Animal models for the preclinical evaluation of candidate influenza vaccines

A quadrivalent recombinant influenza Hemagglutinin vaccine induced strong protective immune responses in animal models

Globally, influenza poses a substantial threat to public health, serving as a major contributor to both morbidity and mortality. The current vaccines for seasonal influenza are not optimal. A novel recombinant hemagglutinin (rHA) protein-based quadrivalent seasonal influenza vaccine, SCVC101, has been developed. SCVC101-S contains standard dose protein (15μg of rHA per virus strain) and an oil-in-water adjuvant, CD-A, which enhances the immunogenicity and cross-protection of the vaccine. Preclinical studies in mice, rats, and rhesus macaques demonstrate that SCVC101-S induces robust humoral and cellular immune responses, surpassing those induced by commercially available vaccines. Notably, a single injection with SCVC101-S can induce a strong immune response in macaques, suggesting the potential for a standard-dose vaccination with a recombinant protein influenza vaccine. Furthermore, SCVC101-S induces cross-protection immune responses against heterologous viral strains, indicating broader protection than current vaccines. In conclusion, SCVC101-S has demonstrated safety and efficacy in preclinical settings and warrants further investigation in human clinical trials. Its potential as a valuable addition to the vaccines against seasonal influenza, particularly for the elderly population, is promising.

Multiple Vaccines and Strategies for Pandemic Preparedness of Avian Influenza Virus

Avian influenza viruses (AIV) are a continuous cause of concern due to their pandemic potential and devasting effects on poultry, birds, and human health. The low pathogenic avian influenza virus has the potential to evolve into a highly pathogenic avian influenza virus, resulting in its rapid spread and significant outbreaks in poultry. Over the years, a wide array of traditional and novel strategies has been implemented to prevent the transmission of AIV in poultry. Mass vaccination is still an economical and effective approach to establish immune protection against clinical virus infection. At present, some AIV vaccines have been licensed for large-scale production and use in the poultry industry; however, other new types of AIV vaccines are currently under research and development. In this review, we assess the recent progress surrounding the various types of AIV vaccines, which are based on the classical and next-generation platforms. Additionally, the delivery systems for nucleic acid vaccines are discussed, since these vaccines have attracted significant attention following their significant role in the fight against COVID-19. We also provide a general introduction to the dendritic targeting strategy, which can be used to enhance the immune efficiency of AIV vaccines. This review may be beneficial for the avian influenza research community, providing ideas for the design and development of new AIV vaccines.

Development of a Mouse Model to Explore CD4 T Cell Specificity, Phenotype, and Recruitment to the Lung after Influenza B Infection

The adaptive T cell response to influenza B virus is understudied, relative to influenza A virus, for which there has been considerable attention and progress for many decades. Here, we have developed and utilized the C57BL/6 mouse model of intranasal infection with influenza B (B/Brisbane/60/2008) virus and, using an iterative peptide discovery strategy, have identified a series of robustly elicited individual CD4 T cell peptide specificities. The CD4 T cell repertoire encompassed at least eleven major epitopes distributed across hemagglutinin, nucleoprotein, neuraminidase, and non-structural protein 1 and are readily detected in the draining lymph node, spleen, and lung. Within the lung, the CD4 T cells are localized to both lung vasculature and tissue but are highly enriched in the lung tissue after infection. When studied by flow cytometry and MHC class II: peptide tetramers, CD4 T cells express prototypical markers of tissue residency including CD69, CD103, and high surface levels of CD11a. Collectively, our studies will enable more sophisticated analyses of influenza B virus infection, where the fate and function of the influenza B-specific CD4 T cells elicited by infection and vaccination can be studied as well as the impact of anti-viral reagents and candidate vaccines on the abundance, functionality, and localization of the elicited CD4 T cells.

Pathology and Immunity After SARS-CoV-2 Infection in Male Ferrets Is Affected by Age and Inoculation Route

Improving COVID-19 intervention strategies partly relies on animal models to study SARS-CoV-2 disease and immunity. In our pursuit to establish a model for severe COVID-19, we inoculated young and adult male ferrets intranasally or intratracheally with SARS-CoV-2. Intranasal inoculation established an infection in all ferrets, with viral dissemination into the brain and gut. Upon intratracheal inoculation only adult ferrets became infected. However, neither inoculation route induced observable COVID-19 symptoms. Despite this, a persistent inflammation in the nasal turbinates was prominent in especially young ferrets and follicular hyperplasia in the bronchi developed 21 days post infection. These effects -if sustained- might resemble long-COVID. Respiratory and systemic cellular responses and antibody responses were induced only in animals with an established infection. We conclude that intranasally-infected ferrets resemble asymptomatic COVID-19 and possibly aspects of long-COVID. Combined with the increasing portfolio to measure adaptive immunity, ferrets are a relevant model for SARS-CoV-2 vaccine research.

Animal Models Utilized for the Development of Influenza Virus Vaccines

Animal models have been an important tool for the development of influenza virus vaccines since the 1940s. Over the past 80 years, influenza virus vaccines have evolved into more complex formulations, including trivalent and quadrivalent inactivated vaccines, live-attenuated vaccines, and subunit vaccines. However, annual effectiveness data shows that current vaccines have varying levels of protection that range between 40–60% and must be reformulated every few years to combat antigenic drift. To address these issues, novel influenza virus vaccines are currently in development. These vaccines rely heavily on animal models to determine efficacy and immunogenicity. In this review, we describe seasonal and novel influenza virus vaccines and highlight important animal models used to develop them.

Transmission and Protection against Reinfection in the Ferret Model with the SARS-CoV-2 USA-WA1/2020 Reference Isolate

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has initiated a global pandemic, and several vaccines have now received emergency use authorization. Using the reference strain SARS-CoV-2 USA-WA1/2020, we evaluated modes of transmission and the ability of prior infection or vaccine-induced immunity to protect against infection in ferrets. Ferrets were semipermissive to infection with the USA-WA1/2020 isolate. When transmission was assessed via the detection of viral RNA (vRNA) at multiple time points, direct contact transmission was efficient to 3/3 and 3/4 contact animals in 2 respective studies, while respiratory droplet transmission was poor to only 1/4 contact animals. To determine if previously infected ferrets were protected against reinfection, ferrets were rechallenged 28 or 56 days postinfection. Following viral challenge, no infectious virus was recovered in nasal wash samples. In addition, levels of vRNA in the nasal wash were several orders of magnitude lower than during primary infection, and vRNA was rapidly cleared. To determine if intramuscular vaccination protected ferrets, ferrets were vaccinated using a prime-boost strategy with the S protein receptor-binding domain formulated with an oil-in-water adjuvant. Upon viral challenge, none of the mock or vaccinated animals were protected against infection, and there were no significant differences in vRNA or infectious virus titers in the nasal wash. Combined, these studies demonstrate direct contact is the predominant mode of transmission of the USA-WA1/2020 isolate in ferrets and that immunity to SARS-CoV-2 is maintained for at least 56 days. Our studies also indicate protection of the upper respiratory tract against SARS-CoV-2 will require vaccine strategies that mimic natural infection or induce site-specific immunity. IMPORTANCE The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) USA-WA1/2020 strain is a CDC reference strain used by multiple research laboratories. Here, we show that the predominant mode of transmission of this isolate in ferrets is by direct contact. We further demonstrate ferrets are protected against reinfection for at least 56 days even when levels of neutralizing antibodies are low or undetectable. Last, we show that when ferrets were vaccinated by the intramuscular route to induce antibodies against SARS-CoV-2, ferrets remain susceptible to infection of the upper respiratory tract. Collectively, these studies suggest that protection of the upper respiratory tract will require vaccine approaches that mimic natural infection.

Aerosolized pH1N1 influenza infection induces less systemic and local immune activation in the lung than combined intrabronchial, nasal and oral exposure in cynomolgus macaques

Non-human primates form an important animal model for the evaluation of immunogenicity and efficacy of novel 'universal' vaccine candidates against influenza virus. However, in most studies a combination of intra-tracheal or intra-bronchial, oral and nasal virus inoculation is used with a standard virus dose of between 1 and 10 million tissue culture infective doses, which differs from typical modes of virus exposure in humans. This paper studies the systemic and local inflammatory and immune effects of aerosolized versus combined-route exposure to pandemic H1N1 influenza virus. In agreement with a previous study, both combined-route and aerosol exposure resulted in similar levels of virus replication in nose, throat and lung lavages. However, the acute release of pro-inflammatory cytokines and chemokines, acute monocyte activation in peripheral blood as well as increased cytokine production and T-cell proliferation in the lungs were only observed after combined-route infection and not after aerosol exposure. Longitudinal evaluation by computed tomography demonstrated persistence of lung lesions after resolution of the infection and a tendency for more lesions in the lower lung lobes after combined-route exposure versus upper and middle lung lobes after aerosol exposure. Computed tomography scores were observed to correlate with fever. In conclusion, influenza virus infection by aerosol exposure is accompanied by less immune-activation and inflammation in comparison with direct virus installation, despite similar levels of virus replication and development of lesions in the lungs.

H7N9 influenza split vaccine with SWE oil-in-water adjuvant greatly enhances cross-reactive humoral immunity and protection against severe pneumonia in ferrets

Until universal influenza vaccines become available, pandemic preparedness should include developing classical vaccines against potential pandemic influenza subtypes. We here show that addition of SWE adjuvant, a squalene-in-water emulsion, to H7N9 split influenza vaccine clearly enhanced functional antibody responses in ferrets. These were cross-reactive against H7N9 strains from different lineages and newly emerged H7N9 variants. Both vaccine formulations protected in almost all cases against severe pneumonia induced by intratracheal infection of ferrets with H7N9 influenza; however, the SWE adjuvant enhanced protection against virus replication and disease. Correlation analysis and curve fitting showed that both VN- and NI-titers were better predictors for protection than HI-titers. Moreover, we show that novel algorithms can assist in better interpretation of large data sets generated in preclinical studies. Cluster analysis showed that the adjuvanted vaccine results in robust immunity and protection, whereas the response to the non-adjuvanted vaccine is heterogeneous, such that the protection balance may be more easily tipped toward severe disease. Finally, cluster analysis indicated that the dose-sparing capacity of the adjuvant is at least a factor six, which greatly increases vaccine availability in a pandemic situation.

Nonclinical data supporting orphan medicinal product designations in the area of rare infectious diseases

This review provides an overview of nonclinical in vivo models that can be used to support orphan designation in selected rare infectious diseases in Europe, with the aim to inform and stimulate the planning of nonclinical development in this area of often neglected diseases.

The tree shrew is a promising model for the study of influenza B virus infection

Background

Influenza B virus is a main causative pathogen of annual influenza epidemics, however, research on influenza B virus in general lags behind that on influenza A viruses, one of the important reasons is studies on influenza B viruses in animal models are limited. Here we investigated the tree shrew as a potential model for influenza B virus studies.

Methods

Tree shrews and ferrets were inoculated with either a Yamagata or Victoria lineage influenza B virus. Symptoms including nasal discharge and weight loss were observed. Nasal wash and respiratory tissues were collected at 2, 4 and 6 days post inoculation (DPI). Viral titers were measured in nasal washes and tissues were used for pathological examination and extraction of mRNA for measurement of cytokine expression.

Results

Clinical signs and pathological changes were also evident in the respiratory tracts of tree shrews and ferrets. Although nasal symptoms including sneezing and rhinorrhea were evident in ferrets infected with influenza B virus, tree shrews showed no significant respiratory symptoms, only milder nasal secretions appeared. Weight loss was observed in tree shrews but not ferrets. V0215 and Y12 replicated in all three animal (ferrets, tree shrews and mice) models with peak titers evident on 2DPI. There were no significant differences in peak viral titers in ferrets and tree shrews inoculated with Y12 at 2 and 4DPI, but viral titers were detected at 6DPI in tree shrews. Tree shrews infected with influenza B virus showed similar seroconversion and respiratory tract pathology to ferrets. Elevated levels of cytokines were detected in the tissues isolated from the respiratory tract after infection with either V0215 or Y12 compared to the levels in the uninfected control in both animals. Overall, the tree shrew was sensitive to infection and disease by influenza B virus.

Conclusion

The tree shrew to be a promising model for influenza B virus research.

Electronic supplementary material

The online version of this article (10.1186/s12985-019-1171-3) contains supplementary material, which is available to authorized users.

Extended delivery of vaccines to the skin improves immune responses

Vaccines prevent 2-3 million childhood deaths annually; however, low vaccine efficacy and the resulting need for booster doses create gaps in immunization coverage. In this translational study, we explore the benefits of extended release of licensed vaccine antigens into skin to increase immune responses after a single dose in order to design improved vaccine delivery systems. By administering daily intradermal injections of inactivated polio vaccine according to six different delivery profiles, zeroth-order release over 28 days resulted in neutralizing antibody titers equivalent to two bolus vaccinations administered one month apart. Vaccinations following this profile also improved immune responses to tetanus toxoid and subunit influenza vaccine but not a live-attenuated viral vaccine, measles vaccine. Finally, using subunit influenza vaccine, we demonstrated that daily vaccination by microneedle patch induced a potent, balanced humoral immunity with an increased memory response compared to bolus vaccination. We conclude that extended presentation of antigen in skin via intradermal injection or microneedle patch can enhance immune responses and reduce the number of vaccine doses, thereby enabling increased vaccination efficacy.

A Review of DNA Vaccines Against Influenza

The challenges of effective vaccination against influenza are gaining more mainstream attention, as recent influenza seasons have reported low efficacy in annual vaccination programs worldwide. Combined with the potential emergence of novel influenza viruses resulting in a pandemic, the need for effective alternatives to egg-produced conventional vaccines has been made increasingly clear. DNA vaccines against influenza have been in development since the 1990s, but the initial excitement over success in murine model trials has been tempered by comparatively poor performance in larger animal models. In the intervening years, much progress has been made to refine the DNA vaccine platform-the rational design of antigens and expression vectors, the development of novel vaccine adjuvants, and the employment of innovative gene delivery methods. This review discusses how these advances have been applied in recent efforts to develop an effective influenza DNA vaccine.

Assessment of Influenza Virus Hemagglutinin Stalk-Specific Antibody Responses

Animal models are essential to examine the pathogenesis and transmission of influenza viruses and for preclinical evaluation of influenza virus vaccines. Among the animal models used in influenza virus research, the domestic ferret (Mustela putorius furo) is the gold standard. As seen in humans, infection with influenza virus or immunization with an influenza virus vaccine induces humoral and cellular immunity in ferrets that provides protection against infection by an antigenically similar influenza virus. Antibodies against the globular head domain of the influenza hemagglutinin can provide sterilizing immunity against virus infection by blocking receptor binding. However, antibodies that bind the stalk region of the hemagglutinin also confer protection by several mechanisms including antibody-dependent cellular cytotoxicity or phagocytosis. Recently, the antigenically and structurally conserved hemagglutinin stalk has become an attractive target for the development of universal influenza virus vaccines that hold the promise to provide protection against influenza epidemics and pandemics. Herein, in vivo and in vitro assays, including optimization of assay conditions to examine hemagglutinin stalk-specific antibody responses in small animal models, are described.

Animal Models in Influenza Research

Animal model systems for human and animal influenza virus infection and transmission have been established to address research questions which cannot be addressed using in vitro models. Several animal models have been established, such as mice, guinea pig, ferret, pig, poultry, nonhuman primates, and others. Each animal model has its own strength and weaknesses, which should be taken into consideration to select the appropriate animal model to use. This chapter will describe standard protocols relevant for in vivo experiment, including procedures required prior to the start of the animal experiment and sample processing. The animal models described in this chapter are mice, guinea pigs, ferrets, pigs, and chickens.

A modified vaccinia Ankara vaccine vector expressing a mosaic H5 hemagglutinin reduces viral shedding in rhesus macaques

The rapid antigenic evolution of influenza viruses requires frequent vaccine reformulations. Due to the economic burden of continuous vaccine reformulation and the threat of new pandemics, there is intense interest in developing vaccines capable of eliciting broadly cross-reactive immunity to influenza viruses. We recently constructed a “mosaic” hemagglutinin (HA) based on subtype 5 HA (H5) and designed to stimulate cellular and humoral immunity to multiple influenza virus subtypes. Modified vaccinia Ankara (MVA) expressing this H5 mosaic (MVA-H5M) protected mice against multiple homosubtypic H5N1 strains and a heterosubtypic H1N1 virus. To assess its potential as a human vaccine we evaluated the ability of MVA-H5M to provide heterosubtypic immunity to influenza viruses in a non-human primate model. Rhesus macaques received an initial dose of either MVA-H5M or plasmid DNA encoding H5M, followed by a boost of MVA-H5M, and then were challenged, together with naïve controls, with the heterosubtypic virus A/California/04/2009 (H1N1pdm). Macaques receiving either vaccine regimen cleared H1N1pdm challenge faster than naïve controls. Vaccination with H5M elicited antibodies that bound H1N1pdm HA, but did not neutralize the H1N1pdm challenge virus. Plasma from vaccinated macaques activated NK cells in the presence of H1N1pdm HA, suggesting that vaccination elicited cross-reactive antibodies capable of mediating antibody-dependent cell-mediated cytotoxicity (ADCC). Although HA-specific T cell responses to the MVA-H5M vaccine were weak, responses after challenge were stronger in vaccinated macaques than in control animals. Together these data suggest that mosaic HA antigens may provide a means for inducing broadly cross-reactive immunity to influenza viruses.

The in vitro MIMIC® platform reflects age-associated changes in immunological responses after influenza vaccination

Increasing research and development costs coupled with growing concerns over healthcare expenditures necessitate the generation of pre-clinical testing models better able to predict the efficacy of vaccines, drugs and biologics. An ideal system for evaluating vaccine immunogenicity will not only be reliable but also physiologically relevant, able to be influenced by immunomodulatory characteristics such as age or previous exposure to pathogens. We have previously described a fully autologous human cell-based MIMIC® (Modular IMmune In vitro Construct) platform which enables the evaluation of innate and adaptive immunity in vitro, including naïve and recall responses. Here, we establish the ability of this module to display reduced antibody production and T cell activation upon in vitro influenza vaccination of cells from elderly adults. In the MIMIC® system, we observe a 2.7-4.2-fold reduction in strain-specific IgG production to seasonal trivalent influenza vaccine (TIV) in the elderly when compared to adults, as well as an age-dependent decline in the generation of functional antibodies. A parallel decline in IgG production with increasing age was detected via short-term ex vivo stimulation of B cells after in vivo TIV vaccination in the same cohort. Using MIMIC®, we also detect a reduction in the number but not proportion of TIV-specific multifunctional CD154⁺IFNγ⁺IL-2⁺TNFα⁺ CD4⁺ T cells in elderly adults. Inefficient induction of multifunctional helper T cells with TIV stimulation in MIMIC® despite a normalized number of initial CD4⁺ T cells suggests a possible mechanism for an impaired anti-TIV IgG response in elderly adults. The ability of the MIMIC® system to recapitulate differential age-associated responses in vitro provides a dynamic platform for the testing of vaccine candidates and vaccine enhancement strategies in a fully human model including the ability to interrogate specific populations, such as elderly adults.

Recent advances in the production of recombinant subunit vaccines in Pichia pastoris

Recombinant protein subunit vaccines are formulated using defined protein antigens that can be produced in heterologous expression systems. The methylotrophic yeast Pichia pastoris has become an important host system for the production of recombinant subunit vaccines. Although many basic elements of P. pastoris expression system are now well developed, there is still room for further optimization of protein production. Codon bias, gene dosage, endoplasmic reticulum protein folding and culture condition are important considerations for improved production of recombinant vaccine antigens. Here we comment on current advances in the application of P. pastoris for the synthesis of recombinant subunit vaccines.

Preclinical Toxicology of Vaccines1

Immunity to targeted infectious diseases may be conferred or enhanced by vaccines, which are manufactured from recombinant forms as well as inactivated or attenuated organisms. These vaccines have to meet requirements for safety, quality, and efficacy. In addition to antigenic components, various adjuvants may be included in vaccines to evoke an effective immune response. To ensure the safety of new vaccines, preclinical toxicology studies are conducted prior to the initiation of, and concurrently with, clinical studies. There are five different types of preclinical toxicology study in the evaluation of vaccine safety: single and/or repeat dose, reproductive and developmental, mutagenicity, carcinogenicity, and safety pharmacology. If any adverse effects are observed in the course of these studies, they should be fully evaluated and a final safety decision made accordingly. Successful preclinical toxicology studies depend on multiple factors including using the appropriate study designs, using the right animal model, and evoking an effective immune response. Additional in vivo and in vitro assays that establish the identity, purity, safety, and potency of the vaccine play a significant role in assessing critical characteristics of vaccine safety.

H7N9 Live Attenuated Influenza Vaccine Is Highly Immunogenic, Prevents Virus Replication, and Protects Against Severe Bronchopneumonia in Ferrets

Avian influenza viruses continue to cross the species barrier, and if such viruses become transmissible among humans, it would pose a great threat to public health. Since its emergence in China in 2013, H7N9 has caused considerable morbidity and mortality. In the absence of a universal influenza vaccine, preparedness includes development of subtype-specific vaccines. In this study, we developed and evaluated in ferrets an intranasal live attenuated influenza vaccine (LAIV) against H7N9 based on the A/Leningrad/134/17/57 (H2N2) cold-adapted master donor virus. We demonstrate that the LAIV is attenuated and safe in ferrets and induces high hemagglutination- and neuraminidase-inhibiting and virus-neutralizing titers. The antibodies against hemagglutinin were also cross-reactive with divergent H7 strains. To assess efficacy, we used an intratracheal challenge ferret model in which an acute severe viral pneumonia is induced that closely resembles viral pneumonia observed in severe human cases. A single- and two-dose strategy provided complete protection against severe pneumonia and prevented virus replication. The protective effect of the two-dose strategy appeared better than the single dose only on the microscopic level in the lungs. We observed, however, an increased lymphocytic infiltration after challenge in single-vaccinated animals and hypothesize that this a side effect of the model.

Evaluating influenza vaccines: progress and perspectives

Severe influenza infections are responsible for 3–5 million cases worldwide and 250,000–500,000 deaths per year. Although vaccination is the primary and most effective means of inducing protection against influenza viruses, it also presents limitations. This review outlines the promising steps that have been taken toward the development of a broadly protective influenza virus vaccine through the use of new technologies. The future challenge is to develop a broadly protective vaccine that is able to induce long-term protection against antigenically variant influenza viruses, regardless of antigenic shift and drift, and thus to protect against seasonal and pandemic influenza viruses.

Translational research on influenza virus infection using a nonhuman primate model

Influenza virus infection is a seasonal infectious disease for humans, whereas it is also a zoonosis that is originally transmitted from animals to humans. Therefore, several animal models are used in research on influenza virus infection. We have used a nonhuman primate (NHP) model to extrapolate pathogenicity of various influenza viruses and efficacy of vaccines and antiviral drugs against the influenza viruses in humans. NHPs have genes, anatomical structure, and immune responses similar to those of humans as compared to other animal models. Using an NHP model, we revealed that the pandemic 2009 influenza A virus caused viral pneumonia as reported in human patients. Thus, it is thought that NHP models can be used to predict replication of emerging viruses in humans. We also examined the pathogenicity of highly pathogenic avian influenza viruses and evaluated a new therapeutic antibody in macaques under an immunocompromised condition. NHP models have provided promising results in research on other infectious diseases including Ebola virus and human/simian immunodeficiency virus infections. Thus, NHPs are important in biomedical research for determining the pathogenesis and for development of treatments, especially when clinical trials are difficult. We summarize the characteristics and advantages of research using NHP models in this review.

Correlation between Virus Replication and Antibody Responses in Macaques following Infection with Pandemic Influenza A Virus

Influenza virus infection of nonhuman primates is a well-established animal model for studying pathogenesis and for evaluating prophylactic and therapeutic intervention strategies. However, usually a standard dose is used for the infection, and there is no information on the relation between challenge dose and virus replication or the induction of immune responses. Such information is also very scarce for humans and largely confined to evaluation of attenuated virus strains. Here, we have compared the effect of a commonly used dose (4 × 10(6) 50% tissue culture infective doses) versus a 100-fold-higher dose, administered by intrabronchial installation, to two groups of 6 cynomolgus macaques. Animals infected with the high virus dose showed more fever and had higher peak levels of gamma interferon in the blood. However, virus replication in the trachea was not significantly different between the groups, although in 2 out of 6 animals from the high-dose group it was present at higher levels and for a longer duration. The virus-specific antibody response was not significantly different between the groups. However, antibody enzyme-linked immunosorbent assay, virus neutralization, and hemagglutination inhibition antibody titers correlated with cumulative virus production in the trachea. In conclusion, using influenza virus infection in cynomolgus macaques as a model, we demonstrated a relationship between the level of virus production upon infection and induction of functional antibody responses against the virus.

IMPORTANCE

There is only very limited information on the effect of virus inoculation dose on the level of virus production and the induction of adaptive immune responses in humans or nonhuman primates. We found only a marginal and variable effect of virus dose on virus production in the trachea but a significant effect on body temperature. The induction of functional antibody responses, including virus neutralization titer, hemagglutination inhibition titer, and antibody-dependent cell-mediated cytotoxicity, correlated with the level of virus replication measured in the trachea. The study reveals a relationship between virus production and functional antibody formation, which could be relevant in defining appropriate criteria for new influenza virus vaccine candidates.

Attenuation of the influenza virus by microRNA response element in vivo and protective efficacy against 2009 pandemic H1N1 virus in mice

BACKGROUND

The 2009 influenza pandemics underscored the need for effective vaccines to block the spread of influenza virus infection. Most live attenuated vaccines utilize cold-adapted, temperature-sensitive virus. An alternative to live attenuated virus is presented here, based on microRNA-induced gene silencing.

METHODS

In this study, miR-let-7b target sequences were inserted into the H1N1 genome to engineer a recombinant virus - miRT-H1N1. Female BALB/c mice were vaccinated intranasally with the miRT-H1N1 and challenged with a lethal dose of homologous virus.

RESULTS

This miRT-H1N1 virus was attenuated in mice, while it exhibited wild-type characteristics in chicken embryos. Mice vaccinated intranasally with the miRT-H1N1 responded with robust immunity that protected the vaccinated mice from a lethal challenge with the wild-type 2009 pandemic H1N1 virus.

CONCLUSIONS

These results indicate that the influenza virus containing microRNA response elements (MREs) is attenuated in vivo and can be used to design a live attenuated vaccine.

Synthetic Long Peptide Influenza Vaccine Containing Conserved T and B Cell Epitopes Reduces Viral Load in Lungs of Mice and Ferrets

Currently licensed influenza vaccines mainly induce antibodies against highly variable epitopes. Due to antigenic drift, protection is subtype or strain-specific and regular vaccine updates are required. In case of antigenic shifts, which have caused several pandemics in the past, completely new vaccines need to be developed. We set out to develop a vaccine that provides protection against a broad range of influenza viruses. Therefore, highly conserved parts of the influenza A virus (IAV) were selected of which we constructed antibody and T cell inducing peptide-based vaccines. The B epitope vaccine consists of the highly conserved HA2 fusion peptide and M2e peptide coupled to a CD4 helper epitope. The T epitope vaccine comprises 25 overlapping synthetic long peptides of 26-34 amino acids, thereby avoiding restriction for a certain MHC haplotype. These peptides are derived from nucleoprotein (NP), polymerase basic protein 1 (PB1) and matrix protein 1 (M1). C57BL/6 mice, BALB/c mice, and ferrets were vaccinated with the B epitopes, 25 SLP or a combination of both. Vaccine-specific antibodies were detected in sera of mice and ferrets and vaccine-specific cellular responses were measured in mice. Following challenge, both mice and ferrets showed a reduction of virus titers in the lungs in response to vaccination. Summarizing, a peptide-based vaccine directed against conserved parts of influenza virus containing B and T cell epitopes shows promising results for further development. Such a vaccine may reduce disease burden and virus transmission during pandemic outbreaks.

Analysis of full-length mitochondrial DNA D-loop sequences from Macaca fascicularis of different geographical origin reveals novel haplotypes

BACKGROUND

Cynomolgus macaques are indigenous to Asia occupying a range of geographical areas. A non-indigenous population established on Mauritius approximately 500 years ago. Mauritian cynomolgus macaques are recognised as having low genetic diversity compared to Indonesian macaques, from which they originated. As cynomolgus macaques are widely used as a biomedical model, there have been many studies of their genetic relationships. However, population diversity and relationships have only been assessed through analysis of either the hypervariable region I or II separately within the D-loop region of the mitochondrial genome in these macaques.

METHODS

Using sequencing, we defined haplotypes encompassing the full D-loop sequence for Mauritian and Indonesian cynomolgus macaques.

RESULTS

We evaluated the haplotype relationships by constructing a median-joining network based on full-length D-loop sequences, which has not been reported previously.

CONCLUSION

Our data allow a complete D-loop haplotype, including a hereto unreported polymorphic region, to be defined to aid the resolution of populations of cynomolgus macaques and which highlights the value in analysing both D-loop hypervariable regions in concert.

Repeated Low-Dose Influenza Virus Infection Causes Severe Disease in Mice: a Model for Vaccine Evaluation

Influenza infection causes severe disease and death in humans. In traditional vaccine research and development, a single high-dose virus challenge of animals is used to evaluate vaccine efficacy. This type of challenge model may have limitations. In the present study, we developed a novel challenge model by infecting mice repeatedly in short intervals with low doses of influenza A virus. Our results show that compared to a single high-dose infection, mice that received repeated low-dose challenges showed earlier morbidity and mortality and more severe disease. They developed higher vial loads, more severe lung pathology, and greater inflammatory responses and generated only limited influenza A virus-specific B and T cell responses. A commercial trivalent influenza vaccine protected mice against a single high and lethal dose of influenza A virus but was ineffective against repeated low-dose virus challenges. Overall, our data show that the repeated low-dose influenza A virus infection mouse model is more stringent and may thus be more suitable to select for highly efficacious influenza vaccines.

IMPORTANCE

Influenza epidemics and pandemics pose serious threats to public health. Animal models are crucial for evaluating the efficacy of influenza vaccines. Traditional models based on a single high-dose virus challenge may have limitations. Here, we describe a new mouse model based on repeated low-dose influenza A virus challenges given within a short period. Repeated low-dose challenges caused more severe disease in mice, associated with higher viral loads and increased lung inflammation and reduced influenza A virus-specific B and T cell responses. A commercial influenza vaccine that was shown to protect mice from high-dose challenge was ineffective against repeated low-dose challenges. Overall, our results show that the low-dose repeated-challenge model is more stringent and may therefore be better suited for preclinical vaccine efficacy studies.

Pandemic Swine-Origin H1N1 Influenza Virus Replicates to Higher Levels and Induces More Fever and Acute Inflammatory Cytokines in Cynomolgus versus Rhesus Monkeys and Can Replicate in Common Marmosets

The close immunological and physiological resemblance with humans makes non-human primates a valuable model for studying influenza virus pathogenesis and immunity and vaccine efficacy against infection. Although both cynomolgus and rhesus macaques are frequently used in influenza virus research, a direct comparison of susceptibility to infection and disease has not yet been performed. In the current study a head-to-head comparison was made between these species, by using a recently described swine-origin pandemic H1N1 strain, A/Mexico/InDRE4487/2009. In comparison to rhesus macaques, cynomolgus macaques developed significantly higher levels of virus replication in the upper airways and in the lungs, involving both peak level and duration of virus production, as well as higher increases in body temperature. In contrast, clinical symptoms, including respiratory distress, were more easily observed in rhesus macaques. Expression of sialyl-α-2,6-Gal saccharides, the main receptor for human influenza A viruses, was 50 to 73 times more abundant in trachea and bronchus of cynomolgus macaques relative to rhesus macaques. The study also shows that common marmosets, a New World non-human primate species, are susceptible to infection with pandemic H1N1. The study results favor the cynomolgus macaque as model for pandemic H1N1 influenza virus research because of the more uniform and high levels of virus replication, as well as temperature increases, which may be due to a more abundant expression of the main human influenza virus receptor in the trachea and bronchi.

Visualizing real-time influenza virus infection, transmission and protection in ferrets

Influenza transmission efficiency in ferrets is vital for risk-assessment studies. However, the inability to monitor viral infection and transmission dynamics in real time only provides a glimpse into transmissibility. Here we exploit a replication-competent influenza reporter virus to investigate dynamics of infection/transmission in ferrets. Bioluminescent imaging of ferrets infected with A/California/04/2009 H1N1 virus (CA/09) encoding NanoLuc (NLuc) luciferase provides the first real-time snapshot of influenza infection/transmission. Luminescence in the respiratory tract and in less well-characterized extra-pulmonary sites is observed, and imaging identifies infections in animals that would have otherwise been missed by traditional methods. Finally, the reporter virus significantly increases the speed and sensitivity of virological and serological assays. Thus, bioluminescent imaging of influenza infections rapidly determines intra-host dissemination, inter-host transmission and viral load, revealing infection dynamics and pandemic potential of the virus. These results have important implications for antiviral drug susceptibility, vaccine efficacy, transmissibility and pathogenicity studies.

Ferrets are the main animal model used for research on influenza transmission. Here, the authors investigate the dynamics of infection and transmission in ferrets using a replication-competent influenza reporter virus and real-time bioluminescence imaging.

Molecular determinants of influenza virus pathogenesis in mice

Mice are widely used for studying influenza virus pathogenesis and immunology because of their low cost, the wide availability of mouse-specific reagents, and the large number of mouse strains available, including knockout and transgenic strains. However, mice do not fully recapitulate the signs of influenza infection of humans: transmission of influenza between mice is much less efficient than in humans, and influenza viruses often require adaptation before they are able to efficiently replicate in mice. In the process of mouse adaptation, influenza viruses acquire mutations that enhance their ability to attach to mouse cells, replicate within the cells, and suppress immunity, among other functions. Many such mouse-adaptive mutations have been identified, covering all 8 genomic segments of the virus. Identification and analysis of these mutations have provided insight into the molecular determinants of influenza virulence and pathogenesis, not only in mice but also in humans and other species. In particular, several mouse-adaptive mutations of avian influenza viruses have proved to be general mammalian-adaptive changes that are potential markers of pre-pandemic viruses. As well as evaluating influenza pathogenesis, mice have also been used as models for evaluation of novel vaccines and anti-viral therapies. Mice can be a useful animal model for studying influenza biology as long as differences between human and mice infections are taken into account.

M2e-Based Universal Influenza A Vaccines

The successful isolation of a human influenza virus in 1933 was soon followed by the first attempts to develop an influenza vaccine. Nowadays, vaccination is still the most effective method to prevent human influenza disease. However, licensed influenza vaccines offer protection against antigenically matching viruses, and the composition of these vaccines needs to be updated nearly every year. Vaccines that target conserved epitopes of influenza viruses would in principle not require such updating and would probably have a considerable positive impact on global human health in case of a pandemic outbreak. The extracellular domain of Matrix 2 (M2e) protein is an evolutionarily conserved region in influenza A viruses and a promising epitope for designing a universal influenza vaccine. Here we review the seminal and recent studies that focused on M2e as a vaccine antigen. We address the mechanism of action and the clinical development of M2e-vaccines. Finally, we try to foresee how M2e-based vaccines could be implemented clinically in the future.

Pathogenesis of infection with 2009 pandemic H1N1 influenza virus in isogenic guinea pigs after intranasal or intratracheal inoculation

To elucidate the pathogenesis and transmission of influenza virus, the ferret model is typically used. To investigate protective immune responses, the use of inbred mouse strains has proven invaluable. Here, we describe a study with isogenic guinea pigs, which would uniquely combine the advantages of the mouse and ferret models for influenza virus infection. Strain 2 isogenic guinea pigs were inoculated with H1N1pdm09 influenza virus A/Netherlands/602/09 by the intranasal or intratracheal route. Viral replication kinetics were assessed by determining virus titers in nasal swabs and respiratory tissues, which were also used to assess histopathologic changes and the number of infected cells. In all guinea pigs, virus titers peaked in nasal secretions at day 2 after inoculation. Intranasal inoculation resulted in higher virus excretion via the nose and higher virus titers in the nasal turbinates than intratracheal inoculation. After intranasal inoculation, infectious virus was recovered only from nasal epithelium; after intratracheal inoculation, it was recovered also from trachea, lung, and cerebrum. Histopathologic changes corresponded with virus antigen distribution, being largely limited to nasal epithelium for intranasally infected guinea pigs and more widespread in the respiratory tract for intratracheally infected guinea pigs. In summary, isogenic guinea pigs show promise as a model to investigate the role of humoral and cell-mediated immunities to influenza and their effect on virus transmission.

An AGM model for changes in complement during pregnancy: neutralization of influenza virus by serum is diminished in late third trimester

Pregnant women in the third trimester are at increased risk of severe influenza disease relative to the general population, though mechanisms behind this are not completely understood. The immune response to influenza infection employs both complement (C') and antibody (Ab). The relative contributions of these components to the anti-viral response are difficult to dissect because most humans have pre-existing influenza-specific Abs. We developed the African green monkey (AGM) as a tractable nonhuman primate model to study changes in systemic innate immunity to influenza during pregnancy. Because the AGMs were influenza-naïve, we were able to examine the role of C' in influenza virus neutralization using serum from non-pregnant animals before and after influenza infection. We determined that serum from naïve AGMs neutralized influenza via C', while post-infection neutralization did not require C', suggesting an Ab-mediated mechanism. The latter mimicked neutralization using human serum. Further, we found that ex vivo neutralization of influenza with both naïve and influenza-immune AGM serum occurred by virus particle aggregation and lysis, with immune serum lysing virus at a much higher rate than naïve serum. We hypothesized that the anti-influenza C' response would diminish late in AGM pregnancy, corresponding with the time when pregnant women suffer increased influenza severity. We found that influenza neutralization capacity is significantly diminished in serum collected late in the third trimester. Strikingly, we found that circulating levels of C3, C3a, and C4 are diminished late in gestation relative to nonpregnant animals, and while neutralization capacity and serum C3a return to normal shortly after parturition, C3 and C4 levels do not. This AGM model system will enable further studies of the role of physiologic and hormonal changes in downregulating C'-mediated anti-viral immunity during pregnancy, and it will permit the identification of therapeutic targets to improve outcomes of influenza virus infection in pregnant women.

Animal models for influenza viruses: implications for universal vaccine development

Influenza virus infections are a significant cause of morbidity and mortality in the human population. Depending on the virulence of the influenza virus strain, as well as the immunological status of the infected individual, the severity of the respiratory disease may range from sub-clinical or mild symptoms to severe pneumonia that can sometimes lead to death. Vaccines remain the primary public health measure in reducing the influenza burden. Though the first influenza vaccine preparation was licensed more than 60 years ago, current research efforts seek to develop novel vaccination strategies with improved immunogenicity, effectiveness, and breadth of protection. Animal models of influenza have been essential in facilitating studies aimed at understanding viral factors that affect pathogenesis and contribute to disease or transmission. Among others, mice, ferrets, pigs, and nonhuman primates have been used to study influenza virus infection in vivo, as well as to do pre-clinical testing of novel vaccine approaches. Here we discuss and compare the unique advantages and limitations of each model.

Characterization and immunogenicity in mice of recombinant influenza haemagglutinins produced in Leishmania tarentolae

The membrane displayed antigen haemagglutinin (HA) from several influenza strains were expressed in the Leishmania tarentolae system. This non-conventional expression system based on a parasite of lizards, can be readily propagated to high cell density (>10(8)cells/mL) in a simple incubator at 26°C. The genes encoding HA proteins were cloned from six influenza strains, among these being a 2009 A/H1N1 pandemic strain from swine origin, namely A/California/07/09(H1N1). Soluble HA proteins were secreted into the cell culture medium and were easily and successfully purified via a His-Tag domain fused to the proteins. The overall process could be conducted in less than 3 months and resulted in a yield of approximately 1.5-5mg of HA per liter of biofermenter culture after purification. The recombinant HA proteins expressed by L. tarentolae were characterized by dynamic light scattering and were observed to be mostly monomeric. The L. tarentolae recombinant HA proteins were immunogenic in mice at a dose of 10μg when administered twice with an oil-in-water emulsion-based adjuvant. These results suggest that the L. tarentolae expression system may be an alternative to the current egg-based vaccine production.

Development and pre-clinical evaluation of two LAIV strains against potentially pandemic H2N2 influenza virus

H2N2 Influenza A caused the Asian flu pandemic in 1957, circulated for more than 10 years and disappeared from the human population after 1968. Given that people born after 1968 are naïve to H2N2, that the virus still circulates in wild birds and that this influenza subtype has a proven pandemic track record, H2N2 is regarded as a potential pandemic threat. To prepare for an H2N2 pandemic, here we developed and tested in mice and ferrets two live attenuated influenza vaccines based on the haemagglutinins of the two different H2N2 lineages that circulated at the end of the cycle, using the well characterized A/Leningrad/134/17/57 (H2N2) master donor virus as the backbone. The vaccine strains containing the HA and NA of A/California/1/66 (clade 1) or A/Tokyo/3/67 (clade 2) showed a temperature sensitive and cold adapted phenotype and a reduced reproduction that was limited to the respiratory tract of mice, suggesting that the vaccines may be safe for use in humans. Both vaccine strains induced haemagglutination inhibition titers in mice. Vaccination abolished virus replication in the nose and lung and protected mice from weight loss after homologous and heterologous challenge with the respective donor wild type strains. In ferrets, the live attenuated vaccines induced high virus neutralizing, haemagglutination and neuraminidase inhibition titers, however; the vaccine based on the A/California/1/66 wt virus induced higher homologous and better cross-reactive antibody responses than the A/Tokyo/3/67 based vaccine. In line with this observation, was the higher virus reduction observed in the throat and nose of ferrets vaccinated with this vaccine after challenge with either of the wild type donor viruses. Moreover, both vaccines clearly reduced the infection-induced rhinitis observed in placebo-vaccinated ferrets. The results favor the vaccine based on the A/California/1/66 isolate, which will be evaluated in a clinical study.

Intranasal H5N1 vaccines, adjuvanted with chitosan derivatives, protect ferrets against highly pathogenic influenza intranasal and intratracheal challenge

We investigated the protective efficacy of two intranasal chitosan (CSN and TM-CSN) adjuvanted H5N1 Influenza vaccines against highly pathogenic avian Influenza (HPAI) intratracheal and intranasal challenge in a ferret model.

Six groups of 6 ferrets were intranasally vaccinated twice, 21 days apart, with either placebo, antigen alone, CSN adjuvanted antigen, or TM-CSN adjuvanted antigen. Homologous and intra-subtypic antibody cross-reacting responses were assessed. Ferrets were inoculated intratracheally (all treatments) or intranasally (CSN adjuvanted and placebo treatments only) with clade 1 HPAI A/Vietnam/1194/2004 (H5N1) virus 28 days after the second vaccination and subsequently monitored for morbidity and mortality outcomes. Clinical signs were assessed and nasal as well as throat swabs were taken daily for virology. Samples of lung tissue, nasal turbinates, brain, and olfactory bulb were analysed for the presence of virus and examined for histolopathological findings.

In contrast to animals vaccinated with antigen alone, the CSN and TM-CSN adjuvanted vaccines induced high levels of antibodies, protected ferrets from death, reduced viral replication and abrogated disease after intratracheal challenge, and in the case of CSN after intranasal challenge. In particular, the TM-CSN adjuvanted vaccine was highly effective at eliciting protective immunity from intratracheal challenge; serologically, protective titres were demonstrable after one vaccination. The 2-dose schedule with TM-CSN vaccine also induced cross-reactive antibodies to clade 2.1 and 2.2 H5N1 viruses. Furthermore ferrets immunised with TM-CSN had no detectable virus in the respiratory tract or brain, whereas there were signs of virus in the throat and lungs, albeit at significantly reduced levels, in CSN vaccinated animals.

This study demonstrated for the first time that CSN and in particular TM-CSN adjuvanted intranasal vaccines have the potential to protect against significant mortality and morbidity arising from infection with HPAI H5N1 virus.

Low dose influenza virus challenge in the ferret leads to increased virus shedding and greater sensitivity to oseltamivir

Ferrets are widely used to study human influenza virus infection. Their airway physiology and cell receptor distribution makes them ideal for the analysis of pathogenesis and virus transmission, and for testing the efficacy of anti-influenza interventions and vaccines. The 2009 pandemic influenza virus (H1N1pdm09) induces mild to moderate respiratory disease in infected ferrets, following inoculation with 10⁶ plaque-forming units (pfu) of virus. We have demonstrated that reducing the challenge dose to 10² pfu delays the onset of clinical signs by 1 day, and results in a modest reduction in clinical signs, and a less rapid nasal cavity innate immune response. There was also a delay in virus production in the upper respiratory tract, this was up to 9-fold greater and virus shedding was prolonged. Progression of infection to the lower respiratory tract was not noticeably delayed by the reduction in virus challenge. A dose of 10⁴ pfu gave an infection that was intermediate between those of the 10⁶ pfu and 10² pfu doses. To address the hypothesis that using a more authentic low challenge dose would facilitate a more sensitive model for antiviral efficacy, we used the well-known neuraminidase inhibitor, oseltamivir. Oseltamivir-treated and untreated ferrets were challenged with high (10⁶ pfu) and low (10² pfu) doses of influenza H1N1pdm09 virus. The low dose treated ferrets showed significant delays in innate immune response and virus shedding, delayed onset of pathological changes in the nasal cavity, and reduced pathological changes and viral RNA load in the lung, relative to untreated ferrets. Importantly, these observations were not seen in treated animals when the high dose challenge was used. In summary, low dose challenge gives a disease that more closely parallels the disease parameters of human influenza infection, and provides an improved pre-clinical model for the assessment of influenza therapeutics, and potentially, influenza vaccines.

Animal models for influenza virus pathogenesis, transmission, and immunology

In humans, infection with an influenza A or B virus manifests typically as an acute and self-limited upper respiratory tract illness characterized by fever, cough, sore throat, and malaise. However, influenza can present along a broad spectrum of disease, ranging from sub-clinical or even asymptomatic infection to a severe primary viral pneumonia requiring advanced medical supportive care. Disease severity depends upon the virulence of the influenza virus strain and the immune competence and previous influenza exposures of the patient. Animal models are used in influenza research not only to elucidate the viral and host factors that affect influenza disease outcomes in and spread among susceptible hosts, but also to evaluate interventions designed to prevent or reduce influenza morbidity and mortality in man. This review will focus on the three animal models currently used most frequently in influenza virus research - mice, ferrets, and guinea pigs - and discuss the advantages and disadvantages of each.

Fast vaccine design and development based on correlates of protection (COPs)

New and reemerging infectious diseases call for innovative and efficient control strategies of which fast vaccine design and development represent an important element. In emergency situations, when time is limited, identification and use of correlates of protection (COPs) may play a key role as a strategic tool for accelerated vaccine design, testing, and licensure. We propose that general rules for COP-based vaccine design can be extracted from the existing knowledge of protective immune responses against a large spectrum of relevant viral and bacterial pathogens. Herein, we focus on the applicability of this approach by reviewing the established and up-coming COPs for influenza in the context of traditional and a wide array of new vaccine concepts. The lessons learnt from this field may be applied more generally to COP-based accelerated vaccine design for emerging infections.

A novel nonhuman primate model for influenza transmission

Studies of influenza transmission are necessary to predict the pandemic potential of emerging influenza viruses. Currently, both ferrets and guinea pigs are used in such studies, but these species are distantly related to humans. Nonhuman primates (NHP) share a close phylogenetic relationship with humans and may provide an enhanced means to model the virological and immunological events in influenza virus transmission. Here, for the first time, it was demonstrated that a human influenza virus isolate can productively infect and be transmitted between common marmosets (Callithrix jacchus), a New World monkey species. We inoculated four marmosets with the 2009 pandemic virus A/California/07/2009 (H1N1pdm) and housed each together with a naïve cage mate. We collected bronchoalveolar lavage and nasal wash samples from all animals at regular intervals for three weeks post-inoculation to track virus replication and sequence evolution. The unadapted 2009 H1N1pdm virus replicated to high titers in all four index animals by 1 day post-infection. Infected animals seroconverted and presented human-like symptoms including sneezing, nasal discharge, labored breathing, and lung damage. Transmission occurred in one cohabitating pair. Deep sequencing detected relatively few genetic changes in H1N1pdm viruses replicating in any infected animal. Together our data suggest that human H1N1pdm viruses require little adaptation to replicate and cause disease in marmosets, and that these viruses can be transmitted between animals. Marmosets may therefore be a viable model for studying influenza virus transmission.

Ferret thoracic anatomy by 2-deoxy-2-(18F)fluoro-D-glucose (18F-FDG) positron emission tomography/computed tomography (18F-FDG PET/CT) imaging

The domestic ferret (Mustela putorius furo) has been a long-standing animal model used in the evaluation and treatment of human diseases. Molecular imaging techniques such as 2-deoxy-2-(¹⁸F)fluoro-D-glucose (¹⁸F-FDG) positron emission tomography (PET) would be an invaluable method of tracking disease in vivo, but this technique has not been reported in the literature. Thus, the aim of this study was to establish baseline imaging characteristics of PET/computed tomography (CT) with ¹⁸F-FDG in the ferret model. Twelve healthy female ferrets were anesthetized and underwent combined PET/CT scanning. After the images were fused, volumes of interest (VOIs) were generated in the liver, heart, thymus, and bilateral lung fields. For each VOI, standardized uptake values (SUVs) were calculated. Additional comparisons were made between radiotracer uptake periods (60, 90, and >90 minutes), intravenous and intraperitoneal injections of ¹⁸F-FDG, and respiratory gated and ungated acquisitions. Pulmonary structures and the surrounding thoracic and upper abdominal anatomy were readily identified on the CT scans of all ferrets and were successfully fused with PET. VOIs were created in various tissues with the following SUV calculations: heart (maximum standardized uptake value [SUV_Max] 8.60, mean standardized uptake value [SUV_Mean] 5.42), thymus (SUV_Max 3.86, SUV_Mean 2.59), liver (SUV_Max 1.37, SUV_Mean 0.99), right lung (SUV_Max 0.92, SUV_Mean 0.56), and left lung (SUV_Max 0.88, SUV_Mean 0.51). Sixty- to 90-minute uptake periods were sufficient to separate tissues based on background SUV activity. No gross differences in image quality were seen between intraperitoneal and intravenous injections of ¹⁸F-FDG. Respiratory gating also did not have a significant impact on image quality of lung parenchyma. The authors concluded that ¹⁸F-FDG PET and CT imaging can be performed successfully in normal healthy ferrets with the parameters identified in this study. They obtained similar imaging features and uptake measurements with and without respiratory gating as well as with intraperitoneal and intravenous ¹⁸F-FDG injections. ¹⁸F-FDG PET and CT can be a valuable resource for the in vivo tracking of disease progression in future studies that employ the ferret model.

Comparison of the effectiveness of antibody and cell-mediated immunity against inhaled and instilled influenza virus challenge

BACKGROUND

To evaluate immunity against influenza, mouse challenge studies are typically performed by intranasal instillation of a virus suspension to anesthetized animals. This results in an unnatural environment in the lower respiratory tract during infection, and therefore there is some concern that immune mechanisms identified in this model may not reflect those that protect against infectious virus particles delivered directly to the lower respiratory tract as an aerosol.

METHOD

To evaluate differences in protection against instilled and inhaled virus, mice were immunized with influenza antigens known to induce antibody or cell-mediated responses and then challenged with 100 LD50 A/PR/8/34 (PR8) in the form of aerosol (inhaled) or liquid suspension (instilled).

RESULTS

Mice immunized with recombinant adenovirus (Ad) expressing hemagglutinin were protected against weight loss and death in both challenge models, however immunization with Ad expressing nucleoprotein of influenza A (NPA) or M2 resulted in greater protection against inhaled aerosolized virus than virus instilled in liquid suspension. Ad-M2, but not Ad-NPA-immunized mice were protected against a lower instillation challenge dose.

CONCLUSIONS

These results demonstrate differences in protection that are dependent on challenge method, and suggest that cell-mediated immunity may be more accurately demonstrated in mouse inhalation studies. Furthermore, the data suggest immune mechanisms generally characterized as incomplete or weak in mouse models using liquid intranasal challenge may offer greater immunity against influenza infection than previously thought.

Immune responses to infection with H5N1 influenza virus

Influenza A H5N1 viruses remain a substantial threat to global public health. In particular, the expanding genetic diversity of H5N1 viruses and the associated risk for human adaptation underscore the importance of better understanding host immune responses that may protect against disease or infection. Although much emphasis has been placed on investigating early virus-host interactions and the induction of innate immune responses, little is known of the consequent adaptive immune response to H5N1 virus infection. In this review, we describe the H5N1 virus-specific and cross-reactive antibody and T cell responses in humans and animal models. Data from limited studies suggest that although initially robust, there is substantial waning of the serum antibody responses in survivors of H5N1 virus infection. Characterization of monoclonal antibodies generated from memory B cells of survivors of H5N1 virus infection has provided an understanding of the fine specificity of the human antibody response to H5N1 virus infection and identified strategies for immunotherapy. Human T cell responses induced by infection with seasonal influenza viruses are directed to relatively conserved internal proteins and cross-react with the H5N1 subtype. A role for T cell-based heterosubtypic immunity against H5N1 viruses is suggested in animal studies. Further studies on adaptive immune responses to H5N1 virus infection in both humans and animals are needed to inform the design of optimal immunological treatment and prevention modalities.

Evaluation of the humoral and cellular immune responses elicited by the live attenuated and inactivated influenza vaccines and their roles in heterologous protection in ferrets

The humoral and cellular immune responses elicited by the trivalent live attenuated influenza vaccine (LAIV) and the trivalent inactivated influenza vaccine (TIV) were evaluated in the ferret model, using newly developed ferret immunological reagents and assays. In contrast to the TIV, which only induced immune responses in primed animals, LAIV induced strong influenza virus-specific serum antibody and T-cell responses in both naive and influenza-seropositive animals. The LAIV offered significant protection against a heterologous H1N1 virus challenge infection in the upper respiratory tract. Influenza virus-specific immunoglobulin A (IgA) and immunoglobulin G (IgG) antibody-secreting cells (ASCs) and influenza virus-specific CD4(+) and CD8(+) T cells were detected in the circulation and local paratracheal draining lymph nodes. The frequency of the influenza-specific ASCs in the local lymph nodes appeared to correlate with the degree of protection in the upper respiratory tract. The protection conferred by the LAIV could be attributed not only to the antibody response but also to the cell-mediated and local mucosal immune responses, particularly in naive ferrets. These findings may explain why the LAIV is immunologically superior and offers immediate protection after a single dose in children.

Influenza virus infection in nonhuman primates

To determine whether nonhuman primates are infected with influenza viruses in nature, we conducted serologic and swab studies among macaques from several parts of the world. Our detection of influenza virus and antibodies to influenza virus raises questions about the role of nonhuman primates in the ecology of influenza.

Advances in the development of universal influenza vaccines

Despite the widespread availability and use of influenza vaccines, influenza still poses a considerable threat to public health. Vaccines against seasonal influenza do not offer protection against pandemic viruses, and vaccine efficacy against seasonal viruses is reduced in seasons when the vaccine composition is not a good match for the predominant circulating viruses. Vaccine efficacy is also reduced in older adults, who are one of the main target groups for vaccination. The continual threat of pandemic influenza, with the known potential for rapid spread around the world and high mortality rates, has prompted researchers to develop a number of novel approaches to providing immunity to this virus, focusing on target antigens which are highly conserved between different influenza A virus subtypes. Several of these have now been taken into clinical development, and this review discusses the progress that has been made, as well as considering the requirements for licensing these new vaccines and how they might be used in the future.