Influenza virus infection of the guinea pig: immune response and resistance

Influenza antivirals and animal models

Influenza A and B viruses are among the most prominent human respiratory pathogens. About 3-5 million severe cases of influenza are associated with 300 000-650 000 deaths per year globally. Antivirals effective at reducing morbidity and mortality are part of the first line of defense against influenza. FDA-approved antiviral drugs currently include adamantanes (rimantadine and amantadine), neuraminidase inhibitors (NAI; peramivir, zanamivir, and oseltamivir), and the PA endonuclease inhibitor (baloxavir). Mutations associated with antiviral resistance are common and highlight the need for further improvement and development of novel anti-influenza drugs. A summary is provided for the current knowledge of the approved influenza antivirals and antivirals strategies under evaluation in clinical trials. Preclinical evaluations of novel compounds effective against influenza in different animal models are also discussed.

Animal Models for Influenza Research: Strengths and Weaknesses

Influenza remains one of the most significant public health threats due to its ability to cause high morbidity and mortality worldwide. Although understanding of influenza viruses has greatly increased in recent years, shortcomings remain. Additionally, the continuous mutation of influenza viruses through genetic reassortment and selection of variants that escape host immune responses can render current influenza vaccines ineffective at controlling seasonal epidemics and potential pandemics. Thus, there is a knowledge gap in the understanding of influenza viruses and a corresponding need to develop novel universal vaccines and therapeutic treatments. Investigation of viral pathogenesis, transmission mechanisms, and efficacy of influenza vaccine candidates requires animal models that can recapitulate the disease. Furthermore, the choice of animal model for each research question is crucial in order for researchers to acquire a better knowledge of influenza viruses. Herein, we reviewed the advantages and limitations of each animal model-including mice, ferrets, guinea pigs, swine, felines, canines, and non-human primates-for elucidating influenza viral pathogenesis and transmission and for evaluating therapeutic agents and vaccine efficacy.

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.

Virus replication kinetics and pathogenesis of infection with H7N9 influenza virus in isogenic guinea pigs upon intratracheal inoculation

Since 2013, avian influenza viruses of subtype H7N9 have been transmitted from poultry to humans in China and caused severe disease. Concerns persist over the pandemic potential of this virus and further understanding of immunity and transmission is required. The isogenic guinea pig model uniquely would allow for investigation into both. Eighteen female isogenic guinea pigs 12-16 weeks were inoculated intratracheally with either A/H7N9 virus (n=12) or PBS (n=6) and sacrificed on days 2 and 7 post-inoculation. Nasal and pharyngeal swabs were taken daily to assess viral replication kinetics and necropsies were performed to study pathogenesis. All animals showed peak virus titers in nasal secretions at day 2 post-inoculation and by day 7 post-inoculation infectious virus titers had decreased to just above detectable levels. At day 2, high virus titers were found in nasal turbinates and lungs and moderate titers in trachea and cerebrum. At day 7, infectious virus was detected in the nasal turbinates only. Histology showed moderate to severe inflammation in the entire respiratory tract and immunohistochemistry (IHC) demonstrated large numbers of viral antigen positive cells in the nasal epithelium at day 2 and fewer at day 7 post-inoculation. A moderate number of IHC positive cells was observed in the bronchi(oli) and alveoli at day 2 only. This study indicates that isogenic guinea pigs are a promising model to further study immunity to and transmission of H7N9 influenza virus.

Animal models for influenza virus transmission studies: a historical perspective

Animal models are used to simulate, under experimental conditions, the complex interactions among host, virus, and environment that affect the person-to-person spread of influenza viruses. The three species that have been most frequently employed, both past and present, as influenza virus transmission models-ferrets, mice, and guinea pigs-have each provided unique insights into the factors governing the efficiency with which these viruses pass from an infected host to a susceptible one. This review will highlight a few of these noteworthy discoveries, with a particular focus on the historical contexts in which each model was developed and the advantages and disadvantages of each species with regard to the study of influenza virus transmission among mammals.

Transmission in the guinea pig model

The ability of an influenza virus to transmit efficiently from human-to-human is a major factor in determining the epidemiological impact of that strain. The use of a relevant animal model to identify viral determinants of transmission, as well as host and environmental factors affecting transmission efficiency, is therefore critical for public health. The characterization of newly emerging influenza viruses in terms of their potential to transmit in a mammalian host is furthermore an important part of pandemic risk assessment. For these reasons, a guinea pig model of influenza virus transmission was developed in 2006. The guinea pig provides an important alternative to preexisting models for influenza. Most influenza viruses do not readily transmit among mice. Ferrets, while highly relevant, are expensive and can be difficult to obtain in high numbers. Moreover, it is generally accepted that efforts to accurately model human disease are strengthened by the use of multiple animal species. Herein, we provide an overview of influenza virus infectivity, growth, and transmission in the guinea pig and highlight knowledge gained on the topic of influenza virus transmission using the guinea pig model.

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.

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.

Animal models to assess the toxicity, immunogenicity and effectiveness of candidate influenza vaccines

INTRODUCTION

Every year, > 100 million doses of licensed influenza vaccine are administered worldwide, with relatively few serious adverse events reported. Initiatives to manufacture influenza vaccines on different platforms have come about to ensure timely production of strain-specific as well as universal vaccines. To prevent adverse events that may be associated with these new vaccines, it is important to evaluate the toxicity of new formulations in animal models.

AREAS COVERED

This review outlines preclinical studies that evaluate safety, immunogenicity and effectiveness of novel products to support further development and clinical trials. This has been done through a review of the latest literature describing vaccines under development.

EXPERT OPINION

The objective of preclinical safety tests is to demonstrate the absence of toxic contaminants and adventitious agents. Additional tests that characterize vaccine content more completely, or demonstrate the absence of exacerbated disease following virus challenge in vaccinated animals, may provide additional data to ensure the safety of new vaccine strategies.

The Role of Animal Models In Influenza Vaccine Research

A major challenge for research on influenza vaccines is the selection of an appropriate animal model that accurately reflects the disease and the protective immune response to influenza infection in humans. Vaccines for seasonal influenza have been available for decades and there is a wealth of data available on the immune response to these vaccines in humans, with well-established correlates of protection for inactivated influenza virus vaccines. Many of the seminal studies on vaccines for epidemic influenza have been conducted in human subjects. Studies in humans are performed less frequently now than they were in the past. Therefore, as the quest for improved influenza vaccines continues, it is important to consider the use of animal models for the evaluation of influenza vaccines, and a major challenge is the selection of an appropriate animal model that accurately reflects the disease and the protective immune response to influenza infection in humans.

The emergence of highly pathogenic H5N1 avian influenza (AI) viruses and the threat of a pandemic caused by AI viruses of this or another subtype has resulted in a resurgence of interest in influenza vaccine research. The development of vaccines for pandemic influenza presents a unique set of obstacles, not the least of which is that the demonstration of efficacy in humans is not possible. As the correlates of protection from pandemic influenza are not known, we rely on extrapolation of the lessons from seasonal influenza vaccines and on data from the evaluation of pandemic influenza vaccines in animal models to guide our decisions on vaccines for use in humans. The features and contributions of commonly used animal models for influenza vaccine research are discussed. The recent emergence of the pandemic 2009 H1N1 influenza virus underscores the unpredictable nature of influenza viruses and the importance of pandemic preparedness.

Serological characterization of guinea pigs infected with H3N2 human influenza or immunized with hemagglutinin protein

Background

Recent and previous studies have shown that guinea pigs can be infected with, and transmit, human influenza viruses. Therefore guinea pig may be a useful animal model for better understanding influenza infection and assessing vaccine strategies. To more fully characterize the model, antibody responses following either infection/re-infection with human influenza A/Wyoming/03/2003 H3N2 or immunization with its homologous recombinant hemagglutinin (HA) protein were studied.

Results

Serological samples were collected and tested for anti-HA immunoglobulin by ELISA, antiviral antibodies by hemagglutination inhibition (HI), and recognition of linear epitopes by peptide scanning (PepScan). Animals inoculated with infectious virus demonstrated pronounced viral replication and subsequent serological conversion. Animals either immunized with the homologous HA antigen or infected, showed a relatively rapid rise in antibody titers to the HA glycoprotein in ELISA assays. Antiviral antibodies, measured by HI assay, were detectable after the second inoculation. PepScan data identified both previously recognized and newly defined linear epitopes.

Conclusions

Infection and/or recombinant HA immunization of guinea pigs with H3N2 Wyoming influenza virus resulted in a relatively rapid production of viral-specific antibody thus demonstrating the strong immunogenicity of the major viral structural proteins in this animal model for influenza infection. The sensitivity of the immune response supports the utility of the guinea pig as a useful animal model of influenza infection and immunization.

Animal Models for Influenza Virus Pathogenesis and Transmission

Influenza virus infection of humans results in a respiratory disease that ranges in severity from sub-clinical infection to primary viral pneumonia that can result in death. The clinical effects of infection vary with the exposure history, age and immune status of the host, and also the virulence of the influenza strain. In humans, the virus is transmitted through either aerosol or contact-based transfer of infectious respiratory secretions. As is evidenced by most zoonotic influenza virus infections, not all strains that can infect humans are able to transmit from person-to-person. Animal models of influenza are essential to research efforts aimed at understanding the viral and host factors that contribute to the disease and transmission outcomes of influenza virus infection in humans. These models furthermore allow the pre-clinical testing of antiviral drugs and vaccines aimed at reducing morbidity and mortality in the population through amelioration of the virulence or transmissibility of influenza viruses. Mice, ferrets, guinea pigs, cotton rats, hamsters and macaques have all been used to study influenza viruses and therapeutics targeting them. Each model presents unique advantages and disadvantages, which will be discussed herein.

Animal models for the preclinical evaluation of candidate influenza vaccines

At present, new influenza A (H1N1)2009 viruses of swine origin are responsible for the first influenza pandemic of the 21st Century. In addition, highly pathogenic avian influenza A/H5N1 viruses continue to cause outbreaks in poultry and, after zoonotic transmission, cause an ever-increasing number of human cases, of which 59% have a fatal clinical outcome. It is also feared that these viruses adapt to replication in humans and become transmissible from human to human. The development of effective vaccines against epidemic and (potentially) pandemic viruses is therefore considered a priority. In this review, we discuss animal models that are used for the preclinical evaluation of novel candidate influenza vaccines. In most cases, a tier of multiple animal models is used before the evaluation of vaccine candidates in clinical trials is considered. Commonly, vaccines are tested for safety and efficacy in mice, ferrets and/or macaques. The use of each of these species has its advantages and limitations, which are addressed here.

The ferret model for influenza

A major challenge in influenza research is the selection of an appropriate animal model that accurately reflects the disease and protective immune response to influenza infection in humans. Ferrets are exquisitely susceptible to infection with human influenza viruses and are widely believed to be the ideal small animal model for influenza research. Mice have also been used for influenza vaccine research for decades. Ferrets are used as an animal model for the study of influenza because they are susceptible to human influenza viruses and develop some of the symptoms of influenza that are seen in humans. Although they are not discussed in detail in this unit, hamsters, guinea pigs, and both cotton rats (Sigmodon) and rats (Rattus) have also been used for influenza research.

The mouse model for influenza

A major challenge in influenza research is the selection of an appropriate animal model that accurately reflects the disease and protective immune response to influenza infection in humans. Ferrets are exquisitely susceptible to infection with human influenza viruses and are widely believed to be the ideal small animal model for influenza research. Mice have also been used for influenza vaccine research for decades. Although human influenza viruses generally cause disease in mice only if they are first adapted to the species, the ready availability of mice, their relatively low cost, and the variety of genetic backgrounds and targeted defects, and the immunologic reagents available make the mouse an attractive and heavily utilized animal model for studies of influenza. Although they are not discussed in detail in this unit, hamsters, guinea pigs, cotton rats (Sigmodon), and rats (Rattus) have also been used for influenza research.

Bronchointerstitial pneumonia in guinea pigs following inoculation with H5N1 high pathogenicity avian influenza virus

The H5N1 high-pathogenicity avian influenza (HPAI) viruses have caused widespread disease of poultry in Asia, Africa and the Middle East, and sporadic human infections. The guinea pig model has been used to study human H3N2 and H1N1 influenza viruses, but knowledge is lacking on H5N1 HPAI virus infections. Guinea pigs were inoculated intranasally or intragastrically with A/Vietnam/1203/04 (VN/04) or A/Muscovy duck/Vietnam/209/05 (MDk/VN/05) viruses. Mild listlessness was seen at 2 and 3 days postinoculation (DPI) in guinea pigs inoculated intranasally with VN/04 virus. At 5 DPI, the guinea pigs had bronchointerstitial pneumonia and virus was identified in bronchiolar epithelium and alveolar macrophages. Virus was isolated from the lungs but was lacking from other organs. Minimal lung lesions were seen in intranasal MDk/VN/06 group and virus was not detected, but serologic evidence of infection was observed. Intragastric exposure failed to produce infection or lesions with either virus. The localized respiratory disease in guinea pigs with H5N1 viruses was very similar to that of H3N2 and H1N1 influenza in humans and was less severe than reported for H5N1 human cases.

Histopathology and growth kinetics of influenza viruses (H1N1 and H3N2) in the upper and lower airways of guinea pigs

Recent investigations have shown that guinea pigs are important for the study of influenza A virus (IAV) transmission. However, very little is known about IAV replication and histopathology in the guinea pig respiratory tract. Here, we describe viral growth kinetics, target cells and histopathology in the nasosinus, trachea and lungs of IAV-infected guinea pigs. We found that guinea pigs infected with either A/Puerto Rico/8/34 (H1N1) or A/Hong Kong/8/68 (H3N2) developed a predominantly upper airway infection with high nasal viral titres. IAV grew to moderate titres in the lungs but induced marked inflammatory responses, resulting in severe bronchopneumonia and alveolitis. Although non-lethal at the high dose of 2x10(6) p.f.u., infections with these IAV strains were associated with reduced weight gain. IAV infection in guinea pigs is characterized by extensive viral replication in the ciliated nasal epithelial cells followed by heavy nasal mucus secretion.

Influenza virus NS vectors expressing the mycobacterium tuberculosis ESAT-6 protein induce CD4+ Th1 immune response and protect animals against tuberculosis challenge

Infection with Mycobacterium tuberculosis remains a major cause of morbidity and mortality all over the world. Since the effectiveness of the only available tuberculosis vaccine, Mycobacterium bovis bacillus Calmette-Guérin (BCG), is suboptimal, there is a strong demand to develop new tuberculosis vaccines. As tuberculosis is an airborne disease, the intranasal route of vaccination might be preferable. Live influenza virus vaccines might be considered as potential vectors for mucosal immunization against various viral or bacterial pathogens, including M. tuberculosis. We generated several subtypes of attenuated recombinant influenza A viruses expressing the 6-kDa early secretory antigenic target protein (ESAT-6) of M. tuberculosis from the NS1 reading frame. We were able to demonstrate the potency of influenza virus NS vectors to induce an M. tuberculosis-specific Th1 immune response in mice. Moreover, intranasal immunization of mice and guinea pigs with such vectors induced protection against mycobacterial challenge, similar to that induced by BCG vaccination.

The guinea pig as a transmission model for human influenza viruses

The severity of epidemic and pandemic influenza outbreaks is dictated in part by the efficiency with which the causative strain transmits between human hosts. The mechanisms underlying influenza virus spread are poorly understood, in part because of the lack of a convenient animal model to study this phenomenon. Indeed, despite extremely efficient transmission among humans and virulence in the mouse model, we have shown that even the 1918 pandemic influenza virus does not transmit between mice. We therefore evaluated the guinea pig as a model mammalian host for influenza virus. Using the recent human isolate A/Panama/2007/99 (Pan/99) (H3N2) virus, we found that guinea pigs were highly susceptible to infection with the unadapted virus (ID(50) = 5 plaque-forming units). Pan/99 virus grew to high titers in the upper respiratory tract and was shed in nasal washings of infected animals. Moreover, influenza virus was transmitted from infected guinea pigs to noninfected guinea pigs housed in the same cage, an adjacent cage, and a cage placed 91 cm away. Our results demonstrate that influenza virus can pass between guinea pigs by means of droplet spread and thereby establish the suitability of the guinea pig as a model host for influenza virus transmission studies.

Lung alterations in guinea-pigs infected with influenza virus

Guinea-pigs were infected intranasally with influenza A Hong Kong 68 (H3N2) virus. Infective particles were re-isolated from lung homogenates up to 3 days after inoculation and indicated local replication. The subsequent lung inflammatory stages were studied by light microscopy, scanning and transmission electron microscopy (TEM). Lung alterations appeared after 24 h and intensified up to 7 days after virus inoculation, progressively decreasing until 3 weeks thereafter. The damage was reversible and complete restoration of structure was obtained within 5 weeks. The lesions commenced with the infiltration of bronchiolar and alveolar walls by polymorphonuclear cells, histiocytes and macrophages. A purulent exudate was seen to occupy the bronchiolar lumen. Cilia disappeared from tracheal and bronchiolar epithelia. Tracheal epithelium desquamated in some animals. TEM examination showed deterioration in type I pneumocytes, an increase in type II pneumocytes and concomitant damage to alveolar capillaries. Alveolar oedema and fibrinous deposits were seen. The pleura presented slight modifications. These results show that infection of guinea-pigs with influenza virus is a useful model for the study of lung pathology associated with a non-lethal respiratory viral infection.