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

A single immunization with H5N1 virus-like particle vaccine protects chickens against divergent H5N1 influenza viruses and vaccine efficacy is determined by adjuvant and dosage

The H5N1 subtype highly pathogenic avian influenza virus (HPAIV) reveals high variability and threatens poultry production and public health. To prevent the spread of H5N1 HPAIV, we developed an H5N1 virus-like particle (VLP) vaccine based on the insect cell-baculovirus expression system. Single immunization of the H5N1 VLP vaccines induced high levels of HI antibody titres and provided effective protection against homologous virus challenge comparable to the commercial inactivated vaccine. Meanwhile, we assessed the relative efficacy of different adjuvants by carrying out a head-to-head comparison of the adjuvants ISA 201 and ISA 71 and evaluated whether the two adjuvants could induce broadly protective immunity. The ISA 71 adjuvanted vaccine induced significantly higher levels of Th1 and Th2 immune responses and provided superior cross-protection against antigenically divergent H5N1 virus challenge than the ISA 201 adjuvanted vaccine. Importantly, increasing the vaccine dose could further enhance the cross-protective efficacy of H5N1 VLP vaccine and confer completely sterilizing protection against antigenically divergent H5N1 virus challenge, which was mediated by neutralizing antibodies. Our results suggest that the H5N1 VLP vaccine can provide broad-spectrum protection against divergent H5N1 influenza viruses as determined by adjuvant and vaccine dose.

M2e nanovaccines supplemented with recombinant hemagglutinin protect chickens against heterologous HPAI H5N1 challenge

Current poultry vaccines against influenza A viruses target the globular head region of the hemagglutinin (HA1), providing limited protection against antigenically divergent strains. Experimental subunit vaccines based on the conserved ectodomain of the matrix protein 2 (M2e) induce cross-reactive antibody responses, but fail to fully prevent virus shedding after low pathogenic avian influenza (LPAI) virus challenge, and are ineffective against highly pathogenic avian influenza (HPAI) viruses. This study assessed the benefits of combining nanoparticles bearing three tandem M2e repeats (NR-3M2e nanorings or NF-3M2e nanofilaments) with an HA1 subunit vaccine in protecting chickens against a heterologous HPAI H5N1 virus challenge. Chickens vaccinated with the combined formulations developed M2e and HA1-specific antibodies, were fully protected from clinical disease and mortality, and showed no histopathological lesions or virus shedding, unlike those given only HA1, NR-3M2e, or NF-3M2e. Thus, the combined vaccine formulations provided complete cross-protection against HPAI H5N1 virus, and prevented environmental virus shedding, crucial for controlling avian influenza outbreaks.

Dual N-linked glycosylation at residues 133 and 158 in the hemagglutinin are essential for the efficacy of H7N9 avian influenza virus like particle vaccine in chickens and mice

H7N9 subtype avian influenza virus (AIV) poses a great challenge to poultry industry. Virus-like particle (VLP) is a prospective alternative for the traditional egg-based influenza vaccines. N-linked glycosylation (NLG) regulates the efficacy of influenza vaccines, whereas the impact of NLG modifications on the efficacy of influenza VLP vaccines remains unclear. Here, H7N9 VLPs were assembled in insect cells through co-infection with the baculoviruses expressing the NLG-modified hemagglutinin (HA), neuraminidase and matrix proteins, and the VLP vaccines were assessed in chickens and mice. NLG modifications significantly enhanced hemagglutination-inhibition and virus neutralization antibody responses in mice, rather than in chickens, because different immunization strategies were used in these animal models. The presence of dual NLG at residues 133 and 158 significantly elevated HA-binding IgG titers in chickens and mice. The VLP vaccines conferred complete protection and significantly suppressed virus replication and lung pathology post challenge with H7N9 viruses in chickens and mice. VLP immunization activated T cell immunity-related cytokine response and inhibited inflammatory cytokine response in mouse lung. Of note, the presence of dual NLG at residues 133 and 158 optimized the capacity of the VLP vaccine to stimulate interleukin-4 expression, inhibit virus shedding or alleviate lung pathology in chickens or mice. Intriguingly, the VLP vaccine with NLG addition at residue 133 provided partial cross-protection against the H5Nx subtype AIVs in chickens and mice. In conclusion, dual NLG at residues 133 and 158 in HA can be potentially used to enhance the efficacy of H7N9 VLP vaccines in chickens and mammals.

Re-evaluating efficacy of vaccines against highly pathogenic avian influenza virus in poultry: A systematic review and meta-analysis

The global spread of highly pathogenic avian influenza (HPAI) A (H5N1) clade 2.3.4.4b virus since 2021 necessitates a re-evaluation of the role of vaccination in controlling HPAI outbreaks among poultry, which has been controversial because of the concern of silent spread with viral mutation and spillover to human. We systematically reviewed and meta-analyzed all existing data from experimental challenge trials to assess the efficacy of HPAI vaccines against mortality in specific pathogen free (SPF) chickens, with evaluation of the certainty of evidence (CoE) using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach. Out of 223 screened publications, 46 trials met our eligibility criteria. Inactivated vaccines showed an efficacy of 95% (risk ratio [RR] = 5% [95% CI: 1% to 17%], I2 = 0%, CoE high) against homologous strains and an efficacy of 78% (RR = 22% [95% CI: 14% to 37%], I2 = 18%, CoE high) against heterologous strains (test for subgroup difference p = 0.02). Live recombinant vaccines exhibited the highest efficacy at 97% (RR = 3% [95% CI: 1% to 13%], I2 = 0%, CoE high). Inactivated recombinant vaccines had an overall efficacy of 90% (RR = 10% [95% CI: 6% to 16%], I2 = 47%, CoE high). Commercial vaccines showed an overall efficacy of 91% (RR = 9% [95% CI: 5% to 17%], I2 = 23%, CoE high), with 96% efficacy (RR = 4% [95% CI: 1% to 21%], I2 = 0%, CoE high) against homologous strains and 90% efficacy (RR = 10% [95% CI: 5% to 20%], I2 = 31%, CoE moderate) against heterologous strains. Our systematic review offers an updated and unbiased assessment of vaccine efficacy against HPAI-related mortality, providing timely and crucial information for re-evaluating the role of vaccination in poultry avian influenza control policy amist the global HPAI outbreak post-2021.

The Potential of Plant-Produced Virus-like Particle Vaccines for African Horse Sickness and Other Equine Orbiviruses

African horse sickness is a devastating viral disease of equids. It is transmitted by biting midges of the genus Culicoides with mortalities reaching over 90% in naïve horses. It is endemic to sub-Saharan Africa and is seasonally endemic in many parts of southern Africa. However, outbreaks in Europe and Asia have occurred that caused significant economic issues. There are attenuated vaccines available for control of the virus but concerns regarding the safety and efficacy means that alternatives are sought. One promising alternative is the use of virus-like particles in vaccine preparations, which have the potential to be safer and more efficacious as vaccines against African horse sickness. These particles are best made in a complex, eukaryotic system, but due to technical challenges, this may cause significant economic strain on the developing countries most affected by the disease. Therefore, this review also summarises the success so far, and potential, of recombinant protein expression in plants to reduce the economic strain of production.

Efficacy of recombinant H5 vaccines delivered in ovo or day of age in commercial broilers against the 2015 U.S. H5N2 clade 2.3.4.4c highly pathogenic avian Influenza virus

BACKGROUND

Avian influenza is a highly contagious, agriculturally relevant disease that can severely affect the poultry industry and food supply. Eurasian-origin H5Nx highly pathogenic avian influenza viruses (HPAIV) (clade 2.3.4.4) have been circulating globally in wild birds with spill over into commercial poultry operations. The negative impact to commercial poultry renewed interest in the development of vaccines against these viruses to control outbreaks in the U.S.

METHODS

The efficacy of three recombinant H5 vaccines delivered in ovo or day of age were evaluated in commercial broilers challenged with the 2015 U.S. H5N2 clade 2.3.4.4c HPAIV. The recombinant vaccines included an alphavirus RNA particle vaccine (RP-H5), an inactivated reverse genetics-derived (RG-H5) and recombinant HVT vaccine (rHVT-AI) expressing H5 hemagglutinin (HA) genes. In the first experiment, in ovo vaccination with RP-H5 or rHVT-AI was tested against HPAI challenge at 3 or 6 weeks of age. In a second experiment, broilers were vaccinated at 1 day of age with a dose of either 107 or 108 RP-H5, or RG-H5 (512 HA units (HAU) per dose).

RESULTS

In experiment one, the RP-H5 provided no protection following in ovo application, and shedding titers were similar to sham vaccinated birds. However, when the RP-H5 was delivered in ovo with a boost at 3 weeks, 95% protection was demonstrated at 6 weeks of age. The rHVT-AI vaccine demonstrated 95 and 100% protection at 3 and 6 weeks of age, respectively, of challenged broilers with reduced virus shedding compared to sham vaccinated birds. Finally, when the RP-H5 and rHVT vaccines were co-administered at one day of age, 95% protection was demonstrated with challenge at either 3 or 6 weeks age. In the second experiment, the highest protection (92%) was observed in the 108 RP-H5 vaccinated group. Significant reductions (p < 0.05) in virus shedding were observed in groups of vaccinated birds that were protected from challenge. The RG-H5 provided 62% protection from challenge. In all groups of surviving birds, antibody titers increased following challenge.

CONCLUSIONS

Overall, these results demonstrated several strategies that could be considered to protected broiler chickens during a H5 HPAI challenge.

Virus-like Particle Vaccines and Platforms for Vaccine Development

Virus-like particles (VLPs) have gained a lot of interest within the past two decades. The use of VLP-based vaccines to protect against three infectious agents-hepatitis B virus, human papillomavirus, and hepatitis E virus-has been approved; they are very efficacious and offer long-lasting immune responses. Besides these, VLPs from other viral infectious agents (that infect humans, animals, plants, and bacteria) are under development. These VLPs, especially those from human and animal viruses, serve as stand-alone vaccines to protect against viruses from which the VLPs were derived. Additionally, VLPs, including those derived from plant and bacterial viruses, serve as platforms upon which to display foreign peptide antigens from other infectious agents or metabolic diseases such as cancer, i.e., they can be used to develop chimeric VLPs. The goal of chimeric VLPs is to enhance the immunogenicity of foreign peptides displayed on VLPs and not necessarily the platforms. This review provides a summary of VLP vaccines for human and veterinary use that have been approved and those that are under development. Furthermore, this review summarizes chimeric VLP vaccines that have been developed and tested in pre-clinical studies. Finally, the review concludes with a snapshot of the advantages of VLP-based vaccines such as hybrid/mosaic VLPs over conventional vaccine approaches such as live-attenuated and inactivated vaccines.

Evaluation of three hemagglutinin-based vaccines for the experimental control of a panzootic clade 2.3.4.4b A(H5N8) high pathogenicity avian influenza virus in mule ducks

In France during winter 2016-2017, 487 outbreaks of clade 2.3.4.4b H5N8 subtype high pathogenicity (HP) avian influenza A virus (AIV) infections were detected in poultry and captive birds. During this epizootic, HPAIV A/decoy duck/France/161105a/2016 (H5N8) was isolated and characterized in an experimental infection transmission model in conventional mule ducks. To investigate options to possibly protect such ducks against this HPAIV, three vaccines were evaluated in controlled conditions. The first experimental vaccine was derived from the hemagglutinin gene of another clade 2.3.4.4b A(H5N8) HPAIV. It was injected at three weeks of age, either alone (Vac1) or after a primer injection at day-old (Vac1 + boost). The second vaccine (Vac2) was a commercial bivalent adjuvanted vaccine containing an expressed hemagglutinin modified from a clade 2.3.2 A(H5N1) HPAIV. Vac2 was administered as a single injection at two weeks of age. The third experimental vaccine (Vac3) also incorporated a homologous 2.3.4.4b H5 HA gene and was administered as a single injection at three weeks of age. Ducks were challenged with HPAIV A/decoy duck/France/161105a/2016 (H5N8) at six weeks of age. Post-challenge virus excretion was monitored in vaccinated and control birds every 2-3 days for two weeks using real-time reverse-transcription polymerase chain reaction and serological analyses (haemagglutination inhibition test against H5N8, H5 ELISA and AIV ELISA) were performed. Vac1 abolished oropharyngeal and cloacal shedding to almost undetectable levels, whereas Vac3 abolished cloacal shedding only (while partially reducing respiratory shedding) and Vac2 only partly reduced the respiratory and intestinal excretion of the challenge virus. These results provided relevant insights in the immunogenicity of recombinant H5 vaccines in mule ducks, a rarely investigated hybrid between Pekin and Muscovy duck species that has played a critical role in the recent H5 HPAI epizootics in France.

Status and Challenges for Vaccination against Avian H9N2 Influenza Virus in China

In China, H9N2 avian influenza virus (AIV) has become widely prevalent in poultry, causing huge economic losses after secondary infection with other pathogens. Importantly, H9N2 AIV continuously infects humans, and its six internal genes frequently reassort with other influenza viruses to generate novel influenza viruses that infect humans, threatening public health. Inactivated whole-virus vaccines have been used to control H9N2 AIV in China for more than 20 years, and they can alleviate clinical symptoms after immunization, greatly reducing economic losses. However, H9N2 AIVs can still be isolated from immunized chickens and have recently become the main epidemic subtype. A more effective vaccine prevention strategy might be able to address the current situation. Herein, we analyze the current status and vaccination strategy against H9N2 AIV and summarize the progress in vaccine development to provide insight for better H9N2 prevention and control.

Dose immunogenicity study of a plant-produced influenza virus-like particle vaccine in layer hens

Avian influenza poses one of the largest known threats to global poultry production and human health, but effective poultry vaccines can reduce infections rates, production losses and prevent mortalities, and reduce viral shed to limit further disease spread. The antigenic match between a vaccine and the circulating field influenza A viruses (IAV) is a critical determinant of vaccine efficacy. Here, an Agrobacterium tumefaciens-mediated transient tobacco plant (Nicotiana benthamiana) system was used to rapidly update an H6 influenza subtype virus-like particle (VLP) vaccine expressing the hemagglutininn (HA) protein of South African H6N2 IAVs circulating in 2020. Specific pathogen free White Leghorn layer hens vaccinated twice with ≥125 hemagglutinating unit (HAU) doses elicited protective antibody responses associated with prevention of viral shedding, i.e. hemaglutination inhibition (HI) mean geometric titres (GMTs) of ≥7 log₂, for at least four months before dropping to approximately 5–6 log₂ for at least another two months. A single vaccination with a 250 HAU dose induced significantly higher HI GMTs compared lower or higher doses, and was thus the optimal dose for chickens. Use of an adjuvant was essential, as the plant-produced H6 HA VLP alone did not induce protective antibody responses. Plant-produced IAV VLPs enable differentiation between vaccinated and infected animals (DIVA principle), and with sucrose density gradient-purified yields of 20,000 doses per kg of plant material, this highly efficacious, safe and economical technology holds enormous potential for improving poultry health in lower and middle-income countries.

Strategies for enhancing immunity against avian influenza virus in chickens: A review

Poultry infection with avian influenza viruses (AIV) is a continuous source of concern for poultry production and human health. Uncontrolled infection and transmission of AIV in poultry increases the potential for viral mutation and reassortment, possibly resulting in the emergence of zoonotic viruses. To this end, implementing strategies to disrupt the transmission of AIVs in poultry, including a wide array of traditional and novel methods, is much needed. Vaccination of poultry is a targeted approach to reduce clinical signs and shedding in infected birds. Strategies aimed at enhancing the effectiveness of AIV vaccines are multi-pronged and include methods directed towards eliciting immune responses in poultry. Strategies include producing vaccines of greater immunogenicity via vaccine type and adjuvant application and increasing bird responsiveness to vaccines by modification of the gastrointestinal tract (GIT) microbiome and dietary interventions. This review provides an in-depth discussion of recent findings surrounding novel AIV vaccines for poultry, including reverse genetics vaccines, vectors, protein vaccines and virus like particles, highlighting their experimental efficacy among other factors such as safety and potential for use in the field. In addition to the type of vaccine employed, vaccine adjuvants also provide an effective way to enhance AIV vaccine efficacy, therefore, research on different types of vaccine adjuvants and vaccine adjuvant delivery strategies is discussed. Finally, the poultry gastrointestinal microbiome is emerging as an important factor in the effectiveness of prophylactic treatments. In this regard, current findings on the effects of the chicken GIT microbiome on AIV vaccine efficacy are summarized here.

Single Dose of Bivalent H5 and H7 Influenza Virus-Like Particle Protects Chickens Against Highly Pathogenic H5N1 and H7N9 Avian Influenza Viruses

Both H5N1 and H7N9 subtype avian influenza viruses cause enormous economic losses and pose considerable threats to public health. Bivalent vaccines against both two subtypes are more effective in control of H5N1 and H7N9 viruses in poultry and novel egg-independent vaccines are needed. Herein, H5 and H7 virus like particle (VLP) were generated in a baculovirus expression system and a bivalent H5+H7 VLP vaccine candidate was prepared by combining these two antigens. Single immunization of the bivalent VLP or commercial inactivated vaccines elicited effective antibody immune responses, including hemagglutination inhibition, virus neutralizing and HA-specific IgG antibodies. All vaccinated birds survived lethal challenge with highly pathogenic H5N1 and H7N9 viruses. Furthermore, the bivalent VLP significantly reduced viral shedding and virus replication in chickens, which was comparable to that observed for the commercial inactivated vaccine. However, the bivalent VLP was better than the commercial vaccine in terms of alleviating pulmonary lesions caused by H7N9 virus infection in chickens. Therefore, our study suggests that the bivalent H5+H7 VLP vaccine candidate can serve as a critical alternative for the traditional egg-based inactivated vaccines against H5N1 and H7N9 avian influenza virus infection in poultry.

Neuraminidase in Virus-like Particles Contributes to the Protection against High Dose of Avian Influenza Virus Challenge Infection

Neuraminidase is an important target for influenza vaccination. In this study, we generated avian influenza VLPs, expressing hemagglutinin (HA), neuraminidase (NA), HA and NA co-expressed (HANA), to evaluate the protective role of NA against a high (10LD₅₀) and low (2LD₅₀) dose of avian influenza virus challenge infections. A single immunization with HANA VLPs elicited the highest level of virus-specific IgG, IgG1, and IgG2a responses from the sera post-vaccination and the lungs post-challenge-infection. Potent antibody-secreting cell responses were observed from the spleens and lungs of HANA-VLP-immunized mice post-challenge-infection. HANA VLPs induced the highest CD4⁺ T cell, CD8⁺ T cell, and germinal center B cells, while strongly limiting inflammatory cytokine production in the lungs compared to other VLP immunization groups. In correlation with these findings, the lowest bodyweight losses and lung virus titers were observed from HANA VLP immunization, and all of the immunized mice survived irrespective of the challenge dose. Contrastingly, VLPs expressing either HA or NA alone failed to elicit complete protection. These results indicated that NA in VLPs played a critical role in inducing protection against a high dose of the challenge infection.

Single dose of multi-clade virus-like particle vaccine protects chickens against clade 2.3.2.1 and clade 2.3.4.4 highly pathogenic avian influenza viruses

Virus-like particles (VLPs) are recognized as an alternative vaccine platform that provide effective protection against various highly pathogenic avian influenza viruses (HPAIVs). Here, we developed multi-clade VLPs expressing two HAs (a chimera of clade 2.3.2.1c and clade 2.3.4.4c HA) within a single vector. We then compared its protective efficacy with that of a monovalent VLP and evaluated its potency against each homologous strain. Chickens vaccinated with the multi-clade VLP shed less virus and were better protected against challenge than birds receiving monovalent vaccines. Single vaccination with a multi-clade VLP resulted in 100% survival, with no clinical symptoms and high levels of pre-challenge protective immunity (7.6–8.5 log₂). Moreover, the multi-clade VLP showed high productivity (128–256 HAU) both in the laboratory and on a large scale, making it cheaper than whole inactivated vaccines produced in eggs. However, the PD₅₀ (protective dose 50%) of the multi-clade VLP against clades 2.3.2.1c and 2.3.4.4c was < 50 PD₅₀ (28 and 42 PD₅₀, respectively), and effective antibody response was maintained for 2–3 months. This multi-clade VLP protects against both clades of HPAI viruses and can be produced in high amounts at low cost. Thus, the vaccine has potential as a pandemic preparedness vaccine.

H7N9 influenza virus-like particle based on BEVS protects chickens from lethal challenge with highly pathogenic H7N9 avian influenza virus

H7N9 avian influenza virus poses a dual threat to both poultry industry and public health. Therefore, it is highly urgent to develop an effective vaccine to reduce its pandemic potential. Virus-like particles (VLP) represent an effective approach for pandemic vaccine development. In this study, a recombinant baculovirus co-expressing the HA, NA and M1 genes of the H7N9 virus was constructed for generation of H7N9 VLP. Single immunization of chickens with 15 μg of the VLP or the commercial whole virus inactivated vaccine stimulates high hemagglutination inhibition, virus neutralizing and HA-specific IgY antibodies. Moreover, the antiserum had a good cross-reactivity with H7N9 field strains isolated in different years. Within 14 days after a lethal challenge with highly pathogenic (HP) H7N9 virus, no clinical symptoms and death were observed in the vaccinated chickens, and no virus was recovered from the organs. Compared to the non-vaccinated chickens, H7N9 VLP significantly reduced the proportion of animals shedding virus. Only 30 % of the VLP-vaccinated birds shed virus, whereas virus shedding was detected in 50 % of the chickens immunized with the commercial vaccine. Moreover, both vaccines dramatically alleviated pulmonary lesions caused by HP H7N9 virus, with a greater degree observed for the VLP. Altogether, our results indicated that the H7N9 VLP vaccine candidate confers a complete clinical protection against a lethal challenge with HP H7N9 virus, significantly inhibits virus shedding and abolishes viral replication in chickens. The VLP generated in this study represents a promising alternative strategy for the development of novel H7N9 avian influenza vaccines for chickens.

Integrating high cell density cultures with adapted laboratory evolution for improved Gag-HA virus-like particles production in stable insect cell lines

Stable insect cell lines are emerging as an alternative to the insect cell-baculovirus expression vector system (IC-BEVS) for protein expression, benefiting from being a virus-free, nonlytic system. Still, the titers achieved are considerably lower. In this study, stable insect (Sf-9 and High Five) cells producing Gag virus-like particles (VLPs) were first adapted to grow under hypothermic culture conditions (22°C instead of standard 27°C), and then pseudotyped with a model membrane protein (influenza hemagglutinin [HA]) for expression of Gag-HA VLPs. Adaptation to lower temperature led to an increase in protein titers of up to 12-fold for p24 (as proxy for Gag-VLP) and sixfold for HA, with adapted Sf-9 cells outperforming High Five cells. Resulting Gag-HA VLPs producer Sf-9 cells were cultured to high cell densities, that is, 100 × 10⁶  cell/ml, using perfusion (ATF® 2) in 1 L stirred-tank bioreactors. Specific p24 and HA production rates were similar to those of batch culture, enabling to increase volumetric titers by 7-8-fold without compromising the assembly of Gag-HA VLPs. Importantly, the antigen (HA) quantity in VLPs generated using stable adapted cells in perfusion was ≈5-fold higher than that from IC-BEVS, with the added benefit of being a baculovirus-free system. This study demonstrates the potential of combining stable expression in insect cells adapted to hypothermic culture conditions with perfusion for improving Gag-HA VLPs production.

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.

Anti-Influenza Protective Efficacy of a H6 Virus-Like Particle in Chickens

H6 avian influenza viruses (AIVs) have a worldwide distribution, and they pose a potential concern for public health. In Taiwan, H6 AIVs have circulated in domestic chickens for more than 40 years, and certain strains have crossed the species barrier to infect mammals. With the goal of containing the disease, there is a pressing need to develop a safe and effective vaccine for pandemic preparedness. In this study, we prepared a virus-like particle (VLP) that consisted of the hemagglutinin (HA) and matrix protein 1 (M1) derived from a H6 AIV as a vaccine antigen, and we examined the immunogenicity and protective efficacy when combined with an adjuvant in a chicken model. Full-length HA and M1 protein genes were cloned and expressed using a baculovirus expression system, and VLPs were purified from the supernatant of insect cell cultures. We performed nanoparticle-tracking analysis and transmission electron microscopy to validate that the particle structure and properties resembled the native virions. In animal experiments, specific-pathogen-free chickens that received the H6 VLPs in combination with an adjuvant showed superior H6N1 virus-specific serum IgG and hemagglutination-inhibition antibody responses, which lasted more than 112 days. Following the H6N1 viral challenge, the vaccinated chickens showed reduced viral replication in the lungs, kidneys and conjunctival/cloacal shedding. The antibodies induced in the chickens by the vaccine were able to cross-react with the H6N1 human isolate and drifted avian H6N1 isolates. In summary, the H6 VLP vaccine elicited superb immunogenicity in vivo, and the use of an adjuvant further enhanced the antiviral protective efficacy. This vaccine formulation could potentially be used to manage H6 influenza virus infections in chickens.

Emerging Role of Mucosal Vaccine in Preventing Infection with Avian Influenza A Viruses

Avian influenza A viruses (AIVs), as a zoonotic agent, dramatically impacts public health and the poultry industry. Although low pathogenic avian influenza virus (LPAIV) incidence and mortality are relatively low, the infected hosts can act as a virus carrier and provide a resource pool for reassortant influenza viruses. At present, vaccination is the most effective way to eradicate AIVs from commercial poultry. The inactivated vaccines can only stimulate humoral immunity, rather than cellular and mucosal immune responses, while failing to effectively inhibit the replication and spread of AIVs in the flock. In recent years, significant progresses have been made in the understanding of the mechanisms underlying the vaccine antigen activities at the mucosal surfaces and the development of safe and efficacious mucosal vaccines that mimic the natural infection route and cut off the AIVs infection route. Here, we discussed the current status and advancement on mucosal immunity, the means of establishing mucosal immunity, and finally a perspective for design of AIVs mucosal vaccines. Hopefully, this review will help to not only understand and predict AIVs infection characteristics in birds but also extrapolate them for distinction or applicability in mammals, including humans.

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.

Rapid Construction and Immunogenicity Testing of a Novel H5 Virus-Like Particle Prototype Vaccine Against Clade 2.3.4.4 H5N8 Highly Pathogenic Avian Influenza Virus

From October 2016 to July 2017, 47 countries have been affected by highly pathogenic avian influenza (HPAI) viruses of the H5N8 clade 2.3.4.4 subtype, including European and African, and it has been the most severe HPAI outbreak ever in Europe. The development of effective influenza vaccines is required to combine preventive and control measures in order to avoid similar avian influenza epidemics taking place. Here we describe a novel prototype recombinant virus-like particle (VLP) vaccine based on a clade 2.3.4.4 H5 HA derived from a French duck HPAI H5N8 isolate of the 2016-2017 epidemics. Prototype vaccines with different antigen content were formulated and the immunogenicity was examined in specific-pathogen-free chickens and in ducks. Serum samples were collected at 3 and 4 weeks postvaccination, and development of the immune response was evaluated by hemagglutination inhibition test and ELISA. The VLP vaccines induced a dose-dependent and high level of antibody response in both chickens and ducks. The results of HPAI H5N8 challenge experiments in ducks are reported separately.

Virus-Like Particle Based Vaccine Provides High Level of Protection Against Homologous H5N8 HPAIV Challenge in Mule and Pekin Duck, Including Prevention of Transmission

The most recent pandemic clade of highly pathogenic avian influenza (HPAI) H5, clade 2.3.4.4, spread widely, with the involvement of wild birds, most importantly wild waterfowl, carrying the virus (even asymptomatically) from Asia to North America, Europe, and Africa. Domestic waterfowl being in regular contact with wild birds played a significant role in the H5Nx epizootics. Therefore, protection of domestic waterfowl from H5Nx avian influenza infection would likely cut the transmission chain of these viruses and greatly enhance efforts to control and prevent disease outbreak in other poultry and animal species, as well as infection of humans. The expectation for such a vaccine is not only to provide clinical protection, but also to control challenge virus transmission efficiently and ensure that the ability to differentiate infected from vaccinated animals is retained. A water-in-oil emulsion virus-like particle vaccine, containing homologous hemagglutinin antigen to the current European H5N8 field strains, has been developed to meet these requirements. The vaccine was tested in commercial Pekin and mule ducks by vaccinating them either once, at 3 wk of age, or twice (at 1 day and at 3 wk of age). Challenge was performed at 6 wk of age with a Hungarian HPAIV H5N8 isolate (2.3.4.4 Group B). Efficacy of vaccination was evaluated on the basis of clinical signs, amount of virus shedding, and transmission. Vaccination resulted in complete clinical protection and prevention of challenge virus transmission from the directly challenged vaccinated ducks to the vaccinated contact animals.

Current knowledge about interactions between avian dendritic cells and poultry pathogens

In poultry production conditions today, birds are surrounded by viral, bacterial, and parasitic agents. DCs are the main antigen-presenting cells located in tissues of the body, and their role involves recognizing antigen structures, engulfing and processing them, and subsequently presenting antigen peptides on their surface by major histocompatibility complex, where T cells and B cells are stimulated and can begin appropriate cellular and antibody immune response. This unique function indicates that these cells can be used in producing vaccines, but first it is necessary to culture DCs in vitro to identify the principles of their interactions with pathogens. The following review summarizes our current knowledge about avian dendritic cells and their interactions with pathogens. It provides a basis for future studies of these unique cells and their use in vaccine development.

Integrating nanoparticle quantification and statistical design of experiments for efficient HIV-1 virus-like particle production in High Five cells

The nature of enveloped virus-like particles (VLPs) has triggered high interest in their application to different research fields, including vaccine development. The baculovirus expression vector system (BEVS) has been used as an efficient platform for obtaining large amounts of these complex nanoparticles. To date, most of the studies dealing with VLP production by recombinant baculovirus infection utilize indirect detection or quantification techniques that hinder the appropriate characterization of the process and product. Here, we propose the application of cutting-edge quantification methodologies in combination with advanced statistical designs to exploit the full potential of the High Five/BEVS as a platform to produce HIV-1 Gag VLPs. The synergies between CCI, MOI, and TOH were studied using a response surface methodology approach on four different response functions: baculovirus infection, VLP production, VLP assembly, and VLP productivity. TOH and MOI proved to be the major influencing factors in contrast with previous reported data. Interestingly, a remarkable competition between Gag VLP production and non-assembled Gag was detected. Also, the use of nanoparticle tracking analysis and flow virometry revealed the existence of remarkable quantities of extracellular vesicles. The different responses of the study were combined to determine two global optimum conditions, one aiming to maximize the VLP titer (quantity) and the second aiming to find a compromise between VLP yield and the ratio of assembled VLPs (quality). This study provides a valuable approach to optimize VLP production and demonstrates that the High Five/BEVS can support mass production of Gag VLPs and potentially other complex nanoparticles.

Adaptive laboratory evolution of stable insect cell lines for improved HIV-Gag VLPs production

Adaptive laboratory evolution (ALE) has been extensively used to modulate the phenotype of industrial model organisms (e.g. Escherichia. coli and Saccharomyces cerevisae) towards a specific trait. Nevertheless, its application to animal cells, and in particular to insect cell lines, has been very limited. In this study, we describe employing an ALE method to improve the production of HIV-Gag virus-like particles (VLPs) in stable Sf-9 and High Five cell lines. Serial batch transfer was used for evolution experiments. During the ALE process, cells were cultured under controlled hypothermic conditions (22 °C instead of standard 27 °C) for a prolonged period of time (over 3 months), which allowed the selection of a population of cells with improved phenotype. Adapted cells expressed up to 26-fold (Sf-9 cells) and 10-fold (High Five cells) more Gag-VLPs than non-adapted cells cultured at standard conditions. The production of HIV Gag-VLPs in adapted, stable insect Sf-9 cell lines was successfully demonstrated at bioreactor scale. The Gag-VLPs produced at 22 °C and 27 °C were comparable, both in size and morphology, thus confirming the null impact of adaptation process and hypothermic culture conditions on VLP's quality. This work demonstrates the suitability of ALE as a powerful method for improving yields in stable insect cell lines producing VLPs.

Development of Universal Influenza Vaccines Targeting Conserved Viral Proteins

Vaccination is still the most efficient way to prevent an infection with influenza viruses. Nevertheless, existing commercial vaccines face serious limitations such as availability during epidemic outbreaks and their efficacy. Existing seasonal influenza vaccines mostly induce antibody responses to the surface proteins of influenza viruses, which frequently change due to antigenic shift and or drift, thus allowing influenza viruses to avoid neutralizing antibodies. Hence, influenza vaccines need a yearly formulation to protect against new seasonal viruses. A broadly protective or universal influenza vaccine must induce effective humoral as well as cellular immunity against conserved influenza antigens, offer good protection against influenza pandemics, be safe, and have a fast production platform. Nanotechnology has great potential to improve vaccine delivery, immunogenicity, and host immune responses. As new strains of human epidemic influenza virus strains could originate from poultry and swine viruses, development of a new universal influenza vaccine will require the immune responses to be directed against viruses from different hosts. This review discusses how the new vaccine platforms and nanoparticles can be beneficial in the development of a broadly protective, universal influenza vaccine.

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.

Enhancing Protective Efficacy of Poultry Vaccines through Targeted Delivery of Antigens to Antigen-Presenting Cells

Avian viral diseases including avian influenza, Marek's disease and Newcastle disease are detrimental to economies around the world that depend on the poultry trade. A significant zoonotic threat is also posed by avian influenza viruses. Vaccination is an important and widely used method for controlling these poultry diseases. However, the current vaccines do not provide full protection or sterile immunity. Hence, there is a need to develop improved vaccines. The major aim of developing improved vaccines is to induce strong and specific humoral and cellular immunity in vaccinated animals. One strategy used to enhance the immunogenicity of vaccines is the selective delivery of protective antigens to antigen-presenting cells (APCs) including dendritic cells, macrophages and B cells. APCs have a central role in the initiation and maintenance of immune responses through their ability to capture, process and present antigens to T and B cells. Vaccine technology that selectively targets APCs has been achieved by coupling antigens to monoclonal antibodies or ligands that are targeted by APCs. The aim of this review is to discuss existing strategies of selective delivery of antigens to APCs for effective vaccine development in poultry.

Structural analysis of influenza vaccine virus-like particles reveals a multicomponent organization

Influenza virus continues to be a major health problem due to the continually changing immunodominant head regions of the major surface glycoprotein, hemagglutinin (HA). However, some emerging vaccine platforms designed by biotechnology efforts, such as recombinant influenza virus-like particles (VLPs) have been shown to elicit protective antibodies to antigenically different influenza viruses. Here, using biochemical analyses and cryo-electron microscopy methods coupled to image analysis, we report the composition and 3D structural organization of influenza VLPs of the 1918 pandemic influenza virus. HA molecules were uniformly distributed on the VLP surfaces and the conformation of HA was in a prefusion state. Moreover, HA could be bound by antibody targeting conserved epitopes in the stem region of HA. Taken together, our analysis suggests structural parameters that may be important for VLP biotechnology such as a multi-component organization with (i) an outer component consisting of prefusion HA spikes on the surfaces, (ii) a VLP membrane with HA distribution permitting stem epitope display, and (iii) internal structural components.

H7 virus-like particles assembled by hemagglutinin containing H3N2 transmembrane domain and M1 induce broad homologous and heterologous protection in mice

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H7 VLPs-WT and H7 VLPs-TM have similar morphological and cleavage characteristics.

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H7 VLPs-TM has more HA trimers and better resists thermal changes than H7 VLPs-WT.

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H7 VLPs-TM induces stronger Th1 immune response than H7 VLPs-WT.

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H7 VLPs-TM induces broad homologous and heterologous protection in mice.

Influenza A H7N9 virus has caused five outbreak waves of human infections in China since 2013 and posed a dual challenge to public health and poultry industry. There is an urgent need to develop an effective vaccine to reduce its pandemic potential. In the present study, we evaluated the biochemical characteristics and immunogenicity of two H7 virus-like particles (VLPs) composed of the matrix 1 (M1) and hemagglutinin of wild-type (HA-WT) or hemagglutinin of whose transmembrane domain replaced by that from H3N2 subtype (HA-TM). H7 VLPs-WT and H7 VLPs-TM could assemble and release into the supernatant of Sf9 cells and they had similar morphological characteristics. However, compared to H7 VLPs-WT, H7 VLPs-TM had more trimeric HA proteins and could better resist thermal changes. In mice H7 VLPs-TM induced higher titers of HI, IgG, IgG2a and IFN-γ, and provided better protection against homologous and heterologous H7N9 viruses (no matter belonging to Yangtze River Delta or Pearl River Delta) challenge with less weight loss and higher survival rate. In summary, H7 VLPs-TM represents a potential strategy for the development of H7N9 vaccines.

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.

Induction of a high-titered antibody response using HIV gag-EV71 VP1-based virus-like particles with the capacity to protect newborn mice challenged with a lethal dose of enterovirus 71

Enterovirus 71 (EV71) is the most frequently detected causative agent in hand, foot, and mouth disease (HFMD) and is a serious threat to public health in the Asia-Pacific region. Many EV71 vaccines are under development worldwide, and although both inactivated virus vaccines and virus-like particles (VLPs) are considered to be effective, the main focus has been on inactivated EV71vaccines. There have been very few studies on EV71 VLPs. In this study, using a strategy based on HIV gag VLPs, we constructed a gag-VP1 fusion gene to generate a recombinant baculovirus expressing the EV71 structural protein VP1 together with gag, which was then used to infect TN5 cells to form VLPs. The VLPs were characterized using transmission electron microscopy, electrophoresis and staining with Coomassie blue, and Western blotting. Mice immunized with gag-VP1 VLPs showed strong humoral and cellular immune responses. Finally, immunization of female mice with gag-VP1 VLPs provided effective protection of their newborn offspring against challenge with a lethal dose EV71. These results demonstrate a successful approach for producing EV71 VP1 VLPs based on the ability of HIV gag to self-assemble, thus providing a good foundation for producing high-titered anti-EV71 antibody by immunization with VLP-based gag EV71 VP1 protein.

Incorporation of CD40 ligand enhances the immunogenicity of tumor‑associated calcium signal transducer 2 virus‑like particles against lung cancer

The cell surface glycoprotein Trop‑2 is overexpressed in various types of cancer, including in lung cancer, and has recently been used as an effective immunotherapeutic target. CD40 ligand (CD40L), a tumor necrosis factor superfamily member, is a promising immune adjuvant. Human immunodeficiency virus (HIV) gag‑based virus‑like particles (VLPs) are highly immunogenic, and foreign antigens can be incorporated onto their membrane envelope for cancer vaccine development. In the present study, a HIV gag‑based VLP strategy and Bac‑to‑Bac system were utilized to construct Trop‑2, CD40L and gag recombinant baculoviruses, which were then used to infect TN5 cells in order to form Trop‑2 VLPs or Trop‑2‑CD40L VLPs. These VLPs were characterized using transmission electron microscopy and western blot analysis methods. VLPs incorporating murine Trop‑2 only or incorporating Trop‑2 and CD40L were used to immunize C57BL/6 mice. Immunized mice demonstrated high humoral and cellular immunity responses, whereas the Trop‑2‑CD40L VLPs led to higher immune responses in comparison with Trop‑2 only VLPs. Immunization with Trop‑2‑CD40L VLPs also reduced tumor growth more effectively compared with Trop‑2 VLPs. Furthermore, Trop‑2‑CD40L VLP immunization increased the survival rate of Lewis tumor‑bearing mice more significantly when compared with Trop‑2 only VLPs. In conclusion, the present study provided a novel vaccine design by combination of a tumor antigen and an immune adjuvant based on a VLP strategy, which may be potentially applied as an alternative immunotherapeutic option in the treatment of lung cancer.

Universal Vaccines and Vaccine Platforms to Protect against Influenza Viruses in Humans and Agriculture

Influenza virus infections pose a significant threat to public health due to annual seasonal epidemics and occasional pandemics. Influenza is also associated with significant economic losses in animal production. The most effective way to prevent influenza infections is through vaccination. Current vaccine programs rely heavily on the vaccine's ability to stimulate neutralizing antibody responses to the hemagglutinin (HA) protein. One of the biggest challenges to an effective vaccination program lies on the fact that influenza viruses are ever-changing, leading to antigenic drift that results in escape from earlier immune responses. Efforts toward overcoming these challenges aim at improving the strength and/or breadth of the immune response. Novel vaccine technologies, the so-called universal vaccines, focus on stimulating better cross-protection against many or all influenza strains. However, vaccine platforms or manufacturing technologies being tested to improve vaccine efficacy are heterogeneous between different species and/or either tailored for epidemic or pandemic influenza. Here, we discuss current vaccines to protect humans and animals against influenza, highlighting challenges faced to effective and uniform novel vaccination strategies and approaches.

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.

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

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

Tackling a novel lethal virus: a focus on H7N9 vaccine development

INTRODUCTION

Avian-origin H7N9 influenza viruses first detected in humans in China in 2013 continue to cause severe human infections with a mortality rate close to 40%. These viruses are acknowledged as the subtype most likely to cause the next influenza pandemic. Areas covered: Here we review published data on the development of H7N9 influenza vaccine candidates and their evaluation in preclinical and clinical trials identified on PubMed database with the term 'H7N9 influenza vaccine'. In addition, a search with the same term was done on ClinicalTrials.gov to find ongoing clinical trials with H7N9 vaccines. Expert commentary: Influenza vaccines are the most powerful tool for protecting the human population from influenza infections, both seasonal and pandemic. During the past four years, a large number of promising H7N9 influenza vaccine candidates have been generated using traditional and advanced gene engineering techniques. In addition, with the support of WHO's GAP program, influenza vaccine production capacities have been established in a number of vulnerable low- and middle-income countries with a high population density, allowing the countries to be independent of vaccine supply from high-income countries. Overall, it is believed that the world is now well prepared for a possible H7N9 influenza pandemic.

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.