus and investigated its immunogenicity in mice. The HA and M1 proteins from the H3N2 influenza virus and the PRRSV GP5 protein fused to the cytoplasmic and transmembrane domains of the NA protein were both incorporated into the chimeric VLPs. Analysis of the immune responses showed that the chimeric VLPs elicited serum antibodies specific for both PRRSV GP5 and the H3N2 HA protein, and they stimulated cellular immune responses compared to the responses to equivalent amounts of inactivated viruses. Taken together, the results suggested that the chimeric VLP vaccine represents a potential strategy for the development of a safe and effective vaccine to control PRRSV and H3N2 influenza virus.